Enhanced photochemical efficiency of PSII in Prosopis juliflora suggests contribution to invasion advantage over native C3 xero-halophytes under salt stress.

Chlorophyll a fluorescence parameters related to PSII photochemistry, photoprotection and photoinhibition were investigated in four C3 plant species growing in their natural habitat: Prosopis juliflora ; Abutilon indicum ; Salvadora persica ; and Phragmites karka . This study compared the light reaction responses of P. juliflora , an invasive species, with three native co-existing species, which adapt to varying water deficit and high salt stress. Chlorophyll a fluorescence quenching analyses revealed that P. juliflora had the highest photochemical quantum efficiency and yield, regulated by higher fraction of open reaction centres and reduced photoprotective energy dissipation without compromising the integrity of photosynthetic apparatus due to photoinhibition. Moreover, the elevated values of parameters obtained through polyphasic chlorophyll a fluorescence induction kinetics, which characterise the photochemistry of PSII and electron transport, highlighted the superior performance index of energy conservation in the transition from excitation to the reduction of intersystem electron carriers for P. juliflora compared to other species. Enhanced pigment contents and their stoichiometry in P. juliflora apparently contributed to upregulating fluxes and yields of energy absorbance, trapping and transport. This enhanced photochemistry, along with reduced non-photochemical processes, could explain the proclivity for invasion advantage in P. juliflora across diverse stress conditions.

Nitrate modulates the physiological tolerance responses of the halophytic species Sarcocornia fruticosa to copper excess.

Coexistence impact of pollutants of different nature on halophytes tolerance to metal excess has not been thoroughly examined, and plant functional responses described so far do not follow a clear pattern. Using the Cu-tolerant halophyte Sarcocornia fruticosa as a model species, we conducted a greenhouse experiment to evaluate the impact of two concentration of copper (0 and 12 mM CuSO4) in combination with three nitrate levels (2, 14 and 50 mM KNO3) on plant growth, photosynthetic apparatus performance and ROS-scavenging enzymes system. The results revealed that S. fruticosa was able to grow adequately even when exposed to high concentrations of copper and nitrate. This response was linked to the plant capacity to uptake and retain a large amount of copper in its roots (up to 1500 mg kg-1 Cu), preventing its transport to aerial parts. This control of translocation was further magnified with nitrate concentration increment. Likewise, although Cu excess impaired S. fruticosa carbon assimilation capacity, the plant was able to downregulate its light-harvesting complexes function, as indicated its lowers ETR values, especially at 12 mM Cu + 50 mM NO3. This downregulation would contribute to avoid excess energy absorption and transformation. In addition, this strategy of avoiding excess energy was accompanied by the upregulation of all ROS-scavenging enzymes, a response that was further enhanced by the increase in nitrate concentration. Therefore, we conclude that the coexistence of nitrate would favor S. fruticosa tolerance to copper excess, and this effect is mediated by the combined activation of several tolerance mechanisms.

Transcription Factor ChbZIP1 from Alkaliphilic Microalgae Chlorella sp. BLD Enhancing Alkaline Tolerance in Transgenic Arabidopsis thaliana.

Saline-alkali soil has become an important environmental problem for crop productivity. One of the most effective approaches is to cultivate new stress-tolerant plants through genetic engineering. Through RNA-seq analysis and RT-PCR validation, a novel bZIP transcription factor ChbZIP1, which is significantly upregulated at alkali conditions, was obtained from alkaliphilic microalgae Chlorella sp. BLD. Overexpression of ChbZIP1 in Saccharomyces cerevisiae and Arabidopsis increased their alkali resistance, indicating ChbZIP1 may play important roles in alkali stress response. Through subcellular localization and transcriptional activation activity analyses, we found that ChbZIP1 is a nuclear-localized bZIP TF with transactivation activity to bind with the motif of G-box 2 (TGACGT). Functional analysis found that genes such as GPX1, DOX1, CAT2, and EMB, which contained G-box 2 and were associated with oxidative stress, were significantly upregulated in Arabidopsis with ChbZIP1 overexpression. The antioxidant ability was also enhanced in transgenic Arabidopsis. These results indicate that ChbZIP1 might mediate plant adaptation to alkali stress through the active oxygen detoxification pathway. Thus, ChbZIP1 may contribute to genetically improving plants' tolerance to alkali stress.

