Acyl-glucose-dependent glucosyltransferase catalyzes the final step of anthocyanin formation in Arabidopsis

The biosynthetic pathways that produce anthocyanins, the principal pigments for flower and leaf coloration in plants, have been extensively investigated. As a result, many of the enzymes involved in these pathways have been identified. Here, we make use of an inducible Arabidopsis thaliana system and demonstrate that the final step in the formation of the major anthocyanin molecule occurs via a glucosylation step catalyzed by acyl-glucose-dependent anthocyanin glucosyltransferase (AAGT). The glucosylation occurs at the 4-coumarate moiety of the anthocyanin molecule cyanidin 3-O-[2″-O-(2′″-O-(sinapoyl) xylosyl) 6″-O-(p-coumaroyl) glucoside] 5-O-[6″″-O-(malonyl) glucoside] leading to completion of the main anthocyanin structure, a reaction that has not previously been identified in studies of Arabidopsis anthocyanins. Earlier studies on flower AAGTs showed that they conjugate a glucose directly to the basic skeleton of anthocyanin. The present study provides the first evidence that an AAGT of Arabidopsis can conjugate a glucose to an acyl moiety of an anthocyanin modified with sugars and organic acids. The results from analyses of gene expression and of anthocyanin composition in a knock-out (KO) mutant and from a complementation test indicate that AtBGLU10 might encode this AAGT.