Infection behavior of Alternaria alternata Japanese pear pathotype and localization of 1,3-β- d -glucan in compatible and incompatible interactions between the pathogen and host plants

Abstract

The infection behavior of Alternaria alternata Japanese pear pathotype and host responses were examined cytologically and ultrastructurally in compatible and incompatible interactions between Japanese pear plants and the pathogen during the infection process. No differences in spore germination, appressorial formation, or cuticle penetration were detected between susceptible and resistant leaves inoculated with spores of the pathogen. On the other hand, infection hyphae emerged from penetration pegs and invaded the pectin layers of susceptible leaves but not those of resistant leaves. The results indicate that expression of resistance to the pathogen may be induced before the formation of infection hyphae in pear tissues. Plasma membrane modifications were observed ultrastructurally 3 h after inoculation only in susceptible leaf cells. Papillae were usually formed in epidermal cells underneath the appressoria in resistant leaves at the stage when the progress of the pegs was stopped 9 h after inoculation. Callose deposition was observed fluorescence-microscopically as papillae at the infection sites in abaxial epidermis of both cultivars. The immunoelectron microscopic results demonstrated that the components of the papillae and the extracellular polysaccharides that had accumulated in plasma membrane modification sites were compatible with 1,3-β-d-glucan. These results suggest that the glucan deposition results from host cuticle damage by fungal invasion and from plasma membrane damage by AK-toxin.