Phosphate Import in Plants: Focus on the PHT1 Transporters

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

A Novel fry1 Allele Reveals the Existence of a Mutant Phenotype Unrelated to 5′->3′ Exoribonuclease (XRN) Activities in Arabidopsis thaliana Roots

BACKGROUND Mutations in the FRY1/SAL1 Arabidopsis locus are highly pleiotropic, affecting drought tolerance, leaf shape and root growth. FRY1 encodes a nucleotide phosphatase that in vitro has inositol polyphosphate 1-phosphatase and 3',(2'),5'-bisphosphate nucleotide phosphatase activities. It is not clear which activity mediates each of the diverse biological functions of FRY1 in planta. PRINCIPAL FINDINGS A fry1 mutant was identified in a genetic screen for Arabidopsis mutants deregulated in the expression of Pi High affinity Transporter 1;4 (PHT1;4). Histological analysis revealed that, in roots, FRY1 expression was restricted to the stele and meristems. The fry1 mutant displayed an altered root architecture phenotype and an increased drought tolerance. All of the phenotypes analyzed were complemented with the AHL gene encoding a protein that converts 3'-polyadenosine 5'-phosphate (PAP) into AMP and Pi. PAP is known to inhibit exoribonucleases (XRN) in vitro. Accordingly, an xrn triple mutant with mutations in all three XRNs shared the fry1 drought tolerance and root architecture phenotypes. Interestingly these two traits were also complemented by grafting, revealing that drought tolerance was primarily conferred by the rosette and that the root architecture can be complemented by long-distance regulation derived from leaves. By contrast, PHT1 expression was not altered in xrn mutants or in grafting experiments. Thus, PHT1 up-regulation probably resulted from a local depletion of Pi in the fry1 stele. This hypothesis is supported by the identification of other genes modulated by Pi deficiency in the stele, which are found induced in a fry1 background. CONCLUSIONS/SIGNIFICANCE Our results indicate that the 3',(2'),5'-bisphosphate nucleotide phosphatase activity of FRY1 is involved in long-distance as well as local regulatory activities in roots. The local up-regulation of PHT1 genes transcription in roots likely results from local depletion of Pi and is independent of the XRNs.This work was supported by an ANR-GENOPLANT grant (RIBOROOT-ANR06 GPLA 011) and the CEA agency. Array hybridizations have been partly supported by RNG (Réseau National des Génopoles, Evry, France). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. No additional external funding received for this study

Roles of type II H+-PPases and PPsPase1/PECP2 in early developmental stages and PPi homeostasis of Arabidopsis thaliana

The regulation of intracellular pyrophosphate (PPi) level is crucial for proper morphogenesis across all taxonomic kingdoms. PPi is released as a byproduct from ~200 metabolic reactions, then hydrolyzed by either membrane-bound (H+-PPase) or soluble pyrophosphatases (PPases). In Arabidopsis, the loss of the vacuolar H+-PPase/FUGU5, a key enzyme in PPi homeostasis, results in delayed growth and a number of developmental defects, pointing to the importance of PPi homeostasis in plant morphogenesis. The Arabidopsis genome encodes several PPases in addition to FUGU5, such as PPsPase1/PECP2, VHP2;1 and VHP2;2, although their significance regarding PPi homeostasis remains elusive. Here, to assess their contribution, phenotypic analyses of cotyledon aspect ratio, palisade tissue cellular phenotypes, adaxial side pavement cell complexity, stomatal distribution, and etiolated seedling length were performed, provided that they were altered due to excess PPi in a fugu5 mutant background. Overall, our analyses revealed that the above five traits were unaffected in ppspase1/pecp2, vhp2;1 and vhp2;2 loss-of-function mutants, as well as in fugu5 mutant lines constitutively overexpressing PPsPase1/PECP2. Furthermore, metabolomics revealed that ppspase1/pecp2, vhp2;1 and vhp2;2 etiolated seedlings exhibited metabolic profiles comparable to the wild type. Together, these results indicate that the contribution of PPsPase1/PECP2, VHP2;1 and VHP2;2 to PPi levels is negligible in comparison to FUGU5 in the early stages of seedling development

