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

CRISPR-Cas9 mediated understanding of plants' abiotic stress-responsive genes to combat changing climatic patterns

Multiple abiotic stresses like extreme temperatures, water shortage, flooding, salinity, and exposure to heavy metals are confronted by crop plants with changing climatic patterns. Prolonged exposure to these adverse environmental conditions leads to stunted plant growth and development with significant yield loss in crops. CRISPR-Cas9 genome editing tool is being frequently employed to understand abiotic stress-responsive genes. Noteworthy improvements in CRISPR-Cas technology have been made over the years, including upgradation of Cas proteins fidelity and efficiency, optimization of transformation protocols for different crop species, base and prime editing, multiplex gene-targeting, transgene-free editing, and graft-based heritable CRISPR-Cas9 approaches. These developments helped to improve the knowledge of abiotic stress tolerance in crops that could potentially be utilized to develop knock-out varieties and over-expressed lines to tackle the adverse effects of altered climatic patterns. This review summarizes the mechanistic understanding of heat, drought, salinity, and metal stress-responsive genes characterized so far using CRISPR-Cas9 and provides data on potential candidate genes that can be exploited by modern-day biotechnological tools to develop transgene-free genome-edited crops with better climate adaptability. Furthermore, the importance of early-maturing crop varieties to withstand abiotic stresses is also discussed in this review.

Differentially expressed genes related to oxidoreductase activity and glutathione metabolism underlying the adaptation of Phragmites australis from the salt marsh in the Yellow River Delta, China

The common reed (Phragmites australis) is a dominant species in the coastal wetlands of the Chinese Yellow River Delta, where it tolerates a wide range of salinity. Recent environmental changes have led to the increase of soil salinity in this region, which has degraded much of the local vegetation. Clones of common reeds from the tidal marsh may have adapted to local high salinity habitat through selection on genes and metabolic pathways conferring salt tolerance. This study aims to reveal molecular mechanisms underlying salt tolerance in the tidal reed by comparing them to the salt-sensitive freshwater reed under salt stress. We employed comparative transcriptomics to reveal the differentially expressed genes (DEGs) between these two types of common reeds under different salinity conditions. The results showed that only three co-expressed genes were up-regulated and one co-expressed gene was down-regulated between the two reed types. On the other hand, 1,371 DEGs were exclusively up-regulated and 285 DEGs were exclusively down-regulated in the tidal reed compared to the control, while 115 DEGs were exclusively up-regulated and 118 DEGs were exclusively down-regulated in the freshwater reed compared to the control. From the pattern of enrichment of transcripts involved in salinity response, the tidal reed was more active and efficient in scavenging reactive oxygen species (ROS) than the freshwater reed, with the tidal reed showing significantly higher gene expression in oxidoreductase activity. Furthermore, when the reeds were exposed to salt stress, transcripts encoding glutathione metabolism were up-regulated in the tidal reed but not in the freshwater reed. DEGs related to encoding glutathione reductase (GR), glucose-6-phosphate 1-dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PD), glutathione S-transferase (GST) and L-ascorbate peroxidase (LAP) were revealed as especially highly differentially regulated and therefore represented candidate genes that could be cloned into plants to improve salt tolerance. Overall, more genes were up-regulated in the tidal reed than in the freshwater reed from the Yellow River Delta when under salt stress. The tidal reed efficiently resisted salt stress by up-regulating genes encoding for oxidoreductase activity and glutathione metabolism. We suggest that this type of common reed could be extremely useful in the ecological restoration of degraded, high salinity coastal wetlands in priority.

An Overview of Hazardous Impacts of Soil Salinity in Crops, Tolerance Mechanisms, and Amelioration through Selenium Supplementation

Soil salinization is one of the major environmental stressors hampering the growth and yield of crops all over the world. A wide spectrum of physiological and biochemical alterations of plants are induced by salinity, which causes lowered water potential in the soil solution, ionic disequilibrium, specific ion effects, and a higher accumulation of reactive oxygen species (ROS). For many years, numerous investigations have been made into salinity stresses and attempts to minimize the losses of plant productivity, including the effects of phytohormones, osmoprotectants, antioxidants, polyamines, and trace elements. One of the protectants, selenium (Se), has been found to be effective in improving growth and inducing tolerance against excessive soil salinity. However, the in-depth mechanisms of Se-induced salinity tolerance are still unclear. This review refines the knowledge involved in Se-mediated improvements of plant growth when subjected to salinity and suggests future perspectives as well as several research limitations in this field.

