Plasma membrane H-ATPase activity is involved in adaptation of tomato calli to NaCl

Effect of salt stress on proximate composition of duckweed (Lemna minor L.)

The shortage of conventional feedstuff is one of the rising issues faced by the developing countries of the world. To bridge the gap between supply and demand of the major feedstuff it is desirable to practice the use of non-conventional feed resources. Duckweeds are the aquatic macrophytes growing in stagnant water bodies that offer a choice to be used as an alternate feed. Before the use of any alternate feed, it is vital to know the nutritional composition of the feed under diverse environmental conditions. The objective of this study was to investigate the influence of salinity, abiotic stress, on the proximate composition of duckweed (Lemna minor L.). The experiment was laid out in Completely Randomized Design (CRD) with 3 repeats. Data was collected on protein, lipid, carbohydrate, and mineral contents. In the laboratory trial plants were grown under the saline condition of different concentrations ranging from 2 g NaCl L⁻¹ to 12 g NaCl L⁻¹ for a growing period of 20 days. The biomasses obtained were tested for proximate composition. ANOVA of the result exhibited a significant effect of salinity on the proximate composition of the plant. Protein residues of the plant started declining above the concentration of 4 g NaCl L⁻¹ until the lowest value was obtained at 12 g NaCl L⁻¹. Lipid composition showed more sensitivity to the stress with a sharp decline above 2 g NaCl L⁻¹ having a minimum value at 12 g NaCl L⁻¹. Carbohydrate contents increased with increasing salinity up to 6 g NaCl L⁻¹ above which a decrease was observed. The highest accumulation of the macronutrients i.e., Ca, Mg, took place in the lower range of concentration of the salt. The percentage compositions of micronutrients such as Fe, Mn, and Zn percentage were reduced at a higher range of salinity while the optimum level was recorded in plants treated with 2 g NaCl L⁻¹, followed by control. The total accumulation of both macro and micronutrients was higher in the plant material treated with a lower level of salt concentration, concluding a significant effect of salinity on proximate composition. As for the Indus water salinity level, the plant has the capacity of tolerance and can be grown without affecting its proximate composition.

Highlights to phytosterols accumulation and equilibrium in plants: Biosynthetic pathway and feedback regulation

Phytosterols are a group of sterols exclusive to plants and fungi, but are indispensable to humans because of their medicinal and nutritional values. However, current raw materials used for phytosterols extraction add to the cost and waste in the process. For higher sterols production, major attention is drawn to plant materials abundant in phytosterols and genetic modification. To provide an insight into phytosterols metabolism, the research progress on key enzymes involved in phytosterols biosynthesis and conversions were summarized. CAS, SSR2, SMT, DWF1 and CYP710A, the enzymes participating in the biosynthetic pathway, and PSAT, ASAT and SGT, the enzymes involved in the conversion of free sterols to conjugated ones, were reviewed. Specifically, SMT and CYP710A were emphasized for their function on modulating the percentage composition of different kinds of phytosterols. The thresholds of sterol equilibrium and the resultant phytosterols accumulation, which vary in plant species and contribute to plasma membrane remodeling under stresses, were also discussed. By retrospective analysis of the previous researches, we proposed a feedback mechanism regulating sterol equilibrium underlying sterols metabolism. From a strategic perspective, we regard salt tolerant plant as an alternative to present raw materials, which will attain higher phytosterols production in combination with gene-modification.

Involvement of Phosphatidylserine and Triacylglycerol in the Response of Sweet Potato Leaves to Salt Stress

