Increased cadmium tolerance and accumulation by plants expressing bacterial arsenate reductase

Phytoremediation strategies for mitigating environmental toxicants

In natural environments, persistent pollutants such as heavy metals and organic compounds, are frequently sequestered in sediments, soils, and mineral deposits, rendering them biologically unavailable. This study examines phytoremediation, a sustainable technology that uses plants to remove pollutants from soil, water, and air. It discusses enhancing techniques such as plant selection, the use of plant growth-promoting bacteria, soil amendments, and genetic engineering. The study highlights the slow removal rates and the limited availability of plant species that are effective for specific pollutants. Furthermore, it investigates bioavailability and the mechanisms underlying root exudation and hyperaccumulation. Applications across diverse environments and innovative technologies, such as transgenic plants and nanoparticles, are also explored. Additionally, the potential for phytoremediation with bioenergy production is considered. The purpose of this study is to provide researchers, practitioners, and policymakers with valuable resources for sustainable solutions.

Iron homeostasis as a cell detoxification mechanism in Mesorhizobium qingshengii J19 under yttrium exposure

Yttrium (Y), an important rare earth element (REE), is increasingly prevalent in the environment due to industrial activities, raising concerns about its toxicity. Understanding the effects of Y on microorganisms is essential for bioremediation and biorecovery processes. This study investigates how Mesorhizobium qingshengii J19, a strain with notable resistance to Y, manages iron homeostasis as a detoxifying mechanism under Y stress. Using comparative genomic and transcriptomic analyses, we explored the gene expression profile of strain J19 to identify the mechanisms underlying its high Y resistance and effective Y removal from the medium. Genome-wide transcriptional profiling revealed 127 significantly differentially expressed genes out of 6,343 under Y stress, with 36.2 % up-regulated and 63.8 % down-regulated. Notably, Y exposure significantly affects cellular iron homeostasis and activates arsenic detoxifying mechanisms. A key finding was the 7.6-fold up-regulation of a TonB transporter gene, indicating its crucial role in Y detoxification. Real-time PCR (RT-PCR) analysis of the selected gene confirmed the accuracy of RNA sequencing results. Further validation showed that iron supplementation mitigates Y-induced growth inhibition, leading to reduced ROS production in strain J19. This study elucidates the molecular mechanisms by which strain M. qingshengii J19 adapts to Y stress, emphasizing the importance of iron in controlling ROS and protecting against Y toxicity. It also highlights critical pathways and adaptive responses involved in the strain's resilience to metal stress.

Root Zn sequestration transporter heavy metal ATPase 3 from Odontarrhena chalcidica enhance Cd tolerance and accumulation in Arabidopsis thaliana

The Ni hyperaccumulator Odontarrhena chalcidica (formerly Alyssum murale), exhibits a significant capacity to accumulate Zn in the roots. However, the molecular mechanisms underlying the variation in Ni and Zn accumulation are poorly understood. Here, we isolated a homolog of heavy metal ATPase 3 from O. chalcidica (OcHMA3) and characterized its functions using heterologous systems. Phylogenetic analysis revealed that OcHMA3 protein shares 87.6 % identity with AtHMA3, with similar metal binding sites to other HMA3 proteins. Heterologous expression of OcHMA3 in yeast increased sensitivity to Cd, Ni and Zn, suggesting it functions as a broad-specificity transporter. Further investigation showed OcHMA3 is constitutively expressed in the roots and localized to the tonoplast. Overexpression of OcHMA3 in A. thaliana shoots increased its roots Zn concentrations by 41.9 % - 74.1 %. However, overexpression of OcHMA3 in roots enhanced its tolerance to Cd and increased roots Cd concentrations by 50.9 % - 90.6 %. Our findings indicated OcHMA3 is responsible for Zn sequestration in root vacuoles, likely leading to Zn retention in roots and subsequent Ni hyperaccumulation in shoots. This study elucidates the molecular mechanism of Ni and Zn accumulation in O. chalcidica, and identifies OcHMA3 as a potential gene for developing Zn-rich plants and for phytoextraction in Cd-contaminated soils.

