Characterization of two phosphate transporters from barley; evidence for diverse function and kinetic properties among members of the Pht1 family

Abstract

Putative phosphate transporters have been identified in a barley (Hordeum vulgare L.) genomic library by their homology to known phosphate transporters from dicot species. The genes designated HORvu;Pht1;1 and HORvu;Pht1;6 encode proteins of 521 and 535 amino acids respectively with 12 predicted membrane-spanning domains and other motifs common to the Phtl family of phosphate transporters. HORvu;Pht1;1 is expressed exclusively in roots and is strongly induced by phosphate deprivation. HORvu;Pht1;6 is expressed in the aerial parts of the plant with strongest expression in old leaves and flag leaves. In situ hybridization showed that HORvu;Pht1;6 is expressed in the phloem of vascular bundles in leaves and ears. In order to study the biochemical properties of HORvu;Pht1;1 and HORvu;Pht1;6, the genes were expressed in transgenic rice (Oryza sativa L.) plants under the control of the rice actin promoter and suspension cell cultures were generated. Cells derived from transgenic plants were able to take up phosphate at a much higher rate than control cells, demonstrating that both genes encode functional phosphate transporters. The estimated Km for phosphate for cells expressing HORvu;Pht1;1 was 9.06 +/- 0.82 microM, which is characteristic of a high-affinity transporter. The rate of phosphate uptake decreased with increasing pH, suggesting that HORvu;Pht1;1 operates as a H+/H2PO4(-) symporter. In contrast, the estimated Km for phosphate for cells expressing HORvu;Pht1;6 was 385 +/- 61 microM, which is characteristic of a low-affinity transporter. Taken together, the results suggest that HORvu;Pht1;1 functions in uptake of phosphate at the root surface, while HORvu;Pht1;6 probably functions in remobilization of stored phosphate from leaves.

A Modified phosphate-carrier protein theory is proposed as a non-target site mechanism For glyphosate resistance in weeds

“…For instance, the Ph1; 6 gene, which encodes a transporter of low affinity of phosphorus and is highly expressed in shoots (Rae et al, 2003), may be a possible candidate gene involved in the cell exclusion of glyphosate. Likewise, the Ph2; 1 gene, that is encoded in the plastids (Rausch et al, 2004) may be involved in the chloroplast exclusion of glyphosate.…”

Stress Response Versus Stress Tolerance: A Transcriptome Analysis of Two Rice Lines Contrasting in Tolerance to Phosphorus Deficiency

“…Furthermore, attempts to improve tolerance to P deficiency through transgenic approaches have so far not yielded great success. Overexpression of the rootspecific high-affinity transporter gene HORvuPht1;1 from barley did not enhance P uptake of transgenic plants grown in soil or in hydroponics although the authors had shown in an initial experiment that HORvuPht1;1 overexpression increased P uptake in cultured rice cells (Rae et al 2003(Rae et al , 2004. A phytase gene from Aspergillus niger in transgenic Trifolium subterraneum improved P uptake of plants supplied with phytate and grown in agar under sterile conditions (Richardson et al 2001), but this positive effect was not always seen in soil-grown plants (George et al 2005), possibly because phytate was bound to soil particles and therefore not available as a substrate for phytase.…”

Phosphate Utilization Efficiency Correlates with Expression of Low-Affinity Phosphate Transporters and Noncoding RNA, IPS1, in Barley

“…Recently, it was confirmed that HvPHT1;1 is a high-affinity Pi transporter with a K m of 1.9 mM when expressed in Xenopus laevis oocytes (Preuss et al, 2011), suggesting that HvPHT1;1 is involved in Pi acquisition. HvPHT1;6 is expressed in both shoots and roots (Huang et al, 2008) and localized in the leaf phloem tissue (Rae et al, 2003). It displays unsaturated uptake in a millimolar range of Pi concentrations when expressed in oocytes (Preuss et al, 2010), suggesting that HvPHT1;6 is involved in Pi retranslocation.…”