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 gested by Keppler et al. (15,16). Bacterial induction of the K+/H+ response is determined by genes within a 31 kb DNA fragment that also controls HR induction (13).We are interested in the mechanism by which the K+/H+ response and active oxygen production are activated during the HR. Rapid stimulation of plasma membrane ion channel, antiporter, and oxidase activities by transmembrane signal transduction pathways has been demonstrated in numerous animal cell systems (10, 22). In addition, phytoalexin synthesis in plants, a phenomenon often associated with the HR, may be triggered by a signaling pathway (20, 30). We therefore postulated that such a pathway is involved in induction of the K+/H+ and hypersensitive responses in tobacco by P. syringae pathovars. In this paper, we show that increased plasmalemma Ca2" influx is associated with and required for these host responses. MATERIALS AND METHODS ChemicalsThe HR3 is a rapid but localized necrotic response of plant cells to microbial invasion and is associated with disease resistance (18). Although its biochemical basis is not known, changes in membrane potential (24), ion fluxes (3, 11), lipid peroxidation (16, 17), and membrane-associated NADPHdependent oxidase activity (6) suggest plasma membrane involvement. In tobacco, initiation of the HR to Pseudomonas syringae pathovars pisi and syringae is marked by active oxygen production (15, 16), K+ efflux, intracellular acidification, and a net uptake of H+ from the extracellular medium (3,4). The K+/H+ response was initially described as a K+ efflux/H+ influx exchange (3), but H+ influx is unproven. Inhibition of this response and HR-associated cell death by increased extracellular pH and ATPase inhibitors (1, 27