Evolution of wheat blast resistance geneRmg8accompanied by differentiation of variants recognizing the powdery mildew fungus

Abstract

AbstractWheat blast, a devastating disease having spread recently from South America to Asia and Africa, is caused byPyricularia oryzaepathotypeTriticumwhich emerged in 1985.Rmg8andRmg7, genes for resistance to wheat blast found in common wheat and tetraploid wheat, respectively, recognize the same avirulence gene,AVR-Rmg8. Here, we show an evolutionary process in which resistance gene(s), which had obtained an ability to recognizeAVR-Rmg8before the differentiation ofTriticumandAegilops, has expanded its target pathogens. Molecular cloning revealed thatRmg7was one of alleles ofPm4(Pm4a), a gene for resistance to wheat powdery mildew on 2AL, whereasRmg8was its homoeolog on 2BL ineffective against wheat powdery mildew.Rmg8variants with the ability to recognizeAVR-Rmg8were distributed not only inTriticumspp. but also inAegilops speltoides,Ae. umbellulata,andAe. comosa. This result suggests that the origin of resistance gene(s) recognizingAVR-Rmg8dates back to the time before differentiation of A, B, S, U, and M genomes, that is, ∼5 million years before the emergence of its current target, the wheat blast fungus. Phylogenetic analyses suggested that, in the evolutionary process thereafter, some of their variants gained the ability to recognize the wheat powdery mildew fungus and evolved into genes for resistance to wheat powdery mildew.