Molecular elucidation of elicitors and suppressors in rice blast disease by using as the marker the Sekiguchi lesion formation in lesion mimic mutants

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An antifungal compound with an infection-inhibiting activity was isolated from a rice Sekiguchi lesion (sl) mutant and identified as the indole alkaloid tryptamine. Tryptamine inhibited not only spore germination and appressorium formation of M.grisea at high concentrations (>600 μg/ml) but also the infection hypha formation in onion cells at low concentrations (150-300 μg/ml). Tryptamine is a normal compound of the rice sl mutant but accumulates further in rice with a mutant-type response (Sekiguchi lesion formation) to inoculation with M.grisea spores. A mutant type of response is involved in light-enhanced resistance. The accumulation of tryptamine was not induced, however, in rice with a wild-type response (blast lesion formation). This study strongly suggests that tryptamine plays an important role as a possible factor in light-enhanced resistance in the rice sl-mutants. Increased activities of tryptophan decarboxylase (TDC) and monoamine oxidase (MAO) were light-dependent and obs … More erved in sl-mutant of rice infected with M.grisea. TDC and MAO activites were already observed before penetration of M.grisea to rice cells and kept at high level even after Sekiguchi lesion formation. Spore germination fluid of M.grisea also induced the Sekiguchi lesion formation with increased activites of both enzymes activities and tryptamine accumulation. Sekiguchi lesion was also induced by the treatments with tryptamine and β-phenylethylamine, which are substrates for MAO, under the light, but not induced by non-substrates such as indole-3-propionic acid, (^^+___)-phenylethylamine and tryptophan even under the light. Light-dependent induction of Sekiguchi lesion by tryptamine was significantly inhibited in the presence of MAO inhibitors, metalaxyl and semicarbazide, and H_2O_2-scavengers, ascorbic acid and catalase. H_2O_2 in leaves with Sekiguchi lesion was demonstrated directly in situ by strong DAB staining. On the other hand, H_2O_2 induced Sekiguchi lesion on leaves of cv. Sekiguchi-asahi under the light, but not under the dark. This difference was associated with decrease of the catalase activity in infected leaves under the light and no decrease under the dark. We hypothesize that H_2O_2-induced breakdown of cellular organelles such as chloroplast and mitochondria in mesophtll cells may bring about high activities of TDC and MAO and development of Sekiguchi lesion, and that the Sl gene products in wild type rice may function as a suppressor of organelle breakdown cased by chemical or environmental stress. Less