The R2R3 MYB transcription factor MYB189 negatively regulates secondary cell wall biosynthesis in Populus.

Abstract

Secondary cell wall biosynthesis during wood formation in trees is controlled by a multilevel regulatory network that coordinates the expression of substantial genes. However, few transcription factors involved in the negative regulation of secondary wall biosynthesis have been characterized in tree species. In this study, we isolated an R2R3 MYB transcription factor MYB189 from Populus trichocarpa, which is expressed predominantly in secondary vascular tissues, especially in the xylem. A novel repression motif was identified in the C-terminal region of MYB189, which indicates this factor was a transcriptional repressor. Overexpression of MYB189 in Arabidopsis and poplar resulted in a significant reduction in the contents of lignin, cellulose and hemicelluloses. Vascular development in stems of MYB189 overexpression lines was markedly inhibited, leading to a dramatic decrease in secondary cell wall thickness of xylem fibres. Gene expression analyses showed that most of the structural genes involved in the biosynthesis of lignin, cellulose and xylans were significantly down-regulated in MYB189-overexpressing poplars compared with the wild-type control. ChIP-qPCR and transient expression assays revealed that MYB189 could directly bind to the promoters of secondary wall biosynthetic genes to repress their expression. Together, these data suggest that MYB189 acts as a repressor to regulate secondary cell wall biosynthesis in poplar.