Abstract
BackgroundPhosphorus is one of the macronutrients essential for plant growth and development. The acquisition and translocation of phosphate are pivotal processes of plant growth. In a large number of plants, phosphate uptake by roots and translocation within the plant are presumed to occur via a phosphate/proton cotransport mechanism.Principal FindingsWe cloned two cDNAs from soybean (Glycine max), GmPT1 and GmPT2, which show homology to the phosphate/proton cotransporter PHO84 from the budding yeast Saccharomyces cerevisiae. The amino acid sequence of the products predicted from GmPT1 and GmPT2 share 61% and 63% identity, respectively, with the PHO84 in amino acid sequence. The deduced structure of the encoded proteins revealed 12 membrane-spanning domains with a central hydrophilic region. The molecular mass values are ∼58.7 kDa for GmPT1 and ∼58.6 kDa for GmPT2. Transiently expressed GFP–protein fusions provide direct evidence that the two Pi transporters are located in the plasma membrane. Uptake of radioactive orthophosphate by the yeast mutant MB192 showed that GmPT1 and GmPT2 are dependent on pH and uptake is reduced by the addition of uncouplers of oxidative phosphorylation. The K m for phosphate uptake by GmPT1 and GmPT2 is 6.65 mM and 6.63 mM, respectively. A quantitative real time RT-PCR assay indicated that these two genes are expressed in the roots and shoots of seedlings whether they are phosphate-deficient or not. Deficiency of phosphorus caused a slight change of the expression levels of GmPT1 and GmPT2.ConclusionsThe results of our experiments show that the two phosphate transporters have low affinity and the corresponding genes are constitutively expressed. Thereby, the two phosphate transporters can perform translocation of phosphate within the plant.
Highlights
Phosphorus is one of the most important macronutrients required for plant growth and metabolism, and is the key component of nucleic acids, phospholipids and energy-providing ATP as well as several enzymes and coenzymes
The results of our experiments show that the two phosphate transporters have low affinity and the corresponding genes are constitutively expressed
Kinetic characterization of the Pi uptake system of whole plants [10,11] and cultured cells [12] suggests a high-affinity transport operating in the low micromolar range and a low-affinity system operating at higher concentrations [7,13,14,15,16]
Summary
Phosphorus is one of the most important macronutrients required for plant growth and metabolism, and is the key component of nucleic acids, phospholipids and energy-providing ATP as well as several enzymes and coenzymes. Plants acquire phosphorus in the form of orthophosphate (Pi) [4,5,6]. Phosphate is the second most frequently limiting macronutrient for plant growth mainly because it exists in the soil in complex, insoluble, inorganic and organic forms that cannot be acquired directly by the plant [4,7]. For this reason, the concentration of Pi in soil solution can be as high as 10 mM but is present more often at concentrations as low as 1 mM [8]. In a large number of plants, phosphate uptake by roots and translocation within the plant are presumed to occur via a phosphate/proton cotransport mechanism
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