Regulation of flavonoid biosynthesis involves an unexpected complex transcriptional regulation of TT8 expression, in Arabidopsis

Summary TT 8 / b HLH 042 is a key regulator of anthocyanins and proanthocyanidins ( PA s) biosynthesis in A rabidopsis thaliana . TT 8 transcriptional activity has been studied extensively, and relies on its ability to form, with several R 2 R 3‐ MYB and TTG 1 ( WD ‐ R epeat protein), different MYB ‐ bHLH ‐ WDR ( MBW ) protein complexes. By contrast, little is known on how TT 8 expression is itself regulated. Transcriptional regulation of TT 8 expression was studied using molecular, genetic and biochemical approaches. Functional dissection of the TT 8 promoter revealed its modular structure. Two modules were found to specifically drive TT 8 promoter activity in PA ‐ and anthocyanin‐accumulating cells, by differentially integrating the signals issued from different regulators, in a spatio‐temporal manner. Interestingly, this regulation involves at least six different MBW complexes, and an unpredicted positive feedback regulatory loop between TT 8 and TTG 2 . Moreover, the results suggest that some putative new regulators remain to be discovered. Finally, specific cis ‐regulatory elements through which TT 8 expression is regulated were identified and characterized. Together, these results provide a molecular model consistent with the specific and highly regulated expression of TT 8 . They shed new light into the transcriptional regulation of flavonoid biosynthesis and provide new clues and tools for further investigation in A rabidopsis and other plant species.