EXAFS study on arsenic species and transformation in arsenic hyperaccumulator

Synchrotron radiation extended X-ray absorption fine structure (SR EXAFS) was employed to study the transformation of coordination environment and the redox speciation of arsenic in a newly discovered arsenic hyperaccumulator, Cretan brake (Pteris cretica L. var nervosa Thunb). It showed that the arsenic in the plant mainly coordinated with oxygen, except that some arsenic coordinated with S as As-GSH in root. The complexation of arsenic with GSH might not be the predominant detoxification mechanism in Cretan brake. Although some arsenic in root presented as As(V) in Na₂HAsO₄ treatments, most of arsenic in plant presented as As(III)-O in both treatments, indicating that As(V) tended to be reduced to As(III) after it was taken up into the root, and arsenic was kept as As(III) when it was transported to the above-ground tissues. The reduction of As(V) primarily proceeded in the root.     

EXAFS study on arsenic species and transformation in arsenic hyperaccumulator

As a cost-effective, efficient and environmental friendly method for the remediation of contaminated soils and waters, phytoremediation of arsenic-con- taminated soils has drawn more and more attention[1]. The plants with the special ability to accumulate arse-nic (hyperaccumulators) are a prerequisite for phy-toremediation. Cretan brake (Pteris cretica L. var nervosa Thunb) has been shown to accumulate arsenic as much as 694 mg/kg in pinna in field investigation[2], and such elevated arsenic…     

Effects of selenium on arsenic uptake in arsenic hyperaccumulator Pteris vittata L

Selenium (Se) is a non-metallic element, which has the capability to increase the antioxidative capacity and stress tolerance of plants to heavy metals. Plants vary considerably in their physiological response to Se. The reported research investigated the effects of Se on arsenic (As) uptake by As hyperaccumulator Pteris vittata L. and determined possible mechanisms of interaction. Pteris vittata plants were exposed hydroponically to 0, 150 or 300 microM of Na(2)HAsO(4) in the presence of 0, 5 or 10 microM of Na(2)SeO(4) for 5 or 10d. Application of 5 microM Se enhanced As concentration by P. vittata fronds by 7-45%. At 5 microM, Se acted as an antioxidant, inhibiting lipid peroxidation (reduced by 26-42% in the fronds) via increased levels of thiols and glutathione (increased by 24% in the fronds). The results suggest that Se is either an antioxidant or it activates plant protective mechanisms, thereby alleviating oxidative stress and improving arsenic uptake in P. vittata.     

Root exudates and arsenic accumulation in arsenic hyperaccumulating Pteris vittata and non-hyperaccumulating Nephrolepis exaltata

This study compared the roles of root exudates collected from two fern species, the As hyperaccumulating Chinese Brake fern (Pteris vittata L.) and the As-sensitive Boston fern (Nephrolepis exaltata L.), on As-mobilization of two As minerals (aluminum arsenate and iron arsenate) and a CCA (chromated copper arsenate)-contaminated soil as well as plant As accumulation. Chinese Brake fern exuded 2 times more dissolved organic carbon (DOC) than Boston fern and the difference was more pronounced under As stress. The composition of organic acids in the root exudates for both ferns consisted mainly of phytic acid and oxalic acid. However, Chinese Brake fern produced 0.46 to 1.06 times more phytic acid than Boston fern under As stress, and exuded 3–5 times more oxalic acid than Boston fern in all treatments. Consequently, root exudates from Chinese Brake fern mobilized more As from aluminum arsenate (3–4 times), iron arsenate (4–6 times) and CCA-contaminated soil (6–18 times) than Boston fern. Chinese Brake fern took up more As and translocated more As to the fronds than Boston fern. The molar ratio of P/As in the roots of Chinese Brake fern was greater than in the fronds whereas the reverse was observed in Boston fern. These results suggested that As-mobilization from the soil by the root exudates (enhancing plant uptake), coupled with efficient As translocation to the fronds (keeping a high molar ratio of P/As in the roots), are both important for As hyperaccumulation by Chinese Brake fern.     

