An NH 4+-sensitive component dominates high-affinity K + uptake in tomato plants

Abstract

High-affinity K + uptake has been studied in tomato plants grown under different conditions. K + starvation induced a high-affinity K + uptake with a K m of 10.5 ± 1.1 μM. This system was inhibited by the presence of NH 4 + in the uptake solution and the inhibition was similar for plants grown in the absence or in the presence of NH 4 +. High-affinity K + uptake was inhibited in plants grown in the presence of NaCl without NH 4 + but not in plants grown with NaCl and with NH 4 +. 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 NH 4 + in the growth solution further increases LeHAK5 expression. In K +-starved plants grown with NaCl in the absence of NH 4 + LeHAK5 expression is almost abolished but when plants are grown with NaCl in the presence of NH 4 + 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.