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Title: Structural Classification and Diversity of Terpenoids: Focus on Monoterpenoids, Sesquiterpenoids, and Other Terpenoid Structural Types

Structural Classification and Diversity of Monoterpenoids Across Plant and Microbial Sources

Structural Classification and Distribution of Terpenoids in Plant and Microbial Sources

Terpenoids constitute a specialized class of organic compounds biosynthesized by most plants, animals, and microorganisms (including bacteria and fungi), with over 90,000 known terpenoids exhibiting diverse biological activities [1]. Coined by August Kekulé in 1863 to describe a compound from Pistacia terebinthus, the term “terpene” refers to molecules derived from isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP) [1]. These precursors are synthesized via the mevalonic acid (MVA) pathway (eukaryotes, archaea) or the 2-C-methylerythritol phosphate (MEP) pathway (higher plants, eubacteria, algae) [1]. Under terpene synthase, IPP and DMAPP undergo head-to-tail condensation to form geranyl diphosphate (GPP, C10; monoterpenoid precursor) and farnesyl diphosphate (FPP, C15; sesquiterpenoid precursor) [1]. Enzymatic cyclization, oxidation, and rearrangement of these chains yield diverse structures, classified by the isoprene principle into monoterpenoids (C10), sesquiterpenoids (C15), diterpenoids (C20), triterpenoids (C30), and tetraterpenoids (C40) [1].

Monoterpenoids

Widely distributed in plants and microbes, monoterpenoids exhibit source-specific profiles, enantiomeric signatures, and regulatory dynamics. Plant sources include Centella asiatica (20.20% monoterpenoid hydrocarbons, 5.46% oxygenated monoterpenoids [2]), extra virgin olive oils (EVOOs; (E)-β-ocimene at high maturation indices [3]), and three unnamed plant sources (93.4–72.8% monoterpenes, dominated by limonene and α-pinene [4]). Microbial sources are more limited: only four monoterpenoids (pestalotiolactones C-D, aspergerthinacids A-B) have been isolated from Aspergillus species in five years [1], while Laetiporus sulphureus NWAFU-1 harbors 8 monoterpene synthases (mono-TPSs) with high similarity to Postia placenta’s STS25 (catalyzes GPP cyclization to myristene and linalool [5]).

Key plant-derived monoterpenoids include α-pinene, β-pinene, limonene, linalool, and carvone [6][7][8][9]. Chiral monoterpenes show species-specific enantiomerism: pine species (e.g., Pinus radiata) emit (2)-enantiomers of α-pinene/limonene, while cypress species (e.g., Cupressus glabra) favor (1)-enantiomers [10]. Genetic regulation is exemplified by Syringa oblata’s SoGPPS2: silencing reduces ocimene release by 68.6%, while overexpression in Antirrhinum majus increases production 3.3-fold [11]. Environmental and seasonal factors influence profiles: lemon grass monoterpenoids (α-ocimene, nerol) are stable across light conditions [12], while juniper monoterpene hydrocarbons peak in fall (except β-pinene, sabinene, etc. [13]).

Rich plant sources include Lauraceae (linalool, 1,8-cineole, citral [14]), citrus peels (linalool, limonene [15]), and lavender taxa (L. angustifolia, L. × intermedia, L. latifolia; variable major compounds: linalyl acetate, 1,8-cineole [16]).

Sesquiterpenoids

Exhibiting greater structural diversity than monoterpenoids, sesquiterpenoids are abundant in plants and microbes. Plant sources include C. asiatica (68.80% sesquiterpenoid hydrocarbons, 3.90% oxygenated sesquiterpenoids [2]), EVOOs (17.7–62.1% sesquiterpene hydrocarbons, α-copaene as main component [3]), and Calypogeia integristipula (46.54–71.19% sesquiterpenes [17]). Microbial sources are highly diverse: 81 Aspergillus sesquiterpenoids span skeletons (bergamotane, bisabolane, drimane [1]), while L. sulphureus NWAFU-1 has 15 sesquiterpene synthases (STSs) clustered into four clades [5].

Plant-derived sesquiterpenoids include β-caryophyllene, α-humulene, and germacrene D [6][18][19][20]. Seasonal variation is less pronounced than in monoterpenoids: only three juniper sesquiterpenes (germacrene-B, germacrene-D, α-bulnesene) peak in fall [13]. Tissue-specific regulation is seen in Melaleuca alternifolia (sesquiterpenoids upregulated in roots [21]). Lauraceae plants yield 362 sesquiterpenoids (megastigmane, eudesmane types [14]), while Achillea species show species-specific skeletons (e.g., A. pratensis: eudesmanolides; A. millefolium: guaianolides, germacranes, eudesmanolides [22]). Zingiberaceae essential oils also feature sesquiterpenes as main components [23].

Other Terpenoid Types

Diterpenoids, sesterterpenoids, triterpenoids, and meroterpenoids expand terpenoid diversity. Aspergillus species yield 24 indole-skeleton diterpenoids (e.g., ascandinines A−D [1]), 44 sesterterpenoids (e.g., asperterpenoids G-P [1]), 4 triterpenoids (e.g., nidulanosides A-B [1]), and meroterpenoids (e.g., aspergienynes A–I [1]). Plant sources include C. asiatica (triterpene saponins: madecassoside, asiaticoside [2]), lemon grass (8 diterpenes, 3 triterpenes; ginkgolide C increased under shade [12]), and Miconia hookeriana (kaurane diterpenes [20]). L. sulphureus produces lanostanoid/ergosteroid triterpenoids (dehydrosulfurenic acid [5]), while Lauraceae diterpenoids have unique cage-like structures (ryanodane-type in C. cassia [14]). Juniper contains two non-seasonal diterpenes [13].

Terpenoid Distribution and Key Characteristics by Class

Terpenoid Class	Key Plant Sources & Profiles	Key Microbial Sources & Profiles	Regulatory/Structural Notes	Citations
Monoterpenoids (C10)	Centella asiatica (20.20% hydrocarbons, 5.46% oxygenated; myrcene predominant); EVOOs ((E)-β-ocimene at high MI); Lauraceae (linalool, 1,8-cineole); lavender taxa (variable major compounds: linalyl acetate, 1,8-cineole); citrus peels (linalool, limonene)	Aspergillus (4 compounds: pestalotiolactones C-D, aspergerthinacids A-B); Laetiporus sulphureus (8 mono-TPSs similar to Postia placenta STS25)	Species-specific enantiomers (pines: (2)-α-pinene/limonene; cypresses: (1)-enantiomers); SoGPPS2 regulates ocimene production; stable under light (lemon grass); fall peaks (juniper, except β-pinene)	[1][2][3][4][6][7][10][8][5][12][11][13][14][15][16]
Sesquiterpenoids (C15)	Centella asiatica (68.80% hydrocarbons, 3.90% oxygenated; α-humulene, β-caryophyllene predominant); EVOOs (17.7–62.1% hydrocarbons; α-copaene main); Calypogeia integristipula (46.54–71.19% sesquiterpenes); Lauraceae (362 compounds: megastigmane, eudesmane); Achillea (species-specific skeletons); Zingiberaceae (main components)	Aspergillus (81 compounds: bergamotane, bisabolane, drimane); Laetiporus sulphureus (15 STSs in 4 clades)	Less seasonal variation (juniper: 3 fall-peaking sesquiterpenes); root upregulation (Melaleuca alternifolia); novel dimers/lactones (Lindera aggregata)	[1][2][3][6][18][17][8][5][19][20][13][21][14][22][23]
Diterpenoids (C20)	C. asiatica (triterpene saponins); lemon grass (8 diterpenes; ginkgolide C increased under shade); Miconia hookeriana (kaurane diterpenes); Lauraceae (cage-like structures: ryanodane-type); juniper (2 non-seasonal diterpenes)	Aspergillus (24 indole-skeleton compounds: ascandinines A−D)	Unique cage-like structures in Lauraceae	[1][2][12][20][13][14]
Sesterterpenoids	—	Aspergillus (44 new compounds: asperterpenoids G-P, ophiobolin P1)	Pentacyclic 5/7/3/6/5 skeleton (asperterpenoids G-P)	[1]
Triterpenoids (C30)	C. asiatica (asiaticoside H/I); lemon grass (3 triterpenes); L. sulphureus (lanostanoid/ergosteroid: dehydrosulfurenic acid)	Aspergillus (4 new compounds: nidulanosides A-B)	Derived from lanosterol-to-ergosterol pathway (L. sulphureus)	[1][2][5][12]
Meroterpenoids	—	Aspergillus (aspergienynes A–I, austalide V/W)	Mixed biosynthetic origins; heptacyclic ring system (austalide V/W)	[1]

Monoterpenoid and Sesquiterpenoid Composition Variability Across Plant Organs and Phenological Stages

Monoterpenoids and sesquiterpenoids exhibit substantial structural diversity and compositional variability across plant organs, phenological stages, and microbial sources. Organ-specific terpenoid profiles are common across plant species, with distinct subclass proportions and key components varying by organ type. Phenological and processing-related changes further drive variability, with terpenoid content, composition, and emission patterns shifting across growth stages, maturation, and post-harvest processing. Microbial terpenoid profiles also vary by strain, environmental factors (e.g., light spectra, flower position), and growth stage, while intraspecific variability is notable in plant and microbial systems, with key components showing negative correlations or bimodal distributions across individuals.

Terpenoid Variability Across Plant Organs

Plant Species	Organ(s)	Key Terpenoid Components & Proportions	Reference
Hemerocallis spp.	Roots, leaves, flower buds	Roots: 14 terpenoids; leaves: 2 terpenoids; H. minor flower buds: 13 terpenoids	[24]
Hemerocallis hybrid cultivars	—	Oxygen-containing monoterpenes (e.g., 1,8-eucalyptol) dominant	[24]
Callicarpa oppositifolia	Inflorescence, leaves	Inflorescence: stronger terpenoid bands; leaves: high α-pinene, carene, caryophyllene	[25]
Heracleum species	Underground parts, fruits	Underground: 23.2–89.2% monoterpenes (β-pinene, (Z)-β-ocimene, limonene, α-pinene); fruits: aliphatic esters	[26]
Rhododendron dauricum	Stems, leaves	Stems: >19% α-pinene, >19% limonene; leaves: 28.3% caryophyllene, 15.0% humulene (no α-pinene/limonene)	[27]
Rhododendron adamsii	Stems, leaves	Stems: germacrene D, germacrene B (absent in leaves)	[27]
Unspecified plant species	Leaves, petals, disc florets/bracts	Total terpenoids: 69.4% (33.7% monoterpenoids, 34.7% sesquiterpenoids); leaves: 2× sesquiterpenoids > monoterpenoids; petals: opposite; disc florets/bracts: even proportions	[28]
Thuja plicata seedlings	Scales, needles	Scales: 4× higher total monoterpenes, 20× higher α-thujone; higher sabinene, myrcene, α-pinene, limonene, α-thujene; needles: higher β-pinene	[29]
Schisandra sphenanthera	Stem, leaf, fruit	Stem: γ-muurolene, δ-cadinol, trans-farnesol; leaf: α-cadinol, neoisolongifolene-8-ol; fruit: isospathulenol, α-santalol, cedrenol, longiverbenone	[30]
Citrus	—	Released VOCs: 16 monoterpenes, 9 sesquiterpenes, 1 oxygenated monoterpene (linalool oxide), 1 sesquiterpene alcohol (elemol)	[31]

Terpenoid Variability Across Phenological Stages & Processing

System/Plant	Stage/Processing Step	Terpenoid Changes	Reference
Grape berries	Maturation (flesh)	Free monoterpenes: ~40% increase (geraniol dominant); bound monoterpenes (linalool dominant) > free forms throughout	[32]
Grape berries	Maturation (tips)	Geranic acid drives monoterpene increases	[32]
Grape berry juice processing	Pressing, enzyme treatment	β-pinene: increase → decrease; 1,8-cineol: decrease post-enzyme/pressing; β-damascenone: significant increase	[33]
Coniferous trees	Current-year foliage (seasonal)	β-3-carene: spring peak → decrease; camphene/bornyl acetate: increase; old foliage: constant	[34]
Citrus	Juvenile vs. mature leaves	Monoterpenes: 70.1% (juvenile) → 93.1% (mature); sabinene most abundant; sesquiterpenes: 21.0% → 2.5% (trans-α-caryophyllene, trans-α-farnesene, α-humulene, valencene reduced)	[31]
Myristica fragrans	Foliation (Fo), flowering (Fl), fruiting (Fr)	Monoterpenoids > sesquiterpenoids; oxygenated monoterpenes (OM): 63.1% Fo, 49.4% Fl, 61.9% Fr; monoterpene hydrocarbons (HM): 11.0% Fo, 12.8% Fl, 18.8% Fr; oxygenated sesquiterpenes (OS) > sesquiterpene hydrocarbons (HS); geranial/neral: 54.7% Fo, 41.4% Fl, 54.0% Fr	[35]
Arum maculatum appendixes	Early (I-2, II) vs. pollination (III)	Sesquiterpenes (dihydro-neoclovene, Z/E-caryophyllene, α-humulene, δ-cadinene): accumulate early → decrease; monoterpenes (dihydro-myrcene, 3,7-dimethyl-1-octene, β-citronellene, limonene): accumulate → peak at III	[36]
Plant galls vs. leaves	Developmental stages	Galls: up to 24× higher monoterpenes (constant levels); leaves: monoterpenes decrease with maturity; galls: ~10× higher biosynthetic capacity; Type 1 trees: similar gall/leaf composition; Type 2 trees: distinct gall composition	[37]
Olive oil	Fruit maturation	Sesquiterpene hydrocarbons: α-copaene (9.0% → 43.3%), cyclosativene increase; (E,E)-α-farnesene: increase → drop; oxygenated sesquiterpenes: constant → decline; monoterpenes ((E)-β-ocimene): only at high maturation indices	[3]
Grape berries	Véraison to maturity/harvest	32 monoterpenoids (free/glycosylated) increase; glycosylated > free at maturity/harvest; glycosylated: decrease until véraison → increase	[38]
Arisaema gigantea flowers	Female vs. male vs. wilted stages	Female: β-myrcene, (E)-β-ocimene; male: linalool, (Z,E)-α-farnesene, (E,E)-α-farnesene + oxygenated derivatives; wilted: no β-myrcene/(E)-β-ocimene	[39]
Coniferous needles	Seasonal (winter vs. spring)	Total terpenes/monoterpenes/sesquiterpenes: highest in winter, lowest in spring; crown-fire species < surface-fire species (all seasons); non-fire species > surface-fire species (winter/summer)	[40]

Microbial Terpenoid Variability

Microbial System	Factor(s)	Terpenoid Changes	Reference
Strains A4, E19, KAN	Strain identity	A4: α-pinene, β-pinene, β-myrcene; E19: linalool, caryophyllene, β-pinene; KAN: ocimene, β-myrcene	[41]
Strain E19	Light spectra (CHD, AP67, SOL)	Higher α-pinene under CHD; higher linalool under AP67/SOL	[41]
Strains E19, KAN	Flower position (MTB, STB, SB)	β-myrcene/ocimene highest in main top buds (MTBs) under AP67/SOL; KAN: β-myrcene gradient across MTBs/STBs/SBs	[41]
Strains Critical Mass, Lemon Wheel, Rockstar Kush	Growth stage	Total monoterpene emission rates: 10, 7, 6 µg dwg⁻¹ h⁻¹ respectively	[42]
Microbial flowering rooms	Plant age (48 vs. 80 days)	Monoterpene concentrations: 150 ppb (48 days) → 1660 ppb (80 days; >10× higher)	[42]

Intraspecific Terpenoid Variability

System	Key Observations	Reference
99 sugi trees	Sabinene (8.8–73.3%) and α-pinene (14.5–73.3%) with strong negative correlation (r = −0.83)	[43]
100 kuromatsu trees	β-pinene (26.5–66.3%) and α-pinene (18.7–46.9%) with negative correlation (r = −0.79)	[43]
Tree foliar terpenes	Main monoterpenes (cineole, α-pinene, limonene): CVs 28–80%; sesquiterpenes (aromadendrene, globulol, β-eudesmol, cryptomeridiol): bimodal distributions for β-eudesmol/cryptomeridiol; two chemical forms (aromadendrene/globulol vs. eudesmols)	[44]
Intact fruit cultivars	Terpenes/terpenoids (m/z 135, 137, 153, 205) distinguish whole vs. cut samples	[45]
Grape varieties 87-1 vs. Kyoho	87-1: 14 monoterpenes; Kyoho: 7 monoterpenes; VvPNLinNer1 expression increases post-veraison in 87-1 (not detected in Kyoho)	[46]
Bryophytes	Gametophytes: monoterpene/sesquiterpene hydrocarbons; sporophytes: irregular terpenoids (e.g., 6-methyl-5-hepten-2-one) exclusive to hypophyses	[47]

Terpenoid Structural Diversity and Quantitative Profiles in Essential Oils

Monoterpenoid structural diversity and quantitative profiles vary significantly across microbial and plant sources, as well as across different strains, genotypes, and extraction conditions. Among microbial strains, terpene profiles show clear differences: strain A4 is dominated by α-pinene with additional higher concentrations of β-pinene and β-myrcene, E19 by linalool, caryophyllene, and β-pinene, and KAN by ocimene and β-myrcene [41]. Light spectra and strain interactions further influence monoterpenoid concentrations: for example, strain A4 has considerably higher α-pinene concentrations than other strains under all light spectra, with E19’s α-pinene levels increasing significantly under CHD compared to LED lights [41]. Linalool concentrations in E19 are significantly higher under AP67 and SOL than CHD, while no such differences are observed for A4 and KAN [41]. In plant sources, six Hedychium accessions show distinct monoterpenoid dominance: monoterpenoids are the primary volatile organic compounds (VOCs) in 'ZS', 'Gaoling', 'Jin', 'Caixia', and 'Zhaoxia', while sesquiterpenoids are foremost in 'KMH' [48].

Monoterpenoids often dominate essential oils across diverse plant species, with key examples shown in the table below:

Key Monoterpenoid Dominance in Plant Essential Oils

Plant Source	Dominant Monoterpenoid(s) & Concentrations	Citation
Peppermint	(–)-menthol (40.53%), (–)-menthone (14.63%), (+)-menthofuran (4.32%), (+)-menthyl acetate (4.07%), 1,8-cineole (6.53%), (–)-limonene (2.57%)	[49]
Kumquat peel	Limonene (96%), myrcene (1.6–1.7%)	[50]
R. officinalis	1,8-cineole (40.80%), α-pinene (26.18%), camphor (19.53%)	[51]
P. ruderale	p-cymene (26.10%), γ-terpinene (20.54%)	[52]
M. alternifolia	γ-terpinene (21.7%, monoterpene hydrocarbons), terpinen-4-ol (38.6%, oxygenated monoterpenes)	[53]
C. martinii	Geraniol (76.9%, oxygenated monoterpenes)	[53]
M. fragrans (all phenological stages)	Geranial + neral (54.7% Fo, 41.4% Fl, 54.0% Fr)	[35]
Z. mezoneurispinosum (leaves)	α-pinene (55.4%), sabinene (10.1%), (E)-β-ocimene (8.9%)	[54]
Z. mezoneurispinosum (trunk bark)	α-pinene (50.9%), sabinene (9.7%), linalool (5.1%)	[54]
Spearmint	Carvone (28%), menthol (23%)	[9]
Peppermint (alternative)	Menthol (40%)	[9]
Rosemary	Cineol (45%)	[9]

