Effects of NaCl on the growth, ion accumulation and photosynthetic parameters of Thellungiella halophila

Low NaCl Concentrations Increase Cotyledon Growth in Chinese White Radish (Raphanus sativus L. var. longipinnatus Bailey) Seedlings via Aquaporin-Mediated Water Transport

This study aimed to elucidate the specific role of low NaCl concentrations, particularly 10 and 20 mM, in stimulating cotyledon growth in Chinese white radish (Raphanus sativus L. var. longipinnatus Bailey) seedlings. Chinese white radish seeds were cultivated in sand culture and subjected to daily watering with solutions containing 0, 10, 20, 50, or 100 mM NaCl. Growth, water status, aquaporin gene expression, ion contents, and physiology-related parameters were assessed 4 days after sowing. Applying 10 and 20 mM NaCl significantly promoted the growth of 4-day-old seedlings. Notably, the cotyledons exhibited the most significant growth, achieving a rate of 177% compared with the 125-138% growth observed in the hypocotyl and root parts. This substantial enhancement in cotyledon growth, including biomass, cotyledon area, cotyledon thickness, and mesophyll cell size, was induced by an optimal concentration of 10 mM NaCl. This induction correlated with the increased water content, degree of succulence, and expression of aquaporin genes, specifically within PIP1-1, PIP1-2, PIP2-1, PIP2-2, and TIP1-1, in addition to the maintenance of the Hill reaction, heightened free radical scavenging, and the elevated accumulation of Na+, Cl-, K+, proline, total N, and C. These findings suggest a beneficial role of low NaCl levels in optimising early-stage seedling growth.

Can lettuce plants grow in saline soils supplemented with biochar?

Salt stress is presently a major environmental concern, given the huge number of soils affected by the presence of dissolved salts. Therefore, it is necessary to find solutions, preferably nature-based ones, to deal with this problem. In this study, biochar, a product made from plant biomass residues through the process of pyrolysis, was tested to alleviate salt stress on lettuce (Lactuca sativa L.) plants. Six different concentrations of NaCl were tested: 0, 50, 100, 200, 300 and 400 mM with and without the addition of 5% (w/w) biochar. Biochar ability to mitigate salinity damage was assessed by means of both biometric (fresh weight), physiological (chlorophyll content), and biochemical (i.e., electrolyte leakage, total antioxidant power, total soluble proteins, free amino acids, and mineral content) parameters. The experiment lasted four weeks. The results showed that NaCl has a negative effect from the concentration of 100-200 mM and that biochar was to some extent effective in mitigating the negative effects of salt on plant physiology; nevertheless, biochar failed to counteract Na accumulation. Similarly, biochar did not influence the content of free amino acids in lettuce leaves, but enhanced the expression of several parameters, such as total antioxidant power, fresh weight, chlorophyll content, total soluble protein, K content, although only clearly evident in some cases. Overall, the present study showed that biochar is a viable solution to counteract the damage caused by high salt concentrations on plant growth.

Adaptative Mechanisms of Halophytic Eutrema salsugineum Encountering Saline Environment

Salt cress (Eutrema salsugineum), an Arabidopsis-related halophyte, can naturally adapt to various harsh climates and soil conditions; thus, it is considered a desirable model plant for deciphering mechanisms of salt and other abiotic stresses. Accumulating evidence has revealed that compared with Arabidopsis, salt cress possesses stomata that close more tightly and more succulent leaves during extreme salt stress, a noticeably higher level of proline, inositols, sugars, and organic acids, as well as stress-associated transcripts in unstressed plants, and they are induced rapidly under stress. In this review, we systematically summarize the research on the morphology, physiology, genome, gene expression and regulation, and protein and metabolite profile of salt cress under salt stress. We emphasize the latest advances in research on the genome adaptive evolution encountering saline environments, and epigenetic regulation, and discuss the mechanisms underlying salt tolerance in salt cress. Finally, we discuss the existing questions and opportunities for future research in halophytic Eutrema. Together, the review fosters a better understanding of the mechanism of plant salt tolerance and provides a reference for the research and utilization of Eutrema as a model extremophile in the future. Furthermore, the prospects for salt cress applied to explore the mechanism of salt tolerance provide a theoretical basis to develop new strategies for agricultural biotechnology.