Metabolic response and correlations between ions and metabolites in Phragmites communis under long-term salinity toxicity

Phragmites communis has a long history in Songnen grassland of China and has a series of biological, ecological as well as genetic characteristics contributing to its adaptation to the specific local climatic and edaphic conditions. The aim of the present study was to investigate the ions balance and their relationship to metabolites in P. communis under three salinity stress conditions. Results showed that the contents of Na⁺, Cl^-, and SO₄^2- significantly increased in P. communis leaves, while K⁺, Mg²⁺, and Mn²⁺ decreased. Moreover, Na⁺ and Cl^- had significant negative correlations with metabolites involved in the tricarboxylic acid cycle (TCA cycle), and significant positive correlations with glycolysis. The metabolite results showed that high contents of sugars and proline played important roles in developing salinity tolerance, indicating that glycolysis and proline biosynthesis were enhanced; however, this consumes large amounts of energy and likely caused the TCA cycle to be inhibited. The results suggested that P. communis might enhance its salinity tolerance mainly through increased glycolysis and energy consumption. In addition, restricting Na⁺ accumulation and increasing of Cl^-, and rearrangement of metabolite production in P. communis tissues are possible causes of salinity tolerance. Therefore, salinity caused systems alterations in widespread metabolic networks involving TCA cycle, glycolysis and proline biosynthesis. These findings provided new insights for the P. communis metabolic adaptation to salinity and demonstrated the ions balance and metabolites in P. communis are possibly attributable to development of salinity tolerance.

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.

Tissue physiological metabolic adaptability in young and old leaves of reed (Phragmites communis) in Songnen grassland

Common reed (Phragmites communis) is widely distributed as the dominant plant species in the Songnen Plain of China. The aim of this study was to investigate different physiological adaptive mechanisms to salinity tolerance between young and old leaves. The profiles of 68 metabolites were measured and studied in reed leaves by gas chromatography-mass spectrometer. The nitrogen, carbon, and pigment contents showed stronger growth inhibition for older leaves with salinity stress. In young leaves, high K⁺ contents not only promoted cell growth, but also prevented influx of superfluous Na⁺ ions in cells; the Ca²⁺ accumulation in old leaves implied that Ca²⁺ triggered the SOS-Na⁺ exclusion system and reduced Na⁺ toxicity. Thus, the mechanism of enhanced tolerance differed between young and old leaves. The metabolite results indicated that the young and old leaves had different mechanisms of osmotic regulation; sugars/polyols and amino acids played important roles in developing salinity tolerance in young leaves but high contents of fatty acids were important for old leaves. These results implied dramatically enhanced sugars and amino acid synthesis but inhibited energy metabolism in young leaves. In contrast, fatty acid synthesis was enhanced in old leaves. The results extended our understanding of the differences in physiological metabolism in adaptive to the salt-alkalization of soil in Songnen grassland between young and old leaves of reeds.

Role of sodium ion transporters and osmotic adjustments in stress alleviation of Cynodon dactylon under NaCl treatment: a parallel investigation with rice

Comparative analyses of the responses to NaCl in Cynodon dactylon and a sensitive crop species like rice could effectively unravel the salt tolerance mechanism in the former. C. dactylon, a wild perennial chloridoid grass having a wide range of ecological distribution is generally adaptable to varying degrees of salinity stress. The role of salt exclusion mechanism present exclusively in the wild grass was one of the major factors contributing to its tolerance. Salt exclusion was found to be induced at 4 days when the plants were treated with a minimum conc. of 200 mM NaCl. The structural peculiarities of the salt exuding glands were elucidated by the SEM and TEM studies, which clearly revealed the presence of a bicellular salt gland actively functioning under NaCl stress to remove the excess amount of Na⁺ ion from the mesophyll tissues. Moreover, the intracellular effect of NaCl on the photosynthetic apparatus was found to be lower in C. dactylon in comparison to rice; at the same time, the vacuolization process increased in the former. Accumulation of osmolytes like proline and glycine betaine also increased significantly in C. dactylon with a concurrent check on the H₂O₂ levels, electrolyte leakage and membrane lipid peroxidation. This accounted for the proper functioning of the Na⁺ ion transporters in the salt glands and also in the vacuoles for the exudation and loading of excess salts, respectively, to maintain the osmotic balance of the protoplasm. In real-time PCR analyses, CdSOS1 expression was found to increase by 2.5- and 5-fold, respectively, and CdNHX expression increased by 1.5- and 2-fold, respectively, in plants subjected to 100 and 200 mM NaCl treatment for 72 h. Thus, the comparative analyses of the expression pattern of the plasma membrane and tonoplast Na⁺ ion transporters, SOS1 and NHX in both the plants revealed the significant role of these two ion transporters in conferring salinity tolerance in Cynodon.

