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.