Nitric oxide synthase-dependent nitric oxide production is associated with salt tolerance in Arabidopsis

Nitric oxide (NO) has emerged as a key molecule involved in many physiological processes in plants. To characterize roles of NO in tolerance of Arabidopsis (Arabidopsis thaliana) to salt stress, effect of NaCl on Arabidopsis wild-type and mutant (Atnoa1) plants with an impaired in vivo NO synthase (NOS) activity and a reduced endogenous NO level was investigated. Atnoa1 mutant plants displayed a greater Na+ to K+ ratio in shoots than wild-type plants due to enhanced accumulation of Na+ and reduced accumulation of K+ when exposed to NaCl. Germination of Atnoa1 seeds was more sensitive to NaCl than that of wild-type seeds, and wild-type plants exhibited higher survival rates than Atnoa1 plants when grown under salt stress. Atnoa1 plants had higher levels of hydrogen peroxide than wild-type plants under both control and salt stress, suggesting that Atnoa1 is more vulnerable to salt and oxidative stress than wild-type plants. Treatments of wild-type plants with NOS inhibitor and NO scavenger reduced endogenous NO levels and enhanced NaCl-induced increase in Na+ to K+ ratio. Exposure of wild-type plants to NaCl inhibited NOS activity and reduced quantity of NOA1 protein, leading to a decrease in endogenous NO levels measured by NO-specific fluorescent probe. Treatment of Atnoa1 plants with NO donor sodium nitroprusside attenuated the NaCl-induced increase in Na+ to K+ ratio. Therefore, these findings provide direct evidence to support that disruption of NOS-dependent NO production is associated with salt tolerance in Arabidopsis.     

Yeast plasma membrane Ena1p ATPase alters alkali-cation homeostasis and confers increased salt tolerance in tobacco cultured cells

In plants, the plasma membrane Na(+)/H(+) antiporter is the only key enzyme that extrudes cytosolic Na(+) and contributes to salt tolerance. But in fungi, the plasma membrane Na(+)/H(+) antiporter and Na(+)-ATPase are known to be key enzymes for salt tolerance. Saccharomyces cerevisiae Ena1p ATPase encoded by the ENA1/PMR2A gene is primarily responsible for Na(+) and Li(+) efflux across the plasma membrane during salt stress and for K(+) efflux at high pH and high K(+). To test if the yeast ATPase would improve salt tolerance in plants, we expressed a triple hemagglutinin (HA)-tagged Ena1p (Ena1p-3HA) in cultured tobacco (Nicotiana tabacum L.) cv Bright Yellow 2 (BY2) cells. The Ena1p-3HA proteins were correctly localized to the plasma membrane of transgenic BY2 cells and conferred increased NaCl and LiCl tolerance to the cells. Under moderate salt stress conditions, the Ena1p-3HA-expressing BY2 clones accumulated lower levels of Na(+) and Li(+) than nonexpressing BY2 clones. Moreover, the Ena1p-3HA expressing BY2 clones accumulated lower levels of K(+) than nonexpressing cells under no-stress conditions. These results suggest that the yeast Ena1p can also function as an alkali-cation (Na(+), Li(+), and K(+)) ATPase and alter alkali-cation homeostasis in plant cells. We conclude that, even with K(+)-ATPase activity, Na(+)-ATPase activity of the yeast Ena1p confers increased salt tolerance to plant cells during salt stress.     

