Cloning and characterization of two phosphate transporters from Medicago truncatula roots: regulation in response to phosphate and to colonization by arbuscular mycorrhizal (AM) fungi

Nylon filter arrays reveal differential gene expression in proteoid roots of white lupin in response to phosphorus deficiency

White lupin (Lupinus albus) adapts to phosphorus deficiency (-P) by the development of short, densely clustered lateral roots called proteoid (or cluster) roots. In an effort to better understand the molecular events mediating these adaptive responses, we have isolated and sequenced 2,102 expressed sequence tags (ESTs) from cDNA libraries prepared with RNA isolated at different stages of proteoid root development. Determination of overlapping regions revealed 322 contigs (redundant copy transcripts) and 1,126 singletons (single-copy transcripts) that compile to a total of 1,448 unique genes (unigenes). Nylon filter arrays with these 2,102 ESTs from proteoid roots were performed to evaluate global aspects of gene expression in response to -P stress. ESTs differentially expressed in P-deficient proteoid roots compared with +P and -P normal roots include genes involved in carbon metabolism, secondary metabolism, P scavenging and remobilization, plant hormone metabolism, and signal transduction.

A phosphate transporter from Medicago truncatula is expressed in the photosynthetic tissues of the plant and located in the chloroplast envelope

• Phosphate is essential for many cellular processes including the light reactions of photosynthesis. Photosynthesis results in the production of triose phosphates that are transported across the chloroplast envelope to the cytosol in counterexchange for phosphate. Until recently, members of the plastid phosphate transport family, which mediate the exchange of phosphate for phosphorylated compounds, were the only proteins known to transport phosphate into the chloroplast. • Here, we characterized a phosphate transporter, MtPHT2;1 of Medicago truncatula. Transient expression of an MtPHT2;1-GFP fusion protein indicates that MtPHT2;1 is located in the chloroplast envelope. • The phosphate transport activity of MtPHT2;1 was assayed in yeast where the protein mediates phosphate uptake with a Km for phosphate of 0.6 m m and a pH optimum of 3-4. • MtPHT2;1 is expressed in all the photosynthetic tissues of the plant and transcript levels are also influenced by light, development and phosphate status of the plant. The phosphate transport activity and location in the chloroplast envelope membrane suggest a role for MtPHT2;1 in phosphate transport into the chloroplast.

Characterization of a general amino acid permease from Hebeloma cylindrosporum

Besides a role in phosphate supply, ectomycorrhizas play a crucial role in nitrogen nutrition of plants. The ectomycorrhizal association between Hebeloma cylindrosporum and Pinus pinaster serves as a model system accessible to molecular manipulation. Hebeloma mycelium is able to take up and use amino acids as the sole nitrogen source. Suppression cloning allowed identification of a Hebeloma transporter (HcGAP1) mediating histidine uptake. HcGAP1 mediates secondary active uptake of a wide spectrum of different amino acids. The secondary active transport mechanism together with the expression in hyphae, but not in mycorrhizas, indicate a role in uptake of organic nitrogen from the soil.

Rice phosphate transporters include an evolutionarily divergent gene specifically activated in arbuscular mycorrhizal symbiosis

Using a genome-wide approach, we asked how many transporter genes contribute to symbiotic phosphate uptake and analyzed their evolutionary conservation. Considering the sequenced rice genome at hand, only the Oryza sativa phosphate transporter (OsPT) gene OsPT11 was specifically induced during the arbuscular mycorrhizal symbiosis. This induction was confined to the root system and was tightly correlated with the degree of root colonization by Glomus intraradices. OsPT11 activation was independent of the nutritional status of the plant and phosphate availability in the rhizosphere. Moreover, infection of roots with the fungal pathogens Rhizoctonia solani and Fusarium moniliforme did not activate OsPT11, corroborating the high signal specificity for OsPT11 activation in the arbuscular mycorrhizal symbiosis. OsPT11 expression complemented a defect in phosphate uptake in a yeast strain mutated in its high-affinity P(i) transporter (pho84), thereby confirming its function. Recently, a phosphate transporter gene in potato was shown to be induced during arbuscular mycorrhizal symbiosis. Assessment of the phylogenetic relationship of the rice and potato protein revealed that the rice is nonorthologous to the potato protein. Further, there are no structural commonalities in the promoter regions. Thus, although cytological and physiological features of the arbuscular mycorrhizal symbiosis seem to be conserved, the molecular components may differ significantly between distantly related plant species.

