Effects of interrupted K+ supply on growth and uptake of K+, Ca2+, Mg2+ and Na+ in spring wheat

Effects of interrupted K⁺ supply on different parameters of growth and mineral cation nutrition were evaluated for spring wheat (Triticum aestivum L. cv. Svenno). K⁺ (2.0 mM) was supplied to the plants during different periods in an otherwise complete nutrient solution. Shoot growth was reduced before root growth after interruption in K⁺ supply. Root structure was greatly affected by the length of the period in K⁺ -free nutrient solution. Root length was minimal, and root branching was maximal within a narrow range of K⁺ status of the roots. This range corresponded to cultivation for the last 1 to 3 days, of 11 in total, in K⁺ -free nutrient solution, or to continuous cultivation in solution containing 0.5 to 2 mM K⁺. In comparison, both higher and lower internal/external K⁺ concentrations had inhibitory effects on root branching. However, the differing root morphology probably had no significant influence on the magnitude of Ca²⁺, Mg²⁺ and Na⁺ uptake. Uptake of Ca²⁺ and especially Mg²⁺ significantly increased after K⁺ interruption, while Na⁺ uptake was constant in the roots and slowly increased in the shoots. The two divalent cations could replace K⁺ in the cells and maintain electroneutrality down to a certain minimal range of K⁺ concentrations. This range was significantly higher in the shoot [110 to 140 μmol (g fresh weight)^?1] than in the root [20 to 30 μmol (g fresh weight)^?1]. It is suggested that the critical K⁺ values are a measure of the minimal amount of K⁺ that must be present for physiological activity in the cells. At the critical levels, K⁺ (⁸⁶Rb) influx and Ca²⁺ and Mg²⁺ concentrations were maximal. Below the critical K⁺ values, growth was reduced, and Ca²⁺ and Mg²⁺ could no longer substitute for K⁺ for electrostatic balance. In a short-term experiment, the ability of Ca²⁺ to compete with K⁺ in maintaining electroneutrality in the cells was studied in wheat seedlings with different K⁺ status. The results indicate that K⁺, which was taken up actively and fastest at the external K⁺ concentration used (2.0 mM), partly determines the size of Ca²⁺ influx.     

The use of phospholipid-impregnated millipore filters for recording nonelectrogenic cation flows in the presence of Me/nH exchangers

It has been shown that with a cation (K⁺, Na⁺, Ca²⁺) concentration gradient on a Millipore filter impregnated with a decane solution of phospholipid, in the presence of a Meⁿ⁺/nH⁺ exchanger (nigericin, monensin, A23187), addition of a protonophore induces the formation of an electric potential positively charged on the side where the concentration of the cation is lower. The formation of the potential is induced by the hydrogen ion concentration gradient in the filter and in the unstirred layers as a result of the Meⁿ⁺/nH⁺ exchange. In such a system, with a pH gradient on the filter in the presence of monensin and valinomycin, a potential is generated with the plus on the side of the lower concentration of hydrogen. The effect is the result of the formation of a potassium ion concentration gradient in the unstirred layers in the course of the K⁺/H⁺ exchange. It is concluded that phospholipid-impregnated filters can be used for search and identification of electroneutral membrane ionophores of the Meⁿ⁺/nH⁺ exchanger type.     

Efficiency,Na+/K+ selectivity and temperature dependence of ion transport through lipid membranes by (221)C10-Cryptand,an ionizable mobile carrier

