Plant Steroid hormones, Brassinosteroids (BRs), play important roles in plant growth, development and responses to various stresses. BR signal through receptor BRI1 and BAK1 and a series signaling intermediates to control the activities of BES1/BZR1 family transcription factors that control the expression of thousands of genes, half of which are induced and the other half repressed by BR. While BES1 is known to activate BR-induced genes by itself or cooperating with co-activators, such as transcription factors, histone modification enzymes and transcription elongation factors, how BES1 mediates the BR-repressed gene expression is not known. In chapter Ⅱ, MYBL2, a small MYB family transcription repressor, was found to interact with BES1 to down-regulate BR-repressed gene expression. The loss-of-function mybl2 mutant enhances the phenotype of a weak allele of bri1 and suppresses the constitutive BR-response phenotype of bes1-D, suggesting that suppression of BR-repressed gene expression is required for optimal BR response. Moreover, MYBL2 is a substrate of GSK3-like kinase BIN2, a negative regulator functioning in inhibiting the activities of BES1/BZR1 through its phosphorylation in BR pathway. Unlike BIN2 phosphorylation of BES1/BZR1 leading to protein degradation, BIN2 phosphorylation stabilizes MYBL2, which demonstrated a dual role of BIN2 phosphorylation in BR pathway, similar to the function of GSK3 in WNT signaling pathway. Our results thus establish the mechanisms for BR-repressed gene expression and the integration of BR signaling and BR transcriptional network.

In addition to promote the growth, BRs are known to be involved in drought response, but the mechanism of interactions between these two pathways remains to be established. In chapter Ⅲ, the NAC family transcription factor RD26 and its close homologs mediate crosstalk between drought and BR signaling pathway. RD26 is a direct target of BES1 and functions to inhibit BR-regulated growth as overexpression of RD26 leads to decreased plant growth and knockout of RD26 and its close homologs results in increased BR response. Global gene expression analysis revealed that RD26 modulates BR-regulated gene expression in a complex way. RD26 represses many BR-induced genes including BR-activated cell elongation genes and activates many BR-repressed genes, thereby inhibiting BR functions. On the other hand, BR signaling also inhibits drought responses through repressing the expression of RD26, its homologs and RD26-mediated drought-induced genes. The reciprocal inhibitory effects of BES1 and RD26 are mediated by their interactions on different promoter elements. This mechanism ensures that BR-induced plant growth is inhibited under drought condition that induced RD26 expression, while this mechanism also prevents unnecessary activation of drought response when plants undergo BR-induced growth, during which BES1 accumulates. Our results thus revealed a previously unknown mechanism coordinating plant growth and drought tolerance.