Regulated expression of Arabidopsis phosphate transporters

Phosphorus deficiency is one of the major abiotic stresses affecting plant growth. Plants respond to the persistent deficiency of phosphate (Pi) by coordinating the expression of genes involved in alleviation of the stress. The high-affinity Pi transporters are among the major molecular determinants that are activated during Pi stress. In this study, using three reporter genes (green fluorescent protein, luciferase, and beta-glucuronidase) regulated by two Pi transporter promoters, we have carried out an extensive analysis of transcriptional and spatial regulation of gene expression. Activation of the genes was rapid, repressible, and specific in response to changes in Pi availability. The phytohormones auxin and cytokinin suppressed the expression of the reporter gene driven by the AtPT1 promoter, and that of the native gene, suggesting that hormones may be involved in regulation of some component(s) of Pi starvation response pathway. These studies also provide molecular evidence for a potential role of high-affinity Pi transporters in mobilizing Pi into reproductive organs. The results suggest that members of the Pi transporter family may have similar but nonredundant functions in plants.

Expression analysis suggests novel roles for members of the Pht1 family of phosphate transporters in Arabidopsis

The completion of the Arabidopsis thaliana genome has revealed that there are nine members of the Pht1 family of phosphate transporters in this species. As a step towards identifying the role of this gene family in phosphorus nutrition, we have isolated the promoter regions from each of these genes, and fused them to the reporter genes beta-glucuronidase and/or green fluorescent protein. These chimeric genes have been introduced into A. thaliana, and reporter gene expression has been assayed in plants grown in soil containing high and low concentrations of inorganic phosphate (Pi). Four of these promoters were found to direct reporter gene expression in the root epidermis, and were induced under conditions of phosphate deprivation in a manner similar to previously characterised Pht1 genes. Other members of this family, however, showed expression in a range of shoot tissues and in pollen grains, which was confirmed by RT-PCR. We also provide evidence that the root epidermally expressed genes are expressed most strongly in trichoblasts, the primary sites for uptake of Pi. These results suggest that this gene family plays a wider role in phosphate uptake and remobilisation throughout the plant than was previously believed.

Arbuscular mycorrhizal fungi confer enhanced arsenate resistance on Holcus lanatus

•  The role of arbuscular mycorrhizal fungi (AMF) in arsenate resistance in arbuscular mycorrhizal associations is investigated here for two Glomus spp. isolated from the arsenate-resistant grass Holcus lanatus. •  Glomus mosseae and Glomus caledonium were isolated from H. lanatus growing on an arsenic-contaminated mine-spoil soil. The arsenate resistance of spores was compared with nonmine isolates using a germination assay. Short-term arsenate influx into roots and long-term plant accumulation of arsenic by plants were also investigated in uninfected arsenate resistant and nonresistant plants and in plants infected with mine and nonmine AMF. •  Mine AMF isolates were arsenate resistant compared with nonmine isolates. Resistant and nonresistant G. mosseae both suppressed high-affinity arsenate/phosphate transport into the roots of both resistant and nonresistant H. lanatus. Resistant AMF colonization of resistant H. lanatus growing in contaminated mine spoil reduced arsenate uptake by the host. •  We conclude that AMF have evolved arsenate resistance, and conferred enhanced resistance on H. lanatus.

Enhancing mineral content in plant food products

Plant foods can serve as dietary sources of all essential minerals required by humans. Unfortunately, mineral concentrations are low in some plants, especially many staple food crops; thus, efforts are underway to increase the mineral content of these foods as a means to ensure adequate attainment of dietary minerals in all individuals. While these efforts have included classical breeding approaches in the past, it is clear that future progress can be made by utilizing the tools of biotechnology to effect directed changes in plant mineral status. Reviewed are the short- and long-distance mineral transport mechanisms responsible for the root acquisition and whole-plant partitioning of mineral ions in crop plants. This background is used to discuss different transgenic strategies with the potential to enhance mineral content in vegetative and/or reproductive tissues. Due to various constraints imposed by plant transport systems on whole-plant mineral movement, it is argued that modifications designed to increase the supply of minerals to edible organs should have the highest chance for success. Examples of previous efforts to manipulate plant mineral nutrition through the introduction of novel transgenes are presented to demonstrate the utility of these approaches.

Phosphorus uptake by arbuscular mycorrhizal hyphae does not increase when the host plant grows under atmospheric CO2 enrichment

• We conducted an experiment to test whether phosphorus (P) uptake by mycorrhizal hyphae could be enhanced by growing the host plant under [CO₂ ] enrichment and whether any response to [CO₂ ] was dependent on C source-sink relationships. • Plant C assimilation, mass allocation, growth and P uptake were measured in pea (Pisum sativum) plants inoculated with 0, 1 or 5% of a mixture of three Glomus spp. Intra- and extra-radical mycorrhizal development was followed and hyphal ³³ P uptake from a root-exclusion compartment was measured. • Total P and ³³ P content measurements indicated that root, not hyphal, P uptake was increased by elevated [CO₂ ] in the mycorrhizal treatments and that hyphal P uptake was actually reduced by elevated [CO₂ ] after 57 d. Neither intra- nor extraradical mycorrhizal development was related to this response. • Plant and fungal measurements suggested positive interactions in plant growth and P uptake only when C source-sink relationships were balanced; high C source (enhanced assimilation at elevated [CO₂ ]) and high C sink (increasing mycorrhizal development). The results also indicated that enhanced plant C supply does not alter growth or function of arbuscular mycorrhizal fungi.

