Exploring the link between heavy metals detoxification and crop improvements.

Phytochelatins (PCs), (gamma-Glu-Cys)n Gly polymers that were formerly considered to be restricted to plants and some fungal systems, are now known to play a critical role in heavy metal (notably Cd2+) detoxification in Caenorhabditis elegans. In view of the functional equivalence of the gene encoding C. elegans PC synthase 1, ce-pcs-1, to its homologs from plant and fungal sources, we have gone on to explore processes downstream of PC fabrication in this organism. Here we describe the identification of a half-molecule ATP-binding cassette transporter, CeHMT-1, from C. elegans with an equivalent topology to that of the putative PC transporter SpHMT-1 from Schizosaccharomyces pombe. At one level, CeHMT-1 satisfies the requirements of a Cd2+ tolerance factor involved in the sequestration and/or elimination of Cd x PC complexes. Heterologous expression of cehmt-1 in S. pombe alleviates the Cd2+-hypersensitivity of hmt- mutants concomitant with the localization of CeHMT-1 to the vacuolar membrane. Suppression of the expression of ce-hmt-1 in intact worms by RNA interference (RNAi) confers a Cd2+-hypersensitive phenotype similar to but more pronounced than that exhibited by ce-pcs-1 RNAi worms. At another level, it is evident from comparisons of the cell morphology of ce-hmt-1 and cepcs-1 single and double RNAi mutants that CeHMT-1 also contributes to Cd2+ tolerance in other ways. Whereas the intestinal epithelial cells of ce-pcs-1 RNAi worms undergo necrosis upon exposure to toxic levels of Cd2+, the corresponding cells of ce-hmt-1 RNAi worms instead elaborate punctate refractive inclusions within the vicinity of the nucleus. Moreover, a deficiency in CeHMT-1 does not interfere with the phenotype associated with CePCS-1 deficiency and vice versa. Double ce-hmt-1; ce-pcs-1 RNAi mutants exhibit both cell morphologies when exposed to Cd2+. These results and those from our previous investigations of the requirement for PC synthase for heavy metal tolerance in C. elegans demonstrate PC-dependent, HMT-1-mediated heavy metal detoxification not only in S. pombe but also in some invertebrates while at the same time indicating that the action of CeHMT-1 does not depend exclusively on PC synthesis.

Detoxification of Heavy Metals: State of Art.- Plants in Heavy Metal Soils.- Functional Significance of Metal Ligands in Hyperaccumulating Plants: What do We Know?.- Progress in Phytoremidiating Heavy-metal Contaminated Soils.- Plant Taxonomy and Metal Phytoremediation.- Reclamation of Contaminated Mine Ponds using Marble Wastes, Organic Amendments and Phytoremediation.- The Role of Membrane Transport in the Detoxification and Accumulation of Zinc in Plants.- Initial Steps of Copper Detoxification: Outside and Inside of the Plant Cell.- Arsenic Tolerance and Detoxification Mechanisms in Plants.- Cadmium Metal Detoxification and Hyperaccumulators.- Transport, Accumulation and Physiological Effects of Vanadium.- Microbial Remediation of Arsenic Contaminated Soil.- Fate of Cadmium in Calcareous Soils under Salinity Conditions.- Organellar Proteomics: A High-Throughput Approach for Obtaining a better Understanding of Heavy Metal Accumulation and Detoxification in Plants.- Sulfur Metabolism as a Support System for Plant Heavy Metal Tolerance.- Cd(II)-activated Synthesis of Phytochelatins.- Tolerance, Accumulation and Detoxification Mechanism of Copper in Elsholtzia splendens.- Role of Aquatic Macrophytes in Biogeochemical Cycling of Heavy Metals - Relevance to Soil - Sediment Continuum Detoxification and Ecosystem Health.- Role of Plant Growth Promoting Bacteria and Fungi in Heavy Metal Detoxification.- Detoxification of Heavy Metals From Soils Through Sugar Crops.- Detoxification of Heavy Metals Using Earthworms.- Heavy Metal Stabilization by Promoting Zeolite Synthesis in Soil.

