this possibility, we studied the cell-specific expression and localization of THCA
synthase in seed coat trichomes (Fig.8.6), and demonstrated that THCA is
biosynthesized in the storage cavity of the glandular trichomes, based on several
lines of evidence previously described (Sirikantaramas et al. 2005 ). These studies
also showed that secretory cells of the glandular trichomes could produce not only
metabolites but also biosynthetic enzymes. To our knowledge, THCA synthase is
thefirst enzyme that is released into the secretory cavity.
It is unknown why this enzyme must be secreted for successful THCA pro-
duction. To our knowledge, we were thefirst to report that THCA is a very toxic
substance to plants, includingCannabis(Sirikantaramas et al. 2005 , 2014 ). In
addition, the THCA synthase reaction produces hydrogen peroxide as well as
THCA during the oxidation of CBGA (Sirikantaramas et al. 2004 ). SinceC. sativa
produces a large concentration of THCA (Shoyama et al. 1975 ), toxic levels of
hydrogen peroxide might also accumulate in the storage cavity as a result of the
THCA synthase reaction. Accordingly, it is reasonable to believe that THCA is
synthesized in an extracellular compartment, such as secretory cavity, to avoid
cellular damage. Furthermore, since cannabinoids, as well as hydrogen peroxide,
have antimicrobial properties (Appendino et al. 2008 ),Cannabis plants might
accumulate these compounds in the glandular trichomes on the plant surface to
allow for effective self-defense.
Fig. 8.6 Glandular trichomes on the seed coat surface (a) and side view of a trichome (b). The
trichome head contains the secretory cavity. The bar represents 0.05 mm. Reproduced with
permission from Taura et al. (2009a). Copyright Wiley-VHCA, Zürich, Switzerland
190 S. Sirikantaramas and F. Taura