Expression of nitrate transporter genes in tomato colonized by an arbuscular mycorrhizal fungus

PCR amplifications using tomato DNA and degenerate oligonucleotide primers allowed identification of a new putative nitrate transporter, termed NRT2;3. Its sequence showed typical motifs of a high affinity nitrate transporter of the Major Facilitator Superfamily (MFS). The formation of its mRNA was positively controlled by nitrate, and negatively by ammonium, but not by glutamine. In situ hybridization experiments showed that this transporter was mainly expressed in rhizodermal cells. Results from expression studies with two other nitrate transporters, LeNRT1;1 and LeNRT2;1, were essentially in accord with data of the literature. In roots colonized by the arbuscular mycorrhizal fungus Glomus intraradices Sy167, transcript formation of NRT2;3 extended to the inner cortical cells where the fungal structures, arbuscules and vesicles, were concentrated. Northern analyses indicated that the expression of only NRT2;3 among the transporters assayed was higher in AMF colonized tomato roots than in non-colonized controls. AMF-colonization caused a significant expression of a nitrate reductase gene of G. intraradices. The results may mean that AMF-colonization positively affects nitrate uptake from soil and nitrate allocation to the plant partner, probably mediated preferentially by LeNRT2;3. In addition, part of the nitrate taken up is reduced by the fungal partner itself and may then be transferred, when in excess, as glutamine to the plant symbiotic partner.

Identification of a cDNA from the arbuscular mycorrhizal fungus Glomus intraradices that is expressed during mycorrhizal symbiosis and up-regulated by N fertilization

A cDNA library was constructed with RNA from Glomus intraradices-colonized lettuce roots and used for differential screening. This allowed the identification of a cDNA (Gi-1) that was expressed only in mycorrhizal roots and was of fungal origin. The function of the gene product is unknown, because Gi-1 contained a complete open reading frame that was predicted to encode a protein of 157 amino acids which only showed little homology with glutamine synthetase from Helicobacter pylori. The time-course analysis of gene expression during the fungal life cycle showed that Gi-1 was expressed only during the mycorrhizal symbiosis and was not detected in dormant or germinating spores of G. intraradices. P fertilization did not significantly change the pattern of Gi-1 expression compared with that in the unfertilized treatment, whereas N fertilization (alone or in combination with P) considerably enhanced the Gi-1 transcript accumulation. This increase in gene expression correlated with plant N status and growth under such conditions. The possible role of the Gi-1 gene product in intermediary N metabolism of arbuscular mycorrhizal symbiosis is further discussed.

A phosphate transporter expressed in arbuscule-containing cells in potato

Arbuscular mycorrhizas are the most common non-pathogenic symbioses in the roots of plants. It is generally assumed that this symbiosis facilitated the colonization of land by plants. In arbuscular mycorrhizas, fungal hyphae often extend between the root cells and tuft-like branched structures (arbuscules) form within the cell lumina that act as the functional interface for nutrient exchange. In the mutualistic arbuscular-mycorrhizal symbiosis the host plant derives mainly phosphorus from the fungus, which in turn benefits from plant-based glucose. The molecular basis of the establishment and functioning of the arbuscular-mycorrhizal symbiosis is largely not understood. Here we identify the phosphate transporter gene StPT3 in potato (Solanum tuberosum). Functionality of the encoded protein was confirmed by yeast complementation. RNA localization and reporter gene expression indicated expression of StPT3 in root sectors where mycorrhizal structures are formed. A sequence motif in the StPT3 promoter is similar to transposon-like elements, suggesting that the mutualistic symbiosis evolved by genetic rearrangements in the StPT3 promoter.

The response of the phosphate uptake system and the organic acid exudation system to phosphate starvation in Sesbania rostrata

It is well known that the P(i) uptake system via the high-affinity P(i) transporter and the organic acid exudation system via PEPC are enhanced in the roots of P(i)-starved plants. In this paper, we compared the expression of these two systems in Sesbania rostrata, a leguminous plant, on whose roots and stems it forms nodules. When S. rostrata plants were transferred to a 0 microM P(i) nutrient solution, the expression of both the high-affinity P(i) transporter and PEPC was enhanced within 2 d. The enhancement of the expression of the high-affinity P(i) transporter genes and the PEPC gene coordinated with the increases in the P(i) uptake rate and the PEPC activity, respectively. This suggests that the expression of the high-affinity P(i) transporters and PEPC is regulated in part at the transcript level. Furthermore, we examined which of the environmental or the endogenous P(i) level regulates the expression of these two systems. The P(i) content in the 6-day-old plants decreased to a lower level than that in the 15-day-old plants when grown in a 30 microM P(i) solution. At that time, the expression of the high-affinity P(i) transporters and PEPC was enhanced only in the 6-day-old plants. Moreover, the P(i) content in plants forming many nodules on their stems decreased. The expression of the high-affinity P(i) transporters and PEPC was then enhanced in the nodulated plants. These facts suggest that the expression of these two systems may be regulated by the P(i) content in the plants, not by the P(i) concentration in the soil.

