Superoxide generation in chemically activated resistance of barley in response to inoculation with the powdery mildew fungus

In a previous work, a phenotype-specific accumulation of superoxide radical anions (O-2(.-)) after attack of the powdery mildew fungus (Blumeria [syn. Erysiphe] graminis f.sp. hordei) in near-isogenic barley (Hordeum vulgare L.) lines bearing different Mis genes for resistance was described (Huckelh oven and Kogel, 1998). We have now a histochemical study of the pathogenesi s-related O-2(.-) generation in the systemic activated resistance (SAR) res ponse induced in barley cv Pallas by the plant activator 2,6-dichloroisonic otinic acid (DCINA). SAR-specific defence was conducted prevalently characterized by penetration resistance. Fungal arrest was observed before haustorium formation by a hi ghly localized cell wall reinforcement (effective papillae) and, in most ca ses, by a subsequent hypersensitive cell death (HR). No O-2(.-) generation was found in association with these plant defence responses. However, a str ong O-2(.-) burst in the attacked epidermal cells was detected in the contr ol plants which were not activated by DCINA. This burst coincided with cell wall penetration and subsequent contact of the pathogen with the host plas ma membrane. A strong SAR-related O-2(.-) burst was induced in the mesophyll tissue bene ath the attacked and hypersensitively reacting epidermal cells in plants tr eated with DCINA. The accumulation of O-2(.-) was confined to chloroplasts The remarkable burst in mesophyll tissue was not followed by mesophyll-HR i ndicating that chloroplastic O-2(.-) generation is not sufficient for the h ypersensitive cell death. Since the same pattern of pathogenesis-related O- 2(.-) accumulation was identified for race-specific response mediated by th e Mlg gene for powdery mildew resistance, the present data are consistent w ith the hypothesis that the SAR phenotype is a phenocopy of the Mlg-type re sistance (Kogel et al., 1994).