Vitamins are vital micronutrients for protecting human and animals health, and also are involved in growth regulation and activation of plant defense responses against biotic and abiotic stresses. In recent years, researches on application of vitamins in various monocot and dicot plants for activating defense responses against pathogens revealed significant effects of vitamins in activating defense mechanisms. In this research, the role of vitamin B2 (riboflavin) as an activator of resistance mechanisms in rice against Rhizoctonia spp. fungi, pathogenic on rice sheath, was investigated at cellular and molecular levels. Foliar application of riboflavin resulted in induction of systemic resistance against the pathogens. Riboflavin didn\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\'t cause any cell death in rice and didn\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\'t have any direct effect on pathogen growth in vitro. Necessity of the time interval between riboflavin application and infection with the pathogen, for reduction of disease severity, indicates induction of resistance by riboflavin in plant. This case was confirmed by investigating cellular and molecular defense responses of the plant. Lignin formation in the cells of rice plants treated with riboflavin in comparison with control was determined using floroglucinol-HCl test and thioglycolic acid assay. In the treated plants, priming in lignin formation was observed compared to control, that is correlated with increasing resistance in treated plants. Results of investigating the expression of rice cationic peroxidase (PO-C1), involved in lignin formation in rice, revealed downregulation of this gene in control compared to treated plant, and induction of resistance by riboflavin was related to upregulation of this gene in treated plants. Therefore, priming in lignification and PO-C1 gene expression are mechanisms involved in riboflavin-induced resistance in rice. There results showed the possibility of using riboflavin in management of Rhizoctonia diseases on rice, as a novel and environmentally safe method that could be applied to decrease the damage of these diseases.

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