Plasmolysis induced by very low concentrations of Cu2+ in Pseudomonas syringae ATCC 12271, and its relation with cation fluxes

Abstract

Very low concentrations of CuSO4 induced a massive leakage of K+ from Pseudomonas syringae ATCC 12271 cells suspended in distilled water, Ca2+/PIPES or Na+/PIPES. Cell suspensions in distilled water, Ca2+/ or Mg2+/PIPES, treated with Cu2+, showed appreciable increases in optical density, whereas suspensions in Na+/PIPES were unaffected. The addition of monovalent cations to suspensions in distilled water, Ca2+/ or Mg2+/PIPES prevented the optical density increase induced by Cu2+, whereas the addition of Ca2+ or Mg2+ to suspensions in distilled water did not have this effect. Cells suspended in Na+/PIPES and treated with Cu2+ showed no major ultrastructural alterations, but cells treated with Cu2+ in distilled water showed pronounced plasmolysis. At all Cu2+ concentrations, two types of cells were observed, normal and heavily plasmolysed. An increase in Cu2+ concentration resulted in an increase in the percentage, but not in the degree of plasmolysis, of the plasmolysed cells. Cells suspended in distilled water or Na+/PIPES bound significant amounts of copper. Cu2+ concentrations that induced leakage of most of the unbound K+ did not saturate the copper-binding sites in the cells. These results indicated that plasmolysis is a direct consequence of the massive K+ leakage from the cells, in agreement with the notion that K+ is the main osmolyte in bacteria grown under normal conditions. Monovalent (but not divalent) cations prevented plasmolysis induced by copper by entry into the cells after the release of K+ ions. Na+ and Li+ probably replaced resident K+ ions in the neutralization of negative charges of cytoplasmic constituents. K+ efflux and plasmolysis, induced by Cu2+, appeared to be essentially ‘all-or-nothing’ effects.