BackgroundDeveloping methods for protecting organisms in metal-polluted environments is contingent upon our understanding of cellular detoxification mechanisms. In this regard, half-molecule ATP-binding cassette (ABC) transporters of the HMT-1 subfamily are required for cadmium (Cd) detoxification. HMTs have conserved structural architecture that distinguishes them from other ABC transporters and allows the identification of homologs in genomes of different species including humans. We recently discovered that HMT-1 from the simple, unicellular organism, Schizosaccharomyces pombe, SpHMT1, acts independently of phytochelatin synthase (PCS) and detoxifies Cd, but not other heavy metals. Whether HMTs from multicellular organisms confer tolerance only to Cd or also to other heavy metals is not known.Methodology/Principal FindingsUsing molecular genetics approaches and functional in vivo assays we showed that HMT-1 from a multicellular organism, Caenorhabditis elegans, functions distinctly from its S. pombe counterpart in that in addition to Cd it confers tolerance to arsenic (As) and copper (Cu) while acting independently of pcs-1. Further investigation of hmt-1 and pcs-1 revealed that these genes are expressed in different cell types, supporting the notion that hmt-1 and pcs-1 operate in distinct detoxification pathways. Interestingly, pcs-1 and hmt-1 are co-expressed in highly endocytic C. elegans cells with unknown function, the coelomocytes. By analyzing heavy metal and oxidative stress sensitivities of the coelomocyte-deficient C. elegans strain we discovered that coelomocytes are essential mainly for detoxification of heavy metals, but not of oxidative stress, a by-product of heavy metal toxicity.Conclusions/SignificanceWe established that HMT-1 from the multicellular organism confers tolerance to multiple heavy metals and is expressed in liver-like cells, the coelomocytes, as well as head neurons and intestinal cells, which are cell types that are affected by heavy metal poisoning in humans. We also showed that coelomocytes are involved in detoxification of heavy metals. Therefore, the HMT-1-dependent detoxification pathway and coelomocytes of C. elegans emerge as novel models for studies of heavy metal-promoted diseases.

Owing to serious heavy metal pollution, much attention has been paid to its effects on soil-plant systems. The research of heavy metal tolerance and hyperaccumulation of higher plants has become a hot topic in the field of pollution ecology. With the development of molecular ecology, research on the mechanisms of heavy metal tolerance, detoxification and accumulation in higher plants has made progress in recent years. There are significant differences in the tolerance to and accumulation of heavy metals among higher plant species and genotypes. Root systems are the first entrance of heavy metal pollutants from the soil into plant. Root exudates reduce the availability and toxicity of metal pollutants and play an important role in ability for plants to absorb heavy metals. Almost all heavy metal ions enter root cells with the help of a metal transporter protein that are subsequently transported to the vacuole. The synthesis of PC in response to the stress caused by heavy metals is one of the adaptive responses common in higher plants. Heavy metal tolerant genotypes have higher levels of PC than non-tolerant genotypes under heavy metal stress. GSH is the substrate that synthesizes PC, which chelates the heavy metals. Heavy metal-PC chelatins are subsequently transported from the cytosol to the vacuole and heavy metal detoxification is thus achieved. MTs play the same role and in the same way as PC under heavy metal stress. The article reviews recent advances in understanding the role of root exudates, metal transporter proteins (MTs, PC and GSH), molecular mechanisms of heavy metal tolerance and hyperaccumulation in higher plants at the molecular level. Existing problems and major topics of future research were discussed.