Effect of arsenic on visible symptom and arsenic concentration in hydroponic Japanese mustard spinach

Responses of Japanese mustard spinach (JM-spinach; Brassica rapa L. var. pervirdis) were investigated at elevated levels of arsenic (As). Plants were grown hydroponically in the greenhouse under 0, 6.7, 33.5 and 67 μM As (equal to 0, 0.5, 2.5 and 5 mg L^?1 As, respectively) for 14 days. Arsenic was used as sodium meta-arsenite (NaAsO₂). Toxicity symptom was solely shown as shoot growth repression at 33.5 and 67 μM As exposures. Dry weight (DW) enhanced by 19.4% in shoot and 38.9% in root in the 6.7 μM As level as compared to control but decreased by 48.1% and 72.1% DW in shoot and 24.1% and 61.1% DW in root in the 33.5 and 67 μM As levels, respectively. This result indicated that As at lower concentration might have slight stimulating effect on JM-spinach growth, but toxicity increased with increasing As. Based on the regression lines between growth and As concentration in the plant tissues, the critical toxicity level (CTL) of As in JM-spinach shoot was 7.85 μg g^?1 DW considering 10% DW reduction. The CTL for the root was almost 2110 μg As g^?1 DW, indicating that shoot of JM-spinach was more sensitive to As-toxicity than that of root. Arsenic concentrations increased in plant parts with increasing As in the medium. Arsenic concentrations were also compared in DW and fresh weight (FW) basis. The JM-spinach concentrated unaccepted level of As in shoots for human consumption in the higher As levels without showing visible toxicity symptom. In spite of decreasing iron (Fe) concentration in shoot in the highest As level, chlorophyll index did not decrease accordingly. Phosphorus (P) concentration also decreased. Phosphorus concentration decreased much more than Fe concentration. Low P might help to mobilize Fe in shoots, resulting in higher chlorophyll index at 67 μM As level. Phosphorus might compete with Fe in shoot tissues of As-stressed JM-spinach.     

Evaluation of amino acid profile in contrasting arsenic accumulating rice genotypes under arsenic stress

Amino acids (AAs) play significant roles in metal binding, antioxidant defense, and signaling in plants during heavy metal stress. In the present study, the essential amino acids (EAAs), non-essential amino acids (NEAAs), as well as the enzymes of proline and cysteine biosynthetic pathways were studied in contrasting arsenic accumulating rice genotypes grown in hydroponic solutions with addition of arsenate (As^V) or arsenite (As^III). Under a mild As stress, the total AAs content significantly increased in both the rice genotypes with a greater increase in a low As accumulating rice genotype (LAARG; IET-19226) than in a high As accumulating rice genotype (HAARG; BRG-12). At the equimolar concentration (10 μM), As^III had a greater effect on EAAs than As^V. Conversely, As^V was more effective in inducing a proline accumulation than As^III. Among NEAAs, As significantly induced the accumulation of histidine, aspartic acid, and serine. In contrast, a higher As concentration (50 μM) reduced the content of most AAs, the effect being more prominent during As^III exposure. The inhibition of glutamate kinase activity was noticed in HAARG, conversely, serine acetyltransferase and cysteine synthase activities were increased which was positively correlated with the cysteine synthesis.     

Resistance to arsenic compounds in microorganisms

Abstract: Arsenic ions, frequently present as environmental pollutants, are very toxic for most microorganisms. Some microbial strains possess genetic determinants that confer resistance. In bacteria, these determinants are often found on plasmids, which has facilitated their study at the molecular level. Bacterial plasmids conferring arsenic resistance encode specific efflux pumps able to extrude arsenic from the cell cytoplasm thus lowering the intracellular concentration of the toxic ions. In Gram-negative bacteria, the efflux pump consists of a two-component ATPase complex. ArsA is the ATPase subunit and is associated with an integral membrane subunit, ArsB. Arsenate is enzymatically reduced to arsenite (the substrate of ArsB and the activator of ArsA) by the small cytoplasmic ArsC polypeptide. In Gram-positive bacteria, comparable arsB and arsC genes (and proteins) are found, but arsA is missing. In addition to the wide spread plasmid arsenic resistance determinant, a few bacteria confer resistance to arsenite with a separate determinant for enzymatic oxidation of more-toxic arsenite to less-toxic arsenate. In contrast to the detailed information on the mechanisms of arsenic resistance in bacteria, little work has been reported on this subject in algae and fungi.     