The toxicity of selenium and mercury in Suaeda salsa after 7-days exposure.

Mercury is one of the major pollutants in the ocean, selenium causes toxicity beyond a certain limit, but there are few comparative toxic studies between them in halophytes. The study was to investigate the toxic effects of selenium (Se4+) and mercury (Hg2+) in halophyte Suaeda salsa at the level of genes, proteins and metabolites after exposure for 7 days. By integrating the results of proteomics and metabolomics, the pathway changed under different treatments were revealed. In Se4+-treated group, the changed 3 proteins and 10 metabolites participated in the process of substance metabolism (amino acid, pyrimidine), citrate cycle, pentose phosphate pathway, photosynthesis, energy, and protein biosynthesis. In Hg2+-treated group, the changed 10 proteins and 10 metabolites were related to photosynthesis, glycolysis, substance metabolism (cysteine and methionine, amino acid, pyrimidine), ATP synthesis and binding, tolerance, sugar-phosphatase activity, and citrate cycle. In Se4++ Hg2+-treated group, the changed 5 proteins an 12 metabolites involved in stress defence, iron ion binding, mitochondrial respiratory chain, structural constituent of ribosome, citrate cycle, and amino acid metabolism. Furthermore, the separate and combined selenium and mercury both inhibited growth of S. salsa, enhanced activity of antioxidant enzymes (superoxide dismutase, peroxidase and catalase), and disturbed osmotic regulation through the genes of choline monoxygenase and betaine aldehyde dehydrogenase. Our experiments also showed selenium could induce synergistic effects in S. salsa. In all, we successfully characterized the effects of selenium and mercury in plant which was helpful to evaluate the toxicity and interaction of marine pollutants.

Comprehensive transcriptome profiling of Caragana microphylla in response to salt condition using de novo assembly.

OBJECTIVE: To investigate the response of Caragana microphylla in salt condition, transcriptome analysis, differentially expressed genes (DEGs) comparison with Arabidopsis thaliana, and the chlorophyll content analysis were performed. RESULTS: Gene Ontology (GO) term, and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses of DEGs indicated that salt condition affected photosynthesis and chlorophyll in C. microphylla. The DEGs compared with salt responsive genes of A. thaliana indicated that C. microphylla's responses to salt differed greatly from those of the model plant and that the results also indicated up-regulated genes related to photosynthesis and chlorophyll in C. microphylla. Moreover, we confirmed that salt-treated C. microphylla increased chlorophyll content, and the genes of protoporphyrin IX downstream in chlorophyll biosynthesis were induced in the heatmap analysis. CONCLUSIONS: These results showed a similar pattern to some halophytes plants with increased chlorophyll at a certain salt concentration, and we assumed that C. microphylla also has a mechanism to adapt or tolerate moderate salt conditions.

Adaptation Strategies of Halophytic Barley Hordeum marinum ssp. marinum to High Salinity and Osmotic Stress.

The adaptation strategies of halophytic seaside barley Hordeum marinum to high salinity and osmotic stress were investigated by nuclear magnetic resonance imaging, as well as ionomic, metabolomic, and transcriptomic approaches. When compared with cultivated barley, seaside barley exhibited a better plant growth rate, higher relative plant water content, lower osmotic pressure, and sustained photosynthetic activity under high salinity, but not under osmotic stress. As seaside barley is capable of controlling Na+ and Cl- concentrations in leaves at high salinity, the roots appear to play the central role in salinity adaptation, ensured by the development of thinner and likely lignified roots, as well as fine-tuning of membrane transport for effective management of restriction of ion entry and sequestration, accumulation of osmolytes, and minimization of energy costs. By contrast, more resources and energy are required to overcome the consequences of osmotic stress, particularly the severity of reactive oxygen species production and nutritional disbalance which affect plant growth. Our results have identified specific mechanisms for adaptation to salinity in seaside barley which differ from those activated in response to osmotic stress. Increased knowledge around salt tolerance in halophytic wild relatives will provide a basis for improved breeding of salt-tolerant crops.

Morphological and physiological responses of two willow species from different habitats to salt stress.