Compatible solute accumulation and stress-mitigating effects in barley genotypes contrasting in their salt tolerance

The accumulation of compatible solutes is often regarded as a basic strategy for the protection and survival of plants under abiotic stress conditions, including both salinity and oxidative stress. In this work, a possible causal link between the ability of contrasting barley genotypes to accumulate/synthesize compatible solutes and their salinity stress tolerance was investigated. The impact of H(2)O(2) (one of the components of salt stress) on K(+) flux (a measure of stress 'severity') and the mitigating effects of glycine betaine and proline on NaCl-induced K(+) efflux were found to be significantly higher in salt-sensitive barley genotypes. At the same time, a 2-fold higher accumulation of leaf and root proline and leaf glycine betaine was found in salt-sensitive cultivars. The total amino acid content was also less affected by salinity in salt-tolerant cultivars. In these, potassium was found to be the main contributor to cytoplasmic osmolality, while in salt-sensitive genotypes, glycine betaine and proline contributed substantially to cell osmolality, compensating for reduced cytosolic K(+). Significant negative correlations (r= -0.89 and -0.94) were observed between Na(+)-induced K(+) efflux (an indicator of salt tolerance) and leaf glycine betaine and proline. These results indicate that hyperaccumulation of known major compatible solutes in barley does not appear to play a major role in salt-tolerance, but rather, may be a symptom of salt-susceptibility.     

Expression of wheat Na+/H+ antiporter TNHXS1 and H+- pyrophosphatase TVP1 genes in tobacco from a bicistronic transcriptional unit improves salt tolerance

Abiotic stress tolerance of plants is a very complex trait and involves multiple physiological and biochemical processes. Thus, the improvement of plant stress tolerance should involve pyramiding of multiple genes. In the present study, we report the construction and application of a bicistronic system, involving the internal ribosome entry site (IRES) sequence from the 5'UTR of the heat-shock protein of tobacco gene NtHSF-1, to the improvement of salt tolerance in transgenic tobacco plants. Two genes from wheat encoding two important vacuolar ion transporters, Na(+)/H(+) antiporter (TNHXS1) and H(+)-pyrophosphatase (TVP1), were linked via IRES to generate the bicistronic construct TNHXS1-IRES-TVP1. Molecular analysis of transgenic tobacco plants revealed the correct integration of the TNHXS1-IRES-TVP1construct into tobacco genome and the production of the full-length bicistronic mRNA from the 35S promoter. Ion transport analyses with tonoplast vesicles isolated from transgenic lines confirmed that single-transgenic lines TVP1cl19 and TNHXS1cl7 had greater H(+)-PPiase and Na(+)/H(+) antiport activity, respectively, than the WT. Interestingly, the co-expression of TVP1 and TNHXS1 increased both Na(+)/H(+) antiport and H(+)-PPiase activities and induced the H(+) pumping activity of the endogenous V-ATPase. Transgenic tobacco plants expressing TNHXS1-IRES-TVP1 showed a better performance than either of the single gene-transformed lines and the wild type plants when subjected to salt treatment. In addition, the TNHXS1-IRES-TVP1 transgenic plants accumulated less Na(+) and more K(+) in their leaf tissue than did the wild type and the single gene-transformed lines. These results demonstrate that IRES system, described herein, can co-ordinate the expression of two important abiotic stress-tolerance genes and that this expression system is a valuable tool for obtaining transgenic plants with improved salt tolerance.     

Molecular insights into the structure and function of plant K(+) transport mechanisms

Our understanding of plant potassium transport has increased in the past decade through the application of molecular biological techniques. In this review, recent work on inward and outward rectifying K(+) channels as well as high affinity K(+) transporters is described. Through the work on inward rectifying K(+) channels, we now have precise details on how the structure of these proteins determines functional characteristics such as ion conduction, pH sensitivity, selectivity and voltage sensing. The physiological function of inward rectifying K(+) channels in plants has been clarified through the analysis of expression patterns and mutational analysis. Two classes of outward rectifying K(+) channels have now been cloned from plants and their initial characterisation is reviewed. The physiological role of one class of outward rectifying K(+) channel has been demonstrated to be involved in long distance transport of K(+) from roots to shoots. The molecular structure and function of two classes of energised K(+) transporters are also reviewed. The first class is energised by Na(+) and shares structural similarities with K(+) transport mechanisms in bacteria and fungi. Structure-function studies suggest that it should be possible to increase the K(+) and Na(+) selectivity of these transporters, which will enhance the salt tolerance of higher plants. The second class of K(+) transporter is comprised of a large gene family and appears to have a dual affinity for K(+). A suite of molecular techniques, including gene cloning, oocyte expression, RNA localisation and gene inactivation, is now being used to fully characterise the biophysical and physiological function of plants K(+) transport mechanisms.     