A phosphate transporter gene from the extra-radical mycelium of an arbuscular mycorrhizal fungus Glomus intraradices is regulated in response to phosphate in the environment

The majority of vascular flowering plants are able to form symbiotic associations with arbuscular mycorrhizal fungi. These symbioses, termed arbuscular mycorrhizas, are mutually beneficial, and the fungus delivers phosphate to the plant while receiving carbon. In these symbioses, phosphate uptake by the arbuscular mycorrhizal fungus is the first step in the process of phosphate transport to the plant. Previously, we cloned a phosphate transporter gene involved in this process. Here, we analyze the expression and regulation of a phosphate transporter gene (GiPT) in the extra-radical mycelium of the arbuscular mycorrhizal fungus Glomus intraradices during mycorrhizal association with carrot or Medicago truncatula roots. These analyses reveal that GiPT expression is regulated in response to phosphate concentrations in the environment surrounding the extra-radical hyphae and modulated by the overall phosphate status of the mycorrhiza. Phosphate concentrations, typical of those found in the soil solution, result in expression of GiPT. These data imply that G. intraradices can perceive phosphate levels in the external environment but also suggest the presence of an internal phosphate sensing mechanism.

Arbuscular mycorrhizal fungi alter phosphorus relations of broomsedge (Andropogon virginicus L.) plants

Broomsedge (Andropogon virginicus L.) is a dominant grass revegetating many abandoned coal-mined lands in West Virginia, USA. Residual soils on such sites are often characterized by low pH, low nutrients, and high aluminium. Experiments were conducted to assess the resistance of broomsedge to limited phosphorus (Pi) availability and to investigate the role that arbuscular mycorrhizal (AM) fungi play in aiding plant growth under low Pi conditions. Pregerminated mycorrhizal and non-mycorrhizal seedlings were grown in a sand-culture system with nutrient solutions containing Pi concentrations ranging from 10 to 100 microM for 8 weeks. Non-mycorrhizal plants exhibited severe inhibition of growth under Pi limitation (<60 microM). Colonization by AM fungi (combined Glomus clarum Nicolson & Schenck and Gigaspora gigantea (Nicol. & Gerd.) Gerd. & Trappe) greatly enhanced host plant growth at low Pi concentrations, but did not benefit growth when Pi was readily available (100 microM). In comparison to non-mycorrhizal plants, mycorrhizal plants had higher phosphorus use efficiency at low Pi concentrations and maintained nearly constant tissue nutrient concentrations across the gradient of Pi concentrations investigated. Manganese (Mn) and sodium (Na) accumulated in shoots of non-mycorrhizal plants under Pi limitation. Mycorrhizal plants exhibited lower instantaneous Pi uptake rates and significantly lower C(min) values compared to non-mycorrhizal plants. These patterns suggest that the symbiotic association between broomsedge roots and AM fungi effectively maintains nutrient homeostasis through changes in physiological properties, including nutrient uptake, allocation and use. The mycorrhizal association is thus a major adaptation that allows broomsedge to become established on infertile mined lands.

A conserved MYB transcription factor involved in phosphate starvation signaling both in vascular plants and in unicellular algae

Plants have evolved a number of adaptive responses to cope with growth in conditions of limited phosphate (Pi) supply involving biochemical, metabolic, and developmental changes. We prepared an EMS-mutagenized M(2) population of an Arabidopsis thaliana transgenic line harboring a reporter gene specifically responsive to Pi starvation (AtIPS1::GUS), and screened for mutants altered in Pi starvation regulation. One of the mutants, phr1 (phosphate starvation response 1), displayed reduced response of AtIPS1::GUS to Pi starvation, and also had a broad range of Pi starvation responses impaired, including the responsiveness of various other Pi starvation-induced genes and metabolic responses, such as the increase in anthocyanin accumulation. PHR1 was positionally cloned and shown be related to the PHOSPHORUS STARVATION RESPONSE 1 (PSR1) gene from Chlamydomonas reinhardtii. A GFP::PHR1 protein fusion was localized in the nucleus independently of Pi status, as is the case for PSR1. PHR1 is expressed in Pi sufficient conditions and, in contrast to PSR1, is only weakly responsive to Pi starvation. PHR1, PSR1, and other members of the protein family share a MYB domain and a predicted coiled-coil (CC) domain, defining a subtype within the MYB superfamily, the MYB-CC family. Therefore, PHR1 was found to bind as a dimer to an imperfect palindromic sequence. PHR1-binding sequences are present in the promoter of Pi starvation-responsive structural genes, indicating that this protein acts downstream in the Pi starvation signaling pathway.