Distributions of arsenic and essential elements in pinna of arsenic hyperaccumulator Pteris vittata L.

The distributions of arsenic and 6 essential elements in the pinna of As hyperaccumulator, Pteris vittata L., were studied using synchrotron radiation X-ray fluorescence (SRXRF). Significant correlation between the distribution and mobility of the elements revealed that SRXRF study on the elemental distribution was feasible to inspect the transportations of elements in plants. The distribution of As in the pinna showed that As had great abilities to be transported in xylem vessels and from xylem to mesophyll. The distribution of K, one of the most mobile elements in plants, was similar to that of As, whereas the distributions of Fe and Ca with less mobility in plants were almost opposite to that of As in the pinna.     

Effect of arsenic trioxide on metallothionein and its conversion to different arsenic metabolites in hen liver

The metabolism of arsenic, its affinity to metallothionein (MT), its influence on selenium levels, and its biotransformation to different metabolites in the liver tissue of laying hens exposed to arsenic trioxide (As2O3) was investigated. The experiment was performed with two groups of hens fed for 19 d with either a standard diet or with the same diet enriched in arsenic (30 microg/g). The major findings were as follows: 1. After 19 d exposure, about 65% of the total liver As was found in the water-soluble phase (100,000g centrifuged supernatant). In liver supernatant, As binding was found mostly in the range of very low-molecular-weight proteins (Mr < 10,000). Although after exposure the amount of MT-like proteins increased, the As bound to it was only in trace amounts. The protein was identified by convential procedures as Zn,Cu-thionein with traces of selenium and arsenic. 2. Arsenic exposure resulted in almost unchanged Se levels regarding its tissue concentrations and distribution between supernatant and pellet, where about 10% of total Se was found in the supernatant. On the contrary, As exposure did affect Cd levels. Tissue Cd concentration was slightly diminished, but the percentage of tissue Cd found in the water-soluble phase was increased from 20% to 40%. 3. In methanol extracts of tissue and supernatant of the As-exposed group, only two arsenic compounds were detected, As(III) and dimethylarsinic acid (DMA), the latter prevailing.     

Biochemistry of arsenic detoxification

All living organisms have systems for arsenic detoxification. The common themes are (a) uptake of As(V) in the form of arsenate by phosphate transporters, (b) uptake of As(III) in the form of arsenite by aquaglyceroporins, (c) reduction of As(V) to As(III) by arsenate reductases, and (d) extrusion or sequestration of As(III). While the overall schemes for arsenic resistance are similar in prokaryotes and eukaryotes, some of the specific proteins are the products of separate evolutionary pathways.     

Systematic engineering of phytochelatin synthesis and arsenic transport for enhanced arsenic accumulation in E. coli

Phytochelatin (PC) is a naturally occurring peptide with high affinity towards arsenic (As). In this article, we demonstrated the systematic engineering of PC‐producing E. coli for As accumulation by addressing different bottlenecks in PC synthesis as well as As transport. Phytochelatin synthase from Schizosaccharomyces pombe (SpPCS) was expressed in E. coli resulting in 18 times higher As accumulation. PC production was further increased by co‐expressing a feedback desensitized γ‐glutamylcysteine synthetase (GshI*), resulting in 30‐fold higher PC levels and additional 2‐fold higher As accumulation. The significantly increased PC levels were exploited further by co‐expressing an arsenic transporter GlpF, leading to an additional 1.5‐fold higher As accumulation. These engineering steps were finally combined in an arsenic efflux deletion E. coli strain to achieve an arsenic accumulation level of 16.8 µmol/g DCW, a 80‐fold improvement when compared to a control strain not producing phytochelatins. Biotechnol. Bioeng. 2010. 105: 780–785. © 2009 Wiley Periodicals, Inc.     

