Phosphate transport in plants

Transport of inorganic phosphate (P_i) through plant membranes is mediated by a number of families of transporter proteins. Studies on the topology, function, regulation and sites of expression of the genes that encode the members of these transporter families are enabling roles to be ascribed to each of them. The Pht1 family, of which there are nine members in the Arabidopsis genome, includes proteins involved in the uptake of P_i from the soil solution and the redistribution of P_i within the plant. Members of this family are H₂PO₄ ^–/H⁺ symporters. Most of the genes of the Pht1 family that are expressed in roots are up-regulated in P-stressed plants. Two members of the Pht1 family have been isolated from the cluster roots of white lupin. These same genes are expressed in non-cluster roots. The evidence available to date suggests that there are no major differences between the types of transport systems that cluster roots and non-cluster roots use to acquire P_i. Differences in uptake rates between cluster and non-cluster roots can be ascribed to more high-affinity P_i transporters in the plasma membranes of cluster roots, rather than any difference in the characteristics of the transporters. The efficient acquisition of P_i by cluster roots arises primarily from their capacity to increase the availability of soil P_i immediately adjacent to the rootlets by excretion of carboxylates, protons and phosphatases within the cluster. This paper reviews P_i transport processes, concentrating on those mediated by the Pht1 family of transporters, and attempts to relate those processes involved in P_i acquisition to likely P_i transport processes in cluster roots.     

Uptake of Selenite by Saccharomyces cerevisiae Involves the High and Low Affinity Orthophosphate Transporters

Although the general cytotoxicity of selenite is well established, the mechanism by which this compound crosses cellular membranes is still unknown. Here, we show that in Saccharomyces cerevisiae, the transport system used opportunistically by selenite depends on the phosphate concentration in the growth medium. Both the high and low affinity phosphate transporters are involved in selenite uptake. When cells are grown at low P_i concentrations, the high affinity phosphate transporter Pho84p is the major contributor to selenite uptake. When phosphate is abundant, selenite is internalized through the low affinity P_i transporters (Pho87p, Pho90p, and Pho91p). Accordingly, inactivation of the high affinity phosphate transporter Pho84p results in increased resistance to selenite and reduced uptake in low P_i medium, whereas deletion of SPL2, a negative regulator of low affinity phosphate uptake, results in exacerbated sensitivity to selenite. Measurements of the kinetic parameters for selenite and phosphate uptake demonstrate that there is a competition between phosphate and selenite ions for both P_i transport systems. In addition, our results indicate that Pho84p is very selective for phosphate as compared with selenite, whereas the low affinity transporters discriminate less efficiently between the two ions. The properties of phosphate and selenite transport enable us to propose an explanation to the paradoxical increase of selenite toxicity when phosphate concentration in the growth medium is raised above 1 mm.     

Nutrient uptake and alkaline phosphatase (ec 3:1:3:1) activity of emiliania huxleyi (PRYMNESIOPHYCEAE) during growth under n and p limitation in continuous cultures

Emiliania huxleyi (strain L) expressed an exceptional P assimilation capability. Under P limitation, the minimum cell P content was 2.6 fmol P·cell^?1, and cell N remained constant at all growth rates at 100 fmol N·cell^?1. Both, calcification of cells and the induction of the phosphate uptake system were inversely correlated with growth rate. The highest (cellular P based) maximum phosphate uptake rate (V_max^P) was 1400 times (i.e. 8.9 h^?1) higher than the actual uptake rate. The affinity of the P‐uptake system (dV/dS) was 19.8 L·μmol^?1·h^?1 at μ = 0.14 d^?1. This is the highest value ever reported for a phytoplankton species. V_max and dV/dS for phosphate uptake were 48% and 15% lower in the dark than in the light at the lowest growth rates. The half‐saturation constant for growth was 1.1 nM. The coefficient for luxury phosphate uptake (Q_maxt/Q_min) was 31. Under P limitation, E. huxleyi expressed two different types of alkaline phosphatase (APase) enzyme kinetics. One type was synthesized constitutively and possessed a V_max and half‐saturation constant of 43 fmol MFP·cell^?1·h^?1 and 1.9 μM, respectively. The other, inducible type of APase expressed its highest activity at the lowest growth rates, with a V_max and half‐saturation constant of 190 fmol MFP·cell^?1·h^?1 and 12.2 μM, respectively. Both APase systems were located in a lipid membrane close to the cell wall. Under N‐limiting growth conditions, the minimum N quotum was 43 fmol N·cell^?1. The highest value for the cell N‐specific maximum nitrate uptake rate (V_max^N) was 0.075 h^?1; for the affinity of nitrate uptake, 0.37 L·μmol^?1·h^?1. The uptake rate of nitrate in the dark was 70% lower than in the light. N‐limited cells were smaller than P‐limited cells and contained 50% less organic and inorganic carbon. In comparison with other algae, E. huxleyi is a poor competitor for nitrate under N limitation. As a consequence of its high affinity for inorganic phosphate, and the presence of two different types of APase in terms of kinetics, E. huxleyi is expected to perform well in P‐controlled ecosystems.     

