CARRIER-MEDIATED CATION TRANSPORT IN BARLEY ROOTS: KINETIC EVIDENCE FOR A SPECTRUM OF ACTIVE SITES

Genomic and phenotypic attributes of novel salinivibrios from stromatolites, sediment and water from a high altitude lake

BACKGROUND

Salinivibrios are moderately halophilic bacteria found in salted meats, brines and hypersaline environments. We obtained three novel conspecific Salinivibrio strains closely related to S. costicola, from Socompa Lake, a high altitude hypersaline Andean lake (approx. 3,570 meters above the sea level).

RESULTS

The three novel Salinivibrio spp. were extremely resistant to arsenic (up to 200 mM HAsO42-), NaCl (up to 15%), and UV-B radiation (19 KJ/m2, corresponding to 240 minutes of exposure) by means of phenotypic tests. Our subsequent draft genome ionsequencing and RAST-based genome annotation revealed the presence of genes related to arsenic, NaCl, and UV radiation resistance. The three novel Salinivibrio genomes also had the xanthorhodopsin gene cluster phylogenetically related to Marinobacter and Spiribacter. The genomic taxonomy analysis, including multilocus sequence analysis, average amino acid identity, and genome-to-genome distance revealed that the three novel strains belong to a new Salinivibrio species.

CONCLUSIONS

Arsenic resistance genes, genes involved in DNA repair, resistance to extreme environmental conditions and the possible light-based energy production, may represent important attributes of the novel salinivibrios, allowing these microbes to thrive in the Socompa Lake.

Transport of Sodium in Plant Tissue

Two mechanisms are implicated in the absorption of alkali cations by barley roots. Mechanism 1 has a high affinity for potassium, but its affinity for sodium is so low that, in the presence of even a low concentration of potassium (1 mM), sodium absorption by this mechanism is all but abolished. Mechanism 2 has a much lower affinity for alkali cations and is not highly selective; it transports sodium as well as potassium.

The potassium transporter AtHAK5 functions in K(+) deprivation-induced high-affinity K(+) uptake and AKT1 K(+) channel contribution to K(+) uptake kinetics in Arabidopsis roots

Potassium is an important macronutrient and the most abundant cation in plants. Because soil mineral conditions can vary, plants must be able to adjust to different nutrient availabilities. Here, we used Affymetrix Genechip microarrays to identify genes responsive to potassium (K(+)) deprivation in roots of mature Arabidopsis (Arabidopsis thaliana) plants. Unexpectedly, only a few genes were changed in their expression level after 6, 48, and 96 h of K(+) starvation even though root K(+) content was reduced by approximately 60%. AtHAK5, a potassium transporter gene from the KUP/HAK/KT family, was most consistently and strongly up-regulated in its expression level across 48-h, 96-h, and 7-d K(+) deprivation experiments. AtHAK5 promoter-beta-glucuronidase and -green fluorescent protein fusions showed AtHAK5 promoter activity in the epidermis and vasculature of K(+) deprived roots. Rb(+) uptake kinetics in roots of athak5 T-DNA insertion mutants and wild-type plants demonstrated the absence of a major part of an inducible high-affinity Rb(+)/K(+) (K(m) approximately 15-24 microm) transport system in athak5 plants. In comparative analyses, uptake kinetics of the K(+) channel mutant akt1-1 showed that akt1-1 roots are mainly impaired in a major transport mechanism, with an apparent affinity of approximately 0.9 mm K(+)(Rb(+)). Data show adaptation of apparent K(+) affinities of Arabidopsis roots when individual K(+) transporter genes are disrupted. In addition, the limited transcriptome-wide response to K(+) starvation indicates that posttranscriptional mechanisms may play important roles in root adaptation to K(+) availability in Arabidopsis. The results demonstrate an in vivo function for AtHAK5 in the inducible high-affinity K(+) uptake system in Arabidopsis roots.

Two types of HKT transporters with different properties of Na+ and K+ transport in Oryza sativa

It is thought that Na+ and K+ homeostasis is crucial for salt-tolerance in plants. To better understand the Na+ and K+ homeostasis in important crop rice (Oryza sativa L.), a cDNA homologous to the wheat HKT1 encoding K+-Na+ symporter was isolated from japonica rice, cv Nipponbare (Ni-OsHKT1). We also isolated two cDNAs homologous to Ni-OsHKT1 from salt-tolerant indica rice, cv Pokkali (Po-OsHKT1, Po-OsHKT2). The predicted amino acid sequence of Ni-OsHKT1 shares 100% identity with Po-OsHKT1 and 91% identity with Po-OsHKT2, and they are 66-67% identical to wheat HKT1. Low-K+ conditions (less than 3 mM) induced the expression of all three OsHKT genes in roots, but mRNA accumulation was inhibited by the presence of 30 mM Na+. We further characterized the ion-transport properties of OsHKT1 and OsHKT2 using an expression system in the heterologous cells, yeast and Xenopus oocytes. OsHKT2 was capable of completely rescuing a K+-uptake deficiency mutation in yeast, whereas OsHKT1 was not under K+-limiting conditions. When OsHKTs were expressed in Na+-sensitive yeast, OsHKT1 rendered the cells more Na+-sensitive than did OsHKT2 in high NaCl conditions. The electrophysiological experiments for OsHKT1 expressed in Xenopus oocytes revealed that external Na+, but not K+, shifted the reversal potential toward depolarization. In contrast, for OsHKT2 either Na+ or K+ in the external solution shifted the reversal potential toward depolarization under the mixed Na+ and K+ containing solutions. These results suggest that two isoforms of HKT transporters, a Na+ transporter (OsHKT1) and a Na+- and K+-coupled transporter (OsHKT2), may act harmoniously in the salt tolerant indica rice.

