Scanning ion‐selective electrode technique and X‐ray microanalysis provide direct evidence of contrasting Na+ transport ability from root to shoot in salt‐sensitive cucumber and salt‐tolerant pumpkin under NaCl stress

Grafting onto salt‐tolerant pumpkin rootstock can increase cucumber salt tolerance. Previous studies have suggested that this can be attributed to pumpkin roots with higher capacity to limit the transport of Na⁺ to the shoot than cucumber roots. However, the mechanism remains unclear. This study investigated the transport of Na⁺ in salt‐tolerant pumpkin and salt‐sensitive cucumber plants under high (200 mM) or moderate (90 mM) NaCl stress. Scanning ion‐selective electrode technique showed that pumpkin roots exhibited a higher capacity to extrude Na⁺, and a correspondingly increased H⁺ influx under 200 or 90 mM NaCl stress. The 200 mM NaCl induced Na⁺/H⁺ exchange in the root was inhibited by amiloride (a Na⁺/H⁺ antiporter inhibitor) or vanadate [a plasma membrane (PM) H⁺‐ATPase inhibitor], indicating that Na⁺ exclusion in salt stressed pumpkin and cucumber roots was the result of an active Na⁺/H⁺ antiporter across the PM, and the Na⁺/H⁺ antiporter system in salt stressed pumpkin roots was sufficient to exclude Na⁺. X‐ray microanalysis showed higher Na⁺ in the cortex, but lower Na⁺ in the stele of pumpkin roots than that in cucumber roots under 90 mM NaCl stress, suggesting that the highly vacuolated root cortical cells of pumpkin roots could sequester more Na⁺, limit the radial transport of Na⁺ to the stele and thus restrict the transport of Na⁺ to the shoot. These results provide direct evidence for pumpkin roots with higher capacity to limit the transport of Na⁺ to the shoot than cucumber roots.     

Hydrogen peroxide as a mediator of 5‐aminolevulinic acid‐induced Na+ retention in roots for improving salt tolerance of strawberries

To explore the mechanisms of 5‐aminolevulinic acid (ALA)‐improved plant salt tolerance, strawberries (Fragaria × ananassa Duch. cv. ‘Benihoppe’) were treated with 10 mg l^?1 ALA under 100 mmol l^?1 NaCl stress. We found that the amount of Na⁺ increased in the roots but decreased in the leaves. Laser scanning confocal microscopy (LSCM) observations showed that ALA‐induced roots had more Na⁺ accumulation than NaCl alone. Measurement of the xylem sap revealed that ALA repressed Na⁺ concentrations to a large extent. The electron microprobe X‐ray assay also confirmed ALA‐induced Na⁺ retention in roots. qRT‐PCR showed that ALA upregulated the gene expressions of SOS1 (encoding a plasma membrane Na⁺/H⁺ antiporter), NHX1 (encoding a vacuolar Na⁺/H⁺ antiporter) and HKT1 (encoding a protein of high‐affinity K⁺ uptake), which are associated with Na⁺ exclusion in the roots, Na⁺ sequestration in vacuoles and Na⁺ unloading from the xylem vessels to the parenchyma cells, respectively. Furthermore, we found that ALA treatment reduced the H₂O₂ content in the leaves but increased it in the roots. The exogenous H₂O₂ promoted plant growth, increased root Na⁺ retention and stimulated the gene expressions of NHX1, SOS1 and HKT1. Diphenyleneiodonium (DPI), an inhibitor of H₂O₂ generation, suppressed the effects of ALA or H₂O₂ on Na⁺ retention, gene expressions and salt tolerance. Therefore, we propose that ALA induces H₂O₂ accumulation in roots, which mediates Na⁺ transporter gene expression and more Na⁺ retention in roots, thereby improving plant salt tolerance.     

