| BES1 and BZR1 were originally identified as two key transcription factors specifically regulating brassinosteroid(BR)-mediated gene expression.They belong to a protein family consisting of six members,BES1,BZR1,BEH1,BEH2,BEH3,and BEH4.bes1 and bzr1 single mutants do not exhibit any characteristic BR phenotypes,suggesting functional redundancy of these proteins.In this study,by generating higher order mutants,we demonstrate that BES1 family members are key downstream transcription factors of the BRI1-BR-BAK1 pathway.During this process,we found some high-order mutants show sterile phenotypes.For example,a quintuple mutant,qui-1,shows no BR defective phenotypes but exhibits a male sterility phenotype.qui-1 was used to analyze the mechanism causing male sterility.Phenotypic analyses at organ,tissue,and cell levels found that the development of tapetum and microsporocytes is abnormal in the anthers of qui-1.As a result,the anthers of qui-1 are pollenless.Molecular and immunological analyses also revealed that tapetum specific genes are significanly down-regulated in qui-1.The fact that BES1 family genes are expressed in the inflorescence and their encoded proteins are localized in the anthers suggests that BES1 family members may be involved in the regulation of tapetum development in Arabidopsis.Our genetic and biochemical results demonstrate that BES1 family members also act as downstream transcription factors in the EMS1-TPD1-SERK1/2 pathway.Anthers from tpd1,ems1 and serk1/2 mutants lack a tapetum cell layer and cannot produce pollen grains,resulting in male sterility.Introducing bes1-D and bzr1-1D by genetic crossing can partially suppress the tapetum defects and male sterility of tpd1,ems1 and serk1/2.Ectopic expression of both TPD1 and EMS1 in bri1-116,a BR receptor null mutant,leads to the accumulation of non-phosphorylated active form of BES1,similar to the activation of BES1 by the BRI1-BR-BAK1 signaling pathway.In addition,the expression levels of tapetum specific genes such as DYT1,A6,and A9 are up-regulated in the backgrounds of tpd1,ems1 and serk1/2 after introducing bes1-D and bzr1-1D.Ch IP analyses showed that BES1 and BZR1 can bind to the promoters of DYT1 and other transcription factors downstream the EMS1-TPD1-SERK1/2 pathway.BES1 and BZR1 may regulate tapetum development via positively regulating the expression of a series of genes necessary for the development of tapetum.These data suggest that two distinctive receptor-like kinase-mediated signaling pathways share BES1 family members as downstream transcription factors to regulate different aspects of plant development. |