Enhancement of Na(+) uptake currents, time-dependent inward-rectifying K(+) channel currents, and K(+) channel transcripts by K(+) starvation in wheat root cells

Excessive low-affinity Na(+) uptake is toxic to the growth of glycophytic plants. Recently, several reports have suggested that the interaction between K(+) and Na(+) uptake might represent a key factor in determining the Na(+) tolerance of plants. We investigated the effects of K(+) starvation on Na(+) and K(+) uptake mechanisms in the plasma membrane of wheat (Triticum aestivum L.) root cortex cells using the patch-clamp technique. Unexpectedly, K(+) starvation of wheat seedlings was found to enhance the magnitude and frequency of occurrence of time-dependent inward-rectifying K(+) channel currents (I(K)(+)(in)). We examined whether the transcription of a wheat root K(+)(in) channel gene is induced by K(+) starvation. A cDNA coding for a wheat root K(+) channel homolog, TaAKT1 (accession no. AF207745), was isolated. TaAKT1 mRNA levels were up-regulated in roots in response to withdrawal of K(+) from the growth medium. Furthermore, K(+) starvation caused an enhancement of instantaneous Na(+) currents (I(Na)(+)). Electrophysiological analyses suggested that I(K)(+)(in) and I(Na)(+) are not mediated by the same transport protein based on: (a) different activation curves, (b) different time dependencies, (c) different sensitivities to external Ca(2+), and (d) different cation selectivities. These data implicate a role for I(Na)(+) in Na(+) uptake and stress during K(+) starvation, and indicate that K(+)(in) channels may contribute to K(+)-starvation-induced K(+) uptake in wheat roots.

Genetic selection of mutations in the high affinity K+ transporter HKT1 that define functions of a loop site for reduced Na+ permeability and increased Na+ tolerance

Potassium is an important macronutrient required for plant growth, whereas sodium (Na+) can be toxic at high concentrations. The wheat K+ uptake transporter HKT1 has been shown to function in yeast and oocytes as a high affinity K+-Na+ cotransporter, and as a low affinity Na+ transporter at high external Na+. A previous study showed that point mutations in HKT1, which confer enhancement of Na+ tolerance to yeast, can be isolated by genetic selection. Here we report on the isolation of mutations in new domains of HKT1 showing further large increases in Na+ tolerance. By selection in a Na+ ATPase deletion mutant of yeast that shows a high Na+ sensitivity, new HKT1 mutants at positions Gln-270 and Asn-365 were isolated. Several independent mutations were isolated at the Asn-365 site. N365S dramatically increased Na+ tolerance in yeast compared with all other HKT1 mutants. Cation uptake experiments in yeast and biophysical characterization in Xenopus oocytes showed that the mechanisms underlying the Na+ tolerance conferred by the N365S mutant were: reduced inhibition of high affinity Rb+ (K+) uptake at high Na+ concentrations, reduced low affinity Na+ uptake, and reduced Na+ to K+ content ratios in yeast. In addition, the N365S mutant could be clearly distinguished from less Na+-tolerant HKT1 mutants by a markedly decreased relative permeability for Na+ at high Na+ concentrations. The new mutations contribute to the identification of new functional domains and an amino acid in a loop domain that is involved in cation specificity of a plant high affinity K+ transporter and will be valuable for molecular analyses of Na+ transport mechanisms and stress in plants.

Rapid Up-regulation of HKT1, a high-affinity potassium transporter gene, in roots of barley and wheat following withdrawal of potassium

High-affinity K+ uptake in plant roots is rapidly up-regulated when K+ is withheld and down-regulated when K+ is resupplied. These processes make important contributions to plant K+ homeostasis. A cDNA coding for a high-affinity K+ transporter, HKT1, was earlier cloned from wheat (Triticum aestivum L.) roots and functionally characterized. We demonstrate here that in both barley (Hordeum vulgare L.) and wheat roots, a rapid and large up-regulation of HKT1 mRNA levels resulted when K+ was withdrawn from growth media. This effect was specific for K+; withholding N caused a modest reduction of HKT1 mRNA levels. Up-regulation of HKT1 transcript levels in barley roots occurred within 4 h of removing K+, which corresponds to the documented increase of high-affinity K+ uptake in roots following removal of K+. Increased expression of HKT1 mRNA was evident before a decline in total root K+ concentration could be detected. Resupply of 1 mM K+ was sufficient to strongly reduce HKT1 transcript levels. In wheat root cortical cells, both membrane depolarizations in response to 100 &mgr;M K+, Cs+, and Rb+, and high-affinity K+ uptake were enhanced by K+ deprivation. Thus, in both plant systems the observed physiological changes associated with manipulating external K+ supply were correlated with levels of HKT1 mRNA expression. Implications of these findings for K+ sensing and regulation of the HKT1 mRNA levels in plant roots are discussed.

AtKUP1: an Arabidopsis gene encoding high-affinity potassium transport activity

Because plants grow under many different types of soil and environmental conditions, we investigated the hypothesis that multiple pathways for K+ uptake exist in plants. We have identified a new family of potassium transporters from Arabidopsis by searching for homologous sequences among the expressed sequence tags of the GenBank database. The deduced amino acid sequences of AtKUP (for Arabidopsis thaliana K+ uptake transporter) cDNAs are highly homologous to the non-plant Kup and HAK1 potassium transporters from Escherichia coli and Schwanniomyces occidentalis, respectively. Interestingly, AtKUP1 and AtKUP2 are able to complement the potassium transport deficiency of an E. coli triple mutant. In addition, transgenic Arabidopsis suspension cells overexpressing AtKUP1 showed increased Rb+ uptake at micromolar concentrations with an apparent K(m) of approximately 22 microM, indicating that AtKUP1 encodes a high-affinity potassium uptake activity in vivo. A small, low-affinity Rb+ uptake component was also detected in AtKUP1-expressing cells. RNA gel blot analysis showed that the various members of the AtKUP family have distinct patterns of expression, with AtKUP3 transcript levels being strongly induced by K+ starvation. It is proposed that plants contain multiple potassium transporters for high-affinity uptake and that the AtKUP family may provide important components of high- and low-affinity K+ nutrition and uptake into various plant cell types.