Lipid remodeling plays an important role in the adaptation of plants to environmental factors, but the mechanism by which lipid remodeling mediates salt stress response remains unclear. In this study, we compared the root and leaf lipidome profiles of salt-tolerant and salt-sensitive sweet potato cultivars (Xu 22 and Xu 32, respectively) under salinity stress. After salt treatment, the leaf lipidome showed more significant remodeling than the root lipidome in both cultivars. Compared with Xu 32 leaves, Xu 22 leaves generally maintained higher abundance of phospholipids, glycolipids, sphingolipids, sterol derivatives, and diacylglycerol under salinity conditions. Interestingly, salinity stress significantly increased phosphatidylserine (PS) abundance in Xu 22 leaves by predominantly triggering the increase of PS (20:5/22:6). Furthermore, Xu 32 leaves accumulated higher triacylglycerol (TG) level than Xu 22 leaves under salinity conditions. The exogenous application of PS delayed salt-induced leaf senescence in Xu 32 by reducing salt-induced K+ efflux and upregulating plasma membrane H+-ATPase activity. However, the inhibition of TG mobilization in salinized-Xu 22 leaves disturbed energy and K+/Na+ homeostasis, as well as plasma membrane H+-ATPase activity. These results demonstrate alterations in the leaf lipidome of sweet potato under salinity condition, underscoring the importance of PS and TG in mediating salt-defensive responses in sweet potato leaves.

iTRAQ-Based Comparative Proteomic Analysis Provides Insights into Molecular Mechanisms of Salt Tolerance in Sugar Beet (Beta vulgaris L.)

Salinity is one of the major abiotic stress factors that limit plant growth and crop yield worldwide. To understand the molecular mechanisms and screen the key proteins in response of sugar beet (Beta vulgaris L.) to salt, in the present study, the proteomics of roots and shoots in three-week-old sugar beet plants exposed to 50 mM NaCl for 72 h was investigated by isobaric Tags for Relative and Absolute Quantitation (iTRAQ) technology. The results showed that 105 and 30 differentially expressed proteins (DEPs) were identified in roots and shoots of salt-treated plants compared with untreated plants, respectively. There were 46 proteins up-regulated and 59 proteins down-regulated in roots; and 13 up-regulated proteins and 17 down-regulated proteins found in shoots, respectively. These DEPs were mainly involved in carbohydrate metabolism, energy metabolism, lipid metabolism, biosynthesis of secondary metabolites, transcription, translation, protein folding, sorting, and degradation as well as transport. It is worth emphasizing that some novel salt-responsive proteins were identified, such as PFK5, MDH, KAT2, ACAD10, CYP51, F3H, TAL, SRPR, ZOG, V-H⁺-ATPase, V-H⁺-PPase, PIPs, TIPs, and tubulin α-2/β-1 chain. qRT-PCR analysis showed that six of the selected proteins, including BvPIP1-4, BvVP and BvVAP in root and BvTAL, BvURO-D1, and BvZOG in shoot, displayed good correlation between the expression levels of protein and mRNA. These novel proteins provide a good starting point for further research into their functions using genetic or other approaches. These findings should significantly improve the understanding of the molecular mechanisms involved in salt tolerance of sugar beet plants.

Comparative Proteomic Analysis of Soybean Leaves and Roots by iTRAQ Provides Insights into Response Mechanisms to Short-Term Salt Stress

Salinity severely threatens land use capability and crop yields worldwide. Understanding the mechanisms that protect soybeans from salt stress will help in the development of salt-stress tolerant leguminous plants. Here we initially analyzed the changes in malondialdehyde levels, the activities of superoxide dismutase and peroxidases, chlorophyll content, and Na(+)/K(+) ratios in leaves and roots from soybean seedlings treated with 200 mM NaCl at different time points. We found that the 200 mM NaCl treated for 12 h was optimal for undertaking a proteomic analysis on soybean seedlings. An iTRAQ-based proteomic approach was used to investigate the proteomes of soybean leaves and roots under salt treatment. These data are available via ProteomeXchange with the identifier PXD002851. In total, 278 and 440 proteins with significantly altered abundances were identified in leaves and roots of soybean, respectively. From these data, a total of 50 proteins were identified in the both tissues. These differentially expressed proteins (DEPs) were from 13 biological processes. Moreover, protein-protein interaction analysis revealed that proteins involved in metabolism, carbohydrate and energy metabolism, protein synthesis and redox homeostasis could be assigned to four high salt stress response networks. Furthermore, semi-quantitative RT-PCR analysis revealed that some of the proteins, such as a 14-3-3, MMK2, PP1, TRX-h, were also regulated by salt stress at the level of transcription. These results indicated that effective regulatory protein expression related to signaling, membrane and transport, stress defense and metabolism all played important roles in the short-term salt response of soybean seedlings.