Summary The kinetics of Na⁺ and K⁺ transport across the membrane of large unilamellar vesicles (LUV) were determined at two pH's when transport was induced by (221)C₁₀-cryptand (diaza-1,10-decyl-5-pentaoxa-4,7,13,16,21-bicyclo [8.8.5.] tricosane) at various temperatures, and by nonactin at 25°C and (222)C₁₀-cryptand at 20 and 25°C. The rate of Na⁺ and K⁺ transport by (221)C₁₀ saturated with the cation and carrier concentrations. Transport was noncooperative and exhibited selectivity for Na⁺ with respect to K⁺. The apparent affinity of (221)C₁₀ for Na⁺ was higher and less pH-dependent than that for K⁺, and seven times higher than that of (222)C₁₀ for K⁺ ions (20.5vs. 1.7 kcal·mole^–). The efficiency of (221)C₁₀ transport of Na⁺ was pH-and carrier concentration-dependent, and was similar to that of nonactin; its activation energy was similar to that for (222)C₁₀ transport of K⁺ (35.5 and 29.7 kcal · mole^–1, respectively). The reaction orders in cationn(S) and in carrierm(M), respectively, increased and decreased as the temperature rose, and were both independent of carrier or cation concentrations; in most cases they varied slightly with the pH.n(S) varied with the cation at pH 8.7 and with the carrier for Na⁺ transport only, whilem(M) always depended on the type of cation and carrier. Results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes.     

Pyruvate kinase: Activation by and catalytic role of the monovalent and divalent cations

Summary This mini review is primarily concerned with the monovalent and divalent cation activation of pyruvate kinase. All preparations of pyruvate kinase from vertebrate tissue which have been examined require monovalent cations such as K⁺ for catalysis. However, several microbial preparations are not activated by monovalent cations. In fact,E. coli synthesizes depending on growth conditions, 2 different forms of the enzyme; one form is not activated while the other is activated by monovalent cations. The monovalent cation was shown by NMR techniques to bind within 4–8 ? of the divalent cation activat or and apparently plays a direct role in the catalytic process. As with all kinases, pyruvate kinase requires a divalent cation for catalysis. Mg⁺² is optimal for the physiological reaction, however, Co⁺², Mn⁺², and Ni⁺² also activate. The divalent cation activation of several non-physiological reactions catalyzed by pyruvate kinase are reviewed. Several lines of evidence suggest that 2 moles of the divalent cation are required in the catalytic event. However, the specific role of both atoms in the catalytic event have not been thoroughly elucidated.     

Induction of Na+ / K+ exchange in swollen heart mitochondria

Heart mitochondria swollen passively in nitrate salts contract in a respiration-dependent reaction which can be attributed to an endogenous cation/H⁺ exchange component (or components). The rate of contraction increases with increased extent of passive swelling in both Na⁺ and K⁺ salts. Since nearly constant internal cation concentrations are maintained during osmotic swelling, this result suggests that both Na⁺/H⁺ and K⁺/H⁺ exchange is enhanced by increased matrix volume. Endogenous Mg²⁺ is also lost with increased matrix volume, and this observation, in conjunction with other evidence available in the literature, suggests that monovalent cation/H⁺ exchanges may be regulated by divalent cations. Passive exchange of Na⁺/K⁺,⁴²K⁺/K⁺, and²⁴Na⁺/Na⁺ can be readily demonstrated in mitochondria swollen in nitrate. All these exchanges are low or not detectable in unswollen control mitochondria, and it appears that they are manifestations of the activated cation/H⁺ component (or components) functioning in the absence of pH.     

Cell growth and ouabain-sensitive Rb uptake and (Na + K)-ATPase activity in 3T3 and SV40 transformed 3T3 fibroblasts

The uptake of ouabain-sensitive ⁸⁶Rb⁺ uptake measured at 5 min and the uptake measured at 60 min was 4.5- and 2.7-fold greater respectively for SV40 transformed 3T3 cells compared to 3T3 cells during the late log phase of growth. This uptake, however, varied markedly with cell growth. Ouabain-sensitive ⁸⁶Rb⁺ uptake was found to be a sensitive indicator of protein synthesis as measured by total protein content. Cessation of cell growth as measured by total protein content was associated with a decline in ouabain-sensitive ⁸⁶Rb⁺ uptake in both cell types. This increased ouabain-sensitive cation transport was reflected in increased levels of (Na⁺ + K⁺)-ATPase activity for SV40 3T3 cells, which showed a 2.5-fold increase V but the same K_rmm as 3T3 cells.These results are compared with the results of related work. Possible mechanisms for these effects are discussed and how changes in cation transport might be related to alterations in cell growth.     

