OsPHT1;3 Mediates Uptake, Translocation and Remobilization of Phosphate under Extremely Low Phosphate Regimes.

Plant roots rely on phosphate (Pi) transporters to acquire soluble Pi from soil solutions that exists at micro-molar levels in natural ecosystems. Here, we functionally characterized a rice (Oryza sativa) Pi transporter, OsPHT1;3, that mediates Pi uptake, translocation and remobilization. OsPHT1;3 was directly regulated by OsPHR2 and, in response to Pi starvation, showed enhanced expression in young leaf blades and shoot basal regions and even more so in roots and old leaf blades. OsPHT1;3 was able to complement a yeast mutant strain defective in five Pi transporters and mediate Pi influx in Xenopus laevis oocytes. Overexpression of OsPHT1;3 led to increased Pi concentration both in roots and shoots. However, unlike that reported for other known OsPHT1 members which facilitate Pi uptake at relatively higher Pi levels, mutation of OsPHT1;3 impaired Pi uptake and root-to-shoot Pi translocation only when external Pi concentration was below 5 μM. Moreover, in basal nodes, the expression of OsPHT1;3 was restricted to the phloem of regular vascular bundles and enlarged vascular bundles. An isotope labeling experiment with 32P showed that ospht1;3 mutant lines were impaired in remobilization of Pi from source to sink leaves. Furthermore, overexpression and mutation of OsPHT1;3 led to reciprocal alteration in the expression of OsPHT1;2 and several other OsPHT1 genes. Yeast-two-hybrid, bimolecular fluorescence complementation, and co-immunoprecipitation assays all demonstrated a physical interaction between OsPHT1;3 and OsPHT1;2. Taken together, our results indicate that OsPHT1;3 acts as a crucial factor for Pi acquisition, root-to-shoot Pi translocation, and redistribution of phosphorus in plants growing in environments with extremely low Pi levels.