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

摘要

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 plant fungitoxic responses mediated by active oxygen. Both compounds, added to inocula (10 #mu#g/ml) or to soil (40 #mu#g/kg, two days prior to inoculation), induced disease resistance, namely, they diminished the frequency of compatible-type lesions on infected leaves, mainly at the expense of hypersensitive spot appearance,Leaf diffusates of untreated plants possessed a weak fungitoxicity, which increased slightly after a leaf infection or diffusate illumination. The flavins added to inocula, to soil, or to a collected diffusate augmented significantly the light-activatedpart of the diffusate toxicity. In some instances, the light-independent part was stimulated too. The effect was not due to the 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 chelator of iron ions desferrioxamine) protected spores from intoxication in almost all cases. This implies active oxygen involvement in the toxic and, presumably, anti-infective effects of the flavins. Roseoflavin was a better inducer of disease resistance than riboflavin and was alike in stimulation of the diffusate toxicity. However, roseoflavin did not produce superoxide and exhibited only weak fungitoxicity if substituted for riboflavin in the well-known O_2-generating model photosystem containing methionine. Therefore, the superoxide generation during photooxidation of methionine or similar substrates scarcely underlies the rise in fungitoxicity of leaf diffusate fungitoxicity and in resistance of plants treated 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.