The Arabidopsis thaliana transcription factor BRASSINAZOLE-RESISTANT 1 (BZR1) interacts with the transcriptional corepressor TOPLESS (TPL) through the conserved ERF-associated amphiphilic repression (EAR) motif at the carboxy terminus of BZR1 whose deletion or mutation abolishes the ability of BZR1 to regulate gene expression and cell elongation