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
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 mmCl– 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 mmNO3– 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 mmwould 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.
Document Type: Original Article
Affiliations: 1: Universität Potsdam, Institut für Biochemie und Biologie, Lehrstuhl für Molekularbiologie, Karl-Liebknecht-Str. 25, Haus 20, 14476 Golm, Germany. 2: Biozentrum, Am Hubland, 97074 Würzburg, Germany. 3: Albrecht-von-Haller-Institut für Pflanzenwissenschaften der Universität Göttingen, Untere Karspüle 2, 37073 Göttingen, Germany.
Publication date: April 1, 2002