Molecular Mechanism Of Heat Shock Transcrip-Tion Factor HSFB2b In Negative Regulation Of Thermomorphogenesis In Arabidopsis

With the global climate change and the intensification of greenhouse effect,high temperature will continue to have an adverse impact on the growth and development of plants.Under extreme high temperature,short-term heat stress may lead to plant death,while warm temperature usually does not lead to plant death,but will affect the structure and development of plants.This process is called thermomorphogenesis.It is of great significance for agricultural production to study and clarify how plants respond to the increase of environmental temperature.Arabidopsis thermomorphogenesis has a complex regulatory network,in which PIF4(Phytochrome interacting factor 4)is the core positive regulator of thermomorphogenesis,which regulates biological processes such as auxin synthesis and response.Other plant hormones such as gibberellin,brassinosteroid and ethylene also play important roles in thermomorphogenesis in Arabidopsis.Recent study shows that jasmonic acid plays a negative regulatory role in regulating hypocotyl elongation in Arabidopsis.Under warm temperature conditions,plants reduce the content of j asm onic acid by up-regulating the expression of jasmonic acid catabolic genes JOX(Jasmonate-induced oxygenase)and ST2A(Sulfotransferase 2A),thereby promoting hypocotyl elongation.However,the underlying regulatory mechanism is still unclear.Heat shock transcription factors(HSFs)play important roles in stress response and are widely existed in eukaryotes.There are a large number of heat shock transcription factors in plants,which can be divided into class A,class B and class C.HSFA family mainly plays a positive regulatory role in response to high temperature,and HSFB family is considered to play an inhibitory role.The members of the plant heat shock transcription factor family have very similar structures.They have a conserved DNA-binding domain(DBD)at the N-terminus,which can recognize heat shock response elements(HSEs).Studies have shown that under extreme high temperature conditions,heat shock transcription factors regulate gene expression by recognizing HSE cis-acting elements presented on downstream gene promoters.HSF family is involved in plant heat shock responses at extreme high temperatures,but it remains unclear how they are involved in plant thermomorphogenesis.In this study,we found that the expression of heat shock transcription factor family members HSFB2a and HSFB2b were up-regulated by warm temperature(29℃)treatment.Protein subcellular localization study showed that both HSFB2a and HSFB2b proteins were localized in nucleus in tobacco cells.Sequence analysis revealed that HSFB2a and HSFB2b contain transcription repression domain,presumably functioning as transcriptional repressors.Under warm temperature conditions,hsfb2b showed a longer hypocotyl phenotype comparing to the wild-type,suggesting that it functions as a negative regulator in thermomorphogenesis.Genes regulated by HSFB2b at warm temperatures were identified by RNA-Seq analysis.qRT-PCR analysis showed that the expressions of heat shock proteins HSPs,heat shock transcription factor HSFA2,and jasmonic acid catabolic gene ST2A were up-regulated under warm temperature conditions compared with that under normal temperature conditions in wild-type plants.Interestingly,the expression levels of these genes were higher in hsfb2b-1 than in wild-type at warm temperature,indicating that HSFB2b regulates thermomorphogenesis by inhibiting the expression of these heat-responsive genes.By analyzing the ST2A promoter,we found that there was a heat shock response element on the promoter.The Effector-Reporter assay showed that HSFB2b inhibited the ST2A promoter activity.In conclusion,this study demonstrated the important role of HSFB2b in plant thermomorphogenesis.HSFB2b negatively regulates the expression of downstream genes,thereby probably inhibiting the degradation of jasmonic acid,which may negatively regulate thermomorphogenesis in Arabidopsis by increasing the level of j asmonic acid.This study further revealed the molecular mechanism of jasmonic acid in regulating plant thermomorphogenesis,enriching our understanding of plant thermomorphogenesis.