Novel Variation and Evolution of AvrPiz-t of Magnaporthe oryzae in Field Isolates
Qun Wang, Jinbin Li, Lu Lin, C.F. He, Chengyun Li
Abstract
The avirulence (Avr) genes of Magnaporthe oryzae are recognized by the cognate resistance (R) genes of rice and activates immunity to rice mediated by the R gene. The high degree of variability in pathogenicity of the isolates of M. oryzae makes the control of rice blast difficult. The ability to defeat the R gene has been hypothesized to be due to the instability of Avr gene in M. oryzae. Further study on the variation of the Avr genes in naturally-occurring field isolates of M. oryze may yield valuable information for the deployment of R genes in rice production areas. AvrPiz-t and Piz-t are a pair of valuable genes in the Rice-Magnaporthe pathosystem. AvrPiz-t is recognized by Piz-t and determined the effectiveness of Piz-t. For effectively deploying R gene Piz-t, the distribution, variation and evolution of the corresponding Avr gene AvrPiz-t were proceeded among 312 M.oryzae isolates collected from Yunnan rice production areas of China. PCR amplification and pathogenicity assay of AvrPiz-t showed that 202 isolates (64.7%) held AvrPiz-t alleles and were avirulent to IRBLzt-T (holding Piz-t). There were 42.3-83.3% avirulent isolates containing AvrPiz-t among seven regions in Yunnan Province. Meanwhile, 11 haplotypes of AvrPiz-t encoding three novel AvrPiz-t variants were identified among 100 isolates. A 198bps insertion homologous to solo-LTR of the retrotransposon inago2 in promoter region of AvrPiz-t in one isolate and a framshift mutation of CDS in another isolate were identified among 100 isolates, and those two isolates were evolved to virulent from avirulent. Synonymous mutation and non-AUG-initiated N-terminal extension keep AvrPiz-t gene avirulence function in M.oryzae field isolate of Yunnan. Haplotype network showed that H3 was ancestral haplotype. Structure variance for absence (28.2%) or partial fragment loss (71.8%) of AvrPiz-t was founded among 39 virulent isolates and may cause AvrPiz-t avirulence function to be lost. In all, the AvrPiz-t evolved to virulent from avirulent forms via point mutation, retrotransposon, shift mutation and structure variance under field conditions.