Genetic behaviour of physiological traits conferring cytosolic K+/Na+ homeostasis in wheat

A plant's ability to maintain an optimal cytosolic K(+)/Na(+) ratio has long been cited as a key feature of salinity tolerance. As traditional whole-leaf nutrient analysis does not account for tissue and organelle-specific ion sequestration, the predictive value of this index at the whole-plant level is not always satisfactory. Consequently, suitable in situ methods for functionally assessing the activity of the key membrane transporters contributing to this trait at the cellular level need to be developed. The aim of this work was to investigate the extent to which plasma membrane transporter-mediated Na(+) exclusion and KOR-mediated K(+) retention traits, measured with the microelectrode ion flux measuring (MIFE) technique, are inheritable in wheat, and whether the MIFE technique has the potential to be used in combination with molecular markers to determine QTLs for these transporter proteins. Experiments involved two bread (Triticum aestivum) and two durum (Triticum turgidum) wheat lines contrasting in their salinity tolerance. Net Na(+), K(+) and H(+) fluxes were measured from 6-day-old roots of parental lines and their F(1) hybrids upon addition and removal of NaCl. These results were complemented by assessment of whole-plant physiological and agronomic characteristics. We show evidence for a strong heritability of plasma membrane transporter-mediated Na(+) exclusion and K(+) retention traits in wheat at the cellular level. This opens the prospect of using the MIFE technique to map the position of these transporters on particular loci of wheat chromosomes. The next obvious step would be to pyramid these traits in one ideotype with superior salinity tolerance.     

The Wheat cDNA LCT1 Generates Hypersensitivity to Sodium in a Salt-Sensitive Yeast Strain

Salinity affects large areas of agricultural land, and all major crop species are intolerant to high levels of sodium ions. The principal route for Na(+) uptake into plant cells remains to be identified. Non-selective ion channels and high-affinity potassium transporters have emerged as potential pathways for Na(+) entry. A third candidate for Na(+) transport into plant cells is a low-affinity cation transporter represented by the wheat protein LCT1, which is known to be permeable for a wide range of cations when expressed in yeast (Saccharomyces cerevisiae). To investigate the role of LCT1 in salt tolerance we have used the yeast strain G19, which is disrupted in the genes encoding Na(+) export pumps and as a result displays salt sensitivity comparable with wheat. After transformation with LCT1, G19 cells became hypersensitive to NaCl. We show that LCT1 expression results in a strong decrease of intracellular K(+)/Na(+) ratio in G19 cells due to the combined effect of enhanced Na(+) accumulation and loss of intracellular K(+). Na(+) uptake through LCT1 was inhibited by K(+) and Ca(2+) at high concentrations and the addition of these ions rescued growth of LCT1-transformed G19 on saline medium. LCT1 was also shown to mediate the uptake of Li(+) and Cs(+). Expression of two mutant LCT1 cDNAs with N-terminal truncations resulted in decreased Ca(2+) uptake and increased Na(+) tolerance compared with expression of the full-length LCT1. Our findings strongly suggest that LCT1 represents a molecular link between Ca(2+) and Na(+) uptake into plant cells.     

