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Depolarization of Cell Membrane Potential during Trans-Plasma Membrane Electron Transfer to Extracellular Electron Acceptors in Iron-Deficient Roots of Phaseolus vulgaris L. ¹

Departments of Plant Physiology, University of Amsterdam, Kruislaan 318, 1098 SM Amsterdam, The Netherlands, University of Groningen, P.O. Box 14, 9750 AA Haren, The Netherlands

Transfer of electrons from the cytosol of bean (Phaseolus vulgaris L.) root cells to extracellular acceptors such as ferricyanide and Fe^IIIEDTA causes a rapid depolarization of the membrane potential. This effect is most pronounced (30-40 millivolts) with root cells of Fe-deficient plants, which have an increased capacity to reduce extracellular ferric salts. Ferrocyanide has no effect. In the state of ferricyanide reduction, H⁺ (1H⁺/2 electrons) and K⁺ ions are excreted. The reduction of extracellular ferric salts by roots of Fe-deficient bean plants is driven by cellular NADPH (Sijmons, van den Briel, Bienfait 1984 Plant Physiol 75: 219-221). From this and from the membrane potential depolarization, we conclude that trans-plasma membrane electron transfer from NADPH is the primary process in the reduction of extracellular ferric salts.