Trichome biology
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Trichome biology is the study of the trichomes of Cannabis: the epidermal hairs and glands that cover much of the plant and that produce and hold almost all of its cannabinoids and terpenes.[1][2] Two broad classes occur on cannabis. Non-glandular covering trichomes are simple protective hairs, many of them stiffened by a calcium carbonate cystolith at the base; glandular trichomes are secretory structures whose cells make a resin rich in cannabinoids and terpenoids and secrete it into a cavity beneath the cuticle.[2][1]
The glandular trichomes borne on the female inflorescence are where cannabis concentrates its resin, and their number, size and metabolite content largely set the cannabinoid and terpene profile of the mature flower.[3][1] Because the trichomes are the site of cannabinoid and terpene biosynthesis, their development and cell biology have become an active area of cannabis research, drawing on microscopy, proteomics and transcriptomics.[4][5]
Trichome types
Cannabis bears both non-glandular and glandular trichomes, a distinction established in early scanning electron microscopy of the plant.[2] Non-glandular covering trichomes are unicellular, curved hairs found on leaves, stems and bracts; many carry a cystolith of calcium carbonate at the base and act as a physical barrier to small herbivores.[2][1] These cystolithic hairs roughen vegetative surfaces but produce no resin.[2]
Glandular trichomes occur in three forms.[2][1] Bulbous trichomes are the smallest, a head of one to four cells on a short stalk, scattered across vegetative and floral surfaces.[1] Capitate-sessile trichomes have a larger globose head on a very short stalk, with a secretory base of about eight disc cells.[3][1] Capitate-stalked trichomes are the largest and the most productive: a broad resin head is raised on a multicellular stalk above a layer of 12–16 secretory disc cells, and they crowd the bracts of the mature female inflorescence.[3][2] The stalked form accounts for most of the cannabinoid and terpene content of drug-type flowers.[3][1]
Development and maturation
The three glandular forms are linked through development rather than being wholly separate lineages. Working with maturing cannabis flowers, Livingston and colleagues found that capitate-stalked trichomes arise from sessile-like precursors, the stalk elongating and the disc-cell number rising as the trichome matures.[3] Sessile trichomes on vegetative leaves carry about eight disc cells, whereas the stalked trichomes of mature flowers carry 12–16, a difference that tracks this developmental transition rather than marking two unrelated structures.[3]
Trichome number and maturity are not fixed. Punja and colleagues showed that glandular trichome density and the progress of maturation vary with plant age and with genotype, so the same cultivar can present different trichome coverage at different stages of flowering.[6] Trichome initiation is also under hormonal control: methyl jasmonate promotes glandular trichome formation, and transcriptomic work has begun to map the regulatory pathway involved.[7][5]
As a trichome matures, the metabolite content of its head changes and the head shifts in appearance from translucent to milky and finally amber.[3] This colour change tracks the accumulation and later degradation of cannabinoids in the storage cavity, and growers read it as a cue for harvest timing.[3]
Metabolite biosynthesis and storage
The secretory disc cells at the base of a glandular trichome are the main site of specialised-metabolite synthesis in the plant.[1][4] A proteomic survey of isolated cannabis trichomes found the disc cells enriched in the enzymes of cannabinoid and terpenoid synthesis, evidence that these compounds are made in the trichome rather than delivered to it.[4]
Cannabinoid biosynthesis proceeds from two precursors made in different compartments of the disc cell: a polyketide, olivetolic acid, from the cytosol, and geranyl pyrophosphate from the plastid.[5][8] An aromatic prenyltransferase joins them into cannabigerolic acid, the common precursor, which is then converted by tetrahydrocannabinolic acid synthase or cannabidiolic acid synthase into the acidic forms of THC and CBD; which synthase predominates sets the plant's chemotype.[5][8] Terpenes are produced alongside the cannabinoids by terpene synthases in the same cells, and the two classes together make up the bulk of the resin.[8][9]
The resin is secreted into a cavity beneath the cuticle at the top of the trichome, where it accumulates.[1] The trichome's walls are remodelled as this happens: Livingston and colleagues described how the walls of the disc cells and the surrounding cavity change composition as the trichome develops into a storage structure, a specialisation that lets the cell hold large quantities of lipophilic metabolites outside the living cytoplasm.[10]