Medicago truncatula and Glomus intraradices gene expression in cortical cells harboring arbuscules in the arbuscular mycorrhizal symbiosis

<p>Abstract</p> <p>Background</p> <p>Most vascular flowering plants have the capacity to form symbiotic associations with arbuscular mycorrhizal (AM) fungi. The symbiosis develops in the roots where AM fungi colonize the root cortex and form arbuscules within the cortical cells. Arbuscules are enveloped in a novel plant membrane and their establishment requires the coordinated cellular activities of both symbiotic partners. The arbuscule-cortical cell interface is the primary functional interface of the symbiosis and is of central importance in nutrient exchange. To determine the molecular events the underlie arbuscule development and function, it is first necessary to identify genes that may play a role in this process. Toward this goal we used the Affymetrix GeneChip^®Medicago Genome Array to document the <it>M. truncatula </it>transcript profiles associated with AM symbiosis, and then developed laser microdissection (LM) of <it>M. truncatula </it>root cortical cells to enable analyses of gene expression in individual cell types by RT-PCR.</p> <p>Results</p> <p>This approach led to the identification of novel <it>M. truncatula </it>and <it>G. intraradices </it>genes expressed in colonized cortical cells and in arbuscules. Within the arbuscule, expression of genes associated with the urea cycle, amino acid biosynthesis and cellular autophagy was detected. Analysis of gene expression in the colonized cortical cell revealed up-regulation of a lysine motif (LysM)-receptor like kinase, members of the GRAS transcription factor family and a symbiosis-specific ammonium transporter that is a likely candidate for mediating ammonium transport in the AM symbiosis.</p> <p>Conclusion</p> <p>Transcript profiling using the Affymetrix GeneChip^®Medicago Genome Array provided new insights into gene expression in <it>M. truncatula </it>roots during AM symbiosis and revealed the existence of several <it>G. intraradices </it>genes on the <it>M. truncatula </it>GeneChip^®. A laser microdissection protocol that incorporates low-melting temperature Steedman's wax, was developed to enable laser microdissection of <it>M. truncatula </it>root cortical cells. LM coupled with RT-PCR provided spatial gene expression information for both symbionts and expanded current information available for gene expression in cortical cells containing arbuscules.</p

Severe hypertriglyceridaemia during treatment with capecitabine

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A novel role for the root cap in phosphate uptake and homeostasis

The root cap has a fundamental role in sensing environmental cues as well as regulating root growth via altered meristem activity. Despite this well-established role in the control of developmental processes in roots, the root cap's function in nutrition remains obscure. Here, we uncover its role in phosphate nutrition by targeted cellular inactivation or phosphate transport complementation in Arabidopsis, using a transactivation strategy with an innovative high-resolution real-time P-33 imaging technique. Remarkably, the diminutive size of the root cap cells at the root-to-soil exchange surface accounts for a significant amount of the total seedling phosphate uptake (approximately 20%). This level of Pi absorption is sufficient for shoot biomass production (up to a 180% gain in soil), as well as repression of Pi starvation-induced genes. These results extend our understanding of this important tissue from its previously described roles in environmental perception to novel functions in mineral nutrition and homeostasis control

