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 2160 mV, which was consistent with nH(+):HPO42- (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 SO42-, and to a lesser degree for NO3- and Cl-, but not for malate. Inward and outward currents showed linear dependence on the concentration of external HPO42-, 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.