Host-induced silencing of essential genes in Puccinia triticina through transgenic expression of RNAi sequences reduces severity of leaf rust infection in wheat

Leaf rust, caused by the pathogenic fungus Puccinia triticina (Pt), is one of the most serious challenges to sustainable wheat production worldwide. This pathogen is known for rapid adaptive evolution to overcome resistant wheat varieties. Novel disease control approaches are therefore required to minimize the yield losses caused by Pt. This fungus is an obligate biotrophic pathogen, meaning it can only grow on its host and cannot be cultured in vitro in the laboratory. This makes molecular and genetic studies very difficult. Virtually every organism has a RNA silencing mechanism, a molecular defense system that causes destruction of messages coming from transcribed genes when a complement copy is present; together these form a hybrid molecule which is then the target for enzymes slicing it up. The result is that this particular gene is not well or no longer expressed. This technology can therefore be used to test for gene function. Interestingly, we have shown previously that we can express copies or already hybridized parts of genes in wheat plants but that match messages coming from transcribed genes in the Pt fungus. Somehow, this fungus ingests these copies or already hybridized parts when "feeding" on the host plant while infecting, thereby affecting its own genes in this “Trojan Horse” approach. We have thus found a novel technique to study genes in the fungus. When targeting genes in the fungus that are essential for pathogenicity, the fungus is severely weakened upon infection, resulting in less rust disease on wheat plants. In this study, we expressed in a stable fashion in wheat plants already hybridized parts of two essential fungal pathogenicity genes. In the next (T2) generation of plants, the rust disease suppressing phenomenon is still present. In the best wheat lines we generated, growth of the fungus in the plant is severely inhibited, reducing to 20% its biomass compared to control plants. This leads to a major reduction in fungal spore production and hence less potential for spreading the disease to other plants. This work demonstrates that generating such wheat plants that express stably RNA silencing molecules targeted to essential genes in rust fungi, and can provide effective disease resistance. This demonstrates an alternative way for developing rust-resistant crops.