Unraveling the multifaceted resilience of arsenic resistant bacterium Deinococcus indicus

Arsenic (As) is a toxic heavy metal widely found in the environment that severely undermines the integrity of water resources. Bioremediation of toxic compounds is an appellative sustainable technology with a balanced cost-effective setup. To pave the way for the potential use of Deinococcus indicus, an arsenic resistant bacterium, as a platform for arsenic bioremediation, an extensive characterization of its resistance to cellular insults is paramount. A comparative analysis of D. indicus cells grown in two rich nutrient media conditions (M53 and TGY) revealed distinct resistance patterns when cells are subjected to stress via UV-C and methyl viologen (MV). Cells grown in M53 demonstrated higher resistance to both UV-C and MV. Moreover, cells grow to higher density upon exposure to 25 mM As(V) in M53 in comparison with TGY. This analysis is pivotal for the culture of microbial species in batch culture bioreactors for bioremediation purposes. We also demonstrate for the first time the presence of polyphosphate granules in D. indicus which are also found in a few Deinococcus species. To extend our analysis, we also characterized DiArsC2 (arsenate reductase) involved in arsenic detoxification and structurally determined different states, revealing the structural evidence for a catalytic cysteine triple redox system. These results contribute for our understanding into the D. indicus resistance mechanism against stress conditions.

Comparative Analysis of Arsenic Transport and Tolerance Mechanisms: Evolution from Prokaryote to Higher Plants

Arsenic (As) is a toxic metalloid for all living organisms and can cause serious harm to humans. Arsenic is also toxic to plants. To alleviate As toxicity, all living organisms (from prokaryotes to higher plants) have evolved comprehensive mechanisms to reduce cytosolic As concentration through the set of As transporters localized at the plasma and tonoplast membranes, which operate either in arsenite As(III) extrusion out of cells (via ArsB, ACR3, and aquaporins) or by sequestering arsenic into vacuoles (by ABC transporters). In addition, a special arsenate resistance mechanism found in some bacterial systems has evolved in an As hyperaccumulating fern Pteris vittata, which involves transforming arsenate As(V) to an As(V) phosphoglycerate derivative by a glyceraldehyde 3-phosphate dehydrogenase and transporting this complex by an efflux transporter. In the present review, we summarize the evolution of these arsenic resistance mechanisms from prokaryotes to eukaryotes and discuss future approaches that could be utilized to better understand and improve As resistance mechanisms in plants.

Targeted expression of the arsenate reductase HAC1 identifies cell type specificity of arsenic metabolism and transport in plant roots

High Arsenic Concentration 1 (HAC1), an Arabidopsis thaliana arsenate reductase, plays a key role in arsenate [As(V)] tolerance. Through conversion of As(V) to arsenite [As(III)], HAC1 enables As(III) export from roots, and restricts translocation of As(V) to shoots. To probe the ability of different root tissues to detoxify As(III) produced by HAC1, we generated A. thaliana lines expressing HAC1 in different cell types. We investigated the As(V) tolerance phenotypes: root growth, As(III) efflux, As translocation, and As chemical speciation. We showed that HAC1 can function in the outer tissues of the root (epidermis, cortex, and endodermis) to confer As(V) tolerance, As(III) efflux, and limit As accumulation in shoots. HAC1 is less effective in the stele at conferring As(V) tolerance phenotypes. The exception is HAC1 activity in the protoxylem, which we found to be sufficient to restrict As translocation, but not to confer As(V) tolerance. In conclusion, we describe cell type-specific functions of HAC1 that spatially separate the control of As(V) tolerance and As translocation. Further, we identify a key function of protoxylem cells in As(V) translocation, consistent with the model where endodermal passage cells, above protoxylem pericycle cells, form a 'funnel' loading nutrients and potentially toxic elements into the vasculature.