MACRONUTRIENT UTILIZATION BY PHOTOSYNTHETIC EUKARYOTES AND THE FABRIC OF INTERACTIONS

Organisms acclimate to a continually fluctuating nutrient environment. Acclimation involves responses specific for the limiting nutrient as well as responses that are more general and occur when an organism experiences different stress conditions. Specific responses enable organisms to efficiently scavenge the limiting nutrient and may involve the induction of high-affinity transport systems and the synthesis of hydrolytic enzymes that facilitate the release of the nutrient from extracellular organic molecules or from internal reserves. General responses include changes in cell division rates and global alterations in metabolic activities. In photosynthetic organisms there must be precise regulation of photosynthetic activity since when severe nutrient limitation prevents continued cell growth, excitation of photosynthetic pigments could result in the formation of reactive oxygen species, which can severely damage structural and functional features of the cell. This review focuses on ways that photosynthetic eukaryotes assimilate the macronutrients nitrogen, sulfur, and phosphorus, and the mechanisms that govern assimilatory activities. Also discussed are molecular responses to macronutrient limitation and the elicitation of those responses through integration of environmental and cellular cues.

Relative quantitative RT-PCR to study the expression of plant nutrient transporters in arbuscular mycorrhizas

The influence of arbuscular mycorrhizal fungi (AMF) on the expression of plant nutrient transporters was studied using a relative, quantitative reverse-transcription polymerase chain-reaction (RQRT-PCR) technique. Reverse-transcribed 18S rRNA was used to standardize the treatments. The technique had high reproducibility and reflected trends in gene expression as observed by Northern blotting. Using this technique, it was demonstrated that both the high-affinity phosphate transporter MtPt2 and a putative nitrate transporter from Medicago truncatula were down-regulated in roots when colonized by some, but not all AMF. Colonization by the AMF Glomus rosea, in particular, failed to strongly down-regulate these plant genes within the root. This technique may be suitable for the study of plant genes in mycorrhizal roots when Northern blotting is not possible due to low gene expression or when limited amounts of tissue are available for study.

Characterization of Arabidopsis acid phosphatase promoter and regulation of acid phosphatase expression

The expression and secretion of acid phosphatase (APase) was investigated in Indian mustard (Brassica juncea L. Czern.) plants using sensitive in vitro and activity gel assays. Phosphorus (P) starvation induced two APases in Indian mustard roots, only one of which was secreted. Northern-blot analysis indicated transcriptional regulation of APase expression. Polymerase chain reaction and Southern-blot analyses revealed two APase homologs in Indian mustard, whereas in Arabidopsis, only one APase homolog was detected. The Arabidopsis APase promoter region was cloned and fused to the beta-glucuronidase (GUS) and green fluorescent protein (GFP) reporter genes. GUS expression was first evident in leaves of the P-starved Arabidopsis plants. In P-starved roots, the expression of GUS initiated in lateral root meristems followed by generalized expression throughout the root. GUS expression diminished with the addition of P to the medium. Expression of GFP in P-starved roots also initiated in the lateral root meristems and the recombinant GFP with the APase signal peptide was secreted by the roots into the medium. The APase promoter was specifically activated by low P levels. The removal of other essential elements or the addition of salicylic or jasmonic acids, known inducers of gene expression, did not activate the APase promoter. This novel APase promoter may be used as a plant-inducible gene expression system for the production of recombinant proteins and as a tool to study P metabolism in plants.

Intracellular accommodation of microbes by plants: a common developmental program for symbiosis and disease?

Plant cells engage in mutualistic and parasitic endosymbioses with a wide variety of microorganisms, ranging from Gram-negative (Rhizobium, Nostoc) and Gram-positive bacteria (Frankia), to oomycetes (Phytophthora), Chytridiomycetes, Zygomycetes (arbuscular mycorrhizal fungi) and true fungi (Erysiphe, ascomycete; Puccinia, basidiomycete). Endosymbiosis is characterised by the 'symbiosome', a compartment within host cells in which the symbiotic microorganism is either partially or completely enclosed by a host-derived membrane. The analysis of plant mutants indicates that the genetic requirements for the interaction with rhizobia and arbuscular mycorrhiza fungi are partially overlapping. The extent to which plants use similar or identical developmental programs for the intracellular accommodation of different microorganisms is, however, not clear. For example, plant cells actively weaken their cell wall to facilitate bacterial colonisation, whereas penetration by fungal symbionts appears not to be assisted in this manner. Moreover, different transport requirements are imposed on the symbiotic interface of different interactions indicating that additional system-specific components are likely to exist.

