We investigated the mechanisms involved in activation of rice defense responses against the economically important sheath blight disease, caused by the necrotrophic fungus Rhizoctonia solani. Riboflavin-treatment of rice plants led to induction of resistance and suppression of disease progress. Expression of a cationic rice peroxidase, PO-C1, that is involved in lignin production, was examined using reverse transcription-polymerase chain reaction (RT-PCR). PO-C1 was upregulated at 18 h after inoculation in riboflavin treated rice plants. The expression in control plants was lower than that observed in riboflavin-treated plants. Also, marker genes of phenylpropanoid and octadecanoid pathways were upregulated in riboflavin-treated plants following infection. Application of the lipoxygenase (LOX) inhibitor 5, 8, 11, 14-eicosatetraynoic acid (ETYA) led to a strong reduction of riboflavin-induced resistance. In addition, riboflavin and JA protect rice plants against sheath blight pathogen in a similar way. The potentiated expression pattern of the LOX gene (which is a key marker of the JA pathway) by riboflavin, and the suppression of riboflavin-induced resistance by application of ETYA suggest an involvement of the JA signalling pathway as well as lignin deposition in riboflavin-induced resistance against R. solani in the monocot model plant rice.

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