Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: August 11, 2007
Publication Date: October 8, 2007
Citation: Jia, Y. 2007. Understanding the coevolution of rice blast resistance gene Pi-ta and Magnaporthe oryzae avirulence gene AVR-Pita. Intl. Rice Blast Conference, October 8-14, 2007, Changsha, China p. 27. Technical Abstract: Rice blast disease caused by the filamentous ascomycetous fungus Magnaporthe oryzae remains to be one of the most serious threats for food security globally. Using resistance (R) genes in integrated cultural practices has been the most powerful practice for rice crop protection. Genetic analysis suggests that resistance mediated by the R gene Pi-ta in rice is effective in preventing infection of fungal races containing the corresponding avirulence gene AVR-Pita. To develop effective strategies to control rice blast disease, a comprehensive study of the co-evolution of Pi-ta with AVR-Pita has been undertaken at USDA-ARS Dale Bumpers National Rice Research Center in cooperation with scientists from US, China and Colombia. A survey in the USDA rice collection of 52 Oryza species identified a total of 14 haplotypes of the Pi-ta allele. Translation of these Pi-ta haplotypes revealed 10 highly similar Pi-ta proteins. Bootstrapping and neighbor-joining analyses suggest that these Pi-ta haplotypes belong to two major clades. Tijma’s D value suggests that the Pi-ta allele existed before the divergence of Oryza species, and the Pi-ta allele is under neutral mutation. In contrast, the AVR-Pita allele was present in most fungal isolates examined. The virulent isolates, with altered AVR-Pita alleles, were detected from US, China and Colombia. Deletions and transposon insertions in the coding regions were found to be responsible for the structure variation of the AVR-Pita gene in virulent isolates. More point mutations leading to amino acid substitutions (nonsynonymous substitution, Ka) were observed than point mutations leading to silenced mutation (synonymous substitutions, Ks) in avirulent isolates suggesting that AVR-Pita is under diversified selection. These data indicate that the ratios of Ka/Ks is less than 1 for Pi-ta and greater than 1 for AVR-Pita. We suggest that Pi-ta and AVR-Pita evolve through trench warfare and will present the implications for crop protection.