UDP-glucose-dependent glucosyltransferase involved in C-glycosylflavone biosynthesis. Has a strict stereo selectivity and shows a preference for apigenin and luteolin as substrates, but could not recognize flavonols, anthocyanins and xanthones as glucosyl acceptors. No activity with orientin (luteolin 8-C-glucoside) and luteolin 4'-glucoside. Can use UDP-galactose instead of UDP-glucose as a sugar donor, but not UDP-glucuronic acid. Highly expressed in young leaves, with a lower expression in mature leaves. Higher expression at early stages of petals development than at the late stages. Belongs to the UDP-glycosyltransferase family.