The biosynthesis of terpenes and cannabinoids occurs within the extracellular secretory cavity, referred to as a trichome. Trichomes are present on over 300 plants,(Dayanandan & Kaufman, 1976) and over 300 morphologically distinct trichomes have been reported (Wagner, 1991). The TrichOME project (www.planttrichome.org) is a database of EST sequences, microarray hybridizations, mass spectrometry-based metabolite profiles, and literature compiled with funding from the NSF and The Samuel Roberts Nobel Foundation ("TrichOME - A Comparative Omics Database for Plant Tichome," 2014). Trichomes serve several functions in plants that include keeping the frost from the surface cells, reducing transpiration from occurring, and increasing light reflectance in desert species. Specialized glandular trichomes produce and emit compounds that interact with insects deterring pests or promoting pollination (Wagner, 1991). The trichomes act as a first line of defense to protect the plant from the environment (Taura et al., 2007). Interestingly, some of the very cannabinoids (THCA and CBGA) produced within the C. sativa trichome have been found to be toxic to C. sativa cells (Sirikantaramas et al., 2005). Cannabinoids are secreted into the trichome in order to prevent cellular damage to the plant. THCA and CBGA have been shown to be toxic to certain insect species as well (Taura et al., 2007).
The term trichome is derived from the Greek word meaning hair, because of the hair-like appearance. Trichomes found on C. sativa have been divided into two types, glandular and non-glandular. Non-glandular C. sativatrichomes cover the shoot from early seedling stages to the end of flowering and are highly-silicified unicellular hair-like appendages (Dayanandan & Kaufman, 1976). Cystolithic non-glandular trichomes are short hairs ranging from 150 – 220 µm in height, and contain basal deposits of calcium carbonate. In contrast, non-cystolithic non-glandular are generally longer (340 – 500 µm) with a more slender structure, and do not contain a basal deposit of calcium carbonate (Upton et al., 2014). The non-glandular trichomes cover the majority of the plants surface, including the stems, petioles, stipules, leaf blades, bracts, and both surfaces of the tepals (Dayanandan & Kaufman, 1976).
The cannabinoids are produced in two types of glandular trichomes, capitate-stalked and capitate-sensille. The capitate-stalked glandular trichome heads are composed of eight cells, and reside on the bracts of the female plants and anthers of the male plants. They are easily recognized by the mushroom-like stalk and head appearance (Figure 7C) (Dayanandan & Kaufman, 1976). Capitate-stalked trichomes are the primary source of cannabinoid and terpene resin production in flowering C. sativa. Dayanandan and Kaufman (1976) have shown that male C. sativa plants produce equal quantities of glandular trichomes compared to females, and bear similar trichome types. It has also been demonstrated that some resinous cannabinoid-producing glands develop prior to flower formation. For example, capitate-sesille trichomes produce cannabinoids throughout the plant life cycle, but at lower levels compared to the capitate-stalked type. Capitate-sesille trichome heads are composed of between two and four cells. In contrast, the bulbous glandular trichomes are found throughout the plant and do not produce cannabinoids or terpenes (Figure 7B). The head of the trichome is composed of one, two, or four cells with stalks either one or two cells high.
The cannabinoid precursors, olivetolic acid and geranyl diphosphate are transported through the stalk of glandular trichomes via vacuoles and plastids respectively (Figure 7A). The cannabinoid precursors are released into the secretory cavity where they are transformed biosynthetically through resident cannabinoid synthase enzymes in the extracellular space, which was shown in Figure 4. Monoterpenes are formed in the plastids via the enzymatic conversion of geranyl diphosphate. Alternatively, geranyl diphosphate can couple with isopentenyl diphosphate in the cytoplasm to form the parent compound to the sesquiterpenoids, farnesyl pyrophosphate (Russo, 2011). THCA synthase-fused to green fluorescent protein helped visually demonstrate that this enzyme is accumulated in the secretory cavity (Sirikantaramas et al., 2005) located in the head of glandular capitate-stalked and capitate sesille trichomes (Figure 7C).