Hybrid plants can exhibit altered monoterpenoid profiles: mint hybrids show a high level of carvone and almost no menthone or menthol, resembling spearmint [55]. In table grapes, 25 monoterpenoids are identified at harvest, encompassing cyclic acids (e.g., geranic acid), cyclic alkenes (e.g., β-trans-ocimene, β-cis-ocimene), cyclic alcohols (e.g., linalool, nerol, geraniol), cyclic aldehydes (e.g., neral, geranial), ethers (e.g., cis-pyran linalool oxide, trans-rose oxide), acyclic alkenes (e.g., γ-terpinen, limonene), and acyclic alcohols (e.g., α-terpineol, 4-terpineol) [56]. Monoterpenes can also be classified into non-oxygenated (e.g., α-pinene, camphene, β-pinene, myrcene, limonene) and oxygenated (e.g., cineole, camphor, borneol, linalool, terpineol) subclasses [57]. The most abundant monoterpenes in some plant systems include α-pinene (48 ± 4), camphene (13.8 ± 1), limonene (11.1 ± 1.7), and β-pinene (9.4 ± 0.5) in well-watered plants grown on unamended soils [57]. Across E. globulus leaf samples, 15 monoterpenes and 10 monoterpenoids induced an EAG response, with 131 compounds putatively identified in the volatile fraction [58]. Seven major (α-pinene, camphene, myrcene, β-pinene, limonene, bornyl acetate, β-3-carene) and three minor (thujone, terpinolene, α-phelladrene) monoterpene components were identified in needle samples, with seasonal variations observed: β-3-carene peaked in spring expanding foliage and decreased as camphene and bornyl acetate increased, while old foliage showed constant composition [34]. Monoterpenes such as α-pinene, camphene, sabinene, myrcene, β-pinene, 3-carene, limonene, and β-phellandrene were identified in a pine-oak forest, with concentrations highest near the forest floor and decreasing with height, and diurnal patterns showing morning peaks and positive correlation with ambient temperature (r² = 0.61) [59]. A. fragrantissima had high phytochemical richness (17 classes), with monoterpenoids among the main classes [60]. Six monoterpenes were identified in Norway spruce via fastGC-PTR-ToF-MS, with α- and β-pinene abundant in pine species [61]. Sugi trees showed enormous variations in monoterpene proportions: sabinene (8.8–73.3%) and α-pinene (14.5–73.3%) negatively correlated (r = −0.83, P < 0.0001), while kuromatsu trees had β-pinene (26.5–66.3%) and α-pinene (18.7–46.9%) negatively correlated (r = −0.79, P < 0.0001) [43]. The aromatic oil of tea tree contains over 100 phytochemicals, majorly monoterpenes, sesquiterpenes, and their related alcohols [62]. In wines, 14 monoterpenoids were detected, with WTW having the highest number (12) and concentration (1047.6 μg L⁻¹) [63]. Metabolite profiling of four culinary herb species (O. vulgare, R. officinalis, O. basilicum, O. majorana) revealed 25 monoterpenes, with β-myrcene produced in all; R. officinalis had the highest diversity (17 monoterpenes), and O. vulgare the highest total amount (8.99 µmol g FW⁻¹) [64]. Limonene, α-pinene, camphene, myrcene, and β-pinene were consistently present in significant quantities in soil samples, with foliar emission profiles similar except for decreased camphene and increased 3-carene [65]. Mint essential oil analyzed by GC-MS identified 17 components (98.72% of the oil), including monoterpene hydrocarbons (32.82%) and oxygenated hydrocarbons (alcohols 53.42%, aldehydes 4.67%, ketones 7.81%) [66]. Mentha piperita plants contain ~3 % volatile oils, consisting of >50 different compounds. The major EO components, which comprise ~60 % of total oil volume, are 1,8-cineole, linalool, (−) menthone, (−) menthol, and (+) pulegone [67]. Monoterpene accumulation values (μg per g fresh weight) were 4.69 for the control group, 15–20 for the singly-inoculated groups, and 29.2 for the WCS417r+SJ04 group (~6-fold higher than the control value; P=0.001) [67]. Yields of the major EO components ((+) pulegone, (−) menthone, (−) menthol, 1,8-cineole, and linalool) were higher in all four treatment groups than in the control group. Menthol yield increased to 2.09 μg/g fresh weight (P<0.05) in the WCS417r+SJ04 group in comparison with 0.22 μg/g fresh weight in the control group and 1.1–1.2 μg/g fresh weight in the other three treatment groups. Similar trends were observed for menthone, 1,8–cineole, linalool, and pulegone; yield was always higher for the WCS417r+SJ04 group than the other three treatment groups [67]. Components from EO of leaves and stems of R. adamsii were isolated, including α- and β-pinene, β-myrcene, cis-β-ocimene, isoledene, aromadendrene, humulene, β-farnesene, δ-cadinene, trans-nerolidol, spathulenol, β-elemenone, and germacrone. EO from stems contained germacrene D and B not found in leaves [27]. For R. dauricum, ~92% of leaf EO components were identified, and stem EO components were fully identified. Leaf and stem compositions differed: stems of freshly collected plants had α-pinene and limonene (>19% each), absent in leaves which had caryophyllene (28.3%), humulene (15.0%), caryophyllene-α-oxide (6.1%) [27]. R. aureum leaf and stem EO compositions differed but shared nonanoic acid, calamene, α-calacorene, squalene. Sample 4a (leaves) had high-molecular-weight alkanes (23-26 C atoms), major components including calarene, carvacrol, squalene, bakharis oxide, friedelin. Stem EO (4b) had hexanoic acid (6.3%), α-cadinol (10.52%), squalene (3.4%), bakharis oxide (4.64%). Samples 5a/5b differed from 4a/4b, with 5a containing α- and β-pinene, β-myrcene, terpineol-4 not in 4a [27]. Only three ESTs can be implicated in terpene biosynthesis (cycloartenol synthase-like protein, farnesyl pyrophosphate synthase and a limonene synthase-like protein) of which only the latter two are involved in the production of the commercially valuable essential oil. The limonene synthase-like protein is most likely to be a monoterpene synthase for sabinene hydrate, 1,8-cineole or terpinolene, which are the predominant monoterpenes produced by M. alternifolia [68]. The farnesylpyrophosphate synthase synthesises the precursor molecule to all sesquiterpenes via the condensation of three isoprene units [68]. Of the types of terpenes represented by the three ESTs, monoterpenes are the most abundant, representing ~90% of the essential oil as opposed to the sesquiterpenes, which only represent ~10% [68]. The following compounds were resolved and quantified with authentic GC standards: α-pinene, camphene, β-pinene, β-myrcene, 3-carene, limonene, longifolene, β-caryophyllene, and β-farnesene [69]. P-Cymene and β-phellandrene were also dominant contributors to the emission profile, but did not have a corresponding authentic standard. In that case, terpinolene was used as a proxy standard for their quantification [69]. The compounds included in Fig. 3 are all monoterpenes that were characterized with an authentic GC standard and constituted greater than 5% of total emissions by mass during at least one of the experimental phases. These compounds were α-pinene, Δ-3-carene, camphene, β-myrcene, limonene and β-pinene [69]. The determined compounds were categorized into two main classes on the basis of their chemical structures: isoprenoids (oxygenated monoterpenes, terpene hydrocarbons) and nonisoprenoids variously functionalized (alkanes, aldehydes, lactones, ketones, alcohols, furans, fatty acids and esters). Terpene hydrocarbons, esters, fatty acids-esters, and alcohols were the dominant groups of compounds in the essential oils [70]. Volatile terpenes were important to the flavor of Occidental pears. α-farnesene was found in 10 cultivars, with the exceptions 'Clapp Favorite' and 'Alexandrine Douillard', and the concentrations ranged from 4.0 ng/g ('Bartlett') to 312.0 ng/g ('Yubileen Dar') [71]. 3,7,11-trimethl-1,3,6,10-(Z,E)-dodecatetraene and α-farnesene are isomers, and 3,7,11-trimethl-1,3,6,10-(Z,E)-dodecatetraene was detected in 'Doctor Jules Guyot', 'Clapp Favorite', Abate Fetel', 'Red Clapp Favorite', 'Yubileen Dar', 'beurré Hardy', 'La france', 'Bartlett' and 'Bartlett-Max Red' at a lower concentration than that of α-farnesene [71]. Terpenoids, primarily the monoterpenoids and sesquiterpenoids, affect the flavor profiles of most fruits and the scent of flowers at varying levels [71]. 5 monoterpenoids were detected in the 12 Occidental pear cultivars with lower concentration (1.01–20.1 ng/g) [71]. Sesquiterpenes were also detected in the 12 Occidental pear cultivars [71]. The contents of 2-Nonen-1-ol (253.1 ng/g) and α-Farnesene (312.0 ng/g) in 'Yubileen Dar' were high [71]. A total of five enantiomeric pairs were identified, all belonging to the class of monoterpenes and monoterpenoids [72]. The prominent monoterpenes synthesized were α-pinene (33%), β-pinene (28%), and myrcene (26%) with minor fractions of sabinene (10%) and limonene (3%) [73]. The comparison of all data revealed a statistically stable mixing ratio of monoterpene synthase activities in both leaf age classes with α-pinene (36%±3%), β-pinene (27%±2%), and myrcene (24%±4%) as major and sabinene (10%±2%) and limonene (3%±3%) as minor enzyme products [73]. Dos constituintes químicos detectados no óleo essencial das folhas 14,13% são monoterpenos e 85,87% são sesquiterpenos, em que os constituintes majoritários foram o b-cariofileno (41,30%), b-selineno (13,60%) e B germacreno (7,75%). No óleo essencial dos caules também se observou teores menores de monoterpenos (15,40%) em relação aos sesquiterpenos (84,60%), tendo como majoritários b-cariofileno (16,20%), linalol (15,40%) e epi-cedrol (13,30%) [74]. A total of 70 different components (58 HD versus 57 MD) were identified by GC-FID and GC-MS with HP-5 column. The MD oils could be distinguished from the HD oils by their richness in sesquiterpene hydrocarbons (51.5% MD versus 42.4% HD), aldehydes (3.7% MD versus 2.3% HD) and hydrocarbons (4.0% MD versus 3.2% HD). The HD oils could be differentiated from the MD oils by their greater richness in sesquiterpenoids (24.2% HD versus 20.8% MD), monoterpene hydrocarbons (5.5% HD versus 4.1% MD), monoterpenoids (4.7% HD versus 4.3% MD) terpene related compounds (1.2% HD versus 1.0% MD) and esters (1.4% HD versus 0.7% MD) [75]. Forty-five compounds were common for two process but (E)-β-ocimene, β-cyclocitral, cis-muurola-3,5-diene, trans-muurola-4(14),5-diene, δ-amorphene, ledol, globulol, selin-11-en-4-α-ol, (E,Z)-farnesol, β-acoradienol, methyl linolenate, 1-tridecene and linoleic acid were only detected from HD oil, pulegone, γ-amorphene, 7-epi-α-selinene, γ-calacorene, caryophyllene oxide, epi-α-bisabolol, (2E,4Z)-decadienal, hexadecanal, nonadecane, eicosane, heneicosane and docosane were only detected from MD process [75]. The main constituents in HD and MD were germacrene D (17.8% HD versus 23.2% MD), limonene (3.5% HD versus 2.6% MD), linalool (3.3% HD versus 2.9% MD), (E)-caryophyllene (5.4% HD versus 6.4% MD), bicycloelemene (2.1% HD versus 3.5% MD) and γ-cadinol (5.8% HD versus 4.4% MD) [75]. Composition of the essential oils obtained from five Lippia species: Table 1 presents the compounds identified by gas-chromatography followed by mass spectrometry in the essential oils of L. alba geraniol and carvone chemotypes, L. sidoides, L. salviifolia, L.rotundifolia and L. lacunosa. Approximately fourteen main compounds were detected, but to the most of them the contents were very low. For L. alba geraniol chemotype the compounds with the highest percentages were geranial and citral and to carvone chemotype were carvone and limonene. In the oil of L. sidoides the main compounds were thymol and o-cymene. The major constituents of L. salviifolia essential oil were nerolidol and germacrene D and of L. rotundifolia were E-myrcene and (E,Z)-, (E,E)-, and (Z,Z)-farnesol. Finally, the most abundant components for the essential oil of L .lacunosa were myrcenone and E-myrcene [76]. The relative concentrations of the volatile components identified in the essential oils of the six samples are presented in Table 1, according to their retention indices Ki on a HP-5 MS column. A total of 26 compounds were identified in the oils. On the whole, qualitative profiles of all essential oils were characterized by a greater amount of oxygenated monoterpenes (88.3–94.8%) and particularly five compounds were the most representative: menthone (7.6–15.9%), menthofuran (7.8–35.0%), menthol (5.6–56.6%), pulegone (5.6–28.1%) and menthyl acetate (2.1–15.4%) [77]. As regards the single components, the peppermint oil was characterized by higher amounts of menthol, that for the sample MpH1 accounted for more than the half of the total oil (56.6%) but was also abundant in MpH2 and MpH3 (38.5% and 27.5%, respectively). For M. arvensis oil, the compound present in higher percentages was instead menthofuran, accounting for 35.0% of the total oil in MaH1, 25.4% in MaH2 and 34.0% in MaH3 [77]. Other components present in minor concentrations in the six essential oils were monoterpene hydrocarbons (3.6–6.7%), sesquiterpene hydrocarbons (0.6–6.4%), oxygenated sesquiterpenes (0.2–0.3%) and others (0.8–2.3%) [77]. In the flower essential oil, monoterpene hydrocarbons represented the dominant class (68%) followed by sesquiterpene hydrocarbons (12.8%), oxygenated monoterpenes (7.3%) and oxygenated sesquiterpenes (5.9%), (Z)-b-ocimene (54.5%), was the principal component in flowers along with sabinene (10.3%), ß-caryophyllene (9.9%) [78]. On the contrary, only eleven compounds were detected and identified in the leaf essential oil. The dominant class of compounds was sesquiterpene hydrocarbons (87.7%) followed by monoterpene hydrocarbons (3.8%) and oxygenated sesquiterpenes (2.9%). No oxygenated monoterpene was detected in the leaf essential oil [78]. ß‑Caryophyllene was the principal component (51.6%) followed by the germacrene D (18.5%), bicyclogermacrene (7.1%), a‑humulene (4.8%), and a‑copaene (3.1%) in the leaf essential oil [78]. A remarkable feature of the leaf essential oil was that only two monoterpenes i.e., b-phellandrene (2.2%) and (Z)-b-ocimene (1.6%) were present [78]. In grape pomace distillates, varietal differences in monoterpenoid profiles are evident: Loureira and Albarin˜o show distinct monoterpenoid compositions (including R-terpinene, linalyl ethyl ether, R-terpenyl ethyl ether, trans-pyran linalool oxide, 2,6-dimethyl-3,7-octadiene-2,6-diol) compared to other varieties, with p-menthen-9-al isomers reported for the first time in grape pomace distillate and berries [79]. The sum of monoterpenols is highest in Loureira distillate, which is double that of Albarin˜o distillate [79]. Score analysis shows Loureira and Albarin˜o distillates are characterized by high contents of monoterpenols (linalool, R-terpineol, hotrienol), furanic and pyranic oxides, C13-norisoprenoids (vitispirane isomers), and p-menthen-9-al isomers, with Loureira having higher levels [79]. A total of 32 monoterpenoids were identified and quantified in plant sources, including both free and glycosylated forms, with most showing an overall increase in concentrations from véraison to maturity or harvest [38]. The concentration of glycosylated monoterpenoids was significantly greater than that of free-form monoterpenoids at maturity and harvest [38]. In leaves overexpressing VviERF003, glycosylated forms of linalool, α-terpineol, citronellol, β-citral, and geraniol exhibited a significant increase, while free-forms of linalool, α-terpineol, and (±)-menthol were also enhanced [38]. Transgenic tomato fruits had significant increases in most monoterpenoids, such as linalool, geraniol, and α-terpineol, compared to wild-type [38]. Female stage whole flowers of A. gigantea are characterized by the simultaneous emission of lemon-scented terpenoids, including citronellal, (Z)-citral (neral), (E)-citral (geranial), citronellol, nerol, and geraniol [39]. Male stage whole flowers transition to the production of novel and intense emissions of linalool, (Z,E)-α-farnesene and (E,E)-α-farnesene and their oxygenated derivatives [39]. The headspace monoterpenes largely prevailed for three S. nigra L. berry cultivars, representing from 66.5% for “Sabugueira” to 77.2% for “Bastardeira” of the overall terpenic and norisoprenoid content [80]. Within the monoterpenes class, limonene was the most abundant compound for the ripe cultivars “Bastardeira” and “Sabugueiro”, while for “Sabugueira”, p-cymene prevailed [80]. Monoterpenes (CS 93.4%, SE 85.2%, and BL 72.8%) represented the major constituents of essential oils from some sources, followed by smaller quantities of sesquiterpenoids, with limonene (CS 56.5%, SE 50.5%, and BL 43.8%) and α-pinene (CS 31.7%, SE 26.1%, and BL 16.2%) as main constituents [4]. Qianghuo from Sichuan province had a much higher content of monoterpenes in its volatile oil compared to other regions, with monoterpenes and oxygenated sesquiterpenes being the main chemical variables contributing to geographical differentiation [81]. Volatile compounds from C. wightii gum extracts included monoterpenes such as α-Pinene (11.725%), 2-Norpinene 3,6,6-trimethyl- (10.607%), α-Ocimene (2.309%), α-Myrcene (5.289%), Limonene (5.324%), and α-Terpineol (1.056%) [82]. Oxygenated monoterpenes in some EOs included linalool (0.4%-1.0%), β-ionone (0.3%-0.6%), 1,8-cineole (trace-0.4%), L-fenchone (0.2%-0.4%), borneol (0.4%) and camphor (trace-0.1%) [83]. Hydrodistillation of some oils yielded ~30% monoterpenic hydrocarbons and 42% oxygenated monoterpenes, with main compounds including β-thujone (47.72% HE1, 55.4% HE2), camphor (10.54% HE1, 11.28% HE2), and eucalyptol (6.37% HE1) [84]. The oxygenated monoterpene fenchone was one of the most abundant components in some essential oils, with higher abundance in achene EO (18.48%) [85]. The three major compounds of M jeffreyi are limonene, α and β-pinene, with sesquiterpenes less representative [20]. Juniper terpene chemical profile consisted of a rich mixture of 54 hydrocarbon and oxygenated mono- and sesquiterpenes, with 20 monoterpene hydrocarbons identified [13]. Concentrations of three monoterpene alcohols (cis-sabinene hydrate, borneol, and p-cymen-8-ol) did not change across seasons, while others peaked in fall [13]. A total of 241 VOCs were detected in root, stem, and leaf tissues of M. alternifolia, including 82 terpenoids, with monoterpenoids (26) and sesquiterpenoids (51) as major categories [21]. Most sesquiterpenoids were significantly upregulated in roots compared to stems, including δ-cadinene (20.62-fold increase), viridiflorene (20.16-fold increase), and terpinen-4-ol (10.50-fold increase) [21]. A complex volatile fraction, including a very large number of peaks (many of which have not yet been identified) and corresponding mainly to sesquiterpenes and monoterpenes, is present in the studied oils (especially in SCO). From the data obtained (not shown), it appears that the major sesquiterpene hydrocarbon volatiles present in SCO are β-caryophyllene, α-humulene, (E)-γ-bisabolene, and (Z)-γ-bisabolene. Among the monoterpene volatiles, the more prominent compounds in SCO are α-pinene, sabinene, β-myrcene, limonene, γ-terpinene, p-cymene, and terpinolene [86]. The measured monoterpenes were a-pinene, camphene, b-pinene, 3D-carene, p-cymene, limonene and terpinolene and the measured sesquiterpenes were longicyclene, iso-longifolene, b-caryophyllene, aromadendrene, a-humulene and alloaromadendrene [87]. In both oil samples, sesquiterpenoids predominated constituting 73.9 % and 81.9 %, with sesquiterpene hydrocarbons reaching up to 51.8 % and 63.6 %, while oxygenated sesquiterpenes amounted to 22.1 % and 18.3 %, respectively, of the total oils. Monoterpenoids were minor, amounting to 7 % and 4.27 % of the total oils, respectively; among which the oxygenated members were only detected in the flower sample reaching 2.58 % and appear to be absent in that of the leaves. The essential oil of the leaves was typified by the presence of the sesquiterpene hydrocarbons β-caryophyllene (24.6 %) and α-humulene (23.77 %) as well as oxygenated sesquiterpenes namely, caryophyllene oxide (8.50 %) and α-cedrol (8.30 %). Meanwhile, the essential oil of the flowers revealed a higher amount of α-humulene (36.73 %), a lower percentage of β-caryophyllene (19.96 %), an approximately similar amount of α-cedrol (9.48 %) and the presence of an appreciable amount of 10-epi-γ-eudesmol (3.63 % vs. 0.60 % in the leaf oil). The major monoterpene hydrocarbon detected in both oil samples was β-myrcene (7.0 % and 1.67 %), respectively [88]. As shown in Table II, 25 compounds of the essential oils of L. canescens were identified representing > 91.8%. The main compounds with concentration higher than 8.0% were the sesquiterpenes: b-bisabolene (4.4–9.3%), caryophyllene oxide (3.7–9.8%), (Z)-b-farnesene (8.8–13.9%), spathulenol (29.5–14.6%) and a-muurolol (30.0–25.1%). The oils from L. tilcarensis and L. trifolia were rich in sesquiterpene hydrocarbons (> 70%). Thus the main components of L. tilcarensis were (E)-b-farnesene (27.5%), germacrene D (26.8%) and a-cadinene (9.6%), while the oil of L. trifolia was rich in b-elemene (18.5%), a-humulene (15.2%) and g-cadinene (21.7%). The two populations of A. citriodora had unusually high contents of a-thujone. The main components of A. gratissima were 1,8-cineole (45.5%), sabinene (8.3%), carvacryl acetate (8.2%) and spathulenol (8.7%). A. castellanosii was characterized by high content of sabinene (44.8%), thymol (17.8%) and spathulenol (12.8%), while A. catamarcensis was characterized by high content of carvone (98.7%) [89]. Among the detected compounds, 38 were monoterpenoids (46.2%), 30 sesquiterpenoids (31.7%), 2 diterpenes (0.4%) and 30 compounds (14.3%) without terpenoid structure. Oxygenated monoterpenes were by far the main class (45.3%) with α-terpineol (7.5%) endo-borneol (7.2%), pinocarveol (5.9%) and thymol (3.7%) as the major compounds. In sesquiterpenes' group, oxygenated sesquiterpenes dominated (20.3%) with caryophyllene oxide (5.0%), τ -murrolol (3.5%), spathulenol (2.9) and α-cadinol (2.3%) as the major compounds. Caryophyllene (4.2%) was the most abounded sesquiterpene hydrocarbon [90]. Analyzed EOs had higher content of monoterpenoids (80–99%) than sesquiterpenes (1–15%), and they can be categorized into three different groups on the basis of their chemical composition: 1. EOs with large amounts of hydrocarbons monoterpenes from M. communis (M, 63%) and R. officinalis (R, 58%). 2. EOs with large amounts of oxygenated monoterpenes from A. millefolium (A, 52%), F. vulgare (F, 66%), and L. angustifolia (L, 66%). 3. EOs with both significant amounts of oxygenated monoterpenes (49%) and the highest level of sesquiterpenes (15%) from H. italicum (E). Few species showed the hydrocarbon sesquiterpenes in their EOs, and these volatile compounds were generally present in larger quantity, in comparison with the oxygenated forms, which were detected in lower quantity only in H. italicum (E, 1.80%) and in M. communis (M, 1%) [91]. Using analytical standards, we determined that our filtration step almost completely removed aromatic compounds, including benzyl alcohol and methyl salicylate. While quantification of these compounds was desirable, a consistent and clean filtration method was necessary for maintenance of the GC instruments over numerous samples. Two more variables were added, “total monoterpenes” and “total sesquiterpenes,” representing the sum of concentrations of the fourteen monoterpenes and six sesquiterpenes in each sample, respectively. Compounds were identified based on previous work (Pezet et al. 2013), retention time comparison to analytical standards on the GC-FID, and comparison to chromatograms from gas chromatography–mass spectrometry on select samples (GCMS; Shimadzu GC-2010 and GCMS-QP2010 Plus with an HP-5 column). References for mass spectrometry included a software library (Stein 2005), published reference (Adams 2009), and analytical standards (Sigma-Aldrich, Saint Louis, MO, USA). While previous research has described monoterpenes, sesquiterpenes, small phenolics, and green leaf volatiles in eastern hemlock (Lagalante and Montgomery 2003; Pezet et al. 2013), the present method captured only mono- and sesquiterpenes and their derivatives. Calibration curves were determined individually for those compounds that were available as analytical standards (Sigma-Aldrich, Saint Louis, MO, USA): α-pinene, camphene, sabinene, β-pinene, myrcene, α-phellandrene, p-cymene, limonene, eucalyptol, camphor, piperitone, bornyl acetate, β-caryophyllene, and α-humulene. Averaged calibration curves of structurally similar terpenoids were used when no commercial standard was available: tricyclene, γ-muurolene, germacrene D, γ-cadinene, δ-cadinene, and unknown compounds A and B [92]. The primary components in the volatile oil of citrus peels were linalool, limonene, linalool acetate, α-terpineol, geraniol, β-ocimene, nerolidol, β-pinene, farnesol, and geranyl acetate (Table 1). The citrus peel essential oil contains monoterpenoids, monoterpenes, acetylated monoterpenes, and sesquiterpenoids (Table 1). The FTIR patterns of essential oils isolated from citrus peels corroborated the GC–MS findings. The stretching vibrations of OH groups in monoterpenoids like linalool are connected with the 3400–3200 cm⁻¹ bands. The stretching vibrations of CH, CH2, and CH3 in monoterpene hydrocarbons are related to peaks in the 2960–2750 cm⁻¹ area. The bands in the 1200–900 cm⁻¹ area represent the stretching of C=O, C=C, and C=O in the terpenes and terpenoids (Figure S1). According to our findings, linalool, limonene, linalool acetate, and α-terpineol were the primary components of essential oil in citrus peels [15]. The three lavender taxa, L. angustifolia (LA), L. × intermedia (LX) and L. latifolia (LL), are the main EO‐producing plants worldwide (Figure 1a,b). As mono‐ and sesquiterpenoids are the main constituents of lavender EO, we analysed the volatile terpenoid profiles by solid‐phase microextraction coupled to gas chromatography‐mass spectrometry (Figure 1c; Table S1). Principal component analysis revealed that five monoterpenoids varied dramatically among three lavender taxa, including linalool, linalyl acetate, lavandulyl acetate, 1,8‐cineole and camphor (Figure 1d). Besides linalool, linalyl acetate and lavandulyl acetate were identified as major monoterpenoids of LA flower, while 1,8‐cineole and camphor constituted the main monoterpenoids of LL flower (Figure 1c,e). The flower linalool contents of LA and LX were higher 60.26% and 71.61% than LL, respectively. Strikingly, there was no linalyl/lavandulyl acetate in LL flower, while linalyl/lavandulyl acetate dominantly accumulated in flowers of LA and LX (total 38.12 and 25.91 μg/mg in LA and LX). LL flower accumulated 5.23 μg/mg camphor and 8.02 μg/mg 1,8‐cineole, which were ~5011‐ and 61‐fold higher than LA flower and ~3‐ and 4‐fold higher than LX flower, respectively (Figure 1e) [16]. The main components of essential oils found in Zingiberaceae are monoterpenes and sesquiterpenes [23].

Sesquiterpenoids also contribute to structural diversity, often in lower proportions than monoterpenoids but with distinct distribution patterns. For example, Hedychium 'KMH' has sesquiterpenoids as the primary VOCs (87.71%) [48]. In microbial strains, caryophyllene and humulene show strain-specific patterns: E19 has the highest humulene concentration (131.63 µg g DW⁻¹) across light spectra, with a similar distribution observed for caryophyllene [41]. In plant EOs, Helichrysum devium and H. melaleucum are predominant in sesquiterpenes, with (−) β-caryophyllene (15–45%) and γ-curcumene (14–35%) as key components [93]. In certain essential oils, sesquiterpenes can dominate: an oil from an unspecified source consists mainly of total sesquiterpenes (60 compounds, 84.5% of the oil), with sesquiterpene hydrocarbons (53.9%) including (Z)-caryophyllene (11.5%), (E)-caryophyllene (10.6%), and α-humulene (4.8%), and oxygenated sesquiterpenes (30.6%) such as α-eudesmol (3.8%) and caryophyllene oxide (2.7%) [94]. A total of 40 volatiles detected in a study included ten sesquiterpenes, with six (β-caryophyllene, α-curcumene, (E)-α-bergamotene, (E)-β-farnesene, (Z)-β-farnesene, β-sesquiphellandrene) correlating with decreased fruit flavour preference during storage [95]. In P. ruderale, sesquiterpenoids were a minority family (two compounds) [52]. In wines, 11 sesquiterpenoids were detected, with RTW having the highest concentration (116.8 μg L⁻¹) and nerolidol and (E,Z)-β-farnesol as main components; β-chamigrene and guaiazulene were first reported as wine constituents in WSW [63]. The four culinary herb species produced 11 sesquiterpenes, with five unique to a single species and β-caryophyllene shared by three; O. basilicum had the most diverse sesquiterpene spectrum, and O. vulgare the highest amount [64]. According to their molecular weight, the unknown components are consistent with one sesquiterpene (204 amu) and five oxygenated sesquiterpenoids (220 and 222 amu) [72]. In grape pomace distillates, Torronte´s is the variety with the major presence of sesquiterpenoids (β-cadinene, δ-selinene, R-humulene, aromandrene, nerolidol) and one diterpene (manoyl oxide) [79]. Score analysis positions Torronte´s distillates in the second principal component (accounting for 24.7% of variance) due to elevated sesquiterpene concentrations [79]. In a total of 51 volatile components identified in an unspecified source, ~30.1% are sesquiterpene hydrocarbons and ~3.5% are sesquiterpenoids, with major components including α-zingiberene (~12.9%), α-farnesene (~5.7%), β-sesquiphellandrene (~4.9%) [96]. In O. vulgare essential oil, trans-caryophyllene is a significant sesquiterpene component [97]. The most frequent sesquiterpene hydrocarbons include β-caryophyllene, germacrene B, germacrene D, δ-cadinene, while oxygenated sesquiterpenes include caryophyllene oxide, α-cadinol, guaiol, spathulenol [98]. In C. illicioides dried bark essential oil, thirty-six sesquiterpenoids account for 27.9% of the oil, with d-cadinene (5.6%), a-copaene (4.1%), (E)-b-caryophyllene (3.0%), and a-cadinol (1.6%) as the most abundant [99]. In Piperaceae species essential oils, qualitative and quantitative differences in sesquiterpene composition exist among and within species; hierarchical clustering analyses produce five significant clusters based on sesquiterpene profiles, with clusters characterized by tricyclic 5,11-epoxy-5-α,10-β-eudesman-4-(14)-ene skeleton sesquiterpenes, bicyclogermacrene and β-muurolene, trans-δ-guaiene and trans-nerolidol, or high contents of germacrene and trans-caryophyllene [100]. In vacuum-distilled oil, sesquiterpene hydrocarbons and oxygenated sesquiterpenes are minimum (5.5% and 2.9%, respectively), while conventionally distilled oil has maximum oxygenated sesquiterpenes (21%) and residual oil has maximum sesquiterpene hydrocarbons (30.8%) [101]. An essential oil from an unspecified source has sesquiterpenes as the main chemical class (88.9%), with 83.2% sesquiterpene hydrocarbons (major components: α-santalene (15.5%), bicyclogermacrene (12.5%), α-zingiberene (8.7%), (E)-β-farnesene (8.2%), α-trans-bergamotene (6.0%), germacrene D (5.7%)) and 5.7% oxygenated sesquiterpenes [102]. In E. maidenii outer bark, aromadendrene and alloaromadendrene are abundant sesquiterpenes [103]. In T. montanum populations, sesquiterpenes are the highest relative percentage class (34.89%, with 17.50% oxygenated and 17.39% hydrocarbon sesquiterpenes) [104]. Serpentinite populations synthesize twice as much sesquiterpenes as calcareous populations, with the highest total sesquiterpenes in serpentinite population NE (32.73%) and calcareous population KO (19.37%) [104]. In EVOO headspace, sesquiterpene hydrocarbons range from 17.7–62.1%, with α-copaene as the main component (ranging from 9.0% to 43.3% during maturation), and oxygenated sesquiterpenes range from 0.7–3.1% [3]. A total of 43 volatile compounds were identified in some samples, including sesquiterpenes [105]. Samples treated with COS 200 mg L−1 presented a characteristic volatile profile containing sesquiterpenes such as seychellene, cis-β-farnesene and (+)-α-chamigrene not identified in control and COS 400 mg L−1 treated roots [105]. Only two sesquiterpenes (+)-δ-cadinene and (+)-epi-bicyclo-sesquiphellandrene increased after elicitation with increasing COS concentration [105]. α-copaene, α-patchoulene, and cadina-3,5-diene increased after COS 200 mg L−1 and decreased after COS 400 mg L−1 treatment [105]. In Z. psammophilum, the root oil was characterized by a high content of sesquiterpenes (75.3%), with main components including β-caryophyllene (21.8%), α-humulene (13.6%), elemol (5.2%), α-copaene (5.7%), β-selinene (4.9%), caryophyllene oxide (4.7%), δ-cadinene (4.5%), and cyclosativene (4.2%) [54]. The root oil of Z. mezoneurispinosum was mainly composed of sesquiterpenes (81.5%), with main compounds including γ-elemene (24.8%), cyclosativene (11.9%), viridiflorol (7.1%), α-copaene (5.4%), and elemol (5.0%) [54]. Male stage whole flowers of A. gigantea emit (Z,E)-α-farnesene and (E,E)-α-farnesene and their oxygenated derivatives [39]. The high chromatographic resolution of GC × GC-ToFMS allowed detection of sesquiterpenic compounds in S. nigra L. berries [80]. Sesquiterpene hydrocarbons in some EOs were represented by α-cubebene (trace-0.9%) [83]. The oil from M. hagei stems and leaves had β-selinene as the main compound (45.78%) [20]. The oil from M. hookeriana stems had germacrene D (11.14%) as a major constituent [20]. Juniper terpene profile included sesquiterpene hydrocarbons, with only three (germacrene-B, germacrene-D, and alpha-bulnesene) varying seasonally [13]. With the exception of 8-alpha-11-elemodiol which peaked in summer, other sesquiterpene alcohols (beta-eudesemol, alpha-eudesemol, gamma-eudesemol, elemol, and e-nerolidol) peaked in spring [13].

