Phytochelatins play a key role in arsenic accumulation and tolerance in the aquatic macrophyte Wolffia globosa

The Effects and Mechanisms of pH and Dissolved Oxygen Conditions on the Release of Arsenic at the Sediment-Water Interface in Taihu Lake

The pH and dissolved oxygen (DO) conditions are important environmental factors that control the migration of arsenic (As) at the sediment-water interface. This study investigates the distribution differences of reactive iron, manganese, and arsenic at the sediment-water interface under anaerobic and aerobic conditions at different pH levels. The strong buffering capacity of sediment to water pH results in a shift towards neutral pH values in the overlying water under different initial pH conditions. The level of DO becomes a key factor in the release of As from sediment, with lower DO environments exhibiting higher release quantities and rates of As compared to high DO environments. Under low DO conditions, the combined effects of ion exchange and anaerobic reduction lead to the most significant release of As, particularly under pH 9.5 conditions. The formation of amorphous ferrous sulfide compounds under low DO conditions is a significant factor contributing to increased arsenic concentration in the interstitial water. Therefore, the re-migration of endogenous arsenic in shallow lake sediments should consider the combined effects of multiple driving forces.

Duckweeds for Phytoremediation of Polluted Water

Tiny aquatic plants from the Lemnaceae family, commonly known as duckweeds, are often regarded as detrimental to the environment because of their ability to quickly populate and cover the surfaces of bodies of water. Due to their rapid vegetative propagation, duckweeds have one of the fastest growth rates among flowering plants and can accumulate large amounts of biomass in relatively short time periods. Due to the high yield of valuable biomass and ease of harvest, duckweeds can be used as feedstock for biofuels, animal feed, and other applications. Thanks to their efficient absorption of nitrogen- and phosphate-containing pollutants, duckweeds play an important role in the restorative ecology of water reservoirs. Moreover, compared to other species, duckweed species and ecotypes demonstrate exceptionally high adaptivity to a variety of environmental factors; indeed, duckweeds remove and convert many contaminants, such as nitrogen, into plant biomass. The global distribution of duckweeds and their tolerance of ammonia, heavy metals, other pollutants, and stresses are the major factors highlighting their potential for use in purifying agricultural, municipal, and some industrial wastewater. In summary, duckweeds are a powerful tool for bioremediation that can reduce anthropogenic pollution in aquatic ecosystems and prevent water eutrophication in a simple, inexpensive ecologically friendly way. Here we review the potential for using duckweeds in phytoremediation of several major water pollutants: mineral nitrogen and phosphorus, various organic chemicals, and heavy metals.

Are cysteine, glutathione and phytochelatins responses of Myriophyllum alterniflorum to copper and arsenic stress affected by trophic conditions?

The aim of this article is to study the impact of both copper (Cu²⁺) and arsenic (As (V)) at 100 µg/L, with each element being combined with trophic conditions at the level of glutathione, cysteine and phytochelatins in the aquatic macrophyte Myriophyllum alterniflorum, whose potential for bioindication and phytoremediation of metal/metalloid pollution has already been demonstrated. To achieve this goal, a synthetic medium, of a composition similar to the water found in the Vienne River in France's Limousin Region and modified for eutrophic or oligotrophic conditions, is prepared. The analysis of cysteine, glutathione and phytochelatins is performed at 0, 3, 7, 14 and 21 days. Our results indicate that the eutrophic medium without contaminant only induces a significant increase in the glutathione level when compared to the oligotrophic medium. However, the joint presence of As (V) and Cu is able to increase the synthesis of cysteine, glutathione and phytochelatins (PC2 and PC3) under both eutrophic and oligotrophic conditions, with a significant increase in the eutrophic medium compared to the oligotrophic one. Phytochelatins (PC2 and PC3) are induced after as little as 3 days of exposure to copper and arsenic under both trophic conditions. Copper induces the synthesis of more PC3 than PC2, unlike arsenic. Our results confirm the potential use of phytochelatins as a specific biochemical biomarker for metal/metalloid stress. In conclusion, the eutrophic condition combined with copper or arsenic does change the response of Myriophyllum alterniflorum by enhancing its antioxidative defense. Thus, M. alterniflorum phytochelatins represent a potential dedicated biomarker to monitor water quality in terms of metal/metalloid stress regardless of the trophic level.