SOS1, HKT1;5, and NHX1 Synergistically Modulate Na+ Homeostasis in the Halophytic Grass Puccinellia tenuiflora

Puccinellia tenuiflora is a typical salt-excluding halophytic grass with excellent salt tolerance. Plasma membrane Na+/H+ transporter SOS1, HKT-type protein and tonoplast Na+/H+ antiporter NHX1 are key Na+ transporters involved in plant salt tolerance. Based on our previous research, we had proposed a function model for these transporters in Na+ homeostasis according to the expression of PtSOS1 and Na+, K+ levels in P. tenuiflora responding to salt stress. Here, we analyzed the expression patterns of PtSOS1, PtHKT1;5, and PtNHX1 in P. tenuiflora under 25 and 150 mM NaCl to further validate this model by combining previous physiological characteristics. Results showed that the expressions of PtSOS1 and PtHKT1;5 in roots were significantly induced and peaked at 6 h under both 25 and 150 mM NaCl. Compared to the control, the expression of PtSOS1 significantly increased by 5.8-folds, while that of PtHKT1;5 increased only by 1.2-folds in roots under 25 mM NaCl; on the contrary, the expression of PtSOS1 increased by 1.4-folds, whereas that of PtHKT1;5 increased by 2.2-folds in roots under 150 mM NaCl. In addition, PtNHX1 was induced instantaneously under 25 mM NaCl, while its expression was much higher and more persistent in shoots under 150 mM NaCl. These results provide stronger evidences for the previous hypothesis and extend the model which highlights that SOS1, HKT1;5, and NHX1 synergistically regulate Na+ homeostasis by controlling Na+ transport systems at the whole-plant level under both lower and higher salt conditions. Under mild salinity, PtNHX1 in shoots compartmentalized Na+ into vacuole slowly, and vacuole potential capacity for sequestering Na+ would enhance Na+ loading into the xylem of roots by PtSOS1 through feedback regulation; and consequently, Na+ could be transported from roots to shoots by transpiration stream for osmotic adjustment. While under severe salinity, Na+ was rapidly sequestrated into vacuoles of mesophyll cells by PtNHX1 and the vacuole capacity became saturated for sequestering more Na+, which in turn regulated long-distance Na+ transport from roots to shoots. As a result, the expression of PtHKT1;5 was strongly induced so that the excessive Na+ was unloaded from xylem into xylem parenchyma cells by PtHKT1;5.

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.

Using Transcriptomics to Identify Differential Gene Expression in Response to Salinity among Australian Phragmites australis Clones

Common Reed (Phragmites australis) is a frequent component of inland and coastal wetlands in temperate zones worldwide. Ongoing environmental changes have resulted in the decline of this species in many areas and invasive expansion in others. In the Gippsland Lakes coastal waterway system in south-eastern Australia, increasing salinity is thought to have contributed to the loss of fringing P. australis reed beds leading to increased shoreline erosion. A major goal of restoration in this waterway is to address the effect of salinity by planting a genetically diverse range of salt-tolerant P. australis plants. This has prompted an interest in examining the variation in salinity tolerance among clones and the underlying basis of this variation. Transcriptomics is an approach for identifying variation in genes and their expression levels associated with the exposure of plants to environmental stressors. In this paper we present initial results of the first comparative culm transcriptome analysis of P. australis clones. After sampling plants from sites of varied surface water salinity across the Gippsland Lakes, replicates from three clones from highly saline sites (>18 g L(-1) TDS) and three from low salinity sites (<6 g L(-1)) were grown in containers irrigated with either fresh (<0.1 g L(-1)) or saline water (16 g L(-1)). An RNA-Seq protocol was used to generate sequence data from culm tissues from the 12 samples allowing an analysis of differential gene expression. Among the key findings, we identified several genes uniquely up- or down-regulated in clones from highly saline sites when irrigated with saline water relative to clones from low salinity sites. These included the higher relative expression levels of genes associated with photosynthesis and lignan biosynthesis indicative of a greater ability of these clones to maintain growth under saline conditions. Combined with growth data from a parallel study, our data suggests local adaptation of certain clones to salinity and provides a basis for more detailed studies.

