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PubMed2001

Molecular characterization of functional domains in the protein kinase SOS2 that is required for plant salt tolerance.

The SOS3 (for SALT OVERLY SENSITIVE3) calcium binding protein and SOS2 protein kinase are required for sodium and potassium ion homeostasis and salt tolerance in Arabidopsis. We have shown previously that SOS3 interacts with and activates the SOS2 protein kinase. We report here the identification of a SOS3 binding motif in SOS2 that also serves as the kinase autoinhibitory domain. Yeast two-hybrid assays as well as in vitro binding assays revealed a 21-amino acid motif in the regulatory domain of SOS2 that is necessary and sufficient for interaction with SOS3. Database searches revealed a large family of SOS2-like protein kinases containing such a SOS3 binding motif. Using a yeast two-hybrid system, we show that these SOS2-like kinases interact with members of the SOS3 family of calcium binding proteins. Two-hybrid assays also revealed interaction between the N-terminal kinase domain and the C-terminal regulatory domain within SOS2, suggesting that the regulatory domain may inhibit kinase activity by blocking substrate access to the catalytic site. Removal of the regulatory domain of SOS2, including the SOS3 binding motif, resulted in constitutive activation of the protein kinase, indicating that the SOS3 binding motif can serve as a kinase autoinhibitory domain. Constitutively active SOS2 that is SOS3 independent also was produced by changing Thr(168) to Asp in the activation loop of the SOS2 kinase domain. Combining the Thr(168)-to-Asp mutation with the autoinhibitory domain deletion created a superactive SOS2 kinase. These results provide insights into regulation of the kinase activities of SOS2 and the SOS2 family of protein kinases.

Adaptation, Physiological↗

PubMed2003

A root-specific O-methyltransferase gene expressed in salt-tolerant barley.

A cDNA encoding an O-methyltransferase (OMT) was isolated from salt-tolerant barley roots by subtraction hybridization with cDNAs of salt-tolerant barley roots as a tester cDNA and cDNAs of the salt-sensitive barley roots as a driver cDNA. The deduced amino acid sequence showed significant identity with plant caffeic acid/5-hydroxyferulic acid OMTs. Southern blot analysis showed that the OMT gene was a single copy in both salt-tolerant and -sensitive barley. The cloned gene was expressed in a wheat germ cell-free system to produce the OMT, which had methylating activity for caffeic acid. Northern blot analysis showed that the OMT gene was expressed constitutively in the salt-tolerant barley roots and the expression level was increased 1.5 times by salt stress, but the salt-sensitive barley showed no expression of the gene in roots and leaves.

Amino Acid Sequence↗

PubMed2003

[Advances of salt-tolerant mechanism in yeast].

Yeast is a model eukaryotic organism and salt-tolerant microorganism. The regulative mechanism of gene expression and signal transduction and ion transport of yeast is similar to that of higher eukaryotic organism. The research on salt-tolerant mechanism of yeast will be helpful to the illustrate the salt-tolerant mechanism of higher eukaryotic organism. This review summarized the signal transduction pathway and molecular responses of yeast under salt stress and the major research methods in the research on the salt-tolerant mechanism in yeast.

English Abstract↗

PubMed2002

Overexpression of a Na+/H+ antiporter confers salt tolerance on a freshwater cyanobacterium, making it capable of growth in sea water.

The salt tolerance of a freshwater cyanobacterium, Synechococcus sp. PCC 7942, transformed with genes involved in the synthesis of a Na(+)/H(+) antiporter, betaine, catalase, and a chaperone was examined. Compared with the expression of betaine, catalase, and the chaperone, the expression of the Na(+)/H(+) antiporter gene from a halotolerant cyanobacterium (ApNhaP) drastically improved the salt tolerance of the freshwater cyanobacterium. The Synechococcus cells expressing ApNhaP could grow in BG11 medium containing 0.5 M NaCl as well as in sea water, whereas those expressing betaine, catalase, and the chaperone could not grow under those conditions. The coexpression of ApNhaP with catalase or ApNhaP with catalase and betaine did not further enhance the salt tolerance of Synechococcus cells expressing ApNhaP alone when grown in BG11 medium containing 0.5 M NaCl. Interestingly, the coexpression of ApNhaP with catalase resulted in enhanced salt tolerance of cells grown in sea water. These results demonstrate a key role of sodium ion exclusion by the Na(+)/H(+) antiporter for the salt tolerance of photosynthetic organisms.

