How does NaCl improve tolerance to cadmium in the halophyte Sesuvium portulacastrum?

Gene co-expression networks unravel the molecular responses of freshwater hydrophytes to combined stress of salinity and cadmium

Salinization in freshwater lakes is becoming a serious global environmental problem, especially in lakes of plateaus such as south-western plateau of China. However, limited information is available about the molecular response of freshwater hydrophytes to salinity under multiple stress. In the present study, a weighted gene co-expression network (WGCNA) was used to identify the modules of co-expressed genes in the physiological and biochemical indicators of Pistia stratiotes to determine its molecular response to salinity (NaCl) alone and when combined with cadmium (Cd). The physiological and biochemical indicators showed that P. stratiotes improved its salt tolerance by enhancing photosynthetic abilities, reducing oxidative stress, and inducing osmoprotectant generation. Morever, addition of NaCl reduced the Cd accumulation in P. stratiotes. Transcriptome and WGCNA analysis revealed that the pathways of alpha-linolenic acid metabolism, ribosomal, flavonoid biosynthesis, and phenylpropanoid biosynthesis were significantly enriched in both treatments. Genes associated with photosynthesis-antenna proteins, nitrogen metabolism, and the acid cycle pathways were only expressed under salinity stress alone, while the proteasome pathway was only significantly enriched in the combined salinity and Cd treatment. Our findings provide novel insights into the effects of salinization on aquatic plants in freshwater ecosystems and the management of aquatic ecosystems under global change.

Physiological and Cellular Ultrastructural Responses of Sesuvium portulacastrum under Cd Stress Grown Hydroponically

This study aimed to investigate the physiological and cellular mechanisms of Sesuvium portulacastrum under heavy metal stress to evaluate possible tolerance and adaptation mechanisms in a metal-polluted environment. The physiological and cellular ultrastructural responses of S. portulacastrum were studied hydroponically under exposure to a range of cadmium (Cd) concentrations (50 µM to 600 µM) for 28 days. The activity of antioxidant enzymes like catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD), changes in chlorophyll, and cellular ultrastructural content were examined. There was no significant difference in chlorophyll content in the leaf under the stress of 300 μM, but 400 μM and 600 μM Cd stress showed significantly decreased chlorophyll content. The SOD activity indicates an increase under the Cd stress of 100 μM for leaves, 300 μM for stems, and 50 μM for roots; after that, the SOD activity gradually decreased with increasing Cd concentrations. But POD activity was considerably increased with increasing Cd stress. CAT activity showed a gradual increase in concentrations until 300 μM of Cd stress and then decreased sharply in roots, stems, and leaf tissues. Cd stress had a considerable impact on the structure of the roots, stems, and leaves cells, such as distorted and thinner cell walls and the deformation of chloroplasts, mitochondria, and other organelles. Therefore, the increased number of nucleolus in the cell nucleus suggests that cells may be able to maintain their protein synthesis in a stressful environment. This study concludes that SOD is the dominant antioxidant enzyme activity during low Cd toxicity (<100 μM), while POD is the dominant enzyme activity during higher Cd toxicity (>100 μM).

NaCl effect on Cd accumulation and cell compartmentalization in barley

The effect of sodium chloride (NaCl) on cadmium (Cd) tolerance, uptake, translocation, and compartmentation was investigated in 3 barley genotypes. Seedlings were cultivated hydroponically in the absence of NaCl and Cd (control), in the presence of 50 mM NaCl alone, in the presence of 10 µM Cd alone, and in the combined addition of NaCl (50 mM) and Cd (10 µM). Plants were cultivated during one month under 16 h light period at a minimal light intensity of 250 µmol m-2 s-1, a temperature of 25 ± 3 °C, and 70-80% of relative humidity. Results showed that NaCl alone did not significantly affect plant development and biomass production; however, Cd alone reduced plant development rate leading to a decline in biomass production in Raihane and Giza 127 but did not affect that in Amalou. NaCl addition in Cd-treated plants accentuated the Cd effect on plant growth. NaCl limited Cd accumulation in the roots and in the shoots in all tested barley varieties by reducing Cd-absorption efficiency and the translocation of Cd from the root to the shoot. In all Cd-treated plants, cell Cd compartmentalization showed the following gradient: organelles < cell wall < vacuole. NaCl in the medium increased Cd accumulation in the soluble fraction and reduced that in organelle and cell wall fractions. Globally our results showed that, although NaCl reduces Cd accumulation in barley, it accentuates the Cd toxic effects, hence limiting the plant yield. We advise farmers to avoid barley cultivation near mine sites and its irrigation with moderately salty water, although this plant is considered as salt tolerant.

Halophytes for the sustainable remediation of heavy metal-contaminated sites: Recent developments and future perspectives

Increasing land degradation by high level of metal wastes is of prime concern for the global research communities. In this respect, halophytes having specific features like salt glands, exclusion of excess ions, heavy metals (HMs) compartmentalization, large pool of antioxidants, and associations with metal-tolerant microbes are of great promise in the sustainable clean-up of contaminated sites. However, sustainable clean-up of HMs by a particular halophyte plant species is governed considerably by physico-chemical characteristics of soil and associated microbial communities. The present review has shed light on the superiority of halophytes over non-halophytes, mechanisms of metal-remediation, recent developments and future perspectives pertaining to the utilization of halophytes in management of HM-contaminated sites with the aid of bibliometric analysis. The results revealed that the research field is receiving considerable attention in the last 5-10 years by publishing ∼50-90% documents with an annual growth rate of 15.41% and citations per document of 29.72. Asian (viz., China, India, and Pakistan) and European (viz., Spain, Portugal, Belgium, Argentina) countries have been emerged as the major regions conducting and publishing extensive research on this topic. The investigations conducted both under in vitro and field conditions have reflected the inherent potential of halophyte as sustainable research tool for successfully restoring the HM-contaminated sites. The findings revealed that the microbial association with halophytes under different challenging conditions is a win-win approach for metal remediation. Therefore, exploration of new halophyte species and associated microorganisms (endophytic and rhizospheric) from different geographical locations, and identification of genes conferring tolerance and phytoremediation of metal contaminants would further advance the intervention of halophytes for sustainable ecological restoration.

Phytoremediation of Potentially Toxic Elements from Contaminated Saline Soils Using Salvadora persica L.: Seasonal Evaluation

Plants in coastal ecosystems are primarily known as natural sinks of trace metals and their importance for phytoremediation is well established. Salvadora persica L., a medicinally important woody crop of marginal coasts, was evaluated for the accumulation of metal pollutants (viz. Fe, Mn, Cu, Pb, Zn, and Cr) from three coastal areas of Karachi on a seasonal basis. Korangi creek, being the most polluted site, had higher heavy metals (HM's) in soil (Fe up to 17,389, Mn: 268, Zn: 105, Cu: 23, Pb: 64.7 and Cr up to 35.9 mg kg^-1) and S. persica accumulated most of the metals with >1 TF (translocation factor), yet none of them exceeded standard permissible ranges except for Pb (up to 3.1 in roots and 3.37 mg kg^-1 in leaves with TF = 11.7). Seasonal data suggested that higher salinity in Clifton and Korangi creeks during pre- and post-monsoon summers resulted in lower leaf water (ΨW_o) and osmotic potential at full turgor (ΨS_o) and bulk elasticity (ε), higher leaf Na⁺ and Pb but lower extractable concentrations of other toxic metals (Cr, Cu, and Zn) in S. persica. Variation in metal accumulation may be linked to metal speciation via specific transporters and leaf water relation dynamics. Our results suggested that S. persica could be grown on Zn, Cr and Cu polluted soils but not on Pb affected soils as its leaves accumulated higher concentrations than the proposed limits.

