Biochemistry and evolution of the shikimate dehydrogenase/quinate dehydrogenase gene family in plants

Gene duplication and functional diversification is a central driving force in the evolution of plant biochemical diversity. However, the latter process is not well understood. Here the diversification of the plant shikimate/quinate dehydrogenase (S/QDH) gene family was investigated in order to shed light on how duplicate genes functionally diversify. The shikimate pathway is the major biosynthetic route towards the aromatic amino acids, linking vital protein biosynthesis with the production of aromatic secondary metabolites. Dehydroquinate dehydratase/shikimate dehydrogenase (SDH) encodes the central enzyme of this pathway, catalyzing the production of shikimate. Quinate is a secondary metabolite synthesized using the same precursors as shikimate by quinate dehydrogenase (QDH). Gene duplication prior to the gymnosperm / angiosperm split generated two distinct clades in seed plants separating SDH and QDH functions whereas non-seed plants have a single copy SDH. In vitro biochemical characterization of a reconstructed ancestral enzyme was performed alongside extant members separated prior to duplication (from a lycopod, a bryophyte, and a chlorophyte) and afterwards (from a gymnosperm and an angiosperm). This revealed that novel quinate biosynthetic activity was gained in seed plants, providing evidence for the diversification of gene function via neofunctionalization. However, the ability to use both NAD(H) and NADP(H) seems to have developed in both SDH and QDH clade members of angiosperms. Finally, a method is described for analysing quinate and its derivative, chlorogenic acid in transgenic Arabidopsis.