Partial characterization of the trait for enhanced K+−Na+ discrimination in the D genome of wheat

The long arm of chromosome 4D of wheat (Triticum aestivum L.) contains a gene (or genes) which influences the ability of wheat plants to discriminate between Na⁺ and K⁺. This discrimination most obviously affects transport from the roots to the shoots, in which less Na⁺ and more K⁺ accumulate in those plants which contain the long arm of chromosome 4D. Concentrations of Na⁺ and K⁺ in the roots, and Cl⁻ concentrations in the roots and shoots, are not significantly affected by this trait, but Na⁺, K⁺ and Cl⁻ contents of the grain are reduced. The trait operates over a wide range of salinities and appears to be constitutive. At the moment it is not possible to determine accurately the effect of this trait on growth or grain yield because the aneuploid lines which are available are much less vigorous and less fertile than their euploid parents.     

Salinity tolerance of Kosteletzkya virginica. I. Shoot growth, ion and water relations

Abstract. Kosteletzkya virginica (L.) Presl., a dicotyledonous halophyte native to brackish tidal marshes, was grown on nutrient solution containing 0. 85, 170 or 255 mol m^-3 NaCl, and the effects of external salinity on shoot growth and ion content of individual leaves were studied in successive harvests. Growth was stimulated by 85 mol m^-3 NaCl and was progressively reduced at the two higher salinities. Growth suppression at high salinity resulted principally from decreased leaf production and area, not from accelerated leaf death. As is characteristic of halophytic dicots. K. virginica accumulated inorganic ions in its leaves, particularly Na⁺ and K⁺. However, the Na⁺ concentration of individual leaves did not increase with time, but remained constant or even declined, seeming to be well-coordinated with changes in water content. A striking feature of the ion composition of salinized plants was the development of a dramatic gradient in sodium content, with Na⁺ partitioned away from the most actively growing leaves. Salt-treated plants exhibited a strong potassium affinity, with foliar K⁺ levels higher in salinized plants than unsalinized plants after an initial decrease. These results suggest that selective uptake and transport, foliar compartmentation of Na⁺ and K⁺ in opposite directions along the shoot axis, and the regulation of leaf salt loads over time to prevent build-up of toxic concentrations are whole-plant features which enable K. virginica to establish favourable K⁺-Na⁺ relations under saline conditions.     

Salinity tolerance of Kosteletzkya virginica. II. Root growth, lipid content, ion and water relations

Abstract. Kosteletzkya virginica (L.) Presl., a dicot halophyte native to brackish tidal marshes, was grown on nutrient solution containing 0. 85, 170 or 255 mol m ³ NaCl, and the effects of external salinity on root growth, ion and water levels, and lipid content were examined in successive harvests. Root growth paralleled shoot growth trends, with some enhancement observed at 85 mol m ³ NaCl and a reduction noted at the higher salinities. Root Na⁺ content increased with increasing external NaCl, but remained constant with time for each treatment. K⁺ content, although lower in salt-grown plants after 14 d salinization, subsequently increased to levels comparable to unsalinized plants. A strong K⁺ affinity was reflected in the increased K⁺/Na⁺ selectivity of salt-grown plants and by their low Na⁺/K⁺ ratios. Cl levels rose in salinized plants and values were double or more those for Na⁺, indicating the possibility of a sodium-excluding mechanism in roots. Root phospholipids and sterols, principal membrane constituents, were maintained or elevated and the free sterol/phospholipids ratio increased in salinized K. virginica plants, suggesting retention of overall membrane structure and decreased permeability. This response, considered in light of root calcium maintenance and high potassium levels, suggests that salinity-induced changes in membrane lipid composition may be important in preventing K⁺ leakage from cells.     

Effect of Halide Ions on t-[35S]Butylbicyclophosphorothionate Binding

The binding of t-[35S]butylbicyclophosphorothionate [( 35S]TBPS) to a site on the GABAA receptor complex is ion dependent. This study was conducted to determine the effects of ion species and concentration on the time course, affinity, and number of sites of [35S]TBPS binding. At a concentration of 200 mM ion, the time to equilibrium for [35S]TBPS binding was shortest for I-, followed by Br- less than Cl- less than F-. A similar rank order was observed for the concentration of ion required to produce half-maximal [35S]TBPS binding. Saturation binding experiments were conducted to evaluate the effect of increasing ion concentration on the KD and Bmax of [35S]TBPS binding. The Bmax was independent of both ion species and concentration. The receptor affinity, however, increased with increasing concentration for each ion. Calculated maximal affinity values were not different between ions; however, the EC50 to produce those values was different among ions and ranked in the same order as that for time course and maximal binding data. Association and dissociation rates for [35S]TBPS binding were greater in I- than in Cl-. These data emphasize the importance of ion selection and incubation times on [35S]TBPS binding.     

