Mechanism Of The RsmA Gene Regulating On Expression Of The Virulence Related Genes In Xanthomonas Oryzae Pv. Oryzicola

Bacterial leaf streak（BLS）caused by Xanthomonas oryzae pv.Oryzicola（Xoc）has become one of the most serious bacterial diseases in rice growing areas in southern China.Xanthomonas is dependent on the type III secretion system encoded by the hrp gene cluster for the secretion of the type III effector protein into rice cells,leading to disease resistance or susceptibility in rice.RsmA belongs to a conserved family of RNA-binding proteins（RsmA/CsrA family）functioning as a global post-transcriptional regulator and has been demonstrated to regulate carbon metabolism,biofilm formation,motility and the expression of pathogenicity genes.Homology search results showed that there is rsmA_Xococ gene in Xoc,which has 100%homology with Xanthomonas oryzae pv.oryzae（Xoo）.However,the function of the rsmA_Xococ gene in pathogenicity is unknown.To investigate the function of rsmA_Xococ gene in pathogenicity,we constructed a deletion mutant of rsmA gene in Xoc RS105 strain named as RΔrsmA in Xoc RS105 strain.Inoculation assays showed that RΔrsmA can cause pathogenicity on host rice,and trigger a hypersensitive response on nonhost tobacco,which exhibts a different phenotype from that of the muant of rsmA_Xoooo mutant.However,compared with the wild-type strain,significantly reduced virulence symptoms on the susceptible rices were observed in RΔrsmA mutant.Virulence-related phenotype results showed that RsmA positively regulated EPS production,negatively regulated biofilm formation and extracellular protease activity.RsmA regulated the expression of T3SS genes by positively regulating the transcriptional expression of hrpX gene.This suggests that RsmA is an important virulence regulator in Xoc that finely regulates multiple virulence-related phenotypes.Transcriptome analysis of rsmA mutants and wild-type strains of Xoo and Xoc was carried out by comparative transcriptomics analysis.Combined with real-time quantitative PCR,181 genes were found to be upregulated in Xoo and Xoc rsmA mutants,51 genes were down-regulated and 16 genes were significantly differentially expressed.These genes are involved in cellular metabolism,intracellular colonization,cell membrane formation,polysaccharide metabolism and cellulolytic processes.This study is the first to report the function of rsmA gene in pathogenicity in Xoc strain and found that the mechanism of RsmA regulating T3SS gene expression in Xoc is different from the existing results in Xanthomonas.This provided a new clue for analyzing the new regulatory mechanism of T3SS gene by RsmA.Meanwhile,the identification of RsmA downstream regulatory genes in Xoo and Xoc also provided clues for analyzing their differences in function in two rice pathogenic varieties.