Quantitative profiles of terpenoids in essential oils are further influenced by flower position, ploidy, extraction methods, environmental factors, and storage. For β-myrcene and ocimene, significant interactions between light spectra and flower positions are observed: under AP67 and SOL, concentrations are highest in main terminal buds (MTBs) followed by secondary terminal buds (STBs) and side buds (SBs), while CHD shows homogeneous distribution [41]. Strain and flower position interactions affect multiple terpenoids: KAN has the highest β-myrcene concentration in MTBs (256.40 µg g DW⁻¹), with significant reductions in STBs and SBs, while A4 shows similar β-myrcene levels in MTBs and STBs [41]. Autopolyploidy in Cymbopogon species correlates with changes in essential oil concentration and biomass, with aldehyde-rich species showing reduced growth and alcohol-rich species enhanced growth in tetraploids [106]. Extraction methods alter terpenoid profiles: supercritical carbon dioxide extraction of kumquat peel oil yields higher linalyl acetate, neryl acetate, and geranyl acetate compared to steam distillation and solvent extraction [50]. GC–MS analysis of an oil obtained under alkaline, aqueous, and acidic media shows varying monoterpene proportions: sabinene contents are 72.7%, 66.7%, and 3.3% respectively, with higher α-terpinene, γ-terpinene, terpinolene, and limonene in the acid medium extract [107]. Fertilisation affects the percentage of β-pinene (P = 0.006), cineole (P = 0.042), and borneol (P = 0.022), while drought stress decreases α-pinene (P = 0.007) and camphene (P = 0.034) and increases cineole (P = 0.003) and borneol (P = 0.004) [57]. Both 3-month-old and 6-month-stored bolts have higher monoterpene concentrations than live trees, with α-pinene (3.73× after 3 months, 2.93× after 6 months), β-pinene (2.69× after 3 months, 2.17× after 6 months), camphene (3.06× after 3 months, 1.98× after 6 months), limonene (2.63× after 3 months, 2.41× after 6 months), and myrcene (2.63× after 3 months, 2.42× after 6 months) significantly higher in bolts [108]. In grape juice, treatment with DV-BG enzyme increases free monoterpenes from 1492 µg/L (control) to 2685 µg/L, with a notable rise in α-terpineol indicative of acid-catalyzed rearrangements [109]. Enzyme-treated wines were richer in monoterpenes (geraniol, linalool, β-terpineol, p-menthen-7,8-diol) due to hydrolysis of bound monoterpenes, with geraniol and linalool concentrations increasing significantly [110]. In O. vulgare essential oil, exposure to different heating temperatures causes no significant change in chemical composition, but carvacrol percentage increases with higher temperatures (80 °C 70.27%; 100 °C 70.6%; 120 °C 74.58%) [97]. In a study of 51 volatile components, PCA shows PC1 corresponds to particle thickness (distancing grated from sliced samples), PC2 discriminates dried from fresh samples, with monoterpenes dominating in fresh samples and sesquiterpenes dominating in grated samples; heating duration shows an antagonistic effect between monoterpenes and sesquiterpenes [96]. In E. globulus true leaves, there are highly significant effects of ontogeny, genetics, leaf age and their interactions on most terpene compounds; population explains the greatest variance for total terpenes, monoterpenes, and sesquiterpenes, with ontogenetic effects next most important for total terpenes and sesquiterpenes [111]. The ontogenetic change in terpene content varies between populations: Tasmanian populations show similar ontogenetic trends in total terpene and monoterpene content, while the mainland Australian population differs in magnitude and trajectory [111]. Monoterpenes like 1,8-cineole, limonene, and α-terpineol increase through ontogeny in all populations but with different timing and rates, while α-terpinyl acetate shows population-specific trends [111]. Sesquiterpenes show population and ontogeny effects, with β-caryophyllene and α-humulene moving in an opposing trajectory to other sesquiterpenes, changing sesquiterpene dominance during seedling leaf ontogeny [111]. In T. montanum populations, PCA shows the first two PCs explain 74.84% of variance, with serpentinite populations differentiated by higher total sesquiterpenes, oxygenated sesquiterpenes, and hydrocarbon sesquiterpenes [104]. In EVOO, sesquiterpene hydrocarbons tend to increase during maturation, with the highest value (62.1%) at MI = 5.27, while oxygenated sesquiterpenes remain constant then decline [3].

Enantiomeric diversity adds another layer to monoterpenoid structural complexity. Pinus genus species, including Pinus sylvestris, P. montana, P. nigra, P. pinaster, and P. cembra, exhibit varying enantiomeric contents of terpenic hydrocarbons, with enantiomeric excess calculated for the dextrorotatory R (+) form [112]. Abies genus species such as Abies alba and A. sibirica also show distinct enantiomeric compositions of terpene hydrocarbons [112]. A total of five enantiomeric pairs were identified, all belonging to the class of monoterpenes and monoterpenoids [72].

Monoterpenoid nucleation thresholds vary by structure: endocyclic compounds like α-pinene and Δ³-carene have slightly higher thresholds (~2 ppbv under dry conditions) than exocyclic ones like β-pinene and sabinene [113]. In aromatic white wines, monoterpenes such as hotrienol, α-terpineol, citronellol, and linalool contribute to floral and citrus-like odours, with yeast strain influencing their release from glycoconjugates; the SC × SK yeast variant produces nearly threefold higher total terpenes than other variants, with hotrienol concentrations above the odour threshold [114]. At ~7 µg m−3 organic aerosol, the yield from β-myrcene was 0.023 while the yield from β-pinene was 0.074—more than three times greater than β-myrcene [69]. This conclusion was supported by our measurements of SOA mass yield curves from five different monoterpene standards representing most of the dominant compounds in the plant emission profile (6c) [69].

In various plant EOs, monoterpenoids often dominate: Hyptis suaveolens leaf essential oil has 5-Caranol (18.5%), α-humulene (9.6%), and camphor (4.7%) [115]; Origanum vulgare subsp. virens EO varies by source, with thymol (59%) dominant in commercial samples, carvacrol (73%) in wild specimens, and similar proportions of both in Tenerife samples [93]; Thymus vulgaris EO belongs to a thymol chemotype with 64% thymol [93]. Cedronella canariensis EO has high pinocarvone (92%), an unusual bicyclic ketone [93]. Mentha pulegium EO is rich in (+)-pulegone (54%) and menthol (32%) [93]. Lavandula dentata essential oil is dominated by 1.8 cineol (50–52%), fenchone (17−16%), and camphor (15−13%) [116]. Nutmeg oil contains about 90% monoterpenoids, with (-)-β-pinene (24–31%), 1-terpinen-4-ol (14–15%), L-α-pinene (13%), and sabinene (10–11%) as main constituents [116]. Eucalyptus essential oil has 1.8 cineole (80.64%), limonene (10.41%), α-pinene (2.52%), and p-cymene (3.65%) [117]. Lemon essential oil is dominated by limonene (69.25%), β-pinene (11.37%), γ-terpinen (7.86%), sabinene (1.98%), and α-pinene (1.75%) [117]. Tea tree essential oil has terpinen-4-ol (35.88%), γ-terpinen (19.65%), α-terpinen (8.64%), p-cymene (4.61%), and 1.8 cineole (4.07%) [117]. Eighty-four components identified in some essential oils include 1,8-cineole (9-37%), camphor (16-30%), and p-cymene (1-27%) [118]. Citronellal (73.6%), isopulegol (4.5%), and citronellol (2.6%) are main components of eucalyptus oil, while methyl chavicol (71.2%), linalool (24%), and geranial (18.9%) dominate Ocimum essential oil, and menthol is the major constituent of mentha essential oil [119]. Underground parts oils of Heracleum sphondylium group representatives are rich in monoterpenes (23.2-89.2%), with (Z)-β-ocimene or β-pinene as dominant constituents [26]. Rosewood oil is mainly composed of monoterpenes (e.g., linalool as the major compound), with sesquiterpenes present in lower quantities [120]. In a total of 51 volatile components, ~17.8% are monoterpenes and ~41.7% are monoterpenoids, with major components including geranial (~15.6%), neral (~11.6%), eucalyptol (~7.8%), camphene (~8.5%), α-pinene (~3.0%) [96]. O. vulgare essential oil kept at room temperature has carvacrol (68.06%), p-cymene (15.91%), α-pinene (2.56%), myrcene (2.03%), γ-terpinene (1.87%) as major monoterpenes and terpenic compounds [97]. Essential oils are predominantly composed of terpenes (pinene, myrcene, terpinene, limonene, p-cymene, phellandrene) and terpenoids (acyclic monoterpene alcohols like geraniol, linalool; monocyclic alcohols like menthol, 4-carvomenthenol, terpineol, carveol, borneol; aldehydes like citral, citronellal; phenols like carvacrol, thymol; ketones like menthone, pulegone; esters like linalyl acetate) [121]. The most frequent monoterpene hydrocarbons include α-pinene, β-pinene, myrcene, limonene, and oxygenated monoterpenes include linalool, α-terpineol, neryl acetate [98]. C. illicioides dried bark essential oil has seventeen monoterpenoids (25% of the oil), with terpinen-4-ol (10.4%) as a major component [99]. In Piperaceae species, some essential oils have only monoterpenes (e.g., P. rivinoides and P. solmsianum from coastal locations), with clusters based on monoterpene composition including those with camphene, limonene or cis-δ-ocymene plus α- and β-pinene, or myrcene, sabinene or δ-phellandrene plus pinene forms [100]. In vacuum-distilled oil, total monoterpenoids (including oxygenated and non-oxygenated) are maximum (88.2%), while conventionally distilled oil has minimum (47.4%) [101]. E. maidenii outer bark has monoterpenes like 1,8-cineole and γ-terpineol as abundant components [103]. T. montanum serpentinite populations have higher monoterpenes than calcareous populations, with the highest total monoterpenes in serpentinite population KJ (20.25%) and calcareous population KO (5.86%) [104]. EVOO headspace has monoterpene hydrocarbons (represented by (E)-β-ocimene) appearing only at higher maturation index (1.3% at MI = 4.09) [3].

Sesquiterpenoids, while less abundant, are key in some profiles: Helichrysum obconicum EO has 40% (+)-pulegone, while H. devium and H. melaleucum have β-caryophyllene and γ-curcumene [93]. In EOs from Macaronesian species, Argyranthemum pinnatifidum has high β-myrcene (46%) and geraniol (23%), while Artemisia argentea has α-phellandrene (67%) and camphor (12%) [93]. The leaf essential oil of Aframomum melegueta contains γ-elemene (8.84%), the stem oil has caryophyllene oxide (19.70%) and β-eudesmene (10.83%), the rhizome oil has caryophyllene oxide (5.97%), and the seed oil has α-caryophyllene (48.78%) and β-caryophyllene (32.50%) [122]. The leaf essential oil of Crassocephalum crepidioides has α-caryophyllene (10.29%) and β-cubebene (13.77%), while the stem oil of Ocimum gratissimum has caryophyllene (10.37%) [122].

Terpenoid profiles also vary by plant part and geographic origin: Pinus sylvestris oils from Poland, Austria, Italy, and Korea show differing enantiomeric contents [112]; Pinus montana oils from Austria, Italy, and Korea have distinct enantiomeric compositions [112]. In Cucurbita extracts, HFV and HFN have monoterpenes at 8.1% and 38.2%, respectively, while albedo extracts (HAN and HAV) have lower monoterpene content (1.9% and 2.2%) [123]. In grape berries, 87-1 berries contain 14 terpenes (mostly monoterpenes), while Kyoho berries have only seven, with no sesquiterpenoids detected in either [46].

Overall, terpenoid structural diversity and quantitative profiles are shaped by genetic, environmental, and processing factors, with monoterpenoids often dominating essential oils but sesquiterpenoids playing critical roles in specific species and strains. The process of preparing nanostructures and/or isolating nutmeg oil influences monoterpenoid quantification due to their susceptibility to volatilization, degradation, and transformation [117]. Small differences in essential oil constituents are attributed to genetic and environmental factors, seasonal and geographical variations, harvest stage, and extraction methods [119]. Monoterpenoids (8, 11) and lotaustralin (3) from plant extracts were inactive [124]. Monoterpene concentrations in some wines were low compared to floral grape varieties (Muscat, Riesling, Gewurztraminer) [110].

Biosynthetic Pathways and Gene Regulation of Monoterpenoids and Sesquiterpenoids

Monoterpenoid structural and biosynthetic diversity spans plant and microbial sources, driven by gene regulation, tissue-specific expression, environmental cues, and conserved/species-specific enzyme families. Compound numbering in monoterpenoid studies often follows the p-menthane convention [125], with key synthetic transformations yielding distinct derivatives:

Key Synthetic Transformations of Monoterpenoids

Substrate Compound	Reagent/Treatment	Product(s)	Yield/Conversion	Additional Details	Citation
(+)-isopulegol (13)	Dess−Martin periodane oxidation	Ketone 14	96%	—	[125]
Ketone 14	Lithium enolate + PhSeBr	2-phenylselenylisopulegone derivatives 15 and 16	—	7:1 diastereomeric mixture favoring (2S)-isomer 15	[125]
Compound 3	LiAlH4	trans/cis-isopiperitenols (2 and 18)	85%	3:2 ratio; trans-isopiperitenol (2) is naturally observed in peppermint	[125]
Compound 4	NaOMe/MeOH	(+)-pulegone (5)	Quantitative	—	[125]
(+)-pulegone (5)	Isolated NtDBR enzyme	Menthone (7) and isomenthone (8)	94% conversion	55:45 ratio	[125]

Plant monoterpenoid diversity is linked to chemopolymorphism and pathway upregulation: Pistacia palaestina galls induced by Baizongia pistaciae exhibit higher monoterpene content than host leaves, with upregulated MEP (plastidial) and MVA (cytosolic) pathway genes; chemopolymorphic trees show differential terpene synthase (TPS) expression matching accumulated monoterpenes [126]. Melaleuca alternifolia tissues contain 241 volatile organic compounds (VOCs), with 82 terpenoids (34.02% of total VOCs) including 26 monoterpenoids (31.71% of terpenoids) and 51 sesquiterpenoids (62.20% of terpenoids); sesquiterpenoids like δ-cadinene (20.62-fold), viridiflorene (20.16-fold), and α-phellandrene (15.35-fold) are upregulated in roots vs. stems, while monoterpenes (e.g., terpinen-4-ol) and sesquiterpenes are upregulated in roots/stems vs. leaves [21].

Biosynthetic pathways for monoterpenoids (MEP) and sesquiterpenoids (MVA) involve distinct gene families: MEP genes include dxs, dxr, cmk, mcs, hds, hdr [127], while MVA genes include hmgs, hmgr [127]. TPS subfamilies are critical: TPSb synthesizes monoterpenes, TPSa produces sesquiterpenes [127][128]. Guava has 15 TPS-b1 (cyclic monoterpenes) and 21 TPS-a (sesquiterpenes) members [128]; grapevine has 69 functional synthases, with monoterpenes stored as glycoconjugates in berries and sesquiterpenes as unsaturated hydrocarbons in epicuticular wax [129]. The MEP pathway is the main route for grape berry monoterpenes, with geranyl diphosphate synthase (GPPS) converting DMAPP to GPP (monoterpene precursor) [46]; transient overexpression of LcGPPS.SSU1 in Litsea cubeba and tobacco enhances monoterpene production [130]. M. alternifolia uses 12 MEP genes (e.g., MaDXS1, MaTPS14) and 7 MVA genes (e.g., MaHMGS1, MaFPS) for terpenoid synthesis [21].

TPS genes often correlate with specific metabolites: In M. alternifolia, 11/16 TPS genes correlate with terpenoids (e.g., TPS2 with linalool, MaTPS4 with germacrene D, TPS2/TPS4 with terpinen-4-ol) [21]. Thyme glandular trichomes produce carvacrol-, thymol-, geraniol-, or α-terpineol-type essential oils, with TPS genes (e.g., γ-terpinene, α-terpineol synthases) correlating with metabolites [131]. Culinary herbs (e.g., Origanum vulgare, Rosmarinus officinalis) have 25 monoterpenes (10 species-specific) synthesized by TPSb members [64].

Environmental factors and tissue specificity shape profiles: Tea tree winter-dormant tissues (WDT) have higher terpenoid content than summer-dormant tissues (SDT), with α-muurolene as a WDT marker; winter upregulates MVA genes, CsIDI, limonene/ocimene synthases, and some TPS genes [132]. Litsea glauca expresses MEP genes (DXS-like) in sarcocarps and MVA genes (HMGR-like) across tissues; linalool/alpha-terpineol synthases show tissue-specific expression correlating with metabolite accumulation [133].

Glycosylation mediates storage and release: Riesling grapes have odourless monoterpene glycosides released by yeast or acid hydrolysis [114][134]; VvGT7 glucosylates nerol, citronellol, geraniol, and terpineol with chiral preference for R-citronellol [135][134]. Light exposure affects grape monoterpenes: constant shading reduces content by 90% (linalool undetectable), while reillumination partially restores levels; VvTPS54/56 and VvGT7 are light-dependent [134].

Microbial sources contribute diversity: Plocamium species produce a pentachlorinated monoterpene (characterized via NMR/MS with ¹H peaks at δ 6.97, 6.38) [136], and the MVA pathway (thiolase, HMGS, HMGR) is conserved for IPP biosynthesis [136]. Terpenoid metabolic modules may enhance cannabinoid bioactivities via the entourage effect [137].

Across species, TPS subfamilies are conserved and expanded: M. alternifolia essential oil is ~90% monoterpenes (sabinene hydrate, 1,8-cineole) and ~10% sesquiterpenes [68]. Wisteria villosa has 66 WvTPSs (TPS-a:24, TPS-b:26) expanded via tandem duplication; 8 TPS-b monoterpene synthases (some bifunctional, e.g., WvTPS35) and most TPS-a are bifunctional [138]; WvTPS14 (TPS-b) is seed-specific, correlating with borneol/camphor levels [138]. Dipterocarpaceae monoterpene synthases (MTSs) have a conserved RR(X)8W motif and rely on Mg²⁺; mutations (e.g., A320N in VOMTS1) alter product profiles [139]. Gamma-terpinene synthase (Ttps2) has RRx8W/DDVYD motifs for GPP cyclization [140]. Monoterpene synthases have N-terminal plastid transit peptides (absent in sesquiterpene synthases) [140]. Santalum album sesquiterpene synthases (SaSQS1/SaSQS2, SaBS) are cytoplasmic and bifunctional (GPP/FPP substrates) [141].

Gene regulation and environmental cues modulate pathways: White tea withering upregulates MVA (HMGR) and MEP (DXS) genes via JA signaling, increasing IPP/DMAPP; TPS10 (linalool synthase) and neomenthol dehydrogenase are upregulated [142]. VviERF003 overexpression in ‘Yeniang-2’ grape leaves enhances glycosylated and free monoterpenoids [38]. Lilium ‘NWAFU-1’ has 8 monoterpene synthases (GPP to myristene/linalool) and 15 sesquiterpene synthases (MVA upstream) [5]. Lilac SoGPPS2 (mitochondrial) silencing reduces ocimene by 68.6%, while overexpression increases it by 3.3-fold [11]. M. alternifolia differentially expressed VOCs are enriched in terpenoid pathways [21].

Terpenoid Diversity in Response to Environmental and Developmental Factors

Terpenoids represent a diverse class of natural products, with monoterpenoids and sesquiterpenoids as prominent structural types across plant and microbial sources. Their diversity is shaped by source organism, tissue type, environmental conditions, and developmental stage, with structural variations spanning monoterpenoids, sesquiterpenoids, triterpenoids, and irregular terpenoids that underpin functional roles like aroma, flavor, and stress response [143][144][145][44][146].

Terpenoid Profile Variation by Source, Tissue, and Clone

Plant terpenoid profiles exhibit tissue-specificity: aerial parts often feature oxygen-containing irregular monoterpenoids (e.g., yomogi alcohol, artemisia alcohol, artemisia acetate), while underground parts are dominated by sesquiterpenes (predominantly carbohydrate-form, with germacrene-D most abundant) [146]. In Litsea glauca, tissue-specific expression of biosynthesis genes drives diversity: monoterpene synthase homologs (linalool, alpha-terpineol synthases) are highly expressed in sarcocarps/leaves (correlating with linalool/alpha-terpineol accumulation), while sesquiterpene synthases (alpha-humulene/beta-caryophyllene synthase) show tissue-specific expression in sarcocarps/leaves, leading to differential sesquiterpenoid accumulation [133]. Terpenoid backbone biosynthesis genes (e.g., DXS in sarcocarps, HMGR in all tissues) also exhibit tissue-specific activity in L. glauca [133].

Microbial sources contribute unique terpenoid dynamics: the green alga Chlamydomonas reinhardtii produces terpenoids as volatile organic compounds (VOCs) during programmed cell death (PCD), with terpenoid content increasing remarkably in this process [147].

Tree clones show contrasting terpenoid profiles across tissues:

Terpenoid Concentration Differences Between Tree Clones (S vs. R) by Tissue

Tissue	Clone S Terpenoid Profile	Clone R Terpenoid Profile	Citation
Leaves	Higher total terpenoids due to higher monoterpenoids (MT) and oxygenated monoterpenoids (MTox)	Higher sesquiterpenoids (ST)	[148]
Bark	Slightly higher MT and MTox	Higher total terpenoids, with much higher ST and oxygenated sesquiterpenoids (STox)	[148]

Monoterpenoid Diversity Across Plant Species

Monoterpenoid diversity is widespread across plant taxa, with key examples including:

Monoterpenoid Composition and Variation in Selected Plant Species

Plant Species	Monoterpenoid Profile and Key Details	Citation
Cannonau grapes	α-terpineol (free: 2.6–15.7 µg kg⁻¹; bound: 0–20.0 µg kg⁻¹ in skins) in skins/juice	[145]
Tree foliar oils	Main monoterpenes: cineole, α-pinene, limonene (coefficients of variation: 28%, 41%, 80% respectively)	[44]
Well-watered plants (unamended soil)	Most abundant: α-pinene (48 ± 4), camphene (13.8 ± 1), limonene (11.1 ± 1.7), β-pinene (9.4 ± 0.5)	[57]
Cryptomeria japonica (sugi)	Sabinene (8.8–73.3%) and α-pinene (14.5–73.3%) of total monoterpenes; strong negative correlation	[43]
Pinus thunbergii (kuromatsu)	β-pinene (26.5–66.3%) and α-pinene (18.7–46.9%) of total monoterpenes; negative correlation	[43]
Mentha piperita	Over 50 compounds; major: 1,8-cineole, linalool, (−) menthone, (−) menthol, (+) pulegone	[67]
Mentha arvensis	Dominated by menthofuran (up to 35.0% of total oil)	[77]
Mentha piperita (oil)	Rich in menthol (up to 56.6% of total oil)	[77]
Citrus leaves	70.1% (juvenile) and 93.1% (mature) of total VOCs; sabinene most abundant in both stages	[31]
Eucalyptus globulus	1,8-cineole, limonene, α-terpineol increase ontogenetically; α-pinene decreases then increases	[111]
Grapevines	Glycosylated monoterpenoids (linalool, α-terpineol, geraniol) peak early, decrease until véraison, then increase; glycosylated forms more abundant at maturity	[38]
S. officinalis plantlets	Main terpenoid group across HZ treatments; 1,8-cineole as major compound	[149]
Afghani plants	Peaks with molecular weight ≤154: α-pinene, β-pinene, β-myrcene, limonene, γ-terpinene, terpinolene, linalool, α-terpineol	[6]
Juniper	20 monoterpene hydrocarbons; beta-pinene, sabinene, verbenene, (e)-beta-ocimene, alpha-terpinene stable seasonally; alcohols (cis-sabinene hydrate, borneol, p-cymen-8-ol) and aldehydes/acetates (except bornyl acetate) stable; verbenone/camphor peak in winter	[13]
Floral system	28 unique floral monoterpenes: citronellal, (Z)-citral (neral), (E)-citral (geranial), citronellol, nerol, geraniol, linalool	[39]

Sesquiterpenoid Diversity Across Plant Species

Sesquiterpenoid diversity is equally notable, with key observations including:

Sesquiterpenoid Composition and Variation in Selected Plant Species

Plant Species	Sesquiterpenoid Profile and Key Details	Citation
Tree foliar oils	Total concentration ~half of monoterpenes (10.6 vs. 24.3 mg g⁻¹ dry mass); dominated by aromadendrene, globulol, β-eudesmol, cryptomeridiol	[44]
Artemisia species	Oxygenated sesquiterpenoids prevail in underground parts (up to 83.5% of total oil)	[146]
Litsea glauca	12 biosynthesis unigenes; alpha-humulene/beta-caryophyllene synthase homologs show tissue-specific expression, correlating with caryophyllene isomer accumulation	[133]
Citrus leaves	Decrease from 21.0% (juvenile) to 2.5% (mature) of total VOCs; trans-α-farnesene, α-humulene, valencene reduced with maturation	[31]
Eucalyptus globulus	Ontogenetic changes; β-caryophyllene/α-humulene follow opposing trajectories to other sesquiterpenes	[111]
Olive oil	Sesquiterpene hydrocarbons (α-copaene, cyclosativene) increase during maturation (62.1% at highest index); oxygenated sesquiterpenes constant then decline	[3]
Root systems (COS-treated)	200 mg L⁻¹ COS induces seychellene, cis-β-farnesene, (+)-α-chamigrene; (+)-δ-cadinene, (+)-epi-bicyclo-sesquiphellandrene increase with COS concentration	[105]
S. officinalis plantlets	Second most abundant terpenoid group in controls; (E)-β-caryophyllene as major component	[149]
Afghani plants	Peaks with molecular weight ≥204: β-caryophyllene, trans-β-farnesene, α-humulene, caryophyllene oxide, guaiol, c-eudesmol, b-eudesmol	[6]
Juniper	Hydrocarbons vary least seasonally; germacrene-B, germacrene-D, alpha-bulnesene peak in fall; alcohols (except 8-alpha-11-elemodiol) peak in spring	[13]
Floral system	Flower-specific compounds: (Z,E)-α-farnesene, (E,E)-α-farnesene, and oxygenated derivatives	[39]

Regulation of Terpenoid Diversity by Environmental and Developmental Factors

Environmental factors dynamically shape terpenoid profiles:

• Fermentation/distillation: Sorghum terpenoids increase post-saccharification, decrease post-fermentation, and ~56% rise in distilled xiaoqu baijiu [143]. Distilled peach spirit has more terpenoids (7 vs. 5 in pervaporation) with higher FD values for citronellol, dihydro-β-ionol, and (E)-β-damascenone; heat treatment liberates terpenoids from glycosyl groups and increases (E)-β-damascenone with time [144].
• Drying methods: Freeze-drying preserves citrus-like terpenes (limonene, citrial, myrcene, pinene), while oven-drying promotes octatriene, trans-ocimene, and alpha-irone [150].
• Nutrients/stress: Zinc (Zn₀.₂₅₀) maximizes monoterpene oil, correlated with photosynthetic rate and saccharides [151]. Fertilisation alters β-pinene, cineole, borneol; drought decreases α-pinene/camphene and increases cineole/borneol [57]. Amino acid supplementation affects limonene/β-myrcene in flowers [152]. Ozone shifts late leaves to sesquiterpene dominance during exposure [153]. HZ treatments in S. officinalis plantlets change terpenoid kind/level (monoterpenes: 52.88–78.4%; sesquiterpenes: 9.31–22.4%) [149]. Shading in lemon grass increases diterpenoid ginkgolide C and tetraterpenoid astaxanthin, with no effect on volatile monoterpenes; transcriptomics shows differential expression of menthol dehydrogenase family genes under light/shade [12].

