Sodium Extrusion Research Articles

Mechanistic insights of salicylic acid-mediated salt stress tolerance in Zea mays L. seedlings

Elevated sodium level (Na+) poses significant threat to crop plant physio-biochemical processes, leading to impaired growth followed by decline in productivity. Addressing this challenge, requires an eco-friendly and cost-effective strategy that enhances plant salt stress tolerance capacity. In this context, the exogenous source of plant growth regulators (PGRs) proved to be an efficient approach. Of various PGRs, salicylic acid (SA) is an emerging signaling molecule that boosts plant stress endurance mechanism. This study investigates SA-mediated salt stress tolerance in maize (Zea mays L.) seedlings, by examining morpho-physiological and biochemical traits. Maize seedlings were subjected to varying levels of salt stress (0, 25, 50, 75, 100, and 150 mM NaCl) for a period of 10-days. The results revealed that, a substantial decline in germination percentage, shoot and root length, plant biomass, vigour index, and various other physiological parameters under salt stress causing concentrations. Conversely, salt stress increased oxidative stress indicators, including hydrogen peroxide (H2O2) and malondialdehyde (MDA), osmolytes and elemental concentrations as well as antioxident enzymes (SOD, CAT, POX, APX, GR, AsA). However, the exogenous supplementation of SA at 0.1 mM significantly restored most morpho-physiological attributes in maize under salt stress conditions. This suggests that SA actively triggers the ascorbate-glutathione (AsA-GSH) pathway and other key enzymes, leading to sodium extrusion and improving antioxidant defense in maize seedlings. This finding provides valuable insights for maize farmers that employing SA could lead to improved maize production in saline soils.

#2704 Effects of high dietary salt on immunome and microbiome composition – results from a randomized clinical trial

Abstract Background and Aims Increased dietary salt intake ranks among the most prominent nutritional risk factors worldwide and has been tied to arterial hypertension and all-cause mortality. The mechanisms responsible are incompletely understood and have recently been extended to include inflammatory and microbiome-associated mechanisms. Building on our preliminary data, the present study investigates whether a moderate increase in salt intake in healthy subjects primes host physiology to a more transiently unstable state, potentially leading to changes in the microbiome-immune axis. Method We conducted a prospective, randomized, double-blinded trial to evaluate the effect of high dietary salt in healthy participants (NCT03024567), where the intervention group (n = 19) was given 6 g of salt (NaCl capsules) in addition to the habitual salt intake, resulting in an average doubling of the salt intake recommended by expert societies. A placebo group (n = 19) received gelatin capsules for 14 days. Clinical parameters, fecal samples and PBMC were collected at baseline and day 14. Shotgun metagenomic sequencing, metabolomics (stool and serum), and single-cell sequencing (Cellular Indexing of Transcriptomes and Epitopes by Sequencing (CITEseq)) of whole PBMCs and CD4+ sorted T cells (∼150 k single cells) was performed. Results Microbiome analysis confirms previous findings showing an increased dissimilarity of microbiome composition (Jensen-Shannon Divergence) under a high-salt. Differential abundance of the microbiome's functional space, as quantified by functional gut-specific modules, picked up a salt-specific module (M00494, NatK-NatR (sodium extrusion) two-component regulatory system) in the salt intervention group. CITEseq revealed a significant reduction of circulating NK cells and an increase in classical monocytes as well as changes to several T cell subsets. Furthermore, within several immune cell subsets, we detected differently expressed genes (DEGs) solely in the salt group. We found 602 DEG in naïve conventional T cells (Tconv) and 202 DEGs in non-naïve Tconv, e.g. belonging to NF-kB and T cell receptor signaling pathways. Conclusion Our placebo-controlled study is the first to utilize different omics techniques to investigate and identify high-salt-induced changes in the microbiome and immunome in healthy individuals that may be relevant for health-to-disease transition.

Does brassinosteroid mitigate salt stress in sorghum?

