Proton-coupled high-affinity phosphate transport revealed from heterologous characterization in Xenopus of barley-root plasma membrane transporter, HvPHT1;1

High-affinity phosphate transporters mediate uptake of inorganic phosphate (P-i) from soil solution under low P-i conditions. The electrophysiological properties of any plant high-affinity P-i transporter have not been described yet. Here, we report the detailed characterization of electrophysiological properties of the barley P-i transporter, HvPHT1;1 in Xenopus laevis oocytes. A very low K-m value (1.9 mu m) for phosphate transport was observed in HvPHT1;1, which falls within the concentration range observed for barley roots. Inward currents at negative membrane potentials were identified as nH+:P-i- (n > 1) co-transport based on simultaneous P-i radiotracer uptake, oocyte voltage clamping and pH dependence. HvPHT1;1 showed preferential selectivity for P-i and arsenate, but no transport of the other oxyanions SO(4)2- and NO3-. In addition, HvPHT1;1 locates to the plasma membrane when expressed in onion (Allium cepa L.) epidermal cells, and is highly expressed in root segments with dense hairs. The electrophysiological properties, plasma membrane localization and cell-specific expression pattern of HvPHT1;1 support its role in the uptake of P-i under low P-i conditions.