Effect of α-Latrotoxin on Acetylcholine Release and Intracellular Ca2+ Concentration in Synaptosomes: Na+-Dependent and Na+-Independent Components

Abstract: We studied the effect of α-latrotoxin (αLTX) on [¹⁴C]acetylcholine ([¹⁴C]ACh) release, intracellular Ca²⁺ concentration ([Ca²⁺]_i), plasma membrane potential, and high-affinity choline uptake of synaptosomes isolated from guinea pig cortex. αLTX (10^?10-10^?8M) caused an elevation of the [Ca²⁺]_i as detected by Fura 2 fluorescence and evoked [¹⁴C]ACh efflux. Two components in the action of the toxin were distinguished: one that required the presence of Na⁺ in the external medium and another that did not. Displacement of Na⁺ by sucrose or N-methylglucamine in the medium considerably decreased the elevation of [Ca²⁺]_i and [¹⁴C]ACh release by αLTX. The Na⁺-dependent component of the αLTX action was obvious in the inhibition of the high-affinity choline uptake of synaptosomes. Some of the toxin action on both [Ca²⁺]_i and [¹⁴C]ACh release remained in the absence of Na⁺. Both the Na⁺-dependent and the Na⁺-independent components of the αLTX-evoked [¹⁴C]ACh release partly required the presence of either Mg²⁺ or Ca²⁺. The nonneurotransmitter [¹⁴C]choline was released along with [¹⁴C]ACh, but this release did not depend on the presence of either Na⁺ or Ca²⁺, indicating nonspecific leakage through the plasma membrane. We conclude that there are two factors in the release of ACh from synaptosomes caused by the toxin: (1) cation-dependent ACh release, which is related to (a) Na⁺-dependent divalent cation entry and (b) Na⁺-independent divalent cation entry, and (2) nonspecific Na⁺- and divalent cation-independent leakage.     

Transport of alkali cations through thin lipid membranes by (222)C10-Cryptand,an ionizable mobile carrier

Summary The kinetics of K⁺ and Na⁺ transport across the membrane of large unilamellar vesicles (L.U.V.) were compared at two pH's, with two carriers: (222)C ₁₀-cryptand (diaza-1, 10-decyl-5-hexaoxa-4,7,13,16,21,24-bicyclo[8.8.8.]hexacosane) and valinomcyin, i.e. an ionizable macrobicyclic amino polyether and a neutral macrocyclic antibiotic. The rate of cation transport by (222)C₁₀ saturated as cation and carrier concentrations rose. The apparent affinity of (222)C₁₀ for K⁺ was higher and less pH dependent than that for Na⁺ but resembled the affinity of valinomycin for K⁺. The efficiency of (222)C₁₀ transport of K⁺ decreased as the pH fell and the carrier concentration rose, and was about ten times lower than that of valinomycin. Noncompetitive K⁺/Na⁺ transport selectivity of (222)C₁₀ decreased as pH, and cation and carrier concentrations rose, and was lower than that of valinomycin. Transport of alkali cations by (222)C₁₀ and valinomycin was noncooperative. Reaction orders in cationn(S) and carrierm(M) varied with the type of cation and carrier and were almost independent of pH;n(S) andm(M) were not respectively dependent on carrier or cation concentrations. The apparent estimated constants for cation translocation by (222)C₁₀ were higher in the presence of Na⁺ than of K⁺ due to higher carrier saturation by K⁺, and decreased as pH and carrier concentration increased. Equilibrium potential was independent of the nature of carrier and transported cation. Results are discussed in terms of the structural, physicochemical and electrical characteristics of carriers and complexes.     

