Study On The Function Of The RS105 VI Secretion System(T6SS)in Rice Bacterial Blight

Rice bacterial leaf streak?BLS?,cause by Xanthomonas oryzae pv.oryzicola?Xoc?,is one of the most destructive rice diseases.The current studies on Xoc are focused on the Type III secretion system?Type III secretion system,T3SS?and its effectors,which is essential to Xoc pathogenesis.Few studies of Xoc VI secretion system?T6SS?were reported.Here,we studied the roles and possible mechnisms of T6SS in the regulation of Xoc virulence,and achieved the following results:?1?With the help of bioinformatics analysis,Xoc RS105 was found to encode two T6SSs.Xoc₂542 and Xoc₁355 encode proteins are homologous of the known T6SS structural protein Hcp?haemolysin co-regulated proein?,which is essential to T6SS function.In order to understand the function of T6SS in Xoc,the deletion mutants of Xoc₂542,Xoc₁355??Xoc2542 and A Xoc1355?and the double mutant??dT6SS?were constructed by homology recombination..Virulences of these strains on rice were tested and results showed that all the mutants had lower pathogenesis than the wild type strain on IRBB24 rice,suggesting that T6SSs are vital for Xoc infection.?2?In order to find what factors regulated by T6SS are involved in Xoc pathogenic,the contents of extracellular enzymes,extracellular polysaccharide and xanthomonacin were compared between T6SSs mutants and wild type strain.?Xoc2542,?Xoc1355 and ?dT6SS produced reduced extracellular polysaccharide and xanthomonacin.While wild type and mutant strains had similar extracellular enzyme activities,indicating that T6SS regulats the extracellular polysaccharide,xanthomonacin,but not extracellular enzyme synthesis.?3?T6SS is involved in the secretion of proteins or polypeptides,and the known secretory proteins are the sheath structure protein Hcp and the VgrG proteins,VgrG functions as a carrier for the effectors and Hcp recognizes and transports the laden VgrG into extracellular components.Therefore,Hcp and VgrG are important partner in T6SSs.We hence analyzed the relationship between hcp and vgrG.Through analysis of expressional correlation,the co-evolution characteristics of hcp and vgrG were obtained and their relationship in transcription was studied.The co-evolution of Hcp with other T6SS structures and the co-evolution characteristics of VgrG were also presented.The co-occurrence of vgrG and hcp in a bacterium species was also obtained.For expression,almost all vgrGs were positively correlated with hcp genes,indicating the correlation between vgrG and hcp functions.In summary,two T6SSs were identified in Xoc in this study.Each of them is important for Xoc virulence.Mutation of each system can reduce the Xoc EPS and pigment synthesis,which might be the reason for the decreased virulence of T6SS mutants.Moreover,T6SSs seem to regulate Quorum sensing in Xoc because the T6SS mutants can not grow in the late growth phase.The proteins or peptides secrected by T6SS might act as Quorum sensing signals,which are not identified in our work.The co-evolution of Hcp and VgrG in Gram-negative bacteria indicates that Hcp and VgrG might coordinate to pump proteins through T6SS structure.hcp can influence the expression of vgrG in Xoc also suggests that their functional relationship.Taken together,we have identified two T6SSs in Xoc,which are vital for bacterial virulence and growth.Our findings are helpful to further elaborate the T6SS function and are of great significance to the prevention and cure of rice BLS.