Arabidopsis ammonium transporters,AtAMT1;1 and AtAMT1;2, have different biochemical properties and functional roles |
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Authors: | Shelden Megan C. Dong Bei de Bruxelles Guy L. Trevaskis Ben Whelan Jim Ryan Peter R. Howitt Susan M. Udvardi Michael K. |
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Affiliation: | (1) Division of Biochemistry and Molecular Biology, The Australian National University, Canberra, ACT, 0200, Australia;(2) Biochemistry Department, University of Western Australia, Perth, 6907 WA, Australia;(3) Division of Plant Industry, CSIRO, GPO Box 1600, Canberra, ACT, 2601, Australia;(4) Present address: Max Plank Institute of Molecular Plant Physiology, Am Mühlenberg 1, 14476 Golm, Germany |
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Abstract: | We have compared the biochemical properties of two different Arabidopsis ammonium transporters, AtAMT1;1 and AtAMT1;2, expressed in yeast, with the biophysical properties of ammonium transport in planta. Expression of the AtAMT1;1 gene in Arabidopsis roots increased approximately four-fold in response to nitrogen deprivation. This coincided with a similar increase in high-affinity ammonium uptake by these plants. The biophysical characteristics of this high-affinity system (Km for ammonium and methylammonium of 8 M and 31 M, respectively) matched those of AtAMT1;1 expressed in yeast (Km for methylammonium of 32 M and Ki for ammonium of 1–10 M). The same transport system was present, although less active, in nitrate-fed roots. Ammonium-fed plants exhibited the lowest rates of ammonium uptake and appeared to deploy a different transporter (Km for ammonium of 46 M). Expression of AtAMT1;2 in roots was insensitive to changes in nitrogen nutrition. In contrast to AtAMT1;1, AtAMT1;2 expressed in yeast exhibited biphasic kinetics for methylammonium uptake: in addition to a high-affinity phase with a Km of 36 M, a low-affinity phase with a Km for methylammonium of 3.0 mM was measured. Despite the presence of a putative chloroplast transit peptide in AtAMT1;2, the protein was not imported into chloroplasts in vitro. The electrophysiological data for roots, together with the biochemical properties of AtAMT1;1 and Northern blot analysis indicate a pre-eminent role for AtAMT1;1 in ammonium uptake across the plasma membrane of nitrate-fed and nitrogen-deprived root cells. |
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Keywords: | ammonium transporter Arabidopsis thaliana biphasic kinetics electrophysiology methylammonium yeast complementation |
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