Critical Evaluation of Specific Efficacy of Preparations Produced According to European Pharmacopeia Monograph 2371

European Pharmacopoeia monograph 2371 describes the production of homeopathic preparations. A specific efficacy of these preparations in high dilution levels is questionable in view of basic scientific principles. There is empirical evidence for such effects, for example in a Lemna-intoxication bioassay published 2010. To test the replicability and robustness of this bioassay, we conducted two experimental series (five independent blinded and randomised experiments each). The specimen of Lemna gibba L., clone-number 9352, were stressed in arsenic solution for 48 h (158 mg/L AsNa2HO4 (250 mg/L in series 2)), then grew in either As2O3 preparations produced according to Eu. Pharm. Monogr. 2371 or control solution. Comparing the area-related relative growth rate of day 3−9 (rgr 3−9) between treatment and control groups for each series showed differences that were not significant in series 1 (p = 0.10), significant in series 2 (p = 0.04) and significant in the pooled data of both series (p < 0.01). The effect direction (rgr 3−9 increase) was comparable to experiments of 2010, but the effect size was smaller, likely due to a changed light cycle. These results are not compatible with the hypothesis that the application of European Pharmacopoeia monograph 2371 results in pharmaceutical preparations without specific effects. Further studies are needed to investigate a potential mode of action explaining these effects.

Frond architecture of the rootless duckweed Wolffia globosa

BACKGROUND

The plant body in duckweed species has undergone reduction and simplification from the ancient Spirodela species towards more derived Wolffia species. Among the five duckweed genera, Wolffia members are rootless and represent the smallest and most reduced species. A better understanding of Wolffia frond architecture is necessary to fully explore duckweed evolution.

RESULTS

We conducted a comprehensive study of the morphology and anatomy of Wolffia globosa, the only Wolffia species in China. We first used X-ray microtomography imaging to reveal the three-dimensional and internal structure of the W. globosa frond. This showed that new fronds rapidly budded from the hollow reproductive pocket of the mother fronds and that several generations at various developmental stages could coexist in a single W. globosa frond. Using light microscopy, we observed that the meristem area of the W. globosa frond was located at the base of the reproductive pocket and composed of undifferentiated cells that continued to produce new buds. A single epidermal layer surrounded the W. globosa frond, and the mesophyll cells varied from small and dense palisade-like parenchyma cells to large, vacuolated cells from the ventral to the dorsal part. Furthermore, W. globosa fronds contained all the same organelles as other angiosperms; the most prominent organelles were chloroplasts with abundant starch grains.

CONCLUSIONS

Our study revealed that the reproductive strategy of W. globosa plants enables the rapid accumulation of biomass and the wide distribution of this species in various habitats. The reduced body plan and size of Wolffia are consistent with our observation that relatively few cell types are present in these plants. We also propose that W. globosa plants are not only suitable for the study of structural reduction in higher plants, but also an ideal system to explore fundamental developmental processes of higher plants that cannot be addressed using other model plants.