Tobacco as an efficient metal accumulator

Tobacco (Nicotiana tabacum L.) is an important industrial crop plant. However, it efficiently accumulates metals, primarily cadmium (Cd) and also zinc (Zn), in its leaves. Therefore, it could be a source of cadmium intake by smokers. On the other hand, as a high leaf metal accumulator, it is widely used for phytoremediation of metal-contaminated soil. Both issues provide an important rationale for investigating the processes regulating metal homeostasis in tobacco. This work summarizes the results of research to date on the understanding of the molecular mechanisms determining the effective uptake of Zn and Cd, their translocation into shoots and accumulation in leaves. It also discusses the current state of research to improve the phytoremediation properties of tobacco through genetic modification and to limit leaf Cd content for the tobacco industry.

Identification of a New Heavy-Metal-Resistant Strain of Geobacillus stearothermophilus Isolated from a Hydrothermally Active Volcanic Area in Southern Italy

Microorganisms thriving in hot springs and hydrothermally active volcanic areas are dynamically involved in heavy-metal biogeochemical cycles; they have developed peculiar resistance systems to cope with such metals which nowadays can be considered among the most permanent and toxic pollutants for humans and the environment. For this reason, their exploitation is functional to unravel mechanisms of toxic-metal detoxification and to address bioremediation of heavy-metal pollution with eco-sustainable approaches. In this work, we isolated a novel strain of the thermophilic bacterium Geobacillus stearothermophilus from the solfataric mud pool in Pisciarelli, a well-known hydrothermally active zone of the Campi Flegrei volcano located near Naples in Italy, and characterized it by ribotyping, 16S rRNA sequencing and mass spectrometry analyses. The minimal inhibitory concentration (MIC) toward several heavy-metal ions indicated that the novel G. stearothermophilus isolate is particularly resistant to some of them. Functional and morphological analyses suggest that it is endowed with metal resistance systems for arsenic and cadmium detoxification.

Molecular mechanisms in phytoremediation of environmental contaminants and prospects of engineered transgenic plants/microbes

Concerns about emerging environmental contaminants have been growing along with industrialization and urbanization around the globe. Among various options for remediating these contaminants, phytotechnology is suggested as a feasible option to maintain the environmental sustainability. The recent advances in phytoremediation, genetic/molecular/omics/metabolic engineering, and nanotechnology are opening new paths for efficient treatment of emerging organic/inorganic contaminants. In this respect, elucidation of molecular mechanisms and genetic engineering of hyperaccumulator plants is expected to enhance remediation of environmental contaminants. This review was organized to offer valuable insights into the molecular mechanisms of phytoremediation and the prospects of transgenic hyperaccumulators with enhanced stress tolerance to diverse contaminants such as heavy metals and metalloids, xenobiotics, explosives, poly aromatic hydrocarbons (PAHs), petroleum hydrocarbons, pesticides, and nanoparticles. The roles of genoremediation and nanoparticles in augmenting the phytoremediation technology are also described in an interrelated framework with biotechnological prospects (e.g., plant molecular nano-farming). Finally, political debate on the preferential use of crops versus non-crop hyperaccumulators in genoremediation, limitations of transgenics in phytotechnologies, and their public acceptance issues are discussed in the policy framework.

Phytoextraction of Heavy Metals: A Promising Tool for Clean-Up of Polluted Environment?

Pollution by heavy metals (HM) represents a serious threat for both the environment and human health. Due to their elemental character, HM cannot be chemically degraded, and their detoxification in the environment mostly resides either in stabilization in situ or in their removal from the matrix, e.g., soil. For this purpose, phytoremediation, i.e., the application of plants for the restoration of a polluted environment, has been proposed as a promising green alternative to traditional physical and chemical methods. Among the phytoremediation techniques, phytoextraction refers to the removal of HM from the matrix through their uptake by a plant. It possesses considerable advantages over traditional techniques, especially due to its cost effectiveness, potential treatment of multiple HM simultaneously, no need for the excavation of contaminated soil, good acceptance by the public, the possibility of follow-up processing of the biomass produced, etc. In this review, we focused on three basic HM phytoextraction strategies that differ in the type of plant species being employed: natural hyperaccumulators, fast-growing plant species with high-biomass production and, potentially, plants genetically engineered toward a phenotype that favors efficient HM uptake and boosted HM tolerance. Considerable knowledge on the applicability of plants for HM phytoextraction has been gathered to date from both lab-scale studies performed under controlled model conditions and field trials using real environmental conditions. Based on this knowledge, many specific applications of plants for the remediation of HM-polluted soils have been proposed. Such studies often also include suggestions for the further processing of HM-contaminated biomass, therefore providing an added economical value. Based on the examples presented here, we recommend that intensive research be performed on the selection of appropriate plant taxa for various sets of conditions, environmental risk assessment, the fate of HM-enriched biomass, economical aspects of the process, etc.

