Alkylguanidines as inhibitors of k transport in isolated barley roots

It has been shown that plants can accumulate K⁺ through an energy-dependent process. The effect of alkylguanidines, in particular octylguanidine on the uptake of ⁸⁶Rb⁺ by excised barley roots (Hordeum vulgare var. Apizaco LV-72), has been studied. ⁸⁶Rb⁺ was used as tracer of K⁺. The uptake of ⁸⁶Rb⁺ which is linear with time and shows saturation kinetics is inhibited by octylguanidine. Half-maximal inhibition of ⁸⁶Rb⁺ uptake is attained at 50 μM octylguanidine. Octylguanidine induces a decrease in the V_max of the process and increases the Km of the system for Rb⁺. When the effects of various alkylguanidines were studied, the following order of effectiveness was encountered; octylguanidine = hexilguanidine > butylguanidine > ethylguanidine > guanidine. This suggests that guanidines inhibit Rb⁺ uptake by interacting through its positively charged guanidinium group with a Rb⁺ carrier while the alkyl chain interacts with the hydrophobic milieu of the membrane.     

Conditions controlling relative uptake of potassium and rubidium by plants from soils

Earlier studies have demonstrated close inverse relationships between Rb⁺ concentrations in plants and pH or base (including K⁺) saturation of soils. This study aims at elucidating conditions in soils influencing plant uptake of Rb⁺. Growth experiments with Carex pilulifera L. were performed, modifying the acidity and K⁺ supply of acid soils and solutions. We were unable to assess any reduction in Rb⁺ uptake by adding precipitated CaCO₃ to acid soil unless pH was raised to near neutrality. Though not fully compensating the loss of soil solution K⁺and exchangeable K⁺ from uptake by the growing plants, soil treated with 0.5 mM K⁺ (as KCl) reduced the Rb⁺ concentration in the shoots by 40% without measurably changing soil pH. Experiments varying the pH and K⁺ concentration of a nutrient solution (20% Hoagland), spiked with 6 uM Rb⁺, clearly demonstrated that plant uptake of Rb⁺ and K⁺ was unaffected by acidity in the pH range 3.6–5.0 tested, whereas Rb⁺ uptake was reduced by ca. 50%, when K⁺ concentration was increased from 1.2 to 3.6 mM. The sensitivity of this reaction indicates that shortage or low availability of K⁺ controls Rb⁺ uptake from acid soils, being probably more important than soil acidity per se. Secondary effects of high soil acidity, such as leaching losses of K⁺, might also be of importance in accounting for the high uptake of Rb⁺ from such soils. It is suggested that leaf analysis of Rb⁺ may be used as a method to assess early stages of K⁺ deficiency in plants on acid soils.     

Allosteric and Non-Allosteric Regulation of Rubidium Influx in Barley Roots

Uptake of Rb⁺ was investigated in 6–8-day-old intact barley plants (Hordeum vulgare cv. Kristina), which had been cultivated or pretreated in nutrient solutions with various K⁺ concentrations. The relationship between Rb⁺ influx and the K⁺ concentration of roots appeared to be sigmoidal for plants grown in solutions containing K⁺, indicating regulation of Rb⁺ uptake by allosteric inhibition of the uptake mechanism. Pretreatment of the roots in K⁺-free solutions changed the pattern of uptake and caused the Rb⁺ influx to become linearly related to the chemical Rb⁺ potential of the uptake solution. Pretreatment in K⁺-free solutions probably abolishes the allosteric inhibition of a carrier system.     