Re-investigation of cadmium accumulation in Mirabilis jalapa L.: evidences from field and laboratory

Mirabilis jalapa L. was identified as a cadmium (Cd) hyperaccumulator, but data were mainly from laboratory conditions. The main aim of the present study was to confirm whether M. jalapa is a Cd hyperaccumulator by field survey and laboratory experiment. The field survey was conducted at 3 sites and 66 samples were collected, and the results showed that although M. jalapa did not exhibit any visible damage when growing on soil containing 139 mg Cd kg^-1, a low concentration of Cd (11.85 ± 3.45 mg kg^-1) in its leaves was observed. Although the translocation factor (TF) was up to 3.24 ± 0.42, the bioconcentration factor (BCF) was only 0.13 ± 0.07. The Cd accumulation in leaves of Lanping (LP, contaminated site) and Kunming (KM, clean site) populations reached 93.88 and 81.76 mg kg^-1 when artificially spiked soil Cd was 175 mg kg^-1, respectively. The BCFs of LP and KM populations were 0.55 and 0.48, and the TFs of the two populations were 3.98 and 4.15, respectively. Under hydroponic condition, the Cd concentration in young leaves of LP and KM populations was 78.5 ± 0.8 and 46.3 ± 1.2 mg kg^-1 at 5 mg L^-1 Cd treatment, respectively. Furthermore, a significantly positive correlation between tissue Cd concentration and total Cd, CaCl₂-extractable Cd, and TCLP-Cd (toxicity characteristic leaching procedure) in soil was established. Therefore, M. jalapa had constitutional characteristics for Cd tolerance and accumulation, but it was not a Cd hyperaccumulator.

Jasmonic acid application triggers detoxification of lead (Pb) toxicity in tomato through the modifications of secondary metabolites and gene expression

Jasmonic acid (JA) is an important phytohormone associated in defense responses against stress. Crop plants experience heavy metal toxicity and needs to be explored to enhance the crop production. Lead (Pb) is one of the dangerous heavy metal that pollutes soil and water bodies and is released from various sources like discharge from batteries, automobile exhaust, and paints. The present study was designed to evaluate the role of JA (100 nM) on photosynthetic pigments, secondary metabolites, organic acids, and metal ligation compounds in tomato seedlings under different concentrations of Pb (0.25, 0.50, and 0.75 mM). It was observed that Pb treatment declined pigment content, relative water content, and heavy metal tolerance index. Expression of chlorophyllase was also enhanced in Pb-treated seedlings. Seeds primed with JA lowered the expression of chlorophyllase under Pb stress. JA application enhanced the contents of secondary metabolites (total phenols, polyphenols, flavonoids, and anthocyanin) which were confirmed with enhanced expression of chalcone synthase and phenylalanine ammonia lyase in Pb-exposed seedlings. Treatment of JA further elevated the levels of organic acids and metal chelating compounds under Pb toxicity. JA up-regulated the expression of succinate dehydrogenase and fumarate hydratase in Pb-exposed seedlings. Results revealed that seeds primed with JA reduced Pb toxicity by elevating, the levels of photosynthetic pigments, secondary metabolites, osmolytes, metal ligation compounds, organic acids, and polyamine accumulation in tomato seedlings.

Duckweed biomarkers for identifying toxic water contaminants?

Surface or ground waters can be contaminated with numerous toxic substances. The duckweeds Lemna minor and Lemna gibba are widely used for assaying waterborne toxicity to higher plants in terms of growth inhibition and photosynthetic pigment reduction. These tests cannot, however, in themselves determine the nature of the agents responsible for toxicity. Morphological, developmental, physiological, biochemical, and genetic responses of duckweeds to exposure to toxic water contaminants constitute biomarkers of toxic effect. In principle, the very detection of these biomarkers should enable the contaminants having elicited them (and being responsible for the toxicity) to be identified. However, in practice, this is severely compromised by insufficient specificity of biomarkers for their corresponding toxicants and by the lack of documentation of biomarker/toxin relationships. The present contribution illustrates the difficulties of using known water contaminant-related duckweed biomarkers to identify toxins, and discusses possibilities for achieving this goal.

