Effects of pH on the interaction of ligands with the (H+ + K+)-ATPase purified from pig gastric mucosa

The effects of K⁺, Na⁺ and ATP on the gastric (H⁺ + K⁺)-ATPase were investigated at various pH. The enzyme was phosphorylated by ATP with a pseudo-first-order rate constant of 3650 min^?1 at pH 7.4. This rate constant increased to a maximal value of about 7900 min^?1 when pH was decreased to 6.0. Alkalinization decreased the rate constant. At pH 8.0 it was 1290 min^?1. Additions of 5 mM K⁺ or Na⁺, did not change the rate constant at acidic pH, while at neutral or alkaline pH a decrease was observed. Dephosphorylation of phosphoenzyme in lyophilized vesicles was dependent on K⁺, but not on Na⁺. Alkaline pH increased the rate of dephosphorylation. K⁺ stimulated the ATPase and p-nitrophenylphosphatase activities. At high concentrations K⁺ was inhibitory. Below pH 7.0 Na⁺ had little or no effect on the ATPase and p-nitrophenylphosphatase, while at alkaline pH, Na⁺ inhibited both activities. The effect of extravesicular pH on transport of H⁺ was investigated. At pH 6.5 the apparent K_m for ATP was 2.7 μM and increased little when K⁺ was added extravesicularly. At pH 7.5, millimolar concentrations of K⁺ increased the apparent K_m for ATP. Extravesicular K⁺ and Na⁺ inhibited the transport of H⁺. The inhibition was strongest at alkaline pH and only slight at neutral or acidic pH, suggesting a competition between the alkali metal ions and hydrogen ions at a common binding site on the cytoplasmic side of the membrane. Two H⁺-producing reactions as possible candidates as physiological regulators of (H⁺ + K⁺)-ATPase were investigated. Firstly, the hydrolysis of ATP per se, and secondly, the hydration of CO₂ and the subsequent formation of H⁺ and HCO₃^?. The amount of hydrogen ions formed in the ATPase reaction was highest at alkaline pH. The H⁺/ATP ratio was about 1 at pH 8.0. When CO₂ was added to the reaction medium there was no change in the rate of hydrogen ion transport at pH 7.0, but at pH 8.0 the rate increased 4-times upon the addition of 0.4 mM CO₂. The results indicate a possible co-operation in the production of acid between the H⁺ + K⁺-ATPase and a carbonic anhydrase associated with the vesicular membrane.     

Comparative analysis of trehalose production by Debaryomyces hansenii and Saccharomyces cerevisiae under saline stress

The comparative analysis of growth, intracellular content of Na⁺ and K⁺, and the production of trehalose in the halophilic Debaryomyces hansenii and Saccharomyces cerevisiae were determined under saline stress. The yeast species were studied based on their ability to grow in the absence or presence of 0.6 or 1.0 M NaCl and KCl. D. hansenii strains grew better and accumulated more Na⁺ than S. cerevisiae under saline stress (0.6 and 1.0 M of NaCl), compared to S. cerevisiae strains under similar conditions. By two methods, we found that D. hansenii showed a higher production of trehalose, compared to S. cerevisiae; S. cerevisiae active dry yeast contained more trehalose than a regular commercial strain (S. cerevisiae La Azteca) under all conditions, except when the cells were grown in the presence of 1.0 M NaCl. In our experiments, it was found that D. hansenii accumulates more glycerol than trehalose under saline stress (2.0 and 3.0 M salts). However, under moderate NaCl stress, the cells accumulated more trehalose than glycerol. We suggest that the elevated production of trehalose in D. hansenii plays a role as reserve carbohydrate, as reported for other microorganisms.     