Statistical optimization of arsenic biosorption by microbial enzyme via Ca-alginate beads

Bioremediation of arsenic using green technology via microbial enzymes has attracted scientists due to its simplicity and cost effectiveness. Statistical optimization of arsenate bioremediation was conducted by the enzyme arsenate reductase extracted from arsenic tolerant bacterium Pseudomonas alcaligenes. Response surface methodology based on Box-Behnken design matrix was performed to determine the optimal operational conditions of a multivariable system and their interactive effects on the bioremediation process. The highest biosorptive activity of 96.2 µg gm^-1 of beads was achieved under optimized conditions (pH = 7.0; As (V) concentration = 1000 ppb; time = 2 h). SEM analysis showed the morphological changes on the surface of enzyme immobilized gluteraldehyde crosslinked Ca-alginate beads. The immobilized enzyme retained its activity for 8 cycles. ANOVA with a high correlation coefficient (R² > 0.99) and lower "Prob > F"value (<0.0001) corroborated the second-order polynomial model for the biosorption process. This study on the adsorptive removal of As (V) by enzyme-loaded biosorbent revealed a possible way of its application in large scale treatment of As (V)-contaminated water bodies.

Dissecting the components controlling root-to-shoot arsenic translocation in Arabidopsis thaliana

Arsenic (As) is an important environmental and food-chain toxin. We investigated the key components controlling As accumulation and tolerance in Arabidopsis thaliana. We tested the effects of different combinations of gene knockout, including arsenate reductase (HAC1), γ-glutamyl-cysteine synthetase (γ-ECS), phytochelatin synthase (PCS1) and phosphate effluxer (PHO1), and the heterologous expression of the As-hyperaccumulator Pteris vittata arsenite efflux (PvACR3), on As tolerance, accumulation, translocation and speciation in A. thaliana. Heterologous expression of PvACR3 markedly increased As tolerance and root-to-shoot As translocation in A. thaliana, with PvACR3 being localized to the plasma membrane. Combining PvACR3 expression with HAC1 mutation led to As hyperaccumulation in the shoots, whereas combining HAC1 and PHO1 mutation decreased As accumulation. Mutants of γ-ECS and PCS1 were hypersensitive to As and had higher root-to-shoot As translocation. Combining γ-ECS or PCS1 with HAC1 mutation did not alter As tolerance or accumulation beyond the levels observed in the single mutants. PvACR3 and HAC1 have large effects on root-to-shoot As translocation. Arsenic hyperaccumulation can be engineered in A. thaliana by knocking out the HAC1 gene and expressing PvACR3. PvACR3 and HAC1 also affect As tolerance, but not to the extent of γ-ECS and PCS1.

OsHAC4 is critical for arsenate tolerance and regulates arsenic accumulation in rice

Soil contamination with arsenic (As) can cause phytotoxicity and elevated As accumulation in rice grain. Here, we used a forward genetics approach to investigate the mechanism of arsenate (As(V)) tolerance and accumulation in rice. A rice mutant hypersensitive to As(V), but not to As(III), was isolated. Genomic resequencing and complementation tests were used to identify the causal gene. The function of the gene, its expression pattern and subcellular localization were characterized. OsHAC4 is the causal gene for the As(V)-hypersensitive phenotype. The gene encodes a rhodanase-like protein that shows As(V) reductase activity when expressed in Escherichia coli. OsHAC4 was highly expressed in roots and was induced by As(V). In OsHAC4pro-GUS transgenic plants, the gene was expressed exclusively in the root epidermis and exodermis. OsHAC4-eGFP was localized in the cytoplasm and the nucleus. Mutation in OsHAC4 resulted in decreased As(V) reduction in roots, decreased As(III) efflux to the external medium and markedly increased As accumulation in rice shoots. Overexpression of OsHAC4 increased As(V) tolerance and decreased As accumulation in rice plants. OsHAC4 is an As(V) reductase that is critical for As(V) detoxification and for the control of As accumulation in rice. As(V) reduction, followed by As(III) efflux, is an important mechanism of As(V) detoxification.