Effect of arsenate on endogenous levels of cytokinins with different existing forms in two Pteris species

Our previous results showed that content of trans-zeatin (tZ) increases in leaves of heavy metal hyperaccumulators but decreases in non-hyperaccumulators growing in multiple heavy metal polluted soils. However, the relationship between arsenic (As) accumulation and endogenous forms of cytokinins (CTKs) in As hyperaccumulators remains unknown. Here a hydroponic experiment was conducted to compare the CTK forms in the As hyperaccumulator Pteris cretica var. nervosa and non-hyperaccumulator Pteris ensiformis under arsenate stress (0, 2, 5, and 10 mg L^-1). A simple and cost-effective procedure for the determination of CTK forms in plants was established, and a stepwise regression analysis was used to study the relationship among total As contents and different forms of endogenous CTKs in fronds of two plants. The results showed that the optimized chromatographic parameters were Zobax SB-C18 column (5 μm × 4.6 mm × 250 mm), UV detection detector at 269 nm, a flow rate of 0.6 mL min^-1, constant temperature of 45 °C and gradient elution with methanol-acetonitrile-1% acetic acid. Contents of chlorophylls in the fronds of P. ensiformis were significantly decreased with addition of As compared to P. cretica var. nervosa. Furthermore, the total As content in fronds of P. cretica var. nervosa was positively correlated to the contents of N6-(2-isopentenyl) adenine-7-β-D-glucoside (iP7G) and N6-(2-isopentenyl) adenosine (iPR). However, the total As content in fronds of P. ensiformis was negatively correlated to its trans-zeatin riboside (ZR) content. Therefore, iP7G and iPR could positively improve As accumulation by P. cretica var. nervosa.

Different Proteomic Processes Related to the Cultivar-Dependent Cadmium Accumulation of Amaranthus gangeticus

To deal with the Cd contaminant of agricultural soil, pollution-safe cultivar (PSC) is developed to minimize the Cd accumulation risk in crops. The present study aimed to investigate the different proteomic responses related to Cd accumulation in different tissues between two Amaranthus gangeticus cultivars, Pen and Nan. A significantly higher Cd accumulation in Pen than in Nan was unraveled, especially in shoot. The proportions of soluble Cd in root and stem of Nan were significantly lower than those of Pen, implying lower Cd transportation from root to shoot in Nan. Higher contents of NaCl-extracted Cd in Pen than in Nan were probably attributed to the enhancement of GSH related metabolism in Pen, which activated the transportation of Cd from root to shoot. Alteration of other proteins involved in Cd detoxification and energy production also demonstrated that Pen had exhibited a stronger tolerance than Nan in dealing with Cd stress. Thus, differences in the proteomic processes associated with biochemical differences between the two typical cultivars suggested a cultivar-dependent capacity of Cd tolerance and accumulation in amaranth for the first time.

Arsenic Uptake, Toxicity, Detoxification, and Speciation in Plants: Physiological, Biochemical, and Molecular Aspects

Environmental contamination with arsenic (As) is a global environmental, agricultural and health issue due to the highly toxic and carcinogenic nature of As. Exposure of plants to As, even at very low concentration, can cause many morphological, physiological, and biochemical changes. The recent research on As in the soil-plant system indicates that As toxicity to plants varies with its speciation in plants (e.g., arsenite, As(III); arsenate, As(V)), with the type of plant species, and with other soil factors controlling As accumulation in plants. Various plant species have different mechanisms of As(III) or As(V) uptake, toxicity, and detoxification. This review briefly describes the sources and global extent of As contamination and As speciation in soil. We discuss different mechanisms responsible for As(III) and As(V) uptake, toxicity, and detoxification in plants, at physiological, biochemical, and molecular levels. This review highlights the importance of the As-induced generation of reactive oxygen species (ROS), as well as their damaging impacts on plants at biochemical, genetic, and molecular levels. The role of different enzymatic (superoxide dismutase, catalase, glutathione reductase, and ascorbate peroxidase) and non-enzymatic (salicylic acid, proline, phytochelatins, glutathione, nitric oxide, and phosphorous) substances under As(III/V) stress have been delineated via conceptual models showing As translocation and toxicity pathways in plant species. Significantly, this review addresses the current, albeit partially understood, emerging aspects on (i) As-induced physiological, biochemical, and genotoxic mechanisms and responses in plants and (ii) the roles of different molecules in modulation of As-induced toxicities in plants. We also provide insight on some important research gaps that need to be filled to advance our scientific understanding in this area of research on As in soil-plant systems.