Diverse responses of halophyte and glycophyte Lepidium species to the salt-mediated amelioration of nickel toxicity and accumulation

Tolerance mechanisms employed by plants under environmental stresses can protect them against other co-occurring stresses. In this study, the effect of pre-exposure and simultaneous salt treatment on nickel (Ni) toxicity tolerance in one halophyte (L. sativum) and one glycophyte (L. latifolium) Lepidium species in hydroponics was investigated. In order to compare the species independent from their salt and Ni tolerance level, the glycophyte was subjected to lower salt and Ni concentrations and for a shorter period of time than the halophyte. Salt (NaCl) was applied at 50 and 100 mM concentrations and Ni was provided at an equal free Ni²⁺ activity by adding 100 and 200 µM Ni as single stresses, but 130 and 300 µM Ni for the treatment of its combination with salt in the glycophyte and halophyte, respectively. Temporal analyses of signaling molecules revealed that the halophyte is characteristically different from the glycophyte in that it exhibits a higher constitutive level of nitric oxide and hydrogen peroxide, a longer duration of response to Ni, and its augmentation by salt. In addition to higher biomass and less Ni accumulation in salt-treated plants, the concentrations of free thiol groups, leaf pigments, proline, free and cell wall-bound phenolics contents, and the activity of phenolic metabolizing enzymes were higher in L. latifolium under the combined salt and Ni treatments than under the single Ni stress. In contrast, the biomass and most biochemical parameters of Ni-stressed L. sativum plants were not enhanced by salt treatment but rather decreased. Our findings shed light on cross-tolerance mechanisms in halophytes and uncovered halophyte survival strategies under multiple stresses.

Salinity mitigates cadmium-induced phytotoxicity in quinoa (Chenopodium quinoa Willd.) by limiting the Cd uptake and improved responses to oxidative stress: implications for phytoremediation

Cadmium (Cd) contamination and soil salinity are the main environmental issues reducing crop productivity. This study aimed to examine the combined effects of salinity (NaCl) and Cd on the physiological and biochemical attributes of quinoa (Chenopodium quinoa Willd.). For this purpose, 30-day-old plants of quinoa genotype "Puno" were transplanted in Hoagland's nutrient solution containing diverse concentrations of Cd: 0, 50, 100, 200 µM Cd, and salinity: 0, 150, and 300 mM NaCl. Results demonstrated that plant growth, stomatal conductance, and pigment contents were significantly lower at all Cd concentrations than the control plants. Quinoa plants exhibited improved growth and tolerance against Cd when grown at a lower level of salinity (150 mM NaCl) combined with Cd. In contrast, the elevated concentration of salinity (300 mM NaCl) combined with Cd reduced shoot and root growth of experimental plants more than 50%. Combined application of salinity and Cd increased Na (25-fold), while lessened the Cd (twofold) and K (1.5-fold) uptake. A blend of high concentrations of Na and Cd caused overproduction of H₂O₂ (eightfold higher than control) contents and triggered lipid peroxidation. The activities of antioxidant enzymes: ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) were 13, 12, 7 and ninefold higher than control to mitigate the oxidative stress. Due to restricted root to shoot translocation, and greater tolerance potential against Cd, the quinoa genotype, Puno, is suitable for phytostabilization of Cd in saline soils.

Transcriptome analysis reveals molecular mechanisms underlying salt tolerance in halophyte Sesuvium portulacastrum

Soil salinity is an important environmental problem that seriously affects plant growth and crop productivity. Phytoremediation is a cost-effective solution for reducing soil salinity and potentially converting the soils for crop production. Sesuvium portulacastrum is a typical halophyte which can grow at high salt concentrations. In order to explore the salt tolerance mechanism of S. portulacastrum, rooted cuttings were grown in a hydroponic culture containing ½ Hoagland solution with or without addition of 400 mM Na for 21 days. Root and leaf samples were taken 1 h and 21 days after Na treatment, and RNA-Seq was used to analyze transcript differences in roots and leaves of the Na-treated and control plants. A large number of differentially expressed genes (DEGs) were identified in the roots and leaves of plants grown under salt stress. Several key pathways related to salt tolerance were identified through KEGG analysis. Combined with physiological data and expression analysis, it appeared that cyclic nucleotide gated channels (CNGCs) were implicated in Na uptake and Na⁺/H⁺ exchangers (NHXs) were responsible for the extrusion and sequestration of Na, which facilitated a balance between Na⁺ and K⁺ in S. portulacastrum under salt stress. Soluble sugar and proline were identified as important osmoprotectant in salt-stressed S. portulacastrum plants. Glutathione metabolism played an important role in scavenging reactive oxygen species. Results from this study show that S. portulacastrum as a halophytic species possesses a suite of mechanisms for accumulating and tolerating a high level of Na; thus, it could be a valuable plant species used for phytoremediation of saline soils.

Effect of Fungal Endophyte Epichloë bromicola Infection on Cd Tolerance in Wild Barley (Hordeum brevisubulatum)

Hydroponic Hordeum brevisubulatum (wild barley) was used as material in the greenhouse to study the effects of endophyte infection on plant growth, Cd absorption and transport, subcellular distribution, and Cd chemical forms under CdCl2 stress. Endophytic fungi respond positively to chlorophyll content and photosynthetic efficiency under Cd stress. The order of Cd absorption in different parts of the plant was: roots > stems > leaves. Endophyte infection increased the plant’s absorption and transport of Cd while causing a significant difference in the stem, which was associated with the distribution density of endophyte hyphae. The proportion of organelle Cd in endophyte-infected wild barley was significantly higher, which facilitated more Cd transport to aboveground. Cd stress showed a slight effect on the chemical forms of Cd in leaves. The proportion of phosphate, oxalate, and residual Cd increased in the stem. Cd existed in the form of inorganic salt, organic acid, pectin, and protein in roots. Endophyte infection reduced the Cd content of the more toxic chemical forms to protect the normal progress of plant physiological functions. Therefore, the isolation of cell walls and vacuoles is a key mechanism for plant Cd tolerance and detoxification. As endophyte infections have more ability to absorb Cd in plants, H. brevisubulatum−Epichloë bromicola symbionts can improve heavy metal contaminated soil and water.

