A polarized supercell produces specialized metabolites in cannabis trichomes.

For centuries, humans have cultivated cannabis for the pharmacological properties that result from consuming its specialized metabolites, primarily cannabinoids and terpenoids. Today, cannabis is a multi-billion-dollar industry whose existence rests on the biological activity of tiny cell clusters, called glandular trichomes, found mainly on flowers. Cannabinoids are toxic to cannabis cells,1 and how the trichome cells can produce and secrete massive quantities of lipophilic metabolites is not known. 1 To address this gap in knowledge, we investigated cannabis glandular trichomes using ultra-rapid cryofixation, quantitative electron microscopy, and immuno-gold labeling of cannabinoid pathway enzymes. We demonstrate that the metabolically active cells in cannabis form a "supercell," with extensive cytoplasmic bridges across the cell walls and a polar distribution of organelles adjacent to the apical surface where metabolites are secreted. The predicted metabolic role of the non-photosynthetic plastids is supported by unusual membrane arrays in the plastids and the localization of the start of the cannabinoid/terpene pathway in the stroma of the plastids. Abundant membrane contact sites connected plastid paracrystalline cores with the plastid envelope, plastid with endoplasmic reticulum (ER), and ER with plasma membrane. The final step of cannabinoid biosynthesis, catalyzed by tetrahydrocannabinolic acid synthase (THCAS), was localized in the cell-surface wall facing the extracellular storage cavity. We propose a new model of how the cannabis cells can support abundant metabolite production, with emphasis on the key role of membrane contact sites and extracellular THCA biosynthesis. This new model can inform synthetic biology approaches for cannabinoid production in yeast or cell cultures. • Glandular cells form a polarized syncytium during THCA production and secretion • GPPS is located in plastids that contain conspicuous membrane fusions • THCAS is located exclusively on the extracellular surface of trichomes • Membrane contacts among plastids, ER, and PM inform a new trafficking model Cannabinoids and terpenes are biosynthesized, secreted, and stored in cannabis glandular trichomes. Livingston et al. reveal the nanoscale distribution of the enzymes involved in cannabinoid biosynthesis and the unique cellular environment where these enzymes operate. A new model of metabolite trafficking from the plastid to the cell wall is proposed. [ABSTRACT FROM AUTHOR]