title: Evidence for Cotransport of Nitrate and Protons in Maize Roots : II. Measurement of NO(3) and H Fluxes with Ion-Selective Microelectrodes.
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 NO(3) (-) and H(+) within the unstirred layer at the root surface. The nitrate-inducibility, pH dependence, and concentration dependence of net NO(3) (-) 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 NO(3) (-) (in PR McClure, LV Kochian, RM Spanswick, JE Shaff [1990] Plant Physiol 93: 281-289), dramatically inhibited NO(3) (-) absorption. These results strongly indicate that the NO(3) (-) electrical response is due to the operation of a NO(3) (-) transport system in the plasmalemma of maize root cells. Furthermore, the results from the H(+)-ATPase inhibitor studies indicate that the NO(3) (-) transport system is linked to the H(+)-ATPase, presumably as a NO(3) (-)/H(+) symport. This is further supported by the pH response of the NO(3) (-) transport system (inhibition at alkaline pH values) and the change in net H(+) flux from a moderate efflux in the absence of NO(3) (-), 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 NO(3) (-)/H(+) flux data is a NO(3) (-)/H(+) symport with a NO(3) (-):H(+) flux stoichiometry >1, whose operation results in the stimulation of the H(+)-ATPase due to the influx of protons through the cotransport system.
authors: Section of Plant Biology, Cornell University, Ithaca, New York 14853.
authors: P R
authors: PR
authors: McClure
authors: L V
authors: LV
authors: Kochian
authors: R M
authors: RM
authors: Spanswick
authors: J E
authors: JE
authors: Shaff