Mechanisms of arsenic hyperaccumulation in<Emphasis Type="Italic"> Pteris</Emphasis> species: root As influx and translocation

Several species of fern from the Pteris genus are able to accumulate extremely high concentrations of arsenic (As) in the fronds. We have conducted short-term unidirectional As influx and translocation experiments with ⁷³As-radiolabeled arsenate, and found that the concentration-dependent influx of arsenate into roots was significantly larger in two of these As-hyperaccumulating species, Pteris vittata (L.) and Pteris cretica cv. Mayii (L.), than in Nephrolepis exaltata (L.), a non-accumulating fern. The arsenate influx could be described by Michaelis-Menten kinetics and the kinetic parameter K _m was found to be lower in the Pteris species, indicating higher affinity of the transport protein for arsenate. Quantitative analysis of kinetic parameters showed that phosphate inhibited arsenate influx in a directly competitive manner, consistent with the hypothesis that arsenate enters plant roots on a phosphate-transport protein. The significantly augmented translocation of arsenic to the shoots that was seen in these As hyperaccumulator species is proposed to be due to a combination of the increased root influx and also decreased sequestration of As in the roots, as a larger fraction of As could be extracted from roots of the Pteris species than from roots of N. exaltata. This leaves a larger pool of mobile As available for translocation to the shoot, probably predominantly as arsenite.Abbreviations As ^V Arsenate - As ^III Arsenite - K _m Michaelis-Menten constant - P _i Phosphate - V _max Maximum rate of an enzyme-catalyzed reaction     

Comparative Proteomic Analysis of Rice Shoots Exposed to High Arsenate

Consumption of arsenic contaminated water and cereals is a serious threat to humans all over the world. Rice (Oryza sativa“Nipponbare”), as a main cereal crop, can accumulate arsenic more than 10-fold ...     

The identification and quantification of arsenic-induced phytochelatins—comparison between plants with varying As sensitivities

The results presented in this paper provide evidence for the role of phytochelatins (PCs) in the detoxification of arsenic in six nonhyperaccumulating plant species, Agropyron repens, Glecoma hederacea, Leonurus marrubiastrum, Lolium perenne, Urtica dioica and Zea mays, in a pot experiment with high phosphate treatment. These plants differed in their arsenic sensitivities and were selected to investigate whether PCs with longer chains in roots of arsenic-tolerant species are synthesized. Raised concentrations of total PCs were measured in plant species with a range of sensitivities to arsenic at equivalent levels of arsenic exposure, determined as the inhibition of root biomass. In addition, the production of PCs as a function of accumulated arsenic was studied. Long-term PC synthesis (over a 5-week period) was positively, but non-linearly correlated with arsenic, suggesting that probably not all As is bound by PCs. Moreover, it could be shown that the synthesis of different chain lengths of PCs is associated with differences in As tolerance. In the more tolerant grasses A. repens and L. perenne it was chiefly the dithiol PC2 which was measured. In contrast, the dominant PC species in the less tolerant plants U. dioica, G. hederacea, L. marrubiastrum and Z. mays was PC3, while PC2 and PC3 were detected as well. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Influence of sulphur on arsenic accumulation and metabolism in rice seedlings

The influence of sulphur on the accumulation and metabolism of arsenic in rice was investigated. Rice seedlings were grown in nutrient solutions with low sulphate (1.8 μM SO₄²⁻) or high sulphate (0.7 mM SO₄²⁻) for 12 or 14 d, before being exposed to 10 μM arsenite or arsenate for 2 or 1 d, respectively. In the arsenite exposure treatment, low sulphate-pretreated rice accumulated less arsenite than high sulphate pretreated plants, but the arsenite concentrations in shoots of low sulphate pretreated rice were higher than those of high sulphate pretreated. In the arsenate exposure treatment, the low sulphate pre-treatments also resulted in less arsenite accumulation in rice roots. Sulphur deprivation in nutrient solution decreased the concentrations of non-protein thiols in rice roots exposed to either arsenite or arsenate. The low sulphate-pretreated plants had a higher arsenic transfer factor than the high sulphate-pretreated plants. The results suggest that rice sulphate nutrition plays an important role in regulating arsenic translocation from roots to shoots, possibly through the complexation of arsenite-phytochelatins.     

Intracellular distribution and chemical forms of arsenic in rabbits exposed to arsenate

Inhibition of the methylation of arsenic in rabbits by ip injection of periodate-oxidized adenosine (PAD) prior to an iv injection of⁷⁴As-arsenate (AsV; 0.4 mg As/kg body wt) caused a marked increase in the retention of⁷⁴As in both the cellular organelles and the soluble fractions of liver and kidney. One day after exposure, almost 30% of the arsenic in the liver and about 40% of the arsenic in the kidney was recovered in the nuclear fraction. In the liver nuclei, the inhibition of the methylation increased the⁷⁴As content of the insoluble fraction and most of this arsenic was protein-bound. The major part of the soluble intranuclear⁷⁴As was in the form of AsIII, formed by reduction of the administered AsV. In the liver, PAD also caused a pronounced increase in the⁷⁴As content of the microsomal fraction. In the kidneys, where most of the arsenic was present as AsV, there was a marked accumulation of arsenic in the mitochondria.     

