A Plasmamembrane Redox System and Proton Transport in Isolated Mesophyll Cells

Abstract

Potassium ferricyanide (K3Fe[CN]6) was added to aerated and stirred nonbuffered suspensions of mechanically isolated photosynthetically competent Asparagus sprengeri Regel mesophyll cells. Rates of Fe(CN)63- reduction and H+ efflux were measured with or without illumination. On the addition of 1 millimolar Fe(CN)63- to nonilluminated cell suspensions acidification of the medium indicated an H+ efflux of 1.54 nanomoles H+/106 cells per minute. Simultaneous Fe(CN)63- reduction occurred at a rate of 1.55 nanomoles Fe(CN)63-/106 cells per minute. Illumination stimulated these rates of 14 to 17 times and corresponding values were 26.1 nanomoles H+/106 cells per minute and 22.9 nanomoles Fe(CN)63-/106 cells per minute. These two processes appeared to be tightly coupled and were rapidly inhibited when illuminated suspensions were transferred to darkness or treated with 1 micromolar 3-(3,4-dichlorophenyl)1,1 dimethylurea. Addition of 0.1 millimolar diethylstilbestrol eliminated ATP dependent H+ efflux in illuminated or nonilluminated cells but had no influence on Fe(CN)63- dependent H+ efflux. Recent reports indicate that a transmembrane redox system spans the plasma membrane of root cells and is coupled to the efflux of H+. The present report extends these observations to photosynthetically competent mesophyll cells. The results indicate a transport process independent of ATP driven H+ efflux which operates with a H+/e- stoichiometry of one.