Closely related members of the Medicago truncatula PHT1 phosphate transporter gene family encode phosphate transporters with distinct biochemical activities

Phosphorus is one of the essential mineral nutrients required by all living cells. Plants assimilate phosphate (P-i) from the soil, and their root systems encounter tremendous variation in P-i concentration, both temporally and spatially. Genome sequence data indicate that plant genomes contain large numbers of genes predicted to encode P-i transporters, the functions of which are largely unexplored. Here we present a comparative analysis of four very closely related P-i transporters of the PHT1 family of Medicago truncatula. Based on their sequence similarity and locations in the genome, these four genes probably arose via recent gene duplication events, and they form a small subfamily within the PHT1 family. The four genes are expressed in roots with partially overlapping but distinct spatial expression patterns, responses to P-i and expression during arbuscular mycorrhizal symbiosis. The proteins are located in the plasma membrane. Three members of the subfamily, MtPT1, MtPT2, and MtPT3, show low affinities for P-i. MtPT5 shares 84% amino acid identity with MtPT1, MtPT2, and MtPT3 but shows a high affinity for P-i with an apparent Km in yeast of 13 mu M. Sequence comparisons and protein modeling suggest that amino acid residues that differ substantially between MtPT5 and the other three transporters are clustered in two regions of the protein. The data provide the first clues as to amino acid residues that impact transport activity of plant P-i transporter proteins.