Phytochelatins play key roles for the difference in root arsenic accumulation of different Triticum aestivum cultivars in comparison with arsenate uptake kinetics and reduction

In the previous studies, we have found that arsenic (As) accumulation in roots of bread wheat (Triticum aestivum L.) seedlings were significantly different among different wheat cultivars, and As(V) tolerant wheat cultivars have much higher capacities of root As accumulation. However, the reason for the difference remains unclear. Four wheat cultivars with high (MM45 and FM8) or low (QF1 and HM29) levels of arsenic (As) accumulation were selected to investigate the relationship between root As(V) uptake kinetics and root As accumulation. MM45 and HM29 were also used to examine As(V) reduction ability and non-protein thiol (cysteine [Cys], glutathione [GSH], and phytochelatins [PCs]) concentrations in wheat seedlings. MM45 had the lowest Michaelis-Menten constant (K_m) and maximum influx rate (V_max). No difference in the K_m values was found among the three other cultivars. No difference in As(V) reduction capacity was observed between MM45 and HM29. GSH and PC₂ were significantly induced by 10 μM As(V) in roots of wheat seedlings, particularly in MM45. Synthesis of GSH and PCs was completely suppressed in the presence of l-buthionine sulfoximine (BSO), a specific inhibitor of γ-glutamylcysteine synthetase. BSO markedly decreased the As tolerance of wheat seedlings and decreased the accumulation of As in roots, but increased As accumulation in shoots. No significant difference in As concentrations was found between MM45 and HM29 under the BSO treatment. GSH and PCs are the reason why As accumulation and As(V) tolerance differ in roots of different wheat cultivars.

Phytochelatin synthesis in Dunaliella salina induced by arsenite and arsenate under various phosphate regimes

This study investigated the dynamic variations in thiol compounds, including cysteine (Cys), glutathione (GSH), and phytochelatins (PCs), in Dunaliella salina samples exposed to arsenite [As(III)] and arsenate [As(V)] under various phosphate (PO₄^3-) regimes. Our results showed that GSH was the major non-protein sulfhydryl compound in D. salina cells. As(III) and As(V) induced PC syntheses in D. salina. PC₂, PC₃, and PC₄ were all found in algal cells; the PC concentrations decreased gradually while exposed to As for 3 d. The synthesis of PC_2-3 was significantly affected by As(III) and As(V) concentrations in the cultures. More PCs were detected in the As(V)-treated algal cells compared with the As(III) treatment. PC levels increased with As(III)/As(V) amount in the medium, but remained stable after 112μgL^-1 As(V) exposure. In contrast, significant (p<0.001) positive correlations were observed between PC synthesis and intracellular As(III) content or As accumulation in As(III)-treated algal cells during the 72-h exposure. PO₄^3- had a significant influence on the PC synthesis in algal cells, irrespective of the As-treated species. Reductions in As uptake and subsequent PC synthesis by D. salina were observed as the PO₄^3- concentration in the growth medium increased. L-Buthionine sulfoximine (BSO) differentially influenced PC synthesis in As-treated D. salina under different extracellular PO₄^3- regimes. Overall, our data demonstrated that the production of GSH and PCs was affected by PO₄^3- and that these thiols played an important role in As detoxification by D. salina.