The objective of the present study was to assess the mitigating effect of brassinosteroid regulator plant on damages caused by salt stress in sorghum. The experiment was carried out in pots containing 5 kg of substrate, on benches in a greenhouse covered with transparent plastic film and with a 50% shade screen on the sides. A completely randomized design with four replications was used, in a 2×5 factorial arrangement consisted of application and non-application of brassinosteroids at concentration of 0.1 mg L-1 applied at the 30, 37, and 45 days after emergence (DAE) and five salinity levels of the irrigation water (electrical conductivities of 0, 2, 4, 8, and 16 dS m-1). The plants were irrigated with water from the public supplying system up to 45 DAE and, then, with brackish water up to 60 (DAE), when they were evaluated. Our results indicate that Brassinosteroids mitigated damages caused by salt stress by increasing the Ca to Na ratio. Sorghum is moderately resistant to salt stress by presenting a sodium extrusion mechanism activated by calcium. Further studies with longer exposure time to salt stress are recommended for a more thorough understanding and recommendation of the use of brackish water in sorghum crops

Genotypic Divergence, Photosynthetic Efficiency, Sodium Extrusion, and Osmoprotectant Regulation Conferred Salt Tolerance in Sorghum

Genotypic Divergence, Photosynthetic Efficiency, Sodium Extrusion, and Osmoprotectant Regulation Conferred Salt Tolerance in Sorghum

Regulation of Na+/H+ exchangers, Na+/K+ transporters, and lignin biosynthesis genes, along with lignin accumulation, sodium extrusion, and antioxidant defense, confers salt tolerance in alfalfa.

Accumulation of high sodium (Na+) leads to disruption of metabolic processes and decline in plant growth and productivity. Therefore, this study was undertaken to clarify how Na+/H+ exchangers and Na+/K+ transporter genes contribute to Na+ homeostasis and the substantial involvement of lignin biosynthesis genes in salt tolerance in alfalfa (Medicago sativa L.), which is poorly understood. In this study, high Na+ exhibited a substantial reduction of morphophysiological indices and induced oxidative stress indicators in Xingjiang Daye (XJD; sensitive genotype), while Zhongmu (ZM; tolerant genotype) remained unaffected. The higher accumulation of Na+ and the lower accumulation of K+ and K+/(Na+ + K+) ratio were found in roots and shoots of XJD compared with ZM under salt stress. The ZM genotype showed a high expression of SOS1 (salt overly sensitive 1), NHX1 (sodium/hydrogen exchanger 1), and HKT1 (high-affinity potassium transporter 1), which were involved in K+ accumulation and excess Na+ extrusion from the cells compared with XJD. The lignin accumulation was higher in the salt-adapted ZM genotype than the sensitive XJD genotype. Consequently, several lignin biosynthesis-related genes including 4CL2, CCoAOMT, COMT, CCR, C4H, PAL1, and PRX1 exhibited higher mRNA expression in salt-tolerant ZM compared with XJD. Moreover, antioxidant enzyme (catalase, superoxide dismutase, ascorbate peroxidase, and glutathione reductase) activity was higher in ZM relative to XJD. This result suggests that high antioxidant provided the defense against oxidative damages in ZM, whereas low enzyme activity with high Na+ triggered the oxidative damage in XJD. These findings together illustrate the ion exchanger, antiporter, and lignin biosysthetic genes involving mechanistic insights into differential salt tolerance in alfalfa.

A Ca2+-sensor switch for tolerance to elevated salt stress in Arabidopsis.

Excessive Na+ in soils inhibits plant growth. Here, we report that Na+ stress triggers primary calcium signals specifically in a cell group within the root differentiation zone, thus forming a "sodium-sensing niche" in Arabidopsis. The amplitude of this primary calcium signal and the speed of the resulting Ca2+ wave dose-dependently increase with rising Na+ concentrations, thus providing quantitative information about the stress intensity encountered. We also delineate a Ca2+-sensing mechanism that measures the stress intensity in order to mount appropriate salt detoxification responses. This is mediated by a Ca2+-sensor-switch mechanism, in which the sensors SOS3/CBL4 and CBL8 are activated by distinct Ca2+-signal amplitudes. Although the SOS3/CBL4-SOS2/CIPK24-SOS1 axis confers basal salt tolerance, the CBL8-SOS2/CIPK24-SOS1 module becomes additionally activated only in response to severe salt stress. Thus, Ca2+-mediated translation of Na+ stress intensity into SOS1 Na+/H+ antiporter activity facilitates fine tuning of the sodium extrusion capacity for optimized salt-stress tolerance.

