Evidence for Cotransport of Nitrate and Protons in Maize Roots : II. Measurement of NO(3) and H Fluxes with Ion-Selective Microelectrodes |
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Authors: | McClure P R Kochian L V Spanswick R M Shaff J E |
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Institution: | Section of Plant Biology, Cornell University, Ithaca, New York 14853. |
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Abstract: | We report here on an investigation of net nitrate and proton fluxes in root cells of maize (Zea mays L.) seedlings grown without (noninduced) and with (induced) 0.1 millimolar nitrate. A microelectrode system described previously (IA Newman, LV Kochian, MA Grusak, WJ Lucas 1987] Plant Physiol 84: 1177-1184) was utilized to quantify net ionic fluxes from the measurement of electrochemical potential gradients for NO3− and H+ within the unstirred layer at the root surface. The nitrate-inducibility, pH dependence, and concentration dependence of net NO3− uptake correlated quite closely with the electrical response of maize roots to nitrate under the same experimental conditions (as described in PR McClure, LV Kochian, RM Spanswick, JE Shaff 1990] Plant Physiol 93: 281-289). Additionally, it was found that potential inhibitors of the plasmalemma H+-ATPase (vandate, diethylstilbestrol), which were shown to abolish the electrical response to NO3− (in PR McClure, LV Kochian, RM Spanswick, JE Shaff 1990] Plant Physiol 93: 281-289), dramatically inhibited NO3− absorption. These results strongly indicate that the NO3− electrical response is due to the operation of a NO3− transport system in the plasmalemma of maize root cells. Furthermore, the results from the H+-ATPase inhibitor studies indicate that the NO3− transport system is linked to the H+-ATPase, presumably as a NO3−/H+ symport. This is further supported by the pH response of the NO3− transport system (inhibition at alkaline pH values) and the change in net H+ flux from a moderate efflux in the absence of NO3−, to zero net H+ flux after exposing the maize root to exogenous nitrate. Although these results can be explained by other interpretations, the simplest model that fits both the electrical responses and the NO3−/H+ flux data is a NO3−/H+ symport with a NO3−:H+ flux stoichiometry >1, whose operation results in the stimulation of the H+-ATPase due to the influx of protons through the cotransport system. |
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