The potential of genetic engineering of plants for the remediation of soils contaminated with heavy metals

The genetic engineering of plants to facilitate the reclamation of soils and waters contaminated with inorganic pollutants is a relatively new and evolving field, benefiting from the heterologous expression of genes that increase the capacity of plants to mobilize, stabilize and/or accumulate metals. The efficiency of phytoremediation relies on the mechanisms underlying metal accumulation and tolerance, such as metal uptake, translocation and detoxification. The transfer of genes involved in any of these processes into fast-growing, high-biomass crops may improve their reclamation potential. The successful phytoextraction of metals/metalloids and their accumulation in aerial organs have been achieved by expressing metal ligands or transporters, enzymes involved in sulfur metabolism, enzymes that alter the chemical form or redox state of metals/metalloids and even the components of primary metabolism. This review article considers the potential of genetic engineering as a strategy to improve the phytoremediation capacity of plants in the context of heavy metals and metalloids, using recent case studies to demonstrate the practical application of this approach in the field.

Molecular characterization of Alr1105 a novel arsenate reductase of the diazotrophic cyanobacterium Anabaena sp. PCC7120 and decoding its role in abiotic stress management in Escherichia coli

This paper constitutes the first report on the Alr1105 of Anabaena sp. PCC7120 which functions as arsenate reductase and phosphatase and offers tolerance against oxidative and other abiotic stresses in the alr1105 transformed Escherichia coli. The bonafide of 40.8 kDa recombinant GST+Alr1105 fusion protein was confirmed by immunoblotting. The purified Alr1105 protein (mw 14.8 kDa) possessed strong arsenate reductase (Km 16.0 ± 1.2 mM and Vmax 5.6 ± 0.31 μmol min⁻¹ mg protein⁻¹) and phosphatase activity (Km 27.38 ± 3.1 mM and Vmax 0.077 ± 0.005 μmol min⁻¹ mg protein⁻¹) at an optimum temperature 37 °C and 6.5 pH. Native Alr1105 was found as a monomeric protein in contrast to its homologous Synechocystis ArsC protein. Expression of Alr1105 enhanced the arsenic tolerance in the arsenate reductase mutant E. coli WC3110 (∆arsC) and rendered better growth than the wild type W3110 up to 40 mM As (V). Notwithstanding above, the recombinant E. coli strain when exposed to CdCl₂, ZnSO₄, NiCl₂, CoCl₂, CuCl₂, heat, UV-B and carbofuron showed increase in growth over the wild type and mutant E. coli transformed with the empty vector. Furthermore, an enhanced growth of the recombinant E. coli in the presence of oxidative stress producing chemicals (MV, PMS and H₂O₂), suggested its protective role against these stresses. Appreciable expression of alr1105 gene as measured by qRT-PCR at different time points under selected stresses reconfirmed its role in stress tolerance. Thus the Alr1105 of Anabaena sp. PCC7120 functions as an arsenate reductase and possess novel properties different from the arsenate reductases known so far.

ARSENIC ACCUMULATION IN PLANTS – OUTLINING STRATEGIES FOR DEVELOPING IMPROVED VARIETY OF CROPS FOR AVOIDING ARSENIC TOXICITY IN FOODS

