| Rice bacterial leaf blight(BLB)and rice bacterial leaf streak(BLS)are important diseases in rice production,which seriously damage rice yield and pose a great threat to food security.The pathogens are Xanthomonas oryzae pv.oryzae(Xoo)and Xanthomonas oryzae pv.oryzicola(Xoc).Both Xoo and Xoc are aerobic microorganisms,therefore,the reactive oxygen species(ROS)is not only produced during the oxygen metabolism process but also the reactive oxygen species released by the plants which are in order to defense during the pathogens’ infection,both of them pose a great threat to the survival of the bacteria.Meanwhile,microorganisms have evolved a complex antioxidant system to reponse the hramful environment and maintain their survival.The research on the antioxidant mechanism of Xoo and Xoc will provide an important theoretical basis for understanding the survival mechanism of pathogens,the occurrence prevention of diseases and the innovation of new pesticides.The precious results in our laboratory showed that although the genomes of Xoo and Xoc are very similar,the sensitivity to hydrogen peroxide(H2O2)is significantly different,and this difference is caused by the diversity of OxyR protein sequence between two bacteria.By comparing the amino acid sequences of OxyR in Xoo and Xoc,we found that there are only 10 amino acid differences between them.In order to explore the role of the above 10 different amino acids in OxyR-mediated differences in the sensitivity ofXoo and Xoc to H2O2,we constructed the OxyR amino acid point mutant strains by homologous double exchange,and these mutant strains were tested for H2O2 sensitivity,total catalase activity,H2O2 accumulation,relative expression levels of anti-oxidation related genes and pathogenicity.The results show that the 23rd amino acid of OxyR is the key residue that is responsive to the H2O2 diversity between Xoc and Xoo.After mutagenesis of 23K to 23T in OxyR of Xoc,the sensitivity of Xoc to H2O2 increased,the H2O2 content in the mutant strain increased,the total catalase activity decreased,the expression level of catalase gene decreased and the pathogenicity was also reduced.Correspondingly,after mutagenesis of 23T to 23K in OxyR of Xoo,Xoo mutants had improved antioxidant capacity and possess the same H2O2 phenotype as Xoc.The results of the protein-DNA binding ability showed that the 23rd amino acid of OxyR in Xoo and Xoc changed the expression level of the target gene in the cell by affecting its binding to the target of the downstream gene promoter,thereby affected the H2O2 sensitivity in strains. |
