Glycosyltransferase that mediates the glucosylation of anthocyanidin 3-O-glucosides to yield anthocyanidin 3-O-sophorosides. 3-O-sophoroside derivatives are required for the bright blue or red color of flowers. an anthocyanidin 3-O-beta-D-glucoside + UDP-alpha-D-glucose = an anthocyanidin 3-O-sophoroside + 2 H(+) + UDP Pigment biosynthesis; anthocyanin biosynthesis. Mainly expressed in the petals and tubes of flower buds at around 24 hours before flower opening. Belongs to the UDP-glycosyltransferase family.