Redox Regulates The Activity Of BZR1 To Control Root Development In Arabidopsis

As sessile organisms,plants have to effectively integrate multiple environmental challenges to adjust their growth and development,in which the maintenance of plant stem cells function plays an important role.Brassinosteroids(BRs)are endogenous plant hormones that are essential for the proper regulation of the root growth.BR-deficient or BR-insensitive mutants display significantly inhibition in root development.Recent studies have found that BR affects the root growth and development by regulating the QC division,but the specific mechanism is still unclear.In this study,we found that BR induces the H2O2 accumulation in the root tip through a BRII-dependent manner.Inhibition of H2O2 production by chemicals treatment or genetic mutation compromised the BR-induced QC division.To investigate how H2O2 is involved in BR signaling pathway,we analyzed the response of BR related mutants to H2O2 scavengers.The result showed that the loss-of-function mutant bin2/bil2/bil3 exhibited higher frequency of QC divisions than that of wild type plant,while that phenotype was repressed by the treatment of DPI.BIN2 has 10 homologous genes,and at least 7 homolog genes are involved in BR signaling.To test whether H2O2 regulates BR signal transduction by targeting BIN2 and its homologs,bikinin and LiCL,the specific GSK3 kinase inhibitors,were used to completely reduce the activity of BIN2 family proteins.The results showed that bikinin or LiCl treatment significantly promoted the division of QC cells,but DPI and KI counter this promotion.Furthermore,chemicals scavengering H2O2 also inhibited the division of QC cells in gain-of function mutants bzr1-1D and bes1-D plants that showed higher QC division phenotype,suggesting that ROS regulate the division of QC cells by regulating BZR1 activity.To investigate how H2O2 regulates the activity of BZR1,we screened the interacting proteins of BZR1,and identified THIOREDOXIN H-TYPE 5(TRXh5)as a redox mediator to catalyze the oxidative BZR1.Overexpression of TRXh5 represses the promotion of BR for QC division and cell elongation by inhibiting the transcription activity of BZR1.In summary,in this study,a series of experimental data demonstrated that BR induce the H2O2 accumulation in the root tip cell through a BRI1-dependent pathway;H2O2 regulates BR signal transduction by acting on the transcriptional factor BZR1;and that thioredoxin TRXh5 interacts with BZR1 to catalyze the conversion of BZR1 from oxidized state to reduced state,which in turn inhibits the activity of BZR1.These studies have initially elucidated the molecular mechanism of that BR and H2O2 crosstalk to regulate the meristem cell activity in root tip.All these provide us solid theoretical basis to understand how plants respond to different signals to regulate plant growth and development.