Cloning of two SOS1 transporters from the seagrass Cymodocea nodosa. SOS1 transporters from Cymodocea and Arabidopsis mediate potassium uptake in bacteria

The Jacalin-Related Lectin HvHorcH Is Involved in the Physiological Response of Barley Roots to Salt Stress

Salt stress tolerance of crop plants is a trait with increasing value for future food production. In an attempt to identify proteins that participate in the salt stress response of barley, we have used a cDNA library from salt-stressed seedling roots of the relatively salt-stress-tolerant cv. Morex for the transfection of a salt-stress-sensitive yeast strain (Saccharomyces cerevisiae YSH818 Δhog1 mutant). From the retrieved cDNA sequences conferring salt tolerance to the yeast mutant, eleven contained the coding sequence of a jacalin-related lectin (JRL) that shows homology to the previously identified JRL horcolin from barley coleoptiles that we therefore named the gene HvHorcH. The detection of HvHorcH protein in root extracellular fluid suggests a secretion under stress conditions. Furthermore, HvHorcH exhibited specificity towards mannose. Protein abundance of HvHorcH in roots of salt-sensitive or salt-tolerant barley cultivars were not trait-specific to salinity treatment, but protein levels increased in response to the treatment, particularly in the root tip. Expression of HvHorcH in Arabidopsis thaliana root tips increased salt tolerance. Hence, we conclude that this protein is involved in the adaptation of plants to salinity.

Saccharomyces cerevisiae as a Tool to Investigate Plant Potassium and Sodium Transporters

Sodium and potassium are two alkali cations abundant in the biosphere. Potassium is essential for plants and its concentration must be maintained at approximately 150 mM in the plant cell cytoplasm including under circumstances where its concentration is much lower in soil. On the other hand, sodium must be extruded from the plant or accumulated either in the vacuole or in specific plant structures. Maintaining a high intracellular K+/Na+ ratio under adverse environmental conditions or in the presence of salt is essential to maintain cellular homeostasis and to avoid toxicity. The baker's yeast, Saccharomyces cerevisiae, has been used to identify and characterize participants in potassium and sodium homeostasis in plants for many years. Its utility resides in the fact that the electric gradient across the membrane and the vacuoles is similar to plants. Most plant proteins can be expressed in yeast and are functional in this unicellular model system, which allows for productive structure-function studies for ion transporting proteins. Moreover, yeast can also be used as a high-throughput platform for the identification of genes that confer stress tolerance and for the study of protein-protein interactions. In this review, we summarize advances regarding potassium and sodium transport that have been discovered using the yeast model system, the state-of-the-art of the available techniques and the future directions and opportunities in this field.

The Role of Na+ and K+ Transporters in Salt Stress Adaptation in Glycophytes

Ionic stress is one of the most important components of salinity and is brought about by excess Na+ accumulation, especially in the aerial parts of plants. Since Na+ interferes with K+ homeostasis, and especially given its involvement in numerous metabolic processes, maintaining a balanced cytosolic Na+/K+ ratio has become a key salinity tolerance mechanism. Achieving this homeostatic balance requires the activity of Na+ and K+ transporters and/or channels. The mechanism of Na+ and K+ uptake and translocation in glycophytes and halophytes is essentially the same, but glycophytes are more susceptible to ionic stress than halophytes. The transport mechanisms involve Na+ and/or K+ transporters and channels as well as non-selective cation channels. Thus, the question arises of whether the difference in salt tolerance between glycophytes and halophytes could be the result of differences in the proteins or in the expression of genes coding the transporters. The aim of this review is to seek answers to this question by examining the role of major Na+ and K+ transporters and channels in Na+ and K+ uptake, translocation and intracellular homeostasis in glycophytes. It turns out that these transporters and channels are equally important for the adaptation of glycophytes as they are for halophytes, but differential gene expression, structural differences in the proteins (single nucleotide substitutions, impacting affinity) and post-translational modifications (phosphorylation) account for the differences in their activity and hence the differences in tolerance between the two groups. Furthermore, lack of the ability to maintain stable plasma membrane (PM) potentials following Na+-induced depolarization is also crucial for salt stress tolerance. This stable membrane potential is sustained by the activity of Na+/H+ antiporters such as SOS1 at the PM. Moreover, novel regulators of Na+ and K+ transport pathways including the Nax1 and Nax2 loci regulation of SOS1 expression and activity in the stele, and haem oxygenase involvement in stabilizing membrane potential by activating H+-ATPase activity, favorable for K+ uptake through HAK/AKT1, have been shown and are discussed.