Glucose-6-phosphate dehydrogenase-dependent hydrogen peroxide production is involved in the regulation of plasma membrane H+-ATPase and Na+/H+ antiporter protein in salt-stressed callus from Carex moorcroftii

Glucose‐6‐phosphate dehydrogenase (G6PDH) is important for the activation of plant resistance to environmental stresses, and ion homeostasis is the physiological foundation for living cells. In this study, we investigated G6PDH roles in modulating ion homeostasis under salt stress in Carex moorcroftii callus. G6PDH activity increased to its maximum in 100 mM NaCl treatment and decreased with further increased NaCl concentrations. K⁺/Na⁺ ratio in 100 mM NaCl treatment did not exhibit significant difference compared with the control; however, in 300 mM NaCl treatment, it decreased. Low‐concentration NaCl (100 mM) stimulated plasma membrane (PM) H⁺‐ATPase and NADPH oxidase activities as well as Na⁺/H⁺ antiporter protein expression, whereas high‐concentration NaCl (300 mM) decreased their activity and expression. When G6PDH activity and expression were reduced by glycerol treatments, PM H⁺‐ATPase and NADPH oxidase activities, Na⁺/H⁺ antiporter protein level and K⁺/Na⁺ ratio dramatically decreased. Simultaneously, NaCl‐induced hydrogen peroxide (H₂O₂) accumulation was abolished. Exogenous application of H₂O₂ increased G6PDH, PM H⁺‐ATPase and NADPH oxidase activities, Na⁺/H⁺ antiporter protein expression and K⁺/Na⁺ ratio in the control and glycerol treatments. Diphenylene iodonium (DPI), the NADPH oxidase inhibitor, which counteracted NaCl‐induced H₂O₂ accumulation, decreased G6PDH, PM H⁺‐ATPase and NADPH oxidase activities, Na⁺/H⁺ antiporter protein level and K⁺/Na⁺ ratio. Western blot result showed that G6PDH expression was stimulated by NaCl and H₂O₂, and blocked by DPI. Taken together, G6PDH is involved in H₂O₂ accumulation under salt stress. H₂O₂, as a signal, upregulated PM H⁺‐ATPase activity and Na⁺/H⁺ antiporter protein level, which subsequently resulted in the enhanced K⁺/Na⁺ ratio. G6PDH played a central role in the process.     

Interferon‐γ suppresses Na+–H+ exchanger in cultured human endolymphatic sac epithelial cells

Adequate regulation of endolymphatic pH is essential for maintaining inner ear function. The Na⁺–H⁺ exchanger (NHE) is a major determinant of intracellular pH (pH_i), and facilitates Na⁺ and fluid absorption in various epithelia. We determined the functional and molecular expression of NHEs in cultured human endolymphatic sac (ES) epithelial cells and examined the effect of IFN‐γ on NHE function. Serial cultures of human ES epithelial cells were generated from tissue samples. The molecular expression of NHE1, ‐2, and ‐3 isoforms was determined by real‐time RT‐PCR. The functional activity of NHE isoforms was measured microfluorometrically using a pH‐sensitive fluorescent dye, 2′,7′‐bis(carbonylethyl)‐5(6)‐carboxyfluorescein (BCECF), and a NHE‐inhibitor, 3‐methylsulfonyl‐4‐piperidinobenzoyl guanidine methanesulfonate (HOE694). NHE1, ‐2, and ‐3 mRNAs were expressed in human ES epithelial cells. Functional activity of NHE1 and ‐2 was confirmed in the luminal membrane of ES epithelial cells by sequentially suppressing Na⁺‐dependent pH_i recovery from intracellular acidification using different concentrations of HOE694. Treatment with IFN‐γ (50 nM for 24 h) suppressed mRNA expression of NHE1 and ‐2. IFN‐γ also suppressed functional activity of both NHE1 and ‐2 in the luminal membrane of ES epithelial cells. This study shows that NHEs are expressed in cultured human ES epithelial cells and that treatment with IFN‐γ suppresses the expression and functional activity of NHE1 and ‐2. J. Cell. Biochem. 107: 965–972, 2009. © 2009 Wiley‐Liss, Inc.     