Thylakoid proteome variation of Eutrema salsugineum in response to drought and salinity combined stress

It is well known that plant responses to stress involve different events occurring at different places of the cell/leaf and at different time scales in relation with the plant development. In fact, the organelles proteomes include a wide range of proteins that could include a wide range of proteins showing a considerable change in cellular functions and metabolism process. On this basis, a comparative proteomics analysis and fluorescence induction measurements were performed to investigate the photosynthetic performance and the relative thylakoid proteome variation in Eutrema salsugineum cultivated under salt stress (200 mM NaCl), water deficit stress (PEG) and combined treatment (PEG + NaCl) as a hyperosmotic stress. The obtained results showed a significant decrease of plant growth under drought stress conditions, with the appearance of some toxicity symptoms, especially in plants subjected to combined treatment. Application of salt or water stress alone showed no apparent change in the chlorophyll a fluorescence transients, primary photochemistry (fluorescence kinetics of the O-J phase), the PQ pool state (J-I phase changes), (Fv/Fm) and (Fk/Fj) ratios. However, a considerable decrease of all these parameters was observed under severe osmotic stress (PEG + NaCl). The thylakoid proteome analysis revealed 58 proteins showing a significant variation in their abundance between treatments (up or down regulation). The combined treatment (PEG + NaCl) induced a decrease in the expression of the whole PSII core subunit (D1, D2, CP43, CP47, PsbE and PsbH), whereas the OEC subunits proteins remained constant. An increase in the amount of PsaD, PsaE, PsaF, PsaH, PsaK and PsaN was detected under drought stress (PEG5%). No significant change in the accumulation of Cyt b6 and Cyt f was observed. Some regulated proteins involved in cellular redox homeostasis were detected (glutamine synthetase, phosphoglycerate kinase, transketolase), and showed a significant decrease under the combined treatment. Some oxidative stress related proteins were significantly up-regulated under salt or drought stress and could play a crucial role in the PSI photoprotection and the control of ROS production level.

Comparative proteomic approach to study the salinity effect on the growth of two contrasting quinoa genotypes

The aim of this study was to investigate the effect of NaCl salinity (0, 100 and 300 mM) on the individual response of the quinoa varieties Kcoito (Altiplano Ecotype) and UDEC-5 (Sea-level Ecotype) with physiological and proteomic approaches. Leaf protein profile was performed using two dimensional gel electrophoresis (2-DE). UDEC-5 showed an enhanced capacity to withstand salinity stress compared to Kcoito. In response to salinity, we detected overall the following differences between both genotypes: Toxicity symptoms, plant growth performance, photosynthesis performance and intensity of ROS-defense. We found a mirroring of these differences in the proteome of each genotype. Among the 700 protein spots reproducibly detected, 24 exhibited significant abundance variations between samples. These proteins were involved in energy and carbon metabolism, photosynthesis, ROS scavenging and detoxification, stress defense and chaperone functions, enzyme activation and ATPases. A specific set of proteins predominantly involved in photosynthesis and ROS scavenging showed significantly higher abundance under high salinity (300 mM NaCl). The adjustment was accompanied by a stimulation of various metabolic pathways to balance the supplementary demand for energy or intermediates. However, the more salt-resistant genotype UDEC-5 presented a beneficial and significantly higher expression of nearly all stress-related altered enzymes than Kcoito.

Two glyoxylate reductase isoforms are functionally redundant but required under high photorespiration conditions in rice

BACKGROUND

The glyoxylate reductase (GR) multigene family has been described in various plant species, their isoforms show different biochemical features in plants. However, few studies have addressed the biological roles of GR isozymes, especially for rice.

RESULTS

Here, we report a detailed analysis of the enzymatic properties and physiological roles of OsGR1 and OsGR2 in rice. The results showed that both enzymes prefer NADPH to NADH as cofactor, and the NADPH-dependent glyoxylate reducing activity represents the major GR activity in various tissues and at different growth stages; and OsGR1 proteins were more abundant than OsGR2, which is also a major contributor to total GR activities. By generating and characterizing various OsGR-genetically modified rice lines, including overexpression, single and double-knockout lines, we found that no phenotypic differences occur among the various transgenic lines under normal growth conditions, while a dwarfish growth phenotype was noticed under photorespiration-promoted conditions.

CONCLUSION

Our results suggest that OsGR1 and OsGR2, with distinct enzymatic characteristics, function redundantly in detoxifying glyoxylate in rice plants under normal growth conditions, whereas both are simultaneously required under high photorespiration conditions.

Salt tolerance mechanisms in three Irano-Turanian Brassicaceae halophytes relatives of Arabidopsis thaliana

Salt tolerance mechanisms were studied in three Irano-Turanian halophytic species from the Brassicaceae ‎‎(Lepidium latifolium, L. perfoliatum and Schrenkiella parvula) and compared with the glycophyte Arabidopsis thaliana. According to seed germination under salt stress, L. perfoliatum was the most tolerant species, while L. latifolium and S. parvula were rather susceptible. Contrastingly, based on biomass production L. perfoliatum was more salt sensitive than the other two species. In S. parvula biomass was increased up to 2.8-fold by 100 mM NaCl; no significant growth reduction was observed even when exposed to 400 mM NaCl. Stable activities of antioxidative defense enzymes, nil or negligible accumulation of superoxide anion and hydrogen peroxide, as well as stable membrane integrity in the three halophytes revealed that no oxidative stress occurred in these tolerant species under salt stress. Proline levels increased in response to salt treatment. However, it contributed only by 0.3‒2.0% to the total osmolyte concentration in the three halophytes (at 400 mM NaCl) and even less (0.04%) in the glycophyte, A. thaliana (at 100 mM NaCl). Soluble sugars in all three halophytes and free amino acids pool in S. parvula decreased under salt treatment in contrast to the glycophyte, A. thaliana. The contribution of organic osmolytes to the total osmolyte pool increased by salt treatment in the roots, while decreased in halophyte and glycophyte, A. thaliana leaves. Interestingly, this reduction was compensated by a higher relative contribution of K in the leaves of the halophytes, but of Na in A. thaliana. Taken together, biomass data and biochemical indicators show that S. parvula is more salt tolerant than the two Lepidium species. Our data indicate that L. latifolium, as a perennial halophyte with a large biomass, is highly suitable for both restoration of saline habitats and saline agriculture.