Molecular mechanisms of phosphate and sulphate transport in plants

The application of molecular techniques in recent years has advanced our understanding of phosphate and sulphate transport processes in plants. Genes encoding phosphate and sulphate transporters have been isolated from a number of plant species. The transporters encoded by these genes are related to the major facilitator superfamily of proteins. They are predicted to contain 12 membrane-spanning domains and function as H(+)/H(2)PO(-4) or H(+)/SO(2/-4) cotransporters. Both high-affinity and low-affinity types have been identified. Most research has concentrated on genes that encode transporters expressed in roots. The expression of many of these genes is transcriptionally regulated by signals that respond to the nutrient status of the plant. Nutrient demand and the availability of precursors needed in the assimilatory pathways also regulate transcription of some of these genes. Information on the cell types in which phosphate and sulphate transporters are expressed is becoming available. These data, together with functional characterisation of the transporters, are enabling the roles of various transporters in the overall phosphate and sulphate nutrition of plants to be defined.

Differential ion accumulation and ion fluxes in the mesophyll and epidermis of barley

In barley (Hordeum vulgare L.) leaves, differential ion accumulation commonly results in inorganic phosphate (Pi) being confined to the mesophyll and Ca(2+) to the epidermis, with preferential epidermal accumulation of Cl(-), Na(+), and some other ions. The pattern was confirmed in this study for major inorganic anions and cations by analysis of barley leaf protoplasts. The work focused on the extent to which differences in plasma membrane ion transport processes underlie these observations. Ion transport across the plasma membrane of barley epidermal and mesophyll protoplasts was investigated electrophysiologically (by microelectrode impalement and patch clamping) and radiometrically. Data from both approaches suggested that similar types of ion-selective channels and membrane transporters, which catalyze the transport of Ca(2+), K(+), Na(+), and Pi, exist in the plasma membrane of the two cell types. In general, the simple presence or absence of ion transporters could not explain cell-type-specific differences in ion accumulation. However, patch-clamp data suggested that differential regulation of instantaneously activating ion channels in the plasma membrane could explain the preferential accumulation of Na(+) in the epidermis.

Pho86p, an endoplasmic reticulum (ER) resident protein in Saccharomyces cerevisiae, is required for ER exit of the high-affinity phosphate transporter Pho84p

In the budding yeast Saccharomyces cerevisiae, PHO84 and PHO86 are among the genes that are most highly induced in response to phosphate starvation. They are essential for growth when phosphate is limiting, and they function in the high-affinity phosphate uptake system. PHO84 encodes a high-affinity phosphate transporter, and mutations in PHO86 cause many of the same phenotypes as mutations in PHO84, including a phosphate uptake defect and constitutive expression of the secreted acid phosphatase, Pho5p. Here, we show that the subcellular localization of Pho84p is regulated in response to extracellular phosphate levels; it is localized to the plasma membrane in low-phosphate medium but quickly endocytosed and transported to the vacuole upon addition of phosphate to the medium. Moreover, Pho84p is localized to the endoplasmic reticulum (ER) and fails to be targeted to the plasma membrane in the absence of Pho86p. Utilizing an in vitro vesicle budding assay, we demonstrate that Pho86p is required for packaging of Pho84p into COPII vesicles. Pho86p is an ER resident protein, which itself is not transported out of the ER. Interestingly, the requirement of Pho86p for ER exit is specific to Pho84p, because other members of the hexose transporter family to which Pho84 belongs are not mislocalized in the absence of Pho86p.

A type 5 acid phosphatase gene from Arabidopsis thaliana is induced by phosphate starvation and by some other types of phosphate mobilising/oxidative stress conditions

Low phosphorous availability, a common condition of many soils, is known to stimulate phosphatase activity in plants; however, the molecular details of this response remain mostly unknown. We purified and sequenced the N-terminal region of a phosphate starvation induced acid phosphatase (AtACP5) from Arabidopsis thaliana, and cloned its cDNA and the corresponding genomic DNA. The nucleotide sequence of the cDNA predicted that AtACP5 is synthesised as a 338 amino acid-long precursor with a signal peptide. AtACP5 was found to be related to known purple acid phosphatases, especially to mammal type 5 acid phosphatases. Other similarities with purple acid phosphatases, which contain a dinuclear metal centre, include the conservation of all residues involved in metal ligand binding and resistance to tartrate inhibition. In addition, AtACP5, like other type 5 acid phosphatases, displayed peroxidation activity. Northern hybridisation experiments, as well as in situ glucuronidase (GUS) activity assays on transgenic plants harbouring AtACP5:GUS translational fusions, showed that AtACP5 is not only responsive to phosphate starvation but also to ABA and salt stress. It is also expressed in senescent leaves and during oxidative stress induced by H2O2, but not by paraquat or salicylic acid. Given its bifunctionality, as it displays both phosphatase and peroxidation activity, we propose that AtACP5 could be involved in phosphate mobilisation and in the metabolism of reactive oxygen species in stressed or senescent parts of the plant.