Carbon-concentrating mechanisms in seagrasses

Seagrasses are unique angiosperms that carry out growth and reproduction submerged in seawater. They occur in at least three families of the Alismatales. All have chloroplasts mainly in the cells of the epidermis. Living in seawater, the supply of inorganic carbon (Ci) to the chloroplasts is diffusion limited, especially under unstirred conditions. Therefore, the supply of CO2 and bicarbonate across the diffusive boundary layer on the outer side of the epidermis is often a limiting factor. Here we discuss the evidence for mechanisms that enhance the uptake of Ci into the epidermal cells. Since bicarbonate is plentiful in seawater, a bicarbonate pump might be expected; however, the evidence for such a pump is not strongly supported. There is evidence for a carbonic anhydrase outside the outer plasmalemma. This, together with evidence for an outward proton pump, suggests the possibility that local acidification leads to enhanced concentrations of CO2 adjacent to the outer tangential epidermal walls, which enhances the uptake of CO2, and this could be followed by a carbon-concentrating mechanism (CCM) in the cytoplasm and/or chloroplasts. The lines of evidence for such an epidermal CCM are discussed, including evidence for special 'transfer cells' in some but not all seagrass leaves in the tangential inner walls of the epidermal cells. It is concluded that seagrasses have a CCM but that the case for concentration of CO2 at the site of Rubisco carboxylation is not proven.

Increased salt tolerance with overexpression of cation/proton antiporter 1 genes: a meta-analysis

Cation/proton antiporter 1 (CPA1) genes encode cellular Na+ /H+ exchanger proteins, which act to adjust ionic balance. Overexpression of CPA1s can improve plant performance under salt stress. However, the diversified roles of the CPA1 family and the various parameters used in evaluating transgenic plants over-expressing CPA1s make it challenging to assess the complex functions of CPA1s and their physiological mechanisms in salt tolerance. Using meta-analysis, we determined how overexpression of CPA1s has influenced several plant characteristics involved in response and resilience to NaCl stress. We also evaluated experimental variables that favour or reduce CPA1 effects in transgenic plants. Viewed across studies, overexpression of CPA1s has increased the magnitude of 10 of the 19 plant characteristics examined, by 25% or more. Among the ten moderating variables, several had substantial impacts on the extent of CPA1 influence: type of culture media, donor and recipient type and genus, and gene family. Genes from monocotyledonous plants stimulated root K+ , root K+ /Na+ , total chlorophyll, total dry weight and root length much more than genes from dicotyledonous species. Genes transformed to or from Arabidopsis have led to smaller CPA1-induced increases in plant characteristics than genes transferred to or from other genera. Heterogeneous expression of CPA1s led to greater increases in leaf chlorophyll and root length than homologous expression. These findings should help guide future investigations into the function of CPA1s in plant salt tolerance and the use of genetic engineering for breeding of resistance.

Variation in tissue Na(+) content and the activity of SOS1 genes among two species and two related genera of Chrysanthemum

BACKGROUND

Chrysanthemum, a leading ornamental species, does not tolerate salinity stress, although some of its related species do. The current level of understanding regarding the mechanisms underlying salinity tolerance in this botanical group is still limited.