Bacterial Proteins↗

PubMed2003

Plant RelA/SpoT homolog confers salt tolerance in Escherichia coli and Saccharomyces cerevisiae.

To analyze the mechanisms of salt tolerance in the halophyte Suaeda japonica, Escherichia coli was used as a host organism to undertake functional screening of cDNAs encoding proteins that may play an important role for the salt-tolerance mechanisms. A transformant expressing RelA/SpoT homolog (Sj-RSH) was found to have enhanced salt tolerance. In E. coli, RelA/SpoT controlled the amount of guanosine tetraphosphate (ppGpp) and guanosine pentaphosphate (pppGpp), which are the effectors of the bacterial stringent response. Complementation analysis using the relA mutant of E. coli showed that Sj-RSH conferred the phenotype associated with (p)ppGpp synthesis. Furthermore, expression of Sj-RSH driven by the GAL1 promoter also gave rise to enhanced salt tolerance in yeast. Northern blot analyses of the yeast transformant revealed that the transcriptional levels of stress responsive genes including GPD1, VMA6, BMH1, HYP1 and HOG1 were clearly enhanced in the Sj-RSH transformant when compared with an empty vector transformant under stress-free and 1.5 M NaCl stress conditions. These results suggest that (p)ppGpp synthesis mediated by plant RelA/SpoT homologs plays a critical role for the transcriptional induction of several stress responsive genes, directly or indirectly in yeast, and that the conserved stress-resistance system may exist in higher plants.

Adaptation, Physiological↗

PubMed2004

Digestion by serine proteases enhances salt tolerance of glutaminase in the marine bacterium Micrococcus luteus K-3.

Salt-tolerant glutaminase (Micrococcus glutaminase, with an apparent molecular mass of 48.3 kDa, intact glutaminase) from the marine bacterium Micrococcus luteus K-3 was digested using protease derived from M. luteus K-3. The digestion products were a large fragment (apparent molecular mass of 38.5 kDa, the glutaminase fragment) and small fragments (apparent molecular mass of 8 kDa). The digestion was inhibited by phenylmethanesulfonyl fluoride (PMSF). Digestion of intact glutaminase by serine proteases including trypsin, elastase, lysyl endopeptidase, and arginylendopeptidase also produced the glutaminase fragment. The N-terminus of the glutaminase fragment was the same as that of intact glutaminase. The N-termini of two small fragments were Ala394 and Ala396, respectively. The enzymological and kinetic properties of the glutaminase fragment were almost the same as those of intact glutaminase except for salt-tolerant behavior. The glutaminase fragment was a higher salt-tolerant enzyme than the intact glutaminase, suggesting that Micrococcus glutaminase is digested in the C-terminal region by serine protease from M. luteus K-3 to confer salt tolerance on glutaminase.

Amino Acid Sequence↗

PubMed2000

SOS3 function in plant salt tolerance requires N-myristoylation and calcium binding.

The salt tolerance gene SOS3 (for salt overly sensitive3) of Arabidopsis is predicted to encode a calcium binding protein with an N-myristoylation signature sequence. Here, we examine the myristoylation and calcium binding properties of SOS3 and their functional significance in plant tolerance to salt. Treatment of young Arabidopsis seedlings with the myristoylation inhibitor 2-hydroxymyristic acid caused the swelling of root tips, mimicking the phenotype of the salt-hypersensitive mutant sos3-1. In vitro translation assays with reticulocyte showed that the SOS3 protein was myristoylated. Targeted mutagenesis of the N-terminal glycine-2 to alanine prevented the myristoylation of SOS3. The functional significance of SOS3 myristoylation was examined by expressing the wild-type myristoylated SOS3 and the mutated nonmyristoylated SOS3 in the sos3-1 mutant. Expression of the myristoylated but not the nonmyristoylated SOS3 complemented the salt-hypersensitive phenotype of sos3-1 plants. No significant difference in membrane association was observed between the myristoylated and nonmyristoylated SOS3. Gel mobility shift and (45)Ca(2)+ overlay assays demonstrated that SOS3 is a unique calcium binding protein and that the sos3-1 mutation substantially reduced the capacity of SOS3 to bind calcium. The resulting mutant SOS3 protein was not able to interact with the SOS2 protein kinase and was less capable of activating it. Together, these results strongly suggest that both N-myristoylation and calcium binding are required for SOS3 function in plant salt tolerance.

