BZR1 Family Transcription Factors Function Redundantly and Indispensably in BR Signaling but Exhibit BRI1-Independent Function in Regulating Anther Development in Arabidopsis

Abstract BRASSINAZOLE RESISTANT 1 family proteins (BZRs) are central transcription factors that govern brassinosteroid (BR)-regulated gene expression and plant growth. However, it is unclear whether there exists a BZRs-independent pathway that mediates BR signaling. In this study, we found that knocking out the expression of all BZR s in Arabidopsis generated a hextuple mutant ( bzr-h ) displays vegetative growth phenotypes that were almost identical to those of the BR receptors null mutant bri1brl1brl3 ( bri-t ). By RNA sequencing, we found that global gene expression in bzr-h was unaffected by 2 h of BR treatment. The anthers of bzr-h plants were loculeless, but a similar phenotype was not observed in bri-t , suggesting that BZRs have a BR signaling-independent regulatory role in anther development. By real-time PCR and in situ hybridization, we found that the expression of SPOROCYTELESS ( SPL ), a transcription factor gene that is essential for anther locule development, was barely detectable in the bzr-h mutant, suggesting that BZRs regulate locule development by regulating SPL expression. Together, our findings not only demonstrate that BZRs are indispensable transcription factors for the regulation of both BR signaling and anther locule development, but also provide new insight into the molecular mechanisms that control microsporogenesis process in Arabidopsis .