High and Low Affinity K Uptake

In the 1950s, of last century Epstein reported that in barley roots, K+ uptake exhibited biphasic kinetics in response to increasing external K+ concentrations (Epstein and Hagen 1952; Epstein et al. 1963). The first system showed high-affinity for K+ (Km for K+ of 21 mM) and was not inhibited by Na+. The second system showed low-affinity for K+ (Km of 11.4 mM) and was inhibited by Na+. In maize roots, the low-affinity component was linear (Kochian and Lucas 1982), and traditionally, it has been thought that ion channels are mostly responsible for the low-affinity component (Maathuis and Sanders 1996).

The high-affinity system has been thought to be mediated by transporters because with the reported data on plasma membrane potentials and gradients for K+ concentrations for barley and Arabidopsis (Maathuis and Sanders 1993, 1994; Walker et al. 1996), a channel would not be operative for K+ influx. A K+:H+ symport with a 1:1 stoichiometry has been suggested as the transport mechanism. In Neurospora crassa, a fungus which exhibited high-affinity K+ uptake similar to that present in plants, the high-affinity K+ uptake was shown to be mediated by a K+:H+ symport (Rodriguez-Navarro et al. 1986). In plant cells, as in fungal cells, the membrane potential is sustained by the H+-ATPase, supporting the idea of the plant K+:H+ symport. Other features observed in the high-affinity K+ uptake in plants are the upregulation by K+ starvation, the lack of discrimination between K+ and Rb+ and the inhibition by NH4+ (Kochian and Lucas 1988; Maathuis and Sanders 1996; Rodríguez-Navarro 2000; Rodríguez-Navarro and Rubio 2006).

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