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, 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+.

Mutation of the regulatory phosphorylation site of tobacco nitrate reductase results in high nitrite excretion and NO emission from leaf and root tissue.

In wild-type Nicotiana plumbaginifolia Viv. and other higher plants, nitrate reductase (NR) is regulated at the post-translational level and is rapidly inactivated in response to, for example, a light-to-dark transition. This inactivation is caused by phosphorylation of a conserved regulatory serine residue, Ser 521 in tobacco, and interaction with divalent cations or polyamines, and 14-3-3 proteins. The physiological importance of the post-translational NR modulation is presently under investigation using a transgenic N. plumbaginifolia line. This line expresses a mutated tobacco NR where Ser 521 has been changed into aspartic acid (Asp) by site-directed mutagenesis, resulting in a permanently active NR enzyme. When cut leaves or roots of this line (S(521)) were placed in darkness in a buffer containing 50 mM KNO(3), nitrite was excreted from the tissue at rates of 0.08-0.2 micromol (g FW)(-1) h(-1) for at least 5 h. For the control transgenic plant (C1), which had the regulatory serine of NR intact, nitrite excretion was low and halted completely after 1-3 h. Without nitrate in the buffer in which the tissue was immersed, nitrite excretion was also low for S(521), although 20-40 micromol (g FW)(-1) nitrate was present inside the tissue. Apparently, stored nitrate was not readily available for reduction in darkness. Leaf tissue and root segments of S(521) also emitted much more nitric oxide (NO) than the control. Importantly, NO emission from leaf tissue of S(521) was higher in the dark than in the light, opposite to what was usually observed when post-translational NR modulation was operating.