The cannabis leaf is iconic, but it is the flowers of cannabis that are consumed for the psychoactive and medicinal effects of their specialized metabolites. Cannabinoid metabolites, together with terpenes, are produced in glandular trichomes. Superficially, stalked and sessile trichomes in cannabis only differ in size and whether they have a stalk. The objectives of this study were: to define each trichome type using patterns of autofluorescence and secretory cell numbers, to test the hypothesis that stalked trichomes develop from sessile-like precursors, and to test whether metabolic specialization occurs in cannabis glandular trichomes. A two-photon microscopy technique using glandular trichome intrinsic autofluorescence was developed which demonstrated that stalked glandular trichomes possessed blue autofluorescence correlated with high cannabinoid levels. These stalked trichomes had 12-16 secretory disc cells and strongly monoterpene-dominant terpene profiles. In contrast, sessile trichomes on mature flowers and vegetative leaves possessed red-shifted autofluorescence, eight secretory disc cells and less monoterpene-dominant terpene profiles. Moreover, intrinsic autofluorescence patterns and disc cell numbers supported a developmental model where stalked trichomes develop from apparently sessile trichomes. Transcriptomes of isolated floral trichomes revealed strong expression of cannabinoid and terpene biosynthetic genes, as well as uncharacterized genes highly co-expressed with CBDA synthase. Identification and characterization of two previously unknown and highly expressed monoterpene synthases highlighted the metabolic specialization of stalked trichomes for monoterpene production. These unique properties and highly expressed genes of cannabis trichomes determine the medicinal, psychoactive and sensory properties of cannabis products.
Keywords: Cannabis sativa; DAPI; cannabinoids; electron microscopy; fluorescence microscopy; glandular trichomes; metabolite profiling; terpenes; transcriptomics.
© 2019 The Authors The Plant Journal © 2019 John Wiley & Sons Ltd.