Octadecanoid or jasmonic acid signal transduction pathway has important role in plant defense responsesagainst biotic and abiotic stresses. Jasmonic acid and its derivatives such as methyl jasmonate are final products of octadecanoid pathway, which are produced after effect of several enzymes on unsaturated fatty acids of chloroplast and their oxidation, and are involved in plant defense responses. Researches on dicot plant, Arabidopsis, revealed that resistance against necrotrophic pathogens is related to jasmonic acid signaling. Arabidopsis mutants, which were deficient in jasmonic acid synthesis, showed more susceptibility to necrotrophic fungi. These results indicate that jasmonic acid is a cellular signal that is involved in defense responses of dicot plants against necrotrophic pathogens. Because of scarce information about the role of this signal in defense mechanisms of monocot plants, more focus on this kind of research seems to be necessary. Therefore, in this study, the role of jasmonic acid as a cellular signal, on plant defense reactions against pathogenic Rhizoctonia fungi was investigated. Treatment of rice leaves with ETYA (an inhibitor of lipoxygenase, LOX, enzyme activity as one of the key enzymes in jasmonic acid signaling) resulted in increasing susceptibility of the leaves to the fungi. After ETYA treatment, the Lox expression and the LOX enzyme activity severely decreased, that is related to decrease of resistance against the fungi. These results were confirmed using rice mutant, Hebiba, deficient in jasmonic acid synthesis. Disease severity on the mutant plants was significantly higher, which is correlated with increasing vegetative growth and penetration of the fungi inside plant cells and increasing cell death. Therefore, in rice plant, jasmonic acid signaling has important role in defense reactions and resistance against necrotrophic pathogens.

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