Revealing Key Interactions in a Metabolic Network: Model of Primary Phosphate Transport in Bacteria

One of the main problems of metabolic engineering is to determine the genetically controlled limiting links of a metabolic network. We have built a model of the primary transport of inorganic phosphates (P _i ), analyzed the P _i metabolic network in Gram-negative bacteria, and determined the factors controlling the phosphate exchange. The model explains why the P _i primary transport is not observed at the release stage. The nonlinearity of primary transport and the differences in its parameters in the membrane and within the cell give rise to transport asymmetry, i.e., the P _i release rate is low as compared with the uptake rate, and is small at the background of secondary transport. Discussed is a general scheme of coordination between primary and secondary transport, which are interconnected through the substrate–product relation.     

Arabidopsis inositol phosphate kinases IPK1 and ITPK1 constitute a metabolic pathway in maintaining phosphate homeostasis

Emerging studies have suggested that there is a close link between inositol phosphate (InsP) metabolism and cellular phosphate (P_i) homeostasis in eukaryotes; however, whether a common InsP species is deployed as an evolutionarily conserved metabolic messenger to mediate P_i signaling remains unknown. Here, using genetics and InsP profiling combined with P_i‐starvation response (PSR) analysis in Arabidopsis thaliana, we showed that the kinase activity of inositol pentakisphosphate 2‐kinase (IPK1), an enzyme required for phytate (inositol hexakisphosphate; InsP₆) synthesis, is indispensable for maintaining P_i homeostasis under P_i‐replete conditions, and inositol 1,3,4‐trisphosphate 5/6‐kinase 1 (ITPK1) plays an equivalent role. Although both ipk1‐1 and itpk1 mutants exhibited decreased levels of InsP₆ and diphosphoinositol pentakisphosphate (PP‐InsP₅; InsP₇), disruption of another ITPK family enzyme, ITPK4, which correspondingly caused depletion of InsP₆ and InsP₇, did not display similar P_i‐related phenotypes, which precludes these InsP species from being effectors. Notably, the level of d /l ‐Ins(3,4,5,6)P₄ was concurrently elevated in both ipk1‐1 and itpk1 mutants, which showed a specific correlation with the misregulated P_i phenotypes. However, the level of d /l ‐Ins(3,4,5,6)P₄ is not responsive to P_i starvation that instead manifests a shoot‐specific increase in the InsP₇ level. This study demonstrates a more nuanced picture of the intersection of InsP metabolism and P_i homeostasis and PSRs than has previously been elaborated, and additionally establishes intermediate steps to phytate biosynthesis in plant vegetative tissues.     

Measurements of 18O‐Pi uptake indicate fast metabolism of phosphate in tree roots