New Drugs for the Na+/H+ Exchanger. Influence of Na+ Concentration and Determination of Inhibition Constants with a Microphysiometer

The NHE-1 isoform of the Na⁺/H⁺ exchanger is excessively activated in cardiac cells during ischemia. Hence NHE-1 specific inhibitors are being developed since they could be of beneficial influence under conditions of cardiac ischemia and reperfusion. In this study, the Cytosensor™ microphysiometer was used to measure the potency of four new drug molecules, i.e., EMD 84021, EMD 94309, EMD 96785 and HOE 642 which are inhibitors of the isoform 1 of the Na⁺/H⁺ exchanger. The experiments were performed with Chinese hamster ovary cells (CHO K1) which are enriched in the NHE-1 isoform of the Na⁺/H⁺ antiporter. The Na⁺/H⁺ exchanger was stimulated with NaCl and the rate of extracellular acidification was quantified with the Cytosensor. The proton exchange rate was measured as a function of the NaCl concentration in the range of 10–138 mm NaCl stimulation. The proton exchange rate followed Michaelis-Menten kinetics with a K _M = 30 ± 4 mm for Na⁺. Addition of either one of the four inhibitors decreased the acidification rate. The IC₅₀ values of the four compounds could be determined as 23 ± 7 nm for EMD 84021, 5 ± 1 nm for EMD 94309, 9 ± 2 nm for EMD 96785 and 8 ± 2 nm for HOE 642 at 138 mm NaCl, in good agreement with more elaborate biological assays. The IC₅₀ values increased with the NaCl concentration indicating competitive binding of the inhibitor. The microphysiometer approach is a fast and simple method to measure the activity of the Na⁺/H⁺ antiporter and allows a quantitative kinetic analysis of the proton excretion rate. Received: 3 September 1998/Revised: 20 November 1998     

Heterologous expression of Arabidopsis H+‐pyrophosphatase enhances salt tolerance in transgenic creeping bentgrass (Agrostis stolonifera L.)

The Arabidopsis vacuolar H⁺‐pyrophosphatase (AVP1), when over‐expressed in transgenic (TG) plants, regulates root and shoot development via facilitation of auxin flux, and enhances plant resistance to salt and drought stresses. Here, we report that TG perennial creeping bentgrass plants over‐expressing AVP1 exhibited improved resistance to salinity than wild‐type (WT) controls. Compared to WT plants, TGs grew well in the presence of 100 mm NaCl, and exhibited higher tolerance and faster recovery from damages from exposure to 200 and 300 mm NaCl. The improved performance of the TG plants was associated with higher relative water content (RWC), higher Na⁺ uptake and lower solute leakage in leaf tissues, and with higher concentrations of Na⁺, K⁺, Cl^‐ and total phosphorus in root tissues. Under salt stress, proline content was increased in both WT and TG plants, but more significantly in TGs. Moreover, TG plants exhibited greater biomass production than WT controls under both normal and elevated salinity conditions. When subjected to salt stress, fresh (FW) and dry weights (DW) of both leaves and roots decreased more significantly in WT than in TG plants. Our results demonstrated the great potential of genetic manipulation of vacuolar H⁺‐pyrophosphatase expression in TG perennial species for improvement of plant abiotic stress resistance.     

Na+ Sensitivity of ROMK1 K+ Channel: Role of the Na+/H+ Antiporter

To examine the extracellular Na⁺ sensitivity of a renal inwardly rectifying K⁺ channel, we performed electrophysiological experiments on Xenopus oocytes or a human kidney cell line, HEK293, in which we had expressed the cloned renal K⁺ channel, ROMK1 (Kir1.1). When extracellular Na⁺ was removed, the whole-cell ROMK1 currents were markedly suppressed in both the oocytes and HEK293 cells. Single-channel ROMK1 activities recorded in the cell-attached patch on the oocyte were not affected by removal of Na⁺ from the pipette solution. However, macro-patch ROMK1 currents recorded on the oocyte were significantly suppressed by Na⁺ removal from the bath solution. A blocker of Na⁺/H⁺ antiporters, amiloride, largely inhibited the Na⁺ removal-induced suppression of whole-cell ROMK1 currents in the oocytes. The pH-insensitive K80M mutant of ROMK1 was much less sensitive to Na⁺ removal. Na⁺ removal was found to induce a significant decrease in intracellular pH in the oocytes using H⁺-selective microelectrodes. Coexpression of ROMK1 with NHE3, which is a Na⁺/H⁺ antiporter isoform of the kidney apical membrane, conferred increased sensitivity of ROMK1 channels to extracellular Na⁺ in both the oocytes and HEK293 cells. Thus, it is concluded that the ROMK1 channel is regulated indirectly by extracellular Na⁺, and that the interaction between NHE transporter and ROMK1 channel appears to be involved in the mechanism of Na⁺ sensitivity of ROMK1 channel via regulating intracellular pH. Received: 13 April 1999/Revised: 15 July 1999     