Cloning of an H+-PPase gene from Thellungiella halophila and its heterologous expression to improve tobacco salt tolerance

An H(+)-pyrophosphatase (PPase) gene named TsVP involved in basic biochemical and physiological mechanisms was cloned from Thellungiella halophila. The deduced translation product has similar characteristics to H(+)-PPases from other species, such as Arabidopsis and rice, in terms of bioinformation. The heterologous expression of TsVP in the yeast mutant ena1 suppressed Na(+) hypersensitivity and demonstrated the function of TsVP as an H(+)-PPase. Transgenic tobacco overexpressing TsVP had 60% greater dry weight than wild-type tobacco at 300 mM NaCl and higher viability of mesophyll protoplasts under salt shock stress conditions. TsVP and AVP1, another H(+)-PPase from Arabidopsis, were heterologously expressed separately in both the yeast mutant ena1 and tobacco. The salt tolerance of TsVP or AVP1 yeast transformants and transgenic tobacco were improved to almost the same level. The TsVP transgenic tobacco lines TL3 and TL5 with the highest H(+)-PPase hydrolytic activity were studied further. These transgenic tobacco plants accumulated 25% more solutes than wild-type plants without NaCl stress and 20-32% more Na(+) under salt stress conditions. Although transgenic tobacco lines TL3 and TL5 accumulated more Na(+) in leaf tissues, the malondialdehyde content and cell membrane damage were less than those of the wild type under salt stress conditions. Presumably, compartmentalization of Na(+) in vacuoles reduces its toxic effects on plant cells. This result supports the hypothesis that overexpression of H(+)-PPase causes the accumulation of Na(+) in vacuoles instead of in the cytoplasm and avoids the toxicity of excessive Na(+) in plant cells.     

The yeast HAL1 gene improves salt tolerance of transgenic tomato

Overexpression of the HAL1 gene in yeast has a positive effect on salt tolerance by maintaining a high internal K(+) concentration and decreasing intracellular Na(+) during salt stress. In the present work, the yeast gene HAL1 was introduced into tomato (Lycopersicon esculentum Mill.) by Agrobacterium tumefaciens-mediated transformation. A sample of primary transformants was self-pollinated, and progeny from both transformed and non-transformed plants (controls) were evaluated for salt tolerance in vitro and in vivo. Results from different tests indicated a higher level of salt tolerance in the progeny of two different transgenic plants bearing four copies or one copy of the HAL1 gene. In addition, measurement of the intracellular K(+) to Na(+) ratios showed that transgenic lines were able to retain more K(+) than the control under salt stress. Although plants and yeast cannot be compared in an absolute sense, these results indicate that the mechanism controlling the positive effect of the HAL1 gene on salt tolerance may be similar in transgenic plants and yeast.     

Identification of two loci in tomato reveals distinct mechanisms for salt tolerance

Salt stress is one of the most serious environmental factors limiting the productivity of crop plants. To understand the molecular basis for salt responses, we used mutagenesis to identify plant genes required for salt tolerance in tomato. As a result, three tomato salt-hypersensitive (tss) mutants were isolated. These mutants defined two loci and were caused by single recessive nuclear mutations. The tss1 mutant is specifically hypersensitive to growth inhibition by Na(+) or Li(+) and is not hypersensitive to general osmotic stress. The tss2 mutant is hypersensitive to growth inhibition by Na(+) or Li(+) but, in contrast to tss1, is also hypersensitive to general osmotic stress. The TSS1 locus is necessary for K(+) nutrition because tss1 mutants are unable to grow on a culture medium containing low concentrations of K(+). Increased Ca(2)+ in the culture medium suppresses the growth defect of tss1 on low K(+). Measurements of membrane potential in apical root cells were made with an intracellular microelectrode to assess the permeability of the membrane to K(+) and Na(+). K(+)-dependent membrane potential measurements indicate impaired K(+) uptake in tss1 but not tss2, whereas no differences in Na(+) uptake were found. The TSS2 locus may be a negative regulator of abscisic acid signaling, because tss2 is hypersensitive to growth inhibition by abscisic acid. Our results demonstrate that the TSS1 locus is essential for K(+) nutrition and NaCl tolerance in tomato. Significantly, the isolation of the tss2 mutant demonstrates that abscisic acid signaling is also important for salt and osmotic tolerance in glycophytic plants.     