Developmental stages also drive variation:

• Rosa varieties have 7.57–22.62% terpenoids, with 37/38 detected terpenoids differing significantly (4 monoterpenes, 2 diterpenes, 2 sesquiterpenes, 29 triterpenoids) [154].
• Artemisia tissue culture samples with developed roots have 2X higher monoterpenoid/sesquiterpenoid ratios in aerial oils than those with inhibited rooting [146].
• Sun-exposed grapevines have higher monoterpene concentrations (juice total: 43.64 µg kg⁻¹) [145]. Methyl jasmonate (MeJ) and urea (Ur) increase terpenoids in ‘Garnacha’/‘Tempranillo’; phenylalanine (Phe) increases C13 norisoprenoids in ‘Graciano’ [155].
• White tea withering upregulates MVA/MEP pathway enzymes (HMGR, DXS) and terpene synthases (neomenthol dehydrogenase, TPS10), boosting terpenoid accumulation [142].
• Floral development shows dynamic shifts: female flowers emit lemon-scented terpenoids (citronellal, neral, geranial, citronellol, nerol, geraniol) plus β-myrcene/(E)-β-ocimene; male flowers shift to linalool, (Z,E)-α-farnesene, (E,E)-α-farnesene; wilted flowers cease β-myrcene/(E)-β-ocimene production. By day 2 (anther dehiscence), linalool, methyl geranate, and farnesenes replace earlier compounds [39].
• Seasonal variation: terpenoid diversity increases annually (20 in Jan, 23 in Apr, 30 in Jul, 35 in Oct), with linalool/1,8-cineol as majors, and germacrene D (summer/spring) or camphor (fall/winter) as third [18].

Microbial Interactions Modulating Terpenoid Diversity

Microbial interactions alter terpenoid profiles:

• Lilium regale roots infected with Fusarium oxysporum show 83% (4 dpi) and 82% (6 dpi) differential terpenoid accumulation, with monoterpenoid pathways enriched at 2/4 dpi and upregulated biosynthesis genes [156].
• Bryophyte gametophytes emit monoterpene/sesquiterpene hydrocarbons; sporophytes produce irregular terpenoids (e.g., 6-methyl-5-hepten-2-one) from hypophyses [47].
• Mountain pine beetle bacterial associates metabolize terpenes, altering monoterpene/diterpene acid concentrations (inhibitory/stimulatory effects by strain) [157].
• Mentha piperita dual-inoculated with WCS417r/SJ04 shows ~6-fold higher monoterpene accumulation (menthol, menthone, etc.) [67].
• Tree clone infection responses: susceptible (S) leaves show reduced total terpenoids (due to lower MT); bark of S/R clones shows strong total terpenoid increases, with R clone bark having 7X higher total terpenoids and 11X higher MT post-infection [148].

Overall, terpenoid diversity reflects a complex interplay of biological and environmental factors, with structural and compositional variations enabling functional adaptation across ecosystems [143][144][145][44][146].

Sesquiterpenoid Structural Types, Biosynthetic Enzymes, and Natural Occurrence

Structural Classification and Natural Occurrence of Monoterpenoids and Sesquiterpenoids in Plant and Fungal Systems

Monoterpenoids and sesquiterpenoids exhibit tissue-specific, temporal, and species-dependent accumulation patterns in plants, with biosynthesis and profiles influenced by developmental stages, stress, and environmental conditions. Below is a consolidated overview of key findings:

Terpenoid Accumulation in C. iria
C. iria shows distinct temporal and tissue-specific terpenoid accumulation:

• Monoterpenes: β-pinene, limonene, and β-ocimene are undetectable before 90 days post-germination; β-pinene accumulates in leaves (90 days) and inflorescences (120 days), limonene in mature leaves, and cis-β-ocimene in mature inflorescences and leaves. No monoterpenes are found in roots over 150 days [158].
• Sesquiterpenoids: Constitutive compounds (β-elemene, E-β-caryophyllene, α-humulene, α-bergamotene, α-farnesene, elemol, hedycaryol, β-eudesmol, α-eudesmol, γ-eudesmol) are present across all stages. Tissue/restricted sesquiterpenes include germacrene D (inflorescences, ≥110 days), α-copaene (inflorescences, ≥110 days), α-gurjunene (roots, ≥70 days), germacrene B (roots, ≥70 days), and farnesol (roots, ≥30 days) [158].
• Biosynthesis: At least four terpene synthase (TPS) enzymes are inferred from cluster analysis of sesquiterpenoid accumulation: clade I (β-elemene, E-β-caryophyllene, α-humulene), clade II (α-bergamotene, α-farnesene), clade III (elemol, hedycaryol), clade IV (γ-eudesmol, β-eudesmol, α-eudesmol) [158]. Stress (MeJA, BAW infestation) alters profiles via TPS gene expression changes [158].

Terpenoid Profiles in Pine Species
Pine species exhibit varying mono/sesquiterpene ratios and dominant compounds:

• P. peuce (soft pine): ~3:1 ratio, with monoterpene hydrocarbons (60.5%), oxygenated monoterpenes (9.5%, including bornyl acetate), and dominant terpenes (α-pinene, germacrene D, camphene, β-pinene, (E)-caryophyllene, bornyl acetate) [159].
• P. nigra and P. heldreichii (hard pines): ~1.5:1 ratio, with monoterpene hydrocarbons (56.5% and 52.8%, respectively) and sesquiterpene hydrocarbons (41.6% and 37.6%, respectively). P. heldreichii includes limonene; P. nigra lacks limonene but shares α-pinene, germacrene D, (E)-caryophyllene, and β-pinene [159].
• P. pinaster: Two chemotypes with similar monoterpene composition (α/β-pinene, myrcene, β-phellandrene + limonene; >70% combined α/β-pinene) and 30 sesquiterpenes (germacrene D, β-caryophyllene as main components; germacrene D-4-ol acetate exclusive to chemotype 2) [160].
• P. mollicomum: Artificially damaged branches release abundant sesquiterpenoids (germacrene D, bicyclogermacrene, E-caryophyllene, β-elemene, α-humulene, β-cubebene) and monoterpenoids (linalool, E-ocimene, Z-ocimene) scarce in undamaged branches [19].

Monoterpenoid Dominance in Essential Oils
Monoterpenoids are prominent in essential oils (EOs) of diverse species:

• Kumquat peel EOs: 98% monoterpenoids, dominated by limonene (96%) [50].
• Cedronella canariensis EO: 92% pinocarvone (unusual bicyclic monoterpenoid ketone) [93].
• Mentha pulegium EO: (+)-pulegone (54%) and menthol (32%) [93].
• Origanum vulgare subsp. virens EOs: Thymol (up to 59%) and carvacrol (up to 73%) [93].
• R. officinalis EO: 1,8-cineole (40.80%), α-pinene (26.18%), camphor (19.53%) [51].
• M. alternifolia EO: Monoterpene hydrocarbons (45.95%, γ-terpinene 21.7%) and oxygenated monoterpenes (44.81%, terpinen-4-ol 38.6%) [53].
• C. martinii EO: 98.40% oxygenated monoterpenes, geraniol (76.9%) [53].

Sesquiterpenoid Dominance and Diversity
Sesquiterpenoids dominate certain extracts and show structural diversity:

• Helichrysum devium and H. melaleucum EOs: (−) β-caryophyllene (15–45%) and γ-curcumene (14–35%) [93].
• Unspecified plant leaf/root oils: Sesquiterpene alcohols (49.1% and 73.7%, respectively) [161].
• Large-scale study (240 samples): Terpenoids (69.4% of compounds), with monoterpenoids (33.7%) and sesquiterpenoids (34.7%) nearly equal. Leaves have twice as many sesquiterpenoids; petals have more monoterpenoids [28].
• C. crepidioides leaf EO: α-caryophyllene (10.29%) and β-cubebene (13.77%) [122].
• H. sphondylium group underground oils: Sesquiterpenes (14.7–35.9%), e.g., intermedeol (7.3–12.8% in H. verticillatum) [26].
• Fungal systems: New sesquiterpenoids isolated from Lactarius and Russula (humulane, caryophyllane, guaiane, eudesmane, aristolane/nardosinane, protoilludane, marasmane/lactarane) [162].
• Marine sponges: Bioactive sesquiterpenoid quinine-like compounds (insecticidal) and merosesquiterpenoids [163].

Biosynthesis and Regulation
Terpenoids form via isoprene unit condensation:

• Monoterpenes (C10): Hydrocarbons (myrcene, camphene), alcohols (menthol), aldehydes (geranial), esters (citronellyl acetate), ketones (camphor), peroxides (ascaridole), phenols (carvacrol), ethers (1,8-cineole) [164].
• Sesquiterpenes (C15): Hydrocarbons (β-bisabolene, germacrene D), oxygenated forms (caryophyllene oxide), alcohols (patchoulol), acids (geranic acid), aldehydes (citral), ketones (germacrone), epoxides (caryophyllene oxide), lactones (bergapten) [164].
• Regulation: Winter dormant tea (WDT) has higher monoterpene/sesquiterpene aroma compounds than summer dormant tea (SDT). Winter up-regulates MVA pathway genes, CsIDI, limonene/ocimene synthases, and four TPS genes, while down-regulating farnesene/nerolidol synthases [132].

Analytical and Environmental Observations

• Serpentinite vs. calcareous populations: Serpentinite populations synthesize twice as much mono- and sesquiterpenes; unique compounds include 3-caren-2-ol (serpentinite) and dodecane (calcareous) [104].
• Maturation effects: In EVOOs, sesquiterpene hydrocarbons (e.g., α-copaene) increase with maturation, while (E)-β-ocimene appears only at higher maturation indices [3].
• Volatile emissions: Aleppo pine twigs emit light-dependent (E)-β-ocimene/linalool and non-light-dependent (α-pinene, β-caryophyllene) terpenes [165]. Rosemary twigs emit equal non-oxygenated/oxygenated monoterpenes and low sesquiterpenes (α-humulene dominant) [165].

Diversity in Additional Species

• Inula japonica and I. britanica: Over 200 compounds, including sesquiterpenes and monoterpenoids [166].
• Lippia species: L. alba (geranial/carvone chemotypes), L. sidoides (thymol/o-cymene), L. salviifolia (nerolidol/germacrene D), L. rotundifolia (E-myrcene/farnesol), L. lacunosa (myrcenone/E-myrcene) [76].
• Yam-3 thalli: Sesquiterpenes (thujopsene, β-chamigrene) localized in oil body cells, higher in field-grown thalli [167].

Sesquiterpenoid Structural Diversity via Biosynthetic Enzymes and Terpene Synthase Gene Families

Sesquiterpenoids, C15 compounds derived from three isoprene units, originate from farnesyl diphosphate (FPP)—a precursor synthesized via the condensation of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) by prenyl converting enzymes [168][169]. Their structural diversity stems from terpene synthase (TPS)-initiated carbocationic cascades, with FPP undergoing up to six initial cyclization modes (e.g., 1,10-cyclization to (E,E)-germacradienyl cation or 1,11-cyclization to (E,E)-humulyl cation) [169]. A key driver of this diversity is TPS promiscuity: nearly half of identified sesquiterpene synthases are multiproduct enzymes [168]. For example, Abies grandis γ-humulene synthase generates 52 sesquiterpenes from FPP, while its δ-selinene synthase produces 34 [168]. Multiproduct activity often relies on conserved motifs: A. grandis γ-humulene synthase requires two DDxxD motifs at the active site entrance, with mutagenesis of one reducing product number [168]. Zea mays TPS4, with only one DDxxD motif, still produces 7-epi-sesquithujene, β-bisabolene, and 12 minor terpenes [168]. Single amino acid substitutions can shift product profiles (e.g., tyrosine-to-phenylalanine at position 526 in Medicago truncatula MtTPS5 eliminates germacrene D formation [168]), and environmental factors like pH alter specificity (e.g., increased pH in MtTPS5 shifts production from cadalane- to germacrene-based sesquiterpenes [168]). Recent research highlights active TPS catalytic roles: metal ion-dependent binding/cleavage of the C–O bond, precise carbocationic cascades, and acid/base catalysis, with mechanisms elucidated via site-directed mutagenesis, aza-analogues, and substrate analogues [170].

Sesquiterpene synthases are primarily classified in the TPS-a subfamily of Class I TPSs, which initiate catalysis via heterolytic diphosphate bond cleavage [168][171][128]. Phylogenetic and functional analyses across species reveal subfamily distribution, conserved motifs, and product profiles:

Sesquiterpene Synthase Classification, Motifs, and Product Profiles Across Species

Species	TPS Subfamily/Clade	Key Genes/Enzymes	Conserved Motifs	Product Profiles	Citations
Ocimum sanctum	TPS-a	18 OsaTPS (clustered), 23 OsaTPS	Not specified	Likely sesquiterpene/diterpene synthases; 23 involved in sesquiterpenoid biosynthesis	[171]
Psidium guajava	TPS-a (largest subfamily)	21 TPS-a members	Not specified	Associated with sesquiterpene production	[128]
Santalum album	Clade related to monoterpene synthases	SaSQS1, SaSQS2, SaBS, SaSS	DDXXD (substrate binding), (D/N)DXX(S/T)XXXE (metal binding), RRX8W (TPS-b characteristic)	SaSS (promiscuous santalene/bergamotene synthase); SaSQS1/SaSQS2 (sesquisabinene synthases); SaBS (β-bisabolene synthase)	[141]
Aquilaria sinensis	TPS-a (16/26 AsTPS genes)	AsTPS1–AsTPS26	R(R)X8W (most AsTPS-a/b except AsTPS7/9); DDxxD (most Class I except AsTPS25)	(−)-germacrene D synthase, alpha-farnesene synthase, (3S,6E)-nerolidol synthase	[172]
Wheat	TPS-a (65 proteins)	TaTPS84, TaTPS27, TaTPS02, TaTPS22, TaTPS102	Not specified	Alpha-humulene synthase like isoform ×1, (+)-delta-cadinene synthase isozyme XC14, (E)-beta-farnesene synthase, alpha-humulene synthase	[173]
Vetiver grass	TPS-a (4 unigenes)	CZTPS1, CZTPS2, CZTPS3	Not specified	CZTPS1: α-cadinol (72.66%), α-muurolol (27.33%); CZTPS2: α-Gurjunene (33.16%), Germacrene D-4-ol (25.54%); CZTPS3: Farnesol (63.31%), Nerolidol isomer (36.68%)	[174]
Medicago truncatula	TPS-a	MtTPS10 (Medtr5g073200)	Not specified	Multiproduct: ≥17 sesquiterpenes/sesquiterpene alcohols; major: himachalol	[175]
Basidiomycetes	Clades I–III	STS sequences	Not specified	Clades I/II: multiple cadinane-type sesquiterpenes; Clade III: single major product (e.g., Δ6-protoilludene); PoSTS-06: novel alliacane-type 'pleostene'	[176]
Daldinia tabescens CPCC 401429	Δ6-protoilludene type (Clade III), 1,10-cyclization, Clade IV	DtSTS1–DtSTS12	Not specified	DtSTS9: cis-thujopsene (11.25%), β-himachalene (4.82%); DtSTS10: α-himachalene (83.59%)	[177]
Ostiole bracts	TPS-a (94 FTPSs)	94 FTPSs	Not specified	Matched 18 known synthases (e.g., (−)-germacrene D synthase, E-beta-caryophyllene synthase); 15 cyclic, 5 acyclic sesquiterpenes	[178]
Laetiporus sulphureus NWAFU-1	4 clades with mushroom STSs	15 STSs	Not specified	Clade I (most STSs), II, III, IV	[5]
Cerrena unicolor	4 clades (cyclization mechanisms)	Not specified	Not specified	Clades I/II: α-muurolene, α-copaene (1,10-ring closure); Clade III: β-curcumene, α-bisabolol (1,6-cyclization); Clade IV: humulene	[179]
TPS-d2 group	TPS-d2 (sesquiterpene-restricted)	TcTPS1–TcTPS6	Not specified (TcTPS5-8 lack XII intron near DDxxD)	TcTPS1: zingiberene; TcTPS3: δ-cadinene; TcTPS4: germacrene-4-ol; TcTPS5: longifolene; TcTPS6: cedrol	[180]
Multi-cation STSs	Not specified	MT636927, MW384854	Not specified	MT636927 (55% nerolidyl specificity): trans-α-bergamotene, β-bisabolene, α-bisabolol; MW384854 (26% nerolidyl specificity): farnesyl-derived caryophyllene	[181]

The sesquiterpene synthase reaction cascade begins with metal-mediated diphosphate removal from (E,E)-FPP to form the transoid farnesyl cation, which may isomerize to the cisoid nerolidyl cation; both can cyclize (1,10 for farnesyl/nerolidyl, 1,6 for nerolidyl) and undergo further rearrangements [181]. Only four true multi-cation STSs are reported, producing acyclic farnesyl products alongside nerolidyl-derived compounds [181].

Sesquiterpenoids occur widely, with notable diversity in agarwood (Aquilaria species), which contains over 200 identified sesquiterpenes [169]. Agarwood is a stress-induced resin, and transient overexpression of AsZFP9 in A. sinensis stems increases sesquiterpene production and upregulates key biosynthetic genes [182]. A. sinensis encodes 26 TPS genes (AsTPS1–AsTPS26) [169][172]; functional assays show AsTPS9 produces β-farnesene, cis-sesquisabinene hydrate, citronellol, and geraniol; AsTPS12 generates β-farnesene, nerolidol, γ-eudesmol, and hinesol; AsTPS14 (first Aquilaria selinene synthase) catalyzes 4,11-selinadiene and α-selinene; and AsTPS13 produces β-eudesmol, γ-eudesmol, and hinesol [169].

In other plants: Litsea glauca has 12 sesquiterpenoid biosynthesis unigenes, including two tissue-specific α-humulene/β-caryophyllene synthase homologs [133]; Wurfbainia villosa TPS-a members are mostly bifunctional (GPP/FPP), except WvTPS42 (exclusive sesquiterpenoids) [138]; Ilex hylonoma has one sesquiterpene among 35 terpenoids [183]; vetiver grass varieties ‘CIM-Vridhi’ (khusimol 33.6%) and ‘CIMAP-Khusinolika’ (khusinol 50.53%) have distinct sesquiterpene profiles [174]; Dendrobium dendrobine and related alkaloids are sesquiterpene-derived (MVA pathway) [184]; ECG (plant species) has 16 sesquiterpenes (e.g., nerolidol 20.57% in petals, germacrene D 18.46% in fruits) [185]; Atractylodes lancea has seven main sesquiterpenoids with tissue-specific distributions (e.g., β-caryophyllene highest in stems, zingiberene in roots) [186][187], with root accumulation peaking at week 8 [186]. Transcriptomic analysis of A. lancea reveals 96 sesquiterpenoid/triterpenoid biosynthesis unigenes (34 direct synthesis) [186], and endophyte-activated GA/ABA signaling enhances essential oil accumulation [187]. Inflorescences show developmental stage-dependent sesquiterpene profiles, with differential expression of synthases (e.g., (−)-germacrene D synthase) correlating with metabolite levels [188].

Fungal sources contribute sesquiterpenoids: Gloeophyllum trabeum upregulates four sesquiterpene synthases (protoilludene/sativene homologs) in interaction zones [189]; Cerrena unicolor submerged cultures produce 13 cyclic sesquiterpenes (e.g., α-muurolene, α-copaene), with rapeseed press cake or unsaturated C18 fatty acids enhancing yields via MVA pathway acetyl-CoA supply [179]; Laetiporus sulphureus uses the mevalonate pathway (MVP) for terpenoids, with CYP51 family enzymes modifying lanostanoids [5].

Sesquiterpenoid biosynthesis involves three stages: upstream precursor generation, skeleton formation, and post-modification [190]. FPP forms from DMAPP/IPP (MVA [cytoplasmic] and MEP [plastid] pathways; sesquiterpenes mostly use MVA) [190]. The MVA pathway has six steps to IPP (isomerized to DMAPP, then condensed to FPP); the MEP pathway has seven steps to IPP [190]. TPSs bind FPP to form skeletons, modified by post-modification enzymes like cytochrome P450s (CYP450s)—only nine CYP71 subfamilies are directly involved (hydroxylation, 羰基化, carboxylation), e.g., CYP71AV1 (artemisinin) and CYP706B1 (gossypol) [190]. Evolutionary analysis shows CYP71 subfamily sequences cluster by function, with exceptions, promiscuity, and continuous oxidation [190].

Upstream pathway genes are characterized across species: A. sinensis has 11 MVA (six enzymes) and 11 MEP (nine enzymes) genes [184]; ECG has 29 terpenoid backbone and 26 sesquiterpene/triterpene biosynthesis DEGs [185]; A. lancea has 69 MVA (nine enzymes) and 28 MEP (seven enzymes) unigenes [186], with CYP450s and peroxisomes contributing to diversity [186]; ‘CIMAP-Khusinolika’ vetiver has 46 terpene backbone, 1 monoterpenoid, 10 diterpenoid, and 3 sesquiterpenoid/triterpenoid unigenes [174]; I. hylonoma has expanded specialized metabolism gene families [183]; A. sinensis AsTPS1/2/3/5/11/13/15/23 are wound-induced, with MeJA upregulating AsTPS13 (164.9×) and AsTPS5 (102.5×) [172]; AsZFP9 activates AsHMGR1 and AsTPS1 promoters [182]; lilac has 16 trans-isoprenyl diphosphate synthase (IDS) genes, including SoFPPS1-3 (FPP production) [11]; transient Pogostemon cablin patchoulol synthase expression in Nicotiana benthamiana yields low patchoulol [191].

Quantitative and Qualitative Variation of Terpenoid Profiles in Response to Environmental and Biotic Factors

Terpenoid structural diversity encompasses monoterpenoids, sesquiterpenoids, and their derivatives, with natural occurrence and profiles varying across plant species, tissues, processing conditions, and other biotic/abiotic factors. Monoterpenoids—including oxygenated forms—are prominent in plant essential oils, while sesquiterpenoids (hydrocarbons and oxygenated derivatives) exhibit distinct taxonomic and tissue-specific distributions. Both classes are synthesized via two primary pathways, with their profiles shaped by genetic, environmental, and biotic drivers.

Terpenoid Profiles Across Biological Systems

System	Terpenoid Class	Key Components	Citations
Peppermint (Mentha × piperita) essential oils	Oxygenated monoterpenes (88.3–94.8%)	Menthol (5.6–56.6%), menthone (7.6–15.9%), menthofuran (7.8–35.0%)	[77]
Mentha arvensis essential oils	Monoterpenoids	Menthofuran (25.4–35.0%)	[77]
Artemisia aerial parts	Oxygenated irregular monoterpenes; carbohydrate-bound sesquiterpenes	Yomogi alcohol, artemisia alcohol, artemisia acetate; germacrene-D	[146]
Artemisia underground parts	Oxygenated sesquiterpenoids (prevail over monoterpenes)	Germacrene D	[146][75]
Torrontés grape pomace distillates	Sesquiterpenoids	β-cadinene, δ-selinene, nerolidol	[79]
Auvergne highland milk	Sesquiterpenoids	β-caryophyllene, δ-elemene	[192]
Field-grown Yam-3 thalli	Sesquiterpenoids	Thujopsene, β-chamigrene	[167]
Fungal cultures	Sesquiterpenoids	β-gurjunene, β-caryophyllene, β-elemene, longicyclene, β-bisabolol, α-copaene, longifolene, germacrene	[193]
Various systems	Monoterpenoids	2-carene, α-terpinene, limonene, (E)-β-ocimene, (±)-linalool, β-myrcene, 1,8-cineole, geranyl acetone, α-pinene, β-pinene, camphene, 3Δ-carene, p-cymene, mentha-1(7),8-dien-2-ol, carene, limonen-10-ol, 3-caren-2-ol, carvone oxide, pyronene	[194][153][104]
Additional sesquiterpenoids	Sesquiterpenoids	Zingiberene, β-sesquiphellandrene, caryophyllene oxide, hinesol, β-eudesmol, atractylone	[195]

Biosynthetic enzymes for terpenoids operate via two primary pathways: the methylerythritol phosphate (MEP) pathway for monoterpenes, involving enzymes like deoxyxylulose-5-phosphate synthase (dxs) and 4-hydroxy-3-methylbut-2-enyl diphosphate reductase (hdr), and the mevalonate (MVA) pathway for sesquiterpenes, with hydroxymethylglutaryl-CoA synthase (hmgs) and hydroxymethylglutaryl-CoA reductase (hmgr) [127]. Terpene synthases (TPS) catalyze the final step, with TPSa subfamily enzymes synthesizing sesquiterpenes and TPSb subfamily enzymes producing monoterpenes [127]. Sesquiterpene biosynthesis can be induced with methyl jasmonate (MeJA) and jasmonic acid (JA) in grape cell cultures, upregulating MVA pathway genes (HMGS, HMGR, MPDC) and one MEP pathway gene (IspE) [196]. The endophytic fungus AL12 induces sesquiterpenoid biosynthesis through nitric oxide (NO), hydrogen peroxide (H₂O₂), ethylene (ET), salicylic acid (SA), JA, brassinosteroid (BR), and calcium (Ca²⁺) signals, while endophytic bacteria ALEB16 uses abscisic acid (ABA) and SA signals [195].

Quantitative and qualitative variation of terpenoid profiles is driven by environmental, biotic, and processing factors. Genetic differences shape baseline and induced terpenoid levels: the resistant Eucalyptus grandis (Egr) has higher baseline monoterpene (16.05±2.61 mg/g dry weight) and sesquiterpene (0.26±0.05 mg/g dry weight) concentrations than the susceptible E. camaldulensis (GC); following Leptocybe invasa oviposition, Egr monoterpenes increase slightly, while GC sesquiterpenes triple [127]. Grape varieties differ in terpenoid content: Loureira grape pomace distillates have double the monoterpenol content of Albariño (with high linalool, α-terpineol, and p-menthen-9-al isomers), while Torrontés distillates are rich in sesquiterpenoids [79].

Processing methods alter terpenoid profiles: distilled peach spirit contains more terpenoids (7 aroma-active terpenes) than pervaporation-produced spirit (5 terpenes), with citronellol, dihydro-β-ionol, and (E)-β-damascenone showing higher flavor dilution (FD) values in distilled spirit; distillation heat liberates terpenoids from glycosyl conjugates and promotes norisoprenoid formation (e.g., (E)-β-damascenone concentration increases with distillation time), whereas pervaporation concentrates hydrophobic terpenoids like eugenol [144].