Reactions of dichloroaluminium acetylacetonate with lewis bases. 1. Ionic complex of Cl2alacac with tetrahydrofurane

The reaction of dichloroaluminium acetylacetonate with THF has been studied. The ionic complex [(acac)₂Al·2THF]⁺[AlCl₄]⁻ was found to result from the reaction. The structure of the complex has been investigated by the variable temperature ¹H NMR technique, as well as by ¹³C and ²⁷Al NMR spectroscopy. The cation complex [(acac)₂Al·2THF]⁺ is predominantly trans in dichloromethane solution (75% trans and 25% cis). In the presence of an excess of THF a fast exchange proceeds at room temperature in the cation complex between the free THF molecules and those present in the complex, which is accompanied by a stereo-chemical rearrangement of the cation complex.     

Distinct Ca<Superscript>2+</Superscript>-permeable Cation Currents Are Activated by Internal Ca<Superscript>2+</Superscript>-Store Depletion in RBL-2H3 Cells and Human Salivary Gland Cells,HSG and HSY

Store-operated Ca²⁺ influx, suggested to be mediated via store-operated cation channel (SOC), is present in all cells. The molecular basis of SOC, and possible heterogeneity of these channels, are still a matter of controversy. Here we have compared the properties of SOC currents (I _SOC) in human submandibular glands cells (HSG) and human parotid gland cells (HSY) with I _CRAC (Ca²⁺ release-activated Ca²⁺ current) in RBL cells. Internal Ca²⁺ store-depletion with IP₃ or thapsigargin activated cation channels in all three cell types. 1 μM Gd³⁺ blocked channel activity in all cells. Washout of Gd³⁺ induced partial recovery in HSY and HSG but not RBL cells. 2-APB reversibly inhibited the channels in all cells. I _CRAC in RBL cells displayed strong inward rectification with E _rev(Ca) = >+90 mV and E _rev (Na) = +60 mV. I _SOC in HSG cells showed weaker rectification with E _rev(Ca) = +25 mV and E _rev(Na) = +10 mV. HSY cells displayed a linear current with E _rev = +5 mV, which was similar in Ca²⁺- or Na⁺-containing medium. pCa/pNa was >500, 40, and 4.6 while pCs /pNa was 0.1,1, and 1.3, for RBL, HSG, and HSY cells, respectively. Evidence for anomalous mole fraction behavior of Ca²⁺/Na⁺ permeation was obtained with RBL and HSG cells but not HSY cells. Additionally, channel inactivation with Ca²⁺ + Na⁺ or Na⁺ in the bath was different in the three cell types. In aggregate, these data demonstrate that distinct store-dependent cation currents are stimulated in RBL, HSG, and HSY cells. Importantly, these data suggest a molecular heterogeneity, and possibly cell-specific differences in the function, of these channels.This revised version was published online in June 2005 with a corrected cover date.     

The K+ channel in the plasma membrane of rye roots has a multiple ion residency pore

The permeation of K⁺ and Na⁺ through the pore of a K⁺ channel from the plasma membrane of rye roots was studied in planar 1-palmitoyl-2-oleoyl phosphatidylethanolamine bilayers. The pore contains at least two ion-binding sites which can be occupied simultaneously. This was indicated by: (i) biphasic relationships with increasing cation concentration of both channel conductance at the zero-current (reversal) potential of the channel (E _rev) and unitary-current at a specified voltage and (ii) a decline in E _rev in the presence of equimolar Na⁺ (cis):K⁺ (trans) as the cation concentration was increased. To determine the spatial characteristics and energy profiles for K⁺ and Na⁺ permeation, unitary-current/ voltage data for the channel were fitted to a three energy-barrier, two ion-binding site (3B2S) model. The model allowed for simultaneous occupancy of binding sites and ionic repulsion within the pore, as well as surface potential effects. Results suggested that energy peaks and energy wells (ion binding sites) were situated asymmetrically within the electrical distance of the pore, the trans energy-well being closer to the center of the pore than its cis counterpart; that the energy profile for K⁺ permeation differed significantly from that of Na⁺ in having a higher cis energy peak and a deeper cis energy well; that cations repelled each other within the pore and that vestibule surface charge was negligible. The model successfully simulated various aspects of K⁺ and Na⁺ permeation including: (i) the complexities in current rectification over a wide range of contrasting ionic conditions; (ii) the biphasic relationships with increasing cation concentration of both channel conductance at E _rev and unitary-current at a specified voltage; (iii) the decline in E _rev in equimolar Na⁺ (cis):K⁺ (trans) as cation concentrations were increased and (iv) the complex relationships between mole fraction and E _rev at total cation concentrations of 100 and 300 mm.We thank Prof. O. Alvarez (Universidad de Chile, Santiago, Chile) for supplying the computer program AJUSTE and Prof. D. Sanders (University of York, UK), Prof. D. Gradmann and Dr. G. Thiel (University of Göttingen, Germany) for stimulating ideas. This work was supported by the Agricultural and Food Research Council.     