Growth performance, organ-level ionic relations and organic osmoregulation of Elaeagnus angustifolia in response to salt stress

Elaeagnus angustifolia is one of the most extensively afforested tree species in environment-harsh regions of northern China. Despite its exceptional tolerance to saline soil, the intrinsic adaptive physiology has not been revealed. In this study, we investigated the growth, organ-level ionic relations and organic osmoregulation of the seedlings hydroponically treated with 0, 100 and 200 mM NaCl for 30 days. We found that the growth characteristics and the whole-plant dry weight were not obviously stunted, but instead, were even slightly stimulated by the treatment of 100 mM NaCl. In contrast, these traits were significantly inhibited by 200 mM NaCl treatment. Interestingly, as compared with the control (0 mM NaCl), both 100 and 200 mM NaCl treatments had a promotional effect on root growth as evidenced by 26.3% and 2.4% increases in root dry weight, respectively. Roots had the highest Na+ and Cl- concentrations and obviously served as the sink for the net increased Na+ and Cl-, while, stems might maintain the capacity of effective Na+ constraint, resulting in reduced Na+ transport to the leaves. K+, Ca2+ and Mg2+ concentrations in three plant organs of NaCl-treated seedlings presented a substantial decline, eventually leading to an enormously drop of K+/Na+ ratio. As the salt concentration increased, proline and soluble protein contents continuously exhibited a prominent and a relatively tardy accumulation, respectively, whereas soluble sugar firstly fell to a significant level and then regained to a level that is close to that of the control. Taken together, our results provided quantitative measures that revealed some robust adaptive physiological mechanisms underpinning E. angustifolia's moderately high salt tolerance, and those mechanisms comprise scalable capacity for root Na+ and Cl- storage, effectively constrained transportation of Na+ from stems to leaves, root compensatory growth, as well as an immediate and prominent leaf proline accumulation.

Differential Mechanisms of Photosynthetic Acclimation to Light and Low Temperature in Arabidopsis and the Extremophile Eutrema salsugineum

Photosynthetic organisms are able to sense energy imbalances brought about by the overexcitation of photosystem II (PSII) through the redox state of the photosynthetic electron transport chain, estimated as the chlorophyll fluorescence parameter 1-qL, also known as PSII excitation pressure. Plants employ a wide array of photoprotective processes that modulate photosynthesis to correct these energy imbalances. Low temperature and light are well established in their ability to modulate PSII excitation pressure. The acquisition of freezing tolerance requires growth and development a low temperature (cold acclimation) which predisposes the plant to photoinhibition. Thus, photosynthetic acclimation is essential for proper energy balancing during the cold acclimation process. Eutrema salsugineum (Thellungiella salsuginea) is an extremophile, a close relative of Arabidopsis thaliana, but possessing much higher constitutive levels of tolerance to abiotic stress. This comparative study aimed to characterize the photosynthetic properties of Arabidopsis (Columbia accession) and two accessions of Eutrema (Yukon and Shandong) isolated from contrasting geographical locations at cold acclimating and non-acclimating conditions. In addition, three different growth regimes were utilized that varied in temperature, photoperiod and irradiance which resulted in different levels of PSII excitation pressure. This study has shown that these accessions interact differentially to instantaneous (measuring) and long-term (acclimation) changes in PSII excitation pressure with regard to their photosynthetic behaviour. Eutrema accessions contained a higher amount of photosynthetic pigments, showed higher oxidation of P700 and possessed more resilient photoprotective mechanisms than that of Arabidopsis, perhaps through the prevention of PSI acceptor-limitation. Upon comparison of the two Eutrema accessions, Shandong demonstrated the greatest PSII operating efficiency (ΦPSII) and P700 oxidizing capacity, while Yukon showed greater growth plasticity to irradiance. Both of these Eutrema accessions are able to photosynthetically acclimate but do so by different mechanisms. The Shandong accessions demonstrate a stable response, favouring energy partitioning to photochemistry while the Yukon accession shows a more rapid response with partitioning to other (non-photochemical) strategies.

