The white barley mutant Albostrians shows a supersusceptible but symptomless interaction phenotype with the hemibiotrophic fungus Bipolaris sorokiniana

Bipolaris sorokiniana (teleomorph: Cochliobolus sativus) is a cereal pathogen of increasing global concern, with most significance in Asiatic cropping systems. In order to gain insight into the mechanism of host resistance, we studied fungal development on the supersusceptible barley mutant albostrians and its parent cv. Haisa. A microscopic dissection of early fungal growth on Haisa and green albostrians leaves revealed a distinct epidermis-localized biotrophic and a mesophyll-based necrotrophic phase. White, green, and striped white-green albostrians leaves showed extreme differences in disease development. When comparing cellular defense responses, we found restriction of fungal spreading after successful infection of host mesophyll tissue to be the most important mechanism limiting outbreak of the disease. Colonization of susceptible green leaves, but not extreme colonization of supersusceptible white albostrians leaves, was associated with macroscopically visible lesion formation and mesophyll accumulation of hydrogen peroxide (H2O2), implying a symptomless growth of the pathogen in supersusceptible host tissue. In contrast, early epidermal papilla-based resistance was closely linked to H2O2 accumulation in all leaf types. In white leaves, ascorbate peroxidase (APX), glutathione-S-transferase (GST), and the cell death regulator Bax-inhibitor-1 (BI-1) showed a stronger constitutive or pathogen responsive activation, whereas glycolate oxidase (GLOX) and catalase (CAT2) expression was stronger in green leaves. We discuss supersusceptibility and symptomless growth on the basis of the histochemical and the gene expression data.