Kosteletzkya pentacarpos: A Potential Halophyte Candidate for Phytoremediation in the Meta(loid)s Polluted Saline Soils

Kosteletzkya pentacarpos (L.) Ledebour is a perennial facultative halophyte species from the Malvacea family that grows in coastal areas with high amounts of salt. The tolerance of K. pentacarpos to the high concentration of salt (0.5–1.5% salinity range of coastal saline land) has been widely studied for decades. Nowadays, with the dramatic development of the economy and urbanization, in addition to the salt, the accumulation of mate(loid)s in coastal soil is increasing, which is threatening the survival of halophyte species as well as the balance of wetland ecosystems. Recently, the capacity of K. pentacarpos to cope with either single heavy metal stress or a combination of multiple meta(loid) toxicities was studied. Hence, this review focused on summarizing the physiological and biochemical behaviors of K. pentacarpos that has been simultaneously exposed to the combination of several meta(loid) toxicities. How the salt accumulated by K. pentacarpos impacts the response to meta(loid) stress was discussed. We conclude that as a potential candidate for phytoremediation, K. pentacarpos was able to cope with various environmental constrains such as multiple meta(loid) stresses due to its relative tolerance to meta(loid) toxicity.

Endogenous Taurine Downregulation Is Required for Renal Injury in Salt-Sensitive Hypertensive Rats via CBS/H2S Inhibition

Although taurine is known to exert an antihypertensive effect, it is unclear whether it is involved in the mechanism for hypertension-related target organ injury. To reveal the role of endogenous taurine in renal injury formation during salt-sensitive hypertension and clarify its mechanisms, both salt-sensitive Dahl rats and salt-resistant SS-13BN rats were fed a high-salt diet (8% NaCl) and given 2% taurine for 6 weeks. Rat systolic blood pressure (SBP) was measured by the tail-cuff method and artery catheterization. Kidney ultrastructure was observed under an electron microscope. Taurine content and mRNA and protein levels of taurine synthases, cysteine dioxygenase type 1 (CDO1) and cysteine sulfinic acid decarboxylase (CSAD), were decreased in Dahl rats fed a high-salt diet. However, taurine supplementation and the resulting increase in renal taurine content reduced the increased SBP and improved renal function and structural damage in high-salt diet-fed Dahl rats. In contrast, taurine did not affect SS-13BN SBP and renal function and structure. Taurine intervention increased the renal H₂S content and enhanced cystathionine-β-synthase (CBS) expression and activity in Dahl rats fed a high-salt diet. Taurine reduced the renin, angiotensin II, and aldosterone contents and the levels of oxidative stress indices in Dahl rat renal tissues but increased antioxidant capacity, antioxidant enzyme activity, and protein expression. However, taurine failed to achieve this effect in the renal tissue of SS-13BN rats fed a high-salt diet. Pretreatment with the CBS inhibitor HA or renal CBS knockdown inhibited H₂S generation and subsequently blocked the effect of taurine on renin, superoxide dismutase 1 (SOD1), and superoxide dismutase 2 (SOD2) levels in high-salt-stimulated Dahl renal slices. In conclusion, the downregulation of endogenous taurine production resulted in a decrease in the renal CBS/H₂S pathway. This decrease subsequently promoted renin-angiotensin-aldosterone system (RAAS) activation and oxidative stress in the kidney, ultimately contributing to renal injury in salt-sensitive Dahl rats.

Salinity alleviates the arsenic toxicity in the facultative halophyte Salvadora persica L. by the modulations of physiological, biochemical, and ROS scavenging attributes

Heavy metal(loid)s contamination in soil is a major environmental concern that limits agricultural yield and threatens human health worldwide. Arsenic (As) is the most toxic non-essential metalloid found in soil which comes from various natural as well as human activities. S. persica is a facultative halophyte found abundantly in dry, semiarid and saline areas. In the present study, growth, mineral nutrient homeostasis, MDA content, phytochelatin levels, and ROS-scavenging attributes were examined in S. persica imposed to solitary treatments of salinity (250 mM and 750 mM NaCl), solitary treatments of arsenic (200 μM and 600 μM As), and combined treatments of As with 250 mM NaCl with an aim to elucidate salinity and As tolerance mechanisms. The results demonstrated that S. persica plants sustained under high levels of As (600 μM As) as well as NaCl (750 mM). The activity of superoxide dismutase, catalase, peroxidase, and glutathione reductase were either elevated or unaffected under salt or As stress. However, ascorbate peroxidase activity declined under both solitary and combination of As with NaCl. Furthermore, the cellular redox status measured in terms of reduced ascorbate/dehydroascorbate, and reduced glutathione/oxidized glutathione ratios also either increased or remained unaffected in seedlings treated with both solitary and combined treatments of As + NaCl. Significant accumulation of various oxidative stress indicators (H₂O₂ and O₂^-) were observed under high As stress condition. However, presence of salt with high As significantly reduced the levels of ROS. Furthermore, exogenous salt improved As tolerance index (Ti) under high As stress condition. The values of translocation factor (Tf) and As bioaccumulation factor (BF) were >1 in all the treatments. From this study, it can be concluded that the facultative halophyte S. persica is a potential As accumulator and may find application for phytoextraction of arsenic-contaminated saline soil.

Cadmium accumulation, subcellular distribution and chemical fractionation in hydroponically grown Sesuvium portulacastrum [Aizoaceae]

Sesuvium portulacastrum is a well-known halophyte with considerable Cd accumulation and tolerance under high Cd stress. This species is also considered as a good candidate of Cd phytoremediation in the polluted soils. However, the mechanism of Cd accumulation, distribution and fractionation in different body parts still remain unknown. Seedlings of Sesuvium portulacastrum were studied hydroponically under exposure to a range of Cd concentrations (50 μM or μmol/L to 600 μM or μmol/L) for 28 days to investigate the potential accumulation capability and tolerance mechanisms of this species. Cd accumulation in roots showed that the bio-concentration factor was > 10, suggesting a strong ability to absorb and accumulate Cd. Cd fractionation in the aboveground parts showed the following order of distribution: soluble fraction > cell wall > organelle > cell membrane. In roots, soluble fraction was mostly predominant than other fractions. Cd speciation in leaves and stems was mainly contained of sodium chloride and deionised water extracted forms, suggesting a strong binding ability with pectin and protein as well as with organic acids. In the roots, inorganic form of Cd was dominant than other forms of Cd. It could be suggested that sodium chloride, deionised water and inorganic contained form of Cd are mainly responsible for the adaption of this plant in the Cd stress environment and alleviating Cd toxicity.

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.