H2O2 and cytosolic Ca2+ signals triggered by the PM H+‐coupled transport system mediate K+/Na+ homeostasis in NaCl‐stressed Populus euphratica cells

Using confocal microscopy, X‐ray microanalysis and the scanning ion‐selective electrode technique, we investigated the signalling of H₂O₂, cytosolic Ca²⁺ ([Ca²⁺]_cyt) and the PM H⁺‐coupled transport system in K⁺/Na⁺ homeostasis control in NaCl‐stressed calluses of Populus euphratica. An obvious Na⁺/H⁺ antiport was seen in salinized cells; however, NaCl stress caused a net K⁺ efflux, because of the salt‐induced membrane depolarization. H₂O₂ levels, regulated upwards by salinity, contributed to ionic homeostasis, because H₂O₂ restrictions by DPI or DMTU caused enhanced K⁺ efflux and decreased Na⁺/H⁺ antiport activity. NaCl induced a net Ca²⁺ influx and a subsequent rise of [Ca²⁺]_cyt, which is involved in H₂O₂‐mediated K⁺/Na⁺ homeostasis in salinized P. euphratica cells. When callus cells were pretreated with inhibitors of the Na⁺/H⁺ antiport system, the NaCl‐induced elevation of H₂O₂ and [Ca²⁺]_cyt was correspondingly restricted, leading to a greater K⁺ efflux and a more pronounced reduction in Na⁺/H⁺ antiport activity. Results suggest that the PM H⁺‐coupled transport system mediates H⁺ translocation and triggers the stress signalling of H₂O₂ and Ca²⁺, which results in a K⁺/Na⁺ homeostasis via mediations of K⁺ channels and the Na⁺/H⁺ antiport system in the PM of NaCl‐stressed cells. Accordingly, a salt stress signalling pathway of P. euphratica cells is proposed.     

The Na+/H+ antiporter of alkaliphilic Bacillus sp.

The Na⁺/H⁺ antiporter, which appears to predominantly contribute to the alkaliphily of Bacillus halodurans C-125, was studied in an alkali-sensitive mutant of this strain and a transformant with restored alkaliphily. The alkali-sensitive mutant, strain 38154, which has lost the ability to grow above pH 9.5, was found to lack electro-genic Na⁺/H⁺ antiport activity driven by ΔΨ (membrane potential, interior negative), and it showed defective regulation of intracellular pH under alkaline conditions. On the other hand, a transformant carrying a 2.0-kb DNA fragment from the parental genome that complemented this defect was able to maintain an intracellular pH lower than that of the external milieu, and it was found to have recovered the Na⁺/H⁺ antiport activity driven by ΔΨ. Sequence analyses found that a 5.1-kb DNA region contained four open reading frames (ORF-1 to ORF-4). Direct sequencing of the corresponding region in mutant 38154 revealed a G-to-A substitution, which resulted in an amino acid substitution from Gly-393 to Arg in the putative ORF-1 product. It has been recently found that a region homologous to the DNA fragment responsible for the alkaliphily of strain C-125 exists in the genomes of Bacillus subtilis, Sinorhizobium (Rhizobium) meliloti, and Staphylococcus aureus. These homologues are present as a cluster of seven ORFs in each case. The shaA gene product of B. subtilis shows significant similarity to the ORF-1 product of strain C-125. Disruption of the shaA gene resulted in a decrease in Na⁺/H⁺ antiport activity, and growth of the shaA-disrupted strain was impaired when the external Na⁺ concentration was increased. We conclude that the shaA gene encodes a Na⁺/H⁺ antiporter, which plays an important role in extrusion of cytotoxic Na⁺. Received: May 29, 2000 / Accepted: July 18, 2000     