Na+ and K+ transport in Nitrosomonas europaea and Nitrobacter agilis

Potassium-depleted cells of Nitrosomonas europaea and Nitrobacter agilis were prepared by diethanolamine treatment and contained less than 5 mM intracellular K⁺. The addition of K⁺ to K⁺-depleted cells of N. europaea and N. agilis resulted in a depolarization of membrane potential (ΔΨ) by about 5 and 10 mV, respectively. This depolarization was, however, compensated by an equivalent increase in transmembrane pH gradient (ΔpH), so that the total proton-motive force (Δp) remained constant, indicating that K⁺ transport was electrogenic in both bacteria. Using ²²Na⁺-loaded cells, it is shown that both bacteria lack a respiration-dependent Na⁺ pump; however, antiporters for Na⁺/H⁺, K⁺/Na⁺ and K⁺/H⁺ were detected. Of these, at least the K⁺/Na⁺ antiporter required an electrochemical gradient for its operation. It is also shown that the unprotonated form of NH₄⁺ is transported into these bacteria by a simple diffusion mechanism.     

Effects of salt stress and exogenous Ca2+ on Na+ compartmentalization,ion pump activities of tonoplast and plasma membrane in Nitraria tangutorum Bobr. leaves

Nitraria tangutorum Bobr. is a typical halophyte with superior tolerance to salinity. However, little is known about its physiological adaptation mechanisms to the salt environment. In the present study, N. tangutorum seedlings were treated with different concentrations of NaCl (100, 200, 300 and 400 mmol L^?1) combined with five levels of Ca²⁺ (0, 5, 10, 15 and 20 mmol L^?1) to investigate the effects of salt stress and exogenous Ca²⁺ on Na⁺ compartmentalization and ion pump activities of tonoplast and plasma membrane (PM) in leaves. Na⁺ and Ca²⁺ treatments increased the fresh weight and dry weight of N. tangutorum seedlings. The absorption of Na⁺ in roots, stems and leaves was substantially increased with the increases of NaCl concentration, and Na⁺ was mainly accumulated in leaves. Exogenous Ca²⁺ reduced Na⁺ accumulation in roots but promoted Na⁺ accumulation in leaves. The absorption and transportation of Ca²⁺ in N. tangutorum seedlings were inhibited under NaCl treatments. Exogenous Ca²⁺ promoted Ca²⁺ accumulation in the plant. Na⁺ contents in apoplast and symplast of leaves were also significantly increased, and symplast was the main part of Na⁺ intracellular compartmentalization. The tonoplast H⁺-ATPase and H⁺-PPase activities were significantly promoted under salt stress (NaCl concentrations ≤300 mmol L^?1). PM H⁺-ATPase activities gradually increased under salt stress (NaCl concentrations ≤200 mmol L^?1) followed by decreases with NaCl concentration increasing. The tonoplast H⁺-ATPase, H⁺-PPase and PM H⁺-ATPase activities increased first with the increasing exogenous Ca²⁺ concentration, reached the maximums at 15 mmol L^?1 Ca²⁺, and then decreased. The tonoplast and PM Ca²⁺-ATPase activities showed increasing trends with the increases of NaCl and Ca²⁺ concentration. These results suggested that certain concentrations of exogenous Ca²⁺ effectively enhanced ion pump activities of tonoplast and PM as well as promoted the intracellular Na⁺ compartmentalization to improve the salt tolerance of N. tangutorum.     

Characteristics of glutamic acid transport by rabbit intestinal brush-border membrane vesicles. Effects of Na+-, K+-and H+-gradients

