Competition between uptake of ammonium and potassium in barley and arabidopsis roots: Molecular mechanisms and physiological consequences

Plants can use ammonium (NH₄⁺) as the sole nitrogen source, but at high NH₄⁺ concentrations in the root medium, particularly in combination with a low availability of K⁺, plants suffer from NH₄⁺ toxicity. To understand the role of K⁺ transporters and non-selective cation channels in K ⁺/NH₄⁺ interactions better, growth, NH ₄⁺ and K⁺ accumulation and the specific fluxes of NH₄⁺, K⁺, and H⁺ were examined in roots of barley (Hordeum vulgare L.) and Arabidopsis seedlings. Net fluxes of K⁺ and NH₄⁺ 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 (Gd³⁺) reduced NH₄⁺ influx, and the addition of TEA⁺ alleviated the NH₄⁺-induced depression of root growth in germinating Arabidopsis plants. Screening of a barley root cDNA library in a yeast mutant lacking all NH₄⁺ 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 NH₄⁺ 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 NH₄⁺ uptake via these transporters may contribute to NH₄⁺ toxicity at low K⁺ levels, and may explain the alleviation of NH₄ ⁺ toxicity by K⁺.