Functional analysis of a UDP-glucosyltransferase gene contributing to biosynthesis of the flavonol triglycoside in tea plants.

Flavonol glycosides have many prominent benefits to human health and significant contributions to the growth and development of tea plant as well as the color and taste of tea infusion. In this study, a gene isolated from tea plant was found to encode a 52.2-kDa protein located on the plasma membrane and in the cytoplasm with activity of flavonol glycosyltransferase (CsFGT). The prokaryotically expressed recombinant CsFGT (rCsFGT) exhibited its main glucosyl transfer activity towards rutin to produce quercetin 3--β-d-glucopyranosyl-(1→3)-α-l-rhamnopyranosyl-(1→6)-β-d-glucopyranoside (Q-g-r-g), and showed a minor galactosyl transfer activity towards delphinidin to produce delphinidin 3- galactoside. The maximum activity of rCsFGT was observed at 30°C and pH 8.0. The main function of rCsFGT seems to be catalysis of the biosynthesis of Q-g-r-g rather than delphinidin 3- galactoside since its affinity and catalytic efficiency are much higher towards rutin than towards delphinidin. Molecular docking and site-directed mutation reveal that amino acid residues G290, E292, R319, and Q352 play important roles in the catalytic specificity of CsFGT. The Q-g-r-g content in leaves of different tea cultivars was significantly correlated with the expression level. Injection of antisense oligodeoxyribonucleotides remarkably downregulated endogenous expression and consequently reduced the Q-g-r-g content significantly. These findings will help elucidate the differential accumulation mechanism of flavonol glycosides in different tea germplasms.