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

We have examined the response of suspension cultured tobacco cells, after inoculation with zoospores from compatible or incomparible races of the Onmyete pathogen Phytophthora nicotianae. The incompatible interaction, characterized by the production of reactive oxygeu species (ROS) specifically superoxide (HO2 /O-2)(-) and hydrogen peroxide (H2O2), and by a hypersensitive responce (HR) was not dependent upon the prescrice of exogenous calcium. However, perturbation cdcalcil-Ull ron movements by EGTA or LaCl3 suppressed both ROS production and the extent of cell death. The Ca2+-specific ionophore, A23187 slightly enhanced ROS production during the incompatible interaction but had no effect on the responses of susceptible or unchallenged cells. These results confirm 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 efflux 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 all incompatible tobacco cell/zoospore interaction. (C) 2001 Academic Press.