Phosphate Import in Plants: Focus on the PHT1 Transporters

Abstract

The main source of phosphorus for plants is inorganic phosphate (Pi), which is characterized by its poor availability and low mobility. Uptake of this element from the soil relies heavily upon the PHT1 transporters, a specific family of plant plasma membrane proteins that were identified by homology with the yeast PHO84 Pi transporter. Since the discovery of PHT1 transporters in 1996, various studies have revealed that their function is controlled by a highly complex network of regulation. This review will summarize the current state of research on plant PHT1 multigenic families, including physiological, biochemical, molecular, cellular, and genetics studies.

Keywords: PHT1; phosphate; phosphorus; plant; transcriptional and post-translational regulation; transporter; uptake.

Figure 1

Sequence alignment of several PHT1 transporters from Medicago truncatula (Mt), Arabidopsis thaliana (At), Hordeum vulgare (Hv), and Oryza sativa (Os). High or low affinity characterized PHT1 are indicated by orange or green colors respectively. Stars indicate the 7 amino acids with non-conservative changes between the three low affinity (MtPT1, Mt PT2, MtPT3) and the high affinity transporters (MtPT5) identified by Liu et al. (2008). The serine identified in AtPHT1.1 (Bayle et al., 2011) that regulates exit from the ER is boxed and the putative ER exit site is underlined.

Figure 2

Western blot analysis of PHT1;1, PHT1;2, and PHT1;3 on root membrane protein extracts reveal the presence of potential multimers. Plants were grown in vitro on medium supplemented with high (500 μM) or low (10 μM) Pi content (+P or −P respectively). Five micrograms of membrane protein were loaded per lane, on 10% acrylamide SDS PAGE gels before protein transfer onto nitrocellulose membranes. (A) The primary antibody was designed against two peptides common to PHT1;1, PHT1;2, and PHT1;3. In parallel, an anti GFP antibody (Roche) was used for control western blots at the same dilution (B). Lanes were loaded as follows: (1) Transgenic line expressing 35S:PHT1;1:GFP construct; and wild type control on +P (2) or −P (3) and Bio-Rad Precision Plus Protein Standards (4). GFP tagged (*) and untagged (#) PHT1 monomeric form, putative multimers (>) and cleaved GFP (−).

Figure 3

Expression patterns of the nine PHT1 transporters from Arabidopsis thaliana, based on a compilation of histological and transcriptomics data (combining information for both +P and −P conditions). The different plant parts are not presented to scale; in particular, the root system has been simplified so that only a single primary root and lateral root are represented. References corresponding to histological studies (promoter::GFP/GUS fusions or in situ hybridization) are identified on the figure by the following symbols: red square (Mudge et al., 2002), green circle (Karthikeyan et al., 2002), pink rectangle (Misson et al., 2004), black rhombus (Karthikeyan et al., 2009), and gray triangle (Nagarajan et al., 2011). Results based on transcriptomics studies are indicated by a blue T, and they correspond to a summary of several data sets presented on the eFP server (Winter et al., 2007). *Due to the design of most transcriptomic chips, it is not possible to distinguish the expression patterns of PHT1;1/PHT1;2 as well as PHT1;4/PHT1;7. When a PHT1 transporter is predominantly (but not exclusively) expressed in one or several tissues, its name is underlined. The subset of expression data indicated between brackets () are based on RT-PCR and/or transcriptomics only (i.e., not validated by promoter::reporter studies).

Figure 4

Time course for phosphate absorption based on ³²P detection in Arabidopsis. Plants were supplied with 10 μM Pi with ³²P (H₃PO₄, 12.5 kBq/ml). Images of radioactivity were obtained every 3 min; 10 images are presented here. For each panel, two phf mutants (left) and one wild type (Col-0; right) plantlet are shown. Circles on the left are ³²P standards:descending from the upper left corner, they represent 2.5, 1, and 0.5k Bq/μl. The plants shown are 11 days old, grown in vitro using 1/10 MS medium containing 500 μM Pi and subsequently transferred to 10 μM Pi for 3 days before analysis.