Meta-Omics reveal the metabolic acclimation of freshwater anammox bacteria for saline wastewater treatment

Abstract Saline wastewater is a big challenge for the application of anammox processes. Although the feasibility of freshwater anammox bacteria for saline wastewater treatment was revealed, its metabolic acclimation to different saline concentrations is still elusive. Here, one low-strength-fed anammox reactor was operated with the shift of saline concentrations. The results showed that one Ca. Brocadia genome with a relative abundance of 31.5% was recovered. Meta-Omics analyses showed that salinity (around 12.5 g L −1 NaCl) could promote the nitrogen transformation in Ca. Brocadia through the driven of dissimilatory nitrate reduction to ammonium (DNRA). Specific sodium translocations pump (Na + -NADH:quinone oxidoreductase), as well as more genes associated with H + - dependent membrane-bound ATP formation and Na + -dependent membrane-bound ATP formation, was highly expressed in Ca. Brocadia under saline conditions than that under fresh conditions. This implied that such Na + transduction was likely associated with the NADPH/NADH via the hydrazine oxidation pathway. Notably, sodium extrusion was also associated with energy exploitation from the hydrolysis of pyrophosphatase (PPi) through K + -stimulated pyrophosphate-energized sodium pump. This study shed light on the metabolic acclimation, such as nitrogen transformation and cation-induced energy transduction (i.e., Na + , K + , and Ca 2+ ) in Ca. Brocadia, illustrating their feasibility for saline and freshwater wastewater treatment. • Ca. Brocadia AMX1 with low similar (<81.8%) to published strains was enriched. • Salinity of 12.5 g L −1 NaCl promoted nitrogen transformation in Ca. Brocadia AMX1 . • The cation-induced energy transductions are verified in Ca. Brocadia AMX1 . • The hydrolysis of pyrophosphatase (PPi) can provide energy for sodium extrusion. • Metabolic linkage of nitrogen conversion and cations transductions were discussed.

Nanomaterial-modulated cellular sodium extrusion and vacuolar sequestration for salt tolerance

Nanomaterials can induce plant tolerance to abiotic environmental stresses, whereas the sensing mechanism and the resulting response at the cellular level need further exploration.

Comparative de novo transcriptome analysis identifies salinity stress responsive genes and metabolic pathways in sugarcane and its wild relative Erianthus arundinaceus [Retzius] Jeswiet

Erianthus arundinaceus [Retzius] Jeswiet, a wild relative of sugarcane has a high biomass production potential and a reservoir of many genes for superior agronomic traits and tolerance to biotic and abiotic stresses. A comparative physiological, anatomical and root transcriptome analysis were carried out to identify the salt-responsive genes and metabolic pathways associated with salt-tolerant E. arundinaceus genotype IND99-907 and salinity-sensitive sugarcane genotype Co 97010. IND99-907 recorded growth of young leaves, higher proline content, higher relative water content, intact root anatomical structures and lower Na+/K+, Ca2+/K+ and Mg2+/K+ ratio as compared to the sugarcane genotype Co 97010. We have generated four de novo transcriptome assemblies between stressed and control root samples of IND99-907 and Co 97010. A total of 649 and 501 differentially expressed genes (FDR<0.01) were identified from the stressed and control libraries of IND99-907 and Co 97010 respectively. Genes and pathways related to early stress-responsive signal transduction, hormone signalling, cytoskeleton organization, cellular membrane stabilization, plasma membrane-bound calcium and proton transport, sodium extrusion, secondary metabolite biosynthesis, cellular transporters related to plasma membrane-bound trafficking, nucleobase transporter, clathrin-mediated endocytosis were highly enriched in IND99-907. Whereas in Co 97010, genes related to late stress-responsive signal transduction, electron transport system, senescence, protein degradation and programmed cell death, transport-related genes associated with cellular respiration and mitochondrial respiratory chain, vesicular trafficking, nitrate transporter and fewer secondary metabolite biosynthetic genes were highly enriched. A total of 27 pathways, 24 biological processes, three molecular functions and one cellular component were significantly enriched (FDR≤ 0.05) in IND99-907 as compared to 20 pathways, two biological processes without any significant molecular function and cellular components in Co 97010, indicates the unique and distinct expression pattern of genes and metabolic pathways in both genotypes. The genomic resources developed from this study is useful for sugarcane crop improvement through development of genic SSR markers and genetic engineering approaches.