Wheat grain yield on saline soils is improved by an ancestral Na⁺ transporter gene

The ability of wheat to maintain a low sodium concentration ([Na(+)]) in leaves correlates with improved growth under saline conditions. This trait, termed Na(+) exclusion, contributes to the greater salt tolerance of bread wheat relative to durum wheat. To improve the salt tolerance of durum wheat, we explored natural diversity in shoot Na(+) exclusion within ancestral wheat germplasm. Previously, we showed that crossing of Nax2, a gene locus in the wheat relative Triticum monococcum into a commercial durum wheat (Triticum turgidum ssp. durum var. Tamaroi) reduced its leaf [Na(+)] (ref. 5). Here we show that a gene in the Nax2 locus, TmHKT1;5-A, encodes a Na(+)-selective transporter located on the plasma membrane of root cells surrounding xylem vessels, which is therefore ideally localized to withdraw Na(+) from the xylem and reduce transport of Na(+) to leaves. Field trials on saline soils demonstrate that the presence of TmHKT1;5-A significantly reduces leaf [Na(+)] and increases durum wheat grain yield by 25% compared to near-isogenic lines without the Nax2 locus.     

Soil bacteria confer plant salt tolerance by tissue-specific regulation of the sodium transporter HKT1

Elevated sodium (Na(+)) decreases plant growth and, thereby, agricultural productivity. The ion transporter high-affinity K(+) transporter (HKT)1 controls Na(+) import in roots, yet dysfunction or overexpression of HKT1 fails to increase salt tolerance, raising questions as to HKT1's role in regulating Na(+) homeostasis. Here, we report that tissue-specific regulation of HKT1 by the soil bacterium Bacillus subtilis GB03 confers salt tolerance in Arabidopsis thaliana. Under salt stress (100 mM NaCl), GB03 concurrently down- and upregulates HKT1 expression in roots and shoots, respectively, resulting in lower Na(+) accumulation throughout the plant compared with controls. Consistent with HKT1 participation in GB03-induced salt tolerance, GB03 fails to rescue salt-stressed athkt1 mutants from stunted foliar growth and elevated total Na(+) whereas salt-stressed Na(+) export mutants sos3 show GB03-induced salt tolerance with enhanced shoot and root growth as well as reduced total Na(+). These results demonstrate that tissue-specific regulation of HKT1 is critical for managing Na(+) homeostasis in salt-stressed plants, as well as underscore the breadth and sophistication of plant-microbe interactions.     

Extracellular Ca2+ ameliorates NaCl-induced K+ loss from Arabidopsis root and leaf cells by controlling plasma membrane K+ -permeable channels

Calcium can ameliorate Na+ toxicity in plants by decreasing Na+ influx through nonselective cation channels. Here, we show that elevated external [Ca2+] also inhibits Na+ -induced K+ efflux through outwardly directed, K+ -permeable channels. Noninvasive ion flux measuring and patch-clamp techniques were used to characterize K+ fluxes from Arabidopsis (Arabidopsis thaliana) root mature epidermis and leaf mesophyll under various Ca2+ to Na+ ratios. NaCl-induced K+ efflux was not related to the osmotic component of the salt stress, was inhibited by the K+ channel blocker TEA+, was not mediated by inwardly directed K+ channels (tested in the akt1 mutant), and resulted in a significant decrease in cytosolic K+ content. NaCl-induced K+ efflux was partially inhibited by 1 mm Ca2+ and fully prevented by 10 mm Ca2+. This ameliorative effect was at least partially attributed to a less dramatic NaCl-induced membrane depolarization under high Ca2+ conditions. Patch-clamp experiments (whole-cell mode) have demonstrated that two populations of Ca2+ -sensitive K+ efflux channels exist in protoplasts isolated from the mature epidermis of Arabidopsis root and leaf mesophyll cells. The instantaneously activating K+ efflux channels showed weak voltage dependence and insensitivity to external and internal Na+. Another population of K+ efflux channels was slowly activating, steeply rectifying, and highly sensitive to Na+. K+ efflux channels in roots and leaves showed different Ca2+ and Na+ sensitivities, suggesting that these organs may employ different strategies to withstand salinity. Our results suggest an additional mechanism of Ca2+ action on salt toxicity in plants: the amelioration of K+ loss from the cell by regulating (both directly and indirectly) K+ efflux channels.     

