Cloning and Characterization of Two Phosphate Transporters from Medicago truncatula Roots: Regulation in Response to Phosphate and to Colonization by Arbuscular Mycorrhizal (AM) Fungi

Abstract

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.

The arbuscular mycorrhizal fungus Glomus mosseae can enhance arsenic tolerance in Medicago truncatula by increasing plant phosphorus status and restricting arsenate uptake

“…Means followed by the same letter within columns are not significantly different by Duncan's multiple range test at the 5% level. downregulate high-affinity phosphate transporters (MtPT1 and MtPT2) which may be responsible for absorption of phosphate from soil by M. truncatula (Liu et al, 1998;Chiou et al, 2001;Burleigh, 2001). Furthermore, studies on the kinetics of phosphate/arsenate uptake have suggested that AMF can downregulate phosphate/arsenate transporters in both arsenate resistant and non-resistant Holcus lanatus (Gonzalez-Chavez et al, 2002).…”

Phosphorus efficiencies and responses of barley (Hordeum vulgare L.) to arbuscular mycorrhizal fungi grown in highly calcareous soil

“…To sort out these complications requires more detailed investigations of growth parameters and P uptake patterns for both roots and AMF in compartmented systems, beyond the scope of the present study. Such investigations will also help in determining the extent to which P uptake via AMF results in changes of expression of P transporter genes in roots (see Liu et al 1998).…”

The spatial expression patterns of a phosphate transporter (MtPT1) from Medicago truncatula indicate a role in phosphate transport at the root/soil interface

“…Plant material and growth conditions M. truncatula was grown under conditions with varying levels of phosphate as described previously (Liu et al, 1998b). Brie¯y, 10-day-old seedlings were grown on acid washed, sterilized river sand and fertilised with half-strength Hoagland's solution containing either 0, 0.02, 0.1, 1.0, or 5.0 mM KH 2 PO 4 three times a week.…”

“…The RNA was separated on a formaldehye gel, blotted and hybridised with either 32 P-labeled MtPT1 cDNA or 32 P-labeled MtPT1 oligo (Liu et al, 1998b).…”

“…This includes transporters from potato (StPT1 and StPT2) (Leggewie et al, 1997), Arabidopsis thaliana (AtPT1 and AtPT2) (Muchhal et al, 1996;, Medicago truncatula (MtPT1 and MtPT2) (Liu et al, 1998b) and tomato (LePT1 and LePT2) (Daram et al, 1998;Liu et al, 1998a). The encoded proteins share between 75 and 85% identity at the amino acid level and are distinct from PHT2;1.…”