Mechanisms of evolution of the wheat blast fungus
Yukio Tosa: Kobe Univ
<div>Wheat blast, caused by a subgroup (<em>Triticum</em> pathotype) of <em>Pyricularia oryzae</em>, first emerged in Brazil in 1985 and spread to neighboring countries in South America. In 2016 it suddenly appeared in Bangladesh and caused a significant loss of wheat production. Here, we show how this devastating pathogen evolved in Brazil. Genetic analysis revealed that the incompatibility between <em>Lolium</em> isolates and common wheat is governed by an avirulence gene, <em>PWT3</em>, and its corresponding resistance gene <em>Rmg6</em> (<em>Rwt3</em>). We isolated <em>PWT3</em> through map-based cloning. <em>PWT3</em> encoded a small secreted protein composed of 141 amino acids. When <em>PWT3</em> of a <em>Lolium</em> isolate was disrupted, the resulting strain gained virulence on wheat cultivars carrying <em>Rwt3</em>. Among local landraces of common wheat collected worldwide, <em>Rwt3</em> carriers accounted for ~77% and the remaining was lacking <em>Rwt3</em>. These results indicate that mutations or deletions of <em>PWT3</em> would lead to a gain of virulence on a majority of wheat cultivars. Actually, wheat blast isolates had various types of mutations in <em>PWT3</em>, e.g. nucleotide substitutions, insertions of transposable elements into the promoter region or ORF. Historical data on wheat cultivation in Brazil suggest that the wheat blast fungus evolved due to widespread deployment of <em>rwt3</em> wheat followed by the loss of function of <em>PWT3</em>. This implies that the <em>rwt3</em> wheat served as a springboard for the host jump to wheat.</div>
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