Increased tryptophan decarboxylase and monoamine oxidase activities induce Sekiguchi lesion formation in rice infected with Magnaporthe grisea.

Sekiguchi 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 fluid (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 beta-phenylethylamine, which are substrates for MAO, but was not induced by non-substrates such as indole-3-propionic acid, (+/-)-phenylethylamine and tryptophan under light. Light-dependent induction of Sekiguchi lesion by tryptamine was significantly inhibited in the presence of MAO inhibitors, metalaxyl and semicarbazide, and H2O2-scavengers, ascorbic acid and catalase. H2O2 in M. grisea-infected leaves with and without Sekiguchi lesions was demonstrated directly in situ by strong 3,3'-diaminobenzidine (DAB) staining. On the other hand, H2O2 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 H2O2-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.