Rubidium transport in Neurospora crassa

Rb⁺ transport in low-K⁺ cells of Neurospora crassa is biphasic, transport at millimolar Rb⁺ being added to a transport process which saturates in the micromolar range. Both processes exhibit Michaelis-Menten kinetics, but in the micromolar phase the kinetic parameters depend on the K⁺ content of the cell (the lower the K⁺ content the lower the K_m and the higher the V_max). Normal-K⁺ cells, suspended in a buffer with millimolar K⁺, do not present Rb⁺ transport in the micromolar range. Millimolar transport in these cells presents kinetics which depend on the K⁺ in buffer (the higher the K⁺ the higher the K_m), although the K⁺ content of the cells is constant. Na⁺ inhibits competitively Rb⁺ transport in low-K⁺ and normal-K⁺ cells, but, even when the differences between the Rb⁺K_m values are more than three orders of magnitude, the apparent dissociation constant for Na⁺ is the same, and millimolar, in both cases.     

Effect of surface potential on Rb+ uptake in yeast: The effect of pH

The apparent K_m of Rb⁺ uptake and the zeta potential of yeast cells are appreciably affected by changes in the pH, variation of the concentration of the buffer cation Tris⁺ and addition of Ca²⁺ to the suspending medium. Irrespective of the way in which the zeta potential is affected, a direct relationship between the apparent K_m of the Rb⁺ uptake and the zeta potential is observed. A reduction of 8 mV in the zeta potential is accompanied by a 20-fold increase in the apparent K_m, which illustrates that electrostatic effects in ion uptake cannot be ignored. Measured zeta potentials are, to a good approximation, linearly related to surface potentials evaluated from a kinetic analysis of the Rb⁺ uptake. This shows the practical use of the zeta potential as a measure of the surface potential in studies of electrostatic effects in ion uptake by yeast. It is concluded that Tris⁺ and the aikaline earth cations inhibit the Rb⁺ uptake in yeast exclusively via a reduction in the surface potential. Protons, in addition, exert a competitive inhibition.     

Molecular analysis of the mechanism of potassium uptake through the TRK1 transporter of Saccharomyces cerevisiae

The TRK-HKT family of K⁺ transporters mediates K⁺ and Na⁺ uptake in fungi and plants. In this study, we have investigated the molecular mechanism involved in the movement of alkali cations through the TRK1 transporter of Saccharomyces cerevisiae. The model that best explains the activity of ScTRK1 is a cotransport of two K⁺ or Rb⁺, both of which bind the two binding sites of ScTRK1 with very high affinities in K⁺-starved cells. Na⁺ can be transported in the same way but it exhibits a much lower affinity for the second binding site. Therefore, only at critical concentration ratios between K⁺ and Na⁺, or Rb⁺ and Na⁺, the transporter takes up Na⁺ together with K⁺ or Rb⁺. Mutation analyses suggest that the two binding sites are located in the P fragment of the first MPM motif of the transporter, and that Gln⁹⁰ is involved in these binding sites. ScTRK1 can be in two states, medium or high affinity, and we have found that Leu⁹⁴⁹ is involved in the oscillation of the transporter between these two states. ScTRK1 mediates active K⁺ uptake. This is not Na⁺-coupled and direct coupling of ScTRK1 to a source of chemical energy seems more probable than K⁺-H⁺ cotransport.     

Modulation of K+ translocation by AKT1 and AtHAK5 in Arabidopsis plants

Root cells take up K⁺ from the soil solution, and a fraction of the absorbed K⁺ is translocated to the shoot after being loaded into xylem vessels. K⁺ uptake and translocation are spatially separated processes. K⁺ uptake occurs in the cortex and epidermis whereas K⁺ translocation starts at the stele. Both uptake and translocation processes are expected to be linked, but the connection between them is not well characterized. Here, we studied K⁺ uptake and translocation using Rb⁺ as a tracer in wild‐type Arabidopsis thaliana and in T‐DNA insertion mutants in the K⁺ uptake or translocation systems. The relative amount of translocated Rb⁺ to the shoot was positively correlated with net Rb⁺ uptake rates, and the akt1 athak5 T‐DNA mutant plants were more efficient in their allocation of Rb⁺ to shoots. Moreover, a mutation of SKOR and a reduced plant transpiration prevented the full upregulation of AtHAK5 gene expression and Rb⁺ uptake in K⁺‐starved plants. Lastly, Rb⁺ was found to be retrieved from root xylem vessels, with AKT1 playing a significant role in K⁺‐sufficient plants. Overall, our results suggest that K⁺ uptake and translocation are tightly coordinated via signals that regulate the expression of K⁺ transport systems.     