Distinct co-tolerance responses to combined salinity and cadmium exposure in metallicolous and non-metallicolous ecotypes of Silene vulgaris

This study compared co-tolerance to salinity and cadmium and investigated its mechanisms in a facultative metallophyte Silene vulgaris originating from distinct habitats. Shoots of calamine (Cal) and non-metallicolous (N-Cal) ecotypes grown in vitro were exposed to 10 and 100 mM NaCl, 5 μM CdCl₂ and their combinations. Stress effects were evaluated based on growth, oxidative stress parameters, and DNA content and damage. Tolerance mechanisms were assessed by analyzing non-enzymatic antioxidants, osmolytes and ion accumulation. Irrespective of the ecotype, Cd stimulated shoot proliferation (micropropagation coefficients MC = 15.2 and 12.1 for Cal and N-Cal, respectively, growth tolerance index GTI = 148.1 and 156.7%). In Cal ecotype this was attributed to an increase in glutathione content and reorganization of cell membrane structures under Cd exposure, whereas in N-Cal to enhanced synthesis of other non-enzymatic antioxidants, mainly carotenoids and ascorbate. Low salinity stimulated growth of Cal ecotype due to optimizing Cl^- content. High salinity inhibited growth, especially in Cal ecotype, where it enhanced DNA damage and disturbed ionic homeostasis. Species-specific reaction to combined salinity and Cd involved a mutual inhibition of Na⁺, Cl^- and Cd²⁺ uptake. N-Cal ecotype responded to combined stresses by enhancing its antioxidant defense, presumably induced by Cd, whereas the metallicolous ecotype triggered osmotic adjustment. The study revealed that in S. vulgaris Cd application ameliorated metabolic responses to simultaneous salinity exposure. It also shed a light on distinct strategies of coping with combined abiotic stresses in two ecotypes of the species showing high plasticity in environmental conditions.

Salt tolerance and exclusion in the mangrove plant Avicennia marina in relation to root apoplastic barriers

Salt tolerance and the possible functions of suberization on salt exclusion and secretion were examined in a dominant mangrove plant, Avicennia marina. The results showed that low salinities (10‰ and 20‰) almost has no negative effect on A. marina, however significant growth inhibitions were observed in the seedlings grown in higher salinities (30‰ and 40‰). With the increases of salinity, increased tissue Na⁺ content and enhanced salt secretion by glands were observed. Obvious suberization thickening were detected both in the exodermis and endodermis of the roots after salt pretreatment when compared to the roots without salt treatment. More importantly, the present data further confirmed that these root apoplastic barriers would directly decrease Na⁺ loading into xylem. Higher salt tolerance was observed in the seedlings pre-cultivated by salty tide when compared to fresh water cultivated A. marina. In summary, this study suggests a barrier property of suberization in dealing with salt exclusion in mangroves, a moderate salt pre-treatment may benefit plant withstanding high salinity.

Rice intercropping with alligator flag (Thalia dealbata): A novel model to produce safe cereal grains while remediating cadmium contaminated paddy soil

Phytoremediation has been employed as a cost-effective technique to remove the cadmium (Cd) from soil and water in several ecosystems. However, little is known about whether intercropping the remediating plants with rice (Oryza sativa) crop could reduce Cd accumulation in rice grains. We conducted greenhouse pot and concrete pond trials to explore the effects of intercropping alligator flag (Thalia dealbata, Marantaceae) on soil Cd remediation, paddy soil and microbial properties, and rice production. Our results suggest that intercropping with alligator flag significantly decreased Cd absorption, transportation, and accumulation from the soil to the rice grains (under 0.2 mg kg^-1 at a soil Cd content below 2.50 mg kg^-1). This decrease was due to the lowered Cd availability and higher soil pH in the rice-alligator flag intercropping system. Although planting alligator flag resulted in the reduction of soil NH₄-N and NO₃-N, Cd content in the rhizosphere was the main factor restricting microbial biomass, species, and community composition. Alligator flag could tolerate higher Cd contamination, and accumulate and stabilize more Cd in its tissues than rice. Our study suggests that alligator flag intercropped with rice has potential as a phytostabilization plant to produce rice safely for human consumption in moderately Cd-contaminated soils.

Nickel tolerance and toxicity mechanisms in the halophyte Sesuvium portulacastrum L. as revealed by Ni localization and ligand environment studies

Halophytes are able to tolerate relatively high concentrations of hazardous metals in a growing substrate, what makes them suitable candidates for phytoremediation of metal-contaminated soils. In this work, we aimed to study the physiological responses of the halophyte Sesuvium portulacastrum L. to Ni, with main focus on Ni localization, compartmentation and ligand environment, to decipher Ni tolerance and toxicity mechanisms. Seedlings were grown in hydroponic nutrient solution containing 0, 25, 50 and 100 μM Ni as NiCl₂ for 3 weeks. Ni localization in leaves was assessed by micro-proton-induced X-ray emission (micro-PIXE). Ni ligand environment was studied by Ni K-edge X-ray absorption near edge structure (XANES). In addition, Ni-soluble, weakly bound/exchangeable and insoluble leaf tissue fractions were determined by sequential extraction. Results show that S. portulacastrum is able to tolerate up to ~ 500 μg g^-1 dry weight (DW) of Ni in the shoots without significant growth reduction. At higher Ni concentrations (> 50 μM Ni in nutrient solution), chloroses were observed due to the accumulation of Ni in photosynthetically active chlorenchyma as revealed by micro-PIXE. Water storage tissue represented the main pool for Ni storage. Incorporation of Ni into Ca-oxalate crystals was also observed in some specimens, conferring tolerance to high leaf Ni concentrations. The majority of Ni (> 70%) was found in soluble tissue fraction. Ni K XANES revealed Ni bound mainly to O- (55%) and N-ligands (45%). Ni toxicity at higher Ni levels was associated with Ni binding to amino groups of proteins in cytosol of chlorenchyma and increased level of lipid peroxidation. Proline levels also increased at high Ni exposures and were associated with Ni-induced oxidative stress and alteration of water regime.

Comparative assessment of heavy metal accumulation and bio-indication in coastal dune halophytes

This study aimed at assessing heavy metals (Fe, Mn, Zn, Cu, Cr and Pb) in four perennial halophytes (viz. Heliotropium bacciferum, Halopyrum mucronatum, Ipomoea pes-caprae and Salsola imbricata) growing at two sites on the Karachi coast. Site - II, closer to the Industrial area had higher bioavailability as well as translocation factor (TF) for most of the heavy metals and Na⁺ where soil sediments had lower pH (approximately 7.5), higher salinity (EC) and organic matter (OM). Site - I which was far from Industrial area had comparatively higher bio-concentration factor (BCF) and lower TF for metal ions and soil pH of 8.1-9. Metal accumulation in plants was both site and species specific. Extractable concentration of shoot Pb in all tested halophytes was above normal of the threshold values (i.e., >0.3 mg kg^-1) while Mn (<50 mg kg^-1) and Cu (<40 mg kg^-1) were within permissible limits. Salsola imbricata had highest Na⁺ at both sites (site - I = 73; site - II = 98 mg kg^-1) with and 10 mg kg^-1 extractable shoot Pb at site - I. Ipomea pes-caprae also accumulated shoot Pb higher than normal (site - I = 3.3; site - II = 0.8 mg kg^-1) with lowest Na⁺ content. Heliotropium bacciferum had higher extractable Pb (site - I = 10.5; II = 2.75) with >20 mg kg^-1 Na⁺ in shoot while maintaining > 1 TF for Pb, Cu, Mn and Zn at site - I and all tested metals at site - II. Halopyrum mucronatum had highest shoot Fe (644 mg kg^-1), Zn (63 mg kg^-1) and Cr (9.2 mg kg^-1) at site - II and above threshold values of Pb at both sites (site - I = 8.2; site - II = 2.5 mg kg^-1) which makes this species an ideal bio-indicator candidate while other species could be potentially used for Pb phytoremediation.

