Rice cv. Sekiguchi-asahi was identified as a lesion mimic mutant from cv. Asahi. This mutant showed light-enhanced resistance to Magnaporthe grisea infection, inducing Sekiguchi lesion (sl) formation and tryptamine accumulation. To elucidate the role of chloroplasts on the activation of tryptamine pathway, the effect of some photosynthetic inhibitors on light-dependant Sekiguchi lesion formation, tryptamine accumulation, and enzyme activities was investigated. When detached leaf blades which had been pretreated with Linuron, 1, 10-Phenanthroline or Poly-L-lysine were inoculated with M. grisea, respectively, Sekiguchi lesion formation and typtamine accumulation were suppressed even under light, suppressing gene expression of tryptophan decarboxylase and monoamine oxidase activity. On the other hand, catalase activity was significantly inhibited in control leaves infected with M. grisea under light, but kept at high level in chemical-pretreated leaves. These results suggested that photosynthetic activity in chloroplasts was involved in inhibition of antioxidant sysytem and activation of tryptamine pathway responsible for light-enhanced resistance in sl mutant to M. grisea infection.