Reassessment of tissue Na(+) concentration as a criterion for salinity tolerance in bread wheat

Wheat is the most important crop grown on many of world's saline and sodic soils, and breeding for improved salinity tolerance (ST) is the only feasible way of improving yield and yield stability under these conditions. There are a number of possible mechanisms by which cereals can tolerate high levels of salinity, but these can be considered in terms of Na(+) exclusion and tissue tolerance. Na(+) exclusion has been the focus of much of the recent work in wheat, but with relatively little progress to date in developing high-yielding, salt-tolerant genotypes. Using a diverse collection of bread wheat germplasm, the present study was conducted to assess the value of tissue Na(+) concentration as a criterion for ST, and to determine whether ST differs with growth stage. Two experiments were conducted, the first with 38 genotypes and the second with 21 genotypes. A wide range of Na(+) concentrations within the roots and shoots as well as in ST were observed in both experiments. However, maintenance of growth and yield when grown with 100 mM NaCl was not correlated with the ability of a genotype to exclude Na(+) either from an individual leaf blade or from the whole shoot. The K(+) : Na(+) ratio also showed a wide range among the genotypes, but it did not explain the variation in ST among the genotypes. The results suggested that Na(+) exclusion and tissue tolerance varied independently, and there was no significant relationship between Na(+) exclusion and ST in bread wheat. Consequently, similar levels of ST may be achieved through different combinations of exclusion and tissue tolerance. Breeding for improved ST in bread wheat needs to select for traits related to both exclusion and tissue tolerance.     

Characterization of two HKT1 homologues from Eucalyptus camaldulensis that display intrinsic osmosensing capability

Plants have multiple potassium (K(+)) uptake and efflux mechanisms that are expressed throughout plant tissues to fulfill different physiological functions. Several different classes of K(+) channels and carriers have been identified at the molecular level in plants. K(+) transporters of the HKT1 superfamily have been cloned from wheat (Triticum aestivum), Arabidopsis, and Eucalyptus camaldulensis. The functional characteristics as well as the primary structure of these transporters are diverse with orthologues found in bacterial and fungal genomes. In this report, we provide a detailed characterization of the functional characteristics, as expressed in Xenopus laevis oocytes, of two cDNAs isolated from E. camaldulensis that encode proteins belonging to the HKT1 superfamily of K(+)/Na(+) transporters. The transport of K(+) in EcHKT-expressing oocytes is enhanced by Na(+), but K(+) was also transported in the absence of Na(+). Na(+) is transported in the absence of K(+) as has been demonstrated for HKT1 and AtHKT1. Overall, the E. camaldulensis transporters show some similarities and differences in ionic selectivity to HKT1 and AtHKT1. One striking difference between HKT1 and EcHKT is the sensitivity to changes in the external osmolarity of the solution. Hypotonic solutions increased EcHKT induced currents in oocytes by 100% as compared with no increased current in HKT1 expressing or uninjected oocytes. These osmotically sensitive currents were not enhanced by voltage and may mediate water flux. The physiological function of these osmotically induced increases in currents may be related to the ecological niches that E. camaldulensis inhabits, which are periodically flooded. Therefore, the osmosensing function of EcHKT may provide this species with a competitive advantage in maintaining K(+) homeostasis under certain conditions.     