In the presence of a Na⁺-gradient (out > in), l-glutamic acid and l-and d-aspartic acids were equally well concentrated inside the vesicles, while no transport above simple diffusion levels was seen by replacement of Na⁺ by K⁺. Equilibrium uptake values were found inversely proportional to the medium osmolarity, thus demonstrating uptake into an osmotically sensitive intravesicular space. The extrapolation of these lines to infinite medium osmolarity (zero space) showed only a small binding component in acidic amino-acid transport. When the same experiment was performed at saturating substrate concentrations, linear relationships extrapolating through the origin but showing smaller slope values were recorded, thus indicating that the binding component could be more important than suspected above. However, binding to the membrane was neglected in our studies as it was absent from initial rate measurements. Na⁺-dependent uphill transport of l-glutamic acid was stimulated by K⁺ present on the intravesicular side only but maximal stimulation was recorded under conditions of an outward K⁺-gradient (in > out). Quantitative and qualitative differences in the K⁺ effect were noted between pH 6.0 and 8.0. Initial uptake rates showed pH dependency in $\text{Na}{}^{\mathrm{+}}\text{-(out > in) + K}{}^{\mathrm{+}}\text{-(in > out)}$ gradient conditions only with a physiological pH optimum between 7.0 and 7.5. It was also found that a pH-gradient (acidic outside) could stimulate both the Na⁺-gradient and the Na⁺ + K⁺-gradient-dependent transport of l-glutamic acid. However, pH- or K⁺-gradient alone were ineffective in stimulating uptake above simple diffusion level. Finally, it was found that increased rates of efflux were always observed with an acidic pH outside, whatever the conditions inside the vesicles. From these results, we propose a channel-type mechanism of l-glutamic acid transport in which Na⁺ and K⁺ effects are modulated by the surrounding pH. The model proposes a carrier with high or low affinity for Na⁺ in the protonated or unprotonated forms, respectively. We also propose that K⁺ binding occurs only to the unprotonated carrier and allows its fast recycling as compared to the free form of the carrier. Such a model would be maximally active and effective in the intestine in the in vivo physiological situations.     

Effects of Ca2+ on the salt-stress response of barley roots as observed by in-vivo 31P-nuclear magnetic resonance and in-vitro analysis

Calcium has been demonstrated to ameliorate the inhibitory effects of high salinity on nutrient transport in plants. Time-course experiments were carried out to study the effect of high Ca²⁺ (6 mM) supply under saline conditions (100 mM NaCl) on the regulation of intracellular pH in excised barley (Hordeum vulgare L. cv Arivat) roots. In-vivo ³¹P-nuclear magnetic resonance measurements showed an alkalinization of the vacuolar pH after salt treatment. In the presence of high Ca²⁺ the extent of salt-induced vacuolar alkalinization was lower. High Ca²⁺ partially mitigated the salt-induced increase in Na⁺ content and decrease in K⁺ content of the root. The pattern of change in the vacuolar pH paralleled that of Na⁺ accumulation in the root. This correlation is consistent with the involvement of a tonoplast Na⁺/H⁺ antiporter in Na⁺ transport and the role of Ca²⁺ in Na⁺ uptake. High salt appeared to decrease the Pi content of the vacuole while high Ca²⁺ increased this content irrespective of the salt treatment.Abbreviation NMR nuclear magnetic resonance We are grateful to Dr. T.W.M. Fan and R.M. Highasi (University of California, Davis, USA) for their valuable help with the NMR experiments. We also thank Dr. J. Norlyn for his technical assistance. V. Martinez was supported by a Fulbright fellowship.     

Mitochondria from the salt-tolerant yeast <Emphasis Type="Italic">Debaryomyces hansenii</Emphasis> (halophilic organelles?)

The yeast Debaryomyces hansenii is considered a marine organism. Sea water contains 0.6 M Na⁺ and 10 mM K⁺; these cations permeate into the cytoplasm of D. hansenii where proteins and organelles have to adapt to high salt concentrations. The effect of high concentrations of monovalent and divalent cations on isolated mitochondria from D. hansenii was explored. As in S. cerevisiae, these mitochondria underwent a phosphate-sensitive permeability transition (PT) which was inhibited by Ca²⁺ or Mg²⁺. However, D. hansenii mitochondria require higher phosphate concentrations to inhibit PT. In regard to K⁺ and Na⁺, and at variance with mitochondria from all other sources known, these monovalent cations promoted closure of the putative mitochondrial unspecific channel. This was evidenced by the K⁺/Na⁺-promoted increase in: respiratory control, transmembrane potential and synthesis of ATP. PT was equally sensitive to either Na⁺ or K⁺. In the presence of propyl-gallate PT was still observed while in the presence of cyanide the alternative pathway was not active enough to generate a ΔΨ due to a low AOX activity. In D. hansenii mitochondria K⁺ and Na⁺ optimize oxidative phosphorylation, providing an explanation for the higher growth efficiency in saline environments exhibited by this yeast.     

