High-affinity potassium transporter that could play a major role in the uptake of potassium from the rhizosphere. May act as a low-affinity potassium transporter under high potassium concentrations. Could also transport rubidium. Detected in the whole mature plant but preferentially expressed in roots and stems, and in potassium-starved plants. Low-affinity activity is strongly inhibited by barium or cesium. Belongs to the HAK/KUP transporter (TC 2.A.72.3) family.