NADPH-dependent O2− generation in membrane fractions isolated from wounded potato tubers inoculated with Phytophthora infestans

A membrane-rich fraction from wounded potato tubers showed increasing activity of NADPH-dependent cytochrome c, but not of acetylated cytochrome c reducing activity, during ageing after slicing (wound-induced activity, WIA). In the fraction from aged tissues inoculated with an incompatible, but not a compatible, race of Phyiophthora infestans, as increase in native and acetylated cytochrome c reducing activities was closely associated with the occurrence of hypersensitive cell death and phytoalexin production (infection-induced activity, IIA). Treatment of aged tissues with hyphal wall components (HWC), a hypersensitivity-eliciting factor of the fungus, also activated both native and acetylated cytochrome c reducing activities similarly to infection. IIA and WIA were inhibited by NADP⁺, but only the former was appreciably inhibited by superoxide dismutase (SOD).

Water-soluble glucans (WSG), a hypersensitivity-inhibiting factor from compatible, but not incompatible, races of P. infestans, significantly inhibited the IIA but not the WIA in vitro.

These results suggest that a novel O₂⁻ generating NADPH oxidase system in the membrane of potato tissues may be activated following an incompatible cell reaction, which results in hypersensitive cell death and phytoalexin production. The lack of IIA activation in the compatible interaction may result from an inhibition of the reaction system by the water-soluble glucans from the fungus, thus resulting in the establishment of a compatible interaction.