Effects of cesium on in vitro myoblast differentiation: An electron microscopic study

Summary This paper describes the microscopic evidence supporting a cesium-induced delay in the fusion of chick embryo myoblast membranes during in vitro myogenic differentiation. We have recently demonstrated that the sharp decrease in the conductivity and permittivity of the membranes of these myogenic cells at the time of fusion is delayed 30 h by the addition of cesium to the culture medium (Santini et al., Biochim. Biophys. Acta 945:56–64; 1988). We report here that this delay in fusion is substantiated by direct microscopic observation and that cesium also induces ultrastructural changes in the myoblast cells themselves. Possible mechanisms by which cesium may cause both the delay in fusion as well as the ultrastructural changes observed are discussed. This investigation was partially supported by an Italian Consiglio Nazionale delle Ricerche grant 85.00.304.02 (to P. L. I.).     

Two Components of Glutamate Exocytosis Differentially Affected by Presynaptic Modulation

Abstract: The total Ca²⁺-dependent release of glutamate induced by depolarization of cerebrocortical nerve terminals with KCl was analyzed into a fast and a slow component. The fast component exhibited a decay time of <1 s and accounted for 0.95 ± 0.10 nmol of glutamate, whereas the slow component, which exhibited a decay time of 52 ± 7 s, accounted for the release of 2.48 ± 0.19 nmol of glutamate. These two components were differentially affected by the Ca²⁺ chelator BAPTA, the divalent cation Sr²⁺, or the botulinum neurotoxin A. The adenosine A₁ receptor agonist N ⁶-cyclohexyladenosine strongly reduced the fast component without altering the slow component. In contrast, the inhibitory effect of arachidonic acid and the facilitatory action of the metabotropic glutamate receptor agonist (1 S ,3 R )-1-aminocyclopentane-1,3-dicarboxylic acid were observed as a decrease and an increase, respectively, in the two components. It is concluded, first, that the fast and slow components correspond to the release of docked and mobilized vesicles, respectively, and second, that presynaptic modulation more significantly alters the fast component of release.     

High-conductance calcium-activated potassium channels; Structure,pharmacology, and function

High-conductance calcium-activated potassium (maxi-K) channels comprise a specialized family of K⁺ channels. They are unique in their dual requirement for depolarization and Ca²⁺ binding for transition to the open, or conducting, state. Ion conduction through maxi-K channels is blocked by a family of venom-derived peptides, such as charybdotoxin and iberiotoxin. These peptides have been used to study function and structure of maxi-K channels, to identify novel channel modulators, and to follow the purification of functional maxi-K channels from smooth muscle. The channel consists of two dissimilar subunits, and . The subunit is a member of theslo Ca²⁺-activated K⁺ channel gene family and forms the ion conduction pore. The subunit is a structurally unique, membrane-spanning protein that contributes to channel gating and pharmacology. Potent, selective maxi-K channel effectors (both agonists and blockers) of low molecular weight have been identified from natural product sources. These agents, together with peptidyl inhibitors and site-directed antibodies raised against and subunit sequences, can be used to anatomically map maxi-K channel expression, and to study the physiologic role of maxi-K channels in various tissues. One goal of such investigations is to determine whether maxi-K channels represent novel therapeutic targets.     

Expression of a moderately anionic peroxidase is induced by aluminum treatment in tobacco cells: Possible involvement of peroxidase isozymes in aluminum ion stress

To clarify the mechanism of aluminum (Al) toxicity and Al tolerance, we isolated a new clone (pAL201) from a tobacco cDNA library. Northern blot hybridization analysis indicated that the expression of pAL201 is induced by Al treatment and phosphate (P₁) starvation. The complete cDNA sequence suggested that this clone encodes a moderately anionic peroxidase (EC 1.11.1.7). Analysis by isoelectric focussing indicated that a moderately anionic peroxidase (approximately pI 6.7) and two cationic peroxidases (pI 9.2 and 9.7) in the soluble fraction are activated by Al treatment and P₁ starvation, while two moderately anionic isozymes are repressed by these stresses. We suppose that Al ion stress can control the activity of some peroxidase isozymes, one of which is probably induced by enhanced gene expression of pAL201. There is a possibility that some of these isozymes have some functions in Al ion stress.     

Transmembrane movements of artificial redox mediators in relation to electron transport and ionic currents in chloroplasts

Electrochemical data obtained with TMPD⁺-sensitive electrodes indicate that ammonium-uncoupled chloroplasts retain TMPD (N,N,N',N'-tetramethyl- p -phenylenediamine) mainly in the reduced form during illumination, whereas uncoupled DCMU-treated chloroplasts accumulate TMPD in the oxidized form (TMPD⁺). This observation indicates that the reduced plastoquinol is the preferred electron donor for photosystem I (PSI) and TMPD can only compete efficiently when plastoquinone reduction is blocked. After adding DCMU the formation of a transmembrane gradient for TMPD⁺ is reflected by a slow-down of the electrogenic electron transport and by the emerging of the overshoot of the membrane current in the light-off response. A light-dependent increase in photoelectric current generated by chloroplasts in the presence of NH₄Cl and TMPD is observed and considered to be caused by a reversible release of current limitation in the interfacial conductance barriers in the lumen.