The Hypersensitive Reaction of Tobacco to Pseudomonas syringae pv. pisi: Activation of a Plasmalemma K/H Exchange Mechanism

Abstract

Net electrolyte efflux from suspension-cultured tobacco cells undergoing the hypersensitive reaction to Pseudomonas syringae pv. pisi resulted from a specific efflux of K(+) which was accompanied by an equimolar net influx of H(+). These fluxes began 60 to 90 minutes after inoculation of tobacco cells with bacteria, reached maximum rates of 6 to 9 micromoles per gram fresh weight tobacco cells per hour within 2.5 to 3 hours, and dropped below 4 micromoles per gram per hour within 5 hours. Tobacco cells lost approximately 35% of total K(+) during this period, and average cellular pH declined by approximately 0.75 pH unit. These events were accompanied by a 30% decrease in cellular ATP. K(+) and H(+) fluxes were inhibited by the protonophore (p-trifluoromethoxy)carbonyl cyanide phenylhydrazone and by increasing the K(+) concentration of the external solution. Tobacco leaf discs inoculated with the bacterium also exhibited a specific net K(+) efflux and H(+) influx. These results suggest that induction of the hypersensitive reaction in tobacco proceeds through the activation of a passive plasmalemma K(+)/H(+) exchange mechanism. It is hypothesized that activation of this exchange is a major contributing factor in hypersensitive plant cell death.