A phosphate transporter from Medicago truncatula is expressed in the photosynthetic tissues of the plant and located in the chloroplast envelope

Phosphate is essential for many cellular processes including the light reactions of photosynthesis. Photosynthesis results in the production of triose phosphates that are transported across the chloroplast envelope to the cytosol in counterexchange for phosphate. Until recently, members of the plastid phosphate transport family, which mediate the exchange of phosphate for phosphorylated compounds, were the only proteins known to transport phosphate into the chloroplast. Here, we characterized a phosphate transporter, MtPHT2;1 of Medicago truncatula. Transient expression of an MtPHT2;1-GFP fusion protein indicates that MtPHT2;1 is located in the chloroplast envelope. The phosphate transport activity of MtPHT2;1 was assayed in yeast where the protein mediates phosphate uptake with a Km for phosphate of 0.6 mM and a pH optimum of 3-4. MtPHT2;1 is expressed in all the photosynthetic tissues of the plant and transcript levels are also influenced by light, development and phosphate status of the plant. The phosphate transport activity and location in the chloroplast envelope membrane suggest a role for MtPHT2;1 in phosphate transport into the chloroplast.