Halophytes as a source of salt tolerance genes and mechanisms: a case study for the Salt Lake area, Turkey

The worst case scenario of global climate change predicts both drought and salinity would be the first environmental factors restricting agriculture and natural ecosystems, causing decreased crop yields and plant growth that would directly affect human population in the next decades. Therefore, it is vital to understand the biology of plants that are already adapted to these extreme conditions. In this sense, extremophiles such as the halophytes offer valuable genetic information for understanding plant salinity tolerance and to improve the stress tolerance of crop plants. Turkey has ecological importance for its rich biodiversity with up to 3700 endemic plants. Salt Lake (Lake Tuz) in Central Anatolia, one of the largest hypersaline lakes in the world, is surrounded by salty marshes, with one of the most diverse floras in Turkey, where arid and semiarid areas have increased due to low rainfall and high evaporation during the summer season. Consequently, the Salt Lake region has a large number of halophytic, xerophytic and xero-halophytic plants. One good example is Eutrema parvulum (Schrenk) Al-Shehbaz & Warwick, which originates from the Salt Lake region, can tolerate up to 600mM NaCl. In recent years, the full genome of E. parvulum was published and it has been accepted as a model halophyte due to its close relationship (sequence identity in range of 90%) with Arabidopsis thaliana (L. Heynh.). In this context, this review will focus on tolerance mechanisms involving hormone signalling, accumulation of compatible solutes, ion transporters, antioxidant defence systems, reactive oxygen species (ROS) signalling mechanism of some lesser-known extremophiles growing in the Salt Lake region. In addition, current progress on studies conducted with E. parvulum will be evaluated to shed a light on future prospects for improved crop tolerance.

Titanium dioxide nanoparticles (100-1000 mg/l) can affect vitamin E response in Arabidopsis thaliana

In the present study we analyze the effect of seed treatment by a range of nano-TiO2 concentrations on the growth of Arabidopsis thaliana plants, on the vitamin E content and the expression of its biosynthetic genes, as well as activity of antioxidant enzymes and lipid peroxidation. To conduct the mechanistic analysis of nano-TiO2 on plants growth and antioxidant status we applied nanoparticles concentrations that are much higher than those reported in the environment. We find that as the concentration of nano-TiO2 increases, the biomass, and chlorophyll content in 5-week-old Arabidopsis thaliana plants decrease in a concentration dependent manner. In opposite, higher nano-TiO2 concentration enhanced root growth. Our results indicate that a high concentration of nano-TiO2 induces symptoms of toxicity and elevates the antioxidant level. We also find that the expression levels of tocopherol biosynthetic genes were either down- or upregulated in response to nano-TiO2. Thermoluminescence analysis shows that higher nano-TiO2 concentrations cause lipid peroxidation. To the best of our knowledge, this is the first report concerning the effect of nano-TiO2 on vitamin E status in plants. We conclude that nano-TiO2 affects the antioxidant response in Arabidopsis thaliana plants. This could be an effect of a changes in vitamin E gene expression that is diminished under lower tested nano-TiO2 concentrations and elevated under 1000 μg/ml.

A Different Pattern of Production and Scavenging of Reactive Oxygen Species in Halophytic Eutrema salsugineum (Thellungiella salsuginea) Plants in Comparison to Arabidopsis thaliana and Its Relation to Salt Stress Signaling

Isolated thylakoids from halophytic Eutrema salsugineum (Thellungiella salsuginea) produces more H2O2 in comparison to glycophytic Arabidopsis thaliana. The first objective of this study was to verify whether this feature is relevant also to the intact chloroplasts and leaves. Enhanced H2O2 levels in chloroplasts and leaves of E. salsugineum were positively verified with several methods (electron microscopy, staining with Amplex Red and with diaminobenzidine). This effect was associated with a decreased ratio of [Formula: see text]/H2O2 in E. salsugineum in comparison to A. thaliana as detected by electron paramagnetic resonance method. As a next step, we tested how this specific ROS signature of halophytic species affects the antioxidant status and down-stream components of ROS signaling. Comparison of enzymatic antioxidants revealed a decreased activity of ascorbate peroxidase (APX), enhanced activity of glutathione peroxidase, and the presence of thylakoid-bound forms of iron superoxide dismutase (FeSOD) and APX in E. salsugineum. These cues were, however, independent from application of salt stress. The typical H2O2-dependent cellular responses, namely the levels of glucosinolates and stress-related hormones were determined. The total glucosinolate content in E. salsugineum water-treated leaves was higher than in A. thaliana and increased after salinity treatment. Treatment with salinity up-regulated all of tested stress hormones, their precursors and catabolites [abscisic acid (ABA), dihydrophaseic acid, phaseic acid, 1-aminocyclopropane-1-carboxylic acid, salicylic acid, jasmonic acid, cis-(+)-12-oxo-phytodienoic acid and jasmonoyl-L-isoleucine] in A. thaliana, whereas in E. salsugineum only a stimulation in ethylene synthesis and ABA catabolism was noted. Obtained results suggest that constitutively enhanced H2O2 generation in chloroplasts of E. salsugineum might be a crucial component of stress-prepardeness of this halophytic species. It shapes a very efficient antioxidant protection (in which glucosinolates might play a specific role) and a fine tuning of hormonal signaling to suppress the cell death program directed by jasmonate pathway.