Glucose inhibits 45Ca efflux from pancreatic β-cells also in the absence of Na+-Ca2+ countertransport

During perifusion with medium deprived of Ca²⁺, addition of glucose or omission of Na⁺ resulted in prompt and quantitatively similar inhibitions of ⁴⁵Ca efflux from β-cell rich pancreatic islets microdissected from ob / ob mice. Glucose had no additional inhibitory effect when Na⁺ was isoosmotically replaced by sucrose or choline⁺. When K⁺ was used as a substitute for Na⁺, the inhibitory effect of Na⁺ removal on ⁴⁵Ca efflux became additive to that of glucose. The observation that glucose can be equally effective in inhibiting ⁴⁵Ca efflux in the presence or absence of Na⁺ is difficult to reconcile with the postulate that the Na⁺-Ca²⁺ countertransport mechanism is a primary site of action for glucose.     

Electrogenic proton ejection coupled to electron transport through the energy-conserving site 2 and K+/H+ exchange in yeast mitochondria

The proton ejection coupled to electron flow from succinate and/or endogenous substrate(s) to cytochrome c using the impermeable electron acceptor ferricyanide is studied in tightly coupled mitochondria isolated from two strains of the yeast Saccharomyces cerevisiae. (1) The observed H⁺ ejection/2e^? ratio approaches an average value of 3 when K⁺ (in the presence of valinomycin) is used as charge-compensating cation. (2) In the presence of the proton-conducting agent carbonyl cyanide m-chlorophenylhydrazone, an H⁺ ejection/2e^? ratio of 2 is observed. (3) The low stoichiometry of 3H⁺ ejected (instead of 4) per 2e^? and the high rate of H⁺ back-decay (0.1615 $\text{ln}\text{δ-}\text{(}\text{ngatom}\text{)}\text{H}{}^{\mathrm{+}}\text{s}$ and a half-time of 4.6 s for 10 mg protein) into the mitochondrial matrix are related to the presence of an electroneutral K⁺/H⁺ antiporter which is demonstrated by passive swelling experiments in isotonic potassium acetate medium.     

Herbivore exposure alters ion fluxes and improves salt tolerance in a desert shrub

Plants have evolved complex mechanisms that allow them to withstand multiple environmental stresses, including biotic and abiotic stresses. Here, we investigated the interaction between herbivore exposure and salt stress of Ammopiptanthus nanus, a desert shrub. We found that jasmonic acid (JA) was involved in plant responses to both herbivore attack and salt stress, leading to an increased NaCl stress tolerance for herbivore-pretreated plants and increase in K⁺/Na⁺ ratio in roots. Further evidence revealed the mechanism by which herbivore improved plant NaCl tolerance. Herbivore pretreatment reduced K⁺ efflux and increased Na⁺ efflux in plants subjected to long-term, short-term, or transient NaCl stress. Moreover, herbivore pretreatment promoted H⁺ efflux by increasing plasma membrane H⁺-adenosine triphosphate (ATP)ase activity. This H⁺ efflux creates a transmembrane proton motive force that drives the Na⁺/H⁺ antiporter to expel excess Na⁺ into the external medium. In addition, high cytosolic Ca²⁺ was observed in the roots of herbivore-treated plants exposed to NaCl, and this effect may be regulated by H⁺-ATPase. Taken together, herbivore exposure enhance s A. nanus tolerance to salt stress by activating the JA-signalling pathway, increasing plasma membrane H ⁺ - ATPase activity, promoting cytosolic Ca²⁺ accumulation, and then restricting K⁺ leakage and reducing Na⁺ accumulation in the cytosol.     