Phosphorus (P) nutrition of beech ecosystems depends on soil processes, plant internal P cycling and P acquisition. P uptake of trees in the field is currently not validated due to the lack of an experimental approach applicable in natural forests. Application of radiolabelled tracers such as ³³P and ³²P is limited to special research sites and not allowed in natural environments. Moreover, only one stable isotope of P, namely ³¹P, exists. One alternative tool to measure P acquisition in the field could be the use of ¹⁸O‐labelled ³¹P‐phosphate (³¹P¹⁸O₄ ^3?). Phosphate (P_i) uptake rates calculated from the ¹⁸O enrichment of dried root material after application of ³¹P_i ¹⁸O₄ ^3? via nutrient solution was always lower compared to ³³P incorporation, did not show increasing rates of P_i uptake at P deficiency under controlled conditions, and did not reveal seasonal fluctuations in the field. Consequently, a clear correlation between ³³P‐based and ¹⁸O‐based P_i uptake by roots could not be established. Comparison of P_i uptake rates achieved from ³³P‐P_i and ¹⁸O‐P_i application led to the conclusion of high P_i metabolism in roots after P_i uptake. The replacement of ¹⁸O by ¹⁶O from water in ¹⁸O‐P_i during root influx, but most probably after P_i uptake into roots, due to metabolic activities, indicates high and fast turnover of P_i. Hence, the use of ¹⁸O‐P_i as an alternative tool to estimate P_i acquisition of trees in the field must consider the increase of ¹⁸O abundance in root water that was disregarded in dried root material.     

Phosphate uptake and utilization by bacteria and algae

Bacterial uptake of inorganic phosphate (closely investigated in Escherichia coli) is maintained by two different uptake systems. One (Pst system) is P_i-repressible and used in situations of phosphorus deficiency. The other system (Pit system) is constitutive. The Pit system also takes part in the phosphate exchange process where orthophosphate is continuously exchanged between the cell and the surrounding medium.Algal uptake mechanisms are less known. The uptake capacity increases during starvation but no clearly defined transport systems have been described. Uptake capacity seems to be regulated by internal phosphorus pools, e.g., polyphosphates. In mixed algal and bacterial populations, bacteria generally seem to be more efficient in utilizing low phosphate concentrations. The second half of this paper discusses how bacteria and algae can share limiting amounts of phosphate provided that the bacteria have pronouncedly higher affinity for phosphate. Part of the solution to this problem may be that bacteria are energy-limited rather than phosphate-limited and dependent on algal organic exudates for their energy supply.The possible phosphate exchange mechanism so convincingly demonstrated in Escherichia coli is here suggested to play a key role for the flux of phosphorus between bacteria and algae. Such a mechanism can also be used to explain the rapid phosphate exchange between the particulate and the dissolved phase which always occurs in short-term ³²P-uptake experiments in lake waters.     

Characterization of Na+-Dependent Phosphate Uptake in Cultured Fetal Rat Cortical Neurons

Abstract: Our laboratory has recently cloned and expressed a brain- and neuron-specific Na⁺-dependent inorganic phosphate (P_i) cotransporter that is constitutively expressed in neurons of the rat cerebral cortex, hippocampus, and cerebellum. We have now characterized Na⁺-dependent ³²P_i cotransport in cultured fetal rat cortical neurons, where >90% of saturable P_i uptake is Na⁺-dependent. Saturable, Na⁺-dependent ³²P_i uptake was first observed in primary cultures of cortical neurons at 7 days in vitro (DIV) and was maximal at 12 DIV. Na⁺-dependent P_i transport was optimal at physiological temperature (37°C) and pH (7.0–7.5), with apparent K_m values for P_i and Na⁺ of 54 ± 12.7 µM and 35 ± 4.2 mM, respectively. A reduction in extracellular Ca²⁺ markedly reduced (>60%) Na⁺-dependent P_i uptake, with a threshold for maximal P_i import of 1–2.5 mM CaCl₂. Primary cultures of fetal cortical neurons incubated in medium where equimolar concentrations of choline were substituted for Na⁺ had lower levels of ATP and ADP and higher levels of AMP than did those incubated in the presence of Na⁺. Furthermore, a substantial fraction of the ³²P_i cotransported with Na⁺ was concentrated in the adenine nucleotides. Inhibitors of oxidative metabolism, such as rotenone, oligomycin, or dinitrophenol, dramatically decreased Na⁺-dependent P_i import rates. These data establish the presence of a Na⁺-dependent P_i cotransport system in neurons of the CNS, demonstrate the Ca²⁺-dependent nature of ³²P_i uptake, and suggest that the neuronal Na⁺-dependent P_i cotransporter may import P_i required for the production of high-energy compounds vital to neuronal metabolism.     

