Uniport of NH4+ by the root hair plasma membrane ammonium transporter LeAMT1;1

The transport of ammonium/ammonia is a key process for the acquisition and metabolism of nitrogen. Ammonium transport is mediated by the AMT/MEP/Rh family of membrane proteins which are found in microorganisms, plants, and animals, including the Rhesus blood group antigens in humans. Although ammonium transporters from all kingdoms have been functionally expressed and partially characterized, the transport mechanism, as well as the identity of the true substrate (NH4+ or NH3) remains unclear. Here we describe the functional expression and characterization of LeAMT1:1, a root hair ammonium transporter from tomato (Lycopersicon esculentum) in Xenopus oocytes. Micromolar concentrations of external ammonium were found to induce concentration- and voltage-dependent inward currents in oocytes injected with LeAMT1;1 cRNA, but not in water-injected control oocytes. The NH4+-induced currents were more than 3-fold larger than methylammonium currents and were not subject to inhibition by Na+ or K+. The voltage dependence of the affinity of LeAMT1;1 toward its substrate strongly suggests that charged NH4+, rather than NH3, is the true 4 transport substrate. Furthermore, ammonium transport was independent of the external proton concentration between pH 5.5 and pH 8.5. LeAMT1;1 is concluded to mediate potential-driven NH4+ uptake and retrieval depending on root membrane potential and NH4+ concentration gradient.