Thiols as biomarkers of heavy metal tolerance in the aquatic macrophytes of Middle Urals, Russia

Aquatic macrophytes, viz. Sagittaria sagittifolia L., Lemna gibba L., Elodea canadensis Michx., Batrachium trichophyllum (Chaix.) Bosch., Ceratophyllum demersum L. and Potamogeton sp. (P. perfoliatus L., P. alpinus Balb., P. crispus L., P. berchtoldii Fieber, P. friesii Rupr., P. pectinatus L.) were collected from 11 sites for determining their metal accumulation and thiols content. Cu(2+), Ni(2+), Mn(2+), Zn(2+), and Fe(3+) exceeded maximum permissible concentrations in chosen sites. Significant transfer of metals from water to leaves is observed in the order of Ni(2+) < Cu(2+) < Zn(2+) < Fe(3+) < Mn(2+). The maximum variation of bioconcentration factor was noticed for manganese. The accumulation of heavy metals in leaves was correlated with non-protein and protein thiols, confirming their important role in metal tolerance. The largest contribution was provided by Cu(2+) (on the average r = 0.88, p < 0.05), which obviously can be explained as an important role of these ions in thiols synthesis. Increased synthesis of thiols in the leaves allows the usage of SH-containing compounds as biomarkers of metal tolerance. Considering accumulation of metals and tolerance, B. trichophyllum, C. demersum and L. gibba are the most suitable species for phytoremediation of highly multimetal contamination, while E. canadensis and some species of Potamageton are suitable for moderately metal-polluted sites.

Construction of a Modular Arsenic-Resistance Operon in E. coli and the Production of Arsenic Nanoparticles

Arsenic is a widespread contaminant of both land and water around the world. Current methods of decontamination such as phytoremediation and chemical adsorbents can be resource and time intensive, and may not be suitable for some areas such as remote communities where cost and transportation are major issues. Bacterial decontamination, with strict controls preventing environmental release, may offer a cost-effective alternative or provide a financial incentive when used in combination with other remediation techniques. In this study, we have produced Escherichia coli strains containing arsenic-resistance genes from a number of sources, overexpressing them and testing their effects on arsenic resistance. While the lab E. coli strain JM109 (the “wild-type”) is resistant up to 20 mM sodium arsenate, the strain containing our plasmid pEC20 is resistant up to 80 mM. When combined with our construct pArsRBCC arsenic-­containing nanoparticles were observed at the cell surface; the elements of pEC20 and pArsRBCC were therefore combined in a modular construct, pArs, in order to evaluate the roles and synergistic effects of the components of the original plasmids in arsenic resistance and nanoparticle formation. We have also investigated introducing the lac operator in order to more tightly control expression from pArs. We demonstrate that our strains are able to reduce toxic forms of arsenic into stable, insoluble metallic As(0), providing one way to remove arsenate contamination, and which may also be of benefit for other heavy metals.

Integrated phytobial remediation for sustainable management of arsenic in soil and water

Arsenic (As), cited as the most hazardous substance by the U.S. Agency for Toxic Substance and Disease Registry (ATSDR, 2005), is an ubiquitous metalloid which when ingested for prolonged periods cause extensive health effects leading to ultimate untimely death. Plants and microbes can help mitigate soil and groundwater As problem since they have evolved elaborate detoxification machineries against this toxic metalloid as a result of their coexistence with this since the origin of life on earth. Utilization of the phytoremediation and bioremediation potential of the plants and microbes, respectively, is now regarded as two innovative tools that encompass biology, geology, biotechnology and allied sciences with cutting edge applications for sustainable mitigation of As epidemic. Discovery of As hyperaccumulating plants that uptake and concentrate large amounts of this toxic metalloid in their shoots or roots offered new hope to As phytoremediation, solar power based nature's own green remediation. This review focuses on how phytoremediation and bioremediation can be merged together to form an integrated phytobial remediation which could synergistically achieve the goal of large scale removal of As from soil, sediment and groundwater and overcome the drawbacks of the either processes alone. The review also points to the feasibility of the introduction of transgenic plants and microbes that bring new hope for more efficient treatment of As. The review identifies one critical research gap on the importance of remediation of As contaminated groundwater not only for drinking purpose but also for irrigation purpose and stresses that more research should be conducted on the use of constructed wetland, one of the most suitable areas of application of phytobial remediation. Finally the review has narrowed down on different phytoinvestigation and phytodisposal methods, which constitute the most essential and the most difficult part of pilot scale and field scale applications of phytoremediation programs.