Reactions of mass-selected Fen cluster ions (n=1-5) with dimethyl carbonate

The reactions of mass-selected iron clusters Fe_n ⁺ (n=1-5) with dimethyl carbonate, (CH₃O)₂CO, are examined by means of Fourier-transform ion-cyclotron-resonance mass spectrometry. For the bare metal cation Fe⁺, loss of a methyl radical prevails which leads to the iron carbonate species FeOC(O)OCH₃ ⁺. For the corresponding Fe_n ⁺ clusters, this type of reaction is not observed anymore. Instead, the clusters show a strong tendency for a formal O-atom abstraction leading to the formation of the corresponding monoxide clusters Fe_nO⁺ In addition, several bond activations of dimethyl carbonate are observed which markedly differ from the behavior of the mononuclear cation. Nevertheless, a mechanistic analysis implies that the initial steps are the same for bare Fe⁺ as well as small Fe_n ⁺ clusters.     

Differential effects of Na+, Mg2+, K+, Ca2+ and osmotic stress on the wild type and the NaCl-tolerant mutants stl1 and stl2 of Ceratopteris richardii

In order to identify physiological components that contribute to salinity tolerance, we compared the effects of Na⁺, Mg²⁺ and K⁺ salts (NaCl, Na₂SO₄, MgCl₂, MgSO₄, KCl and K₂SO₄), Ca₂₊ (CaSO₄), mannitol and melibiose on the wild type and the single-gene NaCl-tolerant mutants stl1 and stl2 of Ceratopteris richardii. Compared with gametophytic growth of the wild type, stl2 showed a low level of tolerance that was restricted to Na⁺ salts and osmotic stress. stl2 exhibited high tolerance to both Na⁺ and Mg²⁺ salts, as well as to osmotic stress. In response to short-term exposure (3 d) to NaCl, accumulation of K⁺ and Na⁺ was similar in the wild type and stl1. In contrast, stl2 accumulated higher levels of K⁺ and lower levels of Na⁺. Ca²⁺ supplementation (1.0 mol m^?3) ameliorated growth inhibition by Na⁺ and Mg²⁺ stress in wild type and stll, but not in stl2. In addition, under Na⁺ stress (175 mol m^?3) wild-type, stll and stl2 gametopbytes maintained higher tissue levels of K⁺ and lower levels of Na⁺ when supplemented with Ca²⁺ (1.0 mol m^?3). stl2 gametophytes were extremely sensitive to K⁺ supplementation. Growth of stl2 was greater than or equal to that of the wild type at trace concentrations of K⁺ but decreased substantially with increasing K⁺ concentration. Supplementation with K⁺ from 0 to 1.85 mol m^?3 alleviated some of the inhibition by 75 mol m^?3 NaCl in the wild type and in stl1. In stl2, growth at 75 mol m^?3 NaCl was similar at 0 and 1.85 mol m^?3 K⁺ supplementation. Although K⁺ supplementation above 1.85 mol m^?3 did not alleviate inhibition of growth by Na⁺ in any genotype, stl2 maintained greater relative tolerance to NaCl at all K⁺ concentrations tested.     