Cytoplasmic pH-Stat during Phenanthrene Uptake by Wheat Roots: A Mechanistic Consideration

Dietary intake of plant-based foods is a major contribution to the total exposure of polycyclic aromatic hydrocarbons (PAHs). However, the mechanisms underlying PAH uptake by roots remain poorly understood. This is the first study, to our knowledge, to reveal cytoplasmic pH change and regulation in response to PAH uptake by wheat roots. An initial drop of cytoplasmic pH, which is concentration-dependent upon exposure to phenanthrene (a model PAH), was followed by a slow recovery, indicating the operation of a powerful cytoplasmic pH regulating system. Intracellular buffers are prevalent and act in the first few minutes of acidification. Phenanthrene activates plasmalemma and tonoplast H(+) pump. Cytolasmic acidification is also accompanied by vacuolar acidification. In addition, phenanthrene decreases the activity of phosphoenolpyruvate carboxylase and malate concentration. Moreover, phenanthrene stimulates nitrate reductase. Therefore, it is concluded that phenanthrene uptake induces cytoplasmic acidification, and cytoplasmic pH recovery is achieved via physicochemical buffering, proton transport outside cytoplasm into apoplast and vacuole, and malate decarboxylation along with nitrate reduction. Our results provide a novel insight into PAH uptake by wheat roots, which is relevant to strategies for reducing PAH accumulation in wheat for food safety and improving phytoremediation of PAH-contaminated soils or water by agronomic practices.

Identification of early salt stress responsive proteins in seedling roots of upland cotton (Gossypium hirsutum L.) employing iTRAQ-based proteomic technique

Soil salinity is a major abiotic stress that limits plant growth and agricultural productivity. Upland cotton (Gossypium hirsutum L.) is highly tolerant to salinity; however, large-scale proteomic data of cotton in response to salt stress are still scant. Here, an isobaric tag for relative and absolute quantitation (iTRAQ)-based proteomic technique was employed to identify the early differentially expressed proteins (DEPs) from salt-treated cotton roots. One hundred and twenty-eight DEPs were identified, 76 of which displayed increased abundance and 52 decreased under salt stress conditions. The majority of the proteins have functions related to carbohydrate and energy metabolism, transcription, protein metabolism, cell wall and cytoskeleton metabolism, membrane and transport, signal transduction, in addition to stress and defense. It is worth emphasizing that some novel salt-responsive proteins were identified, which are involved in cell cytoskeleton metabolism (actin-related protein2, ARP2, and fasciclin-like arabinogalactan proteins, FLAs), membrane transport (tonoplast intrinsic proteins, TIPs, and plasma membrane intrinsic proteins, PIPs), signal transduction (leucine-rich repeat receptor-like kinase encoding genes, LRR-RLKs) and stress responses (thaumatin-like protein, TLP, universal stress protein, USP, dirigent-like protein, DIR, desiccation-related protein PCC13-62). High positive correlation between the abundance of some altered proteins (superoxide dismutase, SOD, peroxidase, POD, glutathione S-transferase, GST, monodehydroascorbate reductase, MDAR, and malate dehydrogenase, MDH) and their enzyme activity was evaluated. The results demonstrate that the iTRAQ-based proteomic technique is reliable for identifying and quantifying a large number of cotton root proteins. qRT-PCR was used to study the gene expression levels of the five above-mentioned proteins; four patterns are consistent with those of induced protein. These results showed that the proteome of cotton roots under NaCl stress is complex. The comparative protein profiles of roots under salinity vs control improves the understanding of the molecular mechanisms involved in the tolerance of plants to salt stress. This work provides a good basis for further functional elucidation of these DEPs using genetic and/or other approaches, and, consequently, candidate genes for genetic engineering to improve crop salt tolerance.

