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 mini-MbA are potent inhibitors of human pancreatic α-amylase and are being developed as drug candidates for the treatment of type-2 diabetes. MbA occurs in corms of the ornamental plant montbretia (Crocosmia x crocosmiiflora), but a system or process to obtain large quantities of MbA is not currently available. Improved MbA production thus requires knowledge of its biosynthesis from the flavonol myricetin. Metabolite profiling and enzyme assays showed MbA formation and accumulation occurs during early stages of corm development. We established myricetin 3-O-rhamnoside (MR), myricetin 3-O-glucosyl rhamnoside (MRG) and myricetin 3-O-(6′-O-caffeoyl)-glucosyl rhamnoside (mini-MbA) as the first three intermediates during MbA biosynthesis. Contrasting transcriptomes of young and old corms revealed a set of differentially expressed UDP-sugar-dependent glycosyltransferases (UGTs) and BAHD-acyltransferases (BAHD-ATs). cDNA cloning and enzyme characterization identified UGT77B2 and UGT709G2 as the enzymes that catalyze the consecutive glycosylation of myricetin to produce MR and of MR to give MRG, respectively. We identified two BAHD-ATs, CcAT1 and CcAT2 that catalyze the acylation of MRG to complete the formation of mini-MbA.
