Molecular Mechanisms Of Brassinosteroids And Jasmonates In Antagonistically Regulating Arabidopsis Apical Hook Development

Apical hook of dicots can protect shoot apical meristem from mechanical damage during soil emergence.Hookless mutants show reduced emergence rate,which directly threaten the security of food production.Several phytohormones were found to regulate the development of apical hook,including brassinosteroids(BRs)and jasmonates(JAs).It was reported that defects of BR biosynthesis or signaling and exogenous application of Me JA can induce Arabidopsis apical hook disappearance and cotyledon opening.What is the relationship between BRs and JAs in regulating apical hook development is an outstanding question to explore.In this study,we studied the relationship between BRs and JAs in Arabidopsis apical hook development.We discovered that(1)exogenous Me JA supplementation induced apical hook disappearance and cotyledon opening of Arabidopsis wild type Col-0,however,simultaneous exogenous application of the most active BRs,brassinolide(BL),can partially rescue these phenotypes.Moreover,bin2-3 bil1 bil2 and bzr1-1D are insensitive to Me JA,indicating that enhancing BR signaling suppresses JAs-induced apical hook disappearance;(2)Exogenous application of a BR biosynthesis inhibitor,brassinazole(BRZ),induced apical hook disappearance and cotyledon opening of Col-0 and two JA insensitive mutants,coi1-2 and myc2.Moreover,double mutants dwf4-44 myc2 and bri1-701 myc2 showed disappeared apical hook and opened cotyledon,which is similar to the phenotypes of BR defects mutants dwf4-44 and bri1-701,indicating that repressing JA signaling does not affect BR defects in apical hook.These findings suggest that JAs regulate apical hook development via repressing BR signaling.A previous study showed that exogenous Me JA treatment can repress the expression of BR biosynthesis gene DWF4.Our question is whether such repression can induce apical hook disappearance.We then treated Arabidopsis with Me JA,and discovered that(3)Me JA can repress the expression of DWF4 in Col-0,but cannot repress the expression of DWF4 in myc2 myc3 myc4,indicating that JAs repress the expression of DWF4 via transcription factors,MYC2,MYC3 and MYC4.Moreover,transient promoter activity analysis in tabaco leaves showed that MYC2,MYC3 or MYC4 can indeed repress the transcription of DWF4.However,(4)we found that the apical hook and cotyledons of dwf4 RNAi transgenic plants(in which the DWF4 expression was reduced to the level equivalent or much less than the level when WS2 was treated with Me JA)are normal,suggesting that reducing the expression of DWF4 does not affect the development of apical hook.In addition,Me JA application cannot affect the phosphorylation level of BZR1.These findings suggest that JAs inducing apical hook disappearance is not primarily through repressing the expression of DWF4.To explore the molecular mechanisms of BRs and JAs in antagonistically regulating Arabidopsis apical hook development,we performed yeast two-hybrid,bimolecular fluorescence complementation and co-immunoprecipitation,and(5)we discovered that BZR1 can interact with MYC2 both in vivo and in vitro.Additional studies found that MYC2 can competitively repress the interaction between BZR1 and PIF4/PIF5,which then down-regulates the expression of a number of apical hook development related genes,such as,WAG2.In summary,we found that JAs repress apical hook development mainly via repressing the functions of BRs.On one hand,JAs repress the expression of DWF4 via MYC2,MYC3 and MYC4.More importantly,MYC2 competitively represses the interaction between BZR1 and PIFs,and negatively regulates the expression of their downstream responsive genes including the genes involved in apical hook development.