Durum and bread wheat differ in their ability to retain potassium in leaf mesophyll: implications for salinity stress tolerance

Understanding the intrinsic mechanisms involved in the differential salinity tolerance between bread wheat and durum wheat is essential for breeding salt-tolerant varieties to cope with the global salinity issue threatening future food supply. In the past, higher salinity tolerance in bread wheat compared with durum wheat has been attributed to its better ability to exclude Na(+) from uptake. Here we show that another mechanism, namely more superior K(+) retention ability in the leaf mesophyll, also contributes to this difference. A strong positive correlation (R(2) > 0.41, P < 0.001) was found between NaCl-induced K(+) efflux in the leaf mesophyll and overall salinity tolerance in 48 wheat varieties. However, while the above correlation was strong in bread wheat, it was statistically insignificant in durum wheat. Consistent with these findings, a significantly higher relative leaf K(+) content was found in bread wheat than in durum wheat. In contrast to root tissues, the role of voltage-gated K(+) channels in K(+) retention in the wheat mesophyll was relatively small, and non-selective cation channels played a major role in controlling intracellular K(+) homeostasis. Moreover, a significant negative correlation between NaCl-induced mesophyll H(+) flux and mesophyll K(+) retention was found, and interpreted as a compensatory mechanism employed by sensitive varieties to regain K(+) leaked into the apoplast. It is concluded that bread wheat and durum wheat show different strategies of coping with salinity, and that targeting mechanisms conferring K(+) retention in the leaf mesophyll may be a promising way to improve the overall salinity tolerance in these species.

Salicylic acid improves salinity tolerance in Arabidopsis by restoring membrane potential and preventing salt-induced K+ loss via a GORK channel

Despite numerous reports implicating salicylic acid (SA) in plant salinity responses, the specific ionic mechanisms of SA-mediated adaptation to salt stress remain elusive. To address this issue, a non-invasive microelectrode ion flux estimation technique was used to study kinetics of NaCl-induced net ion fluxes in Arabidopsis thaliana in response to various SA concentrations and incubation times. NaCl-induced K(+) efflux and H(+) influx from the mature root zone were both significantly decreased in roots pretreated with 10-500 μM SA, with strongest effect being observed in the 10-50 μM SA range. Considering temporal dynamics (0-8-h SA pretreatment), the 1-h pretreatment was most effective in enhancing K(+) retention in the cytosol. The pharmacological, membrane potential, and shoot K(+) and Na(+) accumulation data were all consistent with the model in which the SA pretreatment enhanced activity of H(+)-ATPase, decreased NaCl-induced membrane depolarization, and minimized NaCl-induced K(+) leakage from the cell within the first hour of salt stress. In long-term treatments, SA increased shoot K(+) and decreased shoot Na(+) accumulation. The short-term NaCl-induced K(+) efflux was smallest in the gork1-1 mutant, followed by the rbohD mutant, and was highest in the wild type. Most significantly, the SA pretreatment decreased the NaCl-induced K(+) efflux from rbohD and the wild type to the level of gork1-1, whereas no effect was observed in gork1-1. These data provide the first direct evidence that the SA pretreatment ameliorates salinity stress by counteracting NaCl-induced membrane depolarization and by decreasing K(+) efflux via GORK channels.

Overexpression of SlSOS2 (SlCIPK24) confers salt tolerance to transgenic tomato

The Ca(2+)-dependent SOS pathway has emerged as a key mechanism in the homeostasis of Na(+) and K(+) under saline conditions. We have identified and functionally characterized the gene encoding the calcineurin-interacting protein kinase of the SOS pathway in tomato, SlSOS2. On the basis of protein sequence similarity and complementation studies in yeast and Arabidopsis, it can be concluded that SlSOS2 is the functional tomato homolog of Arabidopsis AtSOS2 and that SlSOS2 operates in a tomato SOS signal transduction pathway. The biotechnological potential of SlSOS2 to provide salt tolerance was evaluated by gene overexpression in tomato (Solanum lycopersicum L. cv. MicroTom). The better salt tolerance of transgenic plants relative to non-transformed tomato was shown by their faster relative growth rate, earlier flowering and higher fruit production when grown with NaCl. The increased salinity tolerance of SlSOS2-overexpressing plants was associated with higher sodium content in stems and leaves and with the induction and up-regulation of the plasma membrane Na(+)/H(+) (SlSOS1) and endosomal-vacuolar K(+), Na(+)/H(+) (LeNHX2 and LeNHX4) antiporters, responsible for Na(+) extrusion out of the root, active loading of Na(+) into the xylem, and Na(+) and K(+) compartmentalization.