The first evidence showing that ABC transporters are involved in heavy metal resistance in eukaryotic cells has been obtained from experiments in Schizosaccharomyces pombe and Saccharomyces cerevisae, where a half-size transporter of the ABCB subclass and an ABCC-type transporter, respectively, have been shown to confer heavy metal tolerance. Biochemical studies have indicated that vacuolar ABC transporters should also play an important role in heavy metal detoxification in plants. But it was only recently that two ABCC-type transporters, AtABCC1 and AtABCC2, have been identified as major apo-phytochelatin and phytochelatin-heavy metal(oid) complex transporters. Several plasma membrane transporters have also been shown to confer heavy metal resistance. However, with the exception of STAR1, an UDP glucose exporter, which—by altering cell wall composition—confers aluminum tolerance, the substrates required to be transported to confer heavy metal resistance by these plasma membrane-localized ABC proteins are still not elucidated. A mitochondrial ABC transporter AtATM3 was shown to be required for plant growth and development. The different studies indicate that this transporter is important for the production of cytosolic iron sulfur complexes and molybdenum cofactors, prosthetic groups required for several enzymes. However, the final proof as to which substrate is transported by AtATM3 is still missing. Several laboratories took advantage of the fact that ABC transporters are involved in heavy metal tolerance to generate transgenic plants suitable for phytoremediation. The results show that overexpression of ABC proteins alone is not sufficient to produce plants that can efficiently decontaminate soils, but they indicate that this class of transporters, when combined with other transporters and enzymes involved in heavy metal transport and detoxification, may prove a good solution to produce plants that can stabilize, and in the long term clean up, soils contaminated with heavy metals.

Crustacean hepatopancreatic lysosomes are organelles of heavy metal sequestration and detoxification. Previous studies have shown that zinc uptake by lysosomal membrane vesicles (LMV) occurred by a vanadate- and thapsigargin-sensitive ATPase that was stimulated by a transmembrane proton gradient established by a co-localized V-ATPase associated with this organelle. In the present study, hepatopancreatic LMV from the American lobster Homarus americanus were prepared by standard centrifugation methods and 65Zn2+, 36Cl-, 35SO(4)2- and 14C-oxalate2- were used to characterize the interactions between the metal and anions during vesicular detoxification events. Vesicles loaded with SO4(2-) or PO(4)3- led to a threefold greater steady-state accumulation of Zn2+ than similar vesicles loaded with mannitol, Cl- or oxalate2-. The stimulation of 65Zn2+ uptake by intravesicular sulfate was SO(4)2- concentration dependent with a maximal enhancement at 500 micromol l(-1). Zinc uptake in the presence of ATP was proton-gradient enhanced and electrogenic, exhibiting an apparent exchange stoichiometry of 1Zn+/3H+. 35SO4(2-) and 14C-oxalate2- uptakes were both enhanced in vesicles loaded with intravesicular Cl- compared to vesicles containing mannitol, suggesting the presence of anion countertransport. 35SO4(2-) influx was a sigmoidal function of external [SO(4)2-] with 25 mmol l(-1) internal [Cl-], or with several intravesicular pH values (e.g. 7.0, 8.0 and 9.0). In all instances Hill coefficients of approximately 2.0 were obtained, suggesting that 2 sulfate ions exchange with single Cl- or OH- ions. 36Cl- influx was a sigmoidal function of external [Cl-] with intravesicular pH of 7.0 and 9.0. A Hill coefficient of 2.0 was also obtained, suggesting the exchange of 2 Cl- for 1 OH-. 14C-oxalate influx was a hyperbolic function of external [oxalate2-] with 25 mmol l(-1) internal [Cl-], suggesting a 1:1 exchange of oxalate2- for Cl-. As a group, these experiments suggest the presence of an anion exchange mechanism exchanging monovalent for polyvalent anions. Polyvalent inorganic anions (SO4(2-) and PO4(3-)) are known to associate with metals inside vesicles and a detoxification model is presented that suggests how these anions may contribute to concretion formation through precipitation with metals at appropriate vesicular pH.