Identification of a putative P-transporter operon in the genome of a Burkholderia strain living inside the arbuscular mycorrhizal fungus Gigaspora margarita

This article reports the identification of a putative P-transporter operon in the genome of a Burkholderia sp. living in the cytoplasm of the arbuscular mycorrhizal fungus Gigaspora margarita. Its presence suggests that Burkholderia sp. has the potential for P uptake from this environment. This finding raises new questions concerning the importance of intracellular bacteria for mycorrhizal symbiosis.

Chlamydomonas reinhardtii mutants abnormal in their responses to phosphorus deprivation

P-starved plants scavenge inorganic phosphate (Pi) by developing elevated rates of Pi uptake, synthesizing extracellular phosphatases, and secreting organic acids. To elucidate mechanisms controlling these acclimation responses in photosynthetic organisms, we characterized the responses of the green alga Chlamydomonas reinhardtii to P starvation and developed screens for isolating mutants (designated psr [phosphorus-stress response]) abnormal in their responses to environmental levels of Pi. The psr1-1 mutant was identified in a selection for cells that survived exposure to high concentrations of radioactive Pi. psr1-2 and psr2 were isolated as strains with aberrant levels of extracellular phosphatase activity during P-deficient or nutrient-replete growth. The psr1-1 and psr1-2 mutants were phenotypically similar, and the lesions in these strains were recessive and allelic. They exhibited no increase in extracellular phosphatase activity or Pi uptake upon starvation. Furthermore, when placed in medium devoid of P, the psr1 strains lost photosynthetic O2 evolution and stopped growing more rapidly than wild-type cells; they may not be as efficient as wild-type cells at scavenging/accessing P stores. In contrast, psr2 showed elevated extracellular phosphatase activity during growth in nutrient-replete medium, and the mutation was dominant. The mutant phenotypes and the roles of Psr1 and Psr2 in P-limitation responses are discussed.

Plasma membrane transport in context - making sense out of complexity

Major advances in our understanding of the transport of inorganic nutrient ions across plant plasma membranes have emerged from recent studies on the control of the dominant H+-pumping ATPase and from identification of a range of new transporters for divalent cations, potassium, phosphate and nitrate. In many cases, multiple transporter isoforms have been described. An appreciation of the physiological roles of these transporters demands combined genetic and physiological approaches, which, in the case of an outward rectifying K+ channel, have already been used to yield an intriguing insight into root-mediated K+ release into the xylem. In this review we attempt to place some of those developments in a physiological context.

PHOSPHATE ACQUISITION

Phosphorus is one of the major plant nutrients that is least available in the soil. Consequently, plants have developed numerous morphological, physiological, biochemical, and molecular adaptations to acquire phosphate (Pi). Enhanced ability to acquire Pi and altered gene expression are the hallmarks of plant adaptation to Pi deficiency. The intricate mechanisms involved in maintaining Pi homeostasis reflect the complexity of Pi acquisition and translocation in plants. Recent discoveries of multiple Pi transporters have opened up opportunities to study the molecular basis of Pi acquisition by plants. An increasing number of genes are now known to be activated under Pi starvation. Some of these genes may be involved in Pi acquisition, transfer, and signal transduction during Pi stress. This review provides an overview of plant adaptations leading to enhanced Pi acquisition, with special emphasis on recent developments in the molecular biology of Pi acquisition.

MOLECULAR AND CELLULAR ASPECTS OF THE ARBUSCULAR MYCORRHIZAL SYMBIOSIS

Arbuscular mycorrhizae are symbiotic associations formed between a wide range of plant species including angiosperms, gymnosperms, pteridophytes, and some bryophytes, and a limited range of fungi belonging to a single order, the Glomales. The symbiosis develops in the plant roots where the fungus colonizes the apoplast and cells of the cortex to access carbon supplied by the plant. The fungal contribution to the symbiosis is complex, but a major aspect includes the transfer of mineral nutrients, particularly phosphate from the soil to the plant. Development of this highly compatible association requires the coordinate molecular and cellular differentiation of both symbionts to form specialized interfaces over which bi-directional nutrient transfer occurs. Recent insights into the molecular events underlying these aspects of the symbiosis are discussed.

Transcriptional regulation of plant phosphate transporters

Phosphorus is acquired by plant roots primarily via the high-affinity inorganic phosphate (Pi) transporters. The transcripts for Pi transporters are highly inducible upon Pi starvation, which also results in enhanced Pi uptake when Pi is resupplied. Using antibodies specific to one of the tomato Pi transporters (encoded by LePT1), we show that an increase in the LePT1 transcript under Pi starvation leads to a concurrent increase in the transporter protein, suggesting a transcriptional regulation for Pi acquisition. LePT1 protein accumulates rapidly in tomato roots in response to Pi starvation. The level of transporter protein accumulation depends on the Pi concentration in the medium, and it is reversible upon resupply of Pi. LePT1 protein accumulates all along the roots under Pi starvation and is localized primarily in the plasma membranes. These results clearly demonstrate that plants increase their capacity for Pi uptake during Pi starvation by synthesis of additional transporter molecules.