Differential salinity-induced variations in the activity of H⁺-pumps and Na⁺/H⁺ antiporters that are involved in cytoplasm ion homeostasis as a function of genotype and tolerance level in rice cell lines

The characterisation of cellular responses to salinity in staple crops is necessary for the reliable identification of physiological markers of salinity tolerance. Under saline conditions, variations in proton gradients that are generated by membrane-bound H⁺ pumps are crucial for maintaining cytoplasm homeostasis. We examined short (15 h) and longer term effects (4 days) of NaCl stress on the H⁺ pumping activities that are associated with the plasma membrane (P-ATPase) and the tonoplast (V-ATPase and V-PPase) in rice (Oryza sativa L.) callus lines that displayed different levels of NaCl tolerance and were established from two japonica rice cultivars. The applied stress conditions were based on those that were used in the induction of a stress-responsive polyubiquitin gene promoter (UBI1) in transgenic rice calli. The most remarkable effect of NaCl stress on H⁺ pumping was the rapid activation of tonoplast-bound pumps; this was particularly observed in cv. Bomba, in which the response of the P-ATPase was slower and showed a higher level of activity after 4 days of stress. The responses were cultivar-dependent; however, in general, a stronger activation occurred in the lines that had a higher tolerance (L-T) than in the less-tolerant (L-S) lines. Substrate hydrolysis was less affected than H⁺ pumping, and it yielded higher H⁺/substrate coupling ratios, which is indicative of an enhanced H⁺ pumping efficiency under saline conditions. The Na⁺/H⁺ antiport activity was generally limited to salt-stressed calli, and higher values and stronger activation of the tonoplast antiporter were observed in the L-T lines than in the L-S lines. The results that were obtained with the NaCl-stressed transgenic lines confirmed the close relationship between metabolic activity, H⁺ pumping and the induction of Na⁺/H⁺ exchange activities.

cGMP regulates hydrogen peroxide accumulation in calcium-dependent salt resistance pathway in Arabidopsis thaliana roots

3',5'-cyclic guanosine monophosphate (cGMP) is an important second messenger in plants. In the present study, roles of cGMP in salt resistance in Arabidopsis roots were investigated. Arabidopsis roots were sensitive to 100 mM NaCl treatment, displaying a great increase in electrolyte leakage and Na(+)/K(+) ratio and a decrease in gene expression of the plasma membrane (PM) H(+)-ATPase. However, application of exogenous 8Br-cGMP (an analog of cGMP), H(2)O(2) or CaCl(2) alleviated the NaCl-induced injury by maintaining a lower Na(+)/K(+) ratio and increasing the PM H(+)-ATPase gene expression. In addition, the inhibition of root elongation and seed germination under salt stress was removed by 8Br-cGMP. Further study indicated that 8Br-cGMP-induced higher NADPH levels for PM NADPH oxidase to generate H(2)O(2) by regulating glucose-6-phosphate dehydrogenase (G6PDH) activity. The effect of 8Br-cGMP and H(2)O(2) on ionic homeostasis was abolished when Ca(2+) was eliminated by glycol-bis-(2-amino ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA, a Ca(2+) chelator) in Arabidopsis roots under salt stress. Taken together, cGMP could regulate H(2)O(2) accumulation in salt stress, and Ca(2+) was necessary in the cGMP-mediated signaling pathway. H(2)O(2), as the downstream component of cGMP signaling pathway, stimulated PM H(+)-ATPase gene expression. Thus, ion homeostasis was modulated for salt tolerance.