HvAKT2 and HvHAK1 confer drought tolerance in barley through enhanced leaf mesophyll H+ homoeostasis

Plant K⁺ uptake typically consists low—affinity mechanisms mediated by Shaker K⁺ channels (AKT/KAT/KC) and high‐affinity mechanisms regulated by HAK/KUP/KT transporters, which are extensively studied. However, the evolutionary and genetic roles of both K⁺ uptake mechanisms for drought tolerance are not fully explored in crops adapted to dryland agriculture. Here, we employed evolutionary bioinformatics, biotechnological and electrophysiological approaches to determine the role of two important K⁺ transporters HvAKT2 and HvHAK1 in drought tolerance in barley. HvAKT2 and HvHAK1 were cloned and functionally characterized using barley stripe mosaic virus‐induced gene silencing (BSMV‐VIGS) in drought‐tolerant wild barley XZ5 and agrobacterium‐mediated gene transfer in the barley cultivar Golden Promise. The hallmarks of the K⁺ selective filters of AKT2 and HAK1 are both found in homologues from strepotophyte algae, and they are evolutionarily conserved in strepotophyte algae and land plants. HvAKT2 and HvHAK1 are both localized to the plasma membrane and have high selectivity to K⁺ and Rb⁺ over other tested cations. Overexpression of HvAKT2 and HvHAK1 enhanced K⁺ uptake and H⁺ homoeostasis leading to drought tolerance in these transgenic lines. Moreover, HvAKT2‐ and HvHAK1‐overexpressing lines showed distinct response of K⁺, H⁺ and Ca²⁺ fluxes across plasma membrane and production of nitric oxide and hydrogen peroxide in leaves as compared to the wild type and silenced lines. High‐ and low‐affinity K⁺ uptake mechanisms and their coordination with H⁺ homoeostasis play essential roles in drought adaptation of wild barley. These findings can potentially facilitate future breeding programs for resilient cereal crops in a changing global climate.     

PHOSPHATE UPTAKE AND GROWTH KINETICS OF TRICHODESMIUM (CYANOBACTERIA) ISOLATES FROM THE NORTH ATLANTIC OCEAN AND THE GREAT BARRIER REEF,AUSTRALIA1

We compared inorganic phosphate (P_i) uptake and growth kinetics of two cultures of the diazotrophic cyanobacterium Trichodesmium isolated from the North Atlantic Ocean (IMS101) and from the Great Barrier Reef, Australia (GBRTRLI101). Phosphate‐limited cultures had up to six times higher maximum P_i uptake rates than P‐replete cultures in both strains. For strain GBRTRLI101, cell‐specific P_i uptake rates were nearly twice as high, due to larger cell size, but P‐specific maximum uptake rates were similar for both isolates. Half saturation constants were 0.4 and 0.6 μM for P_i uptake and 0.1 and 0.2 μM for growth in IMS101 and GBRTRLI101, respectively. Phosphate uptake in both strains was correlated to growth rates rather than to light or temperature. The cellular phosphorus quota for both strains increased with increasing P_i up to 1.0 μM. The C:P ratios were 340–390 and N:P ratios were 40–45 for both strains under severely P‐limited growth conditions, similar to reported values for natural populations from the tropical Atlantic and Pacific Oceans. The C:P and N:P ratios were near Redfield values in medium with >1.0 μM P_i. The North Atlantic strain IMS101 is better adapted to growing on P_i at low concentrations than is GBRTRLI101 from the more P_i‐enriched Great Barrier Reef. However, neither strain can achieve appreciable growth at the very low (nanomolar) P_i concentrations found in most oligotrophic regimes. Phosphate could be an important source of phosphorus for Trichodesmium on the Great Barrier Reef, but populations growing in the oligotrophic open ocean must rely primarily on dissolved organic phosphorus sources.     

