Loading of Ions into the Xylem of the Root

Abstract

Anion conductances and an electrogenic pump participating in the loading of ions into the xylem of the root were investigated on protoplasts isolated from the xylem parenchyma of barley roots, using the patch-clamp technique. Previous studies had mainly focused on K+ channels; they showed that loading of the stelar apoplast was thermodynamically downhill, not requiring a second pump at the exit of the symplast of the root to the apoplast. Our work confirmed this view after concentrating on anion conductances and an electrogenic pump. Three major anion conductances were found residing in the plasmalemma of the xylem parenchyma: an inwardly rectifying anion channel (X-IRAC) with open times of up to several seconds, generating the largest currents at hyperpolarization, a quickly activating anion conductance (X-QUAC), important for anion loading at voltages between –50 mV and the equilibrium voltage for the permeating anion, and a slowly activating anion conductance (X-SLAC), activating above –100 mV. Anion currents through X-SLAC and X-QUAC, in combination with K+ currents through the outwardly directed K+ channel, KORC, were estimated to be large enough to account for reported rates of xylem loading. In the presence of nitrate in the xylem, the current-voltage relationship of X-QUAC shifted towards hyperpolarization, exerting positive feedback on loading of nitrate, so that nitrate efflux into the xylem would be maintained even at high concentrations of nitrate in the xylem, which occurs for instance during the night. Current-voltage relationships of the protoplasts showed also the existence of an electrogenic pump. It was stimulated by fusicoccin and inhibited by dicyclohexylcarbodiimide (DCCD): it exhibited features of an H+- ATPase. The pump was short-circuited by other conductances, mainly for anions. Simultaneous activity of pump and anion conductances provided a condition for acid release into the xylem. Beyond participating in controlling the membrane voltage, the pump appears to be involved in energizing the absorption of ions from the xylem during the circulation of nutrients within the plant. Ion conductances in the xylem parenchyma of roots of barley and maize are similar. A brief review is given on what is known about their control.