AtKUP1: an Arabidopsis gene encoding high-affinity potassium transport activity

Because plants grow under many different types of soil and environmental conditions, we investigated the hypothesis that multiple pathways for K+ uptake exist in plants. We have identified a new family of potassium transporters from Arabidopsis by searching for homologous sequences among the expressed sequence tags of the GenBank database. The deduced amino acid sequences of AtKUP (for Arabidopsis thaliana K+ uptake transporter) cDNAs are highly homologous to the non-plant Kup and HAK1 potassium transporters from Escherichia coli and Schwanniomyces occidentalis, respectively. Interestingly, AtKUP1 and AtKUP2 are able to complement the potassium transport deficiency of an E. coli triple mutant. In addition, transgenic Arabidopsis suspension cells overexpressing AtKUP1 showed increased Rb+ uptake at micromolar concentrations with an apparent K(m) of approximately 22 microM, indicating that AtKUP1 encodes a high-affinity potassium uptake activity in vivo. A small, low-affinity Rb+ uptake component was also detected in AtKUP1-expressing cells. RNA gel blot analysis showed that the various members of the AtKUP family have distinct patterns of expression, with AtKUP3 transcript levels being strongly induced by K+ starvation. It is proposed that plants contain multiple potassium transporters for high-affinity uptake and that the AtKUP family may provide important components of high- and low-affinity K+ nutrition and uptake into various plant cell types.

Molecular and functional characterization of a novel low-affinity cation transporter (LCT1) in higher plants

The transport of cations across membranes in higher plants plays an essential role in many physiological processes including mineral nutrition, cell expansion, and the transduction of environmental signals. In higher plants the coordinated expression of transport mechanisms is essential for specialized cellular processes and for adaptation to variable environmental conditions. To understand the molecular basis of cation transport in plant roots, a Triticum aestivum cDNA library was used to complement a yeast mutant deficient in potassium (K+) uptake. Two genes were cloned that complemented the mutant: HKT1 and a novel cDNA described in this report encoding a cation transporter, LCT1 (low-affinity cation transporter). Analysis of the secondary structure of LCT1 suggests that the protein contains 8-10 transmembrane helices and a hydrophilic amino terminus containing sequences enriched in Pro, Ser, Thr, and Glu (PEST). The transporter activity was assayed using radioactive isotopes in yeast cells expressing the cDNA. LCT1 mediated low-affinity uptake of the cations Rb+ and Na+, and possibly allowed Ca2+ but not Zn2+ uptake. LCT1 is expressed in low abundance in wheat roots and leaves. The precise functional role of this cation transporter is not known, although the competitive inhibition of cation uptake by Ca2+ has parallels to whole plant and molecular studies that have shown the important role of Ca2+ in reducing Na+ uptake and ameliorating Na+ toxicity. The structure of this higher plant ion transport protein is unique and contains PEST sequences.

Salt tolerance in plants and microorganisms: toxicity targets and defense responses

Salt tolerance of crops could be improved by genetic engineering if basic questions on mechanisms of salt toxicity and defense responses could be solved at the molecular level. Mutant plants accumulating proline and transgenic plants engineered to accumulate mannitol or fructans exhibit improved salt tolerance. A target of salt toxicity has been identified in Saccharomyces cerevisiae: it is a sodium-sensitive nucleotidase involved in sulfate activation and encoded by the HAL2 gene. The major sodium-extrusion system of S. cerevisiae is a P-ATPase encoded by the ENA1 gene. The regulatory system of ENA1 expression includes the protein phosphatase calcineurin and the product of the HAL3 gene. In Escherichia coli, the Na(+)-H+ antiporter encoded by the nhaA gene is essential for salt tolerance. No sodium transport system has been identified at the molecular level in plants. Ion transport at the vacuole is of crucial importance for salt accumulation in this compartment, a conspicuous feature of halophytic plants. The primary sensors of osmotic stress have been identified only in E. coli. In S. cerevisiae, a protein kinase cascade (the HOG pathway) mediates the osmotic induction of many, but not all, stress-responsive genes. In plants, the hormone abscisic acid mediates many stress responses and both a protein phosphatase and a transcription factor (encoded by the ABI1 and ABI3 genes, respectively) participate in its action.

Inward-Rectifying K+ Channels in Root Hairs of Wheat (A Mechanism for Aluminum-Sensitive Low-Affinity K+ Uptake and Membrane Potential Control)