Salinity influences Cd accumulation and distribution characteristics in two contrasting halophytes, Suaeda glauca and Limonium aureum

The potential for the phytoremediation of halophytes has been widely recognized. However, the effects of salt on Cd accumulation characteristics in different halophytic species, which may also be related to their salt tolerance, are still unclear. This study investigated the effects of salinity on Cd accumulation and distribution in two distinct halophytes, Suaeda glauca (euhalophyte) and Limonium aureum (recretohalophyte). Seedlings of the two species were treated with 0, 3, and 6 mg kg^-1 soil Cd in combination with or without 0.3% NaCl in a pot experiment. The amount of Cd within the rhizosphere and plant tissues, plant biomass, and the subcellular distribution and chemical forms of Cd were examined. Results showed that the addition of NaCl significantly increased Cd bioavailability at high Cd levels due to the rhizosphere acidification effect. Meanwhile, salinity differently impacted plant biomass allocation, and enhanced Cd uptake and translocation in both studied halophytes. Excess Cd was excreted from the leaf surface, possibly by salt glands of L. aureum, with the salinity facilitating this process. Majority of the Cd was found within the cell walls and vacuolar compartments of two species. However, S. glauca plants had higher proportions of inactive Cd (extracted by 2% HAc and 0.6 M HCl) and lower proportions of active Cd (extracted by 80% ethanol and water), as opposed to L. aureum, which would better inform S. glauca's higher Cd accumulation. Based on these results, S. glauca seems more applicable for phytomanagement of Cd-contaminated saline soils due to its higher capacity for Cd enrichment and tolerance amplified by NaCl.

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.

Salinity decreases Cd translocation by altering Cd speciation in the halophytic Cd-accumulator Carpobrotus rossii

BACKGROUND AND AIMS

Salt has been shown to affect Cd translocation and accumulation in plants but the associated mechanisms are unclear. This study examined the effects of salt type and concentration on Cd uptake, translocation and accumulation in Carpobrotus rossii.

METHODS

Plants were grown in nutrient solution with the same Cd concentration or Cd2+ activity in the presence of 25 mm NaNO3, 12.5 mm Na2SO4 or 25 mm NaCl for ≤10 d. Plant growth and Cd uptake were measured and the accumulation of peptides and organic acids, and Cd speciation in plant tissues were analysed.

KEY RESULTS

Salt addition decreased shoot Cd accumulation by >50 % due to decreased root-to-shoot translocation, irrespective of salt type. Synchrotron-based X-ray absorption spectroscopy revealed that, after 10 d, 61-94 % Cd was bound to S-containing ligands (Cd-S) in both roots and shoots, but its speciation was not affected by salt. In contrast, Cd in the xylem sap was present either as free Cd2+ or complexes with carboxyl groups (Cd-OH). When plants were exposed to Cd for ≤24 h, 70 % of the Cd in the roots was present as Cd-OH rather than Cd-S. However, NaCl addition decreased the proportion of Cd-OH in the roots within 24 h by forming Cd-Cl complexes and increasing the proportion of Cd-S. This increase in Cd-S complexes by salt was not due to changes in glutathione and phytochelatin synthesis.

CONCLUSIONS

Salt addition decreased shoot Cd accumulation by decreasing Cd root-to-shoot translocation due to the rapid formation of Cd-S complexes (low mobility) within the root, without changing the concentrations of glutathione and phytochelatins.

NaCl promotes antibiotic resistance by reducing redox states in Vibrio alginolyticus

The development of antibiotic resistance in Vibrio alginolyticus represents a threat to human health and fish farming. Environmental NaCl regulation of bacterial physiology is well documented, but whether the regulation contributes to antibiotic resistance remains unknown. To explore this, we compared minimum inhibitory concentration (MIC) of V. alginolyticus cultured in different media with 0.5%-10% NaCl, and found that the MIC increased as the NaCl concentration increased, especially for aminoglycoside antibiotics. Consistent with this finding, internal NaCl also increased, while intracellular gentamicin level decreased. GC-MS-based metabolomics showed different distributions of pyruvate cycle intermediates among 0.5%, 4% and 10% NaCl. Differential activity of enzymes in the pyruvate cycle and altered expression of Na(+)-NQR led to a reducing redox state, characterized by decreased levels of NADH, proton motive force (PMF) and ATP. Meanwhile, NaCl negatively regulated PMF as a consequence of the reducing redox state. These together are responsible for the decreased intracellular gentamicin level with the increased external level of NaCl. Our study reveals a previously unknown redox state-dependent mechanism regulated by NaCl in V. alginolyticus that impacts antibiotic resistance.

Heavy metal accumulation in Lathyrus sativus growing in contaminated soils and identification of symbiotic resistant bacteria

In this study, two populations of leguminous plants Lathyrus sativus were grown in four soils that were collected from sites differently contaminated by heavy metals. Evaluations included basic soil properties, concentrations of major nutrients and four metals (copper, zinc, lead and cadmium) in these soils. Investigation of Lathyrus sativus response to contamination showed that the increase of heavy metal concentration in soils affected biomass of plant, number of nodules and plant metal uptake. Heavy metal tolerance of 46 isolated bacteria from the root nodules was evaluated and demonstrated that the maximum concentration of Cd, Pb, Cu and Zn tolerated by strains were 0.8, 2.5, 0.2, and 0.5 mM, respectively. Twenty-two isolates were tested for their effects on plant biomass production and nodule formation and showed that only R. leguminosarum nodulated Lathyrus sativus, while some bacteria improved the shoot and root dry biomass. Sequences of their 16S rDNA gene fragments were also obtained and evaluated for tentative identification of the isolates which revealed different bacterial genera represented by Rhizobium sp, Rhizobium leguminosarum, Sinorhizobium meliloti, Pseudomonas sp, Pseudomonas fluorescens, Luteibacter sp, Variovorax sp, Bacillus simplex and Bacillus megaterium. The existence of Pb- and Cd-resistant genes (PbrA and CadA) in these bacteria was determined by PCR, and it showed high homology with PbrA and CadA genes from other bacteria. The tested resistant population was able to accumulate high concentrations of Pb and Cd in all plant parts and, therefore, can be classified as a strong metal accumulator with suitable potential for phytoremediation of Pb and Cd polluted sites. Heavy metal resistant and efficient bacteria isolated from root nodules were chosen with Lathyrus sativus to form symbiotic associations for eventual bioremediation program, which could be tested to remove pollutants from contaminated sites.