Adaptation, Physiological↗

PubMed1999

Characterization of the Na+-ATPase gene (ZENA1) from the salt-tolerant yeast Zygosaccharomyces rouxii.

In order to clarify the relationship between the salt tolerance of Zygosaccharomyces rouxii and the function of Na+-ATPase, a gene which exhibited homology to the Na+-ATPase gene (ZENA1) of Saccharomyces cerevisiae was isolated from Z. rouxii. This newly isolated gene (ZENA1) encoded a product of 1048 amino acids. The predicted amino-acid sequence of Zena1p was highly homologous to that of S. cerevisiae Ena1p and Ena2p, and Schwanniomyces occidentalis Ena1p and Ena2p, but showed low homology to that of Zpma1p, which is the product of the Z. rouxii plasma membrane H+.ATPase gene (ZENA1). Zena1p shares the peptide motifs which have been suggested to participate in the function of ATPase. Expression of ZENA1 was observed, but was independent of NaCl shock. When ZENA1 was expressed in salt-sensitive S. cerevisiae under the regulation of a GAL1 promoter by using the expression vector pYES2, salt tolerance of the transformants was observed. The growth characteristics of Zena1Delta-disruptants of Z. rouxii and the pH profiles of their plasma membrane ATPase activity were almost the same as those of the wild-type strain, indicating that the function of Zena1p is of little relevance to the salt tolerance property of Z. rouxii. By considering the close relationship between the salt tolerance of Z. rouxii and the function of its Na+/H+-antiporter, we concluded that the extrusion of Na+ across the plasma membrane in Z. rouxii cells might be carried out mainly via the function of the Na+/H+-antiporter in a high salinity environment.

Journal Article↗

PubMed2002

Reduction of perchlorate and nitrate by salt tolerant bacteria.

Spent regenerant brine from ion-exchange technology for the removal of perchlorate and nitrate produces a high salt waste stream, which requires remediation before disposal. Bioremediation is an attractive treatment option. In this study, we enriched for salt tolerant bacteria from sediments from Cargill salt evaporation facility (California, USA), the Salton Sea (California, USA), and a high density hydrocarbon oxidizing bacterial cocktail. The bacterial cocktail enrichment culture reduced ClO4- from 500 to 260 mg 1 in 4 weeks. Salt tolerant bacterial isolates from the enrichment cultures and two denitrifying salt tolerant bacteria, Haloferax denitrificans and Parococcus halodenitricans, substantially reduced perchlorate. The highest rate of perchlorate removal was recorded with the isolate, Citrobacter sp.: 32% reduction in 1 week. This bacterium substantially reduced perchlorate in 0-5% NaCl solutions and maximally at 30 degrees C and at an initial pH 7.5. In simulated brines containing 7.5% total solids, the Citrobacter sp. significantly reduced both perchlorate and nitrate with 34.9 and 15.6% reduction, respectively, in 1 week. Coculture of a potent perchlorate reducing, non-salt tolerant (non-saline) bacterium, perclace and the Citrobacter sp. proved most effective for perchlorate removal in the brine (46.4% in 1 week). This study demonstrates that both anions can be reduced in treatment of brines from ion exchange systems.

Biodegradation, Environmental↗

PubMed2006

Low unidirectional sodium influx into root cells restricts net sodium accumulation in Thellungiella halophila, a salt-tolerant relative of Arabidopsis thaliana.