K+ is the most abundant cation in cells of higher plants, and it plays vital roles in plant growth and development. Extensive studies on the kinetics of K+ uptake in roots have shown that K+ uptake is mediated by at least two transport mechanisms, one with a high and one with a low affinity for K+. However, the precise molecular mechanisms of K+ uptake from soils into root epidermal cells remain unknown. In the present study we have pursued the biophysical identification and characterization of mechanisms of K+ uptake into single root hairs of wheat (Triticum aestivum L.), since root hairs constitute an important site of nutrient uptake from the soil. These patch-clamp studies showed activation of a large inward current carried by K+ ions into root hairs at membrane potentials more negative than -75 mV. This K+ influx current was mediated by hyperpolarization-activated K+-selective ion channels, with a selectivity sequence for monovalent cations of K+ > Rb+ [almost equal to] NH4+ >> Na+ [almost equal to] Li+ > Cs+. Kinetic analysis of K+ channel currents yielded an apparent K+ equilibrium dissociation constant (Km) of [almost equal to]8.8 mM, which closely correlates to the major component of low-affinity K+ uptake. These channels did not inactivate during prolonged stimulation and would thus enable long-term K+ uptake driven by the plasma membrane proton-extruding pump. Aluminum, which is known to inhibit cation uptake at the root epidermis, blocked these inward-rectifying K+ channels with half-maximal current inhibition at [almost equal to]8 [mu]M free Al3+. Aluminum block of K+ channels at these Al3+ concentrations correlates closely to Al3+ phytotoxicity. It is concluded that inward-rectifying K+ channels in root hairs can function as both a physiologically important mechanism for low-affinity K+ uptake and as regulators of membrane potential. The identification of this mechanism is a major step toward a detailed molecular characterization of the multiple components involved in K+ uptake, transport, and membrane potential control in root epidermal cells.

Inward-rectifying K+ channels in guard cells provide a mechanism for low-affinity K+ uptake

The molecular mechanisms by which higher plant cells take up K+ across the plasma membrane (plasmalemma) remain unknown. Physiological transport studies in a large number of higher plant cell types, including guard cells, have suggested that at least two distinct types of K(+)-uptake mechanisms exist, permitting low-affinity and high-affinity K+ accumulation, respectively. Recent patch clamp studies have revealed the presence of inward-conducting (inward-rectifying) K+ channels in the plasma membrane of higher plant cells. Research on guard cells has suggested that these K+ channels provide a major pathway for proton pump-driven K+ uptake during stomatal opening. In the present study the contribution of inward-rectifying K+ channels to higher plant cell K+ uptake was investigated by examining kinetic properties of guard cell K+ channels in Vicia faba in response to changes in the extracellular K+ concentration. Increasing the extracellular K+ concentration in the range from 0.3 mM to 11.25 mM led to enhancement of inward K+ currents and changes in current-voltage characteristics of K+ channels. The increase in K+ conductance as a function of the extracellular K+ concentration revealed a K(+)-equilibrium dissociation constant (Km) of approximately 3.5 mM, which suggests that inward-rectifying K+ channels can function as a molecular mechanism for low-affinity K+ uptake. Lowering the extracellular K+ concentration in the range from 11 mM to 1 mM induced negative shifts in the activation potential of K+ channels, such that these channels function as a K+ sensor, permitting only K+ uptake. At low extracellular K+ concentrations of 0.3 mM K+, inward-rectifying K+ channels induce hyperpolarization. Results from the present study suggest that inward-rectifying K+ channels constitute an essential molecular mechanism for plant nutrition and growth control by providing a K(+)-sensing and voltage-dependent pathway for low-affinity K+ uptake into higher plant cells and additionally by contributing to plasma membrane potential regulation.

Studies of the Uptake of Nitrate in Barley: I. Kinetics of NO(3) Influx

(13)NO(3) (-) was used to investigate patterns of NO(3) (-) influx into roots of barley plants (Hordeum vulgare L. cv Klondike) previously grown with (;induced') or without (;uninduced') a source of external NO(3) (-) ([NO(3) (-)](0)). In both induced and uninduced plants, (13)NO(3) (-) influx was biphasic in the range from 0.005 to 50 moles per cubic meter [NO(3) (-)](0). In the low concentration range (<1 mole per cubic meter for induced plants and <0.3 mole per cubic meter for uninduced plants), influx was saturable and V(max) and K(m) values for influx either increased or decreased according to NO(3) (-) pretreatment. By contrast, (13)NO(3) (-) influx in the high concentration range revealed a strictly linear concentration dependence. These fluxes appeared to be mediated by a constitutive, rather than an inducible, transport system.

Generalized kinetic analysis of ion-driven cotransport systems: II. Random ligand binding as a simple explanation for non-michaelian kinetics

Solute uptake in many cells is characterized by a series of additive Michaelis-Menten functions. Several explanations for these kinetics have been advanced: unstirred layers, transport across more than one membrane, effects of solute concentration on membrane potential, numerous carrier systems. Although each of these explanations might suffice for individual cases, none provides a comprehensive basis for interpretation of the kinetics. The most common mechanism of solute absorption involves cotransport of solute with a driver ion. A model is developed in which solute and driver ion bind randomly to a membrane-bound carrier which provides a single transmembrane pathway for transport. The kinetic properties of the model are explored with particular reference to its capacity to generate additive Michaelian functions for initial rate measurements of isotopic solute influx. In accord with previous analysis of ordered binding models (Sanders, D., Hansen, U.-P., Gradmann, D., Slayman, C.L. (1984) J. Membrane Biol. 77:123), the conventional assumption that transmembrane transit rate-limits transport has not been applied. Random binding carriers can exhibit single or multiple Michaelian kinetics in response to changing substrate concentration. These kinetics include high affinity/low velocity and low affinity/high velocity phases (so-called "dual isotherms") which are commonly observed in plant cells. Other combinations of the Michaelis parameters can result in cis-(substrate) inhibition. Despite the generality of the random binding scheme and the complexity of the underlying rate equation, a number of predictive and testable features emerge. If external driver ion concentration is saturating, single Michaelian functions always result and increasing internal substrate concentration causes uncompetitive inhibition of transport. Numerical analysis of the model in conditions thought to resemble those in many experiments demonstrates that small relative differences in a few key component rate constants of the carrier reaction cycle are instrumental in generation of dual isotherms. The random binding model makes the important prediction that the contributions of the two isotherms show opposing dependence on external concentration of driver ion as this approaches saturation. In the one case in which this dependence has been examined experimentally, the model provides a good description of the data. Charge translocation characteristics of the carrier can be determined from steady-state kinetic data on the basis of the response of substrate flux to modulation of internal driver ion concentration.(ABSTRACT TRUNCATED AT 400 WORDS)