Plant salt tolerance is a complex mechanism, and different plant species have different strategies for surviving salt stress. In the present study, we analyzed and compared the morphological and physiological responses of two willow species (Salix linearistipularis and Salix matsudana) from different habitats to salt stress. S. linearistipularis exhibited higher seed germination rates and seedling root Na+ efflux than S. matsudana under salt stress. After salt treatment, S. linearistipularis leaves exhibited less Na+ accumulation, loss of water and chlorophyll, reduction in photosynthetic capacity, and damage to leaf cell structure than leaves of S. matsudana. Scanning electron microscopy combined with gas chromatography mass spectrometry showed that S. linearistipularis leaves had higher cuticular wax loads than S. matsudana leaves. Overall, our results showed that S. linearistipularis had higher salt tolerance than S. matsudana, which was associated with different morphological and physiological responses to salt stress. Furthermore, our study suggested that S. linearistipularis could be a promising tree species for saline-alkali land greening and improvement.

Salinity relief aniline induced oxidative stress in Suaeda salsa: Activities of antioxidative enzyme and EPR measurements.

Wastewater from printing and dyeing processes often contains aniline and high salinity, which are hazardous to aquatic species. Glycophytic plants cannot survive under high-salinity conditions, whereas halophytes grow well in such an environment. In this study, we investigated the influence of NaCl on the antioxidant level in Suaeda salsa affected by aniline stress. The seedlings showed various growth toxicity effects under different concentrations of aniline. The results showed that the effect of the aniline was more severe for the root growth compared to that for the shoot growth. Aniline exposure significantly increased the total free radicals and ·OH radicals in the plants. Suaeda salsa exposure to aniline caused oxidative stress by altering the superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activity, which resulted in the overproduction of H2O2 and the inducement of lipid peroxidation. Analysis revealed that the malondialdehyde (MDA) content was enhanced after aniline exposure and that the chlorophyll content was significantly decreased. The results showed that aniline induced the production of free radicals and reactive oxygen species (ROS), and changed the antioxidant defense system. This ultimately resulted in oxidative damage in S. salsa; however, it was found that moderate salinity could mitigate the effects. In conclusion, salinity may alleviate the growth inhibition caused by aniline by regulating the antioxidant capacity of S. salsa.

Nano-Zinc Oxide and Arbuscular mycorrhiza Effects on Physiological and Biochemical Aspects of Wheat Cultivars under Saline Conditions.

BACKGROUND AND OBJECTIVE: Saline soils are restrictive factors to agriculture in arid and semi-arid regions, plant growth and productivity. Thus, it was important to consider how, nano-zinc oxide or bulk zinc oxide alleviates the oxidative salt stress in the presence of Arbuscular mycorrhiza (AM) fungi by two wheat cultivars (Sids 13 and Sakha 94). MATERIALS AND METHODS: A field experiment was carried out during two winter successive seasons to study the beneficial role of nano-zinc oxide or bulk zinc oxide with different concentrations (5 and 10 mg L-1) in enhancing growth, some biochemical and physiological of two wheat cultivars under saline soil. RESULTS: Soaking both wheat cultivars with nano-zinc oxide or bulk zinc oxide in the presence of AM improved growth parameters. All treatments increased significantly photosynthetic pigments, IAA, phenols contents, organic antioxidant enzyme activities and significant decrease in lipid peroxidation. some changes are observed in protein patterns, so several proteins were disappear, but others were selectively improved and synthesis of the new groups of protein was formed, some of these responses were observed through the effect of nano-zinc oxide or bulk zinc oxide and AM. CONCLUSION: Nano-ZnO (10 mg L-1) in the presence of AM was the most effective treatments on both cultivars. Results showed superiority of Sakha 94 cultivar in most growth parameters and biochemical aspects than Sids 13 cultivar.

The combined effect of Cr(III) and NaCl determines changes in metal uptake, nutrient content, and gene expression in quinoa (Chenopodium quinoa Willd.).

