Variation in the <i>AvrSr35</i> gene determines <i>Sr35</i> resistance against wheat stem rust race Ug99

Abstract

<i>Puccinia graminis</i> f. sp. <i>tritici</i> (<i>Pgt</i>) causes wheat stem rust, a devastating fungal disease. The <i>Sr35</i> resistance gene confers immunity against this pathogen's most virulent races, including Ug99. We used comparative whole-genome sequencing of chemically mutagenized and natural <i>Pgt</i> isolates to identify a fungal gene named <i>AvrSr35</i> that is required for <i>Sr35</i> avirulence. The <i>AvrSr35</i> gene encodes a secreted protein capable of interacting with Sr35 and triggering the immune response. We show that the origin of <i>Pgt</i> isolates virulent on <i>Sr35</i> is associated with the nonfunctionalization of the <i>AvrSr35</i> gene by the insertion of a mobile element. The discovery of <i>AvrSr35</i> provides a new tool for <i>Pgt</i> surveillance, identification of host susceptibility targets, and characterization of the molecular determinants of immunity in wheat.