Proton extrusion is an essential signalling component in the HR of epidermal single cells in the barley-powdery mildew interaction.

We propose a model for activation of the epidermal cell hypersensitive response (HR) in the barley/powdery mildew interaction. The model suggests that the plasma membrane proton pump (H+-ATPase) of epidermal cells is activated following penetration by an avirulent powdery mildew fungus. This will cause an acidification of the apoplast towards the mesophyll cells, thereby activating generation of H2O2 from the mesophyll, which subsequently triggers the epidermal cell to undergo HR. The model is supported by the following data: (1) the earliest HR-related H2O2 is found in the attachment zones between the epidermal cell and underlying mesophyll cells; (2) scavenger treatment reduces HR; (3) treatment of leaves with low-pH (3.5) citrate and succinate buffers causes more cells to undergo HR in the compatible interaction, while treatment with the same buffers at pH 5.5 reduces the number of HR-cells in the incompatible interaction; (4) race-specific proton extrusion is observed underneath epidermal tissue detached from leaves inoculated 15 h earlier; and (5) treatment of leaves with fusicoccin, an activator of the plasma membrane H+-ATPase, increases the number of HR-cells in the compatible interaction.

Up-regulation of the ascorbate-dependent antioxidative system in barley leaves during powdery mildew infection.

Abstract The ascorbate-dependent antioxidative system was studied in leaves of barley infected with the barley powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh). Increased ascorbate peroxidase (APX) activity was detected upon infection, especially in the compatible interaction. APX activity was determined in epidermal and total leaf tissues. A relatively higher increase in APX activity was found in the epidermis compared to total leaf 72 h after inoculation in the compatible interaction, but the increase was not restricted to the epidermis. Activity assays in native gels and Northern blot hybridization indicated that the increase in APX activity was caused by a cytosolic APX isoform. 'Inverse Northern blot' hybridization results with the cDNA of a cytosolic APX supported the relatively higher increase in epidermal APX activity compared to total leaf activity. In the compatible interaction, monodehydroascorbate reductase (MDHAR) activity increased in temporal and spatial patterns similar to that of APX activity. In contrast to this, dehydroascorbate reductase and glutathione reductase activities either decreased or were unaffected by Bgh infection. The increase in APX and MDHAR activities in the compatible interaction continued until severe infection of the leaves. Thus, an up-regulation of the antioxidative system of the host cells could play a role for maintenance of the biotrophic relationship between Bgh and the barley leaf by preventing proliferating oxidative processes, which would otherwise be harmful to the living plant cell on which the biotrophic powdery mildew fungus depends.