THE INVOLVEMENT OF ACTIVATED OXYGEN IN THE EXPRESSION OF THE VERTICALAND HORIZONTAL RESISTANCE OF RICE TO BLAST DISEASE

The ability of intact rice (Oryza sativa L.) plants to excrete metabol ites toxic to blast disease fungus Magnaporthe grisea (Hebert) Barr wa s studied in relation to plant disease resistance. Five fungus strains and seven rice cultivars were tested: a cultivar susceptible to blast disease and cultivars exhibiting vertical, horizontal, or both types of resistance. Diffusates collected from the leaves of resistant, but not susceptible cultivars 1-2 days after inoculation inhibited the ger mination of fungal spores by 60-80%. This effect is believed to be med iated by H2O2 and (. OH) hydroxyl radical, since it was prevented by t reatment with catalase or formate, a scavenger of this radical. Supero xide dismutase (SOD) inhibited the antifungal toxic effect, suggesting the involvement of the superoxide radical (O-2(-)). The contribution of O-2(-) to fungitoxicity depended on the combination of host and pat hogen. The toxicity of diffusates was SOD-insensitive in horizontally resistant cultivars, whereas it was more or less inhibited by SOD in h orizontally and vertically resistant cultivars. Boiling or adding thio urea prior to testing also decreased diffusate toxicity. Exogenous his tidine enhanced fungitoxicity in horizontally resistant cultivars but completely abolished it in vertically resistant cultivars. The inhibit ion of fungal growth by leaf diffusates is suggested to be a constitue nt of varietal rice resistance to blast. Reactive oxygen species are m ainly responsible for the toxicity. The mode of their formation is bel ieved to depend on the type of disease resistance and can be intermedi ate in horizontally and vertically resistant cultivars.