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SummarySekiguchi lesion (sl)-mutant rice infected with Magnaporthe grisea showed increased light-dependent tryptophan decarboxylase (TDC) and monoamine oxidase (MAO) activities. TDC and MAO activities were observed before the penetration of M. grisea to rice cells and maintained high levels even after Sekiguchi lesion formation. Light-dependent expression of TDC gene was observed in leaves inoculated with M. grisea before Sekiguchi lesion formation. Spore germination¯uid (SGF) of M. grisea also induced Sekiguchi lesion formation accompanied by increased enzymes activities and tryptamine accumulation. Sekiguchi lesion was also induced by treatments with tryptamine and b-phenylethylamine, which are substrates for MAO, but was not induced by non-substrates such as indole-3-propionic acid, (AE)-phenylethylamine and tryptophan under light. Light-dependent induction of Sekiguchi lesion by tryptamine was signi®cantly inhibited in the presence of MAO inhibitors, metalaxyl and semicarbazide, and H 2 O 2 -scavengers, ascorbic acid and catalase. H 2 O 2 in M. grisea-infected leaves with and without Sekiguchi lesions was demonstrated directly in situ by strong 3,3 H -diaminobenzidine (DAB) staining. On the other hand, H 2 O 2 induced Sekiguchi lesions on leaves of cv. Sekiguchi-asahi under light, but not in darkness. This difference was associated with the decrease of catalase activity in infected leaves under light and the absence of decrease in darkness. We hypothesize that the H 2 O 2 -induced breakdown of cellular organelles such as chloroplasts and mitochondria in mesophyll cells may cause high TDC and MAO activities and the development of Sekiguchi lesion, and that the sl gene products in wild-type rice may function as a suppressor of organelle breakdown caused by chemical or environmental stress.

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A polyketide synthase gene named PKS1, involved in the melanin biosynthesis pathway of the phytopathogenic fungus Bipolaris oryzae, was isolated using restriction enzyme-mediated integration. Sequence analysis showed that the PKS1 encodes a putative protein that has 2155 amino acids and significant similarity to other fungal polyketide synthases. Targeted disruption of the PKS1 gene showed that it is necessary for melanin biosynthesis in B. oryzae. Northern blot analysis showed that PKS1 transcripts were specifically enhanced by near-ultraviolet radiation (300-400 nm) and that its temporal transcriptional patterns were similar to those of THR1 and SCD1 genes involved in the melanin biosynthesis pathway of B. oryzae.