Wheat stem rust, caused by the fungus
Puccinia graminisf. sp. tritici, afflicts bread wheat ( Triticum aestivum). New virulent races collectively referred to as “Ug99” have emerged, which threaten global wheat production. The wheat gene Sr33, introgressed from the wild relative Aegilops tauschiiinto bread wheat, confers resistance to diverse stem rust races, including the Ug99 race group. We cloned Sr33, which encodes a coiled-coil, nucleotide-binding, leucine-rich repeat protein. Sr33is orthologous to the barley ( Hordeum vulgare) Mlamildew resistance genes that confer resistance to Blumeria graminisf. sp. hordei. The wheat Sr33gene functions independently of RAR1, SGT1, and HSP90chaperones. Haplotype analysis from diverse collections of Ae. tauschiiplaced the origin of Sr33resistance near the southern coast of the Caspian Sea.