Many small brownish lesions developed over the entire leaf of the mutants 20-50 days after sowing. Activation of two defense-related genes, PBZ1 and PR1, was observed in the leaves of the mutant; high expression of PBZ1 was correlated with the lesion formation in the mutants, whereas PR1 was constitutively expressed irrespective of the lesion formation. Levels of momilactone A, a major phytoalexin of rice, were increased approximately 100-400-fold relative to the wild type levels.