Arsenite oxidation by the phyllosphere bacterial community associated with Wolffia australiana

Speciation is a key determinant in the toxicity, behavior, and fate of arsenic (As) in the environment. However, little is known about the transformation of As species mediated by floating macrophytes and the phyllosphere bacteria in aquatic and wetland environment. In this study, Wolffia australiana, a rootless floating duckweed, was cultured with (W+B) or without (W-B) phyllosphere bacteria to investigate its ability in arsenite (As(III)) oxidation. Results showed that sterile W. australiana did not oxidize As(III) in the growth medium or in plant tissue, whereas W. australiana with phyllpsphere bacteria displayed substantial As(III) oxidation in the medium. Quantitative PCR of As redox-related functional genes revealed the dominance of the arsenite oxidase (aioA) gene in the phyllosphere bacterial community. These results demonstrate that the phyllosphere bacteria were responsible for the As(III) oxidation in the W+B system. The rapid oxidation of As(III) by the phyllosphere bacterial community may suppress As accumulation in plant tissues under phosphate rich conditions. The aioA gene library showed that the majority of the phyllosphere arsenite-oxidizing bacteria related either closely to unidentified bacteria found in paddy environments or distantly to known arsenite-oxidizing bacteria. Our research suggests a previously overlooked diversity of arsenite-oxidizing bacteria in the phyllosphere of aquatic macrophytes which may have a substantial impact on As biogeochemistry in water environments, warranting further exploration.

Sulfur metabolism: different tolerances of two aquatic macrophytes exposed to arsenic

The toxicity of arsenic (As) and the mechanisms of response to this pollutant were analyzed in two aquatic plant species, one sensitive and one tolerant to the pollutant, Salvinia minima and Lemna gibba, respectively. The plants, grown in nutrient solution at pH 6.5, were exposed to As concentrations of 0.0 and 1.0mgL(-1) for 3 days. Both species accumulated As in their tissues, which resulted in increases in H2O2 production. L. gibba accumulated eleven times more As than S. minima. However, L. gibba was more tolerant, as shown by the absence of cell membrane damage and, despite greater accumulation, smaller growth reduction than S. minima. Indeed, the index of tolerance to As was twenty percent higher in L. gibba than in S. minima, which most likely results from the presence of a more efficient defense system. This defense system in L. gibba is most likely based on sulfate absorption, assimilation and metabolism. L. gibba showed an increase in sulfate absorption and adenosine-5'-triphosphate (ATP) sulfurylase activity (the first enzyme of the inorganic sulfate assimilation pathway) following exposure to As. Consequently, the plant produced greater concentrations of sulfur-containing compounds that are involved in cellular detoxification, such as glutathione and non-protein thiols, and demonstrated greater enzymatic activity of γ-glutamylcysteine synthetase, glutathione S-transferase and glutathione reductase. Therefore, the plant׳s ability to increase absorption, assimilation and metabolism of sulfur are key steps for tolerance to oxidative stress triggered by metals.

Is arsenic biotransformation a detoxification mechanism for microorganisms?