The beta subunit of glyceraldehyde 3-phosphate dehydrogenase is an important factor for maintaining photosynthesis and plant development under salt stress-Based on an integrative analysis of the structural, physiological and proteomic changes in chloroplasts in Thellungiella halophila

Thellungiella halophila, a new model halophyte, can survive under highly saline conditions. We performed comparative proteomics of chloroplasts from plants grown under different saline conditions. Seventy-five salt-responsive proteins were positively identified by mass spectrometry, which represented 43 unique ones. These proteins were categorized into 7 main pathways: light reaction, carbon fixation, energy metabolism, antenna proteins, cell structure, and protein degradation and folding. Saline conditions increased the abundance of proteins involved in photosynthesis, energy metabolism and cell structure. The results indicated that Thellungiella could withstand high salinity by maintaining normal or high photosynthetic capacity, reducing ROS production, as well as enhancing energy usage. Meanwhile, the ultrastructural and physiological data also agree with chloroplast proteomics results. Subsequently, the glyceraldehydes 3-phosphate dehydrogenase beta subunit (GAPB) involved in carbon fixation was selected and its role in salt tolerance was clarified by over-expressing it in Arabidopsis. ThGAPB-overexpressing plants had higher total chlorophyll contents, dry weights, water contents and survival rates than that of wild type plants. These results indicated that ThGAPB might improve plant salt tolerance by maintaining higher recycling rates of ADP and NADP(+) to decrease ROS production, helping to maintain photosynthetic efficiency and plant development under saline conditions.

Enhanced chloroplastic generation of H2O2 in stress-resistant Thellungiella salsuginea in comparison to Arabidopsis thaliana

In order to find some basis of salinity resistance in the chloroplastic metabolism, a halophytic Thellungiella salsuginea was compared with glycophytic Arabidopsis thaliana. In control T.s. plants the increased ratios of chlorophyll a/b and of fluorescence emission at 77 K (F730 /F685 ) were documented, in comparison to A.t.. This was accompanied by a higher YII and lower NPQ (non-photochemical quenching) values, and by a more active PSI (photosystem I). Another prominent feature of the photosynthetic electron transport (PET) in T.s. was the intensive production of H2 O2 from PQ (plastoquinone) pool. Salinity treatment (0.15 and 0.30 M NaCl for A.t. and T.s., respectively) led to a decrease in ratios of chl a/b and F730 /F685 . In A.t., a salinity-driven enhancement of YII and NPQ was found, in association with the stimulation of H2 O2 production from PQ pool. In contrast, in salinity-treated T.s., these variables were similar as in controls. The intensive H2 O2 generation was accompanied by a high activity of PTOX (plastid terminal oxidase), whilst inhibition of this enzyme led to an increased H2 O2 formation. It is hypothesized, that the intensive H2 O2 generation from PQ pool might be an important element of stress preparedness in Thellungiella plants. In control T.s. plants, a higher activation state of carboxylase ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) was also documented in concert with the attachment of Rubisco activase (RCA) to the thylakoid membranes. It is supposed, that a closer contact of RCA with PSI in T.s. enables a more efficient Rubisco activation than in A.t.

Changes in the alternative electron sinks and antioxidant defence in chloroplasts of the extreme halophyte Eutrema parvulum (Thellungiella parvula) under salinity

BACKGROUND AND AIMS

Eutrema parvulum (synonym, Thellungiella parvula) is an extreme halophyte that thrives in high salt concentrations (100-150 mm) and is closely related to Arabidopsis thaliana. The main aim of this study was to determine how E. parvulum uses reactive oxygen species (ROS) production, antioxidant systems and redox regulation of the electron transport system in chloroplasts to tolerate salinity.

METHODS

Plants of E. parvulum were grown for 30 d and then treated with either 50, 200 or 300 mm NaCl. Physiological parameters including growth and water relationships were measured. Activities of antioxidant enzymes were determined in whole leaves and chloroplasts. In addition, expressions of chloroplastic redox components such as ferrodoxin thioredoxin reductases (FTR), NADPH thioredoxin reductases (NTRC), thioredoxins (TRXs) and peroxiredoxins (PRXs), as well as genes encoding enzymes of the water-water cycle and proline biosynthesis were measured.

KEY RESULTS

Salt treatment affected water relationships negatively and the accumulation of proline was increased by salinity. E. parvulum was able to tolerate 300 mm NaCl over long periods, as evidenced by H2O2 content and lipid peroxidation. While Ca(2+) and K(+) concentrations were decreased by salinity, Na(+) and Cl(-) concentrations increased. Efficient induction of activities and expressions of water-water cycle enzymes might prevent accumulation of excess ROS in chloroplasts and therefore protect the photosynthetic machinery in E. parvulum. The redox homeostasis in chloroplasts might be achieved by efficient induction of expressions of redox regulatory enzymes such as FTR, NTRC, TRXs and PRXs under salinity.

