Arabidopsis ammonium transporters, AtAMT1;1 and AtAMT1;2, have different biochemical properties and functional roles

We have compared the biochemical properties of two different Arabidopsis am monium transporters, AtAMT1;1 and AtAMT1;2, expressed in yeast, with the bi ophysical properties of ammonium transport in planta. Expression of the AtA MT1;1 gene in Arabidopsis roots increased approximately four-fold in respon se to nitrogen deprivation. This coincided with a similar increase in high- affinity ammonium uptake by these plants. The biophysical characteristics o f this high-affinity system (K-m for ammonium and methylammonium of 8 muM a nd 31 muM, respectively) matched those of AtAMT1;1 expressed in yeast (K-m for methylammonium of 32 muM and K-i for ammonium of 1-10 muM). The same tr ansport system was present, although less active, in nitrate-fed roots. Amm onium-fed plants exhibited the lowest rates of ammonium uptake and appeared to deploy a different transporter (K-m for ammonium of 46 muM). 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 kineti cs for methylammonium uptake: in addition to a high-affinity phase with a K -m of 36 muM, a low-affinity phase with a K-m for methylammonium of 3.0 mM was measured. Despite the presence of a putative chloroplast transit peptid e in AtAMT1;2, the protein was not imported into chloroplasts in vitro. The electrophysiological data for roots, together with the biochemical propert ies 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 n itrogen-deprived root cells.