Journal Article

Alleviation of rapid, futile ammonium cycling at the plasma membrane by potassium reveals K<sup>+</sup>-sensitive and -insensitive components of NH<sub>4</sub><sup>+</sup> transport

Mark W. Szczerba, Dev T. Britto, Konstantine D. Balkos and Herbert J. Kronzucker

in Journal of Experimental Botany

Published on behalf of Society for Experimental Biology

Volume 59, issue 2, pages 303-313
Published in print February 2008 | ISSN: 0022-0957
Published online January 2008 | e-ISSN: 1460-2431 | DOI: https://dx.doi.org/10.1093/jxb/erm309

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Futile plasma membrane cycling of ammonium (NH4+) is characteristic of low-affinity NH4+ transport, and has been proposed to be a critical factor in NH4+ toxicity. Using unidirectional flux analysis with the positron-emitting tracer 13N in intact seedlings of barley (Hordeum vulgare L.), it is shown that rapid, futile NH4+ cycling is alleviated by elevated K+ supply, and that low-affinity NH4+ transport is mediated by a K+-sensitive component, and by a second component that is independent of K+. At low external [K+] (0.1 mM), NH4+ influx (at an external [NH4+] of 10 mM) of 92 μmol g−1 h−1 was observed, with an efflux:influx ratio of 0.75, indicative of rapid, futile NH4+ cycling. Elevating K+ supply into the low-affinity K+ transport range (1.5–40 mM) reduced both influx and efflux of NH4+ by as much as 75%, and substantially reduced the efflux:influx ratio. The reduction of NH4+ fluxes was achieved rapidly upon exposure to elevated K+, within 1 min for influx and within 5 min for efflux. The channel inhibitor La3+ decreased high-capacity NH4+ influx only at low K+ concentrations, suggesting that the K+-sensitive component of NH4+ influx may be mediated by non-selective cation channels. Using respiratory measurements and current models of ion flux energetics, the energy cost of concomitant NH4+ and K+ transport at the root plasma membrane, and its consequences for plant growth are discussed. The study presents the first demonstration of the parallel operation of K+-sensitive and -insensitive NH4+ flux mechanisms in plants.

Keywords: Ammonium; barley; efflux; influx; nitrogen-13; non-selective cation channels; potassium

Journal Article.  6980 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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