Golgi‐localized cation/proton exchangers regulate ionic homeostasis and skotomorphogenesis in Arabidopsis

Multiple transporters and channels mediate cation transport across the plasma membrane and tonoplast to regulate ionic homeostasis in plant cells. However, much less is known about the molecular function of transporters that facilitate cation transport in other organelles such as Golgi. We report here that Arabidopsis KEA4, KEA5, and KEA6, members of cation/proton antiporters‐2 (CPA2) superfamily were colocalized with the known Golgi marker, SYP32‐mCherry. Although single kea4,5,6 mutants showed similar phenotype as the wild type under various conditions, kea4/5/6 triple mutants showed hypersensitivity to low pH, high K⁺, and high Na⁺ and displayed growth defects in darkness, suggesting that these three KEA‐type transporters function redundantly in controlling etiolated seedling growth and ion homeostasis. Detailed analysis indicated that the kea4/5/6 triple mutant exhibited cell wall biosynthesis defect during the rapid etiolated seedling growth and under high K⁺/Na⁺ condition. The cell wall‐derived pectin homogalacturonan (GalA)₃ partially suppressed the growth defects and ionic toxicity in the kea4/5/6 triple mutants when grown in the dark but not in the light conditions. Together, these data support the hypothesis that the Golgi‐localized KEAs play key roles in the maintenance of ionic and pH homeostasis, thereby facilitating Golgi function in cell wall biosynthesis during rapid etiolated seedling growth and in coping with high K⁺/Na⁺ stress.     

Substituting manganese for magnesium alters certain reaction properties of the (Na+ + K+)-ATPase

MnCl₂ was partially effective as a substitute for MgCl₂ in activating the K⁺-dependent phosphatase reaction catalyzed by a purified (Na⁺ + K⁺)-ATPase enzyme preparation from canine kidney medulla, the maximal velocity attainable being one-fourth that with MgCl₂. Estimates of the concentration of free Mn²⁺ available when the reaction was half-maximally stimulated lie in the range of the single high-affinity divalent cation site previously identified (Grisham, C.M. and Mildvan, A.S. (1974) J. Biol. Chem. 249, 3187–3197). MnCl₂ competed with MgCl₂ as activator of the phosphatase reaction, again consistent with action through a single site. However, with MnCl₂ appreciable ouabaininhibitable phosphatase activity occurred in the absence of added KCl, and the apparent affinities for K⁺ as activator of the reaction and for Na⁺ as inhibitor were both decreased. For the (Na⁺ + K⁺)-ATPase reaction substituting MnCl₂ for MgCl₂ was also partially effective, but no stimulation in the absence of added KCl, in either the absence or presence of NaCl, was detectable. Moreover, the apparent affinity for K⁺ was increased by the substitution, although that for Na⁺ was decreased as in the phosphatase reaction. Substituting MnCl₂ also altered the sensitivity to inhibitors. For both reactions the inhibition by ouabain and by vanadate was increased, as was binding of [⁴⁸V]-vanadate to the enzyme; furthermore, binding in the presence of MnCl₂ was, unlike that with MgCl₂, insensitive to KCl and NaCl. Inhibition of the phosphatase reaction by ATP was decreased with 1 mM but not 10 mM KCl. Finally, inhibition of the (Na⁺ + K⁺)-ATPase reaction by Triton X-100 was increased, but that by dimethylsulfoxide decreased after such substitution.     

Ionic processes underlying the decrease in osmotic potential of Chara L-cell fragments in dilute electrolyte solutions

