Identification of direct targets of transcription factor MYB46 provides insights into the transcriptional regulation of secondary wall biosynthesis

Secondary wall formation requires coordinated transcriptional regulation of the genes involved in the biosynthesis of the components of secondary wall. Transcription factor (TF) MYB46 (At5g12870) has been shown to function as a central regulator for secondary wall formation in Arabidopsis thaliana, activating biosynthetic genes as well as the TFs involved in the pathways. Recently, we reported that MYB46 directly regulates secondary wall-associated cellulose synthase (CESA4, CESA7, and CESA8) and a mannan synthase (CSLA9) genes. However, it is not known whether MYB46 directly activates the biosynthetic genes for hemicellulose and lignin, which are the other two major components of secondary wall. Based on the observations that the promoter regions of many of the secondary wall biosynthetic genes contain MYB46-binding cis-regulatory motif(s), we hypothesized that MYB46 directly regulates the genes involved in the biosynthesis of the secondary wall components. In this report, we describe several lines of experimental evidence in support of the hypothesis. Electrophoretic mobility shift assay and chromatin immunoprecipitation analysis showed that MYB46 directly binds to the promoters of 13 genes involved in lignin and xylan biosynthesis. We then used steroid receptor-based inducible activation system to confirm that MYB46 directly activates the transcription of the xylan and lignin biosynthetic genes. Furthermore, ectopic up-regulation of MYB46 resulted in a significant increase in xylose and a small increase in lignin content based on acetyl bromide soluble lignin measurements in Arabidopsis. Taken together, we conclude that MYB46 function as a central and direct regulator of the genes involved in the biosynthesis of all three major secondary wall components.