Epigenetic Regulation And OsbZIP23-Mediated Transcriptional Regulation In Drought Response Of Rice

Drought has become one of the most serious problems restricting the worldwide productivity of rice(Oryza sativa L.)due to global climate change,fresh water shortage,and increasing population.Plants commonly respond to drought stress by regulating expression levels of thousands of stress-related genes via abscisic acid(ABA)-dependent and ABA-independent pathways.Histone modifications affect gene expression level,but genome-wide surveys of stress-related histone modification profiles are very limited,especially in crops.The transcription factor OsbZIP23 has been characterized for its essential role in drought resistance in rice,but the mechanism is unknown.In this study,the chromatin regulation and OsbZIP23-mediated transcription regulation under drought stress were investigated in rice.By using ChIP-Seq and RNA-Seq,we investigated the genome-wide distribution patterns of histone H3 lysine4 tri-methylation(H3K4me3)and the pattern's association with whole genome expression profiles of rice under drought stress.We detected 51.1 and 48%of annotated genes with H3K4me3 modification in rice seedlings under normal growth(control)and drought stress conditions,respectively.By RNA-Seq,76.7 and 79%of annotated genes were detected with expression in rice seedlings under the control and drought stress conditions,respectively.Furthermore,4,837 genes were differentially H3K4me3-modified(H3M),(3,927 genes with increased H3M;910 genes with decreased H3M)and 5,866 genes were differentially expressed(2,145 up-regulated;3,721 down-regulated)in drought stress.Differential H3K4me3 methylation only affects a small proportion of stress-responsive genes,and the H3K4me3 modification level was significantly and positively correlated with transcript level only for a subset of genes showing changes both in modification and expression with drought stress.Moreover,for the H3K4me3-regulated stress-related genes,the H3K4me3 modification level was mainly increased in genes with low expression and decreased in genes with high expression under drought stress.We performed genome-wide identification of OsbZIP23 targets by ChIP-Seq analyses in the OsbZIP23-over-expression,osbzip23 mutant,and wild-type rice under normal and drought stress conditions and 681,2128 and 7194 genes were found in these three samples,respectively.Further analyze the genes bound by OsbZIP23 under drought stress,1471 OsbZIP23BGDs(OsbZIP23 binding genes involved in drought stress)were identified.From OsbZIP23BGDs,we found OsbZIP23 directly regulates a large number of reported genes that function in stress response,hormone signaling,and developmental processes.Among these targets,we found that OsbZIP23 could positively regulate OsPP2C49,and over-expression of OsPP2C49 in rice resulted in significantly decreased sensitivity of the abscisic acid(ABA)response and rapid dehydration.Moreover,OsNCED4(9-cis-epoxycarotenoid dioxygenase 4),a key gene in ABA biosynthesis,was also positively regulated by OsbZIP23.We also investigated the transcriptional activation of OsbZIP23,a homolog of SnRK2 Protein kinase(SAPK2)was found to interact with and phosphorylate OsbZIP23 for its transcriptional activation.SAPK2 also interacted with OsPP2C49,which deactivated the SAPK2 to inhibit the transcriptional activation activity of OsbZIP23.We found that a cluster of four dehydrin genes in chromosome 11 could be directly and postively regulated by OsbZIP23 and the H3K4me3 methylation levels of these genes are significantly increased after drought stress.Further experiments based on ChIP-qPCR revealed that H3ace and H4ace acetylation and RNA Polymerase II binding levels on these four genes were significantly increased after drought stress.However,H3K4me2 and H3K27me3 methylation levels and Histone H3 binding levels were significantly decreased after drought.We also found SDG723,a H3K4me3 methyl-transferase in rice,and JMJ703,a H3K4me3 demethylase in rice could affect the expression level of these genes.Moreover,the histone modification levels of these four genes were also significantly affected by OsbZIP23.