Regulation of Rubidium Uptake in Sunflower Roots

Uptake of Rb⁺ was investigated in 12-day-old intact plants of sunflower (Helianthus annum L. var. californicus) which had been cultivated or pretreated in nutrient solutions with various K⁺ concentrations. The relationship between Rb⁺ influx and K⁺ concentration of the roots indicated regulation of Rb⁺ uptake by allosteric inhibition of the uptake mechanism. A constant passive influx occurred contemporaneously with the active uptake as shown by experiments at 0°C or with 2,4-dinitrophenol. The allosteric regulation of ion carrier activity occurred after a time lag of up to 1 h after the change of external solution. In experiments involving Rb⁺ treatments of K⁺-deficient plants, the synthesis of carriers for transport of Rb⁺ could be demonstrated. A model including allosteric regulation of Rb⁺ uptake in roots is discussed.     

Alkali cation selectivity of the wheat root high-affinity potassium transporter HKT1

The wheat root high-affinity K⁺ transporter HKT1 functions as a sodium-coupled potassium co-uptake transporter. At toxic millimolar levels of sodium (Na⁺), HKT1 mediates low-affinity Na⁺ uptake while potassium (K⁺) uptake is blocked. In roots, low-affinity Na⁺ uptake and inhibition of K⁺ uptake contribute to Na⁺ toxicity. In the present study, the selectivity among alkali cations of HKT1 expressed in Xenopus oocytes and yeast was investigated under various ionic conditions at steady state. The data show that HKT1 is highly selective for uptake of the two physiologically significant alkali cations, K⁺ and Na⁺ over Rb⁺, Cs⁺ and Li⁺. In addition, Rb⁺ and Cs⁺, and an excess of extracellular K⁺ over Na⁺, are shown to partially reduce or block HKT1-mediated K⁺-Na⁺ uptake. Furthermore, K⁺, Rb⁺ and Cs⁺ also effectively reduce outward currents mediated by HKT1, thereby causing depolarizations. In yeast, HKT1 can produce high-affinity Rb⁺ uptake at approximately 15-fold lower rates than for K⁺. Rb⁺ influx in yeast can be mediated by the ability of the yeast plasma membrane proton pump to balance the 35-fold lower HKT1 conductance for Rb⁺. A model for HKT1 activity is presented involving a high-affinity K⁺ binding site and a high-affinity Na⁺ binding site, and competitive interactions of K⁺, Na⁺ and other alkali cations for binding to these two sites. Possible implications of the presented results for physiological K⁺ and Na⁺ uptake in plants are discussed.     