Many areas of the world are affected simultaneously by salinity and heavy metal pollution. Halophytes are considered as useful candidates in remediation of such soils due to their ability to withstand both osmotic stress and ion toxicity deriving from high salt concentrations. Quinoa (Chenopodium quinoa Willd) is a halophyte with a high resistance to abiotic stresses (drought, salinity, frost), but its capacity to cope with heavy metals has not yet been fully investigated. In this pot experiment, we investigated phytoextraction capacity, effects on nutrient levels (P and Fe), and changes in gene expression in response to application of Cr(III) in quinoa plants grown on saline or non-saline soil. Plants were exposed for three weeks to 500 mg kg-1 soil of Cr(NO3)3·9H2O either in the presence or absence of 150 mM NaCl. Results show that plants were able tolerate this soil concentration of Cr(III); the metal was mainly accumulated in roots where it reached the highest concentration (ca. 2.6 mg g-1 DW) in the presence of NaCl. On saline soil, foliar Na concentration was significantly reduced by Cr(III). Phosphorus translocation to leaves was reduced in the presence of Cr(III), while Fe accumulation was enhanced by treatment with NaCl alone. A real-time RT-qPCR analysis was conducted on genes encoding for sulfate, iron, and phosphate transporters, a phytochelatin, a metallothionein, glutathione synthetase, a dehydrin, Hsp70, and enzymes responsible for the biosynthesis of proline (P5CS), glycine betaine (BADH), tocopherols (TAT), and phenolic compounds (PAL). Cr(III), and especially Cr(III)+NaCl, affected transcript levels of most of the investigated genes, indicating that tolerance to Cr is associated with changes in phosphorus and sulfur allocation, and activation of stress-protective molecules. Moderately saline conditions, in most cases, enhanced this response, suggesting that the halophytism of quinoa could contribute to prime the plants to respond to chromium stress.

NaCl-responsive ROS scavenging and energy supply in alkaligrass callus revealed from proteomic analysis.

BACKGROUND: Salinity has obvious effects on plant growth and crop productivity. The salinity-responsive mechanisms have been well-studied in differentiated organs (e.g., leaves, roots and stems), but not in unorganized cells such as callus. High-throughput quantitative proteomics approaches have been used to investigate callus development, somatic embryogenesis, organogenesis, and stress response in numbers of plant species. However, they have not been applied to callus from monocotyledonous halophyte alkaligrass (Puccinellia tenuifora). RESULTS: The alkaligrass callus growth, viability and membrane integrity were perturbed by 50 mM and 150 mM NaCl treatments. Callus cells accumulated the proline, soluble sugar and glycine betaine for the maintenance of osmotic homeostasis. Importantly, the activities of ROS scavenging enzymes (e.g., SOD, APX, POD, GPX, MDHAR and GR) and antioxidants (e.g., ASA, DHA and GSH) were induced by salinity. The abundance patterns of 55 salt-responsive proteins indicate that salt signal transduction, cytoskeleton, ROS scavenging, energy supply, gene expression, protein synthesis and processing, as well as other basic metabolic processes were altered in callus to cope with the stress. CONCLUSIONS: The undifferentiated callus exhibited unique salinity-responsive mechanisms for ROS scavenging and energy supply. Activation of the POD pathway and AsA-GSH cycle was universal in callus and differentiated organs, but salinity-induced SOD pathway and salinity-reduced CAT pathway in callus were different from those in leaves and roots. To cope with salinity, callus mainly relied on glycolysis, but not the TCA cycle, for energy supply.

The functional identification of glycine-rich TtASR from Tetragonia tetragonoides (Pall.) Kuntze involving in plant abiotic stress tolerance.

In this study, we reported on an ASR gene (TtASR) related to salt/drought tolerance from the edible halophyte Tetragonia tetragonoides (Pall.) Kuntze (Aizoaceae). A phylogenetic analysis revealed that TtASR was evolutionarily close to other two halophytic glycine-rich ASR members, SbASR-1 (from Salicornia brachiate) and SlASR (from Suaeda liaotungensis), with a typical abscisic acid (ABA)/water-deficit stress (WDS) domain at C-terminal. Quantitative RT-PCR analyses showed that TtASR was expressed in all tested different organs of the T. tetragonoides plant and that expression levels were apparently induced after salt, osmotic stress, and ABA treatments in T. tetragonoides seedlings. An induction of TtASR improved the growth performance of yeast and bacteria more than the control under high salinity, osmotic stress, and oxidative stress. TtASR was not a nuclear-specific protein in plant, and the transcriptional activation assay also demonstrated that TtASR could not activate reporter gene's expression in yeast. TtASR overexpressed Arabidopsis plants exhibited higher tolerance for salt/drought and oxidative stresses and lower ROS accumulation than wild type (WT) plants, accompanied by increased CAT, SOD activities, higher proline content, and lower MDA content in vivo. The results indicated that the TtASR was involved in plant responses to salt and drought, probably by mediating water homeostasis or by acting as ROS scavengers, and that it decreased the membrane damage and improved cellular osmotic adjustment that respond to abiotic stresses in microorganisms and plants.

