Factor Inducing Hypersensitive Resistance of Rice Plants to Blast Fungus, Pyricularia oryzae

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Localized cell death and activation of phenylpropanoid metabolism in suspension-cultured rice cells are induced by factors secreted by blast fungus. Microscopic observation revealed that the death of cells at the periphery of calli appeared within 1 h after addition of a TCA-precipitable fraction (TCAp) of the fungus culture filtrate. The number of dead cells continued to increase for up to 24 h, followed by general browning of the calli after about 48 h. However, the culture still contained uninjured cells within the callus clumps, and there was no significant difference in culture viability from non-treated callus. Phenylalanine ammonia-lyase (PAL) and peroxidase (POX) activities were induced by TCAp and reached a maximum at 6 h and 1-2 days after treatment, respectively. No cultivar-race specificity was found in PAL activity of cultured cells induced by TCAp. Phenolic compounds were found in callus tissue treated with TCAp. Among these phenolic compounds, accumulation of p-coumaric acid was remarkable. Fractions of fungal culture filtrate obtained by C_<18> reverse-phase chromatography were assayd for the ability to induce cell browning, PAL activity in cultured cells : Among the individual fractions, that which had the most potent activity to induce browning stimulated PAL activity.