Electrogenic transport of 5-oxoproline in rabbit renal brush-border membrane vesicles Effect of intravesicular potassium

The Na⁺-dependent transport of 5-oxoproline into rabbit renal brush-border vesicles was stimulated by a K⁺ diffusion potential (interior-negative) induced by valinomycin. Na⁺ salts of two anions of different epithelial permeabilities also affected 5-oxoproline transport. These results show that the Na⁺-dependent 5-oxoproline transport in renal brush-border vesicles is an electrogenic process which results in a net transfer of positive charge. Maximum transport of 5-oxoproline occurred at an extravesicular pH of 6.0 to 8.0 and over that pH range, 5-oxoproline exists completely as an anion with a negative charge. The simplest stoichiometry consistent with this process is, therefore, the cotransport of one 5-oxoproline anion with two sodium ions. The presence of K⁺ inside the vesicles stimulated the Na⁺-dependent transport of 5-oxoproline. This stimulatory effect was specific for K⁺ and required the presence of Na⁺. The presence of Na⁺ gradient was not mandatory for the K⁺ action. The stimulation by the intravesicular K⁺ was seen in the presence as well as in the absence of a K⁺ gradient. Therefore, the increased influx of 5-oxoproline was not coupled to the simultaneous efflux of K⁺. The presence of K⁺ in the extravesicular medium alone did not affect the Na⁺-dependent transport of 5-oxoproline, showing that the site of K⁺ action was intravesicular. Glutamate did not interact with the Na⁺-dependent 5-oxoproline transport even in the presence of an outward K⁺ gradient.     

A Novel Plant Vacuolar Na+/H+ Antiporter Gene Evolved by DNA Shuffling Confers Improved Salt Tolerance in Yeast

Plant vacuolar Na⁺/H⁺ antiporters play important roles in maintaining cellular ion homeostasis and mediating the transport of Na⁺ out of the cytosol and into the vacuole. Vacuolar antiporters have been shown to play significant roles in salt tolerance; however the relatively low V_max of the Na⁺/H⁺ exchange of the Na⁺/H⁺ antiporters identified could limit its application in the molecular breeding of salt tolerant crops. In this study, we applied DNA shuffling methodology to generate and recombine the mutations of Arabidopsis thaliana vacuolar Na⁺/H⁺ antiporter gene AtNHX1. Screening using a large scale yeast complementation system identified AtNHXS1, a novel Na⁺/H⁺ antiporter. Expression of AtNHXS1 in yeast showed that the antiporter localized to the vacuolar membrane and that its expression improved the tolerance of yeast to NaCl, KCl, LiCl, and hygromycin B. Measurements of the ion transport activity across the intact yeast vacuole demonstrated that the AtNHXS1 protein showed higher Na⁺/H⁺ exchange activity and a slightly improved K⁺/H⁺ exchange activity.     

Cloning and characterization of a plasma membrane Na+/H+ antiporter gene from Cucumis sativus

A plasma membrane Na⁺/H⁺ antiporter gene (CsSOS1) was separated from cucumber (Cucumis sativus L.) plants by RT-PCR and RACE methods. Sequence analysis indicated that the full-length CsSOS1 cDNA was 3638 bp long with an open reading frame of 3435 bp long encoding a protein of 1145 amino acids. The deduced protein contained conserved structural domains and shared a high similarity with plasma membrane type Na⁺/H⁺ antiporters from other plants. TMpred prediction showed that CsSOS1 had 11 transmembrane domains. As shown by RT-PCR, the expression of CsSOS1 was tissue-specific and increased in the root but decreased in the leaves with increasing NaCl concentration. In addition, expression of CsSOS1 in ATX3 mutant yeast could grow on medium containing NaCl and enhanced AXT3 salt tolerance. These results suggest that the CsSOS1 plays a key role in cucumber plants under salt stress.     