Identification of distinctive physiological and molecular responses to salt stress among tolerant and sensitive cultivars of broccoli (Brassica oleracea var. Italica)

BackgroundSalt stress is one of the main constraints determining crop productivity, and therefore one of the main limitations for food production. The aim of this study was to characterize the salt stress response at the physiological and molecular level of different Broccoli (Brassica oleracea L. var. Italica Plenck) cultivars that were previously characterized in field and greenhouse trials as salt sensitive or salt tolerant. This study aimed to identify functional and molecular traits capable of predicting the ability of uncharacterized lines to cope with salt stress. For this purpose, this study measured different physiological parameters, hormones and metabolites under control and salt stress conditions.ResultsThis study found significant differences among cultivars for stomatal conductance, transpiration, methionine, proline, threonine, abscisic acid, jasmonic acid and indolacetic acid. Salt tolerant cultivars were shown to accumulate less sodium and potassium in leaves and have a lower sodium to potassium ratio under salt stress. Analysis of primary metabolites indicated that salt tolerant cultivars have higher concentrations of several intermediates of the Krebs cycle and the substrates of some anaplerotic reactions.ConclusionsThis study has found that the energetic status of the plant, the sodium extrusion and the proline content are the limiting factors for broccoli tolerance to salt stress. Our results establish physiological and molecular traits useful as distinctive markers to predict salt tolerance in Broccoli or to design novel biotechnological or breeding strategies for improving broccoli tolerance to salt stress.

Expression and detergent free purification and reconstitution of the plant plasma membrane Na+/H+ antiporter SOS1 overexpressed in Pichia pastoris

The plant plasma membrane Na+/H+ antiporter SOS1 (Salt Overlay Sensitive 1) of Arabidopsis thaliana is the major transporter extruding Na+ out of cells in exchange for an intracellular H+. The sodium extrusion process maintains a low intracellular Na+ concentration and thereby facilitates salt tolerance. A. thaliana SOS1 consists of 1146 amino acids, with the first 450 in a N-terminal membrane transport domain and the balance forming a cytosolic regulatory domain. For studies on characterization of the protein, two different constructs of SOS1 comprising of the residues 28 to 460 and 28 to 990 were cloned and overexpressed in methylotropic yeast strain of Pichia pastoris with a C-terminal histidine tag using the expression vector pPICZA. Styrene malic acid copolymers (SMA) were used as a cost-effective alternative to detergent for solubilization and isolation of this membrane protein. Immobilized Ni2+-ion affinity chromatography was used to purify the expressed protein resulting in a yield of ~0.6–2 mg of SOS1 per liter of Pichia pastoris culture. The SMA purified protein containing amino acids 28 to 990 was directly reconstituted into liposomes for determination of Na+ transport activity and was functionally active. However, similar reconstitution with amino acids 28–460 did not yield a functional protein. Other results have shown that the truncated SOS1 protein at amino acid 481 is active, which infers the presence of an element between residues 461–481 which is necessary for SOS1 activity. This region contains several conserved segments that may be important in SOS1 structure and function.

Patellin1 Negatively Modulates Salt Tolerance by Regulating PM Na+/H+ Antiport Activity and Cellular Redox Homeostasis in Arabidopsis.

Soil salinity significantly represses plant development and growth. Mechanisms involved sodium (Na+) extrusion and compartmentation, intracellular membrane trafficking as well as redox homeostasis regulation play important roles in plant salt tolerance. In this study, we report that Patellin1 (PATL1), a membrane trafficking-related protein, modulates salt tolerance in Arabidopsis. The T-DNA insertion mutant of PATL1 (patl1) with an elevated PATL1 transcription level displays a salt-sensitive phenotype. PATL1 partially associates with the plasma membrane (PM) and endosomal system, and might participate in regulating membrane trafficking. Interestingly, PATL1 interacts with SOS1, a PM Na+/H+ antiporter in the Salt-Overly-Sensitive (SOS) pathway, and the PM Na+/H+ antiport activity is lower in patl1 than in Col-0. Furthermore, the reactive oxygen species (ROS) content is higher in patl1 and the redox signaling of antioxidants is partially disrupted in patl1 under salt stress conditions. Artificial elimination of ROS could partially rescue the salt-sensitive phenotype of patl1. Taken together, our results indicate that PATL1 participates in plant salt tolerance by regulating Na+ transport at least in part via SOS1, and by modulating cellular redox homeostasis during salt stress.

