Accumulation of Cannabinoids in Glandular Trichomes of Cannabis (Cannabaceae)

Abstract Sessile- and capitate-stalked secretory glands are sites of cannabinoid accumulation in Cannabis (Cannabaceae). Analyses show cannabinoids to be abundant in glands isolated from bracts or leaves of pistillate plants. Cannabinoids are concentrated in the secretory cavity formed as an intrawall cavity in the outer wall of the disc cells. Specialized plastids, lipoplasts, in the disc cells synthesize lipophilic substances, such as terpenes, that migrate through the plasma membrane and into the cell wall adjacent to the secretory cavity. These substances enter the cavity as secretory vesicles. An antibody probe for THC shows it to be most abundant along the surface of vesicles, associated with fibrillar material in the cavity, in the cell wall and in the cuticle; little THC was detected in the cytoplasm of disc or other cells. The phenol, phloroglucinol, is abundant in both gland types. A working hypothesis for the site of cannabinoid synthesis is proposed, and must be examined further. Knowledge of the mechanism of cannabinoid synthesis and localization can contribute to efforts to further reduce the THC content in hemp strains for potential agricultural use in the United States and elsewhere.