Environmental factors drive terpenoid variation:

• In milk, production area accounts for over 60% of variance in seven terpenoids (e.g., α-pinene, β-caryophyllene) and over 90% for β-guaiene, while seasonal herd management influences β-gurjunene (85% variance) and δ-elemene (65% variance) [192].
• In ginger, fresh samples dominate in monoterpenes, grated samples in sesquiterpenes, and heating duration creates an antagonistic relationship between the two classes [96].
• In Artemisia, samples with developed root systems (GAIP_0, GAIP_1, GAIP_2) have a 2X higher monoterpenoid/sesquiterpenoid ratio in aerial oils than those with inhibited rooting (GAIP_3, GAIP_4); underground parts of root-developed samples have higher sesquiterpenoid content (78.2–83.5%) than callus samples (53.8–68.5%) [146].
• Carbon source affects fungal sesquiterpene production: amylopectin supplementation leads to higher total sesquiterpenes (158.5 × 10⁻¹² ± 8.8 × 10⁻¹² μg/g DW) compared to glucose (50.3 × 10⁻¹² ± 5.4 × 10⁻¹² μg/g DW), with specific sesquiterpenes (e.g., β-gurjunene on amylopectin, longicyclene on amylose) varying by carbon source [193].
• Ozone exposure alters monoterpene (MT) and sesquiterpene (SQT) ratios: in early leaves, MT dominates except at 120 ppb O₃ where SQT is ~100%, while late leaves show MT dominance at 0 ppb O₃ and SQT dominance during exposure [153].
• Habitat type influences Thymus montanum: serpentinite populations synthesize twice as much mono- and sesquiterpenes as calcareous populations, with 3-caren-2-ol, carvone oxide, and pyronene unique to serpentinite habitats [104].
• In Satureja officinalis plantlets, 0.0% HZ (control) has monoterpenes as the main group (63.3%), 0.1% HZ increases diterpene content (13.41%), and 0.2% HZ elevates monoterpenes to 78.4% [149].
• Temperature impacts terpenoids: at 45°C, monoterpene and sesquiterpene compounds increase, with curzerene+viridiflorene as major components [197].

Biotic factors like infection affect terpenoid levels: in Eucalyptus clones, a susceptible (S) clone shows decreased total terpenoids (from 7514 to 6003 µg/g) post-infection due to reduced MT, while a resistant (R) clone exhibits a >7-fold increase in total terpenoids (from 1727 to 12,542 µg/g) with elevated MT, oxygenated monoterpenoids (MTox), sesquiterpenoids (ST), and oxygenated sesquiterpenoids (STox) [148].

Seasonal and growth condition effects include:

• Plant terpenoid diversity increasing over the year (20 compounds in January, 35 in October) with major components shifting (germacrene D in summer/spring, camphor in autumn/winter) [18].
• Field-grown Yam-3 thalli having 118–202-fold higher thujopsene and β-chamigrene than axenically grown thalli, and mineral-deficient medium increasing C15 compound amounts [167].

Structural Elucidation and Classification of Novel Sesquiterpenoid Skeletons and Derivatives

Sesquiterpenoids exhibit remarkable structural diversity, with numerous skeletons and derivatives identified across natural sources, including essential oils of many plants [198]. A comprehensive review outlines key classes (Farnesane, Bicyclofarnesane, Bisabolane, Trichothecane, Cadinane, Eudesmane, Guaiane, Aromadendrane, Germacrane, etc.) and details new compounds, sources, and structural determinations [199]. Novel sesquiterpenoids with unique skeletons have been characterized, such as tricyclic 5/6/3 and lactonyl derivatives [200], while others have been isolated from diverse species including Chloranthus spicatus, Lactarius, Russula, Artemisia, Elephantopus tomentosus L., I. falconeri, S. fulvorobeus, D. undulata, A. sieversiana, Picrasma quassioides, Inula japonica Thunb., Inula britanica L., C. pinnatifida, C. japonica, and unspecified sources [405, 489, 648, 706, 729, 737, 710, 744, 762, 763, 791, 803, 838, 882, 888].

Isolation, Structural Elucidation, and Biosynthesis of Sesquiterpenoids

Structural elucidation of sesquiterpenoids typically involves multi-step chromatographic purification (e.g., silica gel, Sephadex LH-20, reversed-phase C-18 columns) followed by spectroscopic analyses [405, 489, 605]. For instance, eudesmane derivatives from Guarea guidonia leaves were characterized via ¹³C/¹H NMR, DEPT, and LREIMS/HREIMS [201], while sesquiterpenoids from S. fulvorobeus fermentation broth were isolated via repeated column chromatography and elucidated using HRESIMS, 1D/2D NMR, and literature comparisons [202]. Additional isolation techniques include MPLC, cycling HPLC, UFLC-MSn, HR-MS, and PTLC on silver nitrate precoated plates [203]. Compounds like conyterpenols A−D from C. japonica and melongenaterpenes M−T were elucidated using HRESIMS, IR, 1D/2D NMR, NOESY, and TDDFT-calculated ECD spectra [882, 888].

Sesquiterpenoid synthases play a key role in biosynthesis: vetiver varieties express CZTPS1 (α-cadinol, α-muurolol), CZTPS2 (α-gurjunene, germacrene D-4-ol), and CZTPS3 (farnesol, nerolidol isomer) [174]. Dendrobium alkaloids (sesquiterpene alkaloids) derive from the MVA pathway, with genes encoding MVA and MEP pathway enzymes identified in dendrobine biosynthesis [184]. In ECG, 29 DEGs regulate terpenoid backbone biosynthesis, with 26 involved in sesquiterpene and triterpene biosynthesis [185]. Drimane-type sesquiterpenoids from D. undulata use farnesyl diphosphate as a precursor, with stereochemistry controlled by cyclase substrate folding [204]. Cymbopogon citratus encodes multiple biosynthetic enzymes, including germacrene synthase and achilleol B synthase [12].

Natural Occurrence of Sesquiterpenoids

Sesquiterpenoids are widespread: chaga contains them alongside other terpenoids, though high molecular weight and isomerism complicate classification [205]. Essential oils from Rhododendron adamsii (spathulenol, β-elemenone), R. dauricum (caryophyllene, humulene), Oregano vulgare (trans-caryophyllene), Cinnamomum illicioides (d-cadinene, α-copaene), and Piper species (bicyclogermacrene, trans-δ-guaiene) include diverse sesquiterpenoids [544, 617, 639, 655]. Wood extracts from Sidi Bel Abbès and Tiaret provenances are dominated by sesquiterpenoids like alloaromadendrene oxide-(2) and cubénol [206]. Vetiver varieties ‘CIM-Vridhi’ and ‘CIMAP-Khusinolika’ share eremophilane, eudesmane, and oplapane constituents but differ in major components (khusimol vs. khusinol) [174]. Lettuce contains sesquiterpene lactones (lactucin, lactucopicrin) [207], while ECG tender leaves, fruits, and petals have 16 sesquiterpenes (e.g., nerolidol in petals, germacrene D in fruits) [185]. Volatile components from an unspecified source are ~70% sesquiterpenoids, with bud exudates (~60%) and supercritical CO₂ extracts (47%) also containing them [208]. I. falconeri ethanol extracts and methanolic extracts of an unspecified plant include sesquiterpenoids [737, 792]. Nootkatone is a main sesquiterpene in SREAO (18.2%) and DREAO (5.34%) extracts [209]. Elatol, a marine algal sesquiterpenoid, has trypanocidal activity [198].

Terpenoid Backbone Biosynthesis Pathways and Metabolic Flux Regulation

Terpenoid Backbone Biosynthesis Pathways (MVA and MEP) and Core Enzyme Characterization

Terpenoids (isoprenoids), the largest group of natural bioactive compounds, include clinically recognized agents like the anticancer drug paclitaxel and antimalarial drug artemisinin [210]. Their biosynthesis relies on two distinct plant pathways producing the core building blocks isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP): the cytosolic mevalonate (MVA) pathway and plastidial 2C-methyl-D-erythritol 4-phosphate (MEP) pathway [211]. The MVA pathway typically supplies precursors for sesquiterpenes and triterpenes (including sterols), while the MEP pathway supports isoprene, monoterpenes, diterpenes, and tetraterpenes [211][212]. Dendrobium sesquiterpene alkaloids (e.g., dendrobine, a sesquiterpene-origin compound [184]) are downstream MVA products [184].

Gene and Enzyme Profiles of Terpenoid Backbone Biosynthesis Pathways Across Plant and Fungal Species

Organism	MVA Pathway Details	MEP Pathway Details
Prunellae Spica	75 coding genes for 6 enzymes (more active than MEP) [213]	23 coding genes for 7 enzymes [213]
S. weigelae	16 key enzyme genes (double-copy farnesyl diphosphate synthase; no mevalonate kinase vs. S. sanghuang) [214]	—
A. argyi	43 unigenes for 8 enzymes (AACT, HMGS, HMGR) [215]	62 unigenes for 6 enzymes; DXS (2187 bp) and DXR (1419 bp) cloned [215]
B. papendorfii UM 226	10 core enzymes (2 HMGR copies, 4 acetyl-CoA C-acetyltransferase copies; primary terpenoid pathway) [210]	—
A. cinnamomea	23 transcriptional sequences (HMGR upregulated under specific conditions) [216]	Absent [216]
Dendrobium	11 genes for 6 enzymes (AACT, HMGS, HMGR, MK, PMK, MPDC) [184]	11 genes for 9 enzymes (supplements isoprene units for dendrobine) [184]
ECG	29 DEGs for enzymes (AACT, HMGS, HMGR); 26 DEGs in sesquiterpene/triterpene pathways [185]	DEGs for enzymes (DXS, CMK); 19 DEGs in monoterpene pathways [185]
White spruce	—	DXS, DXR, HDS, MDS, HDR genes highly expressed in CRD cells (mono/diterpene synthesis) [212]
Peppermint	—	DXR catalyzes slow step; fosmidomycin reduces monoterpenes; DXR upregulation increases essential oils [217]

The MVA pathway initiates with thiolase (TH) and 3-hydroxy-3-methylglutaryl-CoA synthase (HMGS) catalyzing two-step acetyl-CoA condensation, followed by 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) reducing the product to mevalonic acid [136]. Functional studies link MVA activity to sesquiterpene production: Cerrena unicolor’s sesquiterpene synthesis depends on an active MVA pathway, with rapeseed press cake supplementation enhancing yields via increased acetyl-CoA from fatty acid β-oxidation [179]; MeJA and JA upregulate MVA genes (HMGS, HMGR, MPDC) to induce sesquiterpenes in grape cell cultures [196]; reduced poly(cis-1,4-isoprene) in transgenic plants decreases MVA-derived triterpenes/sterols, confirming shared precursors [218].

The MEP pathway’s rate-limiting step is plastidial 1-deoxy-D-xylulose-5-phosphate synthase (DXS) activity, producing isoprenoids like chlorophylls, carotenoids, and ABA [219]. Key enzymes include 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR), inhibited by fosmidomycin [220]. The PmDXR protein has conserved domains (PLN02696, NADB_Rossman, DXP_reductoisom) and motifs (for DXR and NADPH binding) and interacts with both MEP (DXPS, HDR, HDS) and MVA (HMGS, HMG, FPS1) proteins [221]. Overexpressing PmHDS (a pivotal MEP gene) in A. thaliana increases chlorophylls, carotenoids, and phytohormones (CTK, ABA, GA) while altering MEP (AtDXR downregulated, AtHDR upregulated) and MVA (AtHMGS/AtHMGR downregulated, AtMK upregulated) gene expression [222]. Transgenic plants express MEP enzymes (DXR, HDR) only early in growth, whereas MVA enzymes are constant [218].

Prenyltransferases catalyze head-to-tail IPP-DMAPP condensations: 1 IPP + 1 DMAPP form geranyl diphosphate (GPP, C10; monoterpene precursor), 2 IPP + 1 DMAPP form farnesyl diphosphate (FPP, C15; sesquiterpene precursor), and 3 IPP + 1 DMAPP form geranylgeranyl diphosphate (GGPP, C20; diterpene/tetraterpene precursor) [211]. Geranyl diphosphate synthase (GPPS) and farnesyl diphosphate synthase (FPPS) synthesize GPP and FPP, respectively [46]. In white spruce, GPPS/GGPPS are CRD cell-specific (mono/diterpene synthesis), while FPPS has no cell-type preference [212]. S. weigelae has a double-copy FPPS gene [214], and A. argyi includes FPPS/GPPS among 16 key terpenoid backbone enzymes [215].

Terpene synthases (TPSs) generate diverse terpenoid carbon skeletons, with 89 putative TPS genes in the grape genome [46]. White spruce TPSs are highly CRD cell-specific [212]; S. weigelae has nine TPS gene clusters [214]; Cerrena unicolor has ten sesquiterpene cyclases grouped by cyclization mechanisms (e.g., 1,10-ring closure for clades I/II) [179]; peppermint TPSs (e.g., (-)-limonene synthase) were identified via EST analysis [217]; and PmHDS overexpression in A. thaliana upregulates AtTPS [222].

Metabolic flux is regulated by precursor competition and transcription. In Narcissus, upregulated CPS/TPS and downregulated PSY/GGR redirect plastid GGPP from chlorophylls/carotenoids to terpenoid/benzenoid pathways [223]. A. cinnamomea’s camphor substrate upregulates SQS/FPPC3 (sesquiterpene/triterpene synthesis) while downregulating MPD via negative feedback [216]. In E. coli, the IspH-derived IPP:DMAPP ratio (5:1) and low idi expression limit isoprene synthesis [224]; an engineered MVA pathway (ispS from P. trichocarpa, mvaE/mvaS from E. faecalis, mvaK2/mvaD from S. pneumoniae, mvaK1 from S. aureus, idi from E. coli) produces isoprene from glycerol [225]. Grape berries’ plastid VvGGPPS acts upstream of norisoprenoid biosynthesis [226], and VvPNLinNer1 (mono-TPS) expression correlates with monoterpenes [46]. Phlox paniculata’s terpenoid emission is reduced by fosmidomycin (DXR inhibitor) and mevinolin (HMGR inhibitor), confirming MEP/MVA roles in mono/sesquiterpene synthesis [220]. LOV treatment sequentially affects pathways: monoterpenoids (2 h), diterpenoids (4 h), carotenoids (6 h), and terpenoid backbones (10 h) [227]. Cerrena unicolor uses 13C-labeled acetate for MVA-derived sesquiterpenes [179], and SA suppresses MeJA-induced MVA gene upregulation and sesquiterpene production in grape cell cultures [196].

TPS Gene Family Classification and Functional Association with Monoterpenoid/Sesquiterpenoid Biosynthesis

Plant terpenoids, the largest and most diverse class of plant secondary metabolites, are synthesized via two primary pathways: the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway in plastids and the mevalonate (MVA) pathway in the cytosol [228][212]. These pathways converge through isopentenyl diphosphate isomerase (IPPI) to produce geranyl diphosphate (GPP), farnesyl diphosphate (FPP), and geranylgeranyl diphosphate (GGPP)—precursors for monoterpenoids (C10), sesquiterpenoids (C15), and diterpenoids (C20), respectively [169][212].

In sesquiterpenoid biosynthesis, FPP ionization forms the farnesyl cation, which follows six initial cyclization modes (e.g., 1,10-cyclization to the (E,E)-germacradienyl cation, 1,11-cyclization to the (E,E)-humulyl cation) to drive structural diversity [169]. For example, FPP cyclization via intermediates B and E yields germacrene B and A (potential precursors for eudesmane- and guaiane-type sesquiterpenes), while intermediates C and F contribute to α-humulene biosynthesis; linear sesquiterpenes like farnesene and nerolidol form via double bond isomerization without further cyclization [169]. In Cerrena unicolor, sesquiterpene synthases are grouped into four clades by cyclization mechanism: clade I/II catalyzes 1,10-ring closure of trans- or cis-farnesyl cations (producing α-muurolene, α-copaene), clade III mediates 1,6-cyclization (β-curcumene, α-bisabolol), and clade IV forms humulene [179].

Terpene synthases (TPSs) are critical for terpenoid structural diversity, with the TPS gene family subdivided into seven subfamilies (TPS-a to -g) based on amino acid sequence similarity and conserved motifs (e.g., DDXXD, DXDD, NSE/DTE, RRX8W) [229][230]. Two aspartate-rich motifs (DDxxD, NSE/DTE) are essential for cleaving prenyl diphosphate substrates by chelating Mg²⁺ or Mn²⁺ at the C-terminus [231]. TPS subfamilies exhibit distinct functional and taxonomic associations, with representative examples and species distributions summarized below:

TPS Subfamily Functional Associations, Representative Enzymes, and Species Examples

TPS Subfamily	Functional & Taxonomic Associations	Representative Enzymes & Species	Citations
TPS-a	Encodes sesquiterpene/diterpene synthases	Aquilaria sinensis AsTPS1/11 (δ-guaiene/α-humulene), Capsicum annuum CaTPS6/18/37/96/98 (4,5-di-epi-aristolochene), Santalum album SaSQS1/2 (sesquisabinene), wheat TaTPS84/27/02/22/102 (α-humulene, (+)-delta-cadinene, (E)-beta-farnesene), Laetiporus sulphureus (15 sesquiterpene synthases)	[171][169][141][229][232][173][5]
TPS-b	Monoterpenoid biosynthesis; contains RRX8W motif for cyclization	Ocimum sanctum OsaTPS, Cinnamomum camphora CmCJTPS5 (1,8-cineol), C. annuum CaTPS76 (linalool), wheat TaTPS (alpha-terpineol), Dendrobium officinale DoTPS10 (linalool), L. sulphureus (8 monoterpene synthases: GPP → myristene/linalool)	[171][233][229][232][231][173][5]
TPS-g	Monoterpenoid biosynthesis; lacks RRX8W motif (produces acyclic monoterpenoids)	C. annuum CaTPS76 (linalool), D. officinale DoTPS10 (linalool), L. sulphureus (8 monoterpene synthases)	[229][232][231][5]
TPS-d	Exclusive to gymnosperms; multi-functional (mono-, sesqui-, diterpenoids)	Grand fir wound-induced (E)-α-bisabolene synthase	[234][232]
TPS-e/f	Copalyl diphosphate/kaurene synthases (gibberellin/phytoalexin biosynthesis)	Wheat TaTPS150 (ent-kaurene synthase like 1), ent-kaurene/syn-pimara-7,15-diene synthases, D. officinale (3 members)	[229][230][231][173]
TPS-c	Class II diterpene synthases	Wheat TaTPS44/12 (ent-copalyl diphosphate), TaTPS64 (syn-copalyl diphosphate), D. officinale DoTPS31 (lacks DDxxD motif)	[171][232][231][173]

TPS gene family size and composition vary across species: Lavandula has 100 TPS genes (five subclades, excluding TPS-d/h) with expanded TPS-b (monoterpene synthases highly expressed in glandular trichomes) [235]; D. officinale has 34 TPS proteins (four groups: TPS-a/b/c/e/f, 88.2% in TPS-a/b) [231]; wheat has 153 TaTPS genes (six subfamilies, TPS-a largest with 65 members, TPS-b smallest with 13) [173].

Functional diversity of TPSs is evident across species: A. sinensis AsTPS9 converts FPP to β-farnesene/cis-sesquisabinene hydrate, AsTPS12 produces β-farnesene/γ-eudesmol, and AsTPS14 acts as a selinene synthase [169]; S. album SaBS catalyzes β-bisabolene formation, while SaSS is a promiscuous enzyme producing α/β-santalene and exo-α-bergamotene [141]; C. camphora CmCJTPS1 synthesizes γ-selinene, CmCJTPS2 produces β-elemene, and CmCJTPS3 converts FPP to α-bisabolol [233]; D. officinale DoTPS10 singly converts GPP to linalool [231].

Metabolic flux regulation involves transcriptional control, substrate pool modulation, and environmental cues. In Grand fir, wounding induces monoterpene synthases first (transcripts at 4 h), followed by sesquiterpene/diterpene synthases (day 3), with enzyme activity lagging mRNA levels [234]. Water deficit in Vitis vinifera upregulates TPS-a, -b, and -g genes, promoting monoterpene accumulation (hotrienol, linalool) via MYB and drought-responsive motifs [228]. Overexpression of Litsea cubeba LcGPPS.SSU1 enhances tobacco monoterpene production by increasing plastid GPP pools [130]. In Picea abies, MEP pathway genes (DXS, DXR) and TPSs are preferentially expressed in cortical resin duct (CRD) cells for specialized oleoresin biosynthesis [212]. In Aquilaria malaccensis, infection upregulates MEP pathway genes (ADXPS: 26-fold, ADXPR: 33-fold), MVA pathway gene AHMGR (19-fold), FPP synthase (AFPS: 40-fold), and sesquiterpene synthase (ASS: 11-fold) [236]. In Paris polyphylla, terpenoid backbone biosynthesis unigenes are enriched in leaves, with upregulated DEGs in rhizome vs. leaf comparisons [237]. In L. sulphureus, the MVA pathway is upstream for terpenoid metabolism [5]. In C. unicolor, rapeseed press cake (high protein/lipid) or unsaturated C18 fatty acids (oleic/linoleic acid) increase sesquiterpenoid yields (via enhanced MVA pathway acetyl-CoA supply), confirmed by 13C-labeled acetate feeding [179]. In Cymbopogon citratus, light conditions affect TPS expression: new menthol dehydrogenase genes show light/shade differences, while linalool synthase is highly expressed in both [12].

TPS gene family expansion correlates with terpenoid diversity: Dendrobium catenatum has 13 TPS genes (OG0000093) and 4 terpineol synthase genes (OG0001951), with terpenoids accounting for 12.12% of metabolites [238]; M. micrantha has 61 TPS-a and 65 TPS-b unigenes (high expression in roots/leaves/flowers) [239]; C. annuum has 103 TPS loci across 12 chromosomes (expansion via tandem/segmental duplications) [232]; Lavandula’s TPS-b expansion links to monoterpenoid biosynthesis [235]. These expansions, plus differential TPS expression and substrate specificity, underpin terpenoid structural/functional diversity. Post-synthesis modifications further expand diversity: Lavandula CYP450s (207 in CYP71 clan) hydroxylate mono- and sesquiterpenes, while BAHD acyltransferases (213 members) form esters [235].

Regulation of Terpenoid Biosynthesis via Transcription Factors, Signaling Molecules, and Environmental Stimuli

Terpenoids, the most diverse family of natural products with over 40,000 distinct structures, include volatile forms (hemiterpenoids [C5], monoterpenoids [C10], sesquiterpenoids [C15], and some diterpenoids [C20]) that mediate plant-insect interactions and stress responses[240]. All terpenoids derive from isopentenyl diphosphate (IDP) and dimethylallyl diphosphate (DMADP), synthesized via two pathways: the cytosolic mevalonic acid (MEV) pathway (precursors for sesquiterpenes [C15] and triterpenes [C30]) and the plastidial 2C-methyl-D-erythritol-4-phosphate (MEP) pathway (precursors for monoterpenes [C10], diterpenes [C20], and tetraterpenes [C40])[240][228]. Fosmidomycin inhibits the MEP pathway enzyme 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR), reducing monoterpenoid formation, while mevinolin targets the MEV pathway enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), decreasing sesquiterpene synthesis[220].

Structural diversity of terpenoids is evident across species. For example, Cymbopogon citratus (lemon grass) contains 23 terpenoid species, including 4 monoterpenes, 4 sesquiterpenes, 8 diterpenes, 3 triterpenes, and 4 tetraterpenes[12]; its four dominant volatile monoterpenes (ordered by relative content) are α-ocimene, dehydroverrucosidin tetraacetate, nerol, and α-pinene[12]. The fungus Cerrena unicolor produces 13 cyclic sesquiterpene hydrocarbons, alcohols, and aldehydes (e.g., α-muurolene, α-copaene, β-cubebene, epicubenol, β-curcumene, α-bisabolol, humulene) in submerged cultures[179]. These sesquiterpenes arise via distinct cyclization mechanisms: α-muurolene, α-copaene, β-cubebene, and epicubenol from 1,10-ring closure of trans- or cis-farnesyl cations (clade I/II cyclases); β-curcumene and α-bisabolol from 1,6-cyclization of farnesyl cations (clade III cyclases); and humulene from clade IV cyclases[179]. Sesquisabinene and its hydrate are proposed to form via 1,6 closure of nerolidyl pyrophosphate, a pathway also observed in Indian sandalwood oil[179].

