Glucosyltransferase involved in montbretin A (MbA) biosynthesis (PubMed:29967287, PubMed:31004005). Catalyzes the glucosylation of myricetin 3-O-alpha-L-rhamnoside (MR) to produce myricetin 3-O-[beta-D-glucosyl-(1->2)-alpha-L-rhamnoside] (MRG), a precursor of MbA (PubMed:29967287, PubMed:31004005). MbA is a potent inhibitor of human pancreatic alpha-amylase and is being developed as drug candidate to treat type-2 diabetes (PubMed:29967287, PubMed:31004005). In vitro, is able to transfer UDP-xylose with 50-fold less efficiency compared with UDP-glucose (PubMed:29967287). In vitro, can use myricetin 3-O-glucoside and quercetin 3-O-glucoside as substrates, although these two flavonoids may not be physiological substrates in vivo (PubMed:29967287). myricetin 3-O-alpha-L-rhamnoside + UDP-alpha-D-glucose = H(+) + myricetin 3-O-[beta-D-glucosyl-(1->2)-alpha-L-rhamnoside] + UDP Flavonoid metabolism. Expressed in young cromes. Belongs to the UDP-glycosyltransferase family.