Loading of nitrate into the xylem: apoplastic nitrate controls the voltage dependence of X-QUAC, the main anion conductance in xylem-parenchyma cells of barley roots


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We report here that NO3 in the xylem exerts positive feedback on its loading into the xylem through a change in the voltage dependence of the Quickly Activating Anion Conductance, X-QUAC. Properties of this conductance were investigated on xylem-parenchyma protoplasts prepared from roots of Hordeum vulgare by applying the patch-clamp technique. Chord conductances were minimal around − 40 mV and increased with plasma membrane depolarisation as well as with hyperpolarisation. Two gates with opposite voltage dependences were postulated. When 30 mm Cl in the bath was replaced by NO3, a shift in the midpoint potential of the depolarisation-activated gate by about − 60 mV from 43 to − 16 mV occurred (Km = 3.4 mm). No such effect was seen when chloride was replaced by malate. Addition of 10 mm NO3 to the pipette solution and reduction of [Cl] from 124 to 4 mm (to simulate cytoplasmic concentrations) did not interfere with the voltage dependence of X-QUAC activation, nor was it affected by changes in external [K+]. If only the NO3 effect on gating was considered, an increase of the NO3 concentration in the xylem sap to 5 mm would result in an enhancement of NO3 efflux by about 30%. Although the driving force for NO3 efflux would be reduced simultaneously, NO3 efflux into the xylem through X-QUAC would be maintained with high NO3 concentrations in the xylem sap; a situation which occurs for instance during the night.