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

Abstract

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.

Sekiguchi Lesion Gene Encodes a Cytochrome P450 Monooxygenase That Catalyzes Conversion of Tryptamine to Serotonin in Rice

“…Infection by two races of compatible blast fungus caused the activation of SL gene expression in rice plants 4 The abbreviations used are: HPLC, high pressure liquid chromatography; TDC, tryptophan decarboxylase; ER, endoplasmic reticulum; GFP, green fluorescent protein; SECFP, super enhanced cyan fluorescent protein. 5 M. Isshiki, unpublished results.…”

“…These results suggest that SL has a role in rice innate immunity. SL Protein Possesses Tryptamine 5-Hydroxylase Activity-It was shown previously that tryptamine levels were higher in an sl mutant than in wild type plants and that serotonin levels were very low (4,5,15). These results suggest that SL converts tryptamine to serotonin.…”

“…PCR experiments indicated that the CM998 mutant has a 30-kb deletion containing genes for a P450 and a zinc finger protein. 5 Sequence analyses in the other mutants revealed that SL encodes a P450 monooxygenase (Fig. 1A).…”

“…Most of lesion mimic mutants are known to activate immune responses in the absence of pathogens (2). A rice lesion mimic mutant, Sekiguchi lesion (sl), exhibits unique orange colored lesions that are induced by inoculation of pathogens, including Magnaporthe grisea (4,5). In the sl mutants, increased activities of tryptophan decarboxylase and monoamine oxidase are correlated with lesion formation (5).…”

“…A rice lesion mimic mutant, Sekiguchi lesion (sl), exhibits unique orange colored lesions that are induced by inoculation of pathogens, including Magnaporthe grisea (4,5). In the sl mutants, increased activities of tryptophan decarboxylase and monoamine oxidase are correlated with lesion formation (5). However, the identity of the SL gene and its function in cell death and disease resistance remain to be studied.…”