Activation of superoxide generation and enhancement of resistance against compatible races of Phytophthora infestans in potato plants treated with digitonin

Application of digitonin to the leaf surface of potato plants was demonstrated to activate O₂⁻ generation of the leaf tissues as determined by extracellular cytochrome c reduction which was inhibited by superoxide dismutase (SOD). Protoplasts prepared from stem shoots of potato enhanced NADPH-dependent reduction of extracellular cytochrome c immediately after being treated with digitonin. The reduction was also inhibited by SOD. The similar activation of SOD-sensitive cytochrome c reducing activity was observed in leaves or protoplasts of some other plants treated with digitonin.

Potato leaf tissues pre-treated with digitonin by application to the upper or lower surfaces, or through the petiole, were protected from disease caused by infection with compatible races of Phytophthora infestans. Treatment of the wound surface ofpotato tuber tissue with digitonin induced the generation of the superoxide anion and, at the same time, sporangial germination, appressorial formation and invasion by P. infestans was greatly reduced. This effect was partially negated by the presence of SOD in the inoculum and seemed not to depend on antifungal compounds.

These results suggest that plant tissues possess an O₂⁻ generating NADPH oxidase which is activated by digitonin and that its activation in potato may contribute to an enhanced resistance against attack by compatible races of P. infestans at pre- or post-infection stages.