Complex Regulation of Proanthocyanidin Biosynthesis in Plants by R2R3 MYB Activators and Repressors

MYB transcription factors are central players in the regulation of phenylpropanoid and flavonoid metabolism. In proanthocyanidin and anthocyanin biosynthesis, MYBs act within a MBW (MYB-bHLH-WD40) complex. However, the MYB provides the specificity and ultimately determines where in the plant these pathways are activated. While work on the transcriptional regulation of proanthocyanidin biosynthesis in Arabidopsis seed coat has been instrumental in the discovery of MYB transcription factors, research in other plants such as grapevine and poplar has also made substantial contributions. In these systems, several types of MYB transcriptional activators of proanthocyanidin biosynthesis in poplar have been described, which in turn induce the expression of MYB repressors. The role of the MYB repressors may be to help modulate stress-induced PA synthesis, or to fine-tune flavonoid and phenylpropanoid metabolism. Characterizing both types of MYB genes via transgenic overexpression and promoter activation assays in transiently transformed cells has allowed us to construct a model for this gene regulatory network of MYB activators and repressors for regulation of proanthocyanidins in poplar. A long-term aim is to understand the mechanisms of repressor action, and to identify the linkages to upstream signals or environmental stimuli that induce proanthocyanidin accumulation.