Arsenic (As) is extremely toxic to living organisms at high concentration. In aquatic systems, As exists in different chemical forms. The two major inorganic As (iAs) species are As(V), which is thermodynamically stable in oxic waters, and As(III), which is predominant in anoxic conditions. Photosynthetic microorganisms (e.g., phytoplankton and cyanobacteria) take up As(V), biotransform it to As(III), then biomethylate it to methylarsenic (MetAs) forms. Although As(III) is more toxic than As(V), As(III) is much more easily excreted from the cells than As(V). Therefore, majority of researchers consider the reduction of As(V) to As(III) as a detoxification process. The biomethylation process results in the conversion of toxic iAs to the less toxic pentavalent MetAs forms (monomethylarsonate; MMA(V), dimethylarsonate; DMA(V), and trimethylarsenic oxide; TMAO(V)) and trimethylarsine (TMAO(III)). However, biomethylation by microorganisms also produces monomethylarsenite (MMA(III)) and dimethylarsenite (DMA(III)), which are more toxic than iAs, as a result of biomethylation by the microorganisms, demonstrates the need to reconsider to what extent As biomethylation contributes to a detoxification process. In this review, we focused on the discussion of whether the biotransformation of As species in microorganisms is really a detoxification process with recent data.

Accumulation, transformation, and release of inorganic arsenic by the freshwater cyanobacterium Microcystis aeruginosa

Arsenic (As) as a major hazardous metalloid was affected by phytoplankton in many aquatic environments. The toxic dominant algae Microcystis aeruginosa was exposed to different concentrations of inorganic arsenic (arsenate or arsenite) for 15 days in BG11 culture media. Arsenic accumulation, toxicity, and speciation in M. aeruginos as well as the changes of As species in media were examined. M. aeruginosa has a general well tolerance to arsenate and a definite sensitivity to arsenite. Additionally, arsenate actively elevated As methylation by the algae but arsenite definitely inhibited it. Interestingly, the uptake of arsenite was more pronounced than that of arsenate, and it was correlated to the toxicity. Arsenate was the predominant species in both cells and their growth media after 15 days of exposure to arsenate or arsenite. However, the amount of the methylated As species in cells was limited and insignificantly affected by the external As concentrations. Upon uptake of the inorganic arsenic, significant quantities of arsenate as well as small amounts of arsenite, DMA, and MMA were produced by the algae and, in turn, released back into the growth media. Bio-oxidation was the first and primary process and methylation was the minor process for arsenite exposures, while bioreduction and the subsequent methylation were the primary metabolisms for arsenate exposures. Arsenic bioaccumulation and transformation by M. aeruginosa in aquatic environment should be paid more attention during a period of eutrophication.

Sample preparation for arsenic speciation in terrestrial plants--a review

Arsenic is an element widely present in nature. Additionally, it may be found as different species in several matrices and therefore it is one of the target elements in chemical speciation. Although the number of studies in terrestrial plants is low, compared to matrices such as fish or urine, this number is raising due to the fact that this type of matrix are closely related to the human food chain. In speciation analysis, sample preparation is a critical step and several extraction procedures present drawbacks. In this review, papers dealing with extraction procedures, analytical methods, and studies of species conservation in plants cultivated in terrestrial environment are critically discussed. Analytical procedures based on extractions using water or diluted acid solutions associated with HPLC-ICP-MS are good alternatives, owing to their versatility and sensitivity, even though less expensive strategies are shown as feasible choices.

A central role for thiols in plant tolerance to abiotic stress

Abiotic stress poses major problems to agriculture and increasing efforts are being made to understand plant stress response and tolerance mechanisms and to develop new tools that underpin successful agriculture. However, the molecular mechanisms of plant stress tolerance are not fully understood, and the data available is incomplete and sometimes contradictory. Here, we review the significance of protein and non-protein thiol compounds in relation to plant tolerance of abiotic stress. First, the roles of the amino acids cysteine and methionine, are discussed, followed by an extensive discussion of the low-molecular-weight tripeptide, thiol glutathione, which plays a central part in plant stress response and oxidative signalling and of glutathione-related enzymes, including those involved in the biosynthesis of non-protein thiol compounds. Special attention is given to the glutathione redox state, to phytochelatins and to the role of glutathione in the regulation of the cell cycle. The protein thiol section focuses on glutaredoxins and thioredoxins, proteins with oxidoreductase activity, which are involved in protein glutathionylation. The review concludes with a brief overview of and future perspectives for the involvement of plant thiols in abiotic stress tolerance.