TOPLESS mediates brassinosteroid-induced transcriptional repression through interaction with BZR1.

Brassinosteroid (BR) regulates plant development by activating the transcription factor brassinazole resistant 1 (BZR1), which activates and represses different target genes to switch cellular programmes. The mechanisms that determine BZR1's transcriptional activities remain largely unknown. Here we show that BZR1 represses target genes by recruiting the Groucho/TUP1-like transcriptional corepressor TOPLESS (TPL). Specific deletion or mutation of an evolutionarily conserved ERF-associated amphiphilic repression (EAR) motif at the carboxy terminus abolishes BZR1's abilities to regulate gene expression and cell elongation, but these defects are rescued by TPL fusion to the EAR motif-mutated BZR1. The EAR motif in BZR1 mediates recruitment of TPL to BZR1-repressed promoters. A triple tpl mutant (tpl;tpr1;tpr4) shows reduced BR sensitivity and suppresses the gain-of-function bzr1-1D mutant phenotype. BR repression of gene expression also requires histone deacetylases that interact with TPL. Our study demonstrates key roles of the EAR motif and TPL in BR regulation of gene expression and plant growth.

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