CONCLUSIONS

E. parvulum was able to adapt to osmotic stress by an efficient osmotic adjustment mechanism involving proline and was able to regulate its ion homeostasis. In addition, efficient induction of water-water cycle enzymes and other redox regulatory components such as TRXs and PRXs in chloroplasts were able to protect the chloroplasts from salinity-induced oxidative stress.

Comparative proteomics of Thellungiella halophila leaves from plants subjected to salinity reveals the importance of chloroplastic starch and soluble sugars in halophyte salt tolerance

Thellungiella halophila, a close relative of Arabidopsis, is a model halophyte used to study plant salt tolerance. The proteomic/physiological/transcriptomic analyses of Thellungiella plant leaves subjected to different salinity levels, reported herein, indicate an extraordinary ability of Thellungiella to adapt to large concentrations of exogenous saline by compartmentalizing Na(+) into cell vacuoles and accumulating proline and soluble sugars as organic osmolytes. Salinity stress stimulated the accumulation of starch in chloroplasts, which resulted in a greatly increased content of starch and total sugars in leaves. Comparative proteomics of Thellungiella leaves identified 209 salt-responsive proteins. Among these, the sequences of 108 proteins were strongly homologous to Arabidopsis protein sequences, and 30 had previously been identified as Thellungiella proteins. Functional classification of these proteins into 16 categories indicated that the majority are involved in carbohydrate metabolism, followed by those involved in energy production and conversion, and then those involved in the transport of inorganic ions. Pathway analysis revealed that most of the proteins are involved in starch and sucrose metabolism, carbon fixation, photosynthesis, and glycolysis. Of these processes, the most affected were starch and sucrose metabolism, which might be pivotal for salt tolerance. The gene expression patterns of the 209 salt-responsive proteins revealed through hierarchical clustering of microarray data and the expression patterns of 29 Thellungiella genes evaluated via quantitative RT-PCR were similar to those deduced via proteomic analysis, which underscored the possibility that starch and sucrose metabolism might play pivotal roles in determining the salt tolerance ability of Thellungiella. Our observations enabled us to propose a schematic representation of the systematic salt-tolerance phenotype in Thellungiella and suggested that the increased accumulation of starch, soluble sugars, and proline, as well as subcellular compartmentalization of sodium, might collectively denote important mechanisms for halophyte salt tolerance.

A drought-sensitive barley variety displays oxidative stress and strongly increased contents in low-molecular weight antioxidant compounds during water deficit compared to a tolerant variety

Barley displays a great genetic diversity, constituting a valuable source to delineate the responses of contrasted genotypes to environmental constraints. Here, we investigated the level of oxidative stress and the participation of antioxidant systems in two barley genotypes: Express, a variety known to be sensitive to drought, and Saïda, an Algerian landrace selected for its tolerance to water deficit. Soil-grown 15-day-old plants were subjected to water deficit for 8 days and then rewatered. We observed that upon water stress Express exhibits compared to Saïda accelerated wilting and a higher level of oxidative stress evaluated by HPLC measurements of lipid peroxidation and by imaging techniques. In parallel, Express plants also display lower levels of catalase and superoxide dismutase activity. No great difference was observed regarding peroxiredoxins and methionine sulfoxide reductases, enzymes detoxifying peroxides and repairing oxidized proteins, respectively. In contrast, upon water stress and recovery, much higher contents and oxidation ratios of glutathione and ascorbate were measured in Express compared to Saïda. Express also shows during water deficit greater increases in the pools of lipophilic antioxidants like xantophyll carotenoids and α-tocopherol. Altogether, these data show that the differential behavior of the two genotypes involves distinct responses regarding antioxidant mechanisms. Indeed, the drought sensitivity of Express compared with Saïda is associated with oxidative damage and a lower enzymatic ROS-scavenging capacity, but in parallel with a much stronger enhancement of most mechanisms involving low-molecular weight antioxidant compounds.

Involvement of source-sink relationship and hormonal control in the response of Medicago ciliaris - Sinorhizobium medicae symbiosis to salt stress

In order to explore the relationship between leaf hormonal status and source-sink relations in the response of symbiotic nitrogen fixation (SNF) to salt stress, three major phytohormones (cytokinins, abscisic acid and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid), sucrose phosphate synthase activity in source leaves and sucrolytic activities in sink organs were analysed in two lines of Medicago ciliaris (salt-tolerant TNC 1.8 and salt-sensitive TNC 11.9). SNF (measured as nitrogenase activity and amount of N-fixed) was more affected by salt treatment in the TNC 11.9 than in TNC 1.8, and this could be explained by a decrease in nodule sucrolytic activities. SNF capacity was reflected in leaf biomass production and in the sink activity under salinity, as suggested by the higher salt-induced decrease in the young leaf sucrolytic activities in the sensitive line TNC 11.9, while they were not affected in the tolerant line TNC 1.8. As a consequence of maintaining sink activities in the actively growing organs, the key enzymatic activity for synthesis of sucrose (sucrose phosphate synthase) was also less affected in the mature leaves of the more tolerant genotype. Ours results showed also that the major hormone factor associated with the relative tolerance of TNC 1.8 was the stimulation of abscisic acid concentration in young leaves under salt treatment. This stimulation may control photosynthetic organ growth and also may contribute to a certain degree in the maintenance of coordinated sink-source relationships. Therefore, ABA may be an important component which conserves sucrose synthesis in source leaves.

