Involvement of MdUGT75B1 and MdUGT71B1 in flavonol galactoside/glucoside biosynthesis in apple fruit

Apple is rich in flavonol glycosides, which are believed to contribute to putative health benefits associated with apple consumption. Glycosylation, catalyzed by uridine diphospho-glycosyltransferases (UGTs), is the last step in flavonol biosynthesis, which confers molecular stability and solubility to the flavonol. In the present study, the involvement of two UGTs, MdUGT75B1 and MdUGT71B1, in flavonol biosynthesis in apple was investigated. The major flavonols are quercetin 3-O-glycosides, and UV-B and blue light treatment significantly enhanced the accumulation of quercetin 3-O-galactoside, quercetin 3-O-glucoside, and kaempferol 3-O-galactoside. Transcript levels of MdUGT75B1 and MdUGT71B1 in fruit subjected to different treatments were correlated well with flavonol accumulation. MdUGT75B1 showed flavonol-specific activity with a preference for UDP-galactose as the sugar donor, while MdUGT71B1 using UDP-glucose exhibited a wider substrate acceptance. Thus, MdUGT75B1 and MdUGT71B1 are key UGTs involved in flavonol biosynthesis and may have important roles in regulating accumulation of these health-promoting bioactive compounds in apple.