Secondary plant metabolites undergo several modification reactions, including glycosylation. Glycosylation, which is mediated by UDP-glycosyltransferase (UGT), plays a role in the storage of secondary metabolites and in defending plants against stress. In this study, we cloned one of the glycosyltransferases from rice, RUGT-5 resulting in 40-42% sequence homology with UGTs from other plants. RUGT-5 was functionally expressed as a glutathione S-transferase fusion protein in Escherichia coli and was then purified. Eight different flavonoids were used as tentative substrates. HPLC profiling of reaction products displayed at least two peaks. Glycosylation positions were located at the hydroxyl groups at C-3, C-7 or C-4' flavonoid positions. The most efficient substrate was kaempferol, followed by apigenin, genistein and luteolin, in that order. According to in vitro results and the composition of rice flavonoids the in vivo substrate of RUGT-5 was predicted to be kaempferol or apigenin. To our knowledge, this is the first time that the function of a rice UGT has been characterized.

Most flavonoids found in plants exist as glycosides, and glycosylation status has a wide range of effects on flavonoid solubility, stability, and bioavailability. Glycosylation of flavonoids is mediated by Family 1 glycosyltransferases (UGTs), which use UDP-sugars, such as UDP-glucose, as the glycosyl donor. AtGT-2, a UGT from Arabidopsis thaliana, was cloned and expressed in Escherichia coli as a gluthatione S-transferase fusion protein. Several compounds, including flavonoids, were tested as potential substrates. HPLC analysis of the reaction products indicated that AtGT-2 transfers a glucose molecule into several different kinds of flavonoids, eriodictyol being the most effective substrate, followed by luteolin, kaempferol, and quercetin. Based on comparison of HPLC retention times with authentic flavonoid 7-O-glucosides and nuclear magnetic resonance spectroscopy, the glycosylation position in the reacted flavonoids was determined to be the C-7 hydroxyl group. These results indicate that AtGT-2 encodes a flavonoid 7-O-glucosyltransferase.

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