Physiological adjustment to salt stress in Jatropha curcas is associated with accumulation of salt ions,transport and selectivity of K+, osmotic adjustment and K+/Na+ homeostasis

This study assessed the capacity of Jatropha curcas to physiologically adjust to salinity. Seedlings were exposed to increasing NaCl concentrations (25, 50, 75 and 100 mm ) for 15 days. Treatment without NaCl was adopted as control. Shoot dry weight was strongly reduced by NaCl, reaching values of 35% to 65% with 25 to 100 mm NaCl. The shoot/root ratio was only affected with 100 mm NaCl. Relative water content (RWC) increased only with 100 mm NaCl, while electrolyte leakage (EL) was much enhanced with 50 mm NaCl. The Na⁺ transport rate to the shoot was more affected with 50 and 100 mm NaCl. In parallel, Cl^? transport rate increased with 75 and 100 mm NaCl, while K⁺ transport rate fell from 50 mm to 100 mm NaCl. In roots, Na⁺ and Cl^? transport rates fell slightly only in 50 mm (to Na⁺) and 50 and 100 mm (to Cl^?) NaCl, while K⁺ transport rate fell significantly with increasing NaCl. In general, our data demonstrate that J. curcas seedlings present changes in key physiological processes that allow this species to adjust to salinity. These responses are related to accumulation of Na⁺ and Cl^? in leaves and roots, K⁺/Na⁺ homeostasis, transport of K⁺ and selectivity (K–Na) in roots, and accumulation of organic solutes contributing to osmotic adjustment of the species.     

Enhanced V-ATPase activity contributes to the improved salt tolerance of transgenic tobacco plants overexpressing vacuolar Na(+)/H (+) antiporter AtNHX1

AtNHX1, a vacuolar Na⁺/H⁺ antiporter gene from Arabidopsis thaliana, was introduced into tobacco genome via Agrobacterium tumefaciens-mediated transformation to evaluate the role of vacuolar energy providers in plants salt stress response. Compared to the wild-type plants, over-expression of AtNHX1 increased salt tolerance in the transgenic tobacco plants, allowing higher germination rates of seeds and successful seedling establishment in the presence of toxic concentrations of NaCl. More importantly, the induced Na⁺/H⁺ exchange activity in the transgenic plants was closely correlated to the enhanced activity of vacuolar H⁺-ATPase (V-ATPase) when exposed to 200 mM NaCl. In addition, inhibition of V-ATPase activity led to the malfunction of Na⁺/H⁺ exchange activity, placing V-ATPase as the dominant energy provider for the vacuolar Na⁺/H⁺ antiporter AtNHX1. V-ATPase and vacuolar Na⁺/H⁺ antiporter thus function in an additive or synergistic way. Simultaneous overexpression of V-ATPase and vacuolar Na⁺/H⁺ antiporter might be appropriate for producing plants with a higher salt tolerance ability.     

Na+/H+ antiport activity in tonoplast vesicles isolated from sunflower roots induced by NaCl stress

