Possible Involvement of K^＋/Na^＋ in Assessing the Seed Vigor Index

More substances leaked from a higher-vigor seed sample than from a lower-vigor sample. This indicates that, in some cases, electric conductivity does not represent seed vigor level very well, especially for high-vigor seeds. Results from germination, germination index, leachate conductivity, and the ratio of K^＋/Na^＋ from three-seed lots of Chinese cabbage （Brassica pekinensis （Louv.） Rupr） showed that K^＋/Na^＋ correlated well with germination and germination index. The ability of K^＋/Na^＋ to indicate well changes in vigor was further supported by investigation in soybean （Glycine max （L.） Merr.） seeds and another cultivar of Chinese cabbage seeds. Thus, seed leakage of K^＋/Na^＋ can accurately indicate seed vigor, whereas the conductivity test failed to do so. Furthermore, K^＋/Na^＋ showed up bigger quantitative differences in vigor level than did the conductivity test. This findings provide a more sensitive and accurate index for the assessment of seed vigor. The mechanisms of Na^＋ and K^＋ ion transport are also discussed.     

Root Morphology and Zn^2＋ Uptake Kinetics of the Zn Hyperaccumulator of Sedum alfredii Hance

Root morphology and Zn^2＋ uptake kinetics of the hyperaccumulating ecotype （HE） and nonhyperaccumulating ecotype （NHE） of Sedum alfredii Hance were investigated using hydroponic methods and the radiotracer flux technique. The results indicate that root length, root surface area, and root volume of NHE decreased significantly with increasing Zn^2＋ concentration in growth media, whereas the root growth of HE was not adversely affected, and was even promoted, by 500μmol/L Zn^2＋. The concentrations of Zn^2＋ in both ecotypes of S. alfredii were positively correlated with root length, root surface area and root volumes, but no such correlation was found for root diameter. The uptake kinetics for ^65Zn^2＋ in roots of both ecotypes of S. alfredii were characterized by a rapid linear phase during the first 6 h and a slower linear phase during the subsequent period of investigation. The concentration-dependent uptake kinetics of the two ecotypes of S. alfredii could be characterized by the Michaelis-Menten equation, with the Vmax for ^65Zn^2＋ influx being threefold greater in HE compared with NHE, indicating that enhanced absorption into the root was one of the mechanisms involved in Zn hyperaccumulation. A significantly larger Vmax value suggested that there was a higher density of Zn transporters per unit membrane area in HE roots.     

Na^＋ and Water Uptake in Relation to the Radial Reflection Coefficient of Root in Arrowleaf Saltbush Under Salt Stress

The response of halophyte arrowleaf saltbush (Atriplex triangularis Willd) plants to a gradient of salt stress were investigated with hydroponically cultured seedlings. Under salt stress, both the Na+ uptake into root xylem and negative pressures in xylem vessels increased with the elevation of salinity (up to 500 mol/m3) in the root environment. However, the increment in negative pressures in root xylem far from matches the decrease in the osmotic potential of the root bathing solutions, even when the osmotic potential of xylem sap is taken into consideration. The total water potential of xylem sap in arrowleaf saltbush roots was close to the osmotic potential of root bathing solutions when the salt stress was low, but a progressively increased gap between the water potential of xylem sap and the osmotic potential of root bathing solutions was observed when the salinity in the root environment was enhanced. The maximum gap was 1.4 MPa at a salinity level of 500 mol/m3 without apparent dehydration of the tested plants. This discrepancy could not be explained with the current theories in plant physiology. The radial reflection coefficient of root in arrowleaf saltbush decreased with the enhanced salt stress was and accompanied by an increase in the Na+ uptake into xylem sap. However, the relative Na+ in xylem exudates based on the corresponding NaCl concentration in the root bathing solutions showed a tendency of decrease. The results showed that the reduction in the radial reflection coefficient of roots in the arrowleaf saltbush did not lead to a mass influx of NaCl into xylem when the radial reflection coefficient of the root was considerably small; and that arrowleaf saltbush could use small xylem pressures to counterbalance the salt stresses, either with the uptake of large amounts of salt, or with the development of xylem pressures dangerously negative. This strategy could be one of the mechanisms behind the high resistance of arrowleaf saltbush plants to salt stress.     