High affinity k uptake in maize roots: a lack of coupling with h efflux

We report here on the putative coupling between a high affinity K⁺ uptake system which operates at low external K⁺ concentrations (K_m = 10-20 micromolar), and H⁺ efflux in roots of intact, low-salt-grown maize plants. An experimental approach combining electrophysiological measurements, quantification of unidirectional K⁺(⁸⁶Rb⁺) influx, and the simultaneous measurement of net K⁺ and H⁺ fluxes associated with individual cells at the root surface with K⁺- and H⁺-selective microelectrodes was utilized. A microelectrode system described previously (IA Newman, LV Kochian, MA Grusak, and WJ Lucas [1987] Plant Physiol 84: 1177-1184) was used to quantify net ion fluxes from the measurement of electrochemical potential gradients for K⁺ and H⁺ ions within the unstirred layer at the root surface. No evidence for coupling between K⁺ uptake and H⁺ efflux could be found based on: (a) extremely variable K⁺:H⁺ flux stoichiometries, with K⁺ uptake often well in excess of H⁺ efflux; (b) dramatic time-dependent variability in H⁺ extrusion when both fluxes were measured at a particular location along the root over time; and (c) a lack of pH sensitivity by the high affinity K⁺ uptake system (to changes in external pH) when net K⁺ uptake, unidirectional K⁺(⁸⁶Rb⁺) influx, and K⁺-induced depolarizations of the membrane potential were determined in uptake solutions buffered at pH values from pH 4 to 8. Based on the results presented here, we propose that high affinity active K⁺ absorption into maize root cells is not mediated by a K⁺/H⁺ exchange mechanism. Instead, it is either due to the operation of a K⁺-H⁺ cotransport system, as has been hypothesized for Neurospora, or based on the striking lack of sensitivity to changes in extracellular pH, uptake could be mediated by a K⁺-ATPase as reported for Escherichia coli and Saccharomyces.     

Cloning and functional characterization of the high-affinity K<Superscript>+</Superscript> transporter HAK1 of pepper

High-affinity K⁺ uptake in plants plays a crucial role in K⁺ nutrition and different systems have been postulated to contribute to the high-affinity K⁺ uptake. The results presented here with pepper (Capsicum annum) demonstrate that a HAK1-type transporter greatly contributes to the high-affinity K⁺ uptake observed in roots. Pepper plants starved of K⁺ for 3 d showed high-affinity K⁺ uptake (K _m of 6 M K⁺) that was very sensitive to NH and their roots expressed a high-affinity K⁺ transporter, CaHAK1, which clusters in group I of the KT/HAK/KUP family of transporters. When expressed in yeast (Saccharomyces cerevisiae), CaHAK1 mediated high-affinity K⁺ and Rb⁺ uptake with K _m values of 3.3 and 1.9 M, respectively. Rb⁺ uptake was competitively inhibited by micromolar concentrations of NH and Cs⁺, and by millimolar concentrations of Na⁺.     

High salinity tolerance in the stl2 mutation of Ceratopteris richardii is associated with enhanced K+ influx and loss

The roles of K⁺ uptake and loss in the salinity response of the wild type and the salt-tolerant mutant stl2 of Ceratopteris richardii were studied by measuring Rb⁺ influx and loss and the effects of Na⁺, Mg²⁺, Ca²⁺ and K⁺-transport inhibitors. In addition, electrophysiological responses were measured for both K⁺ and Rb⁺ and for the effects of Na⁺ and NH₄⁺ on subsequent K⁺-induced depolarizations. stl2 had a 26–40% higher uptake rate for Rb⁺ than the wild type at 0.5–10 mol m^?3 RbCl. Similarly, membrane depolarizations induced by both RbCl and KCl were consistently greater in stl2. In the presence of 0–180 mol m^?3 NaCl, stl2 maintained a consistently greater Rb⁺ influx than the wild type. stl2 retained a greater capacity for subsequent KCl-induced depolarization following exposure to NaCl. Five mol m^?3 Mg²⁺ decreased Rb⁺ uptake in stl2; however, additional Mg²⁺ up to 40 mol m^?3 did not affect Rb⁺ uptake further. Ca²⁺ supplementation resulted in a very minor decrease of Rb⁺ uptake that was similar in the two genotypes. Tetraethylammonium chloride and CsCl gave similar inhibition of Rb⁺ uptake in both genotypes, but NH₄Cl gave substantially greater inhibition in the wild type than in stl2. NH₄Cl resulted in a greater membrane depolarization in the wild type and the capacity for subsequent depolarization by KCl was markedly reduced. stl2 exhibited a higher Independent loss of Rb⁺ than the wild type, but, in the absence of external K⁺, loss of Rb⁺ was equivalent in the two genotypes. Since constitutive K⁺ contents are nearly identical, we conclude that high K⁺ influx and loss exact a metabolic cost that is reflected in the inhibition of gametophytic growth. Growth inhibition can be alleviated by reduced supplemental K⁺ or by treatments that slightly reduce K⁺ influx, such as moderate concentrations of Na⁺ or Mg²⁺. We propose that high throughput of K⁺ allows maintenance of cytosolic K⁺ under salt stress and that a high uptake rate for K⁺ results in a reduced capacity for the entrance and accumulation of alternative cations such as Na⁺ in the cytosol.     