Individual variations in arsenic metabolism in Vietnamese: the association with arsenic exposure and GSTP1 genetic polymorphism

We investigated the association of As exposure and genetic polymorphism in glutathione S-transferase π1 (GSTP1) with As metabolism in 190 local residents from the As contaminated groundwater areas in the Red River Delta, Vietnam. Total As concentrations in groundwater ranged from <0.1 to 502 μg l(-1). Concentrations of dimethylarsinic acid (DMA(V)), monomethylarsonic acid (MMA(V)), and arsenite (As(III)) in human urine were positively correlated with total As levels in the groundwater, suggesting that people in these areas may be exposed to As through the groundwater. The concentration ratios of urinary As(III)/arsenate (As(V)) and MMA(V)/inorganic As (IA; As(III) + As(V))(M/I), which are indicators of As metabolism, increased with the urinary As level. Concentration and proportion of As(III) were high in the wild type of GSTP1 Ile105Val compared with the hetero type, and these trends were more pronounced in the higher As exposure group (>56 μg l(-1) creatinine in urine), but not in the lower exposure group. In the high As exposure group, As(III)/As(V) ratios in the urine of wild type of GSTP1 Ile105Val were significantly higher than those of the hetero type, while the opposite trend was observed for M/I. These results suggest that the excretion and metabolism of IA may depend on both the As exposure level and the GSTP1 Ile105Val genotype.     

Detoxification of arsenic by phytochelatins in plants

As is a ubiquitous element present in the atmosphere as well as in the aquatic and terrestrial environments. Arsenite and arsenate are the major forms of As intoxication, and these anions are readily taken up by plants. Both anions efficiently induce the biosynthesis of phytochelatins (PCs) ([gamma-glutamate-cysteine](n)-glycine) in vivo and in vitro. The rapid induction of the metal-binding PCs has been observed in cell suspension cultures of Rauvolfia serpentina, in seedlings of Arabidopsis, and in enzyme preparations of Silene vulgaris upon challenge to arsenicals. The rate of PC formation in enzyme preparations was lower compared with Cd-induced biosynthesis, but was accompanied by a prolonged induction phase that resulted finally in higher peptide levels. An approximately 3:1 ratio of the sulfhydryl groups from PCs to As is compatible with reported As-glutathione complexes. The identity of the As-induced PCs and of reconstituted metal-peptide complexes has unequivocally been demonstrated by electrospray ionization mass spectroscopy. Gel filtration experiments and inhibitor studies also indicate a complexation and detoxification of As by the induced PCs.     

外源二甲基砷在土壤中的转化

利用盆栽试验,研究了外源二甲基砷（DMA）进入土壤后的形态及其与土壤胶体结合态的变化.结果表明：土壤中的DMA主要转化为砷酸根［As(V)］,并伴有少量的一甲基砷（MMA）生成;当外源DMA添加浓度为30 mg·kg^-1时,DMA的转化率最高,其在培养时间为10、15、30、40 d时的转化率分别为6.71%、8.11%、11.33%、19.32%;随着外源DMA添加浓度的增加土壤中易溶性砷（AE-As）浓度呈逐渐增加的趋势,但随着培养时间的增加,AE-As浓度不断降低;与不加外源DMA的对照相比,添加不同量的DMA使土壤中铝型砷（Al-As）、铁型砷（Fe-As）、钙型砷（Ca-As）浓度均有所增加,DMA及其转化产物与土壤中铝、铁、钙的氧化物或氢氧化物间的吸附或固定可能是其主要原因.     

Male reproductive effect of arsenic in mice

Arsenic, a known human carcinogen, was given to mice via drinking water as sodium arsenite at a dose 53.39, 133.47, 266.95 and 533.90 mol l for 35 days. A decrease in the activity of 17 HSD along with increase in LDH, GT activity were observed at 533.90 mol l. The observed sperm count, motility and morphological abnormalities in sperm were similar to control at lower dose levels. However at 533.90 mol l a significant decrease in sperm count and motility along with increase in abnormal sperm were noticed. Significant accumulation of arsenic in testes and accessory sex organs may be attributed to the arsenic binding to the tissues or greater cellular uptake. No effects were observed on indices studied for reproductive effects at 53.39 mol l arsenic close to which human being are exposed through drinking water under the present set of experimental conditions.     

