Fungitoxic responses of rice callus culture as an expression of inheritable resistance to blast. Implication of active oxygen

Interactions of rice (Oryza sativa) callus culture with blast fungus (Magna porthe grisea) were investigated using eight plant cultivars and eight fung us strains, different in their virulence to the cultivars. Spores in contac t with calli germinated well in any host-pathogen combination. However, cal lus diffusates collected 6-24 h after inoculation usually affected the germ ination of a new spore population. In the case of uninfected calli, these e ffects (in 18 h) ranged from 12 % stimulation to 20 % inhibition with virul ent strains and, respectively, 12 % stimulation and 64 % inhibition with av irulent ones. After inoculation, the inhibition remained small (normally, b elow 30 %) in compatible combinations but grew significantly (up to 45-85 % ) in incompatible ones. This fungitoxicity increased gradually between 6 an d 24 h post-inoculation. Initially, it was mediated by active oxygen specie s whose effect was diminished by antioxidants added to the diffusate. Later on, it was due to some other toxicants arising from the previous generatio n of active oxygen. Therefore, the fungitoxic response of rice cells to an inoculation with blast spores is peculiar to callus culture (likewise intac t leaves) and active oxygen dependent. By this response, the cultured cells express the host-pathogen specificity characteristic of intact plants from which the cultures were derived. Presumably, the reported effects may be u sed to distinguish blast-resistant rice cell clones in vitro. (C) 2001 Edit ions scientifiques et medicales Elsevier SAS.