Competition between uptake of ammonium and potassium in barley and Arabidopsis roots: molecular mechanisms and physiological consequences

Plants can use ammonium (NH4+) as the sole nitrogen source, but at high NH4+ concentrations in the root medium, particularly in combination with a low availability of K+, plants suffer from NH4+ toxicity. To understand the role of K+ transporters and non-selective cation channels in K+/NH4+ interactions better, growth, NH4+ and K+ accumulation and the specific fluxes of NH4+, K+, and H+ were examined in roots of barley (Hordeum vulgare L.) and Arabidopsis seedlings. Net fluxes of K+ and NH4+ were negatively correlated, as were their tissue concentrations, suggesting that there is direct competition during uptake. Pharmacological treatments with the K+ transport inhibitors tetraethyl ammonium (TEA+) and gadolinium (Gd3+) reduced NH4+ influx, and the addition of TEA+ alleviated the NH4+-induced depression of root growth in germinating Arabidopsis plants. Screening of a barley root cDNA library in a yeast mutant lacking all NH4+ and K+ uptake proteins through the deletion of MEP1–3 and TRK1 and TRK2 resulted in the cloning of the barley K+ transporter HvHKT2;1. Further analysis in yeast suggested that HvHKT2;1, AtAKT1, and AtHAK5 transported NH4+, and that K+ supplied at increasing concentrations competed with this NH4+ transport. On the other hand, uptake of K+ by AtHAK5, and to a lesser extent via HvHKT2;1 and AtAKT1, was inhibited by increasing concentrations of NH4+. Together, the results of this study show that plant K+ transporters and channels are able to transport NH4+. Unregulated NH4+ uptake via these transporters may contribute to NH4+ toxicity at low K+ levels, and may explain the alleviation of NH4+ toxicity by K+

Opening and Closing of KcnkØ Potassium Leak Channels Is Tightly Regulated

Potassium-selective leak channels control neuromuscular function through effects on membrane excitability. Nonetheless, their existence as independent molecular entities was established only recently with the cloning of KCNKØ from Drosophila melanogaster. Here, the operating mechanism of these 2 P domain leak channels is delineated. Single KCNKØ channels switch between two long-lived states (one open and one closed) in a tenaciously regulated fashion. Activation can increase the open probability to ∼1, and inhibition can reduce it to ∼0.05. Gating is dictated by a 700-residue carboxy-terminal tail that controls the closed state dwell time but does not form a channel gate; its deletion (to produce a 300-residue subunit with two P domains and four transmembrane segments) yields unregulated leak channels that enter, but do not maintain, the closed state. The tail integrates simultaneous input from multiple regulatory pathways acting via protein kinases C, A, and G

Freedom Gas to Europe: Scenarios Analyzed Using the Global Gas Model

State-of-the-art, open access numerical modeling of imperfectly competitive energy markets offers a sound and transparent way to address topical research questions in energy and commodity markets. We use an open access equilibrium model, the Global Gas Model (GGM), and sector-specific, politically motivated scenarios to investigate the prospects for sales of liquefied natural gas (LNG) from the U.S. into the European energy market. We discuss the risks and opportunities for U.S. LNG and derive implications for policy, business, and finance in the energy sector. We find that Europe is not an attractive market for US LNG in the base case and in scenarios of moderate support of U.S. LNG flows into Europe. In these scenarios, Asia offers higher prices for US LNG and draws substantially higher import volumes. Our modeling results show that the interconnectedness of global gas markets due to an abundance of LNG import capacity in Europe and other regions—particularly Asia—allows for adjustments to global trade patterns that mitigate the consequences of regional disturbances

Molecular mechanisms and regulation of plant ion channels

Plant ion channel activities are rapidly modulated in response to several environmental and endogenous stimuli. Electrophysiological as well as pharmacological studies provide strong evidence that ion channels are essential for the induction of specific cellular responses and that they are themselves subject to regulation by a variety of cellular factors. Genes anal cDNAs of several plant ion channels have been identified in recent years giving access to molecular mechanisms of such regulatory processes. Cloned inwardly rectifying potassium channels have been investigated in various heterologous expression systems. Two other ion channel classes, namely members of the 'two-pore' K+ channel family as well as of the anion-conducting chloride channel (CIC) family, have been cloned, but a direct link to corresponding plasma membrane or endomembrane ion channel conductances has not been demonstrated yet. Analysis of cellular expression patterns of plant ion channel genes in combination with transgenic approaches now gives access to a detailed ex planta/in planta correlation of channel function, as has recently been demonstrated for proteins of the K-in(+) channel family. This review summarizes current knowledge on molecular structures and some features of structure-function relationships of plant ion channels

New structure and function in plant K+ channels: KCO1, an outward rectifier with a steep Ca2+ dependency.

Potassium (K+) channels mediating important physiological functions are characterized by a common pore-forming (P) domain. We report the cloning and functional analysis of the first higher plant outward rectifying K+ channel (KCO1) from Arabidopsis thaliana. KCO1 belongs to a new class of 'two-pore' K+ channels recently described in human and yeast. KCO1 has four putative transmembrane segments and tandem calcium-binding EF-hand motifs. Heterologous expression of KCO1 in baculovirus-infected insect (Spodoptera frugiperda) cells resulted in outwardly rectifying, K+-selective currents elicited by depolarizing voltage pulses in whole-cell measurements. Activation of KCO1 was strongly dependent on the presence of nanomolar concentrations of cytosolic free Ca2+ [Ca2+]cyt. No K+ currents were detected when [Ca2+]cyt was adjusted to <150 nM. However, KCO1 strongly activated at increasing [Ca2+]cyt, with a saturating activity observed at approximately 300 nM [Ca2+]cyt. KCO1 single channel analysis on excised membrane patches, resulting in a single channel conductance of 64 pS, confirmed outward rectification as well as Ca2+-dependent activation. These data suggest a direct link between calcium-mediated signaling processes and K+ ion transport in higher plants. The identification of KCO1 as the first plant K+ outward channel opens a new field of structure-function studies in plant ion channels

AtTPK4, an Arabidopsis tandem-pore K(+) channel, poised to control the pollen membrane voltage in a pH- and Ca(2+)-dependent manner

The Arabidopsis tandem-pore K(+) (TPK) channels displaying four transmembrane domains and two pore regions share structural homologies with their animal counterparts of the KCNK family. In contrast to the Shaker-like Arabidopsis channels (six transmembrane domains/one pore region), the functional properties and the biological role of plant TPK channels have not been elucidated yet. Here, we show that AtTPK4 (KCO4) localizes to the plasma membrane and is predominantly expressed in pollen. AtTPK4 (KCO4) resembles the electrical properties of a voltage-independent K(+) channel after expression in Xenopus oocytes and yeast. Hyperpolarizing as well as depolarizing membrane voltages elicited instantaneous K(+) currents, which were blocked by extracellular calcium and cytoplasmic protons. Functional complementation assays using a K(+) transport-deficient yeast confirmed the biophysical and pharmacological properties of the AtTPK4 channel. The features of AtTPK4 point toward a role in potassium homeostasis and membrane voltage control of the growing pollen tube. Thus, AtTPK4 represents a member of plant tandem-pore-K(+) channels, resembling the characteristics of its animal counterparts as well as plant-specific features with respect to modulation of channel activity by acidosis and calcium