Sequence variation and recognition specificity of the avirulence gene AvrPiz-T in Magnaporthe Oryzae field populations

Magnaporthe oryzae, the rice blast pathogen, causes significant annual yield loss of rice worldwide. Currently, the most effective disease control approach is deployment of host resistance through introduction of resistance (R) genes into elite cultivars. The function of each R gene relies on the specific recognition of an avirulence (AVR) gene of the pathogen. However, the introduced resistance can be broken down within a few years, due to mutation of the AVR genes in the field populations. It is known from a few case studies that AVR mutation patterns are different from one to another. Therefore, knowledge of AVR genes sequence diversity serves as fundamental background in introducing new resistance to control rice blast. In this study, we focused on a newly identified AVR gene AvrPiz-t. We PCR amplified open reading frames (ORFs) of AvrPiz-t as well as promoter regions to detect size variation at this locus in 711 isolates of M. oryzae collected from 38 countries and regions. Through sequencing and Southern hybridization of the amplified locus, strains with polymorphisms in the ORF were classified into groups based on mutation type and site. Natural selection intensity on this gene was calculated and pathogenicity assays were applied to evaluate the association between AvrPiz-t ORF/promoter polymorphism and virulence. In conclusion, sequences at the AvrPiz-t locus were revealed to contain variations at both promoter and ORF regions. This locus is undergoing a relatively strong positive selection. The diversity in coding sequence and the insertions of transposable elements in the promoter region enable M. oryzae to evade recognition by the cognate Piz-t R gene in the host.