RESULTS

A comparison of the physiological responses to salinity stress was made between Chrysanthemum morifolium 'Jinba' and its more tolerant relatives Crossostephium chinense, Artemisia japonica and Chrysanthemum crassum. The stress induced a higher accumulation of Na(+) and more reduction of K(+) in C. morifolium than in C. chinense, C. crassum and A. japonica, which also showed higher K(+)/Na(+) ratio. Homologs of an Na(+)/H(+) antiporter (SOS1) were isolated from each species. The gene carried by the tolerant plants were more strongly induced by salt stress than those carried by the non-tolerant ones. When expressed heterologously, they also conferred a greater degree of tolerance to a yeast mutant lacking Na(+)-pumping ATPase and plasma membrane Na(+)/H(+) antiporter activity. The data suggested that the products of AjSOS1, CrcSOS1 and CcSOS1 functioned more effectively as Na (+) excluders than those of CmSOS1. Over expression of four SOS1s improves the salinity tolerance of transgenic plants and the overexpressing plants of SOS1s from salt tolerant plants were more tolerant than that from salt sensitive plants. In addition, the importance of certain AjSOS1 residues for effective ion transport activity and salinity tolerance was established by site-directed mutagenesis and heterologous expression in yeast.

CONCLUSIONS

AjSOS1, CrcSOS1 and CcSOS1 have potential as transgenes for enhancing salinity tolerance. Some of the mutations identified here may offer opportunities to better understand the mechanistic basis of salinity tolerance in the chrysanthemum complex.

Measurement of Differential Na(+) Efflux from Apical and Bulk Root Zones of Intact Barley and Arabidopsis Plants

Rapid sodium cycling across the plasma membrane of root cells is widely thought to be associated with Na(+) toxicity in plants. However, the efflux component of this cycling is not well understood. Efflux of Na(+) from root cells is believed to be mediated by Salt Overly-Sensitive-1, although expression of this Na(+)/H(+) antiporter has been localized to the vascular tissue and root meristem. Here, we used a chambered cuvette system in which the distal root of intact salinized barley and Arabidopsis thaliana plants (wild-type and sos1) were isolated from the bulk of the root by a silicone-acrylic barrier, so that we could compare patterns of (24)Na(+) efflux in these two regions of root. In barley, steady-state release of (24)Na(+) was about four times higher from the distal root than from the bulk roots. In the distal root, (24)Na(+) release was pronouncedly decreased by elevated pH (9.2), while the bulk-root release was not significantly affected. In A. thaliana, tracer efflux was about three times higher from the wild-type distal root than from the wild-type bulk root and also three to four times higher than both distal- and bulk-root fluxes of Atsos1 mutants. Elevated pH also greatly reduced the efflux from wild-type roots. These findings support a significant role of SOS1-mediated Na(+) efflux in the distal root, but not in the bulk root.

Sodium efflux in plant roots: what do we really know?

The efflux of sodium (Na(+)) ions across the plasma membrane of plant root cells into the external medium is surprisingly poorly understood. Nevertheless, Na(+) efflux is widely regarded as a major mechanism by which plants restrain the rise of Na(+) concentrations in the cytosolic compartments of root cells and, thus, achieve a degree of tolerance to saline environments. In this review, several key ideas and bodies of evidence concerning root Na(+) efflux are summarized with a critical eye. Findings from decades past are brought to bear on current thinking, and pivotal studies are discussed, both "purely physiological", and also with regard to the SOS1 protein, the only major Na(+) efflux transporter that has, to date, been genetically characterized. We find that the current model of rapid transmembrane sodium cycling (RTSC), across the plasma membrane of root cells, is not adequately supported by evidence from the majority of efflux studies. An alternative hypothesis cannot be ruled out, that most Na(+) tracer efflux from the root in the salinity range does not proceed across the plasma membrane, but through the apoplast. Support for this idea comes from studies showing that Na(+) efflux, when measured with tracers, is rarely affected by the presence of inhibitors or the ionic composition in saline rooting media. We conclude that the actual efflux of Na(+) across the plasma membrane of root cells may be much more modest than what is often reported in studies using tracers, and may predominantly occur in the root tips, where SOS1 expression has been localized.