Patellin1 Negatively Modulates Salt Tolerance by Regulating PM Na+/H+ Antiport Activity and Cellular Redox Homeostasis in Arabidopsis.

Soil salinity significantly represses plant development and growth. Mechanisms involved sodium (Na+) extrusion and compartmentation, intracellular membrane trafficking as well as redox homeostasis regulation play important roles in plant salt tolerance. In this study, we report that Patellin1 (PATL1), a membrane trafficking-related protein, modulates salt tolerance in Arabidopsis. The T-DNA insertion mutant of PATL1 (patl1) with an elevated PATL1 transcription level displays a salt-sensitive phenotype. PATL1 partially associates with the plasma membrane (PM) and endosomal system, and might participate in regulating membrane trafficking. Interestingly, PATL1 interacts with SOS1, a PM Na+/H+ antiporter in the Salt-Overly-Sensitive (SOS) pathway, and the PM Na+/H+ antiport activity is lower in patl1 than in Col-0. Furthermore, the reactive oxygen species (ROS) content is higher in patl1 and the redox signaling of antioxidants is partially disrupted in patl1 under salt stress conditions. Artificial elimination of ROS could partially rescue the salt-sensitive phenotype of patl1. Taken together, our results indicate that PATL1 participates in plant salt tolerance by regulating Na+ transport at least in part via SOS1, and by modulating cellular redox homeostasis during salt stress.

Regulation of Lung Epithelial Sodium Channels by Cytokines and Chemokines.

Acute lung injury leading to acute respiratory distress (ARDS) is a global health concern. ARDS patients have significant pulmonary inflammation leading to flooding of the pulmonary alveoli. This prevents normal gas exchange with consequent hypoxemia and causes mortality. A thin fluid layer in the alveoli is normal. The maintenance of this thin layer results from fluid movement out of the pulmonary capillaries into the alveolar interstitium driven by vascular hydrostatic pressure and then through alveolar tight junctions. This is then balanced by fluid reabsorption from the alveolar space mediated by transepithelial salt and water transport through alveolar cells. Reabsorption is a two-step process: first, sodium enters via sodium-permeable channels in the apical membranes of alveolar type 1 and 2 cells followed by active extrusion of sodium into the interstitium by the basolateral Na+, K+-ATPase. Anions follow the cationic charge gradient and water follows the salt-induced osmotic gradient. The proximate cause of alveolar flooding is the result of a failure to reabsorb sufficient salt and water or a failure of the tight junctions to prevent excessive movement of fluid from the interstitium to alveolar lumen. Cytokine- and chemokine-induced inflammation can have a particularly profound effect on lung sodium transport since they can alter both ion channel and barrier function. Cytokines and chemokines affect alveolar amiloride-sensitive epithelial sodium channels (ENaCs), which play a crucial role in sodium transport and fluid reabsorption in the lung. This review discusses the regulation of ENaC via local and systemic cytokines during inflammatory disease and the effect on lung fluid balance.

Extrusion versus diffusion: mechanisms for recovery from sodium loads in mouse CA1 pyramidal neurons.