Terpenoid backbone biosynthesis is regulated by environmental stimuli, signaling molecules, and nutrient supplementation, with effects on pathway gene expression, precursor accumulation, and terpenoid production:

Regulatory Effects on Terpenoid Biosynthesis Pathways

Regulatory Factor	Organism/Tissue	Pathway/Target Genes/Enzymes	Outcome	Citations
Methyl jasmonate (MeJA) treatment	—	Upregulates 10 DEGs (HMGR, ispH, GGPS)	Boosts IDP, DMADP, GDP, GGDP, FDP accumulation; enhances terpenoid (linalool, geraniol) production	[240]
White tea withering	White tea	Activates JA signaling (AOS, ACX, JAR, JMT) → upregulates MEV (HMGR, ACCT, HMGS, PMK) and MEP (DXS, CMK, MDS, HDS) genes	Increases IPP and DMAPP synthesis	[142]
Water deficit	Vitis vinifera	Modulates MEP genes (VviDXS1, VviDXR); upregulates TPSs	Promotes monoterpene (hotrienol, linalool, nerol, α-terpineol) accumulation	[228]
Baizongia pistaciae-induced galls	Pistacia palaestina	Upregulates key plastidial and cytosolic pathway genes	Increases monoterpene content	[126]
High cobalt conditions	Botryococcus braunii	Alters transcription of IPP/DMAPP biosynthesis enzymes (HMGR, mevalonate kinase) and polyprenyl diphosphate synthases	Most DEGs upregulated early then downregulated later	[241]
PVYN infection	Tobacco	Affects isoprenoid accumulation; tobacco DXS (rate-limiting MEP enzyme) plays a role	—	[219]
Infection	Aquilaria malaccensis	Upregulates MEV (AHMGR, AFPS) and MEP (ADXPS, ADXPR) genes; FPS (40-fold) and ASS (11-fold)	Drives sesquiterpene production	[236]
Endophyte-activated GA/ABA signaling	Atractylodes lancea	Upregulates HMGR and DXR expression/activity	Enhances sesquiterpenoid accumulation	[187]
MeJA/JA treatment	Grape cell cultures	MeJA/JA induce sesquiterpene biosynthesis (JA > MeJA); SA suppresses MeJA-induced production	Upregulates MEV genes (HMGS, HMGR, MPDC); JA also upregulates MEP gene IspE	[196]
Rapeseed press cake supplementation	Cerrena unicolor	—	Increases sesquiterpenoid yields (α-copaene: 4.3 mg/L; cubebol: 4×; sesquisabinene hydrate: 7×; α-bisabolol: 3×; α-muurolene: 100×)	[179]
Unsaturated C18 fatty acids/rapeseed oil supplementation	Cerrena unicolor	β-oxidation of fatty acids increases acetyl-CoA levels → MEV pathway flux	Higher sesquiterpene production	[179]
13C-labeled acetate feeding	Cerrena unicolor	—	Confirms acetyl-CoA surplus drives MEV pathway activity	[179]
Shading	Lemon grass	—	Increases relative content of ginkgolide C (diterpenoid) and astaxanthin (tetraterpenoid); no change in dominant volatile monoterpenes	[12]
Lovastatin (LOV) treatment	—	Time-dependent pathway enrichment: map00902 (2h), map00904 (4h), map00906 (6h), map00909/map00900 (10h)	—	[227]

Biosynthetic enzyme expression is also regulated by environmental factors. In lemon grass, monoterpene biosynthetic enzymes include 9 carboxyindole synthase gene families, 13 neomenthol dehydrogenase gene families, 1 linalool synthase gene, and 1 nepetalactone monooxygenase gene[12]. Under varying light conditions, carboxyindole synthase and nepetalactone monooxygenase genes have low relative expression, linalool synthase is highly expressed, and neomenthol dehydrogenase genes show significant differences (e.g., gene c102703.0 is lower under light than shade)[12]. Diterpene biosynthetic enzymes in lemon grass include 26 gibberellin 2β-dioxygenase gene families, 5 kaurene synthase gene families, 3 families each of ent-kaurene hydroxylase, momilactone-α synthase, and gibberellin 20-oxidase, 2 families each of kaurenoic acid monooxygenase and trimethyltridecatetraene synthase, and 1 gene each for diene 11-hydroxylase, copalyl diphosphate synthase, diene oxidase, and nepetalactone synthase[12]. Shading significantly increases expression of 6 diterpene enzyme genes: gibberellin 2β-dioxygenase (c102697.0, c99152.1), kaurene synthase (c100000.0, c100759.0), momilactone-α synthase (c102803.0), and kaurenoic acid monooxygenase (c96273.0)[12]. Sesquiterpene and triterpene biosynthetic enzymes in lemon grass include 16 germacrene synthase gene families, 15 achillin B synthase gene families, 13 nerolidol synthase gene families, 4 farnesyl diphosphate farnesyl transferase gene families, 3 prenaphthalene diene oxygenase gene families, 2 farnesol dehydrogenase gene families, and 1 gene each for lupeol synthase, acyclic sesquiterpene synthase, β-farnesene synthase, β-caryophyllene synthase, and aristolochene 1,3-dihydroxylase[12]. Most of these genes have low expression under light and shade, except prenaphthalene diene oxygenase (c64786.0, higher under light) and farnesol dehydrogenase (c91185.0, higher under shade)[12]. In Cerrena unicolor, ten sesquiterpene cyclases have been identified and heterologously produced, contributing to its sesquiterpenoid diversity[179].

Transcriptional regulation of terpenoid biosynthesis involves transcription factors (TFs) and microRNAs (miRNAs). In Vitis vinifera, water deficit correlates monoterpene accumulation with TFs like MYB24, C2H2 Zinc finger, and Constans-like 11, whose target genes contain MYB and drought-responsive promoter motifs[228]. In Codonopsis pilosula, CpbHLH2, CpbHLH3, and CpAP2/ERF2 directly regulate triterpenoid and sesquiterpenoid biosynthetic genes (CpLUP4-14, CpSQLE-2, CpFDFT1-8)[242]. In Artemisia argyi, five bZIP TFs (AarbZIP29, AarbZIP55, AarbZIP81, AarbZIP130, AarbZIP150) homologous to Artemisia annua bZIPs (involved in terpenoid regulation) are highly expressed in leaves and flowers[243]. miRNAs such as csa-miR444b, csa-miR2275-3p, csa-miR2275c, and csa-miR169o-3p regulate terpenoid backbone biosynthesis by targeting pathway enzymes[244]. In Antrodia cinnamomea, adding Cinnamomum camphora or C. kanehirae homogenate upregulates MEV pathway genes (ACS1, ACS2, HMGR, SQS), with camphor substrate promoting sesquiterpene and triterpene biosynthesis[216]. Low nitrogen conditions upregulate triterpene saponin biosynthetic genes (FPS, β-AS, HMG-CoA) in related pathways[245], while scale insect infestation enriches DEGs in sesquiterpenoid and triterpenoid biosynthesis[246]. Additionally, DEGs are enriched in “sesquiterpenoid and triterpenoid biosynthesis” in PF2S vs PF2 comparisons[247].

Structural Diversity of Monoterpenoids and Sesquiterpenoids: Carbocation Cascades, Cyclization Modes, and Tissue-Specific Accumulation

Terpenoids constitute the most diverse family of natural products, with 40,000 [240] to 60,000 identified structures [168], including volatile monoterpenoids (C10), sesquiterpenoids (C15), and some diterpenoids (C20) [240]. All terpenoids derive from the common building units isopentenyl diphosphate (IDP/IPP) and its isomer dimethylallyl diphosphate (DMADP/DMAPP), synthesized via two independent pathways: the cytosolic mevalonic acid (MEV/MVA) pathway and the plastidial 2C-methyl-D-erythritol-4-phosphate (MEP) pathway [240][248][127][133]. The MEV pathway supplies precursors for sesquiterpenes (C15) and triterpenes (C30), while the MEP pathway provides precursors for monoterpenes (C10), diterpenes (C20), and tetraterpenes (C40) [240][127]. Key rate-limiting enzymes include hydroxymethylglutaryl-CoA reductase (HMG-CoA reductase) in the MEV pathway [240][133] and 1-deoxy-D-xylulose-5-phosphate synthase (DXS) in the MEP pathway [133]. For example, MeJA treatment upregulates HMG-CoA reductase (2.88-fold) and ispH (EC:1.17.1.2) in tea leaves, increasing IDP/DMADP production [240], while DXS and HMG-CoA synthase (HMGS) are encoded by multigene families in Lindera glauca [133]. Prenyltransferases assemble IPP and DMAPP into linear prenyl diphosphates (e.g., geranyl diphosphate [GPP], farnesyl diphosphate [FPP], geranylgeranyl diphosphate [GGPP]) [248][169], which are then reorganized by terpene synthases (TPSs) into diverse terpenoid structures [248][168][169].

Sesquiterpene biosynthesis involves upstream precursor generation, skeleton formation, and post-modification stages [190]. FPP, the precursor for sesquiterpenes, is formed by condensation of DMAPP and IPP from the MVA (cytosolic) and MEP (plastidial) pathways, with synthesis predominantly relying on the MVA pathway [190]. The MVA pathway proceeds through six enzyme-catalyzed reactions to form IPP, which is isomerized to DMAPP and condensed into FPP; the MEP pathway involves seven plastidial steps to generate IPP [190]. TPSs catalyze GPP and FPP to form monoterpene (C10) and sesquiterpene (C15) backbones, respectively, followed by modification by CYP450s (e.g., CYP71 clan) and BAHD acyltransferases [235].

The structural diversity of monoterpenoids and sesquiterpenoids arises from complex carbocation cascades and cyclization modes catalyzed by TPSs [168][249][169]. TPSs are classified as Class I (initiating via heterolytic diphosphate cleavage) or Class II (initiating via protonation), with most multiproduct synthases belonging to Class I [168]. Monoterpene synthases convert GPP, often via isomerization to linalyl diphosphate (LPP) and cyclization to the terpinyl cation, producing compounds like α-pinene, β-pinene, and camphene [249]. Sesquiterpene synthases act on FPP, which ionizes to the farnesyl cation and undergoes initial cyclization modes (e.g., 1,10-cyclization to germacradienyl cation, 1,11-cyclization to humulyl cation) [169], generating structures such as germacrene D, δ-guaiene, and α-humulene [169][168]. Multiproduct TPSs contribute significantly to diversity: γ-humulene synthase from Abies grandis produces 52 sesquiterpenes from FDP [168], while AsTPS9 from Aquilaria converts FPP to β-farnesene, cis-sesquisabinene hydrate, citronellol, and geraniol [169]. The TPS active site, including conserved motifs like DDxxD, enables multiple substrate conformations and carbocation stabilization [168]; mutagenesis of DDxxD motifs reduces product diversity [168]. Environmental factors like pH also influence product profiles: higher pH shifts MtTPS5 from Medicago truncatula to favor germacrene-based over cadalane-based products [168]. Sesquiterpene structural diversity is further enriched by CYP450s, with only nine CYP71 family subfamilies directly modifying sesquiterpene backbones via hydroxylation, carbonylation, and carboxylation [190].

Tissue-specific terpenoid accumulation is linked to differential biosynthetic gene expression. In Pistacia palaestina, galls induced by Baizongia pistaciae show upregulated plastidial and cytosolic terpene biosynthetic genes, leading to higher monoterpene content than leaves [126]. In L. glauca, MEP pathway genes (e.g., DXS) are highly expressed in sarcocarps, while MEV pathway genes (e.g., HMGR) are active across all tissues; monoterpene synthases (linalool synthase [LIS], alpha-terpineol synthase [TES]) and sesquiterpene synthase homologs (α-humulene/β-caryophyllene) show tissue-specific expression correlating with terpenoid profiles [133]. In tea trees, winter upregulates MVA pathway genes and specific TPSs (e.g., CsLMS, CsLIM) while downregulating MEP pathway genes, altering monoterpene and sesquiterpene accumulation [132]. In Capsicum, TPS-b subfamily members (monoterpene synthases) have chloroplast transit peptides (plastid localization), while TPS-a subfamily members (sesquiterpene synthases) lack signal peptides (cytosolic localization) [232]. Endophytic fungi also produce diverse volatile terpenoids (e.g., pinene, limonene, caryophyllene) [248], with Phialocephala fortinii emitting β-caryophyllene and Phomopsis (EC-4) producing sabinene [248].

Terpenoid Biosynthesis and Profile Variation Across Species, Tissues, and Genotypes

Organism/Tissue/Genotype	Key Finding	Reference(s)
Protium copal	186 transcripts linked to monoterpenoid/triterpenoid synthesis/transport; 57 TPS genes (subfamilies excluding TPSd monophyletic)	[250]
Protium copal	Shares 32 terpene genes with Bursera sacra and 35 with B. simaruba; higher terpene gene cluster diversity	[250]
Lavandula	100 TPS genes (5 subclades excluding TPS-d/h); TPS-b clade (monoterpene synthases) expanded and highly expressed in flowers	[235]
Lavandula	Flowers have highest volatile terpenoid content (65.55 µg/mg; ~16× leaves, ~205× stems); floral volatiles dominated by linalool/linalyl acetate/lavandulyl acetate (71.78%)	[235]
Lavandula (stems/leaves)	Higher defensive terpenoids (borneol, camphor)	[235]
‘CIM-Vridhi’ (variety)	TPS-a (sesquiterpene synthases) and TPS-e (kaurene synthase) members identified; essential oil rich in khusimol (33.6%), khusimene, khusimone, vetivones	[174]
‘CIMAP-Khusinolika’ (variety)	TPS-a and TPS-e members identified; essential oil dominated by khusinol (50.53%); exclusive components: gamma-cadinene, 1,10-di-epi-cubenol	[174]
‘CIM-Vridhi’/‘CIMAP-Khusinolika’	Common components: eremophilanes, eudesmanes, oplapane	[174]
CZTPS1 (enzyme)	Produces α-cadinol (72.66%) and α-muurolol (27.33%) from FPP	[174]
CZTPS2 (enzyme)	Yields α-gurjunene (33.16%), germacrene D-4-ol (25.54%), and other products from FPP	[174]
CZTPS3 (enzyme)	Generates farnesol (63.31%) and a nerolidol isomer (36.68%) from FPP	[174]
Chrysanthemum (CmCJTPS5)	TPS-b subfamily enzyme converts GPP to 1,8-cineol (main product) and 5 other monoterpenoids	[233]
Cerrena unicolor (TPS clades)	Clade I/II: 1,10-ring closure (α-muurolene, α-copaene); Clade III: 1,6-cyclization (β-curcumene, α-bisabolol); Clade IV: humulene formation	[179]
Cerrena unicolor	Proposed pathway: nerolidyl cation → bisabolol, curcumene, sesquisabinene hydrate (supported by co-occurrence)	[179]
ECG (tender leaves/fruits/petals)	49 volatile compounds including 15 monoterpenes, 16 sesquiterpenes	[185]
ECG (petals)	Rich in β-pinene (7.53%), β-myrcene (15.32%), nerolidol (20.57%)	[185]
ECG (fruits)	Rich in β-myrcene (24.35%), germacrene D (18.46%)	[185]
ECG (leaves)	Rich in β-pinene (8.27%), squalene (9.25%)	[185]
A. lancea (endophyte-activated)	GA/ABA signaling upregulates HMGR/DXR expression/activity, enhancing sesquiterpenoid accumulation	[187]
Leaf tissue (terpenoid synthases)	Three linalool synthase transcripts upregulated; linalool/α-terpineol synthases have multiple activities (sabinene, α-farnesene, (R)-limonene)	[251]
Lemon grass (shading)	Higher diterpenoid ginkgolide C and tetraterpenoid astaxanthin; no significant difference in volatile monoterpenes (α-ocimene, dehydrostrychnine tetraacetate, nerol, α-pinene)	[12]
Lemon grass (light vs. shading)	New menthol dehydrogenase genes show expression differences; c102703.0 expressed lower in light	[12]
CYP450 examples	CYP71AV1 (artemisinin synthesis), CYP706B1 (gossypol synthesis)	[190]

Transcriptomic and Metabolomic Profiling of Terpenoid Biosynthesis in Diverse Plant and Fungal Species

Terpenoids are biosynthesized from the universal precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), generated via the deoxyxylulose phosphate (DXP)/2C-methyl-D-erythritol-4-phosphate (MEP) pathway (plastids) or mevalonate (MVA) pathway (cytosol) in plants [248][252][253][228][133][212]. Prenyltransferases assemble IPP and DMAPP into linear prenyl diphosphates, which terpene synthases reorganize into diverse terpenes [248][252][253]. Key rate-limiting enzymes—1-deoxy-D-xylulose-5-phosphate synthase (DXS) in the MEP pathway and hydroxymethylglutaryl-CoA synthase (HMGR) in the MVA pathway—are often encoded by small multigene families [133]. Transcriptomic and gene family analyses across species reveal pathway-specific gene counts, expression patterns, and functional roles, as summarized below:

Terpenoid Biosynthesis Gene Counts, Expression, and Enzyme Roles Across Species

Species	Pathway/Class	Key Findings	Citations
Litsea glauca	MEP/MVA pathways	6 DXS, 4 HMGR homologs; MEP genes highly expressed in sarcocarps, MVA genes stable across tissues	[133]
Prunellae Spica	MEP/MVA pathways	MVA pathway more active; 75 coding genes (6 enzymes) in MVA vs. 23 (7 enzymes) in MEP	[213]
White spruce	MEP/MVA pathways	Full-length sequences for all steps (including MK/PMK missing in prior annotations); MEP/GPPS/GGPPS transcripts dominant in CRD cells; MVA/FPPS not cell-type specific	[212]
Sanghuangporus weigelae	MVA pathway	16 key enzyme genes; triple-copy protein-S-isoprenylcysteine O-methyltransferase; double-copy FPPS/protein farnesyltransferase subunit β	[214]
Sanghuangporus spp.	Terpenoid backbone synthesis	15 (S. baumii), 14 (S. sanghuang), 15 (S. vaninii), 18 (S. weigelae) genes	[214]
Aquilaria malaccensis	Sesquiterpenoid biosynthesis	Infected plants: ADXPS (26×), ADXPR (33×), AHMGR (19×), AFPS (40×), ASS (11×) upregulation; natural infection > artificial induction	[236]
Paris polyphylla	Terpenoid pathways	38 terpenoid backbone, 32 steroid, 7 sesquiterpenoid/triterpenoid unigenes; 9/7/3 upregulated DEGs in rhizome vs. leaf (higher in leaves)	[237]
Tea plants	Sesquiterpenoid/triterpenoid biosynthesis	Top 10 enriched KEGG pathways; DEPs: GERD, FDFT1, LUP4	[254]
Pfaffia glomerata	Sesquiterpenoid/triterpenoid biosynthesis	Enriched in PF2S vs PF2 comparisons	[247]
LOV-treated plants	Terpenoid backbone biosynthesis	Significantly enriched at 10 h (late-stage effect)	[227]

Monoterpenoids (from geranyl diphosphate, GPP) exhibit structural diversity across plants and fungi. Fungal endophytes like Hypoxylon sp. EC38, CI4A, CO27, Daldinia eschscholzii EC12, Phomopsis (EC-4), and Phialocephala fortinii produce monoterpenes including pinene, limonene, sabinene, and β-caryophyllene [248][255][256][257][258][259][260][261]. Plant studies show expanded terpene synthase (TPS) families (e.g., Dendrobium catenatum [238]), tissue-specific upregulation (e.g., Pistacia palaestina galls [126]), and environmental modulation: water deficit in Vitis vinifera upregulates TPS-b genes and monoterpene accumulation [228]; winter conditions in tea plants upregulate MVA pathway genes and monoterpene synthases (CsLMS, CsLIM, CsOCS) [132]; white tea withering upregulates MEP pathway genes and TPS10 (correlated with linalool accumulation) [142]. Genetic variants (e.g., VvDXS1 SNP1822 in grapevines [262]) and TPS subfamily expansions (e.g., TPS-b in Lavandula [235]) also drive monoterpenoid diversity.

Sesquiterpenoids (from farnesyl diphosphate, FPP) are widespread in fungi and plants. Fungal endophytes produce caryophyllene, chamigrene, and selinene [248][256][257][258][259]. Plant TPS-a subfamily enzymes synthesize specific sesquiterpenoids (e.g., Cichorium intybus CmCJTPS1-4/8 [233]; Capsicum CaTPS6/18/37/93/96/98 [232]; Vetiveria zizanioides CZTPS1-3 [174]). Environmental and genetic factors influence profiles: resistant Eucalyptus genotypes have higher sesquiterpene content [127]; tea plants downregulate sesquiterpene synthases under winter conditions but accumulate α-muurolene [132]; white tea withering upregulates MVA pathway genes after 30 h [142]; Cerrena unicolor submerged cultures produce 13 cyclic sesquiterpenes, with yields enhanced by rapeseed press cake, unsaturated C18 fatty acids, or rapeseed oil [179]; grape cell cultures show MeJA/JA-induced sesquiterpene production [196].

Triterpenoids (from squalene, via FPP dimerization) are regulated by genes like FPS and SQS. Transgenic tobacco overexpressing FPS-SQS-HPG has 3-fold higher squalene [263]; D. catenatum triterpenes increase with TPS expression [238]; low nitrogen upregulates triterpene saponin genes [245]. Codonopsis pilosula roots contain 15 triterpenoids/sesquiterpenoids, with 46 genes/TFs (CpbHLH2/3, CpAP2/ERF2) regulating biosynthesis [242]. Other findings include downregulated sterol 24-C-methyltransferase in Botryococcus braunii under high cobalt [241]; reduced triterpene concentrations in bn/bs-transgenic plants [218]; and LUP4 as a triterpenoid-related DEP in tea plants [254].

Transcriptomic and metabolomic profiling highlights dynamic pathway regulation. Key KEGG-enriched pathways include terpenoid backbone, sesquiterpenoid/triterpenoid, and diterpenoid biosynthesis (e.g., cigar tobacco [264], B. braunii [241], scale insect-infested leaves [246], Pfaffia glomerata [247]). Tissue-specific expression drives diversity (e.g., L. glauca ND-like genes in leaves, LIS-like/TES-like in sarcocarps [133]; Protium copal monoterpenoid synthesis/transport transcripts [250]; white spruce TPS/P450 in CRD cells [212]). Metabolite profiles vary by tissue (e.g., Lavandula flower volatiles 16× higher than leaves [235]) and geography (e.g., A. malaccensis expression highest near Hoollongapar Gibbon Sanctuary [236]). Environmental cues (water deficit in grapes [228], LOV treatment [227]) and species interactions (resistant vs. susceptible Eucalyptus [127]) further shape terpenoid diversity.

Monoterpenoid and Sesquiterpenoid Emission Patterns and Environmental Responses

Structural Classification and Diversity of Monoterpenoids, Sesquiterpenoids, and Derivatives Across Plant Species

Monoterpenoids and sesquiterpenoids exhibit distinct structural diversity and emission patterns across plant species, shaped by species identity, environmental stimuli, and physiological processes. Woody plants display species-specific monoterpenoid profiles in essential oils and ambient emissions, while enantiomeric diversity and treatment-induced variations in terpenoid production are also documented. Other plant groups, including ornamental trees, gametophytes, and herbaceous species, show unique terpenoid compositions and emission dynamics.

Monoterpenoid Profiles and Emissions Across Plant Species

Plant Species	Monoterpenoid Composition & Key Details	Citations
Eucalyptus globulus (leaves)	Dominated by 1,8-cineole (15.2%)	[265]
Pinus halepensis (needles)	Prioritizes α-pinene (21.3%); air samples show (R) limonene enrichment but no α-pinene enantiomeric bias	[265]
Cedrus atlantica (needles)	Prioritizes α-pinene (53.3%); releases (+) cis-pinane (ER ~6.4) and (S) limonene (ER ~0.4)	[265]
Fagus sylvatica L.	Emits sabinene, (E)-β-ocimene, linalool; linalool contributes up to 84% of total monoterpene emissions	[266]
Quercus ilex (foliar)	~90% of emissions from α-pinene, β-pinene, sabinene, myrcene, limonene; some trees enrich limonene (30–60%) or myrcene (65–85%)	[267]
Mediterranean ornamental trees	11 common species emit 0.2–110.0 μg·g⁻¹(dw) h⁻¹ monoterpenes; α-pinene is most abundant	[268]
Pinus halepensis (twigs)	Mainly monoterpenes (MTs) with minor sesquiterpenes (SQTs); light-dependent emissions dominated by (E)-β-ocimene/linalool; non-light-dependent emissions dominated by non-oxygenated MTs/β-caryophyllene	[165]
Rosemary (twigs)	Comparable non-oxygenated/oxygenated MTs (α-pinene, camphene, limonene, eucalyptol, camphor, borneol, verbenone, bornyl-acetate); SQTs ~1 order of magnitude lower (α-humulene major)	[165]
Pinus sylvestris (Scots pine)	~30 isoprenoids; 60–90% from α-pinene, 3-carene, β-pinene, camphene; sesquiterpenes/oxygenated compounds highest in March/April, decreasing by August–October	[269]
Juniper	54 mono- and sesquiterpenes; most monoterpene hydrocarbons peak in fall (4 stable, 2 summer peaks); 3 of 7 sesquiterpene hydrocarbons peak in fall	[13]
Aerial parts (certain species)	Predominantly O-containing irregular monoterpenoids (yomogi alcohol, artemisia alcohol, artemisia acetate)	[146]
Gametophytes (green tissues + rhizoid traces)	Emit 10 VOCs, nearly all monoterpene (C₁₀H₁₆) or sesquiterpene (C₁₅H₂₄) hydrocarbons	[47]

Sesquiterpenoid profiles are similarly species-dependent, with treatments and environmental factors driving variations. In ginseng seedlings, MeJA treatment induces more sesquiterpenes (elemene, farnesane, α-carophyllene, β-carophyllene, germacrene) than wounding alone (elemene, farnesane)[270]. Resistant Eucalyptus genotype Egr has ~fourfold higher total sesquiterpene content (0.26±0.05 mg/g dry weight) than susceptible GC (0.06±0.01 mg/g dry weight), with viridiflorene and α-gurjuene showing genotype-specific variation[127]. Sesquiterpenes are exclusive to forest fire emissions (not crop residue burning)[271], and some species store them predominantly as carbohydrates (germacrene-D most abundant)[146]. Volicitin-treated maize produces β-caryophyllene, (E)-α-bergamotene, and (E)-β-farnesene (75–80% of induced volatiles)[272], while artificially damaged P. mollicomum leaves release sesquiterpenoids (germacrene D, bicyclogermacrene, E-caryophyllene, β-elemene, α-humulene, β-cubebene) and monoterpenoids absent/reduced in undamaged branches[19].

Environmental and experimental treatments modulate terpenoid emissions and accumulation. MeJA treatment of ginseng seedlings maximizes mono- and sesquiterpene accumulation at 48 h, exceeding wounding alone or MeJA + wounding[270]. Eucalyptus oviposition by Leptocybe invasa slightly increases monoterpenes in Egr (16.05±2.61 to 16.60±2.56 mg/g dry weight) and triples sesquiterpenes in GC (0.06±0.01 to 0.17±0.03 mg/g dry weight)[127]. Overexpressing snapdragon GPPS.SSU in tobacco boosts monoterpene emissions (e.g., (E)-β-ocimene, new myrcene) while reducing sesquiterpenes[273]. Grape varieties show distinct ripening trends: N grapes have higher early acid hydrolysis-released monoterpenes, while D grapes peak in acid hydrolysis-released sesquiterpenes at harvest (417.5 µg/kg)[274]. Litter emissions vary: Salix prioritizes cis-linalool-oxide, Cassiope tetragona prioritizes 1,8-cineole[275]. MeJA treatment increases cabbage monoterpene emissions 2–10-fold and induces (E,E)-α-farnesene[276]. Shoot JA application induces sesquiterpenes/homoterpenes, but root induction does not[277]. Strophosoma melanogrammum addition increases spruce monoterpene emissions up to 58×, with compositional changes on day 1[278]. Aerial parts of plants with developed roots have a 2× higher monoterpenoid/sesquiterpenoid ratio than those with inhibited rooting[146]; underground parts favor oxygenated sesquiterpenoids, with roots having higher sesquiterpenoid content than callus (higher monoterpene/sesquiterpenoid ratio in callus)[146]. Pine-oak forest monoterpene fluxes correlate with temperature (r²=0.61) and weakly with PPFD (r²=0.20); isoprene fluxes correlate with temperature (r²=0.50) and PPFD₁₅ₘ (r²=0.45)[59]. Grassland monoterpene emissions (30°C normalized) range 0.2–1.7 mg m⁻² h⁻¹, with limonene/carene each at 37%[279]. Temperature positively associates with sesquiterpene (P<0.05) and diterpene (P<0.05) levels[280]. β-glucan elicitor induces 13 sesquiterpenoids/homoterpenes (TMTT, DMNT); only small monoterpene amounts (α-pinene with LCO-IV (C16:2), linalool with LCO-IV (C16:2, S)) are detected[281]. Ozone exposure shifts dominance: early leaves have monoterpenes except at 120 ppb (sesquiterpenes); late leaves have monoterpenes at 0 ppb, sesquiterpenes during exposure[153]. Shoot JA application increases needle sesquiterpenes 9× 1 day post-treatment but reduces root monoterpenes, with weaker effects under heat[282].

