High-affinity potassium transport into wheat roots involves sodium - a role for HKT1?

When a high-affinity K+ transporter, HKT1, was cloned from the roots of K+- starved wheat and characterized, it seemed that the wheat high-affinity K+- uptake mechanism had been found. We review the present status of HKT1 as a putative component of the wheat root high-affinity mechanism, concluding th at its role remains unclear, as it also does in barley. We describe a new a ttempt to find its role. High-affinity K+ transport in K+-starved wheat see dling roots was studied by measuring K+-evoked depolarization. A single dom inant transport system was shown to carry K+, Rb+ and Cs+, with a binding s ite selectivity of about 1:1:0.15, respectively. We showed that the small e ffect of 1 mM Na+ on K+-evoked depolarization could be inhibition under som e conditions and stimulation under others. We confirmed that the effect of 1 mM Na+ was to raise the K+ affinity and to reduce transport velocity. In 0 Na+, high pH greatly reduced the K+-affinity, but in 1 mM Na+, high pH ha d no effect. So, in a given test, the effect of Na+ was to increase or decr ease K+-evoked depolarization, depending on the test [K+] and the pH. We di scuss simplified, but plausible, kinetic models for this interaction of pH and Na+. The simplest model includes a K+ symporter driven by H+ or Na+ dep ending on their relative concentrations and affinities, with random binding order for the two possible driver ions. This symporter would differ from H KT1 in its selectivity for Rb+ vs K+ and in its inability to carry Na+ alon e. No role has been found for a symporter resembling HKT1 as it is presentl y characterized by heterologous expression.