Potassium and sodium absorption kinetics in roots of two tomato species : lycopersicon esculentum and lycopersicon cheesmanii

Excised roots of the tomato species, Lycopersicon esculentum Mill. cv Walter (the commercial species) and of Lycopersicon cheesmanii ssp. minor (Hook.) C.H. Mull. (a wild species from the Galapagos Islands), were used in comparative studies of their absorption of K(+) and Na(+). Uptake of (86)Rb-labeled K(+) and (22)Na-labeled Na(+) by excised roots of ;Walter' and L. cheesmanii varied as a function of genotype and tissue pretreatment with or without K(+). Excised roots of ;Walter' consistently absorbed more (86)Rb-labeled K(+) than those of L. cheesmanii. Absorption of K(+) from solutions ranging from 0.01 to 0.2 millimolar KCl showed saturation kinetics in both K(+)-pretreated and K(+)-depleted roots of ;Walter,' and for K(+)-depleted roots of L. cheesmanii. K(+)-pretreated roots of L. cheesmanii had exceedingly low rates of K(+) uptake with strikingly different, linear kinetics. Pretreatment with K(+) caused a decrease in rates of K(+) uptake in both genotypes. Potassium depleted roots of L. cheesmanii absorbed Na(+) at a greater rate than those of ;Walter,' whereas K(+)-pretreated roots of ;Walter' absorbed Na(+) at a greater rate than those of L. cheesmanii. The results confirm and extend previous conclusions to the effect that closely related genotypes may exhibit widely different responses to the two alkali cations, K(+) and Na(+).

Soil and plant factors influencing the accumulation of heavy metals by plants

The use of plants to monitor heavy metal pollution in the terrestrial environment must be based on a cognizance of the complicated, integrated effects of pollutant source and soil-plant variables. To be detectable in plants, pollutant sources must significantly increase the plant available metal concentration in soil. The major factor governing metal availability to plants in soils is the solubility of the metal associated with the solid phase, since in order for root uptake to occur, a soluble species must exist adjacent to the root membrane for some finite period. The rate of release and form of this soluble species will have a strong influence on the rate and extent of uptake and, perhaps, mobility and toxicity in the plant and consuming animals. The factors influencing solubility and form of available metal species in soil vary widely geographically and include the concentration and chemical form of the element entering soil, soil properties (endogenous metal concentration, mineralogy, particle size distribution), and soil processes (e.g., mineral weathering, microbial activity), as these influence the kinetics of sorption reactions, metal concentration in solution and the form of soluble and insoluble chemical species. The plant root represents the first barrier to the selective accumulation of ions present in soil solution. Uptake and kinetic data for nutrient ions and chemically related nonnutrient analogs suggest that metabolic processes associated with root absorption of nutrients regulate both the affinity and rate of absorption of specific nonnutrient ions. Detailed kinetic studies of Ni, Cd, and Tl uptake by intact plants demonstrate multiphasic root absorption processes over a broad concentration range, and the use of transport mechanisms in place for the nutrient ions Cu, Zn, and K. Advantages and limitations of higher plants as indicators of increased levels of metal pollution are discussed in terms of these soil and plant phenomena.

The kinetics of calcium uptake by roots

The time course of the uptake of labelled calcium by roots of Hordeum vulgare L., Trifolium subterraneum L., and Phaseolus aureus Roxb. was found to have two distinct phases: an initial rapid adsorption phase of about 1 h duration (Phase I) and a slower accumulation which proceeded at a constant rate for at least 12 h (Phase II). Ions taken up by the mechanism of Phase I, but not Phase II, could be removed by rinsing the roots in cold solutions of unlabelled CaCl2 or KCl.The concentration dependence of calcium uptake by barley roots over the range 0.001 to 10.0 mN was analysed with the aid of Hofstee plots. Phase I consisted of two Donnan adsorption processes. The half-saturation concentrations were 0.065 mN and 0.002 mN and the respective capacities were 1.56 and 0.36 μeq g(-1) (fresh weight). Phase II displayed Michaelis-Menten kinetics for a single mechanism with a Km=0.077 mN and V max=0.15 μeq g(-1) (fresh weight) h(-1).

Multiphasic absorption of glucose and 3-o-methyl glucose by aged potato slices

The isotherm for glucose absorption by aged potato (Solanum tuberosum var. Russet Burbank) discs shows four distinct phases in the concentration ranges 1.0 to 75 mum, 75 mum to 1.5 mm, 1.5 to 15 mm, and 15 to 100 mm, respectively. Each segment of the multiphasic isotherm, when plotted reciprocally by the method of Lineweaver and Burk or of Hofstee, without regard for uptake in earlier phases, indicates absorption rate to be a hyperbolic function of concentration. The observations suggest that glucose uptake is carrier-mediated, and that the transport barrier undergoes a series of all-or-none transformations at critical external concentrations, yielding successive new and higher values for the parameters Km and V(max) 3-O-Methyl glucose, a nonmetabolizable analogue of glucose, shows the same multiphasic absorption isotherm, with Km values essentially similar to those for glucose uptake, and V(max) values somewhat lower than those for glucose absorption. Whereas the first three phases of the absorption isotherm are taken to reflect passage across the plasma membrane, the fourth phase may reflect kinetics of glucose or 3-O-methyl glucose transport to the vacuole.