Na⁺ transport across the tonoplast and its accumulation in the vacuoles is of crucial importance for plant adaptation to salinity. Mild and severe salt stress increased both ATP- and PP_i-dependent H⁺ transport in tonoplast vesicles from sunflower seedling roots, suggesting the possibility that a Na⁺/H⁺ antiport system could be operating in such vesicles under salt conditions (E. Ballesteros et al. 1996. Physiol. Plant. 97: 259–268). During a mild salt stress, Na⁺ was mainly accumulated in the roots. Under a more severe salt treatment, Na⁺ was equally distributed in shoots and roots. In contrast to what was observed with Na⁺, all the salt treatments reduced the shoot K⁺ content. Dissipation by Na⁺ of the H⁺ gradient generated by the tonoplast H⁺-ATPase, monitored as fluorescence quenching of acridine orange, was used to measure Na⁺/H⁺ exchange across tonoplast-enriched vesicles isolated by sucrose gradient centrifugation from sunflower (Helianthus annuus L.) roots treated for 3 days with different NaCl regimes. Salt treatments induced a Na⁺/H⁺ exchange activity, which displayed saturation kinetics for Na⁺ added to the assay medium. This activity was partially inhibited by 125 μM amiloride, a competitive inhibitor of Na⁺/H⁺ antiports. No Na⁺/H⁺ exchange was detected in vesicles from control roots. The activity was specific for Na⁺. since K⁺ added to the assay medium slightly dissipated H⁺ gradients and displayed non-saturating kinetics for all salt treatments. Apparent K_m for Na⁺/H⁺ exchange in tonoplast vesicles from 150 mM NaCl-treated roots was lower than that of 75 mM NaCl-treated roots, V_max remaining unchanged. The results suggest that the existence of a specific Na⁺/H⁺ exchange activity in tonoplast-enriched vesicle fractions, induced by salt stress, could represent an adaptative response in sunflower plants, moderately tolerant to salinity.     

SALINITY ADAPTATION BY DUNALIELLA TERTIOLECTA. I. INCREASES IN CARBONIC ANHYDRASE ACTIVITY AND EVIDENCE FOR A LIGHT-DEPENDENT Na+/H+ EXCHANGE1,2

When Dunaliella tertiolecta, previously adapted to medium containing 0.5 M NaCl, is transferred to higher salinities, there is a lag in growth, suggesting an adaptation period. Since there is no significant difference in the Na⁺ content of cells grown between 0.5 and 3.5 M NaCl, a mechanism for Na⁺ extrusion or exclusion is indicated. Increasing the salinity of cell suspensions stimulates an incorporation of H⁺ by the cells, suggesting an H⁺/Na⁺ exchange. Cells adapted to higher salinities have, increased carbonic anhydrase activity, suggesting that increased CO₂ or HCO₃^? transport may be required at higher salinities. Growth, of D. tertiolecta at salinities above 2.5 M requires continuous illumination; therefore a light-driven H⁺/Na⁺ exchange accompanied by a HCO₃^? influx is proposed.     

Metagenomic cloning and characterization of Na+/H+ antiporter genes taken from sediments in Chaerhan Salt Lake in China

A metagenomic library containing 8,000 clones was constructed by using genomic DNA obtained from Chaerhan Salt Lake in northwest China. Three Na⁺/H⁺ antiporters, C4-NhaG, C47-NhaG and C49-NhaG that grouped to the NhaG family, were screened and cloned from this metagenome by complementing Escherichia coli strain KNabc (ΔnhaA ΔnhaB ΔchaA) in medium containing 0.2 M NaCl. The three putative Na⁺/H⁺ antiporters were membrane proteins with 10, 11 and 11 transmembrane segments, respectively. They enabled E. coli KNabc to grow in medium containing 0.2–0.6 M Na⁺ or 7–14 mM Li⁺. Everted membrane vesicles prepared from E. coli KNabc cells carrying C49-NhaG exhibited Na⁺/H⁺ and Li⁺/H⁺ antiport activities.     

Effects of 24‐epibrassinolide on plant growth,osmotic regulation and ion homeostasis of salt‐stressed canola

This study evaluated effects of foliar spraying 24‐epibrassinoide (24‐EBL) on the growth of salt‐stressed canola. Seedlings at the four‐leaf stage were treated with 150 mm NaCl and different concentrations of 24‐EBL (10^?6, 10^?8, 10^?10, 10^?12 m ) for 15 days. A concentration of 10^?10 m 24‐EBL was chosen as optimal and used in a subsequent experiment on plant biomass and leaf water potential parameters. The results showed that 24‐EBL mainly promoted shoot growth of salt‐stressed plants and also ameliorated leaf water status. Foliar spraying of salt‐stressed canola with 24‐EBL increased osmotic adjustment ability in all organs, especially in younger leaves and roots. This was mainly due to an increase of free amino acid content in upper leaves, soluble sugars in middle leaves, organic acids and proline in lower leaves, all of these compounds in roots, as well as essential inorganic ions. Na⁺ and Cl^? sharply increased in different organs under salt stress, and 24‐EBL reduced their accumulation. 24‐EBL improved the uptake of K⁺, Ca²⁺, Mg²⁺ and NO₃^? in roots, which were mainly transported to upper leaves, while NO₃^? was mainly transported to middle leaves. Thus, 24‐EBL improvements in ion homeostasis of K⁺/Na⁺, Ca²⁺/Na⁺, Mg²⁺/Na⁺ and NO₃^?/Cl^?, especially in younger leaves and roots, could be explained. As most important parts, younger leaves and roots were the main organs protected by 24‐EBL via improvement in osmotic adjustment ability and ion homeostasis. Further, physiological status of growth of salt‐stressed canola was ameliorated after 24‐EBL treatment.     