Subcellular distribution and chemical forms of Cd in Bougainvillea spectabilis Willd. as an ornamental phytostabilizer: An integrated consideration

The effects of Cd stress on the growth and Cd accumulation of Bougainvillea spectabilis Willd. as an ornamental plant and the related mechanisms were investigated in the study. We studied the impact of Cd on the plant ultrastructure, examined the cellular distribution of Cd, explored the Cd chemical forms and transformation, and determined the organic acid secretion in the plants. The results showed that B. spectabilis could grow well in the Cd treatment groups, and the roots could accumulate high concentration of Cd. The soluble fraction (primarily in the vacuole) as the form of citrate in leaves of B. spectabilis was the major compartment for Cd storage. The citric acid secreted by B. spectabilis played an important role in the detoxification of Cd, as well as the growth of plants and Cd accumulation. As an ornamental plant, B. spectabilis has the potential to be used in the phytostabilization of Cd-contaminated soils and can beautify the environment at the same time.

Sodium chloride decreases cadmium accumulation and changes the response of metabolites to cadmium stress in the halophyte Carpobrotus rossii

BACKGROUND AND AIMS

Salinity affects the bioavailability of cadmium (Cd) in soils and Cd accumulation in plants, but the associated mechanisms remain unclear. This study aimed to assess the metabolic response to NaCl and Cd and the relationship between metabolites and Cd accumulation in the halophyte Carpobrotus rossii, which has potential for Cd phytoextraction.

METHODS

Plants were grown in nutrient solution with 0-400 mm NaCl in the presence of 5 or 15 µm Cd, with varied or constant solution Cd2+ activity. Plant growth and Cd uptake were measured, and the accumulation of peptides, and organic and amino acids in plant tissues were assessed.

KEY RESULTS

The addition of NaCl to Cd-containing solutions improved plant growth along with 70-87 % less shoot Cd accumulation, resulting from decreases in Cd root uptake and root-to-shoot translocation irrespective of Cd2+ activity in solutions. Moreover, Cd exposure increased the concentration of phytochelatins, which correlated positively with Cd concentrations in plants regardless of NaCl addition. In comparison, Cd inhibited the synthesis of organic acids in shoots and roots in the absence of NaCl, but increased it in shoots in the presence of NaCl. While Cd increased the concentrations of amino acids in plant shoots, the effect of NaCl on the synthesis of amino acids was inconsistent.

CONCLUSIONS

Our data provide the first evidence that NaCl decreased Cd shoot accumulation in C. rossii by decreasing Cd root uptake and root-to-shoot translocation even under constant Cd2+ activity. The present study also supports the important role of peptides and organic acids, particular of phytochelatins, in Cd tolerance and accumulation although the changes of those metabolites was not the main reason for the decreased Cd accumulation.

Coping With Metal Toxicity - Cues From Halophytes

Being the native flora of saline soil, halophytes are well studied for their salt tolerance and adaptation mechanism at the physiological, biochemical, molecular and metabolomic levels. However, these saline habitats are getting contaminated due to various anthropogenic activities like urban waste, agricultural runoff, mining, industrial waste that are rich in toxic metals and metalloids. These toxic metals impose detrimental effects on growth and development of most plant species. Halophytes by virtue of their tolerance to salinity also show high tolerance to heavy metals which is attributed to the enhanced root to shoot metal translocation and bioavailability. Halophytes rapidly uptake toxic ions from the root and transport them toward aerial parts by using different transporters which are involved in metal tolerance and homeostasis. A number of defense related physiological and biochemical strategies are known to be crucial for metal detoxification in halophytes however; there is paucity of information on the molecular regulators. Understanding of the phenomenon of cross-tolerance of salinity with other abiotic stresses in halophytes could very well boost their potential use in phytoremediation. In this article, we present an overview of heavy metal tolerance in case of halophytes, associated mechanisms and cross-tolerance of salinity with other abiotic stresses.

Tolerance and bioaccumulation of Cd and Cu in Sesuvium portulacastrum

In order to investigate the tolerance and bioaccumulation of Cd and Cu in the halophyte Sesuvium portulacastrum, seedlings were hydroponically cultured for 30 days using the modified 1/2 Hoagland nutrient solution with different concentrations of Cd (0, 5, 10, 15, and 20mgL^-1) and Cu (0, 2.5, 5, 7.5, and 10mgL^-1). Afterwards, the seedling height, leaf area, biomass, and mineral element contents (Fe, Mg, Cu, and Zn) in the roots, stems and leaves were measured, and the tolerance index, bioconcentration factor (BCF), transportation index, and removal rate were calculated. The effects of salinity (0‰-30‰) on the growth and bioaccumulation ability of S. portulacastrum under combined Cu/Cd (5mgL^-1) exposure were also determined. The results showed that, with an increasing Cd concentration, the biomass and seedling height of S. portulacastrum initially increased and then decreased. The highest leaf biomass and seedlings height was observed in the 10mgL^-1 and 5mgL^-1 Cd treatment group, respectively. Salinity did not affect the biomass of S. portulacastrum but decreased Cd concentration in roots and aboveground tissues and Cu concentration in roots of S. portulacastrum. Cu treatment significantly facilitated the absorption of Mg, Cu, and Zn in roots. With an increasing Cu concentration, the Mg and Fe contents increased in the leaves of S. portulacastrum. In comparison to the above-ground portions, the root showed a higher bioaccumulation ability of Cd and Cu, with the BCF of 341.5 and 211.9, respectively. The BCF and translocation factor (TF) values indicated that S. portulacastrum was not a hyperaccumulator for Cd and Cu, but could be used as a phytostablization plant in heavy metal contaminated coastal environments.

Organic matter and salinity modify cadmium soil (phyto)availability

Although Cd availability depends on its total concentration in soil, it is ultimately defined by the processes which control its mobility, transformations and soil solution speciation. Cd mobility between different soil fractions can be significantly affected by certain pedovariables such as soil organic matter (SOM; over formation of metal-organic complexes) and/or soil salinity (over formation of metal-inorganic complexes). Phytoavailable Cd fraction may be described as the proportion of the available Cd in soil which is actually accessible by roots and available for plant uptake. Therefore, in a greenhouse pot experiment Cd availability was observed in the rhizosphere of faba bean exposed to different levels of SOM, NaCl salinity (50 and 100mM) and Cd contamination (5 and 10mgkg^-1). Cd availability in soil does not linearly follow its total concentration. Still, increasing soil Cd concentration may lead to increased Cd phytoavailability if the proportion of Cd²⁺ pool in soil solution is enhanced. Reduced Cd (phyto)availability by raised SOM was found, along with increased proportion of Cd-DOC complexes in soil solution. Data suggest decreased Cd soil (phyto)availability with the application of salts. NaCl salinity affected Cd speciation in soil solution by promoting the formation of CdCl_n^2-n complexes. Results possibly suggest that increased Cd mobility in soil does not result in its increased availability if soil adsorption capacity for Cd has not been exceeded. Accordingly, chloro-complex possibly operated just as a Cd carrier between different soil fractions and resulted only in transfer between solid phases and not in increased (phyto)availability.