Aquaporin-facilitated water uptake in barley (Hordeum vulgare L.) roots

It is not known to what degree aquaporin-facilitated water uptake differs between root developmental regions and types of root. The aim of this study was to measure aquaporin-dependent water flow in the main types of root and root developmental regions of 14- to 17-d-old barley plants and to identify candidate aquaporins which mediate this flow. Water flow at root level was related to flow at cell and plant level. Plants were grown hydroponically. Hydraulic conductivity of cells and roots was determined with a pressure probe and through exudation, respectively, and whole-plant water flow (transpiration) determined gravimetrically in response to the commonly used aquaporin inhibitor HgCl2. Expression of aquaporins was analysed by real-time PCR and in situ hybridization. Hydraulic conductivity of cortical cells in seminal roots was largest in lateral roots; it was smallest in the fully mature zone and intermediate in the not fully mature ‘transition’ zone along the main root axis. Adventitious roots displayed an even higher (3- to 4-fold) cortical cell hydraulic conductivity in the transition zone. This coincided with 3- to 4-fold higher expression of three aquaporins (HvPIP2;2, HvPIP2;5, HvTIP1:1). These were expressed (also) in cortical tissue. The largest inhibition of water flow (83–95%) in response to HgCl2 was observed in cortical cells. Water flow through roots and plants was reduced less (40–74%). It is concluded that aquaporins contribute substantially to root water uptake in 14- to 17-d-old barley plants. Most water uptake occurs through lateral roots. HvPIP2;5, HvPIP2;2, and HvTIP1;1 are prime candidates to mediate water flow in cortical tissue

The phosphate transporters LjPT4 and MtPT4 mediate early root responses to phosphate status in non mycorrhizal roots

Arbuscular mycorrhizal (AM) symbiosis improves host plant phosphorous (P) status and elicits the expression of AM-inducible phosphate transporters (PTs) in arbuscule-containing cells, where they control arbuscule morphogenesis and P release. We confirmed such functions for LjPT4 in mycorrhizal Lotus japonicus. Promoter-GUS experiments showed LjPT4 transcription not only in arbusculated cells but also in root tips, in the absence of the fungus: here LjPT4 transcription profile depended on the phosphate level. In addition, quantitative RT-PCR confirmed the expression of Lotus and Medicago truncatula PT4 in the tips of non-mycorrhizal roots. Starting from these observations, we hypothesized that AM-inducible PTs may have a regulatory role in plant development, irrespective of the fungal presence. Firstly, we focused on root development responses to different phosphate treatments in both plants demonstrating that phosphate starvation induced a higher number of lateral roots. By contrast, Lotus PT4i plants and Medicago mtpt4 mutants did not show any differential response to phosphate levels, suggesting that PT4 genes affect early root branching. Phosphate starvation-induced genes and a key auxin receptor, MtTIR1, showed an impaired expression in mtpt4 plants. We suggest PT4 genes as novel components of the P-sensing machinery at the root tip level, independently of AM fungi

A Modeling Approach towards Identifying Potential Bivalent Sensitizers of Neuromuscular Blocking Agents

ABSTRACT Objective: Anaphylactic reactions induced by neuromuscular blocking agents (NMBAs) can occur at first contact and might be due to cross-sensitization by other drugs or chemicals. Our aim was to investigate whether divalent molecules sharing chemical features with NMBAs might potentially cause cross-sensitization. Methods: We constructed a pharmacophore key from chemical features common to all NMBAs (two positive or ionizable features 1.0807 nm apart) and used the key to screen FDA-approved small drug molecules of the Drug Bank® database (1541 molecules). The selected molecules were categorized on the basis of the values for three main parameters (fit value, relative energy and mean polar surface area). Results: Screening from the pharmacophore key selected 13 NMBAs and 88 non-NMBA drugs. Of these 88 drugs, 42 had high-ranking parameter values and were considered preferential cross-sensitizers. These included the dopamine D2 receptor ligands aripiprazole and domperidone. Pholcodine, as well as nizatidine, ranitidine, antrafenine, cabergoline and, to some extent, chlorhexidine best fulfilled the required criteria of apolar character, bioavailability and ionization rate. Conclusion: Our data support the hypothesis that pholcodine might be a potential NMBA cross-sensitizer. They confirmed the results of inhibition tests on patient serum suggesting that dopamine D2 receptor ligands might be cross-sensitizers. They also identified chlorhexidine, a widely used disinfectant incriminated in several cases of immediate hypersensitivity reactions, as a potential cross-sensitizer. Pharmacophore modelling is an inexpensive, straightforward approach that can be used to identify potential NMBA cross-sensitizing agents