MADS-box BSISTER transcription factors up-regulate proanthocyanidin biosynthesis in grapevine

Proanthocyanidins (PAs) are a group of oligomeric flavonoids also known as condensed tannins. PAs in human diet have antioxidant and health-promoting effects. BSISTER (BS) promotes the accumulation of PAs, but its direct target gene and regulation mechanism in PAs biosynthesis are not clear. Here, we reported VviBS1 and VviBS2 genes, which bind directly to the promoter of PA synthesis key genes VviANR (ANTHOCYANIDIN REDUCTASE) and VviLAR (LEUCOANTHOCYANIDIN REDUCTASE) and up-regulate their expression to promote the PAs biosynthesis in grapevine. VviBS1 and VviBS2 partially rescued the transparent testa phenotype of an Arabidopsis tt16 mutant. Overexpression of BS1 and BS2 in grapevine callus increased the content of PAs, as well as the expression of specific genes related to PA synthesis. VviBS1 and VviBS2 proteins bound directly to the promoter regions of the key PA synthesis genes VviANR1, VviANR2, and VviLAR1. Overexpression of VviBS1 and VviBS2 promotes VviANR1, VviANR2 and VviLAR1 expression and PAs biosynthesis. Some BS1- and BS2-interacting MADS-box proteins have an effect on PAs biosynthesis. This study provides insight into the regulatory mechanisms of PAs biosynthesis in grapevine, which could be effectively employed for metabolic engineering to increase PA content.