Recent strategies of increasing metal tolerance and phytoremediation potential using genetic transformation of plants

Avoidance and reduction of soil contamination with heavy metals is one of the most serious global challenges. Nowadays, science offers us new opportunities of utilizing plants to extract toxic elements from the soil by means of phytoremediation. Plant abilities to uptake, translocate, and transform heavy metals, as well as to limit their toxicity, may be significantly enhanced via genetic engineering. This paper provides a comprehensive review of recent strategies aimed at the improvement of plant phytoremediation potential using plant transformation and employing current achievements in nuclear and cytoplasmic genome transformation. Strategies for obtaining plants suitable for effective soil clean-up and tolerant to excessive concentrations of heavy metals are critically assessed. Promising directions in genetic manipulations, such as gene silencing and cis- and intragenesis, are also discussed. Moreover, the ways of overcoming disadvantages of phytoremediation using genetic transformation approachare proposed. The knowledge gathered here could be useful for designing new research aimed at biotechnological improvement of phytoremediation efficiency.

Alleviation of heavy metals toxicity by the application of plant growth promoting rhizobacteria and effects on wheat grown in saline sodic field

The aim of the study was to determine tolerance of plant growth promoting rhizobacteria (PGPR) in different concentrations of Cu, Cr, Co, Cd, Ni, Mn, and Pb and to evaluate the PGPR-modulated bioavailability of different heavy metals in the rhizosphere soil and wheat tissues, grown in saline sodic soil. Bacillus cereus and Pseudomonas moraviensis were isolated from Cenchrus ciliaris L. growing in the Khewra salt range. Seven-day-old cultures of PGPR were applied on wheat as single inoculum, co-inoculation and carrier-based biofertilizer (using maize straw and sugarcane husk as carrier). At 100 ppm of Cr and Cu, the survival rates of rhizobacteria were decreased by 40%. Single inoculation of PGPR decreased 50% of Co, Ni, Cr and Mn concentrations in the rhizosphere soil. Co-inoculation of PGPR and biofertilizer treatment further augmented the decreases by 15% in Co, Ni, Cr and Mn over single inoculation except Pb and Co where decreases were 40% and 77%, respectively. The maximum decrease in biological concentration factor (BCF) was observed for Cd, Co, Cr, and Mn. P. moraviensis inoculation decreases the biological accumulation coefficient (BAC) as well as translocation factor (TF) for Cd, Cr, Cu Mn, and Ni. The PGPR inoculation minimized the deleterious effects of heavy metals, and the addition of carriers further assisted the PGPR.

Cadmium hampers salt tolerance of Sesuvium portulacastrum

It is well known that salinity reduces cadmium toxicity in halophytes. However, the possible interference of Cd with the mechanisms of salt tolerance is poorly explored. The aim of this study was to see whether Cd affects salt tolerance mechanisms in the halophyte Sesuvium portulacastrum. S. portulacastrum plants obtained from cuttings were grown in hydroponics for 3 weeks and then exposed to low (0.09 mM) or moderate (200 mM) NaCl concentrations, alone or in combination with 25 μM CdCl₂. Microscopy observation revealed two strategies of salt tolerance: euhalophytism and secretion of salt by bladder cells. Cadmium exposure hardly influenced the total leaf Na⁺ concentrations. However, Cd supply delayed the salt-induced upregulation of AHA1 (plasma membrane H⁺-ATPase 1) and SOS1 (plasma membrane Na⁺ transporter "Salt Overly Sensitive 1"), genes that are essential for salt tolerance. Moreover, Cd induced the activation of BADH, coding for betaine aldehyde dehydrogenase, indicating enhanced osmotic stress due to Cd. Sodium-green fluorescence in protoplasts from plants grown with low or high NaCl, alone or in combination with Cd, revealed higher Na⁺ concentrations in the cytoplasm of Cd-exposed plants. Taken together the results indicate interference of Cd with salt tolerance mechanisms in S. portulacastrum. This may have consequences for the efficient use of halophytes in phytoremediation of Cd-contaminated saline soils.

Impact of osmoregulation on the differences in Cd accumulation between two contrasting edible amaranth cultivars grown on Cd-polluted saline soils

This study aimed to investigate the difference of osmoregulation between two edible amaranth cultivars, Liuye (high Cd accumulator) and Quanhong (low Cd accumulator), under salinity stress and determine the effects of such difference on Cd accumulation. A pot experiment was conducted to expose the plants to sewage-irrigated garden soil (mean 2.28 mg kg^-1 Cd) pretreated at three salinity levels. Under salinity stress, the concentrations of Cd in the two cultivars were significantly elevated compared with those in the controls, and the Cd concentration in Liuye was statistically higher than that in Quanhong (p < 0.05). Salinity-induced osmoregulation triggered different biogeochemical processes involved in Cd mobilization in the rhizosphere soil, Cd absorption, and translocation by the two cultivars. Rhizosphere acidification induced by an imbalance of cation over anion uptake was more serious in Liuye than in Quanhong, which obviously increased soil Cd bioavailability. Salinity-induced injuries in the cell wall pectin and membrane structure were worse in Liuye than in Quanhong, increasing the risk of Cd entering the protoplasts. The chelation of more cytoplasmic Cd²⁺ with Cl^- ions in the roots of Liuye promoted Cd translocation into the shoots. Furthermore, the less organic solutes in the root sap of Liuye than in that of Quanhong also favored Cd translocation into the shoots. Hence, osmoregulation processes can be regarded as important factors in reducing Cd accumulation in crop cultivars grown on saline soils.

Nickel tolerance, accumulation and subcellular distribution in the halophytes Sesuvium portulacastrum and Cakile maritima

It has been shown that halophytes are able to successfully cope with heavy metal toxicity, suggesting their possible use for remediation of metal contaminated soils. In this work, Ni tolerance and accumulation in two halophytes, Sesuvium portulacastrum (L.) L. and Cakile maritima Scop. was investigated. Seedlings of both species were subjected hydroponically during 21 days to 0, 25, 50, and 100 μM of NiCl₂. The growth and photosynthesis parameters revealed that S. portulacastrum tolerates Ni better than C. maritima. The photosynthesis activity, chlorophyll content and photosystem II integrity were less impacted in Ni-treated S. portulacastrum as compared to C. maritima, although, Ni accumulated in higher concentrations in the shoots of S. portulacastrum (1050 μg g^-1 DW) than in those of C. maritima (550 μg g^-1 DW). The subcellular fractionation of Ni in the shoots of both species showed that C. maritima accumulated about 65% of Ni in the soluble fraction, while 28% was associated with the cell walls. In S. portulacastrum 44% of the total cellular Ni was seen in the soluble fraction and 43% was bound to the cell walls. It can be concluded that S. portulacastrum tolerates Ni better than C. maritima, most probably due to a better ability to sequester Ni in the cell walls, restricting its accumulation in the soluble fraction.

