The Molecular Mechanisms Of OsbZIP46 In Drought Response Of Rice

Rice(Oryza sativa L.)is one of the most important food crops in the world.In recent years,the scarcity of fresh water resources has severely restricted the growth and yield of rice,which severerly threatens the food security in the world.Many scientists devote themselves to study the molecular mechanisms of abiotic stress resistance and provide new ideas for genetic improvement.There are two bZIP transcription factors,OsbZIP23 and OsbZIP46,have been charactered as two positive regulators in abscisic acid(ABA)signaling and drought response.Although they are homologous proteins,there have a few differences.Overexpression of the native OsbZIP46 did not improve the drought resistance,but overexpression of the constitutive OsbZIP46CAI(remove the suppressive domain D)significantly improve the drought resistance.In this study,we anylazed two interaction proteins of OsbZIP46 and revealed the regulatory mechanisms of the bZIP protein in drought response of rice.MODD(Mediator of OsbZIP46 Deactivation and Degradation)is one of the OsbZIP46 interaction proteins,and the interaction depends on the domain D of OsbZIP46.MODD was induced by ABA and drought stress.The overexpression lines decreased the ABA sensitivity and drought resistance.Contrary to the phenotypes of the MODD overexpression lines,the mutants showed strikingly increased ABA sensitivity and drought resistance.These results together suggest that MODD acts as a negative regulator in ABA signaling and drought response.Here,we found that MODD could repress the transcriptional activity of OsbZIP46,and adversely affected the expression of OsbZIP46 target genes.Anylsis of the MODD protein sequence revealed that an EAR motif exists in its N-terminal,which may lead to the interaction of MODD with OsTPR3.Actually,MODD really repressed OsbZIP46 transcription activity via interaction with the OsTPR3HDA702 corepressor complex and down-regualtion of the histone acetylation level at OsbZIP46 target genes.Furthermore,MODD interacted with E3 ligase OsPUB70 and promoted the degradation of OsbZIP46.These findings together suggested that MODD negatively regulates the activity and stability of OsbZIP46.OsHUB2,another interaction protein of OsbZIP46,and was identified as a E3 ligase for histone H2B monoubiquitination(H2Bub1).Genetic analyses suggested that OsHUB2 was induced by drought stress and ABA treatment,and positively modulated ABA sensitivity and drought resistance.H2Bub1 levels mediated by OsHUB2 were increased under the drought stress and ABA treatments,and the H2Bub1 levels were positively correlated to the expression levels of the target genes.Most of the genes in which H2Bub1 levels were changed under the moderate drought stress treatment are drought responsive genes,and a quarter of these genes are OsbZIP46 target genes.However,we found a dynamic change of H2Bub1 levels during drought stress treatment:the H2Bub1 levels under severe drought stress treatment decreased compared with moderate drought stress treatment.Interestingly,MODD could repress the H2Bub1 level in the promoters of OsbZIP46 target genes by recruiting a putative deubiquitinase OsOTLDl.Suppression of OsOTLD1 in vivo resulted in increased H2Bub1 levels and expression of droughtresponsive genes targeted by OsbZIP46.These findings established an elaborate mechanism of histone monoubiquitination in the fine-turning of ABA signaling and drought response via balancing H2Bub1 deposition and removal.Based on the research results,we summarized a fine regulation network centered on OsbZIP46,including the early amplification of the stress signaling(such as the phosphorylation by SAPKs and the H2Bub1 modification by OsHUB2)and the attenuation of the stress signaling(such as the negative regulation of OsbZIP46 acivity and stability by MODD via recruiting the epigenetic modifiers and E3 ligase,respectively).These fine regualtion mechanisms provide us with new ideas for genetic improvement of drought resistance and insights into the molecular basis of stress respones in rice.