Heavy metal detoxification in higher plants - a review

A set of heavy-metal-complexing peptides was isolated from plants and plant suspension cultures. The structure of these peptides was established as (γ-glutamic acid-cysteine)_n-glycine (n=2–11) [(γ-Glu-Cys)_n-Gly]. These peptides appear upon induction of plants with metals of the transition and main groups (Ib-Va, Z=29−83) of the periodic table of elements. These peptides, called phytochelatins (PC), are induced in all autotrophic plants so far analyzed, as well as in select fungi. Some species of the order Fabales and the family Poaceae synthesize aberrant PC that contain, at their C-terminal end, either β-alanine, serine or glutamic acid. For this group of peptides the name iso-PC is proposed. The biosynthesis of PC proceeds by metal activation of a constitutive enzyme that uses glutathione (GSH) as a substrate; this enzyme is a γ-glutamylcysteine dipeptidyl transpeptidase which was given the trivial name PC synthase. It catalyzes the following reaction: γ-Glu-Cys-Gly+(γ-Glu-Cys)_n-Gly→(γ-Glu-Cys)_n+1-Gly+Gly. The plant vacuole is the transient storage compartment for these peptides. They probably dissociate, and the metal-free peptide is subsequently degraded. Sequestration of heavy metals by PC confers protection for heavy-metal-sensitive enzymes. The isolation of a Cd²⁺-sensitive cadl mutant of Arabidopsis thaliana, that is deficient in PC synthase, demonstrates conclusively the importance of PC for heavy metal tolerance. In spite of the fact that nucleic acid sequences and proteins are found in higher plants that have distant homology to animal metallothioneins, there is absolutely no experimental evidence that these ‘plant metallothioneins’ are involved in the detoxification of heavy metals. PC synthase will be an interesting target for biotechnological modification of heavy metal tolerance in higher plants.     

Cd - tolerance of maize, rye and wheat seedlings

The effect of cadmium on growth parameters of seedlings of maize, rye and wheat as well as the role of phytochelatins in Cd detoxication in these species were studied. Cadmium was found to inhibit root growth and decrease fresh weight and water content in roots and shoots of the studied plants. Although a considerably lower Cd accumulation was shown in maize seedlings than in other species, they were characterized by the highest sensitivity to cadmium. Among γ-Glu-Cys peptides synthetized by plant species, phytochelatins — glutathione derivatives predominated. In maize they were synthetized in amounts sufficient for binding the total pool of the metal taken up, and the detoxication mechanism was localized in their roots. Larger amounts of cadmium were accumulated in roots of wheat and rye, but the quantity of the formed γ-Glu-Cys peptides seems insufficient for detoxication of the metal.     

Effect of glutathione on phytochelatin synthesis in tomato cells

Growth of cell suspension cultures of tomato, Lycopersicon esculentum Mill. cv VFNT-Cherry, in the presence of cadmium is inhibited by buthionine sulfoximine, an inhibitor of glutathione synthesis. Cell growth and phytochelatin synthesis are restored to cells treated with buthionine sulfoximine by the addition of glutathione to the medium. Glutathione stimulates the accumulation of phytochelatins in cadmium treated cells, indicating that availability of glutathione can limit synthesis of these peptides. Exogenous glutathione causes a disproportionate increase in the level of smaller phytochelatins, notably [γ-Glu-Cys]₂-Gly. In the presence of buthionine sulfoximine and glutathione, phytochelatins that are produced upon exposure to cadmium incorporate little [³⁵S]cysteine, indicating that these peptides are probably not synthesized by sequential addition of cysteine and glutamate to glutathione.     