Contamination of food with arsenics is a potential health risk for both humans and animals in many regions of the world, especially in Asia. Arsenics can be accumulated in humans, animals and plants for a longer period and a long-term exposure of humans to arsenics results in severe damage of kidney, lever, heart etc. and many other vascular diseases. Arsenic contamination in human may also lead to development of cancer. In this paper we report our results on data mining approach (an in silico analysis based on searching of the existing genomic databases) for identification and characterization of genes that might be responsible for uptake, accumulation or metabolism of arsenics. For these in silico analyses we have involved the model plant Arabidopsis thaliana in our investigation. By employing a system biology model (a kinetic model) we have studied the molecular mechanisms of these processes in this plant. This model contains equations for uptake, metabolism and sequestration of different types of arsenic; As(V), As(III), MMAA and DMAA. The model was then implemented in the software XPP. The model was also validated against the data existing in the literatures. Based on the results of these in silico studies we have developed some strategies that can be used for reducing arsenic contents in different parts of the plant. Data mining experiments resulted in identification of two candidate genes (ACR2, arsenate reductase 2 and PCS1, phytochelatin synthase 1) that are involved either in uptake, transport or cellular localization of arsenic in A. thaliana. However, our system biology model revealed that by increasing the level of arsenate reductase together with an increased rate of arsenite sequestration in the vacuoles (by involving an arsenite efflux pump MRP1/2), it is possible to reduce the amount of arsenics in the shoots of A. thaliana to 11–12%.

Perspectives for genetic engineering of poplars for enhanced phytoremediation abilities

Phytoremediation potential has been widely accepted as highly stable and dynamic approach for reducing eco-toxic pollutants. Earlier reports endorse remediation abilities both in herbaceous plants as well as woody trees. Poplars are dominant trees to the ecosystem structure and functioning in riparian forests of North America Rivers and also to other part of the world. Understanding of the fact that how genetic variation in primary producer structures communities, affects species distribution, and alters ecosystem-level processes, attention was paid to investigate the perspectives of genetic modification in poplar. The present review article furnishes documented evidences for genetic engineering of Populus tree for enhanced phytoremediation abilities. The versatility of poplar as a consequence of its distinct traits, rapid growth rates, extensive root system, high perennial biomass production, and immense industrial value, bring it in the forefront of phytoremediation. Furthermore, remediative capabilities of Populus can be significantly increased by introducing cross-kingdom, non-resident genes encoding desirable traits. Available genome sequence database of Populus contribute to the determination of gene functions together with elucidating phytoremediation linked metabolic pathways. Adequate understanding of functional genomics in merger with physiology and genetics of poplar offers distinct advantage in identifying and upgrading phytoremediation potential of this model forest tree species for human welfare.

Biotechnological potential of aquatic plant-microbe interactions

The rhizosphere in terrestrial systems is the region of soil surrounding plant roots where there is increased microbial activity; in aquatic plants, this definition may be less clear because of diffusion of nutrients in water, but there is still a zone of influence by plant roots in this environment [1]. Within that zone chemical conditions differ from those of the surrounding environment as a consequence of a range of processes that were induced either directly by the activity of plant roots or by the activity of rhizosphere microflora. Recently, there are a number of new studies related to rhizospheres of aquatic plants and specifically their increased potential for remediation of contaminants, especially remediation of metals through aquatic plant-microbial interaction.

Tissue-specific cadmium accumulation and its effects on nitrogen metabolism in tobacco (Nicotiana tabaccum, Bureley v. Fb9)

Tobacco (Nicotiana Tabaccum, Bureley v. Fb9) seedlings were grown for 30 days on control medium, and then treated for seven days with different concentrations (0, 10, 20, 50 and 100 muM) of CdCl(2). Cadmium (Cd) was mostly accumulated in the leaves. However, nitrate reductase and nitrite reductase activities (NR, EC 1.6.1.6 and NiR, EC 1.7.7.1) were more inhibited by Cd stress in the roots than in leaves. Glutamine synthetase activity (GS, EC 6.3.1.2) was inhibited by Cd treatment in roots and leaves. In both organs, aminating activity of glutamate dehydrogenase (GDH, EC 1.4.1.2) and protease activity were significantly stimulated in the leaves and roots of stressed plants. The lesser extents of Cd stress effects on leaves, despite their high Cd accumulation, suggest that: (i) tobacco leaves may evolve adaptive process to partially inactivate Cd ions; and (ii) tobacco is useful for phytoremediation.

