Relationship between transembrane ion movements, production of reactive oxygen species and the hypersensitive response during the challenge of tobacco suspension cells by zoospores of Phytophthora nicotianae

Abstract

We have examined the responses of suspension cultured tobacco cells after inoculation with zoospores from compatible or incompatible races of the Oomycete pathogen Phytophthora nicotianae. The incompatible interaction, characterized by the production of reactive oxygen species (ROS) (speci®cally superoxide (HO2=Oÿ2 ) and hydrogen peroxide (H2O2)), and by a hypersensitive response (HR), was not dependent upon the presence of exogenous calcium. However, perturbation of calcium ion movements by EGTA or LaCl3 suppressed both ROS production and the extent of cell death. The Ca2‡-speci®c ionophore, A23187, slightly enhanced ROS production during the incompatible interaction but had no e€ect on the responses of susceptible or unchallenged cells. These results con®rm that both ROS production and the HR are potentiated by movement of endogenous calcium across the plasmalemma. While there was no evidence of a transmembrane K‡/H‡ exchange immediately following incompatible zoospore challenge, a later potassium e‚ux from cells coincided with the onset of the HR. Unexpectedly, medium pH drifted downwards during all host cell responses suggesting that alkaline peroxidases are not the major source of ROS generation during an incompatible tobacco cell/zoospore interaction.