Wheat stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is one of the most devastating fungal diseases affecting wheat worldwide. The most economical and environmentally sustainable way to control the spread of Pgt is by using effective resistance genes. Stem rust resistance gene Sr2 is one of the few known race non-specific, adult plant rust resistance (APR) genes used in wheat worldwide and has been effective for over a century. Previously, Sr2 was shown to be tightly linked to the leaf rust resistance gene Lr27 and to powdery mildew resistance on chromosome 3B. Lr27 is one of the few known genes that requires a complementary gene for its expression, which in this case is Lr31 located on chromosome 4B.

To identify Lr27-Lr31, we generated mutants in a genetic background that carries both Sr2 and Lr27-Lr31. Deletion mutants on 4B lost leaf rust resistance as expected but also lacked stem rust resistance suggesting that Sr2 mediated resistance also requires gene(s) on 4B.

We previously fine mapped Sr2 on chromosome 3B and identified two candidate genes within a small interval which are differentially expressed upon infection with stem rust. Transgenic studies using single gene or dual gene constructs failed to complement resistance in susceptible backgrounds. However, RNAi experiments indicated that one of these genes is required for resistance to stem rust, leaf rust and powdery mildew. We are currently confirming these results by editing this gene using TALENs. Our data indicate that more than one gene on chromosome 3B and additional gene(s) on chromosome 4B are required to confer Sr2 mediated resistance to stem rust and Lr27-Lr31 leaf rust resistance.