Abstract
Wood is formed by the successive addition of secondary xylem, which consists of cells with a conspicuously thickened secondary wall composed mainly of cellulose, xylan and lignin. Currently, few transcription factors involved in the direct regulation of secondary wall biosynthesis have been characterized in tree species. Here, we show that PdMYB221, a poplar ortholog of the Arabidopsis R2R3-MYB transcription factor AtMYB4, directly regulates secondary wall biosynthesis during wood formation. PdMYB221 is predominantly expressed in cells of developing wood, and the protein it encodes localizes to the nucleus and acts as a transcriptional repressor. Ectopic expression of PdMYB221 resulted in reduced cell wall thicknesses of fibers and vessels in Arabidopsis inflorescence stems. The amounts of cellulose, xylose, and lignin were decreased and the expression of key genes synthesizing the three components was suppressed in PdMYB221 overexpression plants. Transcriptional activation assays showed that PdMYB221 repressed the promoters of poplar PdCESA7/8, PdGT47C, PdCOMT2 and PdCCR1. Electrophoretic mobility shift assays revealed that PdMYB221 bound directly to the PdCESA8, PdGT47C, and PdCOMT2 promoters. Together, our results suggest that PdMYB221 may be involved in the negative regulation of secondary wall formation through the direct and indirect suppression of the gene expression of secondary wall biosynthesis.
Highlights
We investigated the role of PdMYB221, a poplar ortholog of Arabidopsis MYB4, in secondary wall formation, and determined its targets involved in cellulose, xylan and lignin biosynthesis
PdMYB156 and PdMYB221 fell into the group containing Arabidopsis AtMYB4 and alfalfa Medtr4g073420.1, suggesting that these proteins are more evolutionary conserved within dicots than in monocots, moss or green algae
We provide evidence showing that poplar PdMYB221 may be involved in the negative regulation of secondary wall formation in Arabidopsis
Summary
PtrMYB2, PtrMYB3, PtrMYB20 and PtrMYB21 are direct targets of PtrWNDs and share similar functions to their Arabidopsis orthologs MYB46 and MYB836,7 These PtrMYBs positively regulate secondary wall formation and have the abilities of activating secondary wall biosynthetic pathways. Arabidopsis MYB58, MYB63 and switchgrass PvMYB4, three lignin-specific regulators, have been shown to have the abilities of binding to the AC elements of the promoters of some lignin synthetic genes to directly activate the expression of these genes[12,13,14]. We investigated the role of PdMYB221, a poplar ortholog of Arabidopsis MYB4, in secondary wall formation, and determined its targets involved in cellulose, xylan and lignin biosynthesis. Our results suggest that poplar PdMYB221 may be involved in the negative regulation of secondary wall biosynthesis through the direct and indirect suppression of the expression of secondary wall biosynthetic genes
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