Regulation of flavonoid biosynthesis involves an unexpected complex transcriptional regulation of <i><scp>TT</scp>8</i> expression, in <scp>A</scp>rabidopsis

<jats:title>Summary</jats:title><jats:p> <jats:list list-type="bullet"> <jats:list-item><jats:p><jats:italic><jats:styled-content style="fixed-case">TT</jats:styled-content>8</jats:italic>/<jats:italic>b<jats:styled-content style="fixed-case">HLH</jats:styled-content>042</jats:italic> is a key regulator of anthocyanins and proanthocyanidins (<jats:styled-content style="fixed-case">PA</jats:styled-content>s) biosynthesis in <jats:italic><jats:styled-content style="fixed-case">A</jats:styled-content>rabidopsis thaliana</jats:italic>. <jats:styled-content style="fixed-case">TT</jats:styled-content>8 transcriptional activity has been studied extensively, and relies on its ability to form, with several <jats:styled-content style="fixed-case">R</jats:styled-content>2<jats:styled-content style="fixed-case">R</jats:styled-content>3‐<jats:styled-content style="fixed-case">MYB</jats:styled-content> and <jats:styled-content style="fixed-case">TTG</jats:styled-content>1 (<jats:styled-content style="fixed-case">WD</jats:styled-content>‐<jats:styled-content style="fixed-case">R</jats:styled-content>epeat protein), different <jats:styled-content style="fixed-case">MYB</jats:styled-content>‐<jats:styled-content style="fixed-case">bHLH</jats:styled-content>‐<jats:styled-content style="fixed-case">WDR</jats:styled-content> (<jats:styled-content style="fixed-case">MBW</jats:styled-content>) protein complexes. By contrast, little is known on how <jats:italic><jats:styled-content style="fixed-case">TT</jats:styled-content>8</jats:italic> expression is itself regulated.</jats:p></jats:list-item> <jats:list-item><jats:p>Transcriptional regulation of <jats:italic><jats:styled-content style="fixed-case">TT</jats:styled-content>8</jats:italic> expression was studied using molecular, genetic and biochemical approaches.</jats:p></jats:list-item> <jats:list-item><jats:p>Functional dissection of the <jats:italic><jats:styled-content style="fixed-case">TT</jats:styled-content>8</jats:italic> promoter revealed its modular structure. Two modules were found to specifically drive <jats:italic><jats:styled-content style="fixed-case">TT</jats:styled-content>8</jats:italic> promoter activity in <jats:styled-content style="fixed-case">PA</jats:styled-content>‐ 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 <jats:styled-content style="fixed-case">MBW</jats:styled-content> complexes, and an unpredicted positive feedback regulatory loop between <jats:italic><jats:styled-content style="fixed-case">TT</jats:styled-content>8</jats:italic> and <jats:italic><jats:styled-content style="fixed-case">TTG</jats:styled-content>2</jats:italic>. Moreover, the results suggest that some putative new regulators remain to be discovered. Finally, specific <jats:italic>cis</jats:italic>‐regulatory elements through which <jats:italic><jats:styled-content style="fixed-case">TT</jats:styled-content>8</jats:italic> expression is regulated were identified and characterized.</jats:p></jats:list-item> <jats:list-item><jats:p>Together, these results provide a molecular model consistent with the specific and highly regulated expression of <jats:italic><jats:styled-content style="fixed-case">TT</jats:styled-content>8</jats:italic>. They shed new light into the transcriptional regulation of flavonoid biosynthesis and provide new clues and tools for further investigation in <jats:styled-content style="fixed-case">A</jats:styled-content>rabidopsis and other plant species.</jats:p></jats:list-item> </jats:list> </jats:p>