Exploring the transcriptional network regulating secondary cell wall and wood formation
McCarthy, Ryan Lehman
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Secondary cell wall formation is essential to plant mechanical structure and vascular system function. The evolution of the plant vascular system allowed for efficient water transport and was an essential step in the colonization of land by plants. Wood, composed of cells with roles in water transport and mechanical support, is primarily composed of secondary cells walls and is a material with a dense concentration of biopolymers that can be processed to produce liquid biofuels. By better understanding the process of secondary cell wall formation we can not only improve our understanding of land plant evolution but also develop knowledge and tools for engineering enhanced bioenergy crops for biofuel production. Secondary cell wall formation in Arabidopsis is transcriptionally regulated by a set of homologous secondary wall NAC (SWN) domain proteins that act as master regulators capable of activating the entire pathway. Target gene activation by SWNs is mediated through their binding to secondary wall NAC binding elements (SNBEs). MYB46 has been identified as a master regulator of secondary cell wall formation and a direct target of the SWNs. Here we identify the transcription factor MYB83 as a new master regulator of secondary cell wall formation that functions redundantly with MYB46 as a direct target of the SWNs. Double T-DNA knockout of MYB46 and MYB83 blocked secondary cell wall formation indicating that these genes are necessary for activation of secondary cell wall formation. Expanding our findings to the bioenergy crop poplar we identified PtrMYB3 and PtrMYB20 as functional orthologs of MYB46 and MYB83 with a conserved role as master regulators of secondary cell wall formation indicating an evolutionary conservation of the transcriptional network. To further investigate this evolutionary conservation we analyzed SNBE site conservation and distribution across multiple plant genomes. All vascular plants included in the analysis demonstrated a significantly enriched concentration of SNBE sites in promoters of secondary cell wall related genes. Together, these results identify new master regulatory proteins controlling secondary cell wall formation and demonstrate evidence that the transcriptional network regulating secondary cell wall formation is well conserved in the vascular plant lineage.