Ion pathways in renal brush border membranes

The absorbance change of the weak base dye probe, Acridine orange, was used to monitor alterations of pH gradients across renal brush border membrane vesicles. The presence of Na⁺/H⁺ or Li⁺/H⁺ exchange was demonstrated by diluting Na₂SO₄ or Li₂SO₄ loaded vesicles into Na⁺- or Li⁺-free solutions, which caused dye uptake. About 20% of the uptake was abolished by lipid permeable cations such as valinomycin-K⁺ or tetraphenylphosphonium, indicating perhaps the presence of a finite Na⁺ conductance smaller than electroneutral Na⁺/H⁺ exchange. The protonophore tetrachlorosalicylanilide raised the rate of dye uptake under these conditions, hence the presence of an Na⁺ conductance greater than the H⁺ conductance was suggested. K⁺ gradients also induced changes of pH, at about 10% of the Na⁺ or Li⁺ rate. Partial inhibition (21%) was seen with 0.1 mM amiloride indicating that K⁺ was a low affinity substrate for the Na⁺/H⁺ exchange. Acceleration both by tetrachlorosalicylanilide (2-fold) and valinomycin (4-fold) suggested the presence of 2 classes of vesicles, those with high and those with low K⁺ conductance. The larger magnitude of the valinomycin dependent signal suggested that 75% of the vesicles had a low K⁺ conductance. Inward Cl^? gradients also induced acidification, partially inhibited by the presence of tetraphenylphosphonium, and accelerated by tetrachlorosalicylanilide. Thus both a Cl^? conductance greater than the H⁺ conductance and a Cl^?/OH^? exchange were present. The rate of Na⁺/H⁺ exchange was amiloride sensitive with a pH optimum of 6.5 and an apparent K_m for Na⁺ or Li⁺ of about 10 mM and an E_A of 14.3 kcal per mol. A 61-fold Na₂SO₄ gradient resulted in a pH gradient of 1.64 units which increased to 1.8 with gramicidin. An equivalent NaCl gradient gave a much lower ΔpH even in the presence of gramicidin showing that the H⁺ and Cl^? pathways could alter the effects of the Na⁺/H⁺ exchange.     

Functional Characterization of a Wheat NHX Antiporter Gene TaNHX2 That Encodes a K+/H+ Exchanger

The subcellular localization of a wheat NHX antiporter, TaNHX2, was studied in Arabidopsis protoplasts, and its function was evaluated using Saccharomyces cerevisiae as a heterologous expression system. Fluorescence patterns of TaNHX2-GFP fusion protein in Arabidopsis cells indicated that TaNHX2 localized at endomembranes. TaNHX2 has significant sequence homology to NHX sodium exchangers from Arabidopsis, is abundant in roots and leaves and is induced by salt or dehydration treatments. Western blot analysis showed that TaNHX2 could be expressed in transgenic yeast cells. Expressed TaNHX2 protein suppressed the salt sensitivity of a yeast mutant strain by increasing its K⁺ content when exposed to salt stress. TaNHX2 also increased the tolerance of the strain to potassium stress. However, the expression of TaNHX2 did not affect the sodium concentration in transgenic cells. Western blot analysis for tonoplast proteins indicated that the TaNHX2 protein localized at the tonoplast of transgenic yeast cells. The tonoplast vesicles from transgenic yeast cells displayed enhanced K⁺/H⁺ exchange activity but very little Na^+/H⁺ exchange compared with controls transformed with the empty vector; Na⁺/H⁺ exchange was not detected with concentrations of less than 37.5 mM Na⁺ in the reaction medium. Our data suggest that TaNHX2 is a endomembrane-bound protein and may primarily function as a K⁺/H⁺ antiporter, which is involved in cellular pH regulation and potassium nutrition under normal conditions. Under saline conditions, the protein mediates resistance to salt stress through the intracellular compartmentalization of potassium to regulate cellular pH and K⁺ homeostasis.     

Na+-Ca2+ exchange in the axolemma-rich membrane vesicle preparations from the walking-leg nerves of the american lobster

An axolemma-rich membrane vesicle fraction was prepared from the leg nerve of the lobster, Homerus americanus. In this preparation Ca²⁺ transport across the membrane was shown to require a Na⁺ gradient (Na⁺-Ca²⁺ exchange), and external K⁺ was found to facilitate this Na⁺-Ca²⁺ exchange activity. In addition, at high Ca²⁺ concentrations (20 mM) a Ca²⁺-Ca²⁺ exchange system was shown to operate, which is stimulated by Li⁺. The Na⁺-Ca²⁺ exchange system is capable of operating in the reverse direction, with Ca²⁺ uptake coupled with Na⁺ efflux. Such a vesicular preparation has the potential for providing useful experimental approaches to study the mechanism of this important Ca²⁺ extrusion system in the nervous system.     

