1.3.42
ANTI-BLAST RESPONSES OF RICE LEAVES AND CALLI ARE RELATED TO COMPLETE OR PARTIAL RESISTANCE AND NOT NECESSARILY DEPENDENT ON ACTIVE OXYGEN

AA AVERYANOV1, TD PASECHNIK1, VP LAPIKOVA1 and LM GAIVORONSKAYA2

1Research Institute of Phytopathology, p/o B. Vyazemy, Moscow Region 143050, Russia; 2Russian People's Friendship University, Moscow 117198, Russia

Background and objectives
The overproduction of active oxygen (AO) species (hydrogen peroxide, superoxide and hydroxyl free radicals) is one of the earliest events in the interaction of plants with pathogens. This response is more sound in plants with inheritable or induced disease resistance [1]. We demonstrated such a rise of chemically assayed superoxide production on rice leaf surface as related to blast resistance. Simultaneously, leaf diffusates were rendered toxic to the causal fungus, probably due to AO, since scavengers of AO behaved as antidotes [2, 3].

In the present work, rice cultivars differing in their resistance to blast were studied. We (i) evaluated the fungitoxicity of leaf diffusates, and (ii) using antioxidant protectors, estimated the involvement of particular AO species in the toxicity in connection with the resistance type. We also (iii) tested whether this cultivar-dependent response was peculiar to rice cell cultures and whether it depends on AO.

Materials and methods
Seven Magnaporthe grisea strains were studied. Thirteen rice cultivars were susceptible, completely resistant, partially resistant or possessing combined (complete plus partial) resistance to the strains used. Intact plant leaves or callus cultures were exposed to spore suspension or to water. The liquid, collected and filtered over several hours, was referred to as 'diffusate'. Its fungitoxicity was quantified as an inhibition of germination of another spore population.

Results and conclusions
(i) Diffusates of healthy leaves were sometimes fungitoxic but without a clear correlation with resistance. After inoculation, the level of toxicity increased. It was significantly higher in incompatible than in compatible combinations in all infected counterparts, regardless of the type of resistance.
(ii) The toxic action of all leaf diffusates was strongly diminished by catalase or some scavengers of hydroxyl radicals, suggesting the involvement of hydrogen peroxide and OH free radicals in the action. Superoxide dismutase (SOD) was effective as a protector in the case of completely resistant cultivars, but quite ineffective with partially resistant ones. In cultivars with combined resistance, the extent of SOD effect was intermediate. Like SOD, some other agents allowed discrimination between the groups of cultivars.
(iii) Callus diffusates were also fungitoxic, and this ability was stimulated by spore inoculation or elicitor treatment. Cultures derived from any resistant cultivar produced, after inoculation, more toxic diffusates as compared with susceptible ones. This action was sensitive to anti-oxidant reagents in the case of cultivars with partial resistance, but insensitive in experiments with most completely resistant cultivars.

Therefore, rice leaves respond to blast infection by production of fungitoxic exometabolites. Rice cells detain this ability in callus cultures. This effect is indicative of inheritable resistance and may contribute to its expression. The toxicity of leaf diffusates seems to depend on AO, and the type of resistance determines the particular AO species involved. However, the toxic matter of callus diffusates might be of some other nature, not necessarily involving AO.

References
1. Mehdy M, 1994. Plant Physiology 105, 467-472.
2. Nikolaev ON, Aver'yanov AA, Lapikova VP, Djavakhia VG, 1994. Pesticide Biochemistry and Physiology 50, 219-228.
3. Lapikova VP, Aver'yanov AA, Petelina GG et al., 1994. Russian Journal of Plant Physiology 41, 123-129.