Chloride accumulation by mung bean root tips: a low affinity active transport system at the plasmalemma

Net uptake of Cl(-) into root tips of mung bean (Phaseolus aureus) increases steadily with increasing external concentrations from 1 to 60 mm. Membrane potentials were measured to determine the equilibrium concentration of Cl(-) in the tissue which could be due to diffusion. This concentration was readily exceeded in both the relatively nonvacuolate tips (0 to 1 mm) and the vacuolate, mature upper sectons (1 to 11 mm) of the roots. The activity coefficient of both cytoplasmic and vacuolar Cl(-), measured with Cl(-) sensitive microelectrodes, was approximately the same as that of a pure KCl solution of the same concentration. It is concluded that the "second mechanism" of ion uptake involves a large increase in the rate of active transport at the plasmalemma as the external concentration is increased above 1 mm.

Kinetics of iron absorption by excised rice roots

Studies on the rate of iron absorption by excised rice roots from solutions of different concentrations of FeSO4 showed the presence of two patterns, one in the low (0.005-0.5 mM) and the other in the high (1-30 mM) concentration range. The presence of CaSO4 or MnSO4 at 0.5 mM enhanced Fe(++) absorption in the low concentration range, while CaSO4 at 10 mM inhibited Fe absorption in the high concentration range in a competitive manner. Fe(++) absorption at both low and high concentrations was sensitive to metabolic inhibitors. The isotherm for Fe(++) absorption at O° exhibited an initial absorption shoulder in both low and high concentrations and was suggestive of a latent ion-transport capacity for Fe(++) in rice roots.

Electrical potential differences in cells of barley roots and their relation to ion uptake

Single cell electropotentials of barley (Hordeum vulgare L., cv. ;Compana') root cortex were measured at different external concentrations of KCl in the presence of Ca(2+). The roots were low in salt from seedlings grown on 0.5 mm aerated CaSO(4) solution. Thus, the conditions were equivalent to those used to define the dual mechanisms found with radioactive tracer-labeled ion uptake. In 0.5 mm CaSO(4) alone, there is an increase with time of cell negativity from about -65 millivolts 15 minutes after cutting segments to about -185 millivolts in 6 to 8 hours. Two possible hypotheses, not mutually exclusive, are offered to explain this aging effect: that cutting exposes plasmodesmata which are leaky initially but which seal in time, and that some internal factors, e.g., hormones diffusing from the apex, have a regulatory effect on the cell potential, an influence which becomes dissipated in isolated segments and permits the development of a higher potential difference. In any case changes in selective ion transport must be involved. The cell potentials at KCl concentrations above 2.0 mm are more negative than would be expected for a passive diffusion potential. It is suggested that this discrepancy may be due to an electrogenic pump or to a higher K(+) concentration in the cytoplasm than in the remainder of the cell, or perhaps to both. Whether there is a clear relationship between cell potential and mechanisms 1 and 2 of cation transport depends upon whether the cell potentials of freshly cut or of aged tissue represent the values relevant to intact roots.

Water dependent and water independent fluxes of potassium in exuding root systems of Ricinus communis

The flux of water, [Formula: see text], to the xylem of exuding root systems of Ricinus communis was controlled using a range of mannitol concentrations permitting the influence of this water flux on the potassium flux, f K, to be studied. The relationship between [Formula: see text] and f K thus obtained was investigated, for a number of external concentrations of potassium, Cm, supplied as potassium nitrate. An analysis of these data indicated the presence of a water dependent and a water independent f K both of which varied with Cm. The water dependent f K shows a parabolic relationship with Cm for Cm values <1 mM followed by a sharp inflection and decline at higher Cm values whereas the water independent f K shows an hyperbolic relationship over the same range of Cm values.Uptake of potassium by exuding root systems was measured and shown to be dependent on the solute potential of the medium. The uptake was also shown to exhibit a dual absorption isotherm the kinetics of which indicate a low Km system (system 1) and a high Km system (system 2). The Km value obtained for system 1 is very similar to that obtained for the water independent f K. It is postulated that the water independent f K is contributed by that portion of f K arriving in the stele via the cortical symplast and is directly dependent on Cm. The water dependent f K is contributed by those ions moved across the root in response to centripetal water movement through the cortical cell walls.

[The kinetics of ion uptake by young and old branches of mnium cuspidatum]

Isotherms of K(Rb)-, Cl- and SO4-uptake by young and old branches of the moss Mnium cuspidatum were investigated. Old moss gametophytes from the 1966 vegetation period were collected in the forests surrounding Darmstadt from February to mid-April 1967 and from the 1967 season in late September 1967. Young plants were sampled from mid-April to the end of May 1967 and they were also grown by water culture of old plants.Both young and old branches have hyperbolic isotherms of ion uptake in the low concentration range (0-0.5 mM) (Fig. 1-3), which slightly differ in K mand V max (Table). Isotherms in the high range (1-10 mM), however, are drastically different, changing from linear or exponential with young moss branches to hyperbolic with old gametophytes (Figs. 1-3).The linear or exponential high-range isotherm obtained with young moss plants is compared with other examples reported in the literature (Fig. 4). As the leaflets of the moss plants, which constitute 2/3 of the fresh weight of the material used in the experiments, have well developed vacuoles, the correlation between hyperbolic isotherms and vacuolation does not apply here (Fig. 4a, TORII and LATIES, 1966).The change in shape of the high-range moss isotherm with age resembles the change from exponential to hyperbolic kinetics in isolated potato discs during washing (Fig. 4b, LATIES, MACDONALD and DAINTY, 1964). The events triggered by isolation of potato discs from the interior of the tuber may be similar to the changes in the moss material under the control of the terminal bud, which is only active in the young branches.The suggested influence of the active terminal bud of young moss plants on the ion absorption process of cells in the tissue may be related to effects of growth substances on translocation reported in the literature and may point to a direct effect of these regulatory systems on membrane function.In this respect the comparison of corn root stele and cortex is of interest. Isolated steles, both freshly isolated and after washing, have exponential isotherms in the high range (Fig. 4c), whereas cortex displays a hyperbolic isotherm which changes little with ageing (LüTTGE and LATIES, 1967). In contrast to the case in potato and moss materials, this phenomenon is not simply due to ageing but involves morphogenetic differences.Temperature is another factor which influences the shape of the high range isotherm. All examples discussed so far refer to experiments at room temperature. At low temperatures high-range isotherms for proximal root tissue or aged potato discs have an exponential shape (TORII and LATIES, 1966; LATIES, MACDONALD and DAINTY, 1964). It thus appears that the exponential isotherm of young moss branches indicates that as in freshly isolated potato discs or in corn root stele the metabolic high-range uptake system is not developed.