Base to Tip and Long-Distance Transport of Sodium in the Root of Common Reed [Phragmites australis (Cav.) Trin. ex Steud.] at Steady State Under Constant High-Salt Conditions

We analyzed the directions and rates of translocation of sodium ions (Na(+)) within tissues of a salt-tolerant plant, common reed [Phragmites australis (Cav.) Trin. ex Steud.], and a salt-sensitive plant, rice (Oryza sativa L.), under constant high-salt conditions using radioactive (22)Na tracer and a positron-emitting tracer imaging system (PETIS). First, the test plants were incubated in a nutrient solution containing 50 mM NaCl and a trace level of (22)Na for 24 h (feeding step). Then the original solution was replaced with a fresh solution containing 50 mM NaCl but no (22)Na, in which the test plants remained for >48 h (chase step). Non-invasive dynamic visualization of (22)Na distribution in the test plants was conducted during feeding and chase steps with PETIS. Our results revealed that (22)Na was absorbed in the roots of common reed, but not transported to the upper shoot beyond the shoot base. During the chase step, a basal to distal movement of (22)Na was detected within the root tissue over >5 cm with a velocity of approximately 0.5 cm h(-1). On the other hand, (22)Na that was absorbed in the roots of rice was continuously translocated to and accumulated in the whole shoot. We concluded that the basal roots and the shoot base of common reed have constitutive functions of Na(+) exclusion only in the direction of root tips, even under constant high-salt conditions. This function apparently may contribute to the low Na(+) concentration in the upper shoot and high salt tolerance of common reed.

Expression of major photosynthetic and salt-resistance genes in invasive reed lineages grown under elevated CO2 and temperature

It is important to investigate the molecular causes of the variation in ecologically important traits to fully understand phenotypic responses to climate change. In the Mississippi River Delta, two distinct, sympatric invasive lineages of common reed (Phragmites australis) are known to differ in several ecophysiological characteristics and are expected to become more salt resistant due to increasing atmospheric CO2 and temperature. We investigated whether different patterns of gene expression can explain their ecophysiological differences and increased vigor under future climatic conditions. We compared the transcript abundance of photosynthetic genes of the Calvin cycle (Rubisco small subunit, RbcS; Phosphoglycerate kinase, PGK; Phosphoribulokinase, PRK), genes related with salt transport (Na(+)/H(+) antiporter, PhaNHA) and oxidative stress response genes (Manganese Superoxide dismutase, MnSOD; Glutathione peroxidase, GPX), and the total aboveground biomass production between two genotypes representing the two lineages. The two genotypes (Delta-type, Mediterranean lineage, and EU-type, Eurasian lineage) were grown under an ambient and a future climate scenario with simultaneously elevated CO2 and temperature, and under two different soil salinities (0‰ or 20‰). We found neither differences in the aboveground biomass production nor the transcript abundances of the two genotypes, but soil salinity significantly affected all the investigated parameters, often interacting with the climatic conditions. At 20‰ salinity, most genes were higher expressed in the future than in the ambient climatic conditions. Higher transcription of the genes suggests higher abundance of the protein they code for, and consequently increased photosynthate production, improved stress responses, and salt exclusion. Therefore, the higher expression of these genes most likely contributed to the significantly ameliorated salinity impact on the aboveground biomass production of both P. australis genotypes under elevated temperature and CO2. Although transcript abundances did not explain differences between the lineages, they correlated with the increased vigor of both lineages under anticipated future climatic conditions.

