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.