Thellungiella halophila is a useful model species for research into plant salt tolerance. It is closely related to Arabidopsis thaliana, but shows considerably higher salt tolerance. Comparative analysis of ion homeostasis in the two species allows the identification of ion transport pathways that are critical for salt tolerance and provides the basis for future studies into their molecular features. Previous studies indicated that salt tolerance in T. halophila is accompanied by low accumulation of Na in the leaves. Kinetic analysis of net ion uptake over three days confirmed lower Na uptake and K loss in T. halophila compared with A. thaliana. Differential net Na uptake rates were still apparent after 6 weeks of salt treatment. To assess the contribution of unidirectional Na fluxes to net Na uptake, kinetic studies of (22)Na fluxes were carried out in both species. The results show that unidirectional root Na influx is significantly lower in salt-grown T. halophila plants than in A. thaliana exposed to the same level of salinity (100 mM). Quantitative comparison of unidirectional influx and net Na accumulation suggests that both species operate efficient Na efflux, which partly compensates for Na influx. Kinetic analysis of (22)Na efflux indicated higher root Na efflux in A. thaliana than in T. halophila. Thus A. thaliana appears to spend more energy on Na export while nevertheless accumulating more Na than T. halophila. It is proposed that limitation of Na influx is the main mechanism by which T. halophila secures low net Na accumulation in saline conditions. This strategy provides the basis for a positive balance between growth and net Na uptake rates, which is essential for survival in high salt.

Arabidopsis↗

PubMed1987

Relationship between Salt Tolerance and Resistance to Polyethylene Glycol-Induced Water Stress in Cultured Citrus Cells.

Salt-tolerant selected cells of Shamouti orange (Citrus sinensis) and Sour orange (Citrus aurantium) grew considerably better than nonselected cells at any NaCl concentration tested up to 200 millimolar. Also, the growth response of each treatment was identical in the two species. However, the performance of cells of the two species under osmotic stress induced by polyethylene glycol (PEG), which is presumably a nonabsorbed osmoticum, was significantly different. The nonselected Shamouti cell lines were significantly more sensitive to osmotic stress than the selected cells. The salt adapted Shamouti cells were apparently also adapted to osmotic stress induced by PEG. In Sour orange, however, the selected lines had no advantage over the nonselected line in response to osmotic stress induced by PEG. This response was also similar quantitatively to the response of the selected salt-tolerant Shamouti cell line. It seems that the tolerance to salt in Shamouti, a partial salt excluder, involves an osmotic adaptation, whereas in Sour orange, a salt accumulator, such an adaptation apparently does not occur. PEG-induced osmotic stress causes an increase in the percent dry weight of salt-sensitive and salt-tolerant cells of both species. No such increase was found under salt stress. The size of control and stressed cells is not significantly different.

Journal Article↗

PubMedDate not supplied

Prospects of breeding for salt tolerance in rice.

Variations with regard to salt tolerance were observed in rice varieties, Blue bonnet, IR-8, Jhona-349, and Magnolia. Crosses (F1) between relatively salt-resistant (Jhona-349) and salt-sensitive (Magnolia) varieties were highly resistant to salinity. The F2 population resulted in some salt-resistant combinations. In the F3 and F4 populations, some desirable salt-resistant progenies were selected. Results obtained suggest that selection for salt tolerance may be possible within hybrid populations.

Genetic Engineering↗

PubMed2005

Salt tolerance and salinity effects on plants: a review.

Plants exposed to salt stress undergo changes in their environment. The ability of plants to tolerate salt is determined by multiple biochemical pathways that facilitate retention and/or acquisition of water, protect chloroplast functions, and maintain ion homeostasis. Essential pathways include those that lead to synthesis of osmotically active metabolites, specific proteins, and certain free radical scavenging enzymes that control ion and water flux and support scavenging of oxygen radicals or chaperones. The ability of plants to detoxify radicals under conditions of salt stress is probably the most critical requirement. Many salt-tolerant species accumulate methylated metabolites, which play crucial dual roles as osmoprotectants and as radical scavengers. Their synthesis is correlated with stress-induced enhancement of photorespiration. In this paper, plant responses to salinity stress are reviewed with emphasis on physiological, biochemical, and molecular mechanisms of salt tolerance. This review may help in interdisciplinary studies to assess the ecological significance of salt stress.

Adaptation, Physiological↗

PubMed2000

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.

Adaptation, Physiological↗

PubMed2005

Co-expression of the Na(+)/H(+)-antiporter and H(+)-ATPase genes of the salt-tolerant yeast Zygosaccharomyces rouxii in Saccharomyces cerevisiae.