Proton- and sodium-coupled phosphate transport systems and energy status of Yarrowia lipolytica cells grown in acidic and alkaline conditions

In this study we have used a newly isolated Yarrowia lipolytica yeast strain with a unique capacity to grow over a wide pH range (3.5–10.5), which makes it an excellent model system for studying H⁺- and Na⁺-coupled phosphate transport systems. Even at extreme growth conditions (low concentrations of extracellular phosphate, alkaline pH values) Y. lipolytica preserved tightly-coupled mitochondria with the fully competent respiratory chain containing three points of energy conservation. This was demonstrated for the first time for cells grown at pH 9.5–10.0. In cells grown at pH 4.5, inorganic phosphate (P_i) was accumulated by two kinetically discrete H⁺/P_i-cotransport systems. The low-affinity system is most likely constitutively expressed and operates at high P_i concentrations. The high-affinity system, subjected to regulation by both extracellular P_i availability and intracellular polyphosphate stores, is mobilized during P_i-starvation. In cells grown at pH 9.5–10, P_i uptake is mediated by several kinetically discrete Na⁺-dependent systems that are specifically activated by Na⁺ ions and insensitive to the protonophore CCCP. One of these, a low-affinity transporter operative at high P_i concentrations is kinetically characterized here for the first time. The other two, high-affinity, high-capacity systems, are derepressible and functional during P_i-starvation and appear to be controlled by extracellular P_i. They represent the first examples of high-capacity, Na⁺-driven P_i transport systems in an organism belonging to neither the animal nor bacterial kingdoms. The contribution of the H⁺- and Na⁺-coupled P_i transport systems in Y. lipolytica cells grown at different pH values was quantified. In cells grown at pH values of 4.5 and 6.0, the H⁺-coupled P_i transport systems are predominant. The contribution of the Na⁺/P_i cotransport systems to the total cellular P_i uptake activity is progressively increased with increasing pH, reaching its maximum at pH 9 and higher. Received: 15 December 2000/Revised: 14 May 2001     

PHOSPHATE ACCUMULATION AND METABOLISM BY HETEROSIGMA AKASHIWO (RAPHIDOPHYCEAE) DURING DIEL VERTICAL MIGRATION IN A STRATIFIED MICROCOSM1

Diel vertical migration by Heterosigma akashiwo (Hada) Hada (Raphidophyceae) was monitored in a 1.5 in tall microcosm. Vertical stratification, with low salinity and low orthophosphate (P_i) concentration in the upper layer and high salinity and high P_i concentration in the lower layer, was simulated in the tank, analogous to summer stratification in the Seto Inland Sea. The phosphate metabolism of H. akashiwo during this vertical migration was studied using ³¹P-NMR spectroscopy. At night this species migrated to the lower phosphate-rich layer and took up inorganic phosphate (P_i) which then was accumulated as polyphosphate (PP_i) by an increase in the chain length of PP_i During the daytime this species migrated to the phosphate-depleted surface water and utilized the accumulated PP_i for photophosphorylation by decreasing the chain length of PP_i During the first night after the phosphorus was introduced to the previously impoverished waters, the cells took up inorganic phosphate, accumulating the new phosphorus nutrient internally as P_i But the cells did not convert P_i to PP_i presumably due to their lack of ATP. After the second day of the experiment, conversion of P_i to PP_i at night was much more rapid than on the first day, presumably due to increased ATP availability. Then the cycle continued, with uptake of P_i and conversion to PP_i at night at the bottom and its utilization during the day at the surface. These data suggest that the role of PP_i in the metabolism of this species appears to be as a phosphate pool which regulates the level of P_i and ATP in the cell. Diel vertical migration allows this red tide species to shuttle between the phosphate-rich lower layer and the photic upper layer in stratified waters. ³¹P-NMR is shown to be a valuable tool in studying the phosphorus metabolism in migrating organisms.     

31P NUCLEAR MAGNETIC RESONANCE STUDY OF INTRACELLULAR PHOSPHATE POOLS AND POLYPHOSPHATE METABOLISM IN HETEROSIGMA AKASHIWO (HADA) HADA (RAPHIDOPHYCEAE)1

The effects of starvation and subsequent addition of phosphate-containing medium on the phosphate metabolic intermediates were studied by ³¹P-NMR spectroscope of perchloric acid extracts and intact cells of Heterosigma akashiwo (Hada) Hada. When orthophosphate in the medium was completely depleted the medium was enriched with orthophosphate (4.5 μM). In the phosphate starved condition, the P cell quota was 76 fmol·cell⁻¹ and the major components of phosphate intermediates were phosphodiester, sugar phosphate and orthophosphate (P_i). After addition of P_i, rapid uptake of P_i was observed and the P cell quota increased to 108 fmol·cell⁻¹ in 2 h, 134 fmol·cell⁻¹ in 5 h and 222 fmol·cell⁻¹ in 1 day after addition of phosphate. The ³¹P-NMR spectrum indicated that a major portion of P was stored as polyphosphate, in which the average chain length of polyphosphate increased from 10 to 20 phosphate residues in one day after addition of P_i.     