Role of ubiquitination in arsenic tolerance in plants

Arsenic (As) is harmful to all living organisms, including humans and plants. To limit As uptake and avoid its toxicity, plants employ systems that regulate the uptake of As from the soil and its translocation from roots to grains. Ubiquitination, a highly conserved post-translational modification (PTM) in all eukaryotes, plays crucial roles in modulating As detoxification mechanisms in budding yeast (Saccharomyces cerevisiae), but little is known about its roles in As tolerance and transport in plants. In this opinion article we review recent findings and suggest that ubiquitination plays a crucial role in regulating As transport in plants. We also propose ideas for future research to explore the importance of ubiquitination for enhancing As tolerance in crops.     

Arsenic-resistant bacteria solubilized arsenic in the growth media and increased growth of arsenic hyperaccumulator Pteris vittata L

The role of arsenic-resistant bacteria (ARB) in arsenic solubilization from growth media and growth enhancement of arsenic-hyperaccumulator Pteris vittata L. was examined. Seven ARB (tolerant to 10 mM arsenate) were isolated from the P. vittata rhizosphere and identified by 16S rRNA sequencing as Pseudomonas sp., Comamonas sp. and Stenotrophomonas sp. During 7-d hydroponic experiments, these bacteria effectively solubilized arsenic from the growth media spiked with insoluble FeAsO? and AlAsO? minerals (from < 5 μg L?1 to 5.04-7.37 mg L?1 As) and enhanced plant arsenic uptake (from 18.1-21.9 to 35.3-236 mg kg?1 As in the fronds). Production of (1) pyochelin-type siderophores by ARB (fluorescent under ultraviolet illumination and characterized with thin layer chromatography) and (2) root exudate (dissolved organic C) by P. vittata may be responsible for As solubilization. Increase in P. vittata root biomass from 1.5-2.2 to 3.4-4.2 g/plant dw by ARB and by arsenic was associated with arsenic-induced plant P uptake. Arsenic resistant bacteria may have potential to enhance phytoremediation of arsenic-contaminated soils by P. vittata.     

Effects of arsenic species and concentrations on arsenic accumulation by different fern species in a hydroponic system

Two hydroponic experiments were conducted to evaluate factors affecting plant arsenic (As) hyperaccumulation. In the first experiment; two As hyperaccumulators (Pteris vittata and P. cretica mayii) were exposed to 1 and 10 mg L(-1) arsenite (AsIII) and monomethyl arsenic acid (MMA) for 4 wk. Total As concentrations in plants (fronds and roots) and solution were determined In the second experiment P. vittata and Nephrolepis exaltata (a non-As hyperaccumulator) were exposed to 5 mgL(-1) arsenate (AsV) and 20 mgL(-1) AsIIIfor 1 and 15 d. Total As and AsIII concentrations in plants were determined Compared to P. cretica mayii, P. vittata was more efficient in arsenic accumulation (1075-1666 vs. 249-627mg kg(-1) As in the fronds) partially because it is more efficient in As translocation. As translocation factor (As concentration ratio in fronds to roots) was 3.0-5.6 for P. vittata compared to 0.1 to 4.8 for P. cretica. Compared to N. exaltata, P. vittata was significantly more efficient in arsenic accumulation (38-542 vs. 4.8-71 mg kg(-1) As in thefronds) as well asAs translocation (1.3-5.6 vs. 0.2-0.5). In addition, P. vittata was much more efficient in As reduction from AsV to AsIII (83-84 vs. 13-24% AsIII in the fronds). Little As reduction occurred after 1-d exposure to AsV in both species indicates that As reduction was not instantaneous even in an As hyperaccumulator. Our data were consistent with the hypothesis that both As translocation and As reduction are important for plant As hyperaccumulation.