Competition between uptake of ammonium and potassium in barley and Arabidopsis roots: molecular mechanisms and physiological consequences

Plants can use ammonium (NH(4)(+)) as the sole nitrogen source, but at high NH(4)(+) concentrations in the root medium, particularly in combination with a low availability of K(+), plants suffer from NH(4)(+) toxicity. To understand the role of K(+) transporters and non-selective cation channels in K(+)/NH(4)(+) interactions better, growth, NH(4)(+) and K(+) accumulation and the specific fluxes of NH(4)(+), K(+), and H(+) were examined in roots of barley (Hordeum vulgare L.) and Arabidopsis seedlings. Net fluxes of K(+) and NH(4)(+) were negatively correlated, as were their tissue concentrations, suggesting that there is direct competition during uptake. Pharmacological treatments with the K(+) transport inhibitors tetraethyl ammonium (TEA(+)) and gadolinium (Gd(3+)) reduced NH(4)(+) influx, and the addition of TEA(+) alleviated the NH(4)(+)-induced depression of root growth in germinating Arabidopsis plants. Screening of a barley root cDNA library in a yeast mutant lacking all NH(4)(+) and K(+) uptake proteins through the deletion of MEP1–3 and TRK1 and TRK2 resulted in the cloning of the barley K(+) transporter HvHKT2;1. Further analysis in yeast suggested that HvHKT2;1, AtAKT1, and AtHAK5 transported NH(4)(+), and that K(+) supplied at increasing concentrations competed with this NH(4)(+) transport. On the other hand, uptake of K(+) by AtHAK5, and to a lesser extent via HvHKT2;1 and AtAKT1, was inhibited by increasing concentrations of NH(4)(+). Together, the results of this study show that plant K(+) transporters and channels are able to transport NH(4)(+). Unregulated NH(4)(+) uptake via these transporters may contribute to NH(4)(+) toxicity at low K(+) levels, and may explain the alleviation of NH(4)(+) toxicity by K(+).