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 (Magnaporthe grisea) were investigated using eight plant cultivars and eight fungus strains, different in their virulence to the cultivars. Spores in contact with calli germinated well in any host-pathogen combination. However, callus diffusates collected 6–24 h after inoculation usually affected the germination of a new spore population. In the case of uninfected calli, these effects (in 18 h) ranged from 12 % stimulation to 20 % inhibition with virulent strains and, respectively, 12 % stimulation and 64 % inhibition with avirulent ones. After inoculation, the inhibition remained small (normally, below 30 %) in compatible combinations but grew significantly (up to 45–85 %) in incompatible ones. This fungitoxicity increased gradually between 6 and 24 h post-inoculation. Initially, it was mediated by active oxygen species whose effect was diminished by antioxidants added to the diffusate. Later on, it was due to some other toxicants arising from the previous generation of active oxygen. Therefore, the fungitoxic response of rice cells to an inoculation with blast spores is peculiar to callus culture (likewise intact 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 used to distinguish blast-resistant rice cell clones in vitro.