Heavy metal-associated proteins (HMPs) participate in heavy metal detoxification. Although HMPs have been identified in several plants, no studies to date have identified the HMPs in Brassica rapa (B. rapa). Here, we identified 85 potential HMPs in B. rapa by bioinformatic methods. The promoters of the identified genes contain many elements associated with stress responses, including response to abscisic acid, low-temperature, and methyl jasmonate. The expression levels of BrHMP14, BrHMP16, BrHMP32, BrHMP41, and BrHMP42 were upregulated under Cu2+, Cd2+, Zn2+, and Pb2+ stresses. BrHMP06, BrHMP30, and BrHMP41 were also significantly upregulated after drought treatment. The transcripts of BrHMP06 and BrHMP11 increased mostly under cold stress. After applying salt stress, the expression of BrHMP02, BrHMP16, and BrHMP78 was induced. We observed increased BrHMP36 expression during the self-incompatibility (SI) response and decreased expression in the compatible pollination (CP) response during pollen–stigma interactions. These changes in expression suggest functions for these genes in HMPs include participating in heavy metal transport, detoxification, and response to abiotic stresses, with the potential for functions in sexual reproduction. We found potential co-functional partners of these key players by protein–protein interaction (PPI) analysis and found that some of the predicted protein partners are known to be involved in corresponding stress responses. Finally, phosphorylation investigation revealed many phosphorylation sites in BrHMPs, suggesting post-translational modification may occur during the BrHMP-mediated stress response. This comprehensive analysis provides important clues for the study of the molecular mechanisms of BrHMP genes in B. rapa, especially for abiotic stress and pollen–stigma interactions.

Higher levels of ectopic expression of Arabidopsis phytochelatin synthase do not lead to increased cadmium tolerance and accumulation

Two mulberry phytochelatin synthase genes confer zinc/cadmium tolerance and accumulation in transgenic Arabidopsis and tobacco

Chapter 11 - Bioaccumulation and detoxification of heavy metals: an insight into the mechanism

The mustard family – Brassicaceae – is well known as family of plants, metallophytes, which are able to accumulate wide range of heavy metals and metalloids, especially zinc and cadmium, but also nickel, thallium, chromium and selenium. Ecological importance of this process consists partially in plants themselves to survive negative environmental conditions. There are two basic different strategies, how to survive these conditions – accumulation of heavy metals in plants tissues with different intensity in individual cell types, but also organs, which is partially given by chemical composition of cell walls, and ability to synthesize special defensive – detoxification compounds rich on thiol groups – glutathione and phytochelatins, which are able to bind heavy metals and transport them to the “secure” cell compartment – vacuole. The second principle is based on ability to exclude heavy metals. Role of secondary metabolites rich on sulphur in detoxification of heavy metals is still discussed with unclear conclusions. Members of Brassicaceae family, especially genera Thlaspi and Brassica, are well-known hyperaccumulators of heavy metals with possible utilization in phytoremediation technologies. In this review chapter, mechanisms of cadmium uptake and transport and its deposition in various plant cells and tissues are discussed with respect with possible utilization in phytoremediation. In addition, role of special sulphur metabolites, which are typical for plants of Brassicaceae family – glucosinolates – in detoxification of heavy metals is discussed.

Growth-inhibitory and metal-binding proteins in Chlorella vulgaris exposed to cadmium or zinc

Abstract. Wibowo CNP, Azizah CKG, Risdian DA, Fathurrohman DTA, Tsurayya DA, Rizka DR, Renaldi DR, Saleh DA, Ridwan M, Setyawan AD. 2025. Review: Indigenous peatland bacteria and their role in heavy metal detoxification under extreme biogeochemical constraints. Cell Biol Dev 9: 113-128. Peatlands are among the most efficient natural carbon sinks on Earth, yet their unique physicochemical properties also render them vulnerable to heavy metal accumulation from mining, agriculture, and atmospheric deposition. High organic matter content, persistent acidity, and dynamic redox conditions position peatlands as extreme biogeochemical filters that both immobilize and intermittently remobilize metals, posing complex ecological and restoration challenges. This review synthesizes current knowledge on the role of indigenous peatland bacteria in heavy metal detoxification, emphasizing mechanistic pathways, ecological constraints, and implications for peatland management. We examine how peatland-specific conditions regulate microbial processes such as biosorption, redox transformation, intracellular sequestration, and extracellular polymeric substance production, emphasizing differences between boreal, temperate, and tropical peatlands. Heavy metal detoxification emerges from the integration of multiple mechanisms, such as biosorption, EPS-mediated binding, intracellular sequestration, bioprecipitation, redox transformation, and genetically regulated adaptive networks. The effectiveness of these mechanisms is highly context-dependent and tightly coupled to peatland carbon dynamics, influencing organic matter decomposition and methane fluxes. This review further examines why laboratory-validated microbial processes often fail under field conditions, highlighting the roles of environmental heterogeneity, hydrological fluctuation, scale effects, and ecological mismatch in limiting remediation success. Rather than technology- or strain-centric solutions, evidence supports remediation frameworks that prioritize indigenous microbial consortia, biostimulation, and hydrologically aligned in situ and ex situ pathways. Emerging technologies, including molecular tools, bioelectrochemical systems, and biosensors, are evaluated as enablers that support, but do not replace, ecological processes. By integrating microbial mechanisms with peatland biogeochemistry and restoration science, this review provides a conceptual framework for developing sustainable heavy metal remediation strategies that stabilize contaminants while preserving peatland carbon storage and ecosystem resilience.