Polyamines improve K+/Na+ homeostasis in barley seedlings by regulating root ion channel activities

Polyamines are known to increase in plant cells in response to a variety of stress conditions. However, the physiological roles of elevated polyamines are not understood well. Here we investigated the effects of polyamines on ion channel activities by applying patch-clamp techniques to protoplasts derived from barley (Hordeum vulgare) seedling root cells. Extracellular application of polyamines significantly blocked the inward Na(+) and K(+) currents (especially Na(+) currents) in root epidermal and cortical cells. These blocking effects of polyamines were increased with increasing polycation charge. In root xylem parenchyma, the inward K(+) currents were blocked by extracellular spermidine, while the outward K(+) currents were enhanced. At the whole-plant level, the root K(+) content, as well as the root and shoot Na(+) levels, was decreased significantly by exogenous spermidine. Together, by restricting Na(+) influx into roots and by preventing K(+) loss from shoots, polyamines were shown to improve K(+)/Na(+) homeostasis in barley seedlings. It is reasonable to propose that, therefore, elevated polyamines under salt stress should be a self-protecting response for plants to combat detrimental consequences resulted from imbalance of Na(+) and K(+).     

A comparison of hydroponic and soil-based screening methods to identify salt tolerance in the field in barley

Success in breeding crops for yield and other quantitative traits depends on the use of methods to evaluate genotypes accurately under field conditions. Although many screening criteria have been suggested to distinguish between genotypes for their salt tolerance under controlled environmental conditions, there is a need to test these criteria in the field. In this study, the salt tolerance, ion concentrations, and accumulation of compatible solutes of genotypes of barley with a range of putative salt tolerance were investigated using three growing conditions (hydroponics, soil in pots, and natural saline field). Initially, 60 genotypes of barley were screened for their salt tolerance and uptake of Na(+), Cl(-), and K(+) at 150 mM NaCl and, based on this, a subset of 15 genotypes was selected for testing in pots and in the field. Expression of salt tolerance in saline solution culture was not a reliable indicator of the differences in salt tolerance between barley plants that were evident in saline soil-based comparisons. Significant correlations were observed in the rankings of genotypes on the basis of their grain yield production at a moderately saline field site and their relative shoot growth in pots at EC(e) 7.2 [Spearman's rank correlation (rs)=0.79] and EC(e) 15.3 (rs=0.82) and the crucial parameter of leaf Na(+) (rs=0.72) and Cl(-) (rs=0.82) concentrations at EC(e) 7.2 dS m(-1). This work has established screening procedures that correlated well with grain yield at sites with moderate levels of soil salinity. This study also showed that both salt exclusion and osmotic tolerance are involved in salt tolerance and that the relative importance of these traits may differ with the severity of the salt stress. In soil, ion exclusion tended to be more important at low to moderate levels of stress but osmotic stress became more important at higher stress levels. Salt exclusion coupled with a synthesis of organic solutes were shown to be important components of salt tolerance in the tolerant genotypes and further field tests of these plants under stress conditions will help to verify their potential utility in crop-improvement programmes.     

Root plasma membrane transporters controlling K+/Na+ homeostasis in salt-stressed barley

Plant salinity tolerance is a polygenic trait with contributions from genetic, developmental, and physiological interactions, in addition to interactions between the plant and its environment. In this study, we show that in salt-tolerant genotypes of barley (Hordeum vulgare), multiple mechanisms are well combined to withstand saline conditions. These mechanisms include: (1) better control of membrane voltage so retaining a more negative membrane potential; (2) intrinsically higher H(+) pump activity; (3) better ability of root cells to pump Na(+) from the cytosol to the external medium; and (4) higher sensitivity to supplemental Ca(2+). At the same time, no significant difference was found between contrasting cultivars in their unidirectional (22)Na(+) influx or in the density and voltage dependence of depolarization-activated outward-rectifying K(+) channels. Overall, our results are consistent with the idea of the cytosolic K(+)-to-Na(+) ratio being a key determinant of plant salinity tolerance, and suggest multiple pathways of controlling that important feature in salt-tolerant plants.