The apoplastic pH and its significance in adaptation to salinity in maize (Zea mays L.): Comparison of fluorescence microscopy and pH-sensitive microelectrodes

The apoplastic ionic milieu contains essential determinants for cell expansion and plant growth. Since pH is a multifunctional basic component of this extracellular space, the knowledge of its behaviour during stress situations is of major importance. In detached leaves of maize (Zea mays L. cvs. Pioneer 3906 and SR 03) the effect of salinity on apoplastic pH was measured to investigate its adaptive role to salt stress applying two different methods: an optical approach using pH-sensitive fluorescent dyes (fluorescein isothiocyanate-dextran (FITC), fluorescein tetramethylrhodamine-dextran (FTMR) and Oregon Green(®) 488), and an electrophysiological technique, pH-sensitive microelectrodes. Both approaches yielded similar results. In the presence of 100mM NaCl, which was added to the growth medium, apoplastic pH of the salt-sensitive maize genotype Pioneer 3906 leaves increased in maximum by 0.4 units (pH microelectrodes) and by 0.3 units (fluorescent dyes); the salt-resistant SR 03 hardly responded. The same treatment reduced leaf growth by 60% in Pioneer 3906, but only by 40% in SR 03. Since according to acid growth considerations apoplastic pH is an important factor in elongation growth, we suggest that this pH increase is a main cause for reduced leaf growth under salt stress conditions.

Interactive effects of salinity and iron deficiency in Medicago ciliaris

In calcareous salt-affected soils, iron availability to plants is subjected to the effects of both sodium and bicarbonate ions. Our aim was to study interactive effects of salinity and iron deficiency on iron acquisition and root acidification induced by iron deficiency in Medicago ciliaris L., a species commonly found in saline ecosystems. Four treatments were used: C, control treatment, complete medium (CM) containing 30 microM Fe; S, salt treatment, CM with 75 mM NaCl; D, deficient treatment, CM containing only 1 microM Fe; DS, interactive treatment, CM containing 1 microM Fe with 75 mM NaCl. Our study showed that plant growth and chlorophyll content were much more affected by the interactive treatment than by iron deficiency or by the salt treatment, indicating an additive effect of these constraints in DS plants. These results could be partially explained by Na accumulation in shoots as well as a limitation of nutrient uptake such as Fe and K under salt stress, under iron deficiency, and especially under their combined effect. The study also showed that root acidification was deeply diminished when iron deficiency was associated with salinity. This probably explained the decrease of Fe uptake and suggested that root proton pump activity would be inhibited by salinity.

Properties of plasma membrane H+ -ATPase in salt-treated Populus euphratica callus

The plasma membrane (PM) vesicles from Populus euphratica (P. euphratica) callus were isolated to investigate the properties of the PM H(+)-ATPase. An enrichment of sealed and oriented right-side-out PM vesicles was demonstrated by measurement of the purity and orientation of membrane vesicles in the upper phase fraction. Analysis of pH optimum, temperature effects and kinetic properties showed that the properties of the PM H(+)-ATPase from woody plant P. euphratica callus were consistent with those from herbaceous species. Application of various thiol reagents to the reaction revealed that reduced thiol groups were essential to maintain the PM H(+)-ATPase activity. In addition, there was increased H(+)-ATPase activity in the PM vesicles when callus was exposed to NaCl. Western blotting analysis demonstrated an enhancement of H(+)-ATPase content in NaCl-treated P. euphratica callus compared with the control.