Closure of the yeast mitochondria unspecific channel (YMUC) unmasks a Mg2+ and quinine sensitive K+ uptake pathway in Saccharomyces cerevisiae

The K⁺ uptake pathways in yeast mitochondria are still undefined. Nonetheless, the K⁺-mediated mitochondrial swelling observed in the absence of phosphate (PO₄) and in the presence of a respiratory substrate has led to propose that large K⁺ movements occur in yeast mitochondria. Thus, the uptake of K⁺ by isolated yeast mitochondria was evaluated. Two parallel experiments were conducted to evaluate K⁺ transport; these were mitochondrial swelling and the uptake of the radioactive K⁺ analog ⁸⁶Rb⁺. The opening of the yeast mitochondrial unspecific channel (YMUC) was regulated by different PO₄ concentrations. The high protein concentrations used to measure ⁸⁶Rb⁺ uptake resulted in a slight stabilization of the transmembrane potential at 0.4 mM PO₄ but not at 0 or 4 mM PO₄. At 4 mM PO₄ swelling was inhibited while, in contrast, ⁸⁶Rb⁺ uptake was still observed. The results suggest that an energy-dependent K⁺ uptake mechanism was unmasked when the YMUC was closed. To further analyze the properties of this K⁺ uptake system, the Mg²⁺ and quinine sensitivity of both swelling and ⁸⁶Rb⁺ uptake were evaluated. Under the conditions where the unspecific pore was closed, K⁺ transport sensitivity to Mg²⁺ and quinine increased. In addition, when Zn²⁺ was added as an antiport inhibitor, uptake of ⁸⁶Rb⁺ increased. It is suggested that in yeast mitochondria, the K⁺ concentration is highly regulated by the equilibrium of uptake and exit of this cation through two specific transporters.     

Control of K+ (Rb+) influx and transport with changed internal K+ concentration and age in two varieties of barley

The influx of Rb⁺ into the roots of two barley varieties (Hordeum vulgare L. cv. Salve and cv. Ingrid) from a K⁺-free ⁸⁶Rb-labelled nutrient solution with 2.0 mM Rb⁺, was checked at intervals from day 6 to day 18. The control plants were continuously grown in complete nutrient solution containing 5.0 mM K⁺, while two other groups of plants were grown in K⁺-free nutrient solution starting on day 6 and between day 6 and day 9, respectively. The pattern of Rb⁺ influx was similar for both varieties, although their efficiencies in absorbing Rb⁺ were different. The relationship between Rb⁺ influx and K⁺ concentration of the root could be interpreted in terms of negative feedback through allosteric control of uptake across the plasmalemma of the root cells. Hill plots were bimodal, but in the opposite direction. The Hill coefficients, reflecting the minimum number of interacting allosteric binding sites for K⁺ (Rb⁺), were low (≤–3.0). It is discussed whether the threshold value, that is the breaking point in the Hill plot, is indicative of a changed efficiency of transporting units for K⁺ (Rb⁺) transport to the xylem. Moreover, feedback regulation might be involved in transport of K⁺ between root and shoot. The variation in K⁺ concentrations in the roots and shoots of control plants were cyclic but in phase opposition despite an exponential growth. The average K⁺ concentration varied only slightly with age.     

