In the presence of other necessary color factors, this glycosylation reaction allows the accumulation of anthocyanin pigments. an anthocyanidin + H(+) + UDP-alpha-D-glucose = an anthocyanidin 3-O-beta-D-glucoside + UDP Pigment biosynthesis; anthocyanin biosynthesis. Faintly expressed in cotyledons, roots and leaves. Rare transcripts expressed in cotyledon and roots during the development. Belongs to the UDP-glycosyltransferase family.