Isolation and characterization of a Δ1-pyrroline-5-carboxylate synthetase (NtP5CS) from Nitraria tangutorum Bobr. and functional comparison with its Arabidopsis homologue

Several functional and regulatory proteins play important roles in controlling plant stress tolerance. Proline (Pro) is one of the most accumulated osmolytes correlated with tolerance to stresses. Δ(1)-Pyrroline-5-carboxylate synthetase (P5CS) is a rate-limiting enzyme in Pro biosynthesis. In the present study, we isolated the cDNA for a P5CS gene (NtP5CS) from the halophyte Nitraria tangutorum. Phylogenetic analysis and subcellular localization analysis of NtP5CS-GFP protein in onion cells showed that NtP5CS was a new P5CS gene and was involved in Pro synthesis in N. tangutorum. Expression of the NtP5CS gene was induced by salt stress, dehydration, and high and low temperatures. Escherichia coli overexpressing AtP5CS or NtP5CS exhibited better growth in all treatments, including high salinity, high alkalinity, dehydration, osmotic, heat and cold stresses. Additionally, NtP5CS recombinant E. coli cells grew better than did AtP5CS recombinant cells in response to abiotic stresses. Our data demonstrate that the P5CS from a halophytic species functions more efficiently than its homologue from a glycophytic species in improving the stress tolerance of E. coli.

Bioengineering for salinity tolerance in plants: state of the art

Genetic engineering of plants for abiotic stress tolerance is a challenging task because of its multifarious nature. Comprehensive studies for developing abiotic stress tolerance are in progress, involving genes from different pathways including osmolyte synthesis, ion homeostasis, antioxidative pathways, and regulatory genes. In the last decade, several attempts have been made to substantiate the role of "single-function" gene(s) as well as transcription factor(s) for abiotic stress tolerance. Since, the abiotic stress tolerance is multigenic in nature, therefore, the recent trend is shifting towards genetic transformation of multiple genes or transcription factors. A large number of crop plants are being engineered by abiotic stress tolerant genes and have shown the stress tolerance mostly at laboratory level. This review presents a mechanistic view of different pathways and emphasizes the function of different genes in conferring salt tolerance by genetic engineering approach. It also highlights the details of successes achieved in developing salt tolerance in plants thus far.

Modification of plasma membrane proton pumps in cucumber roots as an adaptation mechanism to salt stress

The effect of salt stress (50mM NaCl) on modification of plasma membrane (PM) H(+)-ATPase (EC 3.6.3.14) activity in cucumber roots was studied. Plants were grown under salt stress for 1, 3 or 6 days. In salt-stressed plants, weak stimulation of ATP hydrolytic activity of PM H(+)-ATPase and significant stimulation of proton transport through the plasma membrane were observed. The H(+)/ATP coupling ratio in the plasma membrane of plants subjected to salt stress significantly increased. The greatest stimulation of PM H(+)-ATPase was in 6-day stressed plants. Increased H2O2 accumulation under salt stress conditions in cucumber roots was also observed, with the greatest accumulation observed in 6-day stressed plants. Additionally, during the sixth day of salinity, there appeared heat shock proteins (HSPs) 17.7 and 101, suggesting that repair processes and adaptation to stress occurred in plants. Under salt stress conditions, fast post-translational modifications took place. Protein blot analysis with antibody against phosphothreonine and 14-3-3 proteins showed that, under salinity, the level of those elements increased. Additionally, under salt stress, activity changes of PM H(+)-ATPase can partly result from changes in the pattern of expression of PM H(+)-ATPase genes. In cucumber seedlings, there was increased expression of CsHA10 under salt stress and the transcript of a new PM H(+)-ATPase gene isoform, CsHA1, also appeared. Accumulation of the CsHA1 transcript was induced by NaCl exposure, and was not expressed at detectable levels in roots of control plants. The appearance of a new PM H(+)-ATPase transcript, in addition to the increase in enzyme activity, indicates the important role of the enzyme in maintaining ion homeostasis in plants under salt stress.

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.

