Identification Of Novel Genes Regulating Brassinosteroid-mediated Arabidopsis Growth And Development

Brassinosteroids(BRs),as the sixth class of phytohormones,regulate multiple processes of plant growth and development.To identify additional components regulating BR signaling pathway,an activation tagging genetic screen was carried out in bril-9,a weak allele of the brassinosteroid receptor gene.Eleven genetic modifiers either suppressing or enhancing the phenotype of bril-9 were isolated.Of which,several genes were cloned.Except for BRL1,all the other genes,including PLT2,SPL,HDG3 and several undefined genes,have not yet been reported to associate with BR signaling.Three novel genes,9S1,HYLIST1 and CUR70 were selected for further in-depth characterization.Preliminary results indicated that 9S1,HYLISTI and CUR70 play important roles in BR,microRNA and auxin signaling pathways,respectively.Overexpression of 9S1,a nuclear-localized gene,suppresses multiple developmental defects of bril-9,likely due to its direct involvement in the BR signal transduction.Constitutive overexpres-sion of 9S1 driven by a CaMV35S promoter results in an early senescence phenotype,indicating that 9S1 may mediate the interaction between the BR signaling and cell-death control pathways.HYLIST1 affects the microRNA process to regulate the translation of AP2 family and the transcription of CUCs,so as to influence the development of leaves,flowers and other organs.It co-regulates a few genes with BRs.The phenotype of 9s3-1D is mainly caused by increasing CUR70 expression.A CUR70 constitutive overexpression mutant(CUR70-OX)displays an auxin-deficient phenotype and reduced sensitivity to BL and auxin.Physiological analysis indicated that auxin signal transduction is partially blocked in CUR70-OX.BR insensitivity of CUR70-OX may be resulted from the crosstalk between auxin and BR.From the data we have collected,we hypothesize that CUR70,an E3 ubiquitin ligase,may mediate the degradation of ARFs(AUXIN RESPONSE FACTORS).