[Investigations on the kinetics of ion uptake into the cytoplasm of mnium leaf cells by means of microautoradiography and electron probe X-ray microanalyser]

Methods of quantitative microautoradiography are described. The kinetics of ion uptake into the cytoplasm (including the plastids and other organelles) were investigated by means of microautoradiography and electron probe X-ray microanalyser.The results of microautoradiography show that under our experimental conditions at the end of the experiments the cytoplasm contains much more label per unit of volume than the vacuoles. The kinetics of this filling - up of the cytoplasm correspond to the system-1 isotherm of ion uptake. It must be concluded from the literature that system-1 reflects the process filling up the cytoplasm (=transport through the plasmalemma). The kinetic data reported here describe directly the actual filling - up of the cytoplasmic phase. For this reason the coincidence of kinetic features such as the hyperbolic shape of the rate versus concentration curves in both instances is of interest for the general model of ion uptake into cells of higher plants. The time course of the filling of the cytoplasm is dependent on the external concentration.Similar kinetic data are obtained with the electron probe X-ray microanalyser.The usefulness of a parallel application of microautoradiographic and electron probe X-ray microanalyser techniques in kinetic studies of transport is discussed.

[Evidence for mobile transport structures (carriers) involved in ion transport in plants and kinetics of anion transport in elodea in light and dark]

Influx and efflux of anions in leaves of Elodea canadensis were investigated. A major component of phosphate efflux was found to be a function of the external phosphate concentration in accord with saturation kinetics, i.e. it was found to be proportional to phosphate influx. This suggests that the transport structures which mediate ion transport in plant membranes are mobile and can work in both directions (Fig. 7C). The higher the external concentration the more bound ions (A b (-) , Fig. 7C) are exchanged for external ions (A a (-) ) instead of being transported back to the inside of the membrane. Thus it was demonstrated that ion transport in plants is mediated by carriers or transportases. The action of the carriers is supported by ATP or another "energy-rich" compound which is regenerated by ATP; ATP may be supplied by the mitochondria or/and chloroplasts. In line with these findings some transport phenomena exhibited features of coenzyme kinetics.Magnitude and molecular nature of the carrier motion are unknown. There is no evidence for rotating or shuttling monovalent carriers. The carriers may be polyvalent and may be part of bigger structural units within the membrane. Nevertheless, these experimental observations are rather consistent with the idea that plant membranes contain tertiary or quarternary formed proteins or lipoproteins than with the conventional lipid bilayer models of cell membranes.

Absorption and long distance transport by isolated stele of maize roots in relation to the dual mechanisms of ion absorption

Ion absorption and transport by intact roots, isolated cortex and isolated stele were compared shortly after tissue isolation and after aging. Absorption isotherms in the low and in the high concentration range show that in stripped-stele, which absorbs at a very low rate immediately after isolation, the capacity of system 1 but not system 2 is built up with aging. In agreement with this result analysis of individual fluxes across plasmamembrane and tonoplast reveals that only the influx from the medium into the cytoplasm increases considerably with aging of stele. Changes observed in aging excised roots and in isolated cortex are much less significant. In spite of the increase of absorption with aging by isolated stele, long distance transport, which is essentially passive through freshly stripped stele, decreases with aging. The reported results reflect the marked permeability of the plasmamembrane of fresh isolated stele, and demonstrate the importance of the cortex as a tissue "collecting" ions for long distance transport. New evidence for the theory of symplasmatic transport of ions into the xylem vessels is thus provided.

Sodium absorption by barley roots: its mediation by mechanism 2 of alkali cation transport

When barley roots absorb Na(+) at concentrations ranging from 1 to 50 mm, in the presence of low concentrations of Ca(2+) and K(+), absorption of Na(+) is mediated by carrier mechanism 2 of alkali cation transport, mechanism 1 being unavailable for Na(+) transport under these conditions. The absorption isotherm depicting the rate of Na(+) absorption as a function of the external Na(+) concentration, over the 1 to 50 mm range of concentrations, shows several inflections. This stepwise response occurs whether Cl(-) or SO(4) (2-) is the counterion, but actual rates of Na(+) absorption are lower in the latter case.When the concentration of Na(+) is 50 mm, and the concentration of either K(+) or Ca(2+) is increased from nil to 50 mm, the rate of absorption of Na(+) is diminished not as a smooth function of increasing concentrations of the interfering ions but stepwise. Similarly, when the concentration of K(+) is 50 mm, and the concentration of either Na(+) or Ca(2+) is increased from nil to 50 mm, the rate of absorption of K(+) is diminished not as a smooth function of increasing concentrations of the interfering ions but stepwise.Together, this evidence supports the previous conclusion to the effect that mechanism 2 of alkali cation transport possesses a spectrum of carrier sites with different ionic affinities.When both K(+) and Na(+) are presented at equivalent concentrations over the 1 to 50 mm range, mechanism 2 transports Na(+) almost exclusively, and mechanism 1 K(+) almost exclusively. These findings support previous conclusions to the effect that the active sites of mechanism 2 have higher affinity for Na(+) than for K(+), whereas the reverse is true for mechanism 1.