Dual-Targeting of Arabidopsis 6-Phosphogluconolactonase 3 （PGL3）to Chloroplasts and Peroxisomes Involves Interaction with Trx m2 in the Cytosol

The oxidative pentose-phosphate pathway （OPPP） represents a central branch of cellular metabolism emanating from glucose-6-phosphate （G6P） to provide reductive power （NADPH） and sugar phosphates for anabolic biosyntheses. In plant cells, the oxidative OPPP branch is found in the cytosol and in plastids, consisting of glucose-6-phosphate dehydrogenase （G6PD）, 6-phosphogluconolactonase （PGL）, and 6-phosphogluconate dehydrogenase （6PGD）. These enzymes are encoded by small gene families in the nuclear genome, which, in Arabidopsis, comprise six G6PD, five 6-PGL, and three 6-PGD isoforms （Kruger and von Schaewen, 2003）. Specific targeting motifs at the C-terminus of 6-PGL and 6-PGD isoforms suggested that the OPPP may also occur in peroxisomes （Reumann et al., 2004）.     

Two Na+ and Cl- Hyperaccumulators of the Chenopodiaceae

The authors found five sodium (Na ) and chloride (Cl-) hyperaccumulating halophytes in the Temperate Desert of Xinjiang, China and studied two of them (Suaeda salsa (L.) Pall. and Kalidium folium (Pall.) Moq.). K. folium and S. salsa had a NaCl content of 32.1% and 29.8%, respectively, on a dry weight basis. X-ray microanalysis of the Na in the vacuole, apoplasts and cytoplasm of the two plants indicated a ratio of 7.3:5.6:1.0 in K. folium and 7.3:6.6:1.0 in S. salsa. These data show that K. folium and S. salsa both have a high Na and Cl-accumulating capacity, which is related to high activity oftonoplast H -ATPase and H -PPase.     

Puccinellia tenuiflora maintains a low Na+ level under salinity by limiting unidirectional Na+ influx resulting in a high selectivity for K+ over Na+

Puccinellia tenuiflora is a useful monocotyledonous halophyte that might be used for improving salt tolerance of cereals. This current work has shown that P. tenuiflora has stronger selectivity for K⁺ over Na⁺ allowing it to maintain significantly lower tissue Na⁺ and higher K⁺ concentration than that of wheat under short- or long-term NaCl treatments. To assess the relative contribution of Na⁺ efflux and influx to net Na⁺ accumulation, unidirectional ²²Na⁺ fluxes in roots were carried out. It was firstly found that unidirectional ²²Na⁺ influx into root of P. tenuiflora was significantly lower (by 31–37%) than in wheat under 100 and 150 m m NaCl. P. tenuiflora had lower unidirectional Na⁺ efflux than wheat; the ratio of efflux to influx was similar between the two species. Leaf secretion of P. tenuiflora was also estimated, and found the loss of Na⁺ content from leaves to account for only 0.0006% of the whole plant Na⁺ content over 33 d of NaCl treatments. Therefore, it is proposed that neither unidirectional Na⁺ efflux of roots nor salt secretion by leaves, but restricting unidirectional Na⁺ influx into roots with a strong selectivity for K⁺ over Na⁺ seems likely to contribute to the salt tolerance of P. tenuiflora .     