Isolation and characterisation of Chrysanthemum crassum SOS1, encoding a putative plasma membrane Na(+) /H(+) antiporter

A full-length cDNA homologue of SOS1 (salt overly sensitive 1) was isolated from the salinity-tolerant species Chrysanthemum crassum and found to encode a Na(+) /H(+) antiporter, using degenerate PCR and RACE-PCR. The 3752-bp sequence comprised a 3438 bp open reading frame, encoding a 127-kDa protein with 12 transmembrane domains within its N terminal portion, and a hydrophilic cytoplasmic tail in its C-terminal portion. CcSOS1 appears to be a plasma membrane protein, and shares ∼62% identity at the peptide level with its Arabidopsis thaliana homologue. Expression of CcSOS1 in the roots of C. crassum was sensitive to salinity stress, while in the leaves CcSOS1 was down-regulated in the presence of abscisic acid. CcSOS1 transcript abundance was reduced in both roots and leaves of plants exposed to low temperature, while it was increased in leaves (but not in roots) after drought stress. CcSOS1 expression was not regulated in the presence of CaCl2 . A heterologous complementation assay in yeast suggested that CcSOS1 directs Na(+) efflux, mimicking the function of the endogenous NHA1 protein. Thus CcSOS1 appears to encode a salinity-inducible plasma membrane Na(+) /H(+) antiporter. This gene may be useful in transgenic approaches to improving the salinity tolerance of related ornamental species.

Molecular cloning and characterization of plasma membrane- and vacuolar-type Na⁺/H⁺ antiporters of an alkaline-salt-tolerant monocot, Puccinellia tenuiflora

A better understanding of salt tolerance in plants might lead to the genetic engineering of crops that can grow in saline soils. Here we cloned and characterized plasma membrane and vacuolar Na⁺/H⁺ antiporters of a monocotyledonous alkaline-tolerant halophyte, Puccinellia tenuiflora. The predicted amino acid sequence of the transporters were very similar to those of orthologs in monocotyledonous crops. Expression analysis showed that (1) NHA was more strongly induced by NaCl in the roots of P. tenuiflora while in rice it was rather induced in the shoots, suggesting that the role of NHA in salt excretion from the roots partly accounts for the difference in the tolerance of the two species, and that (2) NHXs were specifically induced by NaHCO₃ but not by NaCl in the roots of both species, suggesting that vacuolar-type Na⁺/H⁺ antiporters play roles in pH regulation under alkaline salt conditions. Overexpression of the antiporters resulted in increased tolerance of shoots to NaCl and roots to NaHCO₃. Overexpression lines exhibited a lower Na⁺ content and a higher K⁺ content in shoots under NaCl treatments, leading to a higher Na⁺/H⁺ ratio.

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.

Agrobacterium rhizogenes-mediated transformation of Superroot-derived Lotus corniculatus plants: a valuable tool for functional genomics

BACKGROUND

Transgenic approaches provide a powerful tool for gene function investigations in plants. However, some legumes are still recalcitrant to current transformation technologies, limiting the extent to which functional genomic studies can be performed on. Superroot of Lotus corniculatus is a continuous root cloning system allowing direct somatic embryogenesis and mass regeneration of plants. Recently, a technique to obtain transgenic L. corniculatus plants from Superroot-derived leaves through A. tumefaciens-mediated transformation was described. However, transformation efficiency was low and it took about six months from gene transfer to PCR identification.

RESULTS

In the present study, we developed an A. rhizogenes-mediated transformation of Superroot-derived L. corniculatus for gene function investigation, combining the efficient A. rhizogenes-mediated transformation and the rapid regeneration system of Superroot. The transformation system using A. rhizogenes K599 harbouring pGFPGUSPlus was improved by validating some parameters which may influence the transformation frequency. Using stem sections with one node as explants, a 2-day pre-culture of explants, infection with K599 at OD(600) = 0.6, and co-cultivation on medium (pH 5.4) at 22 degrees C for 2 days enhanced the transformation frequency significantly. As proof of concept, Superroot-derived L. corniculatus was transformed with a gene from wheat encoding an Na+/H+ antiporter (TaNHX2) using the described system. Transgenic Superroot plants were obtained and had increased salt tolerance, as expected from the expression of TaNHX2.

CONCLUSION

A rapid and efficient tool for gene function investigation in L. corniculatus was developed, combining the simplicity and high efficiency of the Superroot regeneration system and the availability of A. rhizogenes-mediated transformation. This system was improved by validating some parameters influencing the transformation frequency, which could reach 92% based on GUS detection. The combination of the highly efficient transformation and the regeneration system of Superroot provides a valuable tool for functional genomics studies in L. corniculatus.