Metabolome and water homeostasis analysis of Thellungiella salsuginea suggests that dehydration tolerance is a key response to osmotic stress in this halophyte

Thellungiella salsuginea, a Brassicaceae species closely related to Arabidopsis thaliana, is tolerant to high salinity. The two species were compared under conditions of osmotic stress to assess the relationships between stress tolerance, the metabolome, water homeostasis and growth performance. A broad range of metabolites were analysed by metabolic fingerprinting and profiling, and the results showed that, despite a few notable differences in raffinose and secondary metabolites, the same metabolic pathways were regulated by salt stress in both species. The main difference was quantitative: Thellungiella had much higher levels of most metabolites than Arabidopsis whatever the treatment. Comprehensive quantification of organic and mineral solutes showed a relative stability of the total solute content regardless of the species or treatment, meaning that little or no osmotic adjustment occurred under stress. The reduction in osmotic potential observed in plants under stress was found to result from a passive loss of water. Thellungiella shoots contain less water than Arabidopsis shoots, and have the ability to lose more water, which could contribute to maintain a water potential gradient between soil and plant. Significant differences between Thellungiella and Arabidopsis were also observed in terms of the physicochemical properties of their metabolomes, such as water solubility and polarity. On the whole, the Thellungiella metabolome appears to be more compatible with dehydration. Osmotic stress was also found to impact the metabolome properties in both species, increasing the overall polarity. Together, the results suggest that Thellungiella copes with osmotic stress by tolerating dehydration, with its metabolic configuration lending itself to osmoprotective strategies rather than osmo-adjustment.

A comparative study of salt tolerance parameters in 11 wild relatives of Arabidopsis thaliana

Salinity is an abiotic stress that limits both yield and the expansion of agricultural crops to new areas. In the last 20 years our basic understanding of the mechanisms underlying plant tolerance and adaptation to saline environments has greatly improved owing to active development of advanced tools in molecular, genomics, and bioinformatics analyses. However, the full potential of investigative power has not been fully exploited, because the use of halophytes as model systems in plant salt tolerance research is largely neglected. The recent introduction of halophytic Arabidopsis-Relative Model Species (ARMS) has begun to compare and relate several unique genetic resources to the well-developed Arabidopsis model. In a search for candidates to begin to understand, through genetic analyses, the biological bases of salt tolerance, 11 wild relatives of Arabidopsis thaliana were compared: Barbarea verna, Capsella bursa-pastoris, Hirschfeldia incana, Lepidium densiflorum, Malcolmia triloba, Lepidium virginicum, Descurainia pinnata, Sisymbrium officinale, Thellungiella parvula, Thellungiella salsuginea (previously T. halophila), and Thlaspi arvense. Among these species, highly salt-tolerant (L. densiflorum and L. virginicum) and moderately salt-tolerant (M. triloba and H. incana) species were identified. Only T. parvula revealed a true halophytic habitus, comparable to the better studied Thellungiella salsuginea. Major differences in growth, water transport properties, and ion accumulation are observed and discussed to describe the distinctive traits and physiological responses that can now be studied genetically in salt stress research.

The impact of genotype and salinity on physiological function, secondary metabolite accumulation, and antioxidative responses in lettuce

Salinity inhibits plant growth due to osmotic and ionic effects. However, little is known about the impact of genotype and salinity on biochemical and molecular processes in the leafy vegetable lettuce. We report here evaluations of two lettuce types, Verte (NaCl tolerant) and Romaine (NaCl sensitive), under iso-osmotic 100 mM NaCl and 77 mM Na(2)SO(4) treatments. As compared to Romaine, NaCl-treated Verte displayed better growth, contained lower levels of inorganic cations in leaves, and possessed superior antioxidative capacity due to enhanced carotenoid and phenolics biosynthesis and more active antioxidative enzymes resulting in reduced membrane damage. Both genotypes had relatively similar growth patterns under Na(2)SO(4) treatment, but Romaine showed enhanced root lignification, greater malondialdehyde formation, and suppressed Fe-superoxide dismutase expression in roots as compared with Verte.