Heterogeneous Na+ Sensitivity of Na+,K+-ATPase Isoenzymes in Whole Brain Membranes

Abstract: The Na⁺ sensitivity of whole brain membrane Na⁺,K⁺-ATPase isoenzymes was studied using the differential inhibitory effect of ouabain (α1, low affinity for ouabain; α2, high affinity; and α3, very high affinity). At 100 m M Na⁺, we found that the proportion of isoforms with low, high, and very high ouabain affinity was 21, 38, and 41%, respectively. Using two ouabain concentrations (10⁻⁵ and 10⁻⁷ M ), we were able to discriminate Na⁺ sensitivity of Na⁺, K⁺-ATPase isoenzymes using nonlinear regression. The ouabain low-affinity isoform, α1, exhibited high Na⁺ sensitivity [ K _a of 3.88 ± 0.25 m M Na⁺ and a Hill coefficient ( n ) of 1.98 ± 0.13]; the ouabain high-affinity isoform, α2, had two Na⁺ sensitivities, a high ( K _a of 4.98 ± 0.2 m M Na⁺ and n of 1.34 ± 0.10) and a low ( K _a of 28 ± 0.5 m M Na⁺ and an n of 1.92 ± 0.18) Na⁺ sensitivity activated above a thresh old (22 ± 0.3 m M Na⁺); and the ouabain very-high-affinity isoform, α3, was resolved by two processes and appears to have two Na⁺ sensitivities (apparent K _a values of 3.5 and 20 m M Na⁺). We show that Na⁺ dependence in the absence of ouabain is the result of at least of five Na⁺ reactivities. This molecular functional characteristic of isoenzymes in membranes could explain the diversity of physiological roles attributed to isoenzymes.     

Impairment of Na+,K+-ATPase activity following enterotoxigenic Campylobacter jejuni infection: changes in Na+, Cl− and 3-O-methyl-D-glucose transport in vitro, in rat ileum

Abstract Unidirectional fluxes of Na⁺, Cl⁻ and 3-O-methyl-D-glucose (3-MG) were measured in vitro across Campylobacter jejuni live culture-infected and control rat ileal short-circuited tissues by the Using Chamber technique. Net secretion of Na⁺ and enhanced secretion of Cl⁻ ions was observed in the infected animals ( P < 0.001, n =6) as compared to the net absorption of Na⁺ and marginal secretion of Cl⁻ ions in the control animals. There was a significant decrease in the mucosal-to-serosal fluxes of 3-MG in C. jejuni -infected rat ileum. The specific Na⁺,K⁺-ATPase activity when measured biochemically in the membrane-rich fraction of enterocytes was found to be significantly lower (58%) in the infected group as compared to the control group ( P < 0.001). Our results therefore suggest that infection with an enterotoxigenic C. jejuni inhibits the Na⁺,K⁺-ATPase activity in rat enterocytes. The impairment of Na⁺,K⁺-ATPase activity thus appears to induce a secondary change in Na⁺,Cl⁻ and 3-MG transport in vitro in rat ileum.     

Ouabain Binding, ATP Hydrolysis, and Na+,K+-Pump Activity During Chemical Modification of Brain and Muscle Na+,K+-ATPase

The effects of 16 group-specific, amino acid-modifying agents were tested on ouabain binding, catalytical activity of membrane-bound (rat brain microsomal), sodium dodecyl sulfate-treated Na+,K(+)-ATPase, and Na+,K(+)-pump activity in intact muscle cells. With few exceptions, the potency of various tryptophan, tyrosine, histidine, amino, and carboxy group-oriented drugs to suppress ouabain binding and Na+,K(+)-ATPase activity correlated with inhibition of the Na+,K(+)-pump electrogenic effect. ATP hydrolysis was more sensitive to inhibition elicited by chemical modification than ouabain binding (membrane-bound or isolated enzyme) and than Na+,K(+)-pump activity. The efficiency of various drugs belonging to the same "specificity" group differed markedly. Tyrosine-oriented tetranitromethane was the only reagent that interfered directly with the cardiac receptor binding site as its inhibition of ouabain binding was completely protected by ouabagenin preincubation. The inhibition elicited by all other reagents was not, or only partially, protected by ouabagenin. It is surprising that agents like diethyl pyrocarbonate (histidine groups) or butanedione (arginine groups), whose action should be oriented to amino acids not involved in the putative ouabain binding site (represented by the -Glu-Tyr-Thr-Trp-Leu-Glu- sequence), are equally effective as agents acting on amino acids present directly in the ouabain binding site. These results support the proposal of long-distance regulation of Na+,K(+)-ATPase active sites.     

