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Wang P.,CAS Nanjing Institute of Soil Science | Wang P.,University of Chinese Academy of Sciences | Kinraide T.B.,Appalachian Farming Systems Research Center | Zhou D.,CAS Nanjing Institute of Soil Science | And 3 more authors.
Plant Physiology

Electrical properties of plasma membranes (PMs), partially controlled by the ionic composition of the exposure medium, play significant roles in the distribution of ions at the exterior surface of PMs and in the transport of ions across PMs. The effects of coexisting cations (commonly Al3+, Ca2+,Mg2+,H+, and Na+) on the uptake and toxicity of these and other ions (such as Cu2+, Zn2+, Ni2+, Cd2+, andH2AsO- 4) to plants were studied in terms of the electrical properties of PMs. Increased concentrations of cations or decreased pH in rooting media, whether in solution culture or in soils, reduced the negativity of the electrical potential at the PM exterior surface (ψo O). This reduction decreased the activities of metal cations at the PM surface and increased the activities of anions such as H2AsO- 4). Furthermore, the reduced ψo O negativity increased the surface-to-surface transmembrane potential difference, thus increasing the electrical driving force for cation uptake and decreasing the driving force for anion uptake across PMs. Analysis of measured uptake and toxicity of ions using electrostatic models provides evidence that uptake and toxicity are functions of the dual effects of ψo O (i.e. altered PM surface ion activity and surface-to-surface transmembrane potential difference gradient). This study provides novel insights into the mechanisms of plant-ion interactions and extends current theory to evaluate ion bioavailability and toxicity, indicating its potential utility in risk assessment of metal(loid)s in natural waters and soils. © 2010 American Society of Plant Biologists. Source

Kobayashi Y.,Gifu University | Watanabe T.,Hokkaido University | Shaff J.E.,Cornell University | Ohta H.,Tokyo Institute of Technology | And 4 more authors.
Plant Physiology

Al3+ and H++ toxicities predicted to occur at moderately acidic conditions (pH+ [water] = 5-5.5) in low-Ca soils were characterized by the combined approaches of computational modeling of electrostatic interactions of ions at the root plasma membrane (PM) surface and molecular/physiological analyses in Arabidopsis (Arabidopsis thaliana). Root growth inhibition in known hypersensitive mutants was correlated with computed {Al3++} at the PM surface ({Al3++}PM); inhibition was alleviated by increased Ca, which also reduced {Al3++}PM and correlated with cellular Al responses based on expression analysis of genes that are markers for Al stress. The Al-inducible Al tolerance genes ALUMINUM-ACTIVATED MALATE TRANSPORTER1 and ALUMINUM SENSITIVE3 were induced by levels of {Al3++}PM too low to inhibit root growth in tolerant genotypes, indicating that protective responses are triggered when {Al3++}PM was below levels that can initiate injury. Modeling of the H++ sensitivity of the SENSITIVE TO PROTON RH+IZOTOXICITY1 knockout mutant identified a Ca alleviation mechanism of H++ rhizotoxicity, possibly involving stabilization of the cell wall. The phosphatidate phosphohydrolase1 (pah1) pah2 double mutant showed enhanced Al susceptibility under low-P conditions, where greater levels of negatively charged phospholipids in the PM occur, which increases {Al3++}PM through increased PM surface negativity compared with wild-type plants. Finally, we found that the nonalkalinizing Ca fertilizer gypsum improved the tolerance of the sensitive genotypes in moderately acidic soils. These findings fit our modeling predictions that root toxicity to Al3++ and H++ in moderately acidic soils involves interactions between both toxic ions in relation to Ca alleviation. © 2013 American Society of Plant Biologists. All Rights Reserved. Source

Kinraide T.B.,Appalachian Farming Systems Research Center | Poschenrieder C.,Autonomous University of Barcelona | Kopittke P.M.,University of Queensland
Journal of Inorganic Biochemistry

