Channel-like characteristics of the low-affinity barley phosphate transporter PHT1;6 when expressed in Xenopus oocytes.

Remobilization of inorganic phosphate (P(i)) within a plant is critical for sustaining growth and seed production under external P(i) fluctuation. The barley (Hordeum vulgare) transporter HvPHT1;6 has been implicated in P(i) remobilization. In this report, we expressed HvPHT1;6 in Xenopus laevis oocytes, allowing detailed characterization of voltage-dependent fluxes and currents induced by HvPHT1;6. HvPHT1;6 increased efflux of P(i) near oocyte resting membrane potentials, dependent on external P(i) concentration. Time-dependent inward currents were observed when membrane potentials were more negative than -160 mV, which was consistent with nH(+):HPO(4)(2-) (n > 2) cotransport, based on simultaneous radiotracer and oocyte voltage clamping, dependent upon P(i) concentration gradient and pH. Time- and voltage-dependent inward currents through HvPHT1;6 were also observed for SO(4)(2-)and to a lesser degree for NO(3)(-)Cl(-)but not for malate. Inward and outward currents showed linear dependence on the concentration of external HPO(4)(2-)similar to low-affinity P(i) transport in plant studies. The electrophysiological properties of HvPHT1;6, which locates to the plasma membrane when expressed in onion (Allium cepa) epidermal cells, are consistent with its suggested role in the remobilization of P(i) in barley plants.

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 µm) 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 SO4²â» 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.