Genome-wide identification,characterization, and expression analysis of the monovalent cation-proton antiporter superfamily in maize,and functional analysis of its role in salt tolerance

Na⁺, K⁺ and pH homeostasis are important for plant life and they are controlled by the monovalent cation proton antiporter (CPA) superfamily. The roles of ZmCPAs in salt tolerance are not fully elucidated. In this study, we identified 35 ZmCPAs comprising 13 Na⁺/H⁺ exchangers (ZmNHXs), 16 cation/H⁺ exchanger (ZmCHXs), and 6 K⁺ efflux antiporters (ZmKEAs). All ZmCPAs have transmembrane domains and most of them were localized to plasma membrane or tonoplast. ZmCHXs were specifically highly expressed in anthers, while ZmNHXs and ZmKEAs showed high expression in various tissues. ZmNHX5 and ZmKEA2 were up-regulated in maize seedlings under both NaCl and KCl stresses. Yeast complementation experiments revealed the roles of ZmNHX5, ZmKEA2 in NaCl tolerance. Analysis of the maize mutants further validated the salt tolerance functions of ZmNHX5 and ZmKEA2. Our study highlights comprehensive information of ZmCPAs and provides new gene targets for salt tolerance maize breeding.     

Improved yield,fruit quality,and salt resistance in tomato co-overexpressing LeNHX2 and SlSOS2 genes

The K⁺, Na⁺/H⁺ antiporter LeNHX2 and the regulatory kinase SlSOS2 are important determinants of salt tolerance in tomato plants and their fruit production ability. In this work, we have analyzed the effects of LeNHX2 and SlSOS2 co-overexpression on fruit production, quality in tomato plants (Solanum lycopersicum L. cv. MicroTom), and analyzed physiological parameters related to salt tolerance. Plants overexpressing LeNHX2, SlSOS2 or both were grown in greenhouse. They were treated with 125 mM NaCl or left untreated and their salt tolerance was analyzed in terms of plant biomass and fruit yield. Under NaCl cultivation conditions, transgenic tomato plants overexpressing either SlSOS2 or LeNHX2 or both grew better and showed a higher biomass compared to their wild-type plants. Proline, glucose and protein content in leaves as well as pH and total soluble solid (TSS) in fruits were analyzed. Our results indicate that salinity tolerance of transgenic lines is associated with an increased proline, glucose and protein content in leaves of plants grown either with or without NaCl. Salt treatment significantly reduced yield, pH and TSS in fruits of WT plants but increased yield, pH and TSS in fruits of transgenic plants, especially those overexpressing both LeNHX2 and SlSOS2. All these results indicate that the co-overexpression of LeNHX2 and SlSOS2 improve yield and fruit quality of tomato grown under saline conditions.     

Salt tolerance of a cash crop halophyte Suaeda fruticosa: biochemical responses to salt and exogenous chemical treatments

Suaeda fruticosa Forssk is a leaf succulent obligate halophyte that produces numerous seeds under saline conditions. Seeds are a good source of high quality edible oil and leaves are capable of removing substantial amount of salt from the saline soil besides many other economic usages. Little is known about the biochemical basis of salt tolerance in this species. We studied some biochemical responses of S. fruticosa to different exogenous treatments under non-saline (0 mM), moderate (300 mM) or high (600 mM) NaCl levels. Eight-week-old seedlings were sprayed twice a week with distilled water, hydrogen peroxide (H₂O₂, 100 μM), glycine betaine (GB, 10 mM), or ascorbic acid (AsA, 20 mM) for 30 days. At moderate (300 mM) NaCl, leaf Na⁺, Ca²⁺ and osmolality increased, along with unchanged ROS and antioxidant enzyme activities, possibly causing a better plant growth. Plants grew slowly at 600 mM NaCl to avoid leaf Na⁺ buildup relative to those at 300 mM NaCl. Exogenous application of distilled water and H₂O₂ improved ROS scavenging mechanisms, although growth was unaffected. ASA and GB alleviated salt-induced growth inhibition at 600 mM NaCl through enhancing the antioxidant defense system and osmotic and ion homeostasis, respectively.