Movements of ions are considered to be governed by the electroneutrality rule. Therefore, a cation moving across the cell membrane into the cell either passively or actively should move together with its counterion, an anion, in equal amounts of charge or in exchange for another cation inside the cell. This means that the net influx of the cation in question should be affected by the permeability of its counterion and/or another cation inside the cell. To examine osmotic and ionic regulation in Chara cells, cell fragments of Chara having a lower osmotic pressure than normal (L-cell fragments) were prepared. The L-cell fragments were individually put into various dilute electrolyte solutions and their osmotic potentials were measured with a turgor balance. Concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^?, NO^?₃. and SO^2?₄. in the external electrolyte solutions in which L-cells had been incubated were also analysed by ion chromatography. The results showed that in 0.5 mM KCL + 0.1 mM CaCl₂ solution, Chara L-cell fragments absorbed K⁺ and Cl^? to maintain electroneutrality and then regained their osmotic potential very rapidly. When the anion was Cl, the cation absorbed at the highest rate was K⁺ On the other hand, when the cation was K, the anion absorbed at the highest rate was Cl, Other ions Ca²⁺, SO^2?₄ and NO^?₃ showed much less permeability than K⁺ and Cl ^?for the Chara plasma membrane. The conclusion from these findings was that due to the constraint of electroneutral transport, the uptake rate of a salt into L-cells is limited by the permeability of the least permeable ion.     

Characterization of csi52, a Cs+ resistant mutant of Arabidopsis thaliana altered in K+ transport

Plant roots accumulate potassium from a wide range of soil concentrations, utilizing at least two distinct plasma membrane uptake systems with different affinities for the cation. Although details on the structure and function of these transporters are beginning to emerge many prominent questions remain concerning how these proteins function in planta. Such questions can be addressed through the use of well-defined transport mutants. Csi52, a caesium-insensitive mutant of Arabidopsis thaliana which is defective in potassium transport, is further characterized here using conventional electrophysiology, patch-clamp and radiometric approaches to identify the nature of the potassium transport lesion. Rb⁺ uptake experiments reveal a reduced uptake in csi52 in both the high- and low-affinity uptake range. Patch-clamp analysis indicates that the activity of the predominant inward rectifying channel observed in wild-type cells is extremely low in root protoplasts isolated from csi52, whereas outward rectifying channel activity is comparable between wild-type and mutant. Rb⁺ uptake studies show that in both wild-type and csi52 the high-affinity uptake pathway is considerably less sensitive to Cs⁺ than the low-affinity pathway with K_1/2 values for Cs⁺ of around 1.3 and 0.2 mM, respectively. Furthermore, K⁺ starvation leads to a larger relative increase in high-affinity K⁺ uptake in the mutant than the wild-type. The results demonstrate the Cs⁺ sensitivity of each individual uptake pathway is comparable in wild-type and csi52 but the high-affinity pathway is less Cs⁺ sensitive (in both wild-type and csi52). Therefore, the larger shift toward high-affinity uptake in the mutant compared with the wild-type under K⁺-starvation conditions will endow the mutant with a higher degree of overall Cs⁺ resistance. The data supply evidence for the hypothesis that the csi52 mutation lies within a gene that regulates the activity of several potassium transport systems and coordinates their relative contribution to overall root K⁺ uptake.     

Comparative analysis of cation/proton antiporter superfamily in plants

The cation/proton antiporter superfamily is associated with the transport of monovalent cations across membranes. This superfamily was annotated in the Arabidopsis genome and some members were functionally characterized. In the present study, a systematic analysis of the cation/proton antiporter genes in diverse plant species was reported. We identified 240 cation/proton antiporters in alga, moss, and angiosperm. A phylogenetic tree was constructed showing these 240 members are separated into three families, i.e., Na⁺/H⁺ exchangers, K⁺ efflux antiporters, and cation/H⁺ exchangers. Our analysis revealed that tandem and/or segmental duplications contribute to the expansion of cation/H⁺ exchangers in the examined angiosperm species. Sliding window analysis of the nonsynonymous/synonymous substitution ratios showed some differences in the evolutionary fate of cation/proton antiporter paralogs. Furthermore, we identified over-represented motifs among these 240 proteins and found most motifs are family specific, demonstrating diverse evolution of the cation/proton antiporters among three families. In addition, we investigated the co-expressed genes of the cation/proton antiporters in Arabidopsis thaliana. The results showed some biological processes are enriched in the co-expressed genes, suggesting the cation/proton antiporters may be involved in these biological processes. Taken together, this study furthers our knowledge on cation/proton antiporters in plants.