Differences in epithelial morphology correlate to Na+‐transport: A study of the proximal,mid, and distal regions of the coprodeum from hens on high and low NaCl diet

A study was performed to correlate regional morphology and amiloride inhibitable Na⁺‐transport in the coprodeal epithelium in hens, Gallus domesticus, on low‐NaCl diet and in controls. Proximal (close to colon), mid and distal (close to urodeum) regions were examined using light microscopy, transmission‐ and scanning electron microscopy. Na⁺‐transport was measured electrophysiologically in Ussing‐chambers in the proximal and distal regions. The epithelium, simple and columnar, is composed of absorptive intestinal epithelial cells, goblet cells, brush cells, migrating lymphoid cells, and entero‐endocrine cells. Brush cells, identified in avians for the first time, occur in highest number in the proximal part of the coprodeum in low‐NaCl hens. Na⁺‐transport is high in the low‐NaCl hens, ranging from 347μA/cm² (proximal) to 187μA/cm² (distal). In control hens, which correspond to hens on high‐NaCl diet, it is low in all regions (0–4 μA/cm²). Absorptive intestinal epithelial cells as well as brush cells adapt to variations in transepithelial Na⁺‐transport by regulating height and packing density of their microvilli, number, size, and localization of apical vesicles, and the width of the intercellular space. Regional differences in the epithelial cell composition and ultrastructure are closely correlated to transepithelial Na⁺‐transport but only in low‐NaCl hens, as controls do not show these variations. J. Morphol. 239:75–86, 1999. © 1999 Wiley‐Liss, Inc.     

Functional Analysis of the Na+/H+ Antiporter Encoding Genes of the Cyanobacterium Synechocystis PCC 6803

The role of putative Na⁺/H⁺ antiporters encoded by nhaS1 (slr1727), nhaS3 (sll0689), nhaS4 (slr1595), and nhaS5 (slr0415) in salt stress response and internal pH regulation of the cyanobacterium Synechocystis PCC 6803 was investigated. For this purpose the mutants (single, double, and triple) impaired in genes coding for Na⁺/H⁺ antiporters were constructed using the method of interposon mutagenesis. PCR analyses of DNA demonstrated that mutations in nhaS1, nhaS4, and nhaS5 genes were segregated completely and the mutants contained only inactivated copies of the corresponding genes. Na⁺/H⁺ antiporter encoded by nhaS3 was essential for viability of Synechocystis since no completely segregated mutants were obtained. The steady-state intracellular sodium concentration and Na⁺/H⁺ antiporter activities were found to be the same in the wild type and all mutants. No differences were found in the growth rates of wild type and mutants during their cultivation in liquid media supplemented with 0.68 M or 0.85 M NaCl as well as in media buffered at pH 7.0, 8.0, or 9.0. The expression of genes coding for Na⁺/H⁺ antiporters was studied. No induction of any Na⁺/H⁺ antiporter encoding gene expression was found in wild type or single mutant cells grown under high salt or at different pH values. Nevertheless, in cells of double and triple mutants adapted to high salt or alkaline pH some of the remaining Na⁺/H⁺ antiporter encoding genes showed induction. These results might indicate that some of Na⁺/H⁺ antiporters can functionally replace each other under stress conditions in Synechocystis cells lacking the activity of more than one antiporter.