The role of riboflavin (vitamin B2) as an elicitor of systemic resistance and a plant defense activator in rice was demonstrated in the present study. We conducted studies of the mechanism of riboflavin-induced resistance (IR) and defense responses in rice against sheath blight. Following foliar application of riboflavin, rice plants developed systemic resistance to the pathogen. Riboflavin, at concentrations necessary for induction of resistance (0.1 to 1000 μM), did not cause any cell death in rice; also, it did not have any direct effect on the in vitro growth of the fungus. The necessity of a time interval between riboflavin application and inoculation of pathogen for reduction of disease progress clearly indicates that riboflavin protects rice against R. solani by induction of resistance. This case was further confirmed by investigating the expression of rice defense genes, including genes coding for rice cationic peroxidase (PO-C1), phenylalanine ammonia lyase (PAL), and lipoxygenase (LOX) using reverse transcription-polymerase chain reaction (RTPCR). Riboflavin activated the expression of defense-related genes in treated rice plants especially after challenge inoculation that is correlated with induction of resistance.

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