Cereals have natural resistance genes that can protect them from disease-causing fungi. However, certain fungi, such as powdery mildew, are known to be able to overcome such resistance. A team at the University of Zurich has now discovered a new mechanism that enables powdery mildew to evade the immune system of wheat. These findings open the door to targeted development of resistant varieties with a reduced risk of resistance breakthrough.

Cereals are among the most important staple foods. Wheat alone provides around 20 percent of the global supply of protein and calories. However, its production is threatened by plant diseases, such as the wheat powdery mildew fungus. One sustainable alternative to using fungicides is to grow wheat varieties that are genetically resistant to this pathogen. However, in many cases this is not effective in the long term because powdery mildew evolves quickly and is able to overcome any resistance.

Exploiting natural resistance
A team from the Department of Plant and Microbial Biology at the University of Zurich has now conducted more in-depth studies to establish how the powdery mildew fungus is able to infect wheat despite the presence of resistance genes. The researchers discovered a previously unknown interplay between resistance factors in wheat and disease factors in powdery mildew. “This deeper understanding allows us to deploy resistance genes in a more targeted way and prevents or slows down the breakdown of resistance”, says postdoctoral researcher Zoe Bernasconi, one of the lead authors of the study, which has just been published in Nature Plants.

Wheat is tricked by the fungus in two ways
The powdery mildew fungus produces hundreds of tiny proteins, known as effectors. These effectors are introduced into the cells of the host plant and help establish an infection. Resistance proteins produced by wheat can recognize some of these effectors, thereby triggering an immune response that stops the infection. However, the fungus frequently gets around this by modifying recognized effectors or even losing them entirely.

The research team has now identified a novel powdery mildew effector (called AvrPm4) that is recognized by the known wheat resistance protein Pm4. Yet surprisingly, the fungus is able to overcome the Pm4-mediated resistance − and is able to do so without modifying or losing the AvrPm4 effector. Its clever trick is that it has a second effector that prevents the recognition of AvrPm4. “We suspect that the function of AvrPm4 is essential for the fungus to survive, and that’s why this unusual mechanism arose over the course of evolution,” says Bernasconi.

Intriguingly, the second effector has a dual role: It prevents the recognition of the first effector, AvrPm4, but additionally is recognized by yet another resistance protein of wheat. “This means that, by combining the two resistance proteins in the same variety of wheat, it might be possible to lure the fungus down an evolutionary dead end in which it can no longer escape the immune response”, says postdoctoral researcher Lukas Kunz, another lead author of the study.

New approaches to produce more resistant wheat varieties
“Now that we know the fungal factors involved in the interaction and understand their mode of action, we can take more effective measures to prevent powdery mildew from breaking through wheat’s resistance”, says Beat Keller, the professor who led the research group until he retired last year. By monitoring the powdery mildew pathogen, it is now conceivable to use resistant wheat varieties in a targeted manner in places where they will have the greatest impact.

A clever combination of resistance genes in new varieties of wheat would also be an option. “Theoretically, measures like these could significantly slow down the development of new pathogenic fungal strains,” says Keller. The team has already conducted the first promising experiments in the laboratory. To do so, they combined resistance genes that target both the AvrPm4 effector and the second effector. Whether this approach will be effective in the field remains to be tested.


Dr. Lukas Kunz
Department of Plant and Microbial Biology
University of Zurich
+41 44 63 48210
lukas.kunz@botinst.uzh.ch