Modification of Plant Volatiles via Tissue- and Cellular-Specific Targeting of Terrene Synthesis as Viable Tool to Increase Plant Defense
One promising way to reduce unwanted effects is to specifically engineer metabolite pathways in glandular trichomes. Trichomes are hair-like structures on plant epidermal surfaces (see Chapter 13) that produce, store and secrete upon mechanical disruption large amounts of secondary metabolites, such as terpenoids and phenylpropanoids. By restricting metabolic modifications to trichomes using trichome- specific promoters, unwanted effects on crop yield and plant fitness can be prevented. A comprehensive list of trichome-specific promoters is provided in Kortbeek et al. (2016) and references therein.
In Solanum lycopersicum, 14 out of 45 known terpene synthases are expressed in stem trichomes and two of these terpene synthases (S1TPS5 and S1TPS9) have been shown to be exclusively expressed in type VI glandular trichomes (Spyropoulou, Haring, and Schuurink 2014; Kortbeek et al. 2016). The induction of these TPSs by JA indicates a role for terpene synthesis in plant defense. The introduction of a 7-epizingiberene synthase from Solanum habrochaites (ShZIS) into S. lycopersicum under control of the S1TPS5 promoter led to the production of a novel sesquiterpene 7-epizingiberene when coexpressed with the precursor synthase m-prenyltransferase (zFPS) (Bleeker et al. 2012). The resulting transgenic plants displayed severe reduction in fecundity of Tetranychus urticae (spider mites) with no associated plant growth retardation. Still, levels of 7-epizingiberene were very low in the transgenic plants compared to S. habrochaites (Bleeker et al. 2012). Apparently, the amount of substrate available to ShZIS was limiting, similar as was observed for Z, Z-FPP engineering in tobacco (Sallaud et al. 2009). When endogenous genes or closely related genes are overexpressed, such as zFPS, post-transcriptional regulation and feedback inhibition might attenuate the expected output.
Post-transcriptional regulation or feedback inhibition was observed when farnesyl diphosphate synthase (S1FPS) was expressed in cultivated tomato under the trichome-specific promoter of methyl ketone synthase from S. habrochaites (ShMKSl), as only moderately increased levels of FPS transcripts were obtained. However, transformation of the homolog of FPS from chicken (Gallus gallus) resulted in high expression under the same ShMKSl promoter. The internal levels of FPP were much higher compared to wild type and to transgenic lines expressing S1FPS, resulting in higher levels of (Ej-nerolidol and a-farnesene and a remarkable depletion of monoterpenes. Either the loss of monoterpenes or the gain of sesquiterpenes made the transgenic plants less attractive to B. tabaci (Kortbeek et al. 2016). Both transgenes were engineered to contain a signal peptide for targeting FPS to the plastids, as it was hypothesized that biosynthesis of terpenes can be improved by targeting the metabolism to non-native cellular compartments. These studies show the flexibility of terpene metabolism and the potential of metabolic engineering of terpenes in specific tissues.