The LAP1 MYB transcription factor orchestrates anthocyanidin biosynthesis and glycosylation in Medicago

MYB transcription factors help to control anthocyanin biosynthesis in plants, and ectopic expression of the Arabidopsis Production of Anthocyanin Pigment 1 (PAP1) transcription factor activates the anthocyanin pathway in tobacco, suggesting the general utility of such factors for metabolic engineering of anthocyanins and anthocyanin-derived compounds such as proanthocyanidins (condensed tannins). However, PAP1 does not activate anthocyanin biosynthesis in the model legume Medicago truncatula or in alfalfa (Medicago sativa). A related Legume Anthocyanin Production 1 (LAP1) gene was identified from the genome of M. truncatula. When constitutively expressed in transgenic alfalfa, M. truncatula or white clover, LAP1 induced massive accumulation of anthocyanin pigments comprising multiple glycosidic conjugates of cyanidin. Oligomeric/polymeric compounds with some diagnostic characteristics of proanthocyanidins also accumulated in LAP1-expressing plants, but these compounds were not composed of (epi)catechin units. Over 260 and 70 genes were up-regulated in leaves of alfalfa or M. truncatula, respectively, in response to constitutive expression of LAP1, many of which are involved in anthocyanin biosynthesis. In particular, the glucosyltransferase UGT78G1, previously identified as showing preference for isoflavonoid substrates in vitro, was strongly up-regulated by LAP1, and appears to function as an anthocyanin glycosyltransferase in vivo. Over-expression of UGT78G1 in transgenic alfalfa resulted in increased anthocyanin accumulation when plants were exposed to abiotic stress.