Neuronal activity causes local or global sodium signalling in neurons, depending on the pattern of synaptic activity. Recovery from global sodium loads critically relies on Na(+) /K(+) -ATPase and an intact energy metabolism in both somata and dendrites. For recovery from local sodium loads in dendrites, Na(+) /K(+) -ATPase activity is not required per se. Instead, recovery is predominately mediated by lateral diffusion, exhibiting rates that are 10-fold higher than for global sodium signals. Recovery from local dendritic sodium increases is still efficient during short periods of energy deprivation, indicating that fast diffusion of sodium to non-stimulated regions strongly reduces local energy requirements. Excitatory activity is accompanied by sodium influx into neurones as a result of the opening of voltage- and ligand-activated channels. Recovery from resulting sodium transients has mainly been attributed to Na(+) /K(+) -ATPase (NKA). Because sodium ions are highly mobile, diffusion could provide an additional pathway. We tested this in hippocampal neurones using whole-cell patch-clamp recordings and sodium imaging. Somatic sodium transients induced by local glutamate application recovered at a maximum rate of 8mm min(-1) (∼0.03mm min(-1) μm(-2) ). Somatic sodium extrusion was accelerated at higher temperature and blocked by ouabain, emphasizing its dependence on NKA. Moreover, it was slowed down during inhibition of glycolysis by sodium fluoride (NaF). Local glutamate application to dendrites revealed a 10-fold higher apparent dendritic sodium extrusion rate compared to somata. Recovery was almost unaltered by increased temperature, ouabain or NaF. We found that sodium diffused along primary dendrites with a diffusion coefficient of ∼330μm²/s. During global glutamate application, impeding substantial net diffusion, apparent dendritic extrusion rates were reduced to somatic rates and also affected by NaF. Numerical simulations confirmed the essential role of NKA for the recovery of somatic, but not dendritic sodium loads. Our data show that sodium export upon global sodium increases is largely mediated by NKA and depends on an intact energy metabolism. For recovery from local dendritic sodium increases, diffusion dominates over extrusion, operating efficiently even during short periods of energy deprivation. Although sodium will eventually be extruded by the NKA, its diffusion-based fast dissemination to non-stimulated regions might reduce local energy requirements.

Sodium extrusion associated with enhanced expression of SOS1 underlies different salt tolerance between Medicago falcata and Medicago truncatula seedlings

Medicago falcata L. is a perennial legume species native to semiarid grasslands in northern China, and distinguished by its superior tolerance to abiotic stresses such as drought and cold. But there have been few studies to investigate the physiological mechanism underlying salt tolerance in M. falcata. In the present study, we compared the response of non-nodulated M. falcata with legume model plant Medicago truncatula Garetn at physiological and molecular level by submitting them to a salt stock. M. falcata exhibited higher survival rate and less chlorosis than M. truncatula seedlings upon exposure to salt stress, suggesting that M. falcata is more tolerant to salt stress than M. truncatula. The ability to maintain a high K+ and low Na+ contents by plants is an important trait for tolerance to salt stress. M. falcata accumulated less amount of Na+ in their shoots than M. truncatula when exposed to NaCl, leading to a higher K+/(K++Na+) ratio in M. falcata than in M. truncatula under saline conditions. There was a similar difference in Na+ concentration and K+/(K++Na+) ratio in roots and xylem sap between the two species under saline conditions. Results from grafting experiments revealed that seedlings grafted with M. falcata either as scions or rootstocks can significantly reduce Na+ accumulation and increase K+/(K++Na+) ratio in both shoots and roots under saline conditions. Expression levels of SOS1 that encodes a Na+–H+ antiporter responsible for Na+ extrusion from cells in M. falcata were much higher than in M. truncatula under both control and saline conditions. Abundance of SOS1 transcripts in seedlings grafted with M. falcata as either rootstocks or scions was significantly higher than in seedlings self-grafted with M. truncatula. These results strongly suggested that SOS1-dependent Na+ extrusion is an important mechanism to maintain a relatively low Na+ concentration and high K+/(K++Na+) ratio in non-nodulated M. falcata under saline conditions, thus conferring tolerance to salt stress.

Effects of NaCl and Iso-Osmotic Polyethylene Glycol Stress on Na+/H+ Antiport Activity of Three Malus species with Different Salt Tolerance

Salt stress contains osmotic and ionic stress, while iso-osmotic polyethylene glycol (PEG) has only osmotic stress. This study aimed to compare the different effects on the activity of H+-ATPase, proton pump and Na+/H+ antiport in Malus seedlings between osmotic and ionic stress. Species of salt tolerant Malus zumi, middle salt tolerant Malus xiaojinensis and salt sensitive Malus baccata were used as experimental materials. Malus seedlings were treated with NaCl and iso-osmotic PEG stress. The activity of H+-ATPase, proton pump and Na+/H+ antiport of plasmolemma and tonoplast in Malus seedlings were obviously increased under salt stress, and those in salt-tolerant species increased more. Under the same NaCl concentration, the activity of H+-ATPase, proton pump and Na+/H+ antiport of plasmolemma and tonoplast in salt-tolerant species were all obviously higher than those in salt-sensitive one. Higher Na+/H+ antiport activity of plasmolemma and tonoplast in salt-tolerant species could help to extrude and compartmentalize sodium in roots under salt stress. The ascent rate of activity of H+-ATPase, proton pump and Na+/H+ antiport in Malus seedlings under the three salt concentration stress was all obviously higher than that under the iso-osmotic PEG stress. It indicated that the sodium ion effect had more stimulation on the activity of H+-ATPase, proton pump and Na+/H+ antiport in salt-tolerant species, and salt-tolerant species has higher capability of sodium extrusion and compartmentalization in roots and is therefore more salt tolerant.