Interaction of phosphate with monovalent cation uptake in yeast

The uptake of monovalent cations by yeast via the monovalent cation uptake mechanism is inhibited by phosphate. The inhibition of Rb⁺ uptake shows saturation kinetics and the phosphate concentration at which halfmaximal inhibition is observed is equal to the K_m of phosphate for the sodiumindependent phosphate uptake mechanism. The kinetic coefficients of Rb⁺ and Tl⁺ uptake are affected by phosphate: the maximal rate of uptake is decreased and the apparent affinity constants for the translocation sites are increased.In the case of Na⁺ uptake, the inhibition by phosphate may be partly or completely compensated by stimulation of Na⁺ uptake via a sodium-phosphate cotransport mechanism.Phosphate effects a transient stimulation of the efflux of the lipophilic cation dibenzyldimenthylammonium from preloaded yeast cells and a transient inhibition of dibenzyldimethylammonium eptake. Possibly, the inhibition of monovalent cation uptake in yeast can be explained by a transient depolarization of the cell membrane by phosphate.     

Rubidium transport in the cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. leopoliensis) PCC 7942

Synechococcus R-2 is a unicellular blue-green alga. The cells will grow on Rb⁺ as a substitute for K⁺ but at a slower rate (t₂～ 15 h versus 12 h). Potassium is not, strictly speaking, an essential element for Synechococcus. Rubidium duxes (using ⁸⁶Rb⁺) are much slower than those of potassium, about 1 nmol m^?2 s^?1 in the light (0.35 mol m^?3 Rb⁺). ⁸⁶Rb⁺ fluxes in the dark are about 0.1 nmol m^?2 s^?1. These fluxes are very slow compared to those of Na⁺ and other ions. Isotopic influx of Rb⁺ can supply sufficient Rb⁺ to keep up with the demands for growth, but the net dux needed to keep up with growth in the light is a large proportion of the total observed dux. Kinetic studies of Rb⁺ uptake versus [Rb⁺] show two uptake phases consistent with a high-affinity and a low-affinity system. Both systems appear to be light-activated. Transport of Rb⁺ appears to be passive at pH_o 10 in the light and dark. There is no case for active transport of Rb⁺ at pH_o 7.5 in the light, but a marginal case for active uptake in the dark (about 3 kJ mol^?1). There is only a small effect of Na⁺ upon Rb⁺ transport. ⁸⁶Rb⁺ should not be used in place of ⁴²K⁺ in K⁺ nutrition studies as the details of Rb⁺ transport are different to those of K⁺ transport.     

Activation of Rb+ and Na+ uptake into yeast by monovalent cations

⁸⁶Rb⁺ uptake by yeast was not only stimulated by Rb⁺ or K⁺ but also by Na⁺. The uptake of ²²Na⁺ was enhanced by both Rb⁺ and K⁺, but not by Na⁺, which was inhibitory at all concentrations applied. Inhibition of ²²Na⁺ uptake by inactive Na⁺ occurred in two phases: one phase refers to inhibition at low Na⁺ concentrations and the other to inhibition at high Na⁺ concentrations. Our results can be qualitatively described by a two-site transport mechanism, having two cation binding sites, which must be occupied with monovalent cations before transport can occur.     

