University of Zurich scientists uncovered how wheat powdery mildew can bypass genetic resistance without losing its key effector AvrPm4. The fungus deploys a second effector that masks AvrPm4 from the Pm4 resistance protein, yet this suppressor is itself detectable by another wheat resistance gene. Stacking both genes could trap the pathogen and improve durable, low-fungicide wheat protection.
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Cereals are among the world’s most important staple foods, and wheat alone supplies about 20% of global protein and calorie intake. Yet wheat production is increasingly threatened by plant diseases, including wheat powdery mildew. A more sustainable alternative to fungicides is to grow wheat varieties with genetic resistance to the pathogen, but this strategy often loses effectiveness over time because powdery mildew evolves rapidly and can overcome resistance.
To better harness natural resistance, a team from the University of Zurich’s Department of Plant and Microbial Biology investigated how the fungus is able to infect wheat even when resistance genes are present. Their work uncovered a previously unknown interaction between wheat resistance factors and powdery mildew disease factors.
“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. Delivered into host plant cells, these molecules help the pathogen establish infection. In wheat, certain resistance proteins can detect specific effectors and trigger an immune response that halts disease. The fungus often escapes this defense by altering the effectors that are recognized — or by losing them altogether.
In this study, the researchers identified a previously unknown powdery mildew effector, AvrPm4, which is detected by the well-known wheat resistance protein Pm4. Surprisingly, however, the fungus can still bypass Pm4-based resistance without changing or discarding AvrPm4. Instead, it relies on a second effector that blocks AvrPm4 from being recognized, according to a press release.
“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.
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