Sodium absorption by barley roots: role of the dual mechanisms of alkali cation transport

Radioactively labeled Na(+) absorbed by barley roots was sequestered in an intracellular compartment or compartments ("inner" spaces) in which it was only very slowly exchangeable with exogenous Na(+). Absorption of this fraction proceeded at a constant rate for at least 1 hour.When the rate of Na(+) absorption was examined over the range of concentrations, 0.005 to 50 mm, the isotherm depicting the relation showed dual kinetics as follows. Over the range, 0.005 to 0.2 mm, a single Michaelis-Menten term describes the relation between the concentration of Na(+) and the rate of its absorption. The mechanism of Na(+) absorption operating over this range of concentrations, mechanism 1 of alkali cation transport, is severely inhibited in the presence of Ca(2+) and virtually rendered inoperative for Na(+) transport by the combined presence of Ca(2+) and K(+). The mechanism is equally effective in Na(+) transport whether Cl(-) or F(-) is the anion, but is somewhat inhibited when the anion is SO(4) (2-).Over the high range of concentrations, 0.5 to 50 mm Na(+), a second, low-affinity mechanism of Na(+) absorption comes into play. In the presence of Ca(2+) and K(+), this mechanism 2 is the only one to transport Na(+) effectively, since Na(+) absorption via mechanism 1 is virtually abolished under these conditions.Anaerobic conditions, low temperature, and the uncoupler, 2,4-dinitrophenol, inhibit Na(+) absorption both at low and high Na(+) concentrations.

Selective inhibition of absorption and long distance transport in relation to the dual mechanisms of ion absorption in maize seedlings

The influence of several uncouplers of oxidative phosphorylation and inhibitors of terminal electron transport was studied on absorption and long distance transport of both K and C1 at concentrations within each range of the dual isotherm typical of ion uptake by maize roots. At low concentrations in the range of system 1, the system considered to implement ion movement through the plasma membrane, root absorption and long distance transport are equally inhibited by a given inhibitor. In the high range of system 2, the system considered to mediate ion passage through the tonoplast, long distance transport is markedly less sensitive to inhibitors than is absorption. The observations are in accord with the hypothesis that only system 1 is involved in the uptake of ions from the external solution into the symplast, and hence into the xylem. At high concentrations, entrance into the symplasm is deemed to be largely by diffusion and therefore less inhibitor sensitive. With respect to absorption by the roots, the plasma membrane system is more inhibitor sensitive than is the tonoplast system. It is suggested that the difference in sensitivity is real, and not the consequence of an inequality of inhibitor concentration in the vicinity of the plasma membrane and tonoplast respectively.

Dual mechanisms of ion absorption in relation to long distance transport in plants

The characteristics of ion transport to the shoots of young corn seedlings were studied with respect to the nature of the isotherm through a wide concentration range, the competitive influence of closely related ions upon the transport of a given ion, and the influence of the counter-ion. Both with respect to (36)Cl and (86)Rb transport, the characteristics of the process in every way resemble uptake by non-vacuolate root tips wherein the plasma membrane is the only membrane involved in absorption, and where system 1 - of the 2 systems which can be shown to participate in absorption by vacuolate tissue - is the only system operative. Net ion uptake by the roots per se was shown to display both the high affinity (system 1) and low affinity (system 2) mechanisms. It is concluded that the symplastic theory of ion movement to the xylem is valid, and that the contention that system 1 operates at the plasma membrane while system 2 functions at the tonoplast is strengthened.

A study of rubidium accumulation in Euglena gracillis

The absorption of Rb(+) by Euglena gracillis var. bacillaris was examined, and an active uptake process could be detected within 1 hour. This was established by experiments that included a time course study and metabolic depression by either reducing the temperature or by addition of 2,4-dinitrophenol. By treating the cells with 0.50 mg/ml chloramphenicol, the rate of glycine incorporation and Rb(+) uptake were inhibited. The possible role of protein synthesis in relation to ion accumulation is discussed.

Dual mechanisms of ion uptake in relation to vacuolation in corn roots

Absorption isotherms for chloride and rubidium ions have been determined through a wide concentration range for nonvacuolate root tips, and for vacuolate subapical sections of corn root. In the range 0 to 0.5 mm, chloride absorption is hyperbolic with concentration in both tips and proximal sections. At high concentrations, 1 to 50 mm, a second multiple-hyperbolic isotherm for chloride is noted in vacuolate tissue, while the isotherm for nonvacuolate tips rises exponentially. A linear to exponentially rising isotherm is taken to signify diffusive permeation.The same distinction between tip and subapical tissue characterizes Rb absorption. Rb uptake is indifferent to the nature of the counterion at all concentrations in the tip, while the counterion exerts a predictable influence on Rb absorption in proximal tissue. The effect of a poorly absorbable anion on Rb uptake is greater in the high concentration range. Evidence is presented for the metabolic nature of ion transport into nonvacuolate root tips. Verification is offered that ion uptake is mediated by dual mechanisms, and the thesis is developed that the high-affinity (low K(s)) system mediates ion passage through the plasma membrane while the low-affinity (high K(s)) system implements transport through the tonoplast.