Cadmium stress tolerance in crop plants: Probing the role of sulfur

Plants can''t move away and are therefore continuously confronted with unfavorable environmental conditions (such as soil salinity, drought, heat, cold, flooding and heavy metal contamination). Among heavy metals, cadmium (Cd) is a non-essential and toxic metal, rapidly taken up by roots and accumulated in various plant tissues which hamper the crop growth and productivity worldwide. Plants employ various strategies to counteract the inhibitory effect of Cd, among which nutrient management is one of a possible way to overcome Cd toxicity. Sulfur (S) uptake and assimilation are crucial for determining crop yield and resistance to Cd stress. Cd affects S assimilation pathway which leads to the activation of pathway responsible for the synthesis of cysteine (Cys), a precursor of glutathione (GSH) biosynthesis. GSH, a non-protein thiol acts as an important antioxidant in mitigating Cd-induced oxidative stress. It also plays an important role in phytochelatins (PCs) synthesis, which has a proven role in Cd detoxification. Therefore, S assimilation is considered a crucial step for plant survival under Cd stress. The aim of this review is to discuss the regulatory mechanism of S uptake and assimilation, GSH and PC synthesis for Cd stress tolerance in crop plants.Key words: cadmium, cysteine, glutathione, phytochelatins, stress tolerance, sulfur     

植物螯合肽的转运与功能研究进展

植物螯合肽（phytochelatins,PCs）是由植物螯合肽合酶催化谷胱甘肽合成的一类生物小分子,结构式为（γ-Glu-Cys）n-Gly（n=2-11）,在真菌和高等植物耐受重金属胁迫机制中具有重要作用。近年来,人们在Pc介导重金属脱毒害的分子机理研究上取得了重要进展,发JLSpHMT1和SpABC2是PC在裂殖酵母中介导重金属液泡区室化的主要转运蛋白,鉴定了拟南芥液泡膜PC转运蛋AtABCC1和AtABCC2。此外,PCs也可能在超积累植物细胞内对重金属脱毒害具有重要功能。     

Phytochelatin synthase: of a protease a peptide polymerase made

Of the mechanisms known to protect vascular plants and some algae, fungi and invertebrates from the toxic effects of non-essential heavy metals such as As, Cd or Hg, one of the most sophisticated is the enzyme-catalyzed synthesis of phytochelatins (PCs). PCs, (γ-Glu-Cys)(n) Gly polymers, which serve as high-affinity, thiol-rich cellular chelators and contribute to the detoxification of heavy metal ions, are derived from glutathione (GSH; γ-Glu-Cys-Gly) and related thiols in a reaction catalyzed by phytochelatin synthases (PC synthases, EC 2.3.2.15). Using the enzyme from Arabidopsis thaliana (AtPCS1) as a model, the reasoning and experiments behind the conclusion that PC synthases are novel papain-like Cys protease superfamily members are presented. The status of S-substituted GSH derivatives as generic PC synthase substrates and the sufficiency of the N-terminal domain of the enzyme from eukaryotic and its half-size equivalents from prokaryotic sources, for net PC synthesis and deglycylation of GSH and its derivatives, respectively, are emphasized. The question of the common need or needs met by PC synthases and their homologs is discussed. Of the schemes proposed to account for the combined protease and peptide polymerase capabilities of the eukaryotic enzymes vs the limited protease capabilities of the prokaryotic enzymes, two that will be considered are the storage and homeostasis of essential heavy metals in eukaryotes and the metabolism of S-substituted GSH derivatives in both eukaryotes and prokaryotes.     

植物螯合肽合成酶的研究进展

植物螯合肽（phytochelatins,PCs）在植物解除重金属的毒性方面具有重要作用,其结构为（γ-Glu—Cys）n-Gly（n=2—11）,它不是基因的编码产物,而是在植物螯合肽合成酶（phytochelatin synthase,PCS）的催化下以谷胱甘肽（glutathione,GSH）为底物合成的。PCS能够被金属离子激活,高度保守的N-端是催化结构域,而其C-端则是多变的。本文就PCS的结构,功能与催化机制以及PCS的最新研究进行了介绍。     