Hyperaccumulation of arsenic in the shoots of Arabidopsis silenced for arsenate reductase (ACR2)

Endogenous plant arsenate reductase (ACR) activity converts arsenate to arsenite in roots, immobilizing arsenic below ground. By blocking this activity, we hoped to construct plants that would mobilize more arsenate aboveground. We have identified a single gene in the Arabidopsis thaliana genome, ACR2, with moderate sequence homology to yeast arsenate reductase. Expression of ACR2 cDNA in Escherichia coli complemented the arsenate-resistant and arsenate-sensitive phenotypes of various bacterial ars operon mutants. RNA interference reduced ACR2 protein expression in Arabidopsis to as low as 2% of wild-type levels. The various knockdown plant lines were more sensitive to high concentrations of arsenate, but not arsenite, than wild type. The knockdown lines accumulated 10- to 16-fold more arsenic in shoots (350-500 ppm) and retained less arsenic in roots than wild type, when grown on arsenate medium with <8 ppm arsenic. Reducing expression of ACR2 homologs in tree, shrub, and grass species should play a vital role in the phytoremediation of environmental arsenic contamination.

Phytoextraction and phytofiltration of arsenic

Arsenic, a ubiquitous contaminant in groundwater and soils, is currently drawing much public attention. Arsenic-contaminated soils can be cleaned up via phytoextraction-the use of plants to extract the arsenic from soil and transport it into aboveground tissues. Arsenic removal from polluted soils can be carried out using hyperaccumulator ferns like the Chinese brake fern Pteris vittata, which accumulates very high concentrations of the element in aboveground tissues. The capacity of the plant to take up large concentrations of arsenic, even at low levels in soil, illustrates efficient bioaccumulation. The possibility of using Pteris ferns to remove arsenic from water by phytofiltration has been proposed.

Transgenic plants in phytoremediation: recent advances and new possibilities

Phytoremediation, the use of plants and their associated microbes to remedy contaminated soils, sediments, and groundwater, is emerging as a cost-effective and environmentally friendly technology. Due in large part to its aesthetic appeal, this technology has gained increasing attention over the past 10 years. Phytoremediation uses different plant processes and mechanisms normally involved in the accumulation, complexation, volatilization, and degradation of organic and inorganic pollutants. Certain plants, called hyperaccumulators, are good candidates in phytoremediation, particularly for the removal of heavy metals. Phytoremediation efficiency of plants can be substantially improved using genetic engineering technologies. Recent research results, including overexpression of genes whose protein products are involved in metal uptake, transport, and sequestration, or act as enzymes involved in the degradation of hazardous organics, have opened up new possibilities in phytoremediation. This paper provides a critical review of the recent progress made toward the development of transgenic plants with improved phytoremediation capabilities and their potential use in environmental cleanup.

The plant MT1 metallothioneins are stabilized by binding cadmiums and are required for cadmium tolerance and accumulation

The small Arabidopsis genome contains nine metallothionein-like (MT) sequences with classic, cysteine-rich domains separated by spacer sequences, quite unlike the small conserved MT families found vertebrate genomes. Phylogenetic analysis revealed four ancient and divergent classes of plant MTs that predate the monocot-dicot divergence. A distinct cysteine spacing pattern suggested differential metal ion specificity for each class. The in vivo stability of representatives of the four classes of plant MT proteins and a mouse MT2 control expressed in E. coli were enhanced by cadmium (Cd). Particular MTs were also stabilized by arsenic (As), copper (Cu), and or zinc (Zn). To understand why plants have such a diversity of MT sequences, the Arabidopsis MT1 class, comprised of three genes, MT1a, MT1b, and MT1c, was characterized in more detail in plants. MT1 family transcripts were knocked down to less than 5-10% of wild-type levels in Arabidopsis by expression of a RNA interference (RNAi) construct. The MT1 knockdown plant lines were all hypersensitive to Cd and accumulated several fold lower levels of As, Cd, and Zn than wildtype, while Cu and Fe levels were unaffected. The ancient class of MT1 protein sequences may be preserved in plant genomes, because it has distinct metal-binding properties, confers tolerance to cadmium, and can assist with zinc homeostasis.