Dual Effect of Phosphate Transport on Mitochondrial Ca2+ Dynamics

The large inner membrane electrochemical driving force and restricted volume of the matrix confer unique constraints on mitochondrial ion transport. Cation uptake along with anion and water movement induces swelling if not compensated by other processes. For mitochondrial Ca²⁺ uptake, these include activation of countertransporters (Na⁺/Ca²⁺ exchanger and Na⁺/H⁺ exchanger) coupled to the proton gradient, ultimately maintained by the proton pumps of the respiratory chain, and Ca²⁺ binding to matrix buffers. Inorganic phosphate (P_i) is known to affect both the Ca²⁺ uptake rate and the buffering reaction, but the role of anion transport in determining mitochondrial Ca²⁺ dynamics is poorly understood. Here we simultaneously monitor extra- and intra-mitochondrial Ca²⁺ and mitochondrial membrane potential (ΔΨ_m) to examine the effects of anion transport on mitochondrial Ca²⁺ flux and buffering in P_i-depleted guinea pig cardiac mitochondria. Mitochondrial Ca²⁺ uptake proceeded slowly in the absence of P_i but matrix free Ca²⁺ ([Ca²⁺]_mito) still rose to ∼50 μm. P_i (0.001–1 mm) accelerated Ca²⁺ uptake but decreased [Ca²⁺]_mito by almost 50% while restoring ΔΨ_m. P_i-dependent effects on Ca²⁺ were blocked by inhibiting the phosphate carrier. Mitochondrial Ca²⁺ uptake rate was also increased by vanadate (V_i), acetate, ATP, or a non-hydrolyzable ATP analog (AMP-PNP), with differential effects on matrix Ca²⁺ buffering and ΔΨ_m recovery. Interestingly, ATP or AMP-PNP prevented the effects of P_i on Ca²⁺ uptake. The results show that anion transport imposes an upper limit on mitochondrial Ca²⁺ uptake and modifies the [Ca²⁺]_mito response in a complex manner.     

TcPho91 is a contractile vacuole phosphate sodium symporter that regulates phosphate and polyphosphate metabolism in Trypanosoma cruzi

We have identified a phosphate transporter (TcPho91) localized to the bladder of the contractile vacuole complex (CVC) of Trypanosoma cruzi, the etiologic agent of Chagas disease. TcPho91 has 12 transmembrane domains, an N‐terminal regulatory SPX (named after SYG1, Pho81 and XPR1) domain and an anion permease domain. Functional expression in Xenopus laevis oocytes followed by two‐electrode voltage clamp showed that TcPho91 is a low‐affinity transporter with a K_m for P_i in the millimolar range, and sodium‐dependency. Epimastigotes overexpressing TcPho91‐green fluorescent protein have significantly higher levels of pyrophosphate (PP_i) and short‐chain polyphosphate (polyP), suggesting accumulation of P_i in these cells. Moreover, when overexpressing parasites were maintained in a medium with low P_i, they grew at higher rates than control parasites. Only one allele of TcPho91 in the CL strain encodes for the complete open reading frame, while the other one is truncated encoding for only the N‐terminal domain. Taking advantage of this characteristic, knockdown experiments were performed resulting in cells with reduced growth rate as well as a reduction in PP_i and short‐chain polyP levels. Our results indicate that TcPho91 is a phosphate sodium symporter involved in P_i homeostasis in T. cruzi.     