植物对重金属耐性的分子生态机理

植物适应重金属元素胁迫的机制包括阻止和控制重金属的吸收、体内螯合解毒、体内区室化分隔以及代谢平衡等。近年来,随着分子生物学技术在生态学研究中的深入应用,控制这些过程的分子生态机理逐渐被揭示出来。菌根、根系分泌物以及细胞膜是控制重金属进入植物根系细胞的主要生理单元。外生菌根能显著提高寄主植物的重金属耐性,根系分泌物通过改变根际pH、改变金属物质的氧化还原状态和形成络合物等机理减少植物对重金属的吸收。目前,控制菌根和根系分泌物重金属抗性的分子生态机理还不清楚。但细胞膜跨膜转运器已得到深入研究,相关金属离子转运器被鉴定和分离,一些控制基因如铁锌控制运转相关蛋白(ZIP)类、自然抵抗相关巨噬细胞蛋白(Nramp)类、P_1B-type ATPase类基因已被发现和克隆。金属硫蛋白(MTs)、植物螯合素(PCs)、有机酸及氨基酸等是植物体内主要的螯合物质,它们通过螯合作用固定金属离子,降低其生物毒性或改变其移动性。与MTs合成相关的MT-like基因已经被克隆,PCs合成必需的植物螯合素合酶(PCS), 即γ-Glu-Cys二肽转肽酶(γ-ECS) 的编码基因已经被克隆,控制麦根酸合成的氨基酸尼克烟酰胺(NA)在重金属耐性中的作用和分子机理也被揭示出来。ATP 结合转运器(ABC)和阳离子扩散促进器(CDF) 是植物体内两种主要膜转运器,通过它们和其它跨膜方式,重金属被分隔贮藏于液泡内。控制这些蛋白转运器合成的基因也已经被克隆,在植物中的表达证实其与重金属的体内运输和平衡有关。热休克蛋白(HSP)等蛋白类物质的产生是一种重要的体内平衡机制,其分子机理有待进一步研究。重金属耐性植物在这些环节产生了相关响应基因或功能蛋白质,分子克隆和转基因技术又使它们在污染治理上得到了初步的应用。     

Current Research on Heavy Metal Binding Complexes of Plants

Heavy metal binding complexes found in the cytoplasm of plant cells are widely reported to be responsible fox metal resistance. It is believed that these metal binding complexes may play an important role in the accumulation of heavy metal and preventing them from entering the ptant metabolic pathways. This review summarizes information on purification, characterization and properties of these molecules. In view of their inducibility, low molecular weight, specific optical characteristics (high absorption at 254nm and low absorption at 280 nm), high cysteine content and high capacity for binding heavy metal, these complexes were once proposed to be metallothionein-like molecules. However differing from metallothionein of animal system, heavy metal binding complexes in plants are comprised mainly of three amino acids with a unique structure of (γ-Glu-Cys)n-Gly ((γ-EC)nG) (n=2—11). With the presence of γ-carboxyl group peptide linkage, the complexes are thought not to be synthesized via mRNA but the product of biosynthetic pathway using glutatkione as precursor.     

Algae sequester heavy metals via synthesis of phytochelatin complexes

A Cd-binding complex was isolated from Chlorella fusca and has been shown to be composed of phytochelating peptides, (-Glu-Cys) _n -Gly, n=2–5. Members of six of the ten classes of Phycophyta revealed phytochelatin synthesis after exposure to cadmium ions. Phytochelatin was also induced by ions of lead, zinc, silver, copper and mercury. These experiments uneqiovocally demonstrated that algae sequester heavy metals by an identical mechanism as higher plants, namely via complexation to phytochelatins.     

Three families of thiol peptides are induced by cadmium in maize

Phytochelatins ((γGluCys)_nGly) are synthesized from glutathione by plants exposed to metals like Cd²⁺, Cu²⁺ and Zn²⁺. An intracellular complex formed by phytochelatins with Cd²⁺ and sulfide is thought to detoxify the metal possibly by sequestration in the vacuole. It was found that maize seedlings exposed to Cd²⁺ produced phytochelatins and two additional families of cysteina-containing peptides, (γGluCys)_n and (γGluCys)_nGlu. All thiol peptides with n = 2 and 3 were purified and their structure characterized by tandem mass spectrometry. For maize plants exposed to Cd²⁺ for 7 days, phytochelatins were synthesized preferentially in the first 24 h whereas the amounts of (γGluCys)_n and (γGluCys)_nGlu were the highest thereafter. This was probably due to an initial large pool of glutathione available in control plants compared with a dearth of γGluCys and no detectable γGluCysGlu. The (γGluCys)_nGlu peptides were induced exclusively by Cd²⁺ as they were below the detection limit in control seedlings that contained low amounts of phytochelatins and (γGluCys)_n. Since the Cys moiety of the peptides is essential for binding Cd²⁺, a role for accumulated (γGluCys)_n and (γGluCys)_nGlu in detoxifying Cd²⁺ in plants must be considered.     