We cloned two genes from the salt-tolerant yeast Zygosaccharomyces rouxii: ZrSOD2 for the cell membrane Na(+)/H(+)-antiporter and ZrPMA1 for the cell membrane H(+)-ATPase. The products of these genes play cooperative roles in the salt-tolerance of Z. rouxii, and the function of the ZrPMA1 product is regulated at the transcription level. We constructed a yeast expression vector that is able to co-express the ZrSOD2 and ZrPMA1 genes. Single expression of ZrSOD2 was effective in conferring salt-tolerance, and although a slight synergic effect was observed with co-expression of ZrSOD2 and ZrPMA1, the usefulness of this co-expression is likely to be minimal with regard to salt-tolerance.

Gene Expression Regulation, Fungal↗

PubMed2002

Introduction of a Na+/H+ antiporter gene from Atriplex gmelini confers salt tolerance to rice.

We engineered a salt-sensitive rice cultivar (Oryza sativa cv. Kinuhikari) to express a vacuolar-type Na+/H+ antiporter gene from a halophytic plant, Atriplex gmelini (AgNHX1). The activity of the vacuolar-type Na+/H+ antiporter in the transgenic rice plants was eight-fold higher than that in wild-type rice plants. Salt tolerance assays followed by non-stress treatments showed that the transgenic plants overexpressing AgNHX1 could survive under conditions of 300 mM NaCl for 3 days while the wild-type rice plants could not. These results indicate that overexpression of the Na+/H+ antiporter gene in rice plants significantly improves their salt tolerance.

Caryophyllaceae↗

PubMed2004

[Positional analysis of a gene related to salt tolerance in Sinorhizobium meliloti by transposon rescue].

Salt sensitive mutant 042BML-2 was obtained by mutating Sinorhizobium meliloti 042BM with transposon Tn5 carried on the plasmid pRL1063a. By transposon rescue, a 1.179 kb of DNA sequence of S. meliloti flanking the Tn5 insertion site was obtained. The sequence similarity analysis through BLAST analysis in GenBank revealed the transposon was inserted into a functionally unknown gene, which is 6 270 bp in length, of S. meliloti, and the gene was named rtsC. This study indicated that rtsC was associated with salt tolerance in S. meliloti 042BM. Hydrophobicity profile analysis of the putative amino acid sequence showed that two predicted transmembrane domains existed in N-terminal portion of RtsC. The significance of RtsC protein in the salt-tolerance in S. meliloti was discussed.

Amino Acid Sequence↗

PubMed2001

Salt tolerance in aquatic macrophytes: possible involvement of the antioxidative enzymes.

Possible involvement of the antioxidative enzymes in salt tolerance was investigated in three submerged aquatic macrophytes, viz. Hydrilla verticillata, Najas indica and Najas gramenia, which differed in their tolerance to salinity. NaCl in light resulted in significant decrease in the chlorophyll a/b ratio in H. verticillata, which was significantly neutralized by the presence of DABCO (diazabicyclo[2.2.2]octane), sodium benzoate or mannitol along with the NaCl in the exposure medium indicating induction of oxidative stress upon salt treatment. All the three test species showed enhanced activity of superoxide dismutase (SOD; EC 1.15.1.1) and peroxidases (EC 1.11.1.7) upon exposure to either NaCl or SWS (seawater salinity); the increases in their activity in response to NaCl were positively correlated with the salt tolerance of the plants. Only N. gramenia showed considerable increase in the activity of catalase (EC 1.11.1.6), while only H. verticillata exhibited significant increase in the activity of ascorbate peroxidase (EC 1.11.1.11) in response to the salt treatment. A higher constitutive level of the antioxidative enzymes in the salt-tolerant N. gramenia than in the salt-sensitive H. verticillata coupled with significant increase in the activity of these enzymes, particularly of catalase and SOD, in the former upon salinity treatment suggested their active involvement in salt tolerance. The oxidative stress induced upon the salt treatment also indicated a possible involvement of the antioxidative enzymes in the process. The ion interaction study revealed significant modifying effect of Ca(2+), and also of Mg(2+) and K(+), on the changes induced by Na(+) in the activity of catalase and SOD suggesting that relative build-up of these ions inside cells could be of much importance in regulation of synthesis of antioxidative enzymes.

Journal Article↗