Penetration Behaviour of Venturia nashicola, Associated with Hydrogen Peroxide Generation, in Asian and European Pear Leaves

The penetration behaviour of the pathogen Venturia nashicola, which causes scab disease in Asian pears, was studied at the ultrastructural and cytochemical levels in host and non-host leaves. We show, for the first time, that before V. nashicola penetrated the cuticle of the epidermis of the pear leaf, the appressorial bottom of the pathogen invaginated to form a cavity that contains electron-dense material. The leaf cuticle beneath the cavity also became highly electron dense following penetration by V. nashicola. The location of these electron-dense materials at the sites of penetration of the pathogen into plant cell walls suggests that they might be related to enzymes capable of degrading cell walls and that the cavities might be needed for successful penetration of leaves by V. nashicola. The generation of hydrogen peroxide (H2O2) was observed in penetration-related infection structures of V. nashicola, such as appressorial bottoms, infection sacs, penetration pegs and necks of subcuticular hyphae regardless of whether the interaction of V. nashicola with pear plants was compatible or incompatible. Nonetheless, more H2O2 was generated at the sites of the structures in scab-inoculated susceptible leaves than that in scab-inoculated resistant ones. Furthermore, the decrease in the level of H2O2 generation following treatment with the antioxidant ascorbic acid partially prevented the penetration of the cuticle. Therefore, the generation of H2O2 from the penetration-related structures might be a pathogenicity factor that contributes to strengthening the penetration peg of V. nashicola.