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Ultrastructural Study on Acibenzolar-S-Methyl-Induced Scab Resistance in Epidermal Pectin Layers of Japanese Pear Leaves

The infection behavior of Japanese pear scab pathogen Venturia nashicola race 1 was studied ultrastructurally in acibenzolar-S-methyl (ASM)-pretreated susceptible Japanese pear (cv. Kousui) leaves to determine the mechanism of ASM-induced scab resistance. On ASM-pretreated leaf surfaces, the infection behavior (conidial germination and appressorial formation) was similar to that on distilled water (DW)-pretreated leaves prior to cuticle penetration by the pathogen. However, after penetration, differentiated behavior was found in epidermal pectin layers and middle lamellae of the ASM-pretreated leaves. Subcuticular hyphae in epidermal pectin layers and middle lamellae of ASM-pretreated pear leaves were observed at lower frequency than in DW-treated leaves. The results indicated that fungal growth was suppressed in ASM-pretreated pear leaves. In the pectin layers of ASM- and DW-pretreated leaves, some hyphae showed morphological modifications, which were used as criteria to judge collapse of hyphal cells, including plasmolysis, necrotic cytoplasm, and cell wall destruction. More hyphae had collapsed in ASM-pretreated leaves than in DW-treated ones. In addition, the cell walls of collapsed hyphae broke into numerous fibrous and amorphous pieces, suggesting that ASM-induced scab resistance might be associated with cell-wall-degrading enzymes from pear plants. In addition, results from morphometrical analysis suggested that the activity or production of pectin-degrading enzyme from hyphae were inhibited by ASM application when compared with DW treatment.