Novel genes induced during an arbuscular mycorrhizal (AM) symbiosis formed between Medicago truncatula and Glomus versiforme

Many terrestrial plant species are able to form symbiotic associations with arbuscular mycorrhizal fungi. Here we have identified three cDNA clones representing genes whose expression is induced during the arbuscular mycorrhizal symbiosis formed between Medicago truncatula and an arbuscular mycorrhizal fungus, Glomus versiforme. The three clones represent M. truncatula genes and encode novel proteins: a xyloglucan endotransglycosylase-related protein, a putative arabinogalactan protein (AGP), and a putative homologue of the mammalian p110 subunit of initiation factor 3 (eIF3). These genes show little or no expression in M. truncatula roots prior to formation of the symbiosis and are significantly induced following colonization by G. versiforme. The genes are not induced in roots in response to increases in phosphate. This suggests that induction of expression during the symbiosis is due to the interaction with the fungus and is not a secondary effect of improved phosphate nutrition. In situ hybridization revealed that the putative AGP is expressed specifically in cortical cells containing arbuscules. The identification of two mycorrhiza-induced genes encoding proteins predicted to be involved in cell wall structure is consistent with previous electron microscopy data that indicated major alterations in the extracellular matrix of the cortical cells following colonization by mycorrhizal fungi.

The down-regulation of Mt4-like genes by phosphate fertilization occurs systemically and involves phosphate translocation to the shoots

Mt4 is a cDNA representing a phosphate-starvation-inducible gene from Medicago truncatula that is down-regulated in roots in response to inorganic phosphate (Pi) fertilization and colonization by arbuscular mycorrhizal fungi. Split-root experiments revealed that the expression of the Mt4 gene in M. truncatula roots is down-regulated systemically by both Pi fertilization and colonization by arbuscular mycorrhizal fungi. A comparison of Pi levels in these tissues suggested that this systemic down-regulation is not caused by Pi accumulation. Using a 30-bp region of the Mt4 gene as a probe, Pi-starvation-inducible Mt4-like genes were detected in Arabidopsis and soybean (Glycine max L.), but not in corn (Zea mays L.). Analysis of the expression of the Mt4-like Arabidopsis gene, At4, in wild-type Arabidopsis and pho1, a mutant unable to load Pi into the xylem, suggests that Pi must first be translocated to the shoot for down-regulation to occur. The data from the pho1 and split-root studies are consistent with the presence of a translocatable shoot factor responsible for mediating the systemic down-regulation of Mt4-like genes in roots.

Characterization of the Mt4 gene from Medicago truncatula

Previously we identified Mt4, a phosphate starvation inducible cDNA from Medicago truncatula which is down-regulated in roots in response to phosphate fertilization as well as colonization by arbuscular mycorrhizal (AM) fungi (AM). Here we present further studies of Mt4. Expression was highly sensitive to exogenous applications of phosphate fertilizer; transcripts were abundant in roots fertilized with nutrient solution lacking phosphate, reduced when fertilized with 0.02 or 0.1 mM phosphate and undetectable when fertilized with 1 or 5 mM phosphate. A time course experiment, to study the expression of Mt4 following colonization by AM fungi, revealed that Mt4 transcripts increased in uncolonized roots during the first three weeks of growth and then plateaued, while transcript levels in roots colonized with the AM fungus, Glomas versiforme, increased transiently and then decreased. Although the Mt4 gene is expressed exclusively in roots, transcripts were also detected in M. truncatula cell suspension cultures following phosphate starvation. A genomic clone containing the Mt4 gene and 1133 bp of the 5' flanking sequence was identified from a M. truncatula genomic library. The promoter region contains a conserved cis-element found in the promoters of phosphate starvation inducible genes of yeast and tomato. As Mt4 is the first cDNA reported to show independent regulation by both phosphate and mycorrhizal fungi, the genomic clone may provide a starting point from which to analyze the signal transduction pathways involved in these two processes.

Development of the arbuscular mycorrhizal symbiosis

The arbuscular mycorrhizal (AM) symbiosis formed between plant roots and fungi is one of the most widespread symbiotic associations found in plants, yet our understanding of events underlying its development are limited. The recent integration of biochemical, molecular and genetic approaches into analyses of the symbiosis is providing new insights into various aspects of its development. In the past year there have been advances in our understanding of the signals required for the formation of appressoria, the molecular changes in the root in response to colonisation, and components of the signal transduction pathways common to both the AM and Rhizobium symbioses.