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 Ca(2+) influx of 0.02 to 0.06 micromole per gram per hour as determined from (45)Ca(2+) 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 Ca(2+) influx was prevented by EGTA and calcium channel blockers such as La(3+), Co(2+), and Cd(2+) 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 Ca(2+) influx is required for the K(+)/H(+) and hypersensitive responses in tobacco.