Model wheat genotypes as tools to uncover effective defense mechanisms against the hemibiotrophic fungus Bipolaris sorokiniana

We investigated the interaction of several differentially resistant wheat genotypes with the hemibiotrophic phytopathogenic fungus Bipolaris sorokiniana (teleomorph Cochliobolus sativus). Wheat genotypes Yangmai 6, M 3 (W7976), Shanghai 4, and Chirya 7 showed higher levels of resistance compared with cv. Sonalika, used as a susceptible control. In a quantitative microscopic inspection, we found that fungal penetration into the epidermal layer failed mostly through a cell wall-associated defense mechanism. In cases where the fungus successfully overcame epidermal defense, its spread within the mesophyll tissue (necrotrophic phase) was restricted in the more resistant genotypes. Epidermal cell wall-associated defense, spreading as well as the extent of electrolyte leakage of infected tissue, correlated well with field resistance. We propose that cellular host responses such as formation of cell wall appositions as well as the degree of early mesophyll spreading of fungal hyphae are indicative of the defense potential of the respective host genotype and, therefore, could be used for the characterization of new spot blotch resistance traits in cereals.