Molecular Mechanism Of BZR1-EIN3 Transcription Complex Integrating Brassinolide And Ethylene Signaling To Regulate Plant Growth And Development

The high level of developmental plasticity in plants relies on the regulatory networks of interconnected signal transduction pathways that integrate a wide range of internal and environmental signal.Brassinosteroid(BR)and ethylene(ET)have overlapping functions in a wide range of developmental processes.BR and ET treatment both promote apical hook development in the dark,hypocotyl elongation under light,root growth,stomatal closure,leaf senescence,fruit ripening and salt tolerance.BR and ET have been reported to regulate plant growth and development through controlling each other's biosynthesis.BR could control the biosynthesis of ET through transcription factors BZR1 and BES1.BZR1 and BES1 directly bind to the promoters of several ACS genes to regulate their expression.ET has also been reported to regulate the biosynthesis of BR and establishes a gradient of BR in the apical hook region by regulating the expression pattern of CPD,which encodes a key enzyme in BR synthesis.However,the relationship between the BR and ET signalling pathways has remained unclear.Here,we demonstrated that BR and ET co-regulate apical hook development in the dark and hypocotyl elongation under light partially through the interactions between components of the two signalling pathways.1.BR promotes apical hook development by activated BZR1 and its homologue proteins.The materials with the enhanced BR activities display the exaggerated hook curvature,whereas the materials with the reduced BR activities show the decreased apical hook curvature compared with wild-type plants.The BZRs hextuple mutant(bzr-h)exhibits a nearly hookless phenotype and BL couldn't restore the apical hook of bzr-h.The cells of the convex side of the hook in bri1-116 are significantly shorter than that of the wild-type control,and this difference is suppressed by bzr1-1D.Analysis of the kinetics of hook curvature show that apical hook formation ceased prematurely and proceeded into hook opening without a maintenance phase in bri1-116,while bzr1-1D recovers the formation and maintenance process of bri1-116.2.Genetic analysis show that BR signalling is required for ET promotion of apical hook development.The effect of ACC and EIN3 to promote apical hook formation is impaired in the presence of PPZ and bril-116 mutant.ET quantitatively enhances BR-potentiated growth.Our results show that partially restore the hookless phenotypes of ein3 eill and ein2-5 mutants.The ein2-5 and ein3 eill mutants display a reduced BR sensitivity and EIN3-Ox show increased BR sensitivity on apical hook development.3.BZR1 physically interacts with EIN3 to co-regulate numbers of common target genes that mediate plant growth,development and stress responses.4.BZR1 and EIN3 co-operatively induce the expression of HOOKLESSI to promote apical hook development in the dark.BR promote the apical hook formation depending on HLS1.5.BR promote the synthesis of active gibberellin(GA)in the apical hook and the degradation of RGA protein.BR promotion of hook development requires GA,and GA restore the hookless phenotype of BR-deficient materials by activating EIN3/EIL1.6.PRE-IBH1-HBI1 module is involved in ET-mediated cell elongation.BZR1 and EIN3 co-regulate the expression of PACLOBUTRAZOL RESISTANCE FACTORS(PREs)and hypocotyl elongation under light.In summary,this paper shed light on the molecular mechanism underlying the regulation of plant development by BR,ET and GA signals through the direct interaction of master transcriptional regulators.