Flavanone Glycoside Biosynthesis in Citrus: Chalcone Synthase, UDP-Glucose:Flavanone-7-O-Glucosyl-Transferase and -Rhamnosyl-Transferase Activities in Cell-Free Extracts

Previous indirect evidence suggested that the biosynthesis of flavonoids in Citrus may not proceed via the usual chalcone synthase reaction and that glycosylation occurs during chalcone formation and not afterward, as has been reported in other species. We detected chalcone-synthase and UDP-glucose:flavanone-7-O-glucosyl-transferase activities in cell-free extracts of Citrus. The glucosylated flavanone was further rhamnosylated when exogenous UDP-glucose and NADPH were added to the extract. Chalcone-synthase activity was detected in cell-free extracts derived from young leaves and fruits. Young fruits (2 millimeter diameter) had the highest chalcone synthase activity. UDP-glucose:flavanone-7-O-glucosyl-transferase activity was measured in cell-free extracts derived from young leaves and fruits of Citrus mitis and Citrus maxima. The highest UDP-glucose:flavanone-7-O-glucosyl-transferase activity was found in young C. maxima leaves. These data indicate that Citrus contains a flavonoid pathway similar to that studied in other species.