The Respiratory Burst and Electrolyte Leakage Induced by Sulfhydryl Blockers in Egeria densa Leaves Are Associated with H(2)O(2) Production and Are Dependent on Ca(2+) Influx

In leaves of Egeria densa Planchon, N-ethylmaleimide (NEM) and other sulfhydryl-binding reagents induce a temporary increase in nonmitochondrial respiration (ΔQO(2)) that is inhibited by diphenylene iodonium and quinacrine, two known inhibitors of the plasma membrane NADPH oxidase, and are associated with a relevant increase in electrolyte leakage (M. Bellando, S. Sacco, F. Albergoni, P. Rocco, M.T. Marré [1997] Bot Acta 110: 388–394). In this paper we report data indicating further analogies between the oxidative burst induced by sulfhydryl blockers in E. densa and that induced by pathogen-derived elicitors in animal and plant cells: (a) NEM- and Ag(+)-induced ΔQO(2) was associated with H(2)O(2) production and both effects depended on the presence of external Ca(2+); (b) Ca(2+) influx was markedly increased by treatment with NEM; (c) the Ca(2+) channel blocker LaCl(3) inhibited ΔQO(2), electrolyte release, and membrane depolarization induced by the sulfhydryl reagents; and (d) LaCl(3) also inhibited electrolyte leakage induced by the direct infiltration of the leaves with H(2)O(2). These results suggest a model in which the interaction of sulfhydryl blockers with sulfhydryl groups of cell components would primarily induce an increase in the Ca(2+) cytosolic concentration, followed by membrane depolarization and activation of a plasma membrane NADPH oxidase. This latter effect, producing active oxygen species, might further influence plasma membrane permeability, leading to the massive release of electrolytes from the tissue.