Two Isoenzymes of Na+, K+-ATPase Have Different Kinetics of K+ Dephosphorylation in Normal Cat and Human Brain Cortex

Analysis of purified Na+,K+-ATPase from cat and human cortex by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals two large catalytic subunits called alpha (-) (lower molecular weight) and alpha (+) (higher molecular weight). Differences in K+ dephosphorylation of these two molecular forms have been investigated by measuring the phosphorylation level of each protein after their separation on sodium dodecyl sulfate gels. In the presence of Na+, Mg2+, and ATP, both subunits are phosphorylated. Increasing concentrations (from 0 to 3 mM) of K+ induce progressive dephosphorylation of both alpha-subunits, although the phosphoprotein content of alpha (-) is decreased significantly less than that of alpha (+). Ka values of alpha (-) for K+ are 40% and 50% greater in cat and human cortex, respectively, than values of alpha (+). alpha (-) and alpha (+) are thought to be localized in specific cell types of the brain: alpha (-) is the exclusive form of nonneuronal cells (astrocytes), whereas alpha (+) is the only form of axolemma. Our results support the hypothesis that glial and neuronal Na+,K+-ATPases are different molecular entities differing at least by their K+ sensitivity. Results are discussed in relation to the role of glial cells in the regulation of extracellular K+ in brain.     

Possible Regulation of Caffeine-Induced Intracellular Ca2+ Mobilization by Intracellular Free Na+

To gain some understanding of the regulatory mechanism involved in caffeine-induced Ca2+ release in adrenal chromaffin cells, we took advantage of the paradoxical observation that removal of divalent cations potentiated the secretory response to caffeine. We measured the concentration of cytosolic free Ca2+ ([Ca]in) in isolated cat chromaffin cells, by fura-2 microfluorometry, to see whether there was any correlation between the secretory response and the rise in [Ca]in. The caffeine-induced [Ca]in rise and catecholamine secretion were increased by treatment of cells with a divalent cation-deficient solution. These potentiated responses were strongly inhibited either by pretreatment with ryanodine, by the reduction of the external Na+ concentration, or by the addition of Ca2+ channel blockers. Removal of divalent cations caused a large rise in the cytosolic free Na+ concentration ([Na]in), which was measured using SBFI microfluorometry. This rise in [Na]in was reduced either by adding Ca2+ channel blockers or by reducing the external Na+ concentration. These results show a good correlation between caffeine-induced Ca2+ release and [Na]in at the time of stimulation, suggesting that caffeine-induced Ca2+ release is regulated by [Na]in.     

Evidence for a Na+ / H+ electrogenic antiporter in an alkaliphilic cyanobacterium Synechocystis

Abstract An alkaliphilic cyanobacterium characterized as a Synechocystis species was purified from a soil sample taken from a village in Java, Indonesia, by its preferential growth at elevated pH; it grew optimally at pH 9.5. Phosphorus nuclear magnetic resonance studies showed that the organism can maintain a ΔpH of over 2 pH units at an external pH of 10. It was observed that the viability of the organism in the dark was dependent on sodium ions. Evidence from experiments in which the extrusion of Na⁺ was measured from cells subjected to an alkali shock suggests that the organism possesses a Na⁺ / H⁺ electrogenic antiporter which is used for the maintenance of pH homeostasis.     

Distribution of Na+, K+-ATPase α-Subunit Isoforms in Rat Pituitary

The distributions of alpha-subunit isoforms of the Na+,K(+)-ATPase in rat pituitary were determined by immunoblotting and immunohistochemistry. Immunoreactivity for all three forms is present in the neural lobe, whereas the anterior lobe contains only alpha 1 and alpha 2. Most areas of the intermediate lobe exhibit faint immunoreactivity for only alpha 1, but thin strands of cells which stain strongly for all three isoforms are also present in this lobe. The previously reported ouabain inhibitable Na+,K(+)-ATPase activity in the neural lobe is consistent with the presence of both alpha 2 and alpha 3 subunits.