The significance of amino acids and amino acid-derived molecules in plant responses and adaptation to heavy metal stress

Plants exposed to heavy metals accumulate an array of metabolites, some to high millimolar concentrations. This review deals with N-containing metabolites frequently preferentially synthesized under heavy metal stress such as Cd, Cu, Ni, and Zn. Special focus is given to proline, but certain other amino acids and oligopeptides, as well as betaine, polyamines, and nicotianamine are also addressed. Particularly for proline a large body of data suggests significant beneficial functions under metal stress. In general, the molecules have three major functions, namely metal binding, antioxidant defence, and signalling. Strong correlative and mechanistic experimental evidence, including work with transgenic plants and algae, has been provided that indicates the involvement of metal-induced proline in metal stress defence. Histidine, other amino acids and particularly phytochelatins and glutathione play a role in metal binding, while polyamines function as signalling molecules and antioxidants. Their accumulation needs to be considered as active response and not as consequence of metabolic dys-regulation.     

Cadmium-Responsive Thiols in the Ectomycorrhizal Fungus Paxillus involutus

Molecular and cellular mechanisms underlying the sustained metal tolerance of ectomycorrhizal fungi are largely unknown. Some of the main mechanisms involved in metal detoxification appear to involve the chelation of metal ions in the cytosol with thiol-containing compounds, such as glutathione, phytochelatins, or metallothioneins. We used an improved high-performance liquid chromatography method for the simultaneous measurement of thiol-containing compounds from cysteine and its derivatives (γ-glutamylcysteine, glutathione) to higher-molecular-mass compounds (phytochelatins). We found that glutathione and γ-glutamylcysteine contents increased when the ectomycorrhizal fungus Paxillus involutus was exposed to cadmium. An additional compound with a 3-kDa molecular mass, most probably related to a metallothionein, increased drastically in mycelia exposed to cadmium. The relative lack of phytochelatins and the presence of a putative metallothionein suggest that ectomycorrhizal fungi may use a different means to tolerate heavy metals, such as Cd, than do their plant hosts.     

Cadmium-responsive thiols in the ectomycorrhizal fungus Paxillus involutus

Molecular and cellular mechanisms underlying the sustained metal tolerance of ectomycorrhizal fungi are largely unknown. Some of the main mechanisms involved in metal detoxification appear to involve the chelation of metal ions in the cytosol with thiol-containing compounds, such as glutathione, phytochelatins, or metallothioneins. We used an improved high-performance liquid chromatography method for the simultaneous measurement of thiol-containing compounds from cysteine and its derivatives (gamma-glutamylcysteine, glutathione) to higher-molecular-mass compounds (phytochelatins). We found that glutathione and gamma-glutamylcysteine contents increased when the ectomycorrhizal fungus Paxillus involutus was exposed to cadmium. An additional compound with a 3-kDa molecular mass, most probably related to a metallothionein, increased drastically in mycelia exposed to cadmium. The relative lack of phytochelatins and the presence of a putative metallothionein suggest that ectomycorrhizal fungi may use a different means to tolerate heavy metals, such as Cd, than do their plant hosts.     

植物的硫同化及其相关酶活性在镉胁迫下的调节

植物对土壤中硫的利用包括根系对硫酸盐的吸收、转运、同化、分配等过程,也是由一系列酶和蛋白质参与和调节的代谢过程。近年来的研究表明,在植物体内,硫同化与植物对镉等重金属元素的胁迫反应机制有着密切关系。镉胁迫能调节植物对硫酸盐的吸收、转运、同化,以及半胱氨酸、谷胱甘肽（glutathione,GSH）和植物螯合肽（Dhytochelatins,pc）的合成。植物在镉胁迫下通过多种调节机制,增强对硫酸盐的吸收和还原,迅速合成半胱氨酸和谷胱甘肽等代谢物,从而合成足够的PC,以满足植物生理的需要。     

Insights into the Roles of Melatonin in Alleviating Heavy Metal Toxicity in Crop Plants