Implication of citrate, malate and histidine in the accumulation and transport of nickel in Mesembryanthemum crystallinum and Brassica juncea

Citrate, malate and histidine have been involved in many processes including metal tolerance and accumulation in plants. These molecules have been frequently reported to be the potential nickel chelators, which most likely facilitate metal transport through xylem. In this context, we assess here, the relationship between organics acids and histidine content and nickel accumulation in Mesembryanthemum crystallinum and Brassica juncea grown in hydroponic media added with 25, 50 and 100 µM NiCl2. Results showed that M. crystallinum is relatively more tolerant to Ni toxicity than B. juncea. For both species, xylem transport rate of Ni increased with increasing Ni supply. A positive correlation was established between nickel and citrate concentrations in the xylem sap. In the shoot of B. juncea, citric and malic acids concentrations were significantly higher than in the shoot of M. crystallinum. Also, the shoots and roots of B. juncea accumulated much more histidine. In contrast, a higher root citrate concentration was observed in M. crystallinum. These findings suggest a specific involvement of malic and citric acid in Ni translocation and accumulation in M. crystallinum and B. juncea. The high citrate and histidine accumulation especially at 100µM NiCl2, in the roots of M. crystallinum might be among the important factors associated with the tolerance of this halophyte to toxic Ni levels.

Isobolographic analysis of the interaction between cadmium (II) and sodium sulphate: toxicological consequences

Sulphate is an essential nutrient for autotrophic organisms and has been shown to have important implications in certain processes of tolerance to cadmium toxicity. Sodium sulphate is the main salt of sulphate in the natural environments. The concentration of this salt is increasing in the aquatic environments due to environmental pollution. The aim of this study was to investigate, using an analysis of isobolograms, the type and the degree of the interaction between Cd(II) and sodium sulphate in the freshwater microalga Chlamydomonas moewusii. Two blocks of experiments were performed, one at sub-optimal sodium sulphate concentrations (<14.2 mg/L) and the other at supra-optimal concentrations (>14.2 mg/L). Three fixed ratios (2:1, 1:1, and 1:2) of the individual EC50 for cadmium and sodium sulphate were used within each block. The isobolographic analysis of interaction at sub-optimal concentrations showed a stronger antagonistic effect with values of interaction index (γ) between 1.46 and 3.4. However, the isobologram with sodium sulphate at supra-optimal concentrations revealed a slight but significant synergistic effect between both chemicals with an interaction index between 0.54 and 0.64. This synergic effect resulted in the potentiation of the toxic effects of cadmium, synergy that was related to the increase of the ionic strength and of two species of cadmium, CdSO4 (aq), and Cd(SO4)2(2-) , in the medium. Results of the current study suggest that sodium sulphate is able to perform a dual antagonist/synergist effect on cadmium toxicity. This role was concentration dependent.

How can we take advantage of halophyte properties to cope with heavy metal toxicity in salt-affected areas?

BACKGROUND

Many areas throughout the world are simultaneously contaminated by high concentrations of soluble salts and by high concentrations of heavy metals that constitute a serious threat to human health. The use of plants to extract or stabilize pollutants is an interesting alternative to classical expensive decontamination procedures. However, suitable plant species still need to be identified for reclamation of substrates presenting a high electrical conductivity.

SCOPE

Halophytic plant species are able to cope with several abiotic constraints occurring simultaneously in their natural environment. This review considers their putative interest for remediation of polluted soil in relation to their ability to sequester absorbed toxic ions in trichomes or vacuoles, to perform efficient osmotic adjustment and to limit the deleterious impact of oxidative stress. These physiological adaptations are considered in relation to the impact of salt on heavy metal bioavailabilty in two types of ecosystem: (1) salt marshes and mangroves, and (2) mine tailings in semi-arid areas.

CONCLUSIONS

Numerous halophytes exhibit a high level of heavy metal accumulation and external NaCl may directly influence heavy metal speciation and absorption rate. Maintenance of biomass production and plant water status makes some halophytes promising candidates for further management of heavy-metal-polluted areas in both saline and non-saline environments.

Down-regulated CBS/H2S pathway is involved in high-salt-induced hypertension in Dahl rats

BACKGROUND

The study was designed to explore the significance of endogenous H2S in the development of high-salt-induced hypertension in rats.

METHODS

High-salt-induced hypertension rat model was made by feeding Dahl rat high-salt diet containing 8% NaCl for 8 weeks with SD rats as control. SBP and aorta structure in rats were observed. Endogenous H2S content and expression of cystathionine β-lyase (CBS), cystathionine γ-lyase and mercaptopyruvate sulfurtransferase in renal tissues were detected. Mechanisms for the impact of high-salt on CBS/H2S in renal tissues were studied, targeting HIF-1α pathway. The effect of H2S on RAS in serum and renal tissue of rats were tested.

RESULTS

High-salt reduced endogenous H2S content and inhibited the expression of CBS in renal tissue in salt-sensitive Dahl rats. H2S donor, however, inhibited salt-sensitive hypertension, reversed aortic structural remodeling and inhibited activation of the RAS system in renal tissues in Dahl rats. Expression of HIF-1α was decreased but expression of PHD2 was increased in renal tissue of Dahl rats with high-salt diet, whereas they did not alter in renal tissue of SD rats with high-salt diet. Ex vivo experiment showed that inhibitor of HIF-1α degradation could rescue down-regulated CBS/H2S pathway in renal tissue of Dahl rats with high-salt. In contrast, inhibitor of HIF-1α activity decreased the CBS/H2S pathway in the renal tissue of SD rats treated with high-salt.

CONCLUSIONS

Down-regulated CBS/H2S pathway in renal tissues under high-salt insult might be an important pathogenesis of salt-sensitive hypertension.

Nodulation by Sinorhizobium meliloti originated from a mining soil alleviates Cd toxicity and increases Cd-phytoextraction in Medicago sativa L

Besides their role in nitrogen supply to the host plants as a result of symbiotic N fixation, the association between legumes and Rhizobium could be useful for the rehabilitation of metal-contaminated soils by phytoextraction. A major limitation presents the metal-sensitivity of the bacterial strains. The aim of this work was to explore the usefulness of Sinorhizobium meliloti originated from a mining site for Cd phytoextraction by Medicago sativa. Inoculated and non-inoculated plants were cultivated for 60 d on soils containing 50 and/or 100 mg Cd kg(-1) soil. The inoculation hindered the occurrence of Cd- induced toxicity symptoms that appeared in the shoots of non-inoculated plants. This positive effect of S. meliloti colonization was accompanied by an increase in biomass production and improved nutrient acquisition comparatively to non-inoculated plants. Nodulation enhanced Cd absorption by the roots and Cd translocation to the shoots. The increase of plant biomass concomitantly with the increase of Cd shoot concentration in inoculated plants led to higher potential of Cd-phytoextraction in these plants. In the presence of 50 mg Cd kg(-1) in the soil, the amounts of Cd extracted in the shoots were 58 and 178 μg plant(-1) in non-inoculated and inoculated plants, respectively. This study demonstrates that this association M. sativa-S. meliloti may be an efficient biological system to extract Cd from contaminated soils.