Phosphate transport across biomembranes and cytosolic phosphate homeostasis in barley leaves

Barley (Hordeum vulgare L.) plants were grown hydroponically with or without inorganic phosphate (P_i) in the medium. Leaves were analyzed for the intercellular and the intracellular distribution of P_i. Most of the leaf P_i was contained in mesophyll cells; P_i concentrations were low in the xylem sap, the apoplast and in the cells of the epidermis. The vacuolar concentration of P_i in mesophyll cells depended on P_i availability in the nutrient medium. After infiltrating the intercellular space of leaves with solutions containing P_i, P_i was taken up by the mesophyll at rates higher than 2.5 mol· (g fresh weight)^–1 · h^–1. Isolated mesophyll protoplasts did not possess a comparable capacity to take up P_i from the medium. Phosphate uptake by mesophyll protoplasts showed a biphasic dependence on P_i concentration. Uptake of P_i by P_i-deficient cells was faster than uptake by cells which had P_i stored in their vacuoles, although cytoplasmic P_i concentrations were comparable. Phosphate transport into isolated mesophyll vacuoles was dependent on their P_i content; it was stimulated by ATP. In contrast to the vacuolar P_i concentration, and despite different kinetic characteristics of the uptake systems for p_i of the plasmalemma and the tonoplast, the cytoplasmic p_i concentration was regulated in mesophyll cells within narrow limits under very different conditions of P_i availability in the nutrient medium, whereas vacuolar P_i concentrations varied within wide limits.Dedicated to Professor Wilhelm Simonis on the occasion of his 80th birthdayThis investigation was part of the research efforts of the Sonderforschungsbereich 176 of the Bayerische Julius-Maximilians-Universität Würzburg. We are grateful to Dr. Olaf Wolf for introducing us to the method for preparation of xylem sap of barley plants and to Mr. Yin Zuhua for fluorimetric experiments with the dye pyranine. T. Mimura is indebted to the Alexander-von-Humboldt-Stiftung for a postdoctoral research fellowship.     

Phosphate transporters OsPHT1;9 and OsPHT1;10 are involved in phosphate uptake in rice

We characterized the function of two rice phosphate (Pi) transporters: OsPHT1;9 (OsPT9) and OsPHT1;10 (OsPT10). OsPT9 and OsPT10 were expressed in the root epidermis, root hairs and lateral roots, with their expression being specifically induced by Pi starvation. In leaves, expression of the two genes was observed in both mesophyll and vasculature. High‐affinity Km values for Pi transport of OsPT9 and OsPT10 were determined by yeast experiments and two‐electrode voltage clamp analysis of anion transport in Xenopus oocytes expressing OsPT9 and OsPT10. Pi uptake and Pi concentrations in transgenic plants harbouring overexpressed OsPT9 and OsPT10 were determined by Pi concentration analysis and ³³P‐labelled Pi uptake rate analysis. Significantly higher Pi uptake rates in transgenic plants compared with wild‐type plants were observed under both high‐Pi and low‐Pi solution culture conditions. Conversely, although no alterations in Pi concentration were found in OsPT9 or OsPT10 knockdown plants, a significant reduction in Pi concentration in both shoots and roots was observed in double‐knockdown plants grown under both high‐ and low‐Pi conditions. Taken together, our results suggest that OsPT9 and OsPT10 redundantly function in Pi uptake.     

The uptake of phosphate by Carex species from oligotrophic to eutrophic swamp habitats

Abstract Phosphate uptake by excised roots of Carex species from a range of oligotrophic to eutrophic swamps was investigated. The range of species from oligotrophic to eutrophic is: C.rostrata Stokes, C.limosa L., C.lasiocarpa Ehrh., C.diandra Schrank, C.hudsonii A.Benn. and C.acutiformis Ehrh. All species showed two phases for P_i uptake in the P_i concentration range of 0.01 – 50 μM. In phase 1, C.rostrata and C.lasiocarpa had relatively high Vmax:s and Km:s, whereas the species from richer areas had intermediate values. The lowest Vmax and Km values were found in C.limosa and C.hudsonii. In phase 2, apart from the high Vmax and Km values found for C.lasiocarpa, the kinetic constants showed little variation, indicating a similar P_i carrier mechanism for all the species. Results on phosphate uptake and leakage are discussed against the phosphate requirement of each species in its specific habitat, and against the literature data of agricultural crops, which generally show a much lower affinity for phosphate uptake.