AKT3, A Phloem-Localized K+ Channel, Is Blocked by Protons

The potassium-channel gene, AKT3, has recently been isolated from an Arabidopsis thaliana cDNA library. By using the whole-mount and in situ hybridization techniques, we found AKT3 predominantly expressed in the phloem. To study the physiological role of this channel type, AKT3 was heterologously expressed in Xenopus oocytes, and the electrical properties were examined with voltage-clamp techniques. Unlike the plant inward-rectifying guard cell K+ channels KAT1 and KST1, the AKT3 channels were only weakly regulated by the membrane potential. Furthermore, AKT3 was blocked by physiological concentrations of external Ca2+ and showed an inverted pH regulation. Extracellular acidification decreased the macroscopic AKT3 currents by reducing the single-channel conductance. Because assimilate transport in the vascular tissue coincides with both H+ and K+ fluxes, AKT3 K+ channels may be involved in K+ transport accompanying phloem loading and unloading processes.