Molecular cloning and bioinformatics analysis of a new plasma membrane Na⁺/H⁺ antiporter gene from the halophyte Kosteletzkya virginica

A new plasma membrane Na⁺/H⁺ antiporter gene (named as KvSOS1) was cloned from the halophyte Kosteletzkya virginica by reverse-transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE) technology, which is a homologue of SOS1 (salt overly sensitive 1). The full-length cDNA is 3850 bp and contains an open reading frame (ORF) encoding a protein of 1147 amino acids with a molecular weight of 127.56 kDa and a theoretical pI of 6.18. Bioinformatics analysis indicated that the deduced protein appears to be a transmembrane protein with 12 transmembrane domains at the N-terminal region and a long hydrophilic tail in cytoplasm at its C-terminal region and shares 72–82% identity at the peptide level with other plant plasma membrane Na⁺/H⁺ antiporters.

Functional analyses of a putative plasma membrane Na+/H+ antiporter gene isolated from salt tolerant Helianthus tuberosus

Jerusalem artichokes (Helianthus tuberosus L.) can tolerate relatively higher salinity, drought and heat stress. In this paper, we report the cloning of a Salt Overly Sensitive 1 (SOS1) gene encoding a plasma membrane Na(+)/H(+) antiporter from a highly salt-tolerant genotype of H. tuberosus, NY1, named HtSOS1 and characterization of its function in yeast and rice. The amino acid sequence of HtSOS1 showed 83.4% identity with the previously isolated SOS1 gene from the Chrysanthemum crassum. The mRNA level in the leaves of H. tuberosus was significantly up-regulated by presence of high concentrations of NaCl. Localization analysis using rice protoplast expression showed that the protein encoded by HtSOS1 was located in the plasma membrane. HtSOS1 partially suppressed the salt sensitive phenotypes of a salt sensitive yeast strain. In comparison with wild type (Oryza sativa L., ssp. Japonica. cv. Nipponbare), the transgenic rice expressed with HtSOS1 could exclude more Na(+) and accumulate more K(+). Expression of HtSOS1 decreased Na(+) content much larger in the shoot than in the roots, resulting in more water content in the transgenic rice than WT. These data suggested that HtSOS1 may be useful in transgenic approaches to improving the salinity tolerance of glycophyte.

Different evolutionary histories of two cation/proton exchanger gene families in plants

BACKGROUND

Gene duplication events have been proposed to be involved in the adaptation of plants to stress conditions; precisely how is unclear. To address this question, we studied the evolution of two families of antiporters. Cation/proton exchangers are important for normal cell function and in plants, Na+,K+/H+ antiporters have also been implicated in salt tolerance. Two well-known plant cation/proton antiporters are NHX1 and SOS1, which perform Na+ and K+ compartmentalization into the vacuole and Na+ efflux from the cell, respectively. However, our knowledge about the evolution of NHX and SOS1 stress responsive gene families is still limited.

RESULTS

In this study we performed a comprehensive molecular evolutionary analysis of the NHX and SOS1 families. Using available sequences from a total of 33 plant species, we estimated gene family phylogenies and gene duplication histories, as well as examined heterogeneous selection pressure on amino acid sites. Our results show that, while the NHX family expanded and specialized, the SOS1 family remained a low copy gene family that appears to have undergone neofunctionalization during its evolutionary history. Additionally, we found that both families are under purifying selection although SOS1 is less constrained.

CONCLUSIONS

We propose that the different evolution histories are related with the proteins' function and localization, and that the NHX and SOS1 families are examples of two different evolutionary paths through which duplication events may result in adaptive evolution of stress tolerance.

Selective transport capacity for K+ over Na+ is linked to the expression levels of PtSOS1 in halophyte Puccinellia tenuiflora

