Mitogen-activated protein kinase 6 negatively regulates secondary wall biosynthesis by modulating MYB46 protein stability in Arabidopsis thaliana

The R2R3-MYB transcription factor MYB46 functions as a master switch for secondary cell wall biosynthesis, ensuring the exquisite expression of the secondary wall biosynthetic genes in the tissues where secondary walls are critical for growth and development. At the same time, suppression of its function is needed when/where formation of secondary walls is not desirable. Little is known about how this opposing control of secondary cell wall formation is achieved. We used both transient and transgenic expression of MYB46 and mitogen-activated protein kinase 6 (MPK6) to investigate the molecular mechanism of the post-translational regulation of MYB46. We show that MYB46 is phosphorylated by MPK6, leading to site specific phosphorylation-dependent degradation of MYB46 by the ubiquitin-mediated proteasome pathway. In addition, the MPK6-mediated MYB46 phosphorylation was found to regulate in planta secondary wall forming function of MYB46. Furthermore, we provide experimental evidences that MYB83, a paralog of MYB46, is not regulated by MPK6. The coupling of MPK signaling to MYB46 function provides insights into the tissue- and/or condition-specific activity of MYB46 for secondary wall biosynthesis. Author summary Secondary cell walls are critical for plant growth and of economic importance to humans as fiber, pulp for paper, and as a renewable source of energy. A master switch for secondary wall biosynthesis, MYB46, ensures the exquisite expression of the biosynthetic genes in the tissues where secondary walls are critical. However, suppression of secondary wall formation is needed during impermanent cessation of vegetative growth triggered by environmental stresses. Although the expression of MYB46 is upregulated by abiotic stresses, secondary wall formation may not occur. How is this opposing control achieved? The mitogen-activated protein kinase (MPK) cascade is among the most conserved signal transduction systems in eukaryotes and plays a crucial role in the regulation of biochemical and physiological changes associated with environmental stimuli and phytohormones. In this report, we show that MYB46 is negatively regulated by MPK6 during salt stress, providing a novel insight into a mechanism by which plants incorporate environmental signals into differential regulation of secondary wall biosynthesis.

Automated summary

MYB46 acts as a master switch for secondary cell wall biosynthesis, ensuring precise gene expression in critical tissues but being suppressed where secondary wall formation is not desired. The negative regulation of MYB46 by MPK6 during salt stress reveals a novel mechanism for plants to incorporate environmental signals into differential regulation of secondary wall biosynthesis.