Alleviating heavy metal pollution in farmland soil, and heavy metal toxicity in plants is the focus of global agricultural environmental research. Melatonin is a kind of indoleamine compound that wide exists in organisms; it is currently known as an endogenous free radical scavenger with the strongest antioxidant effect. As a new plant growth regulator and signaling molecule, melatonin plays an important role in plant resistance to biotic or abiotic stress. Recent studies indicate that melatonin can effectively alleviate heavy metal toxicity in crop plants, which provides a new strategy to minimize heavy metal pollution in crop plants. This study summarizes the research progress on the role of melatonin in alleviating heavy metal toxicity in crop plants and the related physiological and ecological mechanisms such as reducing the concentration of heavy metals in the rhizosphere, fixing and regionally isolating of heavy metals, maintaining the mineral element balance, enhancing the antioxidant defense system and interacting with hormonal signaling. Furthermore, future prospects for the mechanism of melatonin in regulating heavy metal toxicity, the pathway regulating synthesis and catabolism, and the interaction mechanism of melatonin signaling and other phytohormones are presented in this paper, with the goal of providing a theoretical basis for controlling heavy metal ion accumulation in crop plants grown in contaminated soil.     

Cadmium toxicity in crop plants and its alleviation by arbuscular mycorrhizal (AM) fungi: An overview

Cadmium (Cd), a toxic metal released into agricultural settings induces numerous changes in plant growth and physiology. The main known mechanisms of Cd toxicity include its affinity for sulfhydryl groups in proteins and its ability to replace some essential metals in active sites of enzymes, thus causing inhibition of enzyme activities and protein denaturation. This article reviews detrimental effects of Cd toxicity on the functional biology of plants and summarizes the mechanisms that are activated by plants to prevent the absorption or to detoxify Cd ions such as synthesis of antioxidants, osmolytes, phytochelatins, metallothioneins, etc. Arbuscular mycorrhizal (AM) fungi are reported to be present on the roots of plants growing in metal-contaminated soils and play an important role in metal tolerance. Through mycorrhizal symbiosis, heavy metals are immobilized in the rhizosphere through precipitation in the soil matrix, adsorption onto the root surface or accumulation within roots, and compartmentalized in aboveground parts of the plant. This article unfolds the potential role of AM fungi in enhancing Cd tolerance of plants.     

Molecular biology of metal tolerances of plants

Abstract. The review discusses some of the important aspects of the molecular biology of metal tolerances in animals, fungi and plants. First, results of classical ecological and genetical studies are briefly outlined. The evidence for the occurrence and properties of metal-binding proteins (metallothioneins) and peptides (phytochelatins) in fungi and plants is described. It is concluded that at present there is no firm evidence to suggest that a protein homologous with the metallothioneins of animals and fungi occurs in plants. The discovery of phytochelatins, γ-glutarnyl peptides, containing only glutamic acid, cysteine and glycine, in plants is described and evidence for their role in heavy metal tolerance is assessed. The difference between sulphur metabolism in animals and plants and its relationship to heavy metal tolerances is discussed in terms of the occurrences of metallothioneins in animals and phytochelalins in plants. Future prospects for research in this area are outlined in terms of identification of plant genes coding for metallothioneins and for the enzymes involved in the synthesis of phytochelatins.     

Heavy metals toxicity in plants: An overview on the role of glutathione and phytochelatins in heavy metal stress tolerance of plants

Plants experience oxidative stress upon exposure to heavy metals that leads to cellular damage. In addition, plants accumulate metal ions that disturb cellular ionic homeostasis. To minimize the detrimental effects of heavy metal exposure and their accumulation, plants have evolved detoxification mechanisms. Such mechanisms are mainly based on chelation and subcellular compartmentalization. Chelation of heavy metals is a ubiquitous detoxification strategy described in wide variety of plants. A principal class of heavy metal chelator known in plants is phytochelatins (PCs), a family of Cys-rich peptides. PCs are synthesized non-translationally from reduced glutathione (GSH) in a transpeptidation reaction catalyzed by the enzyme phytochelatin synthase (PCS). Therefore, availability of glutathione is very essential for PCs synthesis in plants at least during their exposure to heavy metals. Here, I reviewed on effect of heavy metals exposure to plants and role of GSH and PCs in heavy metal stress tolerance. Further, genetic manipulations of GSH and PCs levels that help plants to ameliorate toxic effects of heavy metals have been presented.