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

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 BZR-independent pathway that mediates BR signaling.In this study,we found that disruption of all BZRs in Arabidopsis generated a hextuple mutant(bzr-h)displaying vegetative growth phenotypes that were almost identical to those of the null mutant of three BR receptors,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),which encodes a transcription factor essential for anther locule development,was barely detectable in bzr-h,suggesting that BZRs regulate locule development by affecting SPL expression.Our findings reveal that BZRs are indispensable transcription factors required for both BR signaling and anther locule development,providing new insight into the molecular mechanisms underlying the microsporogenesis in Arabidopsis.

Brassinosteroids regulate outer ovule integument growth in part via the control of INNER NO OUTER by BRASSINOZOLE‐RESISTANT family transcription factors

Brassinosteroids(BRs)play important roles in regulating plant reproductive processes.BR signaling or BR biosynthesis null mutants do not produce seeds under natural conditions,but the molecular mechanism underlying this infertility is poorly understood.In this study,we report that outer integument growth and embryo sac development were impaired in the ovules of the Arabidopsis thaliana BR receptor null mutant bri1-116.Gene expression and RNA-seq analyses showed that the expression of INNER NO OUTER(INO),an essential regulator of outer integument growth,was significantly reduced in the bri1-116 mutant.Increased INO expression due to overexpression or increased transcriptional activity of BRASSINAZOLE-RESISTANT 1(BZR1)in the mutant alleviated the outer integument growth defect in bri1-116 ovules,suggesting that BRs regulate outer integument growth partially via BZR1-mediated transcriptional regulation of INO.Meanwhile,INO expression in bzr-h,a null mutant for all BZR1 family genes,was barely detectable;and the outer integument of bzr-h ovules had much more severe growth defects than those of the bri1-116 mutant.Together,our findings establish a new role for BRs in regulating ovule development and suggest that BZR1 family transcription factors might regulate outer integument growth through both BRI1-dependent and BRI1-independent pathways.