The plasma membrane Na+/H+ antiporter (SOS1) was shown to be a Na+ efflux protein and also involved in K+ uptake and transport. PtSOS1 was characterised from Puccinellia tenuiflora (Griseb.) Scribn. et Merr., a monocotyledonous halophyte that has a high selectivity for K+ over Na+ by roots under salt stress. To assess the contribution of PtSOS1 to the selectivity for K+ over Na+, the expression levels of PtSOS1 and Na+, K+ accumulations in P. tenuiflora exposed to different concentrations of NaCl, KCl or NaCl plus KCl were analysed. Results showed that the expression levels of PtSOS1 in roots increased significantly with the increase of external NaCl (25-150mM), accompanied by an increase of selective transport (ST) capacity for K+ over Na+ by roots. Transcription levels of PtSOS1 in roots and ST values increased under 0.1-1mM KCl, then declined sharply under 5-10mM KCl. Under 150mM NaCl, PtSOS1 expression levels in roots and ST values at 0.1mM KCl was significantly lower than that at 5mM KCl with the prolonging of treatment time. A significant positive correlation was found between root PtSOS1 expression levels and ST values under various concentrations of NaCl, KCl or 150mM NaCl plus 0.1 or 5mM KCl treatments. Therefore, it is proposed that PtSOS1 is the major component of selective transport capacity for K+ over Na+ and hence, salt tolerance of P. tenuiflora. Finally, we hypothesise a function model of SOS1 in regulating K+ and Na+ transport system in the membrane of xylem parenchyma cells by sustaining the membrane integrity; it also appears that this model could reasonably explain the phenomenon of Na+ retrieval from the xylem when plants are exposed to severe salt stress.

The SbSOS1 gene from the extreme halophyte Salicornia brachiata enhances Na(+) loading in xylem and confers salt tolerance in transgenic tobacco

BACKGROUND

Soil salinity adversely affects plant growth and development and disturbs intracellular ion homeostasis resulting cellular toxicity. The Salt Overly Sensitive 1 (SOS1) gene encodes a plasma membrane Na(+)/H(+) antiporter that plays an important role in imparting salt stress tolerance to plants. Here, we report the cloning and characterisation of the SbSOS1 gene from Salicornia brachiata, an extreme halophyte.

RESULTS

The SbSOS1 gene is 3774 bp long and encodes a protein of 1159 amino acids. SbSOS1 exhibited a greater level of constitutive expression in roots than in shoots and was further increased by salt stress. Overexpressing the S. brachiata SbSOS1 gene in tobacco conferred high salt tolerance, promoted seed germination and increased root length, shoot length, leaf area, fresh weight, dry weight, relative water content (RWC), chlorophyll, K(+)/Na(+) ratio, membrane stability index, soluble sugar, proline and amino acid content relative to wild type (WT) plants. Transgenic plants exhibited reductions in electrolyte leakage, reactive oxygen species (ROS) and MDA content in response to salt stress, which probably occurred because of reduced cytosolic Na(+) content and oxidative damage. At higher salt stress, transgenic tobacco plants exhibited reduced Na(+) content in root and leaf and higher concentrations in stem and xylem sap relative to WT, which suggests a role of SbSOS1 in Na(+) loading to xylem from root and leaf tissues. Transgenic lines also showed increased K(+) and Ca(2+) content in root tissue compared to WT, which reflect that SbSOS1 indirectly affects the other transporters activity.

CONCLUSIONS

Overexpression of SbSOS1 in tobacco conferred a high degree of salt tolerance, enhanced plant growth and altered physiological and biochemical parameters in response to salt stress. In addition to Na(+) efflux outside the plasma membrane, SbSOS1 also helps to maintain variable Na(+) content in different organs and also affect the other transporters activity indirectly. These results broaden the role of SbSOS1 in planta and suggest that this gene could be used to develop salt-tolerant transgenic crops.

HAK transporters from Physcomitrella patens and Yarrowia lipolytica mediate sodium uptake

The widespread presence of Na(+)-specific uptake systems across plants and fungi is a controversial topic. In this study, we identify two HAK genes, one in the moss Physcomitrella patens and the other in the yeast Yarrowia lipolytica, that encode Na(+)-specific transporters. Because HAK genes are numerous in plants and are duplicated in many fungi, our findings suggest that some HAK genes encode Na(+) transporters and that Na(+) might play physiological roles in plants and fungi more extensively than is currently thought.

