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

Jong Hee Im,Won-Chan Kim,Kyung-Hwan Han,Brent Crain,Daniel Keathley,Jae-Heung Ko,Gregory P Copenhaver

doi:10.1371/journal.pgen.1009510

Abstract

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.

Highlights

Secondary cell walls, located between the plasma membrane and primary cell wall, are a defining feature of xylem fibers and vessels, providing mechanical support for plants and serving as a conduit for long-distance transport of water and solutes
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
We show that MYB46 is negatively regulated by mitogenactivated protein kinase 6 (MPK6) during salt stress, providing a novel insight into a mechanism by which plants incorporate environmental signals into differential regulation of secondary wall biosynthesis

Summary

Introduction

Secondary cell walls, located between the plasma membrane and primary cell wall, are a defining feature of xylem fibers and vessels, providing mechanical support for plants and serving as a conduit for long-distance transport of water and solutes They constitute the majority of plant biomass and are of economic importance to humans as fiber, animal feed, pulp for manufacture of paper, and as an environmentally desirable, cost-effective, renewable source of energy. The biosynthesis of secondary walls occurs in a highly-coordinated manner by successive deposition of cellulose fibrils, hemicelluloses and lignin upon cessation of cell growth [1,2,3]. This process requires a coordinated transcriptional activation of the biosynthetic genes for the components, suggesting the existence of one or more central transcriptional regulators. How does the plant that is undergoing stress-induced upregulation of MYB46 transcription manage to suppress MYB46 function in cells where secondary wall formation is not needed? How does the plant incorporate extracellular (e.g., environmental, positional) signals into the condition-specific activity of MYB46? Little is known about the mechanism regulating this condition-specific activity of MYB46

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Discussion

Conclusion