Association Analysis of Stem Rust Resistance in U.S. Winter Wheat

Abstract

<div><p>Stem rust has become a renewed threat to global wheat production after the emergence and spread of race TTKSK (also known as Ug99) and related races from Africa. To elucidate U.S. winter wheat resistance genes to stem rust, association mapping was conducted using a panel of 137 lines from cooperative U.S. winter wheat nurseries from 2008 and simple sequence repeat (SSR) and sequence tagged site (STS) markers across the wheat genome. Seedling infection types were evaluated in a greenhouse experiment using six U.S. stem rust races (QFCSC, QTHJC, RCRSC, RKQQC, TPMKC and TTTTF) and TTKSK, and adult plant responses to bulked U.S. races were evaluated in a field experiment. A linearization algorithm was used to convert the qualitative Stakman scale seedling infection types for quantitative analysis. Association mapping successfully detected six known stem rust seedling resistance genes in U.S. winter wheat lines with frequencies: <i>Sr6</i> (12%), <i>Sr24</i> (9%), <i>Sr31</i> (15%), <i>Sr36</i> (9%), <i>Sr38</i> (19%), and <i>Sr1RS^Amigo</i> (8%). Adult plant resistance gene <i>Sr2</i> was present in 4% of lines. <i>SrTmp</i> was postulated to be present in several hard winter wheat lines, but the frequency could not be accurately determined. <i>Sr38</i> was the most prevalent <i>Sr</i> gene in both hard and soft winter wheat and was the most effective <i>Sr</i> gene in the adult plant field test. Resistance to TTKSK was associated <i>with</i> nine markers on chromosome 2B that were in linkage disequilibrium and all of the resistance was attributed to the <i>Triticum timopheevii</i> chromosome segment carrying <i>Sr36</i>. Potential novel rust resistance alleles were associated with markers <i>Xwmc326-203</i> on 3BL, <i>Xgwm160-195</i> and <i>Xwmc313-225</i> on 4AL near <i>Sr7</i>, <i>Xgwm495-182</i> on 4BL, <i>Xwmc622-147</i> and <i>Xgwm624-146</i> on 4DL, and <i>Xgwm334-123</i> on 6AS near <i>Sr8</i>. <i>Xwmc326-203</i> was associated with adult plant resistance to bulked U.S. races and <i>Xgwm495-182</i> was associated with seedling resistance to TTKSK.</p></div