Our interest is focused on the diversification of bifunctional dehydroquinate-dehydratases shikimate dehydrogenases (EC 18.104.22.168) into mono-functional quinate dehydrogenases (QDH). QDHs catalyze reversible oxidation/reduction of 3-dehydroquiante/ quinate with the assistance of NADP+ or NAD+ cofactors. Quinate is a secondary metabolite which participates in the plant immunity and lignin formations, whereas dehydroquinate is an intermediate of shikimate pathway which produces aromatic amino acids. Through the cofactor dependendent interconversion of quinate/dehydroquinate, the metabolite can either be directed to primary or secondary metabolite biosynthesis. So far, we identified several of both NADP+ and NAD+ dependent QDHs from S. lycopersicum, N. tabacum and B. rapa plants and we are working on elucidating roles of those QDHs. S. lycopersicum (tomato) QDHs have interesting expression profiles, with NADP+ dependent QDHs being expressed in green tissues, and NAD+ dependent QDHs being expressed in roots and fruits undergoing ripening. We suspect that NADP+ dependent QDHs catalyze reduction of dehydroquinate into quinate, and therefore direct quinate towards biosynthesis of antifeeding agents and lignin. On the other hand, NAD+ dependent QDHs most likely divert quinate away from the secondary metabolite biosynthesis in cytoplasm.
By modulating levels of quinate through knock out/ overexpression of QDH enzymes we hope to modify plants ability to resist infections and to modify lignification.
Figure 1. Quinate metabolism in plant cells. Dehydroquinate is diverted away from the shikimate pathway via NADPH dependent QDH reduction. In cytoplasm, quinate is incorporated into the CGA biosynthesis by the action of hydroxycinnamoyl quinate/shikimate transferases (HQT/HCT). In vacuoles, additional hydroxycinnamoyl moieties can be crosslinked onto CGAs to make di/trihydroxycinnamoyl-quinate esters by the action of chlorogenate:chlorogenate transferases (CCT). CGAs serve as anti-feeding/fungal agents and lignin biosynthesis precursors. Upon breakdown of CGAs by caffeoyl-shikimate esterase (CSE), the released quinate is converted back into dehydroquinate via NAD+ dependent QDH catalyzed oxidation with NAD+ cofactor. Figure by Victor Tulceanu
Figure 2. Phylogenetic tree of plant quinate dehydrogenases and shikimate dehydrogenases.