Identification and characterization of <i>Sr13</i> , a tetraploid wheat gene that confers resistance to the Ug99 stem rust race group

Abstract

The <i>Puccinia graminis</i> f. sp. <i>tritici</i> (<i>Pgt</i>) Ug99 race group is virulent to most stem rust resistance genes currently deployed in wheat and poses a threat to global wheat production. The durum wheat (<i>Triticum turgidum</i> ssp. <i>durum</i>) gene <i>Sr13</i> confers resistance to Ug99 and other virulent races, and is more effective at high temperatures. Using map-based cloning, we delimited a candidate region including two linked genes encoding coiled-coil nucleotide-binding leucine-rich repeat proteins designated CNL3 and CNL13. Three independent truncation mutations identified in each of these genes demonstrated that only <i>CNL13</i> was required for Ug99 resistance. Transformation of an 8-kb genomic sequence including <i>CNL13</i> into the susceptible wheat variety Fielder was sufficient to confer resistance to Ug99, confirming that <i>CNL13</i> is <i>Sr13</i><i>CNL13</i> transcripts were slightly down-regulated 2-6 days after <i>Pgt</i> inoculation and were not affected by temperature. By contrast, six pathogenesis-related (<i>PR</i>) genes were up-regulated at high temperatures only when both <i>Sr13</i> and <i>Pgt</i> were present, suggesting that they may contribute to the high temperature resistance mechanism. We identified three <i>Sr13</i>-resistant haplotypes, which were present in one-third of cultivated emmer and durum wheats but absent in most tested common wheats (<i>Triticum aestivum</i>). These results suggest that <i>Sr13</i> can be used to improve Ug99 resistance in a large proportion of modern wheat cultivars. To accelerate its deployment, we developed a diagnostic marker for <i>Sr13</i> The identification of <i>Sr13</i> expands the number of <i>Pgt</i>-resistance genes that can be incorporated into multigene transgenic cassettes to control this devastating disease.