Discovery of UDP-Glycosyltransferases and BAHD-Acyltransferases Involved in the Biosynthesis of the Antidiabetic Plant Metabolite Montbretin A

Plant specialized metabolism serves as a rich resource of biologically active molecules for drug discovery. The acylated flavonol glycoside montbretin A (MbA) and its precursor myricetin 3-<i>O</i>-(6'-<i>O</i>-caffeoyl)-glucosyl rhamnoside (mini-MbA) are potent inhibitors of human pancreatic α-amylase and are being developed as drug candidates to treat type-2 diabetes. MbA occurs in corms of the ornamental plant montbretia (<i>Crocosmia x crocosmiiflora</i>), but a system for large-scale MbA production is currently unavailable. Biosynthesis of MbA from the flavonol myricetin and MbA accumulation occur during early stages of corm development. We established myricetin 3-<i>O</i>-rhamnoside (MR), myricetin 3-<i>O</i>-glucosyl rhamnoside (MRG), and mini-MbA as the first three intermediates of MbA biosynthesis. Contrasting the transcriptomes of young and old corms revealed differentially expressed UDP-sugar-dependent glycosyltransferases (UGTs) and BAHD-acyltransferases (BAHD-ATs). UGT77B2 and UGT709G2 catalyze the consecutive glycosylation of myricetin to produce MR and of MR to give MRG, respectively. In addition, two BAHD-ATs, CcAT1 and CcAT2, catalyze the acylation of MRG to complete the formation of mini-MbA. Transcript profiles of UGT77B2, UGT709G2, CcAT1, and CcAT2 during corm development matched the metabolite profile of MbA accumulation. Expression of these enzymes in wild tobacco (<i>Nicotiana benthamiana</i>) resulted in the formation of a surrogate mini-MbA, validating the potential for metabolic engineering of mini-MbA in a heterologous plant system.