GhEIN3, a cotton (Gossypium hirsutum) homologue of AtEIN3, is involved in regulation of plant salinity tolerance.

Ethylene insensitive 3 (EIN3), a key transcription factor in ethylene signal transduction, play important roles in plant stress signaling pathways. In this study, we isolated and characterized an EIN3-like gene from cotton (Gossypium hirsutum), designated as GhEIN3. GhEIN3 is highly expressed in vegetative tissues, and its expression is induced by 1-aminocyclopropane-1-carboxylic acid (ACC) and NaCl. Ectopic expression of GhEIN3 in Arabidopsis elevated plants' response to ethylene, which exhibit smaller leaves, more root hairs, shorter roots and hypocotyls. The germination rate, survival rate and root length of GhEIN3 transgenic plants were significantly improved compared to wild type under salt stress. GhEIN3 transgenic plants accumulated less H2O2 and malondialdehyde (MDA), while higher superoxide dismutase (SOD) and peroxidase (POD) activities were detected under salt stress. In addition, expression of several genes related to reactive oxygen species (ROS) pathway and ABA signaling pathway was increased in the GhEIN3 transgenic plants under salt stress. In contrast, virus-induced gene silencing (VIGS) of GhEIN3 in cotton enhanced the sensitivity of transgenic plants to salt stress, accumulating higher H2O2 and MDA and lower SOD and POD activities compared to control plants. Collectively, our results revealed that GhEIN3 might be involved in the regulation of plant response to salt stress by regulating ABA and ROS pathway during plant growth and development.

Mechanistic elucidation of germination potential and growth of wheat inoculated with exopolysaccharide and ACC- deaminase producing Bacillus strains under induced salinity stress.

The potential of plant growth regulating microorganisms present in the soil can be explored towards the purpose of identifying salt tolerant strategies and crop cultivars. Current study was designed to elucidate the capabilities of salt stress tolerant plant growth promoting rhizobacteria (PGPR) Bacillus siamensis (PM13), Bacillus sp. (PM15) and Bacillus methylotrophicus (PM19) in undermining the effects of salt stress on wheat seedling. Strains were characterized for their IAA (81-113 µM/ml), ACC-deaminase (0.68-0.95 µM/mg protein/h) and exopolysaccharide (EPS) (0.62-0.97 mg/ml) producing activity both under normal and NaCl stressed conditions. Effects of bacterial inoculation on germination and seedling growth of wheat variety Pakistan-13 was observed under induced salinity stress levels (0, 4, 8, 16 dS/m). All the morpho-physiological characteristics of wheat seedlings were affected drastically by the NaCl stress and the growth parameters expressed a negative relationship with increased NaCl levels. PGPR application had a very positive influence on germination rate of wheat seedlings, root and shoot length, photosynthetic pigments etc. Elongated roots and enhanced vegetative shoot growth as well as seedling's fresh and dry weights were highest in plants treated with B. methylotrophicus PM19. Sequestration of Na+ ion by EPS production and degradation of exuded ACC into a-ketobutyrate and ammonia by ACCD bacteria efficiently reduced the impact of salinity stress on wheat growth. Current findings suggested that the used PGPR strains are potential candidates for improving crop growth in salt stressed agricultural systems. However further research validation would be necessary before large scale/field application.

Strawberries under salt stress: ALA and ROS to the rescue.

Generating salt-tolerant plants that can cope with increasing soil salinity is a major goal of crop-breeding programs worldwide. Together with genetic approaches, research efforts are focusing on finding chemical modulators of salt tolerance. The exogenous application of 5-aminolevulinic acid (ALA) has been shown to improve salt tolerance in diverse crop species, but its mechanism of action is not properly understood. Wu et al. (2019) report that ALA treatment enhances reactive oxygen species (ROS) production in the roots of salt-stressed strawberry plants. Activation of several key ion transporters downstream to the ROS signal helps to sequester the toxic Na+ ions in the roots and protects the shoots against salt stress.

The response of a model C3/CAM intermediate semi-halophyte Mesembryanthemum crystallinum L. to elevated cadmium concentrations.