Phytoremediation: novel approaches to cleaning up polluted soils

Environmental pollution with metals and xenobiotics is a global problem, and the development of phytoremediation technologies for the plant-based clean-up of contaminated soils is therefore of significant interest. Phytoremediation technologies are currently available for only a small subset of pollution problems, such as arsenic. Arsenic removal employs naturally selected hyperaccumulator ferns, which accumulate very high concentrations of arsenic specifically in above-ground tissues. Elegant two-gene transgenic approaches have been designed for the development of mercury or arsenic phytoremediation technologies. In a plant that naturally hyperaccumulates zinc in leaves, approximately ten key metal homeostasis genes are expressed at very high levels. This outlines the extent of change in gene activities needed in the engineering of transgenic plants for soil clean-up. Further analysis and discovery of genes for phytoremediation will benefit from the recent development of segregating populations for a genetic analysis of naturally selected metal hyperaccumulation in plants, and from comprehensive ionomics data--multi-element concentration profiles from a large number of Arabidopsis mutants.

Phytoremediation: green technology for the clean up of toxic metals in the environment

The contamination of the environment by toxic metals poses a threat for "Man and biosphere", reducing agricultural productivity and damaging the health of the ecosystem. In developed nations, this problem is being addressed and solved to some extent by using "green technology" involving metal tolerant plants, to clean up the polluted soils. The use of naturally occurring metal tolerant plants and the application of genetic manipulation, should hasten the process of transferring this technology from laboratory to field. Therefore, it is essential to investigate and understand how plants are able to tolerate toxic metals and to identify which metabolic pathways and genes are involved in such a process. Recent advances in knowledge derived from the "omics", have considerable potential in developing this green technology. However, strategies to produce genetically altered plants to remove, destroy or sequester toxic metals from the environment and the long-term implications, must be investigated carefully.

Molecular mechanisms of heavy metal hyperaccumulation and phytoremediation

A relatively small group of hyperaccumulator plants is capable of sequestering heavy metals in their shoot tissues at high concentrations. In recent years, major scientific progress has been made in understanding the physiological mechanisms of metal uptake and transport in these plants. However, relatively little is known about the molecular bases of hyperaccumulation. In this paper, current progresses on understanding cellular/molecular mechanisms of metal tolerance/hyperaccumulation by plants are reviewed. The major processes involved in hyperaccumulation of trace metals from the soil to the shoots by hyperaccumulators include: (a) bioactivation of metals in the rhizosphere through root-microbe interaction; (b) enhanced uptake by metal transporters in the plasma membranes; (c) detoxification of metals by distributing to the apoplasts like binding to cell walls and chelation of metals in the cytoplasm with various ligands, such as phytochelatins, metallothioneins, metal-binding proteins; (d) sequestration of metals into the vacuole by tonoplast-located transporters. The growing application of molecular-genetic technologies led to the well understanding of mechanisms of heavy metal tolerance/accumulation in plants, and subsequently many transgenic plants with increased resistance and uptake of heavy metals were developed for the purpose of phytoremediation. Once the rate-limiting steps for uptake, translocation, and detoxification of metals in hyperaccumulating plants are identified, more informed construction of transgenic plants would result in improved applicability of the phytoremediation technology.

Enhancing the first enzymatic step in the histidine biosynthesis pathway increases the free histidine pool and nickel tolerance inArabidopsis thaliana

Naturally selected nickel (Ni) tolerance in Alyssum lesbiacum has been proposed to involve constitutively high levels of endogenous free histidine. Transgenic Arabidopsis thaliana expressing a Salmonella typhimurium ATP phosphoribosyl transferase enzyme (StHisG) resistant to feedback inhibition by histidine contained approximately 2-fold higher histidine concentrations than wild type plants. Under exposure to a toxic Ni concentration, biomass production in StHisG expressing lines was between 14- and 40-fold higher than in wild-type plants. This suggested that enhancing the first step in the histidine biosynthesis pathway is sufficient to increase the endogenous free histidine pool and Ni tolerance in A. thaliana.

Vacuolar compartmentalization: a second-generation approach to engineering plants for phytoremediation

Engineering plants with greater metal tolerance and accumulation properties is the key to developing phytoremediators. A recent study by Won-Yong Song et al. has shown that overexpressing the yeast vacuolar transporter YCF1 increases Pb and Cd tolerance and consequently increases the accumulation of these metals in shoots of transgenic Arabidopsis plants even though expression levels of YCF1 were relatively low. This technology can be used to engineer advanced phytoremediators, increasing their ability to pump heavy metals into a safe compartment while requiring only a small amount of transporters rather than a large amount of chelating peptide material.