MYB Transcription Factors Regulate Secondary Metabolism

Secondary cell walls play important roles in providing long-distance water transport, mechanical support, and plant defense. The main components of secondary cell walls are cellulose, hemicellulose, and lignin. The synthesis of lignin monomers involves the phenylpropanoid pathway, which is also shared by many other metabolites such as suberin, flavonoids, tannins, and lignans. The aromatic amino acid Phe, the endproduct of the shikimate pathway, serves as the precursor of downstream phenylpropanoids. Lignin biosynthesis is a high-energy-consuming and irreversible process; therefore, it is critical that lignin deposition is tightly controlled to avoid overconsumption of Phe.. Geng et al. (10.1104/pp.19.01070) have uncovered a new level of complexity in the regulatory network underlying plant lignin biosynthesis. More specifically, they have discovered that the MYB transcription factors MYB20, MYB42, MYB43, and MYB85 are transcriptional regulators that directly activate lignin biosynthesis genes and Phe biosynthesis genes during secondary wall formation in Arabidopsis. Disruption of MYB20, MYB42, MYB43, and MYB85 resulted in growth development defects and substantial reductions in lignin biosynthesis. In addition, their data show that these MYB proteins directly activated transcriptional repressors that specifically inhibit flavonoid biosynthesis, which competes with lignin biosynthesis for Phe precursors. Together, these results provide important insights into the molecular framework for the lignin biosynthesis pathway