Abstract: ObjectiveThis study investigated the effects of pH on morphological development of biotrophy-associa-ted secreted protein 4(BAS4)overexpressed strain and expression of defense-related genes in infected rice ,and disco-vered the mechanism of pH on rice blast effect protein in the interaction of rice blast strain and rice,in order to provide important reference for predicting effects of adverse environmental factors on rice planting in agricultural production.MethodOverexpressed strain of Magnaporthe oryzae(35S:BAS4/Mo-1)and wild strain A1343R-7(WT)were pre-treated with different pH values(pH 5.00,pH 6.00,pH 7.00,and pH 8.00) to analyze the effects of pH on colony growth,spore quantity,spore germination rate,dry weight of hypha,and appressorium formation of overexpressed strain. And the disease incidence of overexpressed strain re-infected rice which were pre-treated with pH5.0 and pH8.0 was investigated and the expression of infected rice defense-related genes was analyzed by real-time fluorescent quantita-tive PCR(qRT-PCR).ResultThe result showed that the effect of pH on colony growth,spore quantity,dry weight of hypha,germination rate and appressorium formation of 35S:BAS4/Mo-1 was smaller than the effect on those of wild-type strain. Disease incidence of rice infected by overexpressed strains which were pre-treated with pH 5.00 and pH 8.00was increased. pH 5.00 and pH 8.00 repressed the expression of pathogenesis-related genes,SA-pathway genes,JA-path-way genes and PR5 of overexpressed strain that infected rice,while up-regulated the expression of HSP90 in the early stage of infection in rice.ConclusionAlthough morphologic development of BAS4 overexpressed strains are influenced by pH,the expression of most rice denfense-related genes pre-treated with pH appears down-regulation,which indicates that BAS4 overexpressed strains have strong tolerance to pH.