Sequence variation and recognition specificity of the avirulence gene AvrPiz-t in Magnaporthe oryzae field populations

Authors: CHEN, CHENXI - The Ohio State University; CHEN, MEILIAN - Fujian Agricultural & Forestry University; HU, JINNAN - The Ohio State University; ZHANG, WENJING - Fujian Agricultural & Forestry University; ZHONG, ZHENHUI - Fujian Agricultural & Forestry University; Jia, Yulin; ALLAUX, LUDOVIC - Umr - Bgpi, Biologie Et Génétique Des Interactions Plante-parasite; FOURNIER, ELISABETH - Umr - Bgpi, Biologie Et Génétique Des Interactions Plante-parasite; THARREAU, DIDIER - Umr - Bgpi, Biologie Et Génétique Des Interactions Plante-parasite; WANG, GUO-LIANG - The Ohio State University; WANG, ZONGHUA - Fujian Agricultural & Forestry University; SHEN, WEI-CHIANG - National Taiwan University; LU, GUODONG - Fujian Agricultural & Forestry University; WANG, BAOHUA - Fujian Agricultural & Forestry University; MITCHELL, THOMAS - The Ohio State University

Submitted to: Fungal Genomics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2014
Publication Date: 7/28/2014
Publication URL: http://handle.nal.usda.gov/10113/60700
Citation: Chen, C., Chen, M., Hu, J., Zhang, W., Zhong, Z., Jia, Y., Allaux, L., Fournier, E., Tharreau, D., Wang, G., Wang, Z., Shen, W., Lu, G., Wang, B., Mitchell, T.K. 2014. Sequence variation and recognition specificity of the avirulence gene AvrPiz-t in Magnaporthe oryzae field populations. Fungal Genomics and Biology. 4:113. doi:10.4172/2165-8056.1000113.

Interpretive Summary: Magnaporthe oryzae (M. oryzae), the causal agent of rice blast disease, causes significant loss of rice yield worldwide. The disease is managed with fungicides, cultural practices, and resistant varieties. The use of resistant varieties, cultivars with resistance (R) genes, is one of the most benign and effective methods of disease management currently available. Each R gene’s function relies on the specific recognition of an avirulence (AVR) gene in the pathogen. However, plant resistance can be broken down within a few years, due to mutating AVR genes. It is known that AVR mutation patterns are different from one to another. In this study, we focused on a newly identified AVR gene, AVRPiz-t, from M. oryzae isolates collected from several countries. We found several mutations, or variations, and classified them into groups based on type and DNA sequence location. We used disease screening assays to evaluate the mutations’ effect on causing disease symptoms and found that the mutations are concentrated in only a few locations in the gene. We also found that the AVRPiz-t gene is undergoing mutation that allows the pathogen to avoid recognition by the corresponding R gene in the plant. This information demonstrates that genetic changes in the blast fungus are the major causes of instability in rice cultivar disease resistance.

Technical Abstract: 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 resistance genes can be broken down within a few years, due to mutation of the AVR genes in the field populations of the pathogen. It is known from other studies that AVR mutation patterns differ from one to another. Therefore, knowledge of sequence diversity of AVR genes serves as fundamental background in introducing new resistance genes to control rice blast. In this study, we focused on a newly identified AVR gene, AvrPiz-t. We 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. 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 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.