Nitric oxide enhances salt secretion and Na(+) sequestration in a mangrove plant, Avicennia marina, through increasing the expression of H(+)-ATPase and Na(+)/H(+) antiporter under high salinity

Modulation of nitric oxide (NO) on ion homeostasis, by enhancing salt secretion in the salt glands and Na(+) sequestration into the vacuoles, was investigated in a salt-secreting mangrove tree, Avicennia marina (Forsk.) Vierh. The major results are as follows: (i) under 400 mM NaCl treatment, the application of 100 µM sodium nitroprusside (SNP), an NO donor, significantly increased the density of salt crystals and salt secretion rate of the leaves, along with maintaining a low Na(+) to K(+) ratio in the leaves. (ii) The measurement of element contents by X-ray microanalysis in the epidermis and transversal sections of A. marina leaves revealed that SNP (100 µM) significantly increased the accumulation of Na(+) in the epidermis and hypodermal cells, particularly the Na(+) to K(+) ratio in the salt glands, but no such effects were observed in the mesophyll cells. (iii) Using non-invasive micro-test technology (NMT), both long-term SNP (100 µM) and transient SNP (30 µM) treatments significantly increased net Na(+) efflux in the salt glands. On the contrary, NO synthesis inhibitors and scavenger reversed the effects of NO on Na(+) flux. These results indicate that NO enhanced salt secretion by increasing net Na(+) efflux in the salt glands. (iv) Western blot analysis demonstrated that 100 µM SNP stimulated protein expressions of plasma membrane (PM) H(+)-ATPase and vacuolar membrane Na(+)/H(+) antiporter. (v) To further clarify the molecular mechanism of the effects of NO on enhancing salt secretion and Na(+) sequestration, partial cDNA fragments of PM H(+)-ATPase (HA1), PM Na(+)/H(+) antiporter (SOS1) and vacuolar Na(+)/H(+) antiporter (NHX1) were isolated and transcriptional expression of HA1, SOS1, NHX1 and vacuolar H(+)-ATPase subunit c (VHA-c1) genes were analyzed using real-time quantitative polymerase chain reaction. The relative transcript abundance of the four genes were markedly increased in 100 µM SNP-treated A. marina. Moreover, the increase was reversed by NO synthesis inhibitors and scavenger. Taken together, our results strongly suggest that NO functions as a signal in salt resistance of A. marina by enhancing salt secretion and Na(+) sequestration, which depend on the increased expression of the H(+)-ATPase and Na(+)/H(+) antiporter.

The SOS1 transporter of Physcomitrella patens mediates sodium efflux in planta

• SOS1 is an Na(+)/H(+) antiporter that plays a central role in Na(+) tolerance in land plants. SOS1 mediation of Na(+) efflux has been studied in plasma-membrane vesicles and deduced from the SOS1 suppression of the Na(+) sensitivity of yeast mutants defective in Na(+) -efflux. However, SOS1-mediated Na(+) efflux has not been characterized in either plant or yeast cells. Here, we use Physcomitrella patens to investigate the function of SOS1 in planta. • In P. patens, a nonvascular plant in which the study of ion cellular fluxes is technically simple, the existence of two SOS1 genes suggests that the Na(+) efflux remaining after the deletion of the ENA1 ATPase is mediated by a SOS1 system. Therefore, we cloned the P. patens SOS1 and SOS1B genes (PpSOS1 and PpSOS1B, respectively) and complementary DNAs, and constructed the PpΔsos1 and PpΔena1/PpΔsos1 deletion lines by gene targeting. • Comparison of wild-type, and PpΔsos1 and PpΔena1/PpΔsos1 mutant lines revealed that PpSOS1 is crucial for Na(+) efflux and that the PpΔsos1 line, and especially the PpΔena1/PpΔsos1 lines, showed excessive Na(+) accumulation and Na(+)-triggered cell death. The PpΔsos1 and PpΔena1/PpΔsos1 lines showed impaired high-affinity K(+) uptake. • Our data support the hypothesis that PpSOS1 mediates cellular Na(+) efflux and that PpSOS1 enhances K(+) uptake by an indirect effect.