Many areas exhibiting increased concentrations of soluble salts are simultaneously polluted with heavy metals (HM), and halophytes with extended tolerance to heavy metal toxicity seem to represent a promising tool for their phytoremediation. In this study, the response of the soil-grown C3-CAM (Crassulacean acid metabolism) intermediate halophyte Mesembryanthemum crystallinum (common ice plant) to increased concentrations of Cd (0.01-1 mM) was investigated. None of the tested Cd treatments affected growth parameters or tissue water content of either C3 or CAM-performing plants. Chlorophyll a fluorescence confirmed high tolerance of the photosynthetic apparatus of both metabolic states towards Cd. Plants performing both photosynthesis types accumulated significant Cd amounts only under the highest (1 mM) treatment, and the metal was primarily deposited in the roots, which are features typical of an excluding strategy. Upon the application of 1 mM Cd solution CAM-performing plants, due to the NaCl pre-treatment applied for CAM induction, were exposed to significantly higher amounts of bioavailable Cd in comparison with those of C3-performing plants. As a result, roots of CAM plants accumulated over 4-fold higher Cd amounts when compared with C3 plants. In our opinion, enhanced Cd-accumulating potential observed in CAM-performing plants was the effect of osmotic stress episode and resulting modifications e.g. in the detoxifying capacity of the antioxidative system. Increased antioxidative potential of NaCl pre-treated plants was pronounced with significantly higher activity of CuZnSOD (copper-zinc superoxide dismutase), not achievable in C3 plants subjected to high Cd concentrations. Moreover, the applied Cd doses induced SOD activity in a compartment-dependent manner only in C3 plants. We confirmed that none of the applied Cd concentrations initiated the metabolic shift from C3 to CAM.

Salinity modifies heavy metals and arsenic absorption by the halophyte plant species Kosteletzkya pentacarpos and pollutant leaching from a polycontaminated substrate.

Phytomanagement of polycontaminated soils is challenging, especially in areas simultaneously affected by salinity. The wetland halophyte plant species Kosteletzkya pentacarpos was cultivated in a column device allowing leachate harvest, on a polycontaminated spiked soil containing Cd (6.5 mg kg-1 DW), As (75 mg kg-1 DW), Zn (200 mg kg-1 DW) and Pb (300 mg kg-1 DW) and irrigated with salt water (final soil electrical conductivity 5.0 ms cm-1). Salinity increased Cd bioavailability in the soil and Cd accumulation in the shoots while it had an opposite effect on As. Salinity did not modify Pb and Zn bioavailability and accumulation. Cultivating plants on the polluted soil drastically reduced the volume of leachate. In all cases, salinity reduced the total amounts of heavy metals removed by the leachate and significantly increased the proportion of Cd and Zn removed by the plants. Heavy metal contamination induced a decrease in shoot dry weight and an increase in malondialdehyde (an indicator of oxidative stress); both symptoms were alleviated by the additional presence of NaCl but this positive impact was not related to increase in protecting phytochelatins synthesis. It is concluded i) that bioavailability estimated by the 0.01M CaCl2 extraction procedure is not fully relevant from the heavy metal mobility, ii) that salinity decreased heavy metal percolation, especially in soils cultivated with K. pentacarpos and iii) that salinity improves plant tolerance to heavy metals in K. pentacarpos and that this species is a promising plant material for phytoremediation of polycontaminated soils.

Salinity-induced alterations in plant growth, antioxidant enzyme activities, and lead transportation and accumulation in Suaeda salsa: implications for phytoremediation.

Halophytes have been considered promising candidates for accumulating heavy metals from saline soils; however, little information has been given on plant physiological responses and heavy metal transportation and accumulation in halophytes that grow in heavy metal-polluted saline soils. This study hypothesized that salinity or heavy metals could induce alterations in plant growth, antioxidant enzyme activities and accumulation and transportation of heavy metals or sodium (Na) in Suaeda salsa. Pot experiments were conducted to test the above hypothesis. Lead (Pb) was selected as the representative heavy metal, and NaCl was added to simulate the Pb-polluted saline soil. The results showed that 0.5% NaCl addition alleviated the inhibition of plant growth under moderate Pb stress (35 and 100 mg kg-1 Pb levels), while the phytotoxicity on plants was magnified by 1.0% NaCl addition. NaCl weakened the oxidative stress in Pb-treated plants by increasing the activity levels of antioxidative enzymes (dismutase (SOD), peroxidase (POD) and catalase (CAT)). At all Pb levels, as the NaCl addition increased, significant increases were observed in the concentration of Na. The 100 mg kg-1 Pb induced a greater increase in Na concentrations than the 35 mg kg-1 Pb did, while the latter induced a greater increase than the 300 mg kg-1 Pb did. NaCl improved Pb translocation factor and its accumulation in Suaeda salsa under Pb stress, indicating that NaCl improves Pb uptake and translocation from roots to shoots and enhances the phytoextraction of Pb. Compared with the 0.1% NaCl treatment, the 0.5 and 1.0% NaCl treatments increased the concentrations of bioavailable Pb in the rhizosphere by 15.0-19.2 and 28.6-35.1%, respectively, indicating the contribution of salinity in producing more available Pb for plant uptake. Moderate salinity may be profitable for Pb transportation and accumulation in plants when there are positive effects on plant growth, antioxidant enzyme activities and Pb availability. These facts suggest that the halophyte Suaeda salsa may be exploited to remediate heavy metal-contaminated saline soils.

