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.

Xylem ionic relations and salinity tolerance in barley

“…The NaCl-induced activation of H + efflux observed in our experiments (Figure 8b) is consistent with a role for the H + -ATPase pump present at the plasma membrane of xylem parenchyma cells in restoring membrane potential (Jahn et al, 1998;Zhu et al, 2007).…”

Correlations in concentrations, xylem and phloem flows, and partitioning of elements and ions in intact plants. A summary and statistical re-evaluation of modelling experiments in Ricinus communis

“…Wegner and Raschke (1994) suggested that cations and anions are released simultaneously from the xylem parenchyma into the xylem apoplast and vessels through channels, following electrochemical gradients set up by the ion uptake processes in the cortex. Zhu et al (2007) characterized an electrogenic pump in the plasma membrane of xylem parenchyma that was not directly involved in the loading of ions, but contributed to the establishment of membrane potentials, such that electroneutral salt transport and acid release can proceed. These assumptions can be supported by the present analysis, since the slopes for 'loading of the xylem' from the root tissue are mostly below 1.…”

Hydraulic conductance and K+ transport into the xylem depend on radial volume flow, rather than on xylem pressure, in roots of intact, transpiring maize seedlings

“…A re‐investigation of root water and ion transport at the level of the intact plant will certainly profit from the tremendous progress that has been made in recent years in unraveling membrane transport processes on a molecular scale (De Boer, 1999; Tester & Leigh, 2001; Very & Sentenac, 2003). By applying the patch‐clamp technique to protoplasts derived separately from cortex and stelar cells, respectively, ion channels for ion uptake and release were identified (Wegner & Raschke, 1994; Roberts & Tester, 1995; Wegner & De Boer, 1997, 1999; Maathuis et al ., 1998; Köhler & Raschke, 2000; Köhler et al ., 2002) and proton pump activity was demonstrated in both cell types (Zhu et al ., 2007). The genetic basis of many of these transport proteins has now been resolved, including a K + outward rectifying ion channel in stelar cells (SKOR) that has been shown to be involved in K + loading of xylem vessels (Gaymard et al ., 1998).…”