Na(+)/H(+) antiporter activity of the SOS1 gene: lifetime imaging analysis and electrophysiological studies on Arabidopsis seedlings

Based on sequence analysis, the salt overly sensitive (SOS1) gene has been suggested to function as a Na(+)/H(+) antiporter located at the plasma membrane of plant cells, being expressed mostly in the meristem zone of the root and in the parenchyma cells surrounding the vascular tissue of the stem. In this study, we compared net H(+) and Ca(2+) fluxes and intracellular pH and [Ca(2+)](cyt) in the root meristem zone of Arabidopsis wild-type (WT) and sos mutants before and after salt stress. In addition, we studied the effect of pretreatment with amiloride (an inhibitor of Na(+)/H(+) antiporters) on net ion fluxes, intracellular pH and intracellular Ca(2+) activity ([Ca(2+)](cyt)) in WT plants and sos1 mutants before and after salt stress. Net ion fluxes were measured using microelectrode ion flux estimation (MIFE) and intracellular pH and [Ca(2+)](cyt) using fluorescence lifetime imaging microscopy (FLIM) techniques. During the first 15 min after NaCl application, sos1 mutants showed net H(+) efflux and intracellular alkalinization in the meristem zone, whereas sos2 and sos3 mutants and WT showed net H(+) influx and slight intracellular acidification in the meristem zone. Treatment with amiloride led to intracellular acidification and lower net H(+) flux in WT plants and to a decrease in intracellular Ca(2+) in WT and sos1 plants. WT plants pretreated with amiloride did not show positive net H(+) flux and intracellular acidification. After NaCl application, internal pH shifted to higher values in WT and sos1 plants. However, absolute values of H(+) fluxes were higher and internal pH values were lower in WT plants pretreated with amiloride compared with sos1 mutants. Therefore, the SOS1 transporter is involved in H(+) influx into the meristem zone of Arabidopsis roots, or it may function as a Na(+)/H(+) antiporter. Amiloride affects SOS1 and other Na(+)/H(+) antiporters in plant cells because of its ability to decrease the H(+) gradient across the plasma membrane.

The plasma membrane Na+/H+ antiporter SOS1 is essential for salt tolerance in tomato and affects the partitioning of Na+ between plant organs

We have identified a plasma membrane Na(+)/H(+) antiporter gene from tomato (Solanum lycopersicum), SlSOS1, and used heterologous expression in yeast to confirm that SlSOS1 was the functional homolog of AtSOS1. Using post-transcriptional gene silencing, we evaluated the role played by SlSOS1 in long-distance Na(+) transport and salt tolerance of tomato. Tomato was used because of its anatomical structure, more complex than that of Arabidopsis, and its agricultural significance. Transgenic tomato plants with reduced expression of SlSOS1 exhibited reduced growth rate compared to wild-type (WT) plants in saline conditions. This sensitivity correlated with higher accumulation of Na(+) in leaves and roots, but lower contents in stems of silenced plants under salt stress. Differential distribution of Na(+) and lower net Na(+) flux were observed in the xylem sap in the suppressed plants. In addition, K(+) concentration was lower in roots of silenced plants than in WT. Our results demonstrate that SlSOS1 antiporter is not only essential in maintaining ion homeostasis under salinity, but also critical for the partitioning of Na(+) between plant organs. The ability of tomato plants to retain Na(+) in the stems, thus preventing Na(+) from reaching the photosynthetic tissues, is largely dependent on the function of SlSOS1.

Isolation and characterization of plasma membrane Na(+)/H(+) antiporter genes from salt-sensitive and salt-tolerant reed plants

We isolated cDNAs for Na(+)/H(+) antiporter genes (PhaNHA1s) from salt-sensitive and salt-tolerant reed plants. A phylogenetic analysis and localization analysis using yeast strains expressing PhaNHA1-GFP protein showed that PhaNHA1s were plasma membrane Na(+)/H(+) antiporters. Yeast strains expressing PhaNHA1 from salt-tolerant reed plants (PhaNHA1-n) grew well than yeast strains expressing PhaNHA1 from salt-sensitive reed plants (PhaNHA1-u) in the presence of 100mM NaCl. Furthermore, Na(+) contents of yeast cells expressing PhaNHA1-n were less than half of those of yeast cells expressing PhaNHA1-u. These results suggest that PhaNHA1-n is more efficient at excluding Na(+) from the cells than PhaNHA1-u.