Ion Transporters and Abiotic Stress Tolerance in Plants.

Adaptation of plants to salt stress requires cellular ion homeostasis involving net intracellular Na+ and Cl− uptake and subsequent vacuolar compartmentalization without toxic ion accumulation in the cytosol. Sodium ions can enter the cell through several low- and high-affinity K+ carriers. Some members of the HKT family function as sodium transporter and contribute to Na+ removal from the ascending xylem sap and recirculation from the leaves to the roots via the phloem vasculature. Na+ sequestration into the vacuole depends on expression and activity of Na+/H+ antiporter that is driven by electrochemical gradient of protons generated by the vacuolar H+-ATPase and the H+-pyrophosphatase. Sodium extrusion at the root-soil interface is presumed to be of critical importance for the salt tolerance. Thus, a very rapid efflux of Na+ from roots must occur to control net rates of influx. The Na+/H+ antiporter SOS1 localized to the plasma membrane is the only Na+ efflux protein from plants characterized so far. In this paper, we analyze available data related to ion transporters and plant abiotic stress responses in order to enhance our understanding about how salinity and other abiotic stresses affect the most fundamental processes of cellular function which have a substantial impact on plant growth development.

A kinetic model for sodium transport via a non‐gastric H+(Na+)/K+ ATPase

Experimental results have suggested that macula densa (MD) cells have a unique way of regulating cellular sodium concentration via an apical H+(Na+)/K+‐ATPase. We developed a kinetic model to represent the sodium transport via this transporter. The model assumes a 2:2 stoichiometry for both the hydrogen and sodium cycles. Rate constant values for the hydrogen cycle were obtained from a renal H+/K+‐ATPase model (A.M. Weinstein, Am J Physiol Renal Physiol 274:F856‐F867, 1998) which were, in turn, based on values obtained from a gastric H+/K+‐ATPase model (P. Brzezinski et al., Biochim. Biophys. Acta 942:215–219, 1988). Rate constants for the sodium cycle are assumed to be equal to those in the hydrogen cycle. Results showed that sodium extrusion is very sensitive to changes in luminal potassium concentration from 0 to 20 mM. In addition, sodium extrusion as a function of internal sodium concentration saturates at ~70 mM. This ATPase pump model is essential in the development of larger scale models of ion transport by the MD cells.Research supported by MBRS‐RISE grant 2R25GM061151‐09 and by NIH‐NIGMS grant SC1GM084744.

Arabidopsis sodium dependent and independent phenotypes triggered by H +-PPase up-regulation are SOS1 dependent

Coordinate regulation of transporters at both the plasma membrane and vacuole contribute to plant cell's ability to adapt to a changing environment and play a key role in the maintenance of the chemiosmotic circuits required for cellular growth. The plasma membrane (PM) Na +/H + antiporter (SOS1) is involved in salt tolerance, presumably in sodium extrusion; the vacuolar type I H +-PPase AVP1 is involved in vacuolar sodium sequestration, but its overexpression has also been shown to alter the abundance and activity of the PM H +-ATPase. Here we investigate the relationship between these transporters utilizing loss-of-function mutants of SOS1 ( sos1) and increased expression of AVP1 ( AVP1OX). Heightened expression of AVP1 enhances pyrophosphate-dependent proton pump activity, salt tolerance, ion vacuolar sequestration, K + uptake capacity, root hair development, osmotic responses, and PM ATPase hydrolytic and proton pumping activities. In sos1 lines overexpressing AVP1, these phenotypes are negatively affected demonstrating that sos1 is epistatic to AVP1. Enhanced AVP1 protein levels require SOS1 and this regulation appears to be post-translational.