Involvement of superoxide anion generation in the hypersensitive response of potato tuber tissues to infection with an incompatible race of Phytophthora infestans and to the hyphal wall components

Abstract

Potato tuber tissue discs, which were aged after wounding in order to acquire hypersensitive reactivity, reduced extracellular cytochrome c and nitroblue tetrazolium (NBT) following inoculation with an incompatible, but not compatible, race of Phytophthora infestans. The cytochrome c-reducing activity rapidly increased from l to 4 h after inoculation along with an increase in the percentage of hypersensitively dead cells, and then decreased from the time when most of the penetrated cells had died. A localized activation of NBT reduction around invading hyphae of the incompatible, but not those of the compatible, race was observed at early stages of penetration before cell death. The reductive activity of the discs was also elicited by treatment with a hypersensitivity-eliciting substance, hyphal wall components of the fungus. Superoxide dismutase (SOD), an enzyme catalysing the conversion of the superoxide anion (O 2 −) to H 2O 2 and O 2 inhibited the enhanced reducing activity of the discs when added to the assay solution, indicating that cytochrome c and NBT may be reduced by O 2 − generated from the discs. Pre-infectional, vacuum infiltration of the discs with a solution containing SOD significantly delayed the occurrence of hypersensitive cell death caused by infection with the incompatible race as well as the accumulation of phytoalexin. Application of SH-binding reagents and NADP +, but not respiratory inhibitors, inhibited the elicitation of the reducing activity caused by infection with the incompatible race. These results indicate that an O 2 −-generating system may be activated in potato tissues during the incompatible interaction induced by invading fungi or fungal wall components, and also that the generation of O 2 − may be involved during hypersensitive cell death as a trigger of the sequence of resistance reactions.