Arsenic metabolism in freshwater and terrestrial plants

Freshwater and terrestrial plants differ markedly in their ability to metabolize arsenate. In experiments with higher terrestrial plants, e.g. tomato, Lycopersicon esculentum Mill. cv. Better boy, ⁷⁴As-arsenate was readily taken up and reduced to arsenite. Methylation and reduction to methanearsonic acid, methanearsinic acid (indicated for the first time) and dimethylarsinic acid were apparent only in phosphate deficient plants. Lower and higher freshwater plants, e.g. Nitella tenuissima Kütz. and Lemna minima Phill., not only methylated arsenic but also produced considerable amounts of an arsoniumphospholipid previously identified in marine algae. These differences indicate that freshwater but not terrestrial plants have evolved mechanisms for rapid detoxication of arsenate, arsenite and other toxic arsenic species.     

Arsenate respiratory reductase gene (arrA) for Desulfosporosinus sp. strain Y5

Desulfosporosinus sp. strain Y5 is a spore-forming bacterium capable of dissimilatory arsenate reduction coupled to the oxidation of aromatic compounds. In arsenate respiration, the arsenate respiratory reductase (ARR) catalyzes the reduction of arsenate to arsenite. Our objective is to characterize the arrA gene, encoding the ARR, for Desulfosporosinus sp. strain Y5. Oligonucleotide primers were designed based on the few arrA gene sequences available at the time and validated against positive and negative controls. The resulting arrA-amplicon of approximately 2.0kb was cloned and sequenced. The arrA from Desulfosporosinus sp. Y5 is closely related to Desulfitobacterium hafniense (similarity of 77% and 81% at the nucleotide and amino acid levels, respectively). Phylogenetic topology based on the arrA gene was partially congruent with that of 16S rRNA-based analysis. This arrA sequence will support the development of specific tracking probes for Desulfosporosinus sp. Y5 and the molecular characterization and monitoring of dissimilatory arsenate reducing bacteria.     

Horizontal arsC gene transfer among microorganisms isolated from arsenic polluted soil

The study of recent evolution of the arsC genes amplified from microorganisms inhabiting a Colombian oil-polluted soil with high concentrations of arsenic was performed through the isolation of 26 bacterial morphotypes resistant to 10 mM of arsenate. A 353 bp fragment of the gene coding for arsenate-reductase enzyme (i.e. arsC), and a 500 bp 16S rDNA partial sequence were sequenced for 16 morphotypes of the 26 previously isolated. arsC sequences clustered on the same clade with previously reported arsC chromosomal genes of Escherichia coli and Shigella sp.; while 16S rDNA sequences grouped within the genus Pseudomonas and Bacillus. The GC content and the Codon Adaptation Index (CAI) were calculated and statistically compared, both supported the previous results. The Isolation-Migration model (IM model) was applied to calculate the genetic flux between each clade defined by the phylogenetic analysis. In general, the existence of recent horizontal gene transfer (HGT) events was confirmed, and the presence of the arsC gene in Bacillus sphaericus is reported for the first time.     

Accumulation, speciation, and coordination of arsenic in an inbred line and a wild type cultivar of the desert plant species Chilopsis linearis (Desert willow)

This study investigated the absorption of arsenic (As), sulfur (S), and phosphorus (P) in the desert plant Chilopsis linearis (Desert willow). A comparison between an inbred line (red flowered) and wild type (white flowered) plants was performed to look for differential responses to As treatment. One month old seedlings were treated for 7 days with arsenate (As₂O₅, As^V) at 0, 20, and 40 mg As^V L⁻¹. Results from the ICP-OES analysis showed that at 20 mg As^V L⁻¹, red flowered plants had 280 ± 11 and 98 ± 7 mg As kg⁻¹ dry wt in roots and stems, respectively, while white flowered plants had 196 ± 30 and 103 ± 13 mg As kg⁻¹ dry wt for roots and stems. At this treatment level, the concentration of As in leaves was below detection limits for both plants. In red flowered plants treated with 40 mg As^V L⁻¹, As was at 290 ± 77 and 151 ± 60 mg As kg⁻¹ in roots and stems, respectively, and not detected in leaves, whereas white flowered plants had 406 ± 36, 213 ± 12, and 177 ± 40 mg As kg⁻¹ in roots, stems, and leaves. The concentration of S increased in all As treated plants, while the concentration of P decreased in roots and stems of both types of plants and in leaves of red flowered plants. X-ray absorption spectroscopy analyses demonstrated partial reduction of arsenate to arsenite in the form of As-(SX)₃ species in both types of plants.