Abstract
The product of the avirulence (Avr) gene of Magnaporthe oryzae can be detected by the product of the corresponding resistance (R) gene of rice and activates immunity to rice mediated by the R gene. The high degree of variability of M. oryzae isolates in pathogenicity makes the control of rice blast difficult. That resistance of the R gene in rice has been lost has been ascribed to the instability of the Avr gene in M. oryzae. Further study on the variation of the Avr genes in M. oryze field isolates may yield valuable information on the durable and effective 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 detectable by Piz-t and determines the effectiveness of Piz-t. To effectively deploy the R gene Piz-t, the distribution, variation, and evolution of the corresponding Avr gene AvrPiz-t were found among 312 M. oryzae isolates collected from Yunnan rice production areas of China. PCR amplification and pathogenicity assays 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 198 bps insertion homologous to solo-LTR of the retrotransposon inago2 in the promoter region of AvrPiz-t in one isolate and a frameshift mutation of CDS in another isolate were identified among 100 isolates, and those two isolates had evolved to virulent from avirulent. Synonymous mutation and non-AUG-initiated N-terminal extensions keeps the AvrPiz-t gene avirulence function in M. oryzae field isolates in Yunnan. A haplotype network showed that H3 was an ancestral haplotype. Structure variance for absence (28.2%) or partial fragment loss (71.8%) of AvrPiz-t was found among 39 virulent isolates and may cause the AvrPiz-t avirulence function to be lost. Overall, AvrPiz-t evolved to virulent from avirulent forms via point mutation, retrotransposon, shift mutation, and structure variance under field conditions.
Highlights
During the long history of coevolution between plants and pathogens, two layers of host immunity have developed in plants (Jones and Dangl, 2006)
We report the molecular mechanisms of variance and evolution of AvrPiz-t in M. oryzae field isolates in Yunnan Province of China
These results suggest that these field M. oryzae isolates can still be beneficial to its correspondent resistant gene Piz-t in managing rice blast disease in Yunnan’s rice growing regions, except for limited use in LC of Southwestern Yunnan
Summary
During the long history of coevolution between plants and pathogens, two layers of host immunity have developed in plants (Jones and Dangl, 2006). Avirulence (Avr) proteins can interact directly or indirectly with the corresponding resistance (R) protein or combine to the promoter of the corresponding R genes, trigger the ETI. Race-specific plant R genes have the ability to detect the corresponding Avr protein and induce effective resistance to plant pathogens (Dai et al, 2010). Specific recognition and interaction between Avr and R genes is the key to induce plant resistance. Through mutation or complete loss of effector genes, the pathogens are able to evade recognition of R proteins, infect plants, and counter efforts to control pivotal plant diseases (McDonald and Linde, 2002; Singh et al, 2011). Further analysis of Avr genes of the field isolate in a plant pathogen may provide precious information for the utilization of R genes in field crops (Stukenbrock and McDonald, 2009)
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