Functional ecology
The ecological functions of cannabis trichomes are only partly understood, and much of the thinking rests on comparison with glandular trichomes in other plants.[11][1] The covering trichomes, with their hard cystoliths, act as a structural defence that deters small herbivores and impedes movement across the leaf surface.[2][1]
For the glandular trichomes, the leading hypotheses concern chemical defence and protection from radiation. Pate proposed that cannabinoids screen the developing seed and flower from ultraviolet-B radiation and deter herbivores and pathogens, a reading consistent with the concentration of resin on the reproductive parts of the plant and with the higher trichome density often reported in populations from high-altitude, high-light environments.[11] Several trichome terpenes have measurable antimicrobial and insect-deterrent activity, which supports a defensive interpretation of the resin, though a defensive benefit in the field has been hard to demonstrate directly.[9][1]
Trichome coverage also responds to the environment. Light quality, ultraviolet exposure in particular, can shift trichome density and the metabolite profile,[1] and coverage varies further with plant age and genotype.[6]
Assessment
Because the trichomes carry the cannabinoids, their density and maturity serve as practical proxies for the potency and readiness of a crop.[3][6] Harvest timing is judged from the proportion of trichome heads that have turned milky or amber, and trichome coverage is often taken as a rough guide to cannabinoid yield.[3]
The relationship is looser than it looks. A 2025 review of trichome-density methods found that quantification is inconsistent between studies and that density does not translate cleanly into cannabinoid concentration, because head size, secretory volume and the tissue sampled all intervene.[12] Automated image analysis, including deep-learning methods for counting and classifying trichomes, is being developed to put density measurement on a firmer footing.[7][12]
See also
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 Tanney, C.A.S.; Backer, R.; Geitmann, A.; Smith, D.L. (2021). "Cannabis glandular trichomes: a cellular metabolite factory". Frontiers in Plant Science. 12 721986. doi:10.3389/fpls.2021.721986.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Dayanandan, P.; Kaufman, P.B. (1976). "Trichomes of Cannabis sativa L. (Cannabaceae)". American Journal of Botany. 63 (5): 578–591. doi:10.1002/j.1537-2197.1976.tb11846.x.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 Livingston, S.J.; Quilichini, T.D.; Booth, J.K.; Wong, D.C.J.; Rensing, K.H.; et al. (2020). "Cannabis glandular trichomes alter morphology and metabolite content during flower maturation". The Plant Journal. 101 (1): 37–56. doi:10.1111/tpj.14516.
- ↑ 4.0 4.1 4.2 Conneely, L.J.; Mauleon, R.; Mieog, J.; Barkla, B.J.; Kretzschmar, T. (2021). "Characterization of the Cannabis sativa glandular trichome proteome". PLOS ONE. 16 (4) e0242633. doi:10.1371/journal.pone.0242633.
- ↑ 5.0 5.1 5.2 5.3 Xie, Z.; Mi, Y.; Kong, L.; Gao, M.; Chen, S.; et al. (2023). "Cannabis sativa: origin and history, glandular trichome development, and cannabinoid biosynthesis". Horticulture Research. 10 (9) uhad150. doi:10.1093/hr/uhad150.
- ↑ 6.0 6.1 6.2 Punja, Z.K.; Sutton, D.B.; Kim, T. (2023). "Glandular trichome development, morphology, and maturation are influenced by plant age and genotype in high THC-containing cannabis (Cannabis sativa L.) inflorescences". Journal of Cannabis Research. 5 (1) 12. doi:10.1186/s42238-023-00178-9.
- ↑ 7.0 7.1 Huang, X.; Chen, W.; Zhao, Y.; Chen, J.; Ouyang, Y.; et al. (2024). "Deep learning-based quantification and transcriptomic profiling reveal a methyl jasmonate-mediated glandular trichome formation pathway in Cannabis sativa". The Plant Journal. 118 (4): 1155–1173. doi:10.1111/tpj.16663.
- ↑ 8.0 8.1 8.2 Andre, C.M.; Hausman, J.-F.; Guerriero, G. (2016). "Cannabis sativa: the plant of the thousand and one molecules". Frontiers in Plant Science. 7 19. doi:10.3389/fpls.2016.00019.
- ↑ 9.0 9.1 Russo, E.B. (2011). "Taming THC: potential cannabis synergy and phytocannabinoid-terpenoid entourage effects". British Journal of Pharmacology. 163 (7): 1344–1364. doi:10.1111/j.1476-5381.2011.01238.x.
- ↑ Livingston, S.J.; Bae, E.J.; Unda, F.; Hahn, M.G.; Mansfield, S.D.; et al. (2021). "Cannabis glandular trichome cell walls undergo remodeling to store specialized metabolites". Plant and Cell Physiology. 62 (12): 1944–1962. doi:10.1093/pcp/pcab127.
- ↑ 11.0 11.1 Pate, D.W. (1994). "Chemical ecology of Cannabis". Journal of the International Hemp Association. 1 (2): 29, 32–37.
- ↑ 12.0 12.1 Alberti, T.; Didaran, F.; Sharma, S.; De Sarandy Raposo, R.; Diatta, A.A.; et al. (2025). "Bracts, buds, and biases: uncovering gaps in trichome density quantification and cannabinoid concentration in Cannabis sativa L". Plants. 14 (14) 2220. doi:10.3390/plants14142220.