The ustilago hordei-barley interaction is a versatile system for characterization of fungal effectors

Obligate biotrophic fungal pathogens, such as Blumeria graminis and Puccinia graminis, are amongst the most devastating plant pathogens, causing dramatic yield losses in many economically important crops worldwide. However, a lack of reliable tools for the efficient genetic transformation has hampered studies into the molecular basis of their virulence or pathogenicity. In this study, we present the Ustilago hordei-barley pathosystem as a model to characterize effectors, small proteins that are secreted by many pathogens as virulence factors to subdue its host. We generate a version of an U. hordei strain that now mimics the natural infection hyphae without the need for mating, normally required for infection. This engineered fungal strain can now be used to test the function of secretion of effector proteins from various pathogenic fungi. To this end, the highly efficient CRISPR/Cas9 system has been adapted so it becomes very efficient and fast in modifying genes in this fungus. We also show that the mode of infection of this engineered strain is very similar to that of various other related fungal pathogens such as rust and powdery mildew fungi. This will allow this test system to investigate virulence effector functions from a variety of pathogens. Such knowledge will contribute to the generation and breeding of (cereal) host plants with more resistance to these pathogens.