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

摘要

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, NH₄+ 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 NH₄+, 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 NH₄+. Together, the results of this study show that plant K+ transporters and channels are able to transport NH₄+. 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+.