INVOLVEMENT OF PLASMA-MEMBRANE CALCIUM INFLUX IN BACTERIAL INDUCTION OF THE K+/H+ AND HYPERSENSITIVE RESPONSES IN TOBACCO

An early event in the hypersensitive response of tobacco to Pseudomonas syringae pv syringae is the initiation of a K+/H+ response characterized by specific plasma membrane K+ efflux, extracellular alkalinization, and intracellular acidification. We investigated the role of calcium in induction of these host responses. Suspension-cultured tobacco cells exhibited a baseline Ca2+ influx of 0.02 to 0.06 micromole per gram per hour as determined from 45Ca2+ uptake. Following bacterial inoculation, uptake rates began to increase coincidently with onset of the K+/ H+ response. Rates increased steadily for 2 to 3 hours, reaching 0.5 to 1 micromole per gram per hour. This increased Ca2+ influx was prevented by EGTA and calcium channel blockers such as La3+, Co2+, and Cd2+ but not by verapamil and nifedipine. Lanthanum, cobalt, cadmium, and EGTA inhibited the K+/H+ response in both suspension-cultured cells and leaf discs and prevented hypersensitive cell death in leaf discs. We conclude that increased plasmalemma Ca2+ influx is required for the K+/H+ and hypersensitive responses in tobacco.