Studies on Mechanism of Controlling Action of Fthalide against Rice Blast Disease

CHIDA Tsuneaki

doi:10.1584/jpestics.14.363

Fthalide specifically blocked appressorial penetration by Pyriculayia oryzae without affecting conidial germination or appressorial formation. The scanning electron microscopic observation revealed that fthalide affected the fungus prior to appressorial penetration on rice plants. The appressoria of an albino mutant of P. oryzae and cerulenin-treated wild type fungus lost the ability to penetrate onion epidermal walls, while they recovered melanization and penetration ability when melanin biosynthetic intermediates were added. Fthalide nullified the recovery with scytalone, a melanin intermediate, but not with 1, 8-dihydroxynaphthalene (1, 8-DHN). 1, 8-DHN alone was not sufficient to recover the penetration ability of wild appressoria treated with 1.0 or 10μg/ml of fthalide alone, and the presence of cerulenin was required. Our study suggests: Fthalide causes an accumulation of cytotoxic metabolites at higher concentrations. Fthalide blocks NADPH-dependent reductase reactions involved in the conversion of 1, 3, 6, 8-tetrahydroxynaphthalene to scytalone and the converion of 1, 3, 8-trihydroxynaphthalene to vermelone on the melanin biosynthetic pathway to 1, 8-DHN. The melanin biosynthetic pathway is blocked by fthalide in two antipenetrant actions, interference with melanin synthesis and accumulation of inhibitory metabolites. Sensitivity of P. oryzae P-2 against fthalide did not change during successive inoculations on rice plants treated with fthalide, suggesting that the lasting effectiveness of fthalide may relate to the aforementioned controlling action.

Studies on Mechanism of Controlling Action of Fthalide against Rice Blast Disease

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