The standard electrode potential (E θ) has been known for many decades to predict the toxicity of metal ions. We have compiled acute toxicity data from fifteen studies and find that the toxicity of thirty metal ions correlates with E θ at r 2 = 0.868 when toxicity is expressed as log concentration of comparably effective doses. We have discovered an even stronger relationship between the prooxidant activity (PA) of metal ions and E θ (and electronegativity, χ). Data compiled from thirty-four studies demonstrate that the PA of twenty-five metal ions correlates with E θ at r 2 = 0.983 (and χ at r 2 = 0.968). PA was commonly measured as metal-induced peroxidation of cell membranes or accumulation of reactive oxygen species. None of the redox metals (capable of Fenton-like reactions) in our studies (i.e., Mn, Fe, Co, Ni, and Cu) was prooxidative or toxic beyond what was expected from E θ or χ. We propose that the formation of superoxide-metal ion complexes is greater at greater E θ or χ values and that these complexes, whether free or enzyme-bound, function in PA without redox cycling of the complexed ion. © 2011 Elsevier Inc. Source

Kinraide T.B.,Appalachian Farming Systems Research Center | Wang P.,CAS Nanjing Institute of Soil Science
Journal of Experimental Botany

The electrical potentials at membrane surfaces (ψ0) strongly influence the physiological responses to ions. Ion activities at membrane surfaces may be computed from ψ0, and physiological responses to ions are better interpreted with surface activities than with bulk-phase activities. ψ0 influences the gating of ion channels and the driving force for ion fluxes across membranes. ψ0 may be computed with electrostatic models incorporating the intrinsic surface charge density of the membrane (σ0), the ion composition of the bathing medium, and ion binding to the membrane. Some of the parameter values needed for the models are well established: the equilibrium constants for ion binding were confirmed for several ions using multiple approaches, and a method is proposed for the computation of other binding constants. σ0 is less well established, although it has been estimated by several methods, including computation from the near-surface electrical potentials [zeta (ζ) potentials] measured by electrophoreses. Computation from ζ potentials yields values in the range-2mC m-2 to-8mC m-2, but other methods yield values in the range-15mC m-2 to-40mC m-2. A systematic discrepancy between measured and computed ζ potentials was noted. The preponderance of evidence supports the suitability of σ0=-30mC m-2. A proposed, fully paramatized Gouy-Chapman-Stern model appears to be suitable for the interpretation of many plant responses to the ionic environment. © 2010 The Author(s). Source

Wang P.,CAS Nanjing Institute of Soil Science | Wang P.,University of Queensland | Kinraide T.B.,Appalachian Farming Systems Research Center | Smolders E.,Catholic University of Leuven | And 8 more authors.
Soil Biology and Biochemistry

Toxicity data for microorganism in soil or in soil less cultures have been described with ion competition models, however these models disregard electrostatic and osmotic effects which are known to affect ion sorption and toxicity. Using European soils with diverse characteristics, the factors that influence the toxicity of soil Cu or Ni to potential nitrification rate (PNR) and glucose-induced respiration (GIR) were evaluated based on the electrical potential (ψ0) and ion activities ({M2+}0) at the outer surfaces of bacterial cell membranes (CMs). The zeta potentials (ζ) of bacterial (Escherichia coli) protoplasts, as affected by the ionic composition of the solution, were measured and used to estimate the parameters of a Gouy-Chapman-Stern (GCS) model which was then used to compute ψ0 values. The ψ0 values varied widely with soil type and increased markedly (became less negative) as metal salts were added. Computed ψ0 was then used to predict the surface ion activities from the soil solution composition. The toxicity data (both PNR and GIR) were statistically related to (i) surface activities of free metal ions ({M2+}0), (ii) the ameliorative effect of surface H+ activity ({H+}0), (iii) the ψ0-influenced electrical driving force for cation uptake across CMs, and (iv) osmotic effects. This electrostatic model predicted the observed GIR and PNR with Radj2>0.816 for observed vs. predicted PNR and Radj2>0.861 for observed vs. predicted GIR. These predictions were generally better than those by previous models. The suggestion that metal toxicity in spiked soils is partly related to a spike-induced osmotic increase is corroborated by fitting the model to spiked soils that were or were not leached and aged to reduce the osmotic increase. The predicted soil EC50 values (in mg metal/kg soil) were within a factor of 2.5 for up to nineteen European soils with a wide range of properties. © 2012 Elsevier Ltd. Source

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