Identification of the glucosyltransferase that mediates direct flavone C-glucosylation in Gentiana triflora

•Gentiana triflora accumulates isoorientin and isoorientin 4′-glucoside in leaves.•UDP-glucose:flavone 6-C-glcucosyltransferase activity was detected in leaf extracts.•A cDNA (GtUF6CGT1) encoding flavone 6-C-glucosyltranferase was isolated.•Recombinant GtUF6CGT1 protein catalyzed direct C-glucosylation of flavone.•This reaction is different from C-glucosylation found in cereals and buckwheat. The major flavonoids accumulated in leaves of Japanese gentian (Gentiana triflora) were determined as isoorientin (luteolin 6-C-glucoside) and isoorientin 4′-glucoside. A cDNA (GtUF6CGT1) was isolated that encoded the UDP-glucose-dependent glucosyltransferase that is involved in C-glycosylflavone biosynthesis. The recombinant GtUF6CGT1 protein could transfer a glucose group to the C6 position of a flavone skeleton through C-linkage, using UDP-glucose as the glucosyl donor. These C-glycosylflavones also accumulated in petals. A good correlation was observed between GtUF6CGT1 expression and C-glycosylflavone accumulation in leaves and petals. GtUF6CGT1 is the first reported C-glucosyltransferase that mediates direct C-glucosylation of the flavone skeleton.