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 NO(3)(-) 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 NO(3)(-), a shift in the midpoint potential of the depolarisation-activated gate by about -60 mV from 43 to -16 mV occurred (K(m) = 3.4 mM). No such effect was seen when chloride was replaced by malate. Addition of 10 mM NO(3)(-)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 NO(3)(-) effect on gating was considered, an increase of the NO(3)(-) concentration in the xylem sap to 5 mM would result in an enhancement of NO(3)(-) efflux by about 30%. Although the driving force for NO(3)(-) efflux would be reduced simultaneously, NO(3)(-) efflux into the xylem through X-QUAC would be maintained with high NO(3)(-) concentrations in the xylem sap; a situation which occurs for instance during the night.
Köhler
Barbara
B
Universität Potsdam, Institut für Biochemie und Biologie, Lehrstuhl für Molekularbiologie, Karl-Liebknecht-Str. 25, Haus 20, 14476 Golm, Germany. bakoehl@rz.uni-potsdam.de
Wegner
Lars H
LH
Osipov
Viktor
V
Raschke
Klaus
K
D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D000838:Anions / Q000378:metabolism*; D004553:Electric Conductivity; D006016:Glycolates / Q000494:pharmacology; D001467:Hordeum / Q000187:drug effects / Q000378:metabolism*; D015640:Ion Channel Gating / Q000187:drug effects; D007473:Ion Channels / Q000037:antagonists & inhibitors / Q000378:metabolism; D017136:Ion Transport / Q000187:drug effects; D008293:Malates / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D009566:Nitrates / Q000378:metabolism* / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D018517:Plant Roots / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D011523:Protoplasts / Q000187:drug effects / Q000378:metabolism
D000838:Anions; D006016:Glycolates; D007473:Ion Channels; D008293:Malates; D009566:Nitrates; C015765:MK 473; C030298:malic acid; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D011188:Potassium