Rapid in vitro selection of salt-tolerant genotypes of the potentially medicinal plant Centaurium maritimum (L.) fritsch

We investigated differences of salinity tolerance between 'salt-tolerant' (ST) and 'salt-sensitive' (SS) genotypes of yellow centaury [Centaurium maritimum (L.) Fritsch] selected during the germination phase. The ability of in vitro cultured C. maritimum to complete the whole ontogenetic cycle in less than 6 months enabled us to deterine salinity tolerance during different growth phases. Based on the physiological attributes measured in this study (growth, morphogenesis, photosynthesis, flowering, seed germination), it can be concluded that C. maritimum genotypes differing in salinity tolerance showed a variable response to elevated salt concentrations during both the vegetative and the generative growth phase.

Learning from evolution: Thellungiella generates new knowledge on essential and critical components of abiotic stress tolerance in plants

Thellungiella salsuginea (halophila) is a close relative of Arabidopsis thaliana but, unlike A. thaliana, it grows well in extreme conditions of cold, salt, and drought as well as nitrogen limitation. Over the last decade, many laboratories have started to use Thellungiella to investigate the physiological, metabolic, and molecular mechanisms of abiotic stress tolerance in plants, and new knowledge has been gained in particular with respect to ion transport and gene expression. The advantage of Thellungiella over other extremophile model plants is that it can be directly compared with Arabidopsis, and therefore generate information on both essential and critical components of stress tolerance. Thellungiella research is supported by a growing body of technical resources comprising physiological and molecular protocols, ecotype collections, expressed sequence tags, cDNA-libraries, microarrays, and a pending genome sequence. This review summarizes the current state of knowledge on Thellungiella and re-evaluates its usefulness as a model for research into plant stress tolerance.

Comparative salt tolerance analysis between Arabidopsis thaliana and Thellungiella halophila, with special emphasis on K(+)/Na(+) selectivity and proline accumulation

The eco-physiology of salt tolerance, with an emphasis on K(+) nutrition and proline accumulation, was investigated in the halophyte Thellungiella halophila and in both wild type and eskimo-1 mutant of the glycophyte Arabidopsis thaliana, which differ in their proline accumulation capacity. Plants cultivated in inert sand were challenged for 3 weeks with up to 500mM NaCl. Low salinity significantly decreased A. thaliana growth, whereas growth restriction was significant only at salt concentrations equal to or exceeding 300mM NaCl in T. halophila. Na(+) content generally increased with the amount of salt added in the culture medium in both species, but T. halophila showed an ability to control Na(+) accumulation in shoots. The analysis of the relationship between water and Na(+) contents suggested an apoplastic sodium accumulation in both species; this trait was more pronounced in A. thaliana than in T. halophila. The better NaCl tolerance in the latter was associated with a better K(+) supply, resulting in higher K(+)/Na(+) ratios. It was also noteworthy that, despite highly accumulating proline, the A. thaliana eskimo-1 mutant was the most salt-sensitive species. Taken together, our findings indicate that salt tolerance may be partly linked to the plants' ability to control Na(+) influx and to ensure appropriate K(+) nutrition, but is not linked to proline accumulation.

Salinity tolerance of Arabidopsis: a good model for cereals?

Arabidopsis is a glycophyte species that is sensitive to moderate levels of NaCl. Arabidopsis offers unique benefits to genetic and molecular research and has provided much information about both Na(+) transport processes and Na(+) tolerance. A compilation of data available on Na(+) accumulation and Na(+) tolerance in Arabidopsis is presented, and comparisons are made with several crop plant species. The relationship between Na(+) tolerance and Na(+) accumulation is different in Arabidopsis and cereals, with an inverse relationship often found within cereal species that is not as evident in Arabidopsis ecotypes. Results on salinity tolerance obtained in Arabidopsis should therefore be extrapolated to cereals with caution. Arabidopsis remains a useful model to study and discover plant Na(+) transport processes.

Efficiency of biochemical protection against toxic effects of accumulated salt differentiates Thellungiella halophila from Arabidopsis thaliana

Thellungiella halophila and Arabidopsis thaliana were irrigated with medium containing NaCl at various concentrations. The salt treatment resulted in a restriction of rosette biomass deposition in both species. In A. thaliana leaves, this inhibition was stronger than for T. halophila and was associated with strong inhibition of both leaf initiation and leaf expansion. At highest medium salinity, A. thaliana accumulated Na(+) and Cl(-) at higher levels than T. halophila, but similar leaf dehydration was observed in the two species. Proline accumulation, which increased with NaCl concentration, did not differentiate the two species. The magnitude of the electrolyte leakage and the level of lipid peroxidation (assessed through hydroxy fatty acid content) were modest in T. halophila and quite marked in A. thaliana. The detrimental effects of the salt on photosynthetic activity and stomatal conductance of A. thaliana leaves were much more important than in T. halophila leaves. The abundance of the CDSP32 thioredoxin, a critical component of the defence system against oxidative damage and lipid peroxidation, was found to be higher in T. halophila than in A. thaliana under control conditions and salt treatment. These results suggest that the rosette leaves of T. halophila exhibit more efficient protective mechanisms against Na(+) metabolic toxicity than those of A. thaliana.