Active oxygen-associated control of rice blast disease by riboflavin and roseoflavin

Exogenous riboflavin and its dimethylated amino(nor)-derivative roseoflavin were studied in their ability to protect susceptible rice plants from blast disease and to induce fungitoxicity mediated by active oxygen. Both compounds, either added to the inoculum (10 microg/ml) or to soil (40 mg/kg, two days prior to inoculation), induced disease resistance, i.e., diminished the frequency of compatible-type lesions on infected leaves, mainly at the expense of the appearance of hypersensitive spots. Leaf diffusates of untreated plants possessed a weak fungitoxicity that increased slightly after leaf infection or illumination of diffusate. The flavins added to inoculum, to soil, or to a collected diffusate augmented significantly the light-activated part of the diffusate toxicity. In some instances, the light-independent part was stimulated as well. The effect was not due to direct fungitoxicity of flavins as they alone did not interfere with spores regardless of illumination. Antioxidant reagents (superoxide dismutase, catalase, scavengers of hydroxyl radical, and the iron ion chelator desferrioxamine) protected spores from intoxication in almost all cases. This implies the involvement of active oxygen in the toxic and, probably, disease-controlling effects of the flavins. Roseoflavin was a better inducer of disease resistance than riboflavin but was similar in stimulation of diffusate toxicity. However, roseoflavin did not produce superoxide and exhibited only weak fungitoxicity if substituted for riboflavin in the well-known O2--generating model photosystem containing methionine. Therefore, the superoxide generation due to photo-oxidation of methionine or similar substrates is not the cause of the increase of leaf diffusate fungitoxicity and of disease resistance of plants supplied with roseoflavin. It is suggested that the rise in active oxygen production favors a forthcoming hypersensitive reaction, and both phenomena contribute to resistance induced by flavo-compounds. The light-driven activation of oxygen may be of interest as a mode of action of novel fungicides.