The putative cannabinoid-secreting trichome of Trema micrantha (L.) Blume (Cannabaceae)

Trema, a genus of the popularly known Cannabaceae, has recently been the subject of cannabinoid bioprospection. T. micrantha is a tree with pharmacological potential widely used in folk medicine. It has two types of glandular trichomes, bulbous and filiform, spread throughout the plant body. Considering the proximity of this species to Cannabis sativa and Trema orientalis, species containing cannabinoids, the glandular trichomes of T. micrantha are also expected to be related to the secretion of these compounds. Thus, this study aims to detail the morphology of secretory trichomes during the synthesis, storing and release of metabolites in T. micrantha. We tested the proposition that they could be a putative type of cannabinoid-secreting gland. Pistillate and staminate flowers and leaves were collected and processed for ontogenic, histochemical, and ultrastructural analyses. Both types of glandular trichomes originate from a protodermal cell. They are putative cannabinoid-secreting sites because: (1) terpene-phenols and, more specifically, cannabinoids were detected in situ; (2) their secretory subcellular apparatus is consistent with that found in C. sativa: modified plastids, polyribosomes, an extensive rough endoplasmic reticulum, and a moniliform smooth endoplasmic reticulum. Plastids and smooth endoplasmic reticulum are involved in the synthesis of terpenes, while the rough endoplasmic reticulum acts in the phenolic synthesis. These substances cross the plasma membrane by exocytosis and are released outside the trichome through cuticle pores. The study of the cell biology of the putative cannabinoid glands can promote the advancement of prospecting for natural products in plants.