Terpenoid production links to plant fitness and functional roles. Woody plant terpenoid concentrations show a non-significant positive correlation with fitness, contrasting with negative correlations for phenolics/alkaloids (common in herbaceous plants)[283]. Vitis vinifera 'Callet' produces more terpenes to aid sugar transport, enhancing berry quality[284]. Infested tree monoterpenes increase secondary organic aerosol formation[285]. Grape berry monoterpenes derive from the MEP pathway (GPP via GPPS), while sesquiterpenes use FPP (via FPPS)[46]; no sesquiterpenoid synthase genes are detected in Kyoho or 87-1 berries (consistent with aroma absence)[46]. Grapes and hops have glycosidically bound monoterpene alcohols/polyols; grapes (Muscat of Alexandria, Cabernet Sauvignon) have non-glucosyl sugars bound to sesquiterpene aglycones (sesquiterpenoids not reported in hops)[286]. Hop monoterpenes are impacted by oxidation, hydrolysis, and biotransformation, with linalool/β-damascenone increases during wort boiling/fermentation[286].

Terpenoid Accumulation and Emission Patterns During Plant Development and Phenological Stages

Terpenoid accumulation, emission, and gene expression patterns are shaped by biological rhythms, phenological stages, cultivar/species differences, and environmental factors across plant taxa. Circadian regulation drives monoterpene synthase gene expression in lavender (Lavandula) flowers, with LaLINS, LaLIMS, and LaTPS-l transcripts peaking between mid-day and 15:00 over two consecutive days—correlating with afternoon essential oil (EO) shifts toward higher linalool acetate, lavandulyl acetate, and β-caryophyllene, and lower camphor and 1,8-cineole [287]. In grape varieties, maturation influences monoterpene dynamics, while cultivar specificity affects both monoterpene and sesquiterpene accumulation [452, 461]. Environmental triggers like UVB exposure also modulate terpenoid emissions, with effects dependent on light intensity [288].

Terpenoid Dynamics in Grapes: Maturation and Cultivar-Specific Patterns

Parameter	Finding	Citation
Free monoterpenes (berry flesh/skins)	Increase by ~40% between early and late ripening stages, driven by geraniol and geranic acid	[32]
Bound monoterpenes (glycoside-conjugated)	More abundant than free monoterpenes in grape flesh throughout maturation; linalool shows highest increase in bound form	[32]
Acid hydrolysis-released monoterpene precursors	‘N’ grapes have higher early-stage concentrations; ‘B’ and ‘D’ varieties show lower levels until harvest	[274]
Pre-fermentative sesquiterpene levels at harvest	‘B’ grapes have higher levels	[274]
Acid hydrolysis-released sesquiterpenes	‘D’ grapes accumulate more	[274]

Seasonal and species-specific variations in terpenoid emissions are pronounced in tree taxa. Eucalyptus globulus maintains stable monoterpene composition (dominated by 1,8-cineole, α-pinene, β-pinene, and limonene) year-round but varies emission rates, while E. calophylla and E. wandoo shift dominant emitted monoterpenes between spring and summer [289]. Downy birch (Betula pubescens) emits higher sesquiterpenes (e.g., β-caryophyllene) than monoterpenes except at the growing season’s end, when early-season sesquiterpene emissions decline sharply [290]. Mānuka (Leptospermum scoparium) emits fewer monoterpenoids and sesquiterpenoids in winter than summer, with temperature, herbivory, and nitrogen driving summer emissions [291].

Phenological stages further shape terpenoid profiles: Satureja horvatii EO transitions from high linalool, thymol, and carvacrol in pre-flowering (April) to 65.8% linalool dominance in fruiting stages (September–October), with thymol and carvacrol declining significantly [292]. Arum gigantea flowers shift emissions by reproductive stage: female-stage flowers emit β-myrcene and (E)-β-ocimene, male-stage flowers shift to linalool and α-farnesene derivatives, and wilted flowers cease emitting β-myrcene and (E)-β-ocimene [39]. Oak species (Quercus ilex, Q. suber) show seasonal emission rate changes, with Q. suber emitting more monoterpenes (α-pinene, β-pinene, sabinene, myrcene, limonene) in summer than autumn [293]. Conifers exhibit distinct biosynthesis and storage strategies: Scots pine and Norway spruce rely on de novo monoterpene biosynthesis for 58% and 34% of emissions, respectively, while European larch uses stored monoterpenes for most emissions; bark monoterpene concentrations are higher in European larch (84.5%) and Scots pine (76.4%) [294].

Abiotic and Biotic Stress-Induced Modulation of Terpenoid Biosynthesis and Emission

Monoterpenoid (MT) and sesquiterpenoid (SQT) emissions and biosynthesis are dynamically modulated by abiotic and biotic stressors, with distinct patterns across species and conditions. Abiotic factors—including drying conditions, light, temperature, nutrient treatments, water deficit, freezing temperatures, chemical fumigation, girdling, seasonality, and jasmonic acid (JA) application—drive species-specific terpenoid responses, while biotic stressors such as herbivory, pathogen infection, and elicitor treatments trigger targeted changes in terpenoid profiles and emissions.

Terpenoid Responses to Abiotic Stressors

Stressor	Species/Tissue	Key Observations	Citation
Sun-drying vs. shade-drying	T. mollis	Total terpenoids decreased significantly in sun-dried samples	[252]
Light/temperature	Beech trees	Diurnal MT emissions linked to light; peak emissions: 334 μg m⁻² h⁻¹ (growth room, 27°C), 567 μg m⁻² h⁻¹ (forest, 23°C); 17°C reduced peaks to 34–28 μg m⁻² h⁻¹	[295]
Zinc (Zn) treatment	—	Maximum monoterpene oil levels at Zn0.250; positive correlation with photosynthetic rate (PN)	[151]
Water deficit	Vitis vinifera	Plastidial MEP pathway genes (VviDXS1, VviDXR) down-regulated; TPS-a/b/g family genes up-regulated (93 DAA), promoting monoterpene accumulation (hotrienol, linalool, nerol, α-terpineol)	[228]
Freezing temperatures (-5°C/-7°C)	Fruits	MT peak area increased ~1.1-fold (dl-limonene, sabinene, γ-terpinene, linalool, terpinen-4-ol); SQT levels decreased 2.4–3.0-fold	[296]
Methanol fumigation	Unattacked/previously clipped plants	~14% increase in MT emissions (12.2 → 14 nmol m⁻² s⁻¹); α-pinene/β-pinene dominant	[297]
Girdling	Riesling vines	Increased MTs (α-terpinene, limonene, linalool) and SQT nerolidol	[298]
Seasonal variation	Mānuka	Lower MT, SQT, and total volatile emissions in winter vs. summer	[291]
Long-term water deficit	Mediterranean species (R. officinalis, P. halepensis, C. albidus)	P. halepensis/C. albidus: slow MT emission decreases; R. officinalis: reduced SQT release	[299]
Tree health status	Healthy vs. declining stands	Healthy: more isocaryophyllene; declining: more camphene, D-limonene, caryophyllene, humulene	[300]
JA application	Needles/roots	Needle SQT emissions increased 9-fold (1 day post-treatment); root MT emissions decreased, reducing root total carbon emitted as VOCs from 31% to 4%	[282]
JA + heat	Needles/roots	JA effect on VOC emissions less pronounced	[282]

Terpenoid Responses to Biotic Stressors

Stressor	Species/Tissue	Key Observations	Citation
Gypsy moth feeding	Cranberries	Increased total MT/SQT emissions; SQT emissions varied by variety	[301]
MeJA treatment	Ginseng seedlings	Maximum terpene levels at 48 h (higher than wounding alone/combined wounding-MeJA)	[270]
MeJA/chitosan treatment	P. abies cell cultures	2–4-fold increase in MT accumulation via enhanced biosynthesis	[302]
MeJA treatment	P. pinaster/P. radiata	P. pinaster: ~6-fold linalool increase; P. radiata: induced low levels of new compounds (α-terpinolene, geranyl acetate)	[303]
MeJA treatment	Cabbage (Lennox/Rinda)	2–10-fold MT emission increase; induced DMNT and (E,E)-α-farnesene	[276]
S. melanogrammum weevil herbivory	Spruce trees	Up to 58-fold MT emission increase; composition changes (increased tricyclene/camphene/myrcene; decreased β-pinene/Δ3-carene)	[278]
PPM larval feeding	P. pinaster/P. radiata	P. pinaster: 25%/35%/10% lower total MT/SQT/diterpene concentrations; ~8-fold linalool increase; P. radiata: reduced total MTs/diterpenes	[303]
Mite infection (E. tiliae/E. exilis)	Tilia cordata	1.7–3.1-fold MT emission increase; E. exilis induced α-bourbonene/β-caryophyllene	[304]
Mite infection (E. inangulis)	Alnus glutinosa	1.4-fold MT emission decrease	[304]
Bark beetle attack (Iitti site)	Trees	June: decreased MTs (terpinen-4-ol, α-terpineol) and SQTs (longipinene, β-bourbonene); eliminated α-humulene/germacrene-D/γ-cadinene; induced (E,E)-α-farnesene; August: 20-fold higher total MT emissions (increased α-pinene/camphene/limonene/bornyl acetate)	[305]
Bark beetle attack (Lahti site)	Trees	June: emitted β-phellandrene; August: higher tricyclene/camphene/myrcene/1,8-cineole emissions	[305]
Herbivore attack timing (dusk)	N. attenuata	Delayed SQT (e.g., (E)-α-bergamotene, 5-epi-aristolochene) emission until next day; GLVs peaked within 4 h	[306]
Large pine weevil damage	Scots pine (shoots/bark)	Shoots: 2.8-fold higher normalized MT emission, 2.9-fold higher SQT emission; bark: 3.4-fold total VOC increase, 7.1-fold SQT increase; qualitative differences (e.g., linalool only minor in bark post-damage; 1,8-cineole absent in bark)	[307]
AMV infection	Basil	SQTs increased (19.8% → 72.8%); MTs decreased (35% → 11%); reduced linalool/estragole, increased eugenol/s-Cadinol	[308]
Yeast fermentation	Wine	Pichia membranifaciens/Hanseniaspora vineae (high β-glucosidase) increased terpenes, peaking early-middle fermentation	[309]
Mechanical wounding	Grand fir	Sequential induction: MT synthase upregulated at 4 h, peak MT production (200× control) at 10 days; SQT synthase induced at day 3	[234]
SA/YE treatment	Roots	SA: increased MTs (24 h) and SQTs (72 h); YE: stimulated SQTs (24 h); both upregulated IPPI/FPS	[310]
β-glucan/LCO-IV treatment	—	β-glucan: induced SQTs and homoterpenes (TMTT, DMNT); LCO-IV: small MT (α-pinene, linalool) induction	[281]
P. viticola inoculation	Grape genotypes (SO4, Kober 5BB, Pinot noir)	Resistant (SO4, Kober 5BB): greater SQT emission; susceptible (Pinot noir): no MT emission; SO4: inoculation-induced MT emission	[311]

Altered terpenoid emissions from infested trees can impact atmospheric processes, such as secondary organic aerosol formation [285]. JA treatment also caused a strong decline in net CO₂ assimilation, leading to net daytime respiration, though root respiration remained unaffected [282]. Large pine weevil damage on Scots pine stems did not significantly influence shoot net photosynthesis, though the average rate was 15.4% lower in damaged plants [307]. The sequential induction of terpenoids in mechanically wounded Grand fir may reflect functional specialization, with monoterpenes acting as early insect toxins and sesquiterpenes (e.g., (E)-α-bisabolene converted to fungicidal todomatuic acid) serving as a secondary defense [234].

Regulatory Mechanisms of Monoterpenoid and Sesquiterpenoid Production

Monoterpenoids and sesquiterpenoids exhibit distinct structural diversity, with specific terpene synthases (TPS) catalyzing their formation from respective substrates. TPS subfamily classification reflects functional specialization: TPS-b primarily includes angiosperm monoterpene synthases, TPS-a includes sesquiterpene synthases, and TPS-g has distinct members like CmCJTPS7 [233]. Key TPS enzymes and their products are summarized below:

TPS Enzymes, Substrates, and Product Specificity

TPS Enzyme	TPS Subfamily	Substrate	Main Products	Citations
CmCJTPS5	TPS-b	Geranyl pyrophosphate (GPP)	1,8-cineol (main); α-pinene, camphene (minor)	[233]
CmCJTPS1	TPS-a	(E,E)-farnesyl pyrophosphate (FPP)	γ-selinene and related derivatives	[233]
α-humulene/β-caryophyllene synthase (TPS21/BCS)	—	—	α-humulene, β-caryophyllene (sesquiterpenes)	[301]
(3S,6E)-nerolidol/R-linalool synthase (NER1/LLS)	—	—	Monoterpenes, (3E)-4,8-dimethylnona-1,3,7-triene (DMNT)	[301]

Emission patterns of these terpenoids are dynamic and responsive to biotic and abiotic cues. In cranberries, gypsy moth feeding induces monoterpene (e.g., linalool) and sesquiterpene (e.g., β-caryophyllene) emission: total monoterpene emission is significantly higher in treated versus control plants (with no variety differences for monoterpenes), while sesquiterpene emission varies by variety, with ‘Stevens’ and ‘Crimson Queen’ emitting less than other varieties under herbivory [301]. Methyl jasmonate (MeJA) treatment also induces terpenoid emission in cranberries but elicits different compounds (e.g., α-terpineol, β-farnesene) compared to gypsy moth feeding [301]. In Phlox paniculata flowers, inhibiting the plastidic methylerythritol phosphate (MEP) pathway (via fosmidomycin targeting DXR) or cytosolic mevalonate (MVA) pathway (via mevinolin targeting HMGR) reduces terpenoid emission, with monoterpenoids and germacrene D (a sesquiterpene) driving discrimination between control and treated bouquets [220]. During white tea withering, terpenoid biosynthesis remains active, with upregulation of MEP (e.g., DXS) and MVA (e.g., HMGR) pathway genes, though most TPS gene expressions decrease over time—except for specific members like TPS10, which is linked to linalool accumulation [142]. Floral terpenoid emission also varies with development: in the rose ‘Tianmidemeng’, terpenoids dominate floral scent (98.9% at bud stage, 82.3% at semi-flowering) and peak at the semi-flowering stage, with both monoterpenoids and sesquiterpenoids sensitive to flower opening and senescence [312]. Overexpression of snapdragon GPPS.SSU in tobacco increases monoterpene emission (e.g., (E)-β-ocimene, myrcene) while decreasing sesquiterpene emission, indicating substrate competition between pathways [273].

Regulatory mechanisms of terpenoid production involve multiple layers, including transcription factors (TFs) and long non-coding RNAs (lncRNAs). In Lilium ‘Siberia’, MYB TFs (LiMYB1, LiMYB305, LiMYB330) positively regulate LiTPS2, enhancing monoterpene (ocimene, linalool) emission: overexpression increases these compounds by 3.7–4.9-fold, while silencing reduces them by 1.7–3.2-fold [313]. In white tea, jasmonic acid (JA) signaling upregulates MVA and MEP pathway genes (e.g., HMGR, DXS) via increased JAR and JMT expression, promoting synthesis of terpene precursors (IPP, DMAPP) [142]. In roses, lncRNAs regulate terpenoid production: the greenyellow module correlates with half of terpenoid compounds (e.g., geraniol, β-pinene), while silencing lncRNA TCONS_00008447 increases 7,8-dihydro-β-ionone emission by 3.9-fold [312].

Atmospheric Reactivity and Environmental Fate of Terpenoid Structural Types

Monoterpenoids and sesquiterpenoids exhibit diverse emission patterns across biogenic and anthropogenic sources, with biogenic emissions often dominated by specific compounds. For example, laboratory and field experiments identify α-pinene, β-pinene, Z-β-ocimene, and limonene as primary plant-emitted terpenoids [314]. Product emissions show distinct liquid vs. gas phase profiles, while soil systems highlight the role of litter biomass in sesquiterpene emissions from Mediterranean deciduous forests [315]. Bryophytes emit common sesquiterpenes (SQT, C15H24), a second sesquiterpene type (SQT2, C15H22), oxygenated sesquiterpenes (OSQT, C15H24O), and rare diterpenes (DT, C20H32) [316]. Cannabis cultivation facilities (CCFs) show age-dependent monoterpene concentrations and strain-specific emission capacities [42].

Terpenoid Emission Characteristics Across Sources

Source Category	Key Observations
Plant emissions	Dominant compounds: α-pinene, β-pinene, Z-β-ocimene, limonene [314]
Salix litter	Most abundant terpenoid: cis-linalool-oxide [275]
C. tetragona litter	Dominant terpenoid: 1,8-cineole (eucalyptol), emission rate ~2× cis-linalool-oxide [275]
Product emissions (liquid phase)	11 terpenes detected; major TerVOCs: limonene, eucalyptol, ρ-cymene, β-pinene [317]
Product emissions (gas phase)	6 terpenes identified; major compounds match liquid phase [317]
Product emissions (volatility)	Eucalyptol: highest volatile fraction/liquid mass ratio; cymene: ratio <1 [317]
Product emissions (rates)	Eucalyptol and limonene emission rates equal under realistic conditions [317]
Product emissions (concentrations)	Peak limonene (144 ppb at 0.63 h post-application); peak eucalyptol (138 ppb at 0.38 h); total TerVOCs ~300 ppb within 0.5 h [317]
Soil systems (Mediterranean deciduous forests)	High BVOC diversity including sesquiterpenes; sesquiterpene emissions increase with litter biomass [315]
Bryophytes	Emissions: SQT (C15H24), SQT2 (C15H22), OSQT (C15H24O), rare DT (C20H32) [316]
CCFs (plant age)	80-day-old plants: 1660 ppb monoterpenes; 48-day-old plants: 150 ppb [42]
CCFs (strain emission capacity)	Critical Mass: 10 µg dwg⁻¹ h⁻¹; Lemon Wheel: 7 µg dwg⁻¹ h⁻¹; Rockstar Kush: 6 µg dwg⁻¹ h⁻¹ [42]

Environmental factors significantly influence terpenoid emissions. Monoterpene emissions increase with a temperature response slope (β) of 0.11 ± 0.02, consistent with literature [314]. Terpene (monoterpene and sesquiterpene) emissions are less sensitive to solar shortwave radiation (SSR) changes (up to 0.7% change) than isoprene’s nearly linear response [318]. Wounding from mastication (thinning) dramatically elevates monoterpene emissions and ozone flux, with maximum α-pinene mixing ratios observed when mastication occurs in the tower footprint [319]. Jasmonic acid application increases needle sesquiterpene emissions ninefold 1 day post-treatment but reduces root monoterpene emissions, with a less pronounced effect under heat [282]. Sporadic sawmill activity can also raise monoterpene concentrations, as seen in Hyytiälä where southeasterly winds from a nearby sawmill increase α-pinene levels and enrich the (+)-α-pinene enantiomer [320].

Terpenoid structure dictates atmospheric reactivity and fate. Monoterpene ozonolysis produces oxidation products: ozonolysis of Quercus ilex mixed monoterpene emissions yields acetone with an 18.8% overall yield [314]. Oxidation products at m/z 113 (representing mono- and sesquiterpene oxidation products) have a flux equal to 12% of total monoterpene emissions, with a diurnal cycle matching monoterpene emissions and non-stomatal ozone fluxes [321]. Despite α-pinene’s dominance in total monoterpene mass, monoterpene mixture reaction rates with atmospheric oxidants are 25–30% faster than α-pinene due to more reactive, lower-abundance compounds like limonene [322]. Individual monoterpenes show variability: β-myrcene has an OH radical reaction rate constant (k298 = 3.35 × 10⁻¹⁰ molecules cm⁻³ s⁻¹) 4 times higher than the aggregated TERP category (k298 = 6.77 × 10⁻¹¹ molecules cm⁻³ s⁻¹) and 5.6 times faster than α-pinene [42]. TERP undergoes oxidation with nitrate radical (NO3), hydroxyl radical (OH), ozone (O3), and singlet oxygen [42]. In bryophytes, total ozone reactivity emissions (TOHRE) are ~two-thirds from monoterpenes and ~one-third from sesquiterpenes [316]. Secondary organic aerosol (SOA) growth factors differ by class: pure monoterpene and oxygenated terpene SOA have growth factors of 1.06–1.11 at 85% RH, while sesquiterpene SOA growth factors are 1.01–1.04 under the same conditions [323]. In boreal wetlands, monoterpene-ozone and sesquiterpene-ozone reactions form nitrogen-free highly oxygenated molecules (HOMs), with sesquiterpene-derived HOMs correlated with sub-3 nm particles [324]. Monoterpene lifetimes (11–190 min) are too long for substantial in-canopy ozone reactions, suggesting unmeasured terpenes drive observed ozone flux enhancements [325].

Diverse Terpenoid Subclasses (Diterpenoids, Triterpenoids, and Specialized Derivatives) in Natural Systems

Structural Diversity of Terpenoid Classes in Plant and Marine Systems

Terpenoids, also called isoprenoids, represent the largest family of plant secondary metabolites (approximately 16,000) and originate through the condensation of the universal five-carbon precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) [326]. Monoterpenoids (C10) and sesquiterpenoids (C15) form from two and three IPP/DMAPP units, respectively, and accumulate in grassland families like Labiatae, Rutaceae, and Apiaceae [326]. Iridoids—monoterpenoids with a cyclopentan[c]pyran skeleton, typically as glucosides—are found in plant families (Apocynaceae, Loganiaceae, Lamiaceae, Scrophulariaceae, Rubiaceae, Verbenaceae) and some animals; medicinal plants rich in iridoid glucosides treat infections, rheumatism, and inflammation [165, 39].

Terpenoids exhibit diverse ecological and biological roles: sesquiterpene lactones (Asteraceae) deter herbivores (some detoxify via the liver) [326]; terpenoid phenols (thymol, carvacrol) from thyme/oregano disrupt bacterial membranes, denature proteins, and have antioxidative, anti-inflammatory, and anticarcinogenic properties [326]; carotenoids (C40 liposoluble terpenoids) act as photosynthetic pigments and antioxidants with anticancer and immune-stimulating effects [326].

Beyond monoterpenoids and sesquiterpenoids, diterpenoids, triterpenoids, sesterterpenoids, and specialized derivatives (steroidal saponins, meroterpenoids) occur across natural systems. Terpenoids are widely distributed in plants (e.g., Cassia fistula, Swertia chirayita, Plumeria obtusa, jackfruit peel) [28, 34, 35, 41], plant latex [327], and marine organisms (algae, sponges, corals) [133, 135]. They also feature in archaeological residues, often with triterpenoid components similar to modern Dipterocarpaceae resins [328].

Gene expression and functional analyses link terpenoid biosynthesis to biological responses: DEGs in scale insect-infested leaves enrich sesquiterpenoid/triterpenoid pathways [246], and expanded gene families in I. hylonoma relate to monoterpene, triterpenoid, and saponin biosynthesis [183]. Fire-adapted pine species show lower terpenoid levels in crown-fire vs. surface-fire species [40], and seasonal variation in terpenoid profiles (e.g., juniper monoterpenes peaking in fall/winter) is common [13].

Terpenoid Occurrence, Composition, and Abundance Across Natural Systems

Terpenoid Class	Source Organism/Extract	Key Compounds	Abundance/Composition Details	Citations
Monoterpenoids	Multiple samples (study)	34 components; 11 shared, 12 unique	27–56% of total ion current (TIC)	[329]
Monoterpenoids	C. oppositifolia leaf essential oil	α-pinene	23.85%	[25]
Monoterpenoids	Eucalyptus globulus essential oil	1,8-cineole, limonene	72.3%, 2.3% respectively	[330]
Monoterpenoids	Gardenia jasminoides fruits	15 tentatively identified; monoterpenoid glycosides	Characteristic fragmentations (losses of glycosides, CH₃, H₂O; ions at m/z 179, 161)	[331]
Monoterpenoids	Pinus pinaster needles (both chemotypes)	α-pinene, β-pinene	Together account for over 70% of total monoterpenes	[160]
Monoterpenoids	C. asiatica aerial parts essential oil	11 hydrocarbons, 9 oxygenated	20.20%, 5.46% respectively of total composition	[2]
Monoterpenoids	B. sacra	α-pinene, β-myrcene, p-cymene, limonene (odorants); 1,8-cineole, linalool (oxygenated)	Relevant odorants; oxygenated monoterpenoids as largest detected odorant group	[332]
Monoterpenoids	C. wightii hexane gum extract	α-Pinene, 2-Norpinene 3,6,6-trimethyl-, α-Ocimene, α-Myrcene, Limonene, α-Terpineol	11.725%, 10.607%, 2.309%, 5.289%, 5.324%, 1.056% respectively	[82]
Monoterpenoids	Pine species (e.g., P. rigida trunk resin, P. ponderosa needle resin)	Monoterpene hydrocarbons, oxygenated analogues	Up to ~90% of total terpenes	[333]
Monoterpenoids	S. officinalis plantlets (HZ treatments)	1,8-cineole	15.63% to 47.96% (major compound)	[149]
Sesquiterpenoids	Multiple samples (study)	36 components; 27 unique to one sample	Greater sample-to-sample variation than monoterpenoids	[329]
Sesquiterpenoids	C. oppositifolia leaf oil	Caryophyllene	11.312%	[25]
Sesquiterpenoids	Tehran population essential oil	E-caryophyllene	27.38%	[334]
Sesquiterpenoids	Endophytic fungi	Zopfiellin B (trinor-bisabolane), aspterpenin A (ketal-bridged bisabolane)	Specialized rare structures	[335]
Sesquiterpenoids	C. asiatica aerial parts essential oil	14 hydrocarbons, 5 oxygenated, 1 sulfide	68.80%, 3.90%, 0.76% respectively; α-humulene (21.06%), β-caryophyllene (19.08%) as predominant	[2]
Sesquiterpenoids	B. sacra	Germacrene D, α-copaene, unidentified A/B	Odor-active sesquiterpene hydrocarbons	[332]
Sesquiterpenoids	C. wightii hexane gum extract	Caryophyllene, β-Elemene, α-Curcumene, Sesquithujene, α-Bisabolene	6.128%, 2.381%, 3.961%, 1.693%, 1.033% respectively	[82]
Sesquiterpenoids	Pinus pinaster needles	30 compounds (19 hydrocarbons, 5 alcohols, 6 esters); germacrene D, β-caryophyllene	Main components	[160]
Sesquiterpenoids	Inula genus	Eudesmane-, germacrane-, guaiane-, xanthane-type; eudesmanolides most common	Monomers, dimers, trimers	[336]
Sesquiterpenoids	S. chinensis	12 sesquiterpenoids (among 202 compounds)	Part of diverse terpenoid profile	[337]
Sesquiterpenoids	Pine species (e.g., P. sylvestris needles/female cones)	Trace amounts; up to 20% of total terpenes (some parts)	Germacrene D-4-ol (3.3% in P. sylvestris needles), caryophyllene oxide (4.6% in P. rigida cones)	[333]
Sesquiterpenoids	Eugenia species essential oils	Cyclic sesquiterpenes (predominant)	Main volatile class (among ~500 compounds)	[338]
Diterpenoids	Pinus pinaster needles	40 neutral diterpenes (11 hydrocarbons, 29 oxygenated); isoabienol, 8,13-abietadiene	Chemotype 1: high oxygenated; Chemotype 2: hydrocarbons/anticopalol	[160]
Diterpenoids	C. wightii hexane gum extract	Cembrene, Neocembrene	1.757%, 9.950% respectively	[82]
Diterpenoids	Pine species (e.g., P. sylvestris female cones, P. ponderosa trunk)	Pimaranes, dehydroabietanes, podocarpanes	Up to 15% of total terpenes (some parts)	[333]
Diterpenoids	S. officinalis plantlets (HZ treatments)	Sugiol, podocarpa-8,11,13-trien-7-one,12-hydroxy-13-isopropyl	1.78% (control) to 5.78–14.44% (treatments)	[149]
Triterpenoids	E. globulus leaf extracts	Ursolic acid, oleanolic acid	87.7–89% of identified compounds; 17.46–19.82 g kg⁻¹ dw, 5.45–6.32 g kg⁻¹ dw respectively	[330]
Triterpenoids	Gardenia jasminoides fruits	9 compounds (ursolic acid, oleanolic acid)	Characteristic fragmentations (losses of H₂O, formaldehyde)	[331]
Triterpenoids	C. asiatica	Ursane- or oleanane-type triterpenoids, madecassoside, asiaticoside H/I	Triterpene saponins as main metabolites; 14 characterized/quantified in leaf extract	[2]
Triterpenoids	Archaeological residues (Goddard samples)	Tricyclic terpanes (C23 most abundant)	Dominate m/z 191 chromatograms; extend to C41–C45	[339]
Triterpenoids	S. chinensis	83 triterpenoids (among 202 compounds)	Major terpenoid class	[337]
Sesterterpenoids	Aspergillus terreus	Aspterpenacids A/B (5/3/7/6/5 pentacarbocyclic ring system)	Unusual structural features	[335]
Steroidal Saponins	Yam (Dioscorea sp.)	Diosgenin	Hypoglycemic effects	[340]
Steroidal Saponins	Switchgrass cultivars	72 saponins (44 diosgenin, 20 oxydiosgenin-like aglycones)	Grouped into 7 classes by aglycone/glycosylation	[341]
Meroterpenoids	Aspergillus terreus; Penicillium chermesinum	Spiroterreusnoids A–F (BACE1/AChE inhibitory); chermesins A–D (spiromeroterpenoids)	Spiro-dioxolane adducts; drimane-type sesquiterpene skeleton	[335]