Salt tolerance and salinity effects on plants: a review

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

Nitric oxide functions as a signal in salt resistance in the calluses from two ecotypes of reed

Calluses from two ecotypes of reed (Phragmites communis Trin.) plant (dune reed [DR] and swamp reed [SR]), which show different sensitivity to salinity, were used to study plant adaptations to salt stress. Under 200 mm NaCl treatment, the sodium (Na) percentage decreased, but the calcium percentage and the potassium (K) to Na ratio increased in the DR callus, whereas an opposite changing pattern was observed in the SR callus. Application of sodium nitroprusside (SNP), as a nitric oxide (NO) donor, revealed that NO affected element ratios in both DR and SR calluses in a concentration-dependent manner. N(omega)-nitro-l-arginine (an NO synthase inhibitor) and 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxyde (a specific NO scavenger) counteracted NO effect by increasing the Na percentage, decreasing the calcium percentage and the K to Na ratio. The increased activity of plasma membrane (PM) H(+)-ATPase caused by NaCl treatment in the DR callus was reversed by treatment with N(omega)-nitro-l-arginine and 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxyde. Western-blot analysis demonstrated that NO stimulated the expression of PM H(+)-ATPase in both DR and SR calluses. These results indicate that NO serves as a signal in inducing salt resistance by increasing the K to Na ratio, which is dependent on the increased PM H(+)-ATPase activity.

Enhanced H+/ATP coupling ratio of H+-ATPase and increased 14-3-3 protein content in plasma membrane of tomato cells upon osmotic shock

Modulation of proton extrusion and ATP-dependent H+ transport through the plasma membrane in relation to the presence of 14-3-3 proteins in this membrane in response to osmotic shock was studied in tomato (Lycopersicon esculentum Mill. cv. Pera) cell cultures. In vivo H+ extrusion by cells was activated rapidly and significantly after adding 100 mM NaCl, 100 mM KCl, 50 mM Na2SO4, 1.6% sorbitol or 2 micro M fusicoccin to the medium. The increase in H+ extrusion by cells treated with 100 mM NaCl was correlated with an increase of H+ transport by the plasma membrane H+-ATPase (EC 3.6.1.35), but not with changes in ATP hydrolytic activity of this enzyme, suggesting an increased coupling ratio of the enzyme. Immunoblot experiments showed increased amounts of 14-3-3 proteins in plasma membrane fractions isolated from tomato cells treated with 100 mM NaCl as compared to control cells without changing the amount of plasma membrane H+-ATPase. Together, these data indicate that in tomato cells an osmotic shock could enhance coupling between ATP hydrolysis and proton transport at the plasma membrane through the formation of a membrane 14-3-3/H+-ATPase complex.

Tolerance to NaCl induces changes in plasma membrane lipid composition, fluidity and H+-ATPase activity of tomato calli

Two tomato (Lycopersicon esculentum Mill. cv. Pera) callus lines tolerant to NaCl were obtained by successive subcultures of NaCl-sensitive calli in 50 and 100 mM NaCl-supplemented medium. Growth and ion content, as well as plasma membrane lipid composition, fluidity and H+-ATPase (EC 3.6.1.35) activity, were studied in both NaCl-sensitive and NaCl-tolerant calli. Although calli tolerant to 100 mM NaCl exhibited a reduced growth relative to calli sensitive to NaCl or tolerant to 50 mM NaCl, growth of calli tolerant to 100 mM NaCl was higher than that of NaCl-sensitive calli grown for one subculture in 100 mM NaCl. Growth in the presence of 100 mM NaCl provoked an increase of Na+ and Cl- content, but no significant changes in K+ and Ca2+. As compared with NaCl-sensitive and 50 mM NaCl-tolerant calli, plasma membrane vesicles isolated from calli tolerant to 100 mM NaCl exhibited a higher phospholipid and sterol content as well as a lower phospholipid/free sterol ratio and a lower double bond index (DBI) of phospholipid fatty acids. The changes in plasma membrane lipid composition were correlated with a decrease of plasma membrane fluidity in calli tolerant to 100 mM NaCl, as indicated by fluorimetric studies using diphenylhexatriene (DPH) as probe. Plasma membrane-enriched vesicles isolated from calli tolerant to 100 mM NaCl showed lower ATP hydrolysis and ATP-dependent H+-pumping activities, as well as a lower passive permeability to H+ than plasma membrane from NaCl-sensitive and 50 mM NaCl-tolerant calli. The involvement of the changes in plasma membrane lipid content and composition, fluidity and H+-ATPase activity in salt tolerance of tomato calli is discussed.