Respiratory Pathways Reconstructed by Multi-Omics Analysis in Melioribacter roseus, Residing in a Deep Thermal Aquifer of the West-Siberian Megabasin

Melioribacter roseus, a representative of recently proposed Ignavibacteriae phylum, is a metabolically versatile thermophilic bacterium, inhabiting subsurface biosphere of the West-Siberian megabasin and capable of growing on various substrates and electron acceptors. Genomic analysis followed by inhibitor studies and membrane potential measurements of aerobically grown M. roseus cells revealed the activity of aerobic respiratory electron transfer chain comprised of respiratory complexes I and IV, and an alternative complex III. Phylogeny reconstruction revealed that oxygen reductases belonged to atypical cc(o/b)o₃-type and canonical cbb₃–type cytochrome oxidases. Also, two molybdoenzymes of M. roseus were affiliated either with Ttr or Psr/Phs clades, but not with typical respiratory arsenate reductases of the Arr clade. Expression profiling, both at transcripts and protein level, allowed us to assign the role of the terminal respiratory oxidase under atmospheric oxygen concentration for the cc(o/b)o₃ cytochrome oxidase, previously proposed to serve for oxygen detoxification only. Transcriptomic analysis revealed the involvement of both molybdoenzymes of M. roseus in As(V) respiration, yet differences in the genomic context of their gene clusters allow to hypothesize about their distinct roles in arsenate metabolism with the ‘Psr/Phs’-type molybdoenzyme being the most probable candidate respiratory arsenate reductase. Basing on multi-omics data, the pathways for aerobic and arsenate respiration were proposed. Our results start to bridge the vigorously increasing gap between homology-based predictions and experimentally verified metabolic processes, what is especially important for understudied microorganisms of novel lineages from deep subsurface environments of Eurasia, which remained separated from the rest of the biosphere for several geological periods.

A Hybrid Mechanism for the Synechocystis Arsenate Reductase Revealed by Structural Snapshots during Arsenate Reduction

Evolution of enzymes plays a crucial role in obtaining new biological functions for all life forms. Arsenate reductases (ArsC) are several families of arsenic detoxification enzymes that reduce arsenate to arsenite, which can subsequently be extruded from cells by specific transporters. Among these, the Synechocystis ArsC (SynArsC) is structurally homologous to the well characterized thioredoxin (Trx)-coupled ArsC family but requires the glutaredoxin (Grx) system for its reactivation, therefore classified as a unique Trx/Grx-hybrid family. The detailed catalytic mechanism of SynArsC is unclear and how the "hybrid" mechanism evolved remains enigmatic. Herein, we report the molecular mechanism of SynArsC by biochemical and structural studies. Our work demonstrates that arsenate reduction is carried out via an intramolecular thiol-disulfide cascade similar to the Trx-coupled family, whereas the enzyme reactivation step is diverted to the coupling of the glutathione-Grx pathway due to the local structural difference. The current results support the hypothesis that SynArsC is likely a molecular fossil representing an intermediate stage during the evolution of the Trx-coupled ArsC family from the low molecular weight protein phosphotyrosine phosphatase (LMW-PTPase) family.

Possible roles of plant sulfurtransferases in detoxification of cyanide, reactive oxygen species, selected heavy metals and arsenate

