Most vascular plants can acquire
phosphate from the environment either directly, via the roots, or indirectly, via a fungal symbiont that invades the cortical cells of the root. Here we have identified two
cDNA clones (MtPT1 and MtPT2) encoding
phosphate transporters from a mycorrhizal root cDNA library (Medicago truncatula/Glomus versiforme). The cDNAs represent M. truncatula genes and the encoded
proteins share identity with high-affinity
phosphate transporters from Arabidopsis, potato, yeast, Neurospora crassa, and an arbuscular mycorrhizal (AM) fungus, G. versiforme. The function of the
protein encoded by MtPT1 was confirmed by complementation of a yeast
phosphate transport mutant (pho84). The K(m) of the MtPT1 transporter in this system is 192 microM. MtPT1 and MtPT2 transcripts are present in roots and transcript levels increase in response to
phosphate starvation. MtPT transcripts were not detected in leaves. Following colonization of the roots by the AM fungus G. versiforme, both MtPT1 and MtPT2 transcript levels decrease significantly. Down-regulation of
phosphate starvation-inducible genes in mycorrhizal roots appears to be a common occurrence and a homologue of a
phosphate starvation-inducible
purple acid phosphatase is also down-regulated in the mycorrhizal roots. The functional characteristics and expression patterns of the MtPT transporters are consistent with a role in the acquisition of
phosphate from the environment but suggest that they may not be involved in
phosphate uptake at the symbiotic interface in mycorrhizal roots.