Terpenoid Structural Types and Biosynthetic Pathway Regulation

Terpenoids are classified into five primary subclasses: monoterpenoids, sesquiterpenoids, diterpenoids, triterpenoids, and carotenoids [342]. Their biosynthesis is catalyzed by terpene synthase (TPS) genes, which are organized into functionally distinct subfamilies. Below is a consolidated overview of TPS gene distributions, biosynthetic enzymes, and pathway-specific details across diverse species:

Terpenoid Biosynthetic Gene Distributions and Key Enzymes Across Species

Species	Terpenoid Subclass	Key Genes/Enzymes	Key Observations	Citations
Ocimum sanctum	All	18 TPS-a, 16 TPS-b, 5 TPS-g, 2 TPS-e, 1 TPS-f, 5 TPS-c; 15 monoterpene, 23 sesquiterpene, 8 diterpene TPS genes	TPS subfamilies correspond to specific terpenoid types (e.g., TPS-b = monoterpenes)	[171]
Grand fir	All	30+ TPS-d family members (wound-induced mono-, sesqui-, diterpene synthases)	Wound induction: monoterpene synthases (4h), limonene synthase (24h), (–)-pinene synthase (day7); sesquiterpene synthases (day3)	[234]
Dendrobium catenatum	All	24 expanded genes: 13 TPS, 7 cytochrome P450, 4 terpineol synthase; DcTPS (triterpene production)	Transient DcTPS expression increases triterpenes (dehydrotumulosic acid, etc.)	[238]
Laetiporus sulphureus NWAFU-1	All	25 TPS: 15 sesquiterpene (STS), 8 monoterpene, 1 squalene (SQS), 1 phytoene (PSY) synthases; lanostanoid/ergosteroid biosynthetic genes	STSs clustered into 4 clades; mono-TPSs homologous to Postia placenta STS25 (GPP cyclization to myristene/linalool)	[5]
Litsea glauca	All	12 monoterpene (LIS, TES), 12 sesquiterpene (α-humulene/β-caryophyllene), 17 diterpene (GA2ox, KAO, KO), 4 triterpene (FDFT1, SQLE) unigenes	Tissue-specific expression (e.g., LIS-like in sarcocarps; SQLE-like in kernels)	[133]
Melaleuca alternifolia	All	Limonene synthase-like (monoterpenes), farnesyl pyrophosphate synthase (FPP), cycloartenol synthase-like (triterpenes)	Limonene synthase-like protein produces predominant monoterpenes (sabinene hydrate, etc.)	[68]
Uncharacterized species	All	4 linalool/6 α-terpineol (monoterpene); 4 FPP synthase/4 sesquiterpene (nerolidol, α-humulene) synthases; 4 GGPP synthase/10 momilactone A/1 ent-kaurene (diterpene); 6 cycloartenol/10 squalene/2 squalene epoxidase (triterpene) transcripts	GPP/FPP/GGPP as substrates; 3 linalool synthase transcripts upregulated in leaves	[251]
Cymbopogon citratus	All	9 carboxyindole/13 neomenthol dehydrogenase/1 linalool/1 nepeta lactone monooxygenase (monoterpene); 16 germacrene/15 achilleol B/13 nerolidol (sesquiterpene); 26 gibberellin 2β-dioxygenase/5 kaurene synthase (diterpene) genes	Light/shade affects expression (e.g., linalool synthase high; c102703.0 lower under light)	[12]
Vetiveria zizanioides	Sesquiterpenoid	TPS-a subfamily: CZTPS1 (α-cadinol, α-muurolol), CZTPS2 (multiple), CZTPS3 (farnesol, nerolidol)	Distinct products from different TPS-a genes	[174]
Nasutitermes takasagoensis	Diterpenoid	8 expanded GGPPS genes	Supports diterpene synthesis in frontal gland Class 1 cells	[343]
Conifers	Diterpenoid	CYP720B enzymes (clades I/III)	Convert diterpene precursors to resin acids (abietic, levopimaric, etc.)	[344]
Abies grandis	Diterpenoid	Bifunctional abietadiene synthase (class II/III TPS activity)	Catalyzes GGPP → (+)-copalyl diphosphate → abietadiene/levopimaradiene	[191]
Tomato	Sesqui-, di-, triterpenoid	Pathway enrichment upon pathogen challenge	High pathway impact (PI 1.0 for sesqui-/triterpenoid; 1.0001 for diterpenoid)	[345]

Biosynthetic pathway regulation occurs at transcriptional, tissue-specific, and environmental levels. In Grand fir, wound-induced terpene synthase activity lags mRNA transcripts, with differential regulation of monoterpene synthases [234]. Litsea glauca shows stable MVA pathway (HMGR) expression across tissues, while MEP pathway (DXS) genes are sarcocarp-specific [133]. In grape, plastid-localized VvGGPPS acts upstream of norisoprenoid biosynthesis [226], and cobalt treatment in B. braunii alters terpenoid gene expression (upregulated early, downregulated at 8 days) [241]. Euryodendron excelsum has 29 DEGs regulating terpenoid backbone biosynthesis, with 19 in monoterpene and 26 in sesquiterpene/triterpene pathways [185].

In an uncharacterized species, differential expression analysis revealed two MEP pathway DXS transcripts upregulated in leaves, while only an ISPG isoform was detected in leaves/flowers at the proteomic level [251]. The MVA pathway had no upregulated transcripts, but 5 acetyl-CoA C-acyltransferase proteins were found in leaves, and an MVD isoform only in flowers [251]. In C. citratus, light conditions affected gene expression: carboxyindole synthase and nepeta lactone monooxygenase had low expression under both light and shade, while neomenthol dehydrogenase family genes showed significant differences [12]. In another uncharacterized system, terpenoids (alongside phenylpropanoids and N-containing compounds) were downregulated by AMF + Trichoderma but upregulated by plant hormones [346].

Additional regulatory details include L. sulphureus NWAFU-1’s triterpenoid biosynthetic genes (e.g., CYP51 family for lanosterol conversion) [5], conifer CYP720B enzymes for resin acid production [344], and A. grandis abietadiene synthase’s transient production in Nicotiana benthamiana (yielding abietadiene and derivatives) [191]. Co-production with Picea sitchensis PsCYP720B4 led to glycosylated diterpene resin acids, likely via plant defense mechanisms [191]. Geranylgeranyl diphosphate synthases (GGDPSs) are differentiated into three types, with plant GGDPSs as type II, regulated at gene, transcript, and protein levels [191]. Nitrogen limitation up-regulates triterpene saponin genes (FPS, β-AS, etc.) [245], and scale insect infestation induces sesqui- and triterpenoid pathways [246]. Diterpenoids related to hormone biosynthesis (kauralexin, gibberellins) were highlighted in an uncharacterized system [346].

Detection and Quantification of Terpenoid Subclasses via Analytical Techniques

Terpenoids exhibit structural diversity across subclasses—monoterpenoids, sesquiterpenoids, diterpenoids, and triterpenoids—with analytical techniques critical for their detection, quantification, and classification. Mass spectrometry (MS) and gas chromatography-mass spectrometry (GC-MS) are primary tools for identifying specific terpenoid compounds, while colorimetric assays, thin-layer chromatography (TLC), and high-resolution MSⁿ support qualitative detection and subclass classification.

Terpenoid Subclass Identification, Abundance, and Seasonal Variation Across Samples

Subclass	Key Compounds & Details	Sample/Context	Citations
Monoterpenoids	Camphor (m/z 153; fragments m/z 135, 109, 95); camphene (m/z 137; fragments m/z 95)	Identified via MS	[258]
	Limonene (31.9%), 1,8-cineole (38.3%) (most abundant; monoterpene fraction > sesquiterpene)	Specific samples	[347]
	15 tentatively identified (e.g., jasminoside B: fragments m/z 183, 165, 121)	G. jasminoides fruits (via retention time, accurate masses, MS/MS)	[331]
	Crown-fire species < surface-fire species (fresh: summer p=0.0019, winter p=0.0041, spring p=0.0026; dried: spring p=0.0160, winter p=0.0046; summer dried NS)	Needles (seasonal comparison)	[40]
	Non-fire species: higher median levels than surface-fire pines (winter/summer)	Leaves	[40]
Sesquiterpenoids	12 compounds (e.g., furanogermenone m/z 233; curzeone m/z 229; zedoarondiol m/z 253)	Identified via MS	[258]
	β-elemene, β-caryophyllene, humulene, β-eudesmene (0.4–1.3% mean)	Specific samples	[347]
	43 identified; oxygenated sesquiterpenes (45.2%) dominate (e.g., eudesma-4(15),7-dien-1-β-ol 21.3%)	X. spinosum "Collective oil"	[348]
	Crown-fire species < surface-fire species (fresh: summer/winter p=0.0054, spring p=0.0070; dried: spring p=0.0004, winter p=0.0006; summer dried NS)	Needles (seasonal comparison)	[40]
	Non-fire species: median abundance ~ crown-fire species	Leaves	[40]
Diterpenoids	Retinoic acid (m/z 301; fragments m/z 283, 255); traversianal (m/z 317; fragments m/z 299, 271); retinal (m/z 285; fragments m/z 215, 201)	Identified via MS	[258]
	Neophytadiene (6.3%), phytol (4.1%) (dried samples post-derivatization); squalene (0.9%), thunbergol (0.4%), phytol (0.6%) (DCM extract only)	Specific samples	[347]
	Kaurane diterpenes (9.52% of oil)	M. hookeriana	[20]
	Crown-fire species < surface-fire species (fresh: all seasons p<0.0001; dried: all seasons p<0.0001)	Needles (seasonal comparison)	[40]
	Non-fire species: median abundance ~ crown-fire species; lowest levels in summer fresh needles	Leaves	[40]
Triterpenoids	66.34 μg/mg (methanol extracts); nigranoic acid, ursolic acid (m/z 485.2831 [M–H]⁻; fragments m/z 387.2522, 329.0908)	Identified via MS	[349][350][351]
	β-sitosterol acetate, β-sitosterol, stigmastan-3-5-diene, β-tocopherol (eluted 50–80 min)	Identified via capillary GC	[123]
	Lupeol (8.1%), α-amyrin (1.5%), β-amyrin (1.3%)	Specific samples	[347]
	9 identified (e.g., ursolic acid: [M−H]⁻ m/z 455.3534; fragments m/z 407, 391, 377)	G. jasminoides (via MS/MS; negative ion mode with H₂O/HCHO losses)	[331]
	7 new triterpenes (structures via HREIMS, HRFABMS, NMR)	Active fractions	[352]
	Angiosperm-derived (oleanane/ursane skeletons) and bacterial hopanoids (e.g., 22R-17α(H),21β(H)-homohopane)	"Chukurovo" bitumen	[353]

Qualitative terpenoid detection relies on colorimetric assays: the Salkowski test (red-brown at chloroform-concentrated H₂SO₄ interface for terpenoids) [354][355][356][357], Lieberman–Bouchard test (deep red for triterpenoids, blue-green for steroids) [354][356][358][359][357][360], vanillin-concentrated H₂SO₄ test (purple for terpenoids) [358], and acetic anhydride-concentrated H₂SO₄ test (red for terpenoids) [361][362]. The Salkowski test revealed higher terpenoid accumulation in root extracts [357], while the NADI reagent indicated terpene presence, with maximum strength in full peltate trichomes [360].

TLC aids detection: lichen samples showed reddish charred spots (terpenoids) or treebark-derived spots (reddish with yellow rim) [363]; gynoecium/fruit extracts had pinkish-purple spots (Rf 0.76) matching myrcene [360]. High-resolution MSⁿ tentatively identified iridoids like geniposidic acid (m/z 373 [M−H]⁻; fragments m/z 329, 211, 193) [350], while ESI–QTOF–MS found terpenes/terpenoids (10.32% of constituents) in Portulaca oleracea [364].

GC-MS analyses enabled putative identification of triterpenes/diterpenes in derivatized dried samples [347] and detected phytol (1.51%) in G. verrucosa ethanol extract [359]. Strain 5–4 extracts contained 647 metabolites, including triterpenoids (13, 2.22%), sesquiterpenoids (12, 2.05%), terpene glycosides (21, 3.58%), and terpene lactones (11, 1.88%) [365]. Ginkgo biloba tablets are standardized to 6.7% terpenoids (ginkgolides/bilobalide) [366], and Sargassum polycystum extracts contain terpenoids alongside other metabolites [362]. Terpenoids were detected in ethyl acetate but not ethanol fractions in some samples [361], and co-occurred with saponins, tannins, and alkaloids in others [358]. When individual terpenoids could not be conclusively identified via GC-MS, they were classified as mono- (C₁₀H₁₆), sesqui- (C₁₅H₂₄), or di-terpenes (C₂₀H₃₂) using characteristic ions (m/z 93, 91, 105, 121), retention time, and library matches [40].

Terpenoid Profile Variations Across Species, Tissues, and Environmental Conditions

Terpenoids are a structurally diverse class of metabolites distributed across biological systems, with subclass composition and profiles varying by species, tissue type, and environmental conditions. In plants, terpenoid subclasses (monoterpenoids, sesquiterpenoids, diterpenoids, triterpenoids) are biosynthesized via the mevalonate (MVA) and 2-C-methyl-D-erythritol 4-phosphate (MEP) pathways, whose gene expression drives tissue-specific production [133]. Terpenoid profiles also differ across species, tissues, and fractions, and are shaped by environmental factors. They are additionally present in non-plant systems (lichens, peat organic matter, animal/pear cuticular waxes) and widely reported in phytochemical screenings, underscoring their functional significance in ecology, development, and adaptation [363][367][368][260][369][370][371][372][154][373][374].

Tissue-Specific Terpenoid Biosynthesis and Profile Variation Across Plant Species

Species/Tissue/Context	Terpenoid Class/Pathway	Key Observations	Citation
Litsea glauca sarcocarps	Monoterpenoids, Sesquiterpenoids	High expression of LIS (monoterpenoid) and alpha-humulene/beta-caryophyllene synthase (sesquiterpenoid)	[133]
Litsea glauca leaves	Monoterpenoids, Sesquiterpenoids	High expression of TES (monoterpenoid) and alpha-humulene/beta-caryophyllene synthase (sesquiterpenoid)	[133]
Litsea glauca roots	Diterpenoids	High expression of KO/KAO (GA biosynthetic enzymes) linked to root development	[133]
Litsea glauca kernels	Triterpenoids	Kernel-specific expression of SQLE	[133]
Unspecified plant (leaves vs. roots)	MEP/MVA pathways, Precursor unigenes	MEP (plastidial) enriched in leaves; MVA (cytosolic) and farnesyl-PP/geranyl-geranyl-PP unigenes enriched in roots	[357]
Unspecified plant roots	Specialized terpenoid pathways	Enrichment of monoterpenoid, sesquiterpenoid, diterpenoid, triterpenoid pathways; higher expression of germacrene A synthase and FPS vs. leaves/stems	[357]
Rosa samples	Mixed terpenoids	7.57–22.62% of metabolites; 37/38 terpenoids (4 mono-, 2 di-, 2 sesqui-, 29 triterpenoids) with inter-sample differences	[154]
Red vs. sugar beet roots	Mixed terpenoids	48 terpenoids (6 classes); 6 differential metabolites with varying abundance	[368]
Leea species	Major phytochemicals	Terpenoids alongside flavonoids and steroids	[371]
Mukia javanica stems	Most abundant secondary metabolites	Phytol identified	[370]
Unspecified plant (leaf vs. flower extract)	Mixed terpenoids	Leaf extract (20%) higher terpenoid content than flower extract (9%); rubrinol/arjunolic acid common, others extract-specific	[373]
Pinus pinaster needles (chemotypes)	Neutral/acid components, mono-/sesqui-/diterpenes, resin acids	Chemotype 1: higher acid content, oxygenated diterpenes (isoabienol), levopimaric/palustric acids (exclusive), high neoabietic acid; Chemotype 2: balanced neutral/acid, hydrocarbon diterpenes (8,13-abietadiene), germacrene D-4-ol acetate (exclusive), anticopalic/eperuic acids; α/β-pinene >70% of monoterpenes (qualitatively similar); germacrene D/β-caryophyllene main sesquiterpenes	[160]
Pinus pinaster vs. P. radiata foliage	Foliar terpene concentrations	P. pinaster: ~5× more diterpenes, 14× more monoterpenes, 32× more sesquiterpenes than P. radiata; α/β-pinene dominant (>80% in P. pinaster, >90% in P. radiata)	[303]
Melaleuca alternifolia essential oil	Mono-/sesqui-/triterpenes	Monoterpenes (~90%, e.g., sabinene hydrate, 1,8-cineole); sesquiterpenes (~10%); triterpenes (e.g., ursolic acid) not detected	[68]
Unspecified plant (leaf vs. root oil)	Mono-/sesqui-/diterpenes	Leaf oil: 9 monoterpenes (neo-allo-ocimene 21.91%), 9 sesquiterpenes (alloaromadendrene 2.45%), 1 diterpene (jolkinol D 0.16%); Root oil: 14 monoterpenes (m-cymene 49%), 21 sesquiterpenes	[375]
Pine species (resins)	Mono-/sesqui-/diterpenes	Monoterpenes up to 90% (trunk/needles); sesquiterpenes trace to 20% (needles/cones); diterpenes up to 15% (cones/trunks)	[333]
Juniper	Mono-/sesqui-/diterpenes	54 terpenes; monoterpenes peak fall/winter; sesquiterpene hydrocarbons least variable; diterpenes no seasonal change	[13]
Unspecified plant essential oil	Mixed terpenoids	Sesquiterpenes (78.26%) dominant; monoterpenoids (17.84%); diterpenoids/triterpenoids <2%	[376]

Environmental Effects on Terpenoid Profiles
Exogenous methyl jasmonate (MeJA) increased linalool ~6-fold in P. pinaster but had no significant effect or reduced some terpenes in P. radiata [303]. Pine processionary moth (PPM) feeding reduced total diterpenes (10%), monoterpenes (25%), and sesquiterpenes (35%) in P. pinaster (with 8-fold linalool increase) and caused smaller declines in P. radiata [303]. On poor vs. good sites, P. pinaster had 20% lower total monoterpenes (reduced α/β-pinene) but higher linalool; sesquiterpenes showed mixed changes (geranyl acetate declined, trans-caryophyllene increased), while diterpenes were unchanged [303]. In Satureja officinalis plantlets, 0.1% HZ increased diterpenes (13.41%) and introduced triterpenes (0.07%) vs. control (63.3% monoterpenes), with 1,8-cineole as the major compound across treatments [149]. Shading in lemon grass increased diterpenoid ginkgolide C and tetraterpenoid astaxanthin, while volatile monoterpenes showed no significant difference [12]. Low nitrogen (LN) up-regulated triterpene saponin biosynthesis genes (FPS, β-AS, DS), likely increasing saponin content [245]. Crown-fire-adapted species had lower mono-, sesqui-, and diterpene levels than surface-fire species across seasons, with total terpenes highest in winter and lowest in spring [40].

Terpenoids in Non-Plant Systems
In Pyrenula lichens, thin-layer chromatography (TLC) revealed terpenoid spots (weak short-wave UV visibility, reddish after charring); some Pyrenula species lack these red-charring terpenoids, and profiles differ from Allographa [363]. Peat organic matter from the Ob Fen has depth-uneven terpenoid distribution, with concentration maxima at specific layers and broader component spectra with increasing depth in some boreholes [372]. In animal cuticular waxes, terpenoids are minor (5.3% male, 4.5% female, including squalene) [377]. Pear cuticular wax terpenoids (e.g., δ-tocopherol, β-amyrin, lanosterol) vary by storage treatment; β-sitosterol acetate is detected only under 90–95% relative humidity [367].

Novel Terpenoid Skeletons and Subclass-Specific Structural Features

Terpenoids are a structurally diverse class of natural products, categorized into monoterpenoids (C10), sesquiterpenoids (C15), diterpenoids (C20), triterpenoids (C30), and specialized derivatives (e.g., meroterpenoids, sesterterpenoids). A comprehensive analysis of 70 unique natural products identified terpenoids as the second most abundant class, with 24 compounds documented [378]. Below, key findings on terpenoid distribution, structural diversity, and biological properties across plant and fungal sources are consolidated.

Terpenoid Distribution, Abundance, and Key Examples Across Sources

Terpenoid Class	Source	Key Compounds/Characteristics	Citations
Monoterpenoids	Sunflower	~400 reported molecules; volatile monoterpenes (e.g., alpha-pinene, antifungal)	[379]
	Gardenia jasminoides fruits	Jasminoside B (glycoside; fragmentation: 162 Da glycoside loss, diagnostic ions m/z 179, 161)	[331]
	Lauraceae plants	Linalool, 1,8-cineole, citral, camphene, borneol, camphor, geraniol (fragrant, bioactive)	[14]
	Eucalyptus maidenii (outer bark)	Main lipophilic component (rt 4.14–13.69 min; 1,8-cineole, γ-terpineol most abundant)	[103]
	E. maidenii (inner bark)	Mainly composed of monoterpenes	[103]
Sesquiterpenoids	Sunflower	Helibisabonols, helianuols, heliangolides	[379]
	Vetiveria zizanioides	Khusimol, khusimene, khusimone, khusinol, vetivones (dominant in essential oils)	[174]
	Lauraceae plants	362 compounds from 44 species; megastigmane, eudesmane, germacrane, lindenane-type majority	[14]
	E. maidenii (outer bark)	Detected at rt 15.08–24.56 min; aromadendrene, alloaromadendrene, globulol most abundant	[103]
	E. maidenii (inner bark)	Major component	[103]
	C. pinnatifida leaves	13 sesquiterpenoids (3–15), including novel glycosides (3–4)	[380]
	Switchgrass cultivars	4 tentatively identified	[341]
Diterpenoids	Rosemary	Phenolic diterpenes (carnosic acid, carnosol; catechol class key to antioxidant activity)	[381]
	Croton genus	105 clerodane diterpenes (4 types based on C-9 side chains)	[382]
	Lauraceae plants	Unique cage-like structures (8 subtypes; ryanodane-type in C. cassia, C. verum, P. indica)	[14]
	Switchgrass cultivars	10 tentatively identified	[341]
	C. pinnatifida leaves	2 norditerpenoids (1–2; glycoside and skeleton variants)	[380]
Triterpenoids	Sunflower	ent-kaurenes, ent-trachylobanes, ent-atisarenes	[379]
	Eucalyptus globulus leaves	Predominant lipophilic compounds (87.7–89%); ursolic, oleanolic, betulonic acids	[330]
	Gardenia jasminoides	Ursolic acid, oleanolic acid, 9,19-cyclolanost-24-ene-3,23-dione, gardenic acid B	[331]
	Schisandra chinensis	83 triterpenoids (novel lanostane-type)	[337][352]
	Eucalyptus species (outer bark)	Ursolic (1.9–3.6 g/kg), betulinic (1.3–2.1 g/kg), oleanolic (0.8–1.4 g/kg) acids	[103]
	E. maidenii (inner bark)	α-amyrin (0.4 g/kg) + small amounts of ursolic, betulinic, oleanolic acids	[103]
Specialized Derivatives	Switchgrass cultivars	72 tentatively identified saponins (7 classes based on aglycone/glycosylation)	[341]

Structural diversity is a hallmark of terpenoids, with mass spectral fragmentation patterns aiding characterization: monoterpenoids like camphor (m/z 153) produce fragments at m/z 135 and 109 [258], while sesquiterpenoids (e.g., furanogermenone, curzeone) and diterpenoids (e.g., retinoic acid, traversianal) have distinct fragment profiles [258]. Fungi are a rich source of novel terpenoid skeletons: endophytic fungi yield rare trinor-bisabolane (zopfiellin B) and ketal-bridged bisabolane (aspergoterpenin A) sesquiterpenoids [335], while Trichoderma sp. produces unique 4/5/6/7 tetracyclic diterpenes (harzianelactone) [335]. Sesterterpenoids (e.g., aspterpenacids A–B from Aspergillus terreus) feature unusual pentacarbocyclic rings [335], and meroterpenoids (e.g., spiroterreusnoids A–F, chermesins A–D) combine terpenoid and polyketide moieties with bioactivities like BACE1 inhibition [335].

Mycochemical studies have identified 7 diterpenoids [383] and 108 triterpenoids [383], including novel ent-atisane-type diterpenoids (Ebractenone A–B) with unique moieties [384]. Salvia species contain abietane quinone diterpenes classified into four groups, with tanshinones specific to East Asian species [385]. Schisandra chinensis has novel lanostane-type triterpenes formed via H-17 dehydrogenation [352], while Eucalyptus outer bark triterpenoids include acetyl derivatives (e.g., 3-acetylbetulinic acid) and species-specific compounds (betulonic acid only in E. maidenii) [103]. Switchgrass saponins exhibit structural variation: 44 have diosgenin aglycones (m/z 415), 20 have m/z 431 aglycones, and unique variants include SS1050 (cholesterol-like C27 aglycone) and acetylated saponins (SS1254, SS1089) [341].

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