Plants and animals have evolved various potential mechanisms to surmount the adverse effects of heavy metal toxicity. Plants possess low molecular weight compounds containing sulfhydryl groups (-SH) that actively react with toxic metals. For instance, glutathione (γ-Glu-Cys-Gly) is a sulfur-containing tripeptide thiol and a substrate of cysteine-rich phytochelatins (γ-Glu-Cys)2-11-Gly (PCs). Phytochelatins react with heavy metal ions by glutathione S-transferase in the cytosol and afterwards they are sequestered into the vacuole for degradation. Furthermore, heavy metals induce reactive oxygen species (ROS), which directly or indirectly influence metabolic processes. Reduced glutathione (GSH) attributes as an antioxidant and participates to control ROS during stress. Maintenance of the GSH/GSSG ratio is important for cellular redox balance, which is crucial for the survival of the plants. In this context, sulfurtransferases (Str), also called rhodaneses, comprise a group of enzymes widely distributed in all phyla, paving the way for the transfer of a sulfur atom from suitable sulfur donors to nucleophilic sulfur acceptors, at least in vitro. The best characterized in vitro reaction is the transfer of a sulfane sulfur atom from thiosulfate to cyanide, leading to the formation of sulfite and thiocyanate. Plants as well as other organisms have multi-protein families (MPF) of Str. Despite the presence of Str activities in many living organisms, their physiological role has not been clarified unambiguously. In mammals, these proteins are involved in the elimination of cyanide released from cyanogenic compounds. However, their ubiquity suggests additional physiological functions. Furthermore, it is speculated that a member of the Str family acts as arsenate reductase (AR) and is involved in arsenate detoxification. In summary, the role of Str in detoxification processes is still not well understood but seems to be a major function in the organism.

Investigation of biochemical responses of Bacopa monnieri L. upon exposure to arsenate

Widespread contamination of arsenic (As) is recognized as a global problem due to its well-known accumulation by edible and medicinal plants and associated health risks for the humans. In this study, phytotoxicity imposed upon exposure to arsenate [As(V); 0-250 μM for 1-7 days] and ensuing biochemical responses were investigated in a medicinal herb Bacopa monnieri L. vis-à-vis As accumulation. Plants accumulated substantial amount of As (total 768 μg g(-1) dw at 250 μM As(V) after 7 days) with the maximum As retention being in roots (60%) followed by stem (23%) and leaves (17%). The level of cysteine and total nonprotein thiols (NP-SH) increased significantly at all exposure concentrations and durations. Besides, the level of metalloid binding ligands viz., glutathione (GSH) and phytochelatins (PCs) increased significantly at the studied concentrations [50 and 250 μM As(V)] in both roots and leaves. The activities of various enzymes viz., arsenate reductase (AR), glutathione reductase (GR), superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX), and catalase (CAT) showed differential but coordinated stimulation in leaves and roots to help plants combat As toxicity up to moderate exposure concentrations (50 μM). However, beyond 50 μM, biomass production was found to decrease along with photosynthetic pigments and total soluble proteins, whereas lipid peroxidation increased. In conclusion, As accumulation potential of Bacopa may warrant its use as a phytoremediator but if Bacopa growing in contaminated areas is consumed by humans, it may prove to be toxic for health.

Increased cadmium tolerance and accumulation by plants expressing bacterial arsenate reductase

•  Cadmium (Cd) is a major environmental pollutant that poses a serious threat to natural ecosystems. However, most initial attempts to engineer phytoremediation of Cd have not succeeded in developing sufficient Cd tolerance for vigorous plant growth. •  We found that the bacterial arsenate reductase gene (arsC) provided Cd(II) resistance to Escherichia coli. When ArsC is overexpressed in tobacco (Nicotiana tabacum) and Arabidopsis thaliana, both transgenic plant species showed significantly greater Cd tolerance than wild-type controls. •  At 50, 75, and 100 µm concentrations of Cd (II), the ArsC expressing transgenic lines grew bigger with broader leaves and longer roots than wild-type controls, which were stunted, turned yellow, flowered early, and often died. At the various Cd(II) concentrations, ArsC transgenic plants attained f. wt 2-3-fold higher than the wild-type plants and had roots significantly longer than wild-type plants. These transgenic plants also contained 30-50% higher Cd concentrations than wild-type plants. •  It is likely that the arsC gene directs Cd tolerance via the electrochemical reduction of Cd(II) to Cd(0).