An NH4+-sensitive component dominates high-affinity K+ uptake in tomato plants

High-affinity K+ uptake has been studied in tomato plants grown under different conditions. K+ starvation induced a high-affinity K+ uptake with a Km of 10.5 ± 1.1 μM. This system was inhibited by the presence of NH4+ in the uptake solution and the inhibition was similar for plants grown in the absence or in the presence of NH4+. High-affinity K+ uptake was inhibited in plants grown in the presence of NaCl without NH4+ but not in plants grown with NaCl and with NH4+. A full-length cDNA has been isolated that probably encodes a high-affinity K+ uptake system, LeHAK5. The gene encoding LeHAK5 is induced in K+-starved plants and the presence of NH4+ in the growth solution further increases LeHAK5 expression. In K+-starved plants grown with NaCl in the absence of NH4+LeHAK5 expression is almost abolished but when plants are grown with NaCl in the presence of NH4+ a strong expression of the gene is detected. The LeHAK5 expression pattern parallels the presence of high-affinity K+ uptake in tomato plants which suggests that this gene is encoding an important component of K+ uptake in the micromolar range of K+ concentrations.