Transport of Ammonium and Methylammonium Ions by Azotobacter vinelandii

Ammonium and methylammonium are rapidly taken up by cultures of Azotobacter vinelandii respiring in the presence of succinate. The rate of methylamine uptake increased with external pH from 5.5 to 7.5 but increasing the pH further to 8.5 had little effect on activity, indicating that methylammonium cation rather than uncharged methylamine is the permeant species. The kinetics of methylammonium entry followed the Michaelis-Menten relationship, yielding a K_m of 25 μM and a V_max of 3.8 nmol/min per mg of cell protein. At saturating concentrations ammonium was taken up at rates 30-fold higher than those for methylammonium. Ammonium was a competitive inhibitor of methylammonium uptake and gave an inhibition constant of 1 μM. Ammonium derivatives were inhibitors of methylammonium entry in order of effectiveness: hydrazine > methylhydrazine > formamidine > guanidine > dimethylamine > ethylamine; amides and amino acids did not block uptake. Likewise, metal cations inhibited in the order Tl⁺ > Cs⁺ > Rb⁺, whereas Na⁺, K⁺, and Li⁺ produced no significant effect. Methylammonium uptake was blocked in cells exposed to an uncoupler, p-trifluorome-thoxycarbonyl cyanide-phenyl hydrazone or gramicidin D, but not with dicyclo-hexylcarbodiimide or arsenate. Valinomycin stimulated methylammonium entry into cells in a K⁺-free medium but prevented entry in the presence of 10 mM K⁺. Monensin and nigericin had little effect on transport. These results indicate that methylammonium and ammonium ions enter A. vinelandii electrogenically via a specific transporter.     

Sodium/potassium selectivity and pleiotropy in stl2, a highly salt-tolerant mutation of Ceratopteris richardii

The roles of Na⁺ and K⁺ (Rb⁺) uptake were further studied in a NaCl-tolerant strain of Ceratopteris richardii containing the stl2 mutation by direct comparison with the wild-type strain. In addition to Na⁺ tolerance, stl2 also confers tolerance to Mg²⁺ and sensitivity to K⁺. In addition to higher K⁺ (Rb⁺) uptake at concentrations commonly associated with low-affinity K⁺ transport, stl2 maintained higher uptake down to 0·1 mol m^–3 Rb⁺. Up to a 25-fold excess of Na⁺ had little effect in either genotype on K⁺ (Rb⁺) uptake at low concentrations, i.e. 0·2 and 0·5 mol m^–3 RbCl. Pretreatment with K⁺ (20 mol m^–3) inhibited uptake of K⁺ (Rb⁺) in the wild type, whereas concurrent inclusion of K⁺ inhibited uptake of Rb⁺ more in stl2. In the absence of K⁺, Na⁺ uptake (0·01–60 mol m^–3) was nearly identical in the wild type and stl2. K⁺ inhibited Na⁺ uptake more effectively in stl2 than the wild type, especially at 60 mol m^–3 Na⁺. Greater inhibition of K⁺ uptake in stl2 occurred with MgCl₂ or TEA (tetraethylammonium chloride) preincubation or with simultaneous inclusion of Al³⁺ (Al₂SO₄). The higher effective velocity of K⁺ uptake at a wide range of concentrations and the enhanced selectivity for K⁺ and against Na⁺ contribute to the preservation of higher cytosolic K⁺ and lower Na⁺ under salinity stress.     

Increased Potassium Absorption Confers Resistance to Group IA Cations in Rubidium-Selected Suspension Cells of Brassica napus

Cell lines of suspension cultures of Brassica napus cv. Jet Neuf were identified for their ability to tolerate 100 millimolar Rb⁺, a level which was double the normally lethal concentration. Ten spontaneous isolates were obtained from approximately 5 × 10⁷ cells, one of which was reestablished as a cell suspension. This cell line, JL5, was also resistant to the other group IA cations— Li⁺, Na⁺, K⁺, and Cs⁺—and this trait was stable for at least 30 cell generations in the absence of Rb⁺ selection pressure. The growth characteristics were similar to those of sensitive cells under nonselective conditions. The selected JL5 cells were shown by analysis to have effected more net accumulation of K⁺ and Rb⁺ and less of Na⁺ than did the unselected cells. JL5 and unselected cells after 14 days of culture in basal medium contained 597.2 and 258.2 micromoles of K per gram dry weight, respectively. Michaelis-Menten kinetic analysis of K⁺ influx showed that JL5 possessed an elevated phase 1 V_max, but there was no alteration in its K_m. This is the first time that a plant mutation has been shown to have both increased influx and net absorption of a major essential cation.