Heavy metals are found in nature and are the main component of a variety of enzymes, transcription factors, and other proteins. Excessive level of heavy metal is considered as a pollutant agent. Soil-heavy metals cannot be degraded biologically; they can only be transformed to organic complexes. Remediation techniques have high costs and low efficiency, whereas an alternative technique, phytoremediation has low cost and is environmentally friendly. To stimulate phytoremediation, fast-growing plants with high metal uptake and rapid and high biomass are required. Alternatively, soil microorganisms such as fungi and bacteria are used in heavy metal detoxification. This chapter reviews some recent advances in effect and significance of fungi and rhizobacteria in heavy metal detoxification.

To date, numerous correlative studies have implicated metallothionein in the detoxification of heavy metals and in the regulation of metal distribution within an organism. In the pre sent study cadmium-binding proteins (metallothionein equivalents), cadmium acute toxicity, and cadmium distribution in tissues and subcellular fractions were compared in metalloth-ionein-l and -II deficient (MT−/−) mice and the parental strain carrying intact metalloth ionein genes (MT+/+) to determine if the absence of metallothionein altered any of these parameters. In an uninduced state, MT−/− mice expressed lower levels of cadmium-binding proteins relative to MT+/+ mice in several tissues. Administration of zinc enhanced the levels of cadmium-binding proteins in liver, small intestine, kidney, pancreas, and male sex organs, but not in cecum or brain of MT+/+ mice compared to zinc pretreated MT−/− mice. The cadmium LD50 was similar for MT−/−, MT+/+, and zinc-pretreated MT−/− mice (15-17To date, numerous correlative studies have implicated metallothionein in the detoxification of heavy metals and in the regulation of metal distribution within an organism. In the pre sent study cadmium-binding proteins (metallothionein equivalents), cadmium acute toxicity, and cadmium distribution in tissues and subcellular fractions were compared in metalloth-ionein-l and -II deficient (MT−/−) mice and the parental strain carrying intact metalloth ionein genes (MT+/+) to determine if the absence of metallothionein altered any of these parameters. In an uninduced state, MT−/− mice expressed lower levels of cadmium-binding proteins relative to MT+/+ mice in several tissues. Administration of zinc enhanced the levels of cadmium-binding proteins in liver, small intestine, kidney, pancreas, and male sex organs, but not in cecum or brain of MT+/+ mice compared to zinc pretreated MT−/− mice. The cadmium LD50 was similar for MT−/−, MT+/+, and zinc-pretreated MT−/− mice (15-17 μmol CdCl2/kg body weight delivered ip). However, zinc-pretreated MT+/+ mice had a cadmium LD50 of 58-63 μmol CdCIJkg body weight. Over two-thirds of cadmium was found in liver, cecum, small intestine, and kidney in both MT+/+ and MT−/− mice; therefore, metallothionein levels do not appear to play a major role in the tissue distribution of cad mium. However, after zinc pretreatment, MT+/+ mice accumulated more cadmium in the liver and less in other tissues, whereas the amount of cadmium in the liver was not altered by zinc pretreatment in MT−/− mice. In general, the cytosolic/particulate ratio of cadmium was significantly higher in tissues of noninduced MT+/+ mice relative to MT−/− mice. This difference was accentuated after zinc pretreatment. Together these results indi cate that basal levels of metallothionein do not protect from the acute toxicity of a single ip cadmium challenge. Furthermore, it does not appear that the cytosolic compartmental-ization of cadmium is correlated with reduced toxicity.