Identification and characterization of Camellia sinensis glucosyltransferase, UGT73A17: A possible role in flavonol glucosylation

Abstract

Tea plant (Camellia sinensis) biosynthesizes a wide variety of specialized metabolites, including phenolic compounds such as catechins. Flavonol, one of the major flavonoid subclasses, in C. sinensis is present in the O-glycoside form, such as quercetin 3-O-β-D-glucopyranoside, kaempferol 3-O-β-D-glucopyranoside, and rutin (quercetin 3-O-β-glucopyranosyl-6-O-α-rhamnoside). These flavonol glycosides are highly accumulated, constituting up to 2-3% (w/w dry weight) of tea leaves; however, their biosynthetic machinery in C. sinensis remains elusive. Using high-throughput RNA sequencing from the fresh leaves of a cultivar (C. sinensis var sinensis cv Yabukita) and rapid amplification of cDNA ends (RACE) cloning with degenerate oligonucleotide primers, we identified a full-length cDNA of UDP-glycosyltransferase, designated as UGT73A17, and characterized the biochemical and molecular functions of UGT73A17. Recombinant UGT73A17 protein catalyzed 3-O-glucosylation of quercetin, yielding quercetin 3-O-β-D-glucopyranoside in vitro. The preferential expression of UGT73A17 gene in the mature, relative to young leaves, stems and roots, is roughly consistent with the accumulation pattern of flavonol glycosides in C. sinensis, suggesting that UGT73A17, in part, participates in the biosynthesis of flavonol glycosides in planta.