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 blas t disease and to induce fungitoxicity mediated by active oxygen. Both compo unds, either added to the inoculum (10 mug/ml) or to soil (40 mg/kg, two da ys prior to inoculation), induced disease resistance, i.e,, diminished the frequency of compatible-type lesions on infected leaves. mainly at the expe nse of the appearance of hypersensitive spots. Leaf diffusates of untreated plants possessed a weak fungitoxicity that increased slightly after leaf i nfection or illumination of diffusate. The flavins added to inoculum, to so il, or to a collected diffusate augmented significantly the light-activated part of the diffusate toxicity. In some instances, the light-independent p art was stimulated as well. The effect was not due to direct fungitoxicity of flavins as they alone did not interfere with spores regardless of illumi nation. Antioxidant reagents (superoxide dismutase, catalase, scavengers of hydroxyl radical, and the iron ion chelator desferrioxamine) protected spo res from intoxication in almost ail 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 ribof lavin but was similar in stimulation of diffusate toxicity. However, roseof lavin did not produce superoxide and exhibited only weak fungitoxicity if s ubstituted for riboflavin in the well-known O-2(-)-generating model photosy stem containing methionine. Therefore, the superoxide generation due to pho to-oxidation of methionine or similar substrates is not the cause of the in crease of leaf diffusate fungitoxicity and of disease resistance of plants supplied with roseoflavin. It is suggested that the rise ia active oxygen p roduction favors a forthcoming hypersensitive reaction, and both phenomena contribute to resistance induced by flavo-compounds. The light-driven activ ation of oxygen may be of interest as a mode of action of novel fungicides.