Author(s): Thordal-Christensen H, Gregersen PL, Collinge DB
Barley leaves attacked by the powdery mildew fungus is a pathosystem well suited for studies of plant-pathogen interaction mechanisms. Nearly one hundred specific resistance genes have been identified, many of which are alleles at more or less complex loci. The corresponding avirulence genes are, on the other hand, evenly distributed in the powdery mildew fungus genome. The fungus colonizes only the leaf surface, placing haustoria in the leaf epidermal cells to acquire nutrients in an obligately biotrophic manner. The fungus exhibits a highly synchronous development and the epidermal cells respond accordingly. Papillae, which are formed early, subjacent to fungal germ tubes, arrest a considerable fraction of the attempted penetrations in all combinations of pathogen and plant genotypes (i.e. both incompatible and compatible). Papillae consist of several different components, and inhibitor studies have suggested that at least callose and phenylpropanoids are directly involved in arresting the growth of the penetration peg. In compatible interactions, the haustorial nutrient uptake is believed to be driven by fungal plasma membrane H+-ATPase activity. This is based on the apparent lack of H+-ATPase activity at the invaginated host plasma membrane. In incompatible interactions, the hypersensitive response serves as a back-up resistance mechanism arresting the germlings which have managed to penetrate the papilla. While only limited information is available concerning the expression of pathogenicity genes in the powdery mildew fungus, significantly more is known in relation to host response gene expression. However, the biological role of the host response genes is poorly understood; data suggest that many of them are merely part of a general stress response with no direct involvement in defense.KeywordsPowdery MildewGerm TubePowdery Mildew FungusFormae SpecialesBarley Powdery Mildew
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