An electrogenic pump in the xylem parenchyma of barley roots

Abstract

Analysis of an electrogenic pump in the plasma membrane of xylem‐parenchyma protoplasts from barley roots was performed using the patch‐clamp technique in the whole‐cell configuration. Particularly with regard to understanding xylem loading and unloading, the study of the electrogenic pump from this cell type is important; its functional confirmation was lacking to date. About one‐half of the investigated protoplasts displayed current responses with reversal potentials between −80 and −200 mV. The application of fusicoccin, an H+‐pump stimulator, caused an increase in currents recorded at a membrane potential of 0 mV and a shift of the reversal potential by about −50 mV. Treatment with dicylohexylcarbodiimid, an H+‐pump inhibitor, resulted in the reduction of the current at 0 mV. The Ca2+‐pump inhibitor, erythrosin B, showed no effect on current density at 0 mV and on the polarisation of the membrane potential. Enlarging the transmembrane pH gradient by raising the pH of the extracellular solution from 5.8 to 8.8 stimulated the currents. These are strong indications that the electrogenic pump was an H+‐pump. Neither intracellular pH nor the intracellular Ca2+ concentration affected its activity. Simultaneous activity of the electrogenic pump and anion conductances could produce states in which protoplasts exhibited ‘intermediate’ reversal potentials. It was concluded that the electrogenic pump was not directly involved in the loading of KCl and KNO3 into the xylem but, in combination with anion channel activities, contributed to the establishment of membrane potentials at which electroneutral salt transport and acid release can proceed.