MYB20, MYB42, MYB43 and MYB85 Regulate Phenylalanine and Lignin Biosynthesis during Secondary Cell Wall Formation in Arabidopsis

Lignin is a phenylpropanoid-derived polymer that functions as a major component in plant vascular tissues. The aromatic amino acid phenylalanine biosynthesis provides precursor for many secondary metabolites including lignin and flavonoids. Here we discovered MYB transcription factors MYB20, MYB42, MYB43 and MYB85 are transcriptional regulators directly activating lignin biosynthetic genes and phenylalanine biosynthetic genes during secondary wall formation in Arabidopsis thaliana. Disruption of MYB20, MYB42, MYB43 and MYB85 resulted in growth development defects and significant reduction of lignin biosynthesis. In addition, our data showed that these MYB proteins directly activated the transcriptional repressors specifically inhibiting flavonoids biosynthesis that is competing with lignin biosynthesis for phenylalanine as precursors. Together, our results provide important insights into the molecular framework for lignin biosynthesis pathway.