Functional analysis of McSnRK1 (SNF1-related protein kinase 1) in regulating Na/K homeostasis in transgenic cultured cells and roots of halophyte Mesembryanthemum crystallinum.

KEY MESSAGE: Transgenic callus and roots of ice plant with altered SnRK1 function were established using Agrobacterium-mediated transformation. The role of McSnRK1 in controlling Na+ influx and Na/K ratio was demonstrated. SnRK1 kinases (SNF1-related protein kinase1) control metabolic adaptation during energy deprivation and regulate protective mechanisms against environmental stress. Yeast SNF1 activates a P-type ATPase, the Na+ exclusion pump, under glucose starvation. The involvement of plant SnRK1 in salt stress response is largely unknown. We previously identified a salt-induced McSnRK1 in the halophyte ice plant (Mesembryanthemum crystallinum). In the current study, the function of McSnRK1 in salt tolerance was analyzed in transgenic cultured cells and roots of ice plant. Ice plant callus constitutively expressed a high level of McSnRK1 and introducing the full-length McSnRK1 did not alter the Na/K ratio at 24 h after 200 mM NaCl treatment. However, interfering with McSnRK1 activity by introducing a truncate McSnRK1 to produce a dominant-negative form of McSnRK1 increased cellular Na+ accumulation and Na/K ratio. As a result, the growth of cultured cells diminished under salt treatment. Hydroponically grown ice plants with roots expressing full-length McSnRK1 had better growth and lowered Na/K ratio compared to the wild-type or vector-only plants. Roots expressing a truncate McSnRK1 had reduced growth and high Na/K ratio under 400 mM NaCl treatment. The changes in Na/K ratio in transgenic cells and whole plants demonstrated the function of SnRK1 in controlling Na+ flux and maintaining Na/K homeostasis under salinity. The Agrobacterium-mediated transformation system could be a versatile tool for functional analysis of genes involved in salt tolerance in the ice plant.

Insight into mechanisms of multiple stresses tolerance in a halophyte Aster tripolium subjected to salinity and heavy metal stress.

The study aimed at comparing metabolic reactions of a halophyte Aster tripolium to abiotic stresses. Profiling of endogenous phytohormones, soluble carbohydrates and stress-related amino acids was conducted in plants exposed to moderate and high salinity (150 and 300 mM NaCl), and heavy metal salts CdCl2 or PbCl2 (100 and 200 µM). High NaCl and Pb doses inhibited growth of A. tripolium (Stress Tolerance Index STI) of 37% and 32-35%, respectively. The plants tolerated moderate salinity and Cd (STI = 91% and STI = 83-96%, respectively). Toxic metals accumulated mainly in the roots but Cd translocation to the shoots was also observed. The stressors did not affect total concentrations of the main growth promoting phytohormones but we observed enhanced deactivation of auxins and gibberellins, and reduced accumulation of jasmonate precursor. ABA content increased under stress except for moderate salinity. A common reaction was also activation of osmotic adjustment, however it was disparately manifested under salinity and metallic stress. The distinct responses to salinity and metallic stresses involved changes in carbohydrate profile and altered interplay between salicylic acid content and the pool of active gibberellins. The content of active jasmonates diversified A. tripolium reactions to salt excess and each of the heavy metals. This parameter was linked to the accumulation of ethylene precursor. The results of the study can be used to decipher potential co-tolerance mechanism of this halophyte species to multiple environmental stresses.