Heavy metal detoxification mechanisms in halophytes: an overview

Heavy metals are among the major pollutants from anthropogenic inputs that reach mangrove ecosystem by urban and agricultural runoff, industrial effluents, boating, mining and other processes. To minimize the detrimental effects of heavy metal exposure and their accumulations, plants in general have evolved biological detoxification mechanisms, which include avoidance or exclusion, excretion and accumulation. To protect the cellular components from oxidative damage by heavy metal contamination, biological systems have developed enzymatic and non-enzymatic antioxidant mechanisms. Another detoxification mechanisms produced in plants are osmoprotectants, which are the compatible solutes which maintain a favourable water potential gradient and protect cellular structures from toxic ions. Besides these mechanisms, another heavy metal detoxification system in plants involves the chelation of metals by metal binding molecules like metallothioneins (MTs) and phytochelatins (PCs). To limit the heavy metal toxicity from mangrove ecosystem, it was found that phytoremediation is a most useful technology where in plants are used to remove pollutants from the environment and it is considered as a comparatively new, low-cost and highly promising technology for the remediation of heavy metal. Rhizofiltration, phytovolatilization, phytoextraction and phytostabilization are the important phytoremediation techniques. Among these phytoextraction and phytostabilization are found highly important in the case of mangroves and are promising means of phytoremediation.     

Phytoextraction of toxic metals: a central role for glutathione

Phytoextraction has a promising potential as an environmentally friendly clean-up method for soils contaminated with toxic metals. To improve the development of efficient phytoextraction strategies, better knowledge regarding metal uptake, translocation and detoxification in planta is a prerequisite. This review highlights our current understanding on these mechanisms, and their impact on plant growth and health. Special attention is paid to the central role of glutathione (GSH) in this process. Because of the high affinity of metals to thiols and as a precursor for phytochelatins (PCs), GSH is an essential metal chelator. Being an important antioxidant, a direct link between metal detoxification and the oxidative challenge in plants growing on contaminated soils is observed, where GSH could be a key player. In addition, as redox couple, oxidized and reduced GSH transmits specific information, in this way tuning cellular signalling pathways under environmental stress conditions. Possible improvements of phytoextraction could be achieved by using transgenic plants or plant-associated microorganisms. Joined efforts should be made to cope with the challenges faced with phytoextraction in order to successfully implement this technique in the field.     

A new pathway for heavy metal detoxification in animals. Phytochelatin synthase is required for cadmium tolerance in Caenorhabditis elegans

Increasing emissions of heavy metals such as cadmium, mercury, and arsenic into the environment pose an acute problem for all organisms. Considerations of the biochemical basis of heavy metal detoxification in animals have focused exclusively on two classes of peptides, the thiol tripeptide, glutathione (GSH, gamma-Glu-Cys-Gly), and a diverse family of cysteine-rich low molecular weight proteins, the metallothioneins. Plants and some fungi, however, not only deploy GSH and metallothioneins for metal detoxification but also synthesize another class of heavy metal binding peptides termed phytochelatins (PCs) from GSH. Here we show that PC-mediated heavy metal detoxification is not restricted to plants and some fungi but extends to animals by demonstrating that the ce-pcs-1 gene of the nematode worm Caenorhabditis elegans encodes a functional PC synthase whose activity is critical for heavy metal tolerance in the intact organism.     

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.     

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。     

Effect of cadmium and calcium treatments on phytochelatin and glutathione levels in citrus plants

Industry residues, phosphate fertilisers and wastewater as a source of irrigation have considerably increased levels of heavy metals in the soil, mainly cadmium (Cd²⁺). To test the effects of a calcium (Ca²⁺) treatment on Cd²⁺ accumulation and plant tolerance to this heavy metal, plants of two citrus genotypes, Cleopatra mandarin (CM) and Carrizo citrange (CC), were watered with increasing concentrations of Cd²⁺, and phytochelatin (PC) and glutathione (GSH) content were measured. Both genotypes were able to synthesise PCs in response to heavy metal intoxication, although CM seems to be a better Cd²⁺ excluder than CC. However, data indicate that CC plants had a higher capacity for regenerating GSH than CM plants. In this context, the effects of Ca²⁺ treatment on Cd²⁺ accumulation, plant survival and PC, GSH and oxidised glutathione (GSSG) content were assessed. Data indicate that treatment with Ca²⁺ had two positive effects on citrus physiology: it reduced Cd⁺² uptake into roots and also increased GSH content (even in the absence of Cd²⁺). Overall, the data indicate that although Cd²⁺ exclusion is a powerful mechanism to avoid heavy metal build‐up into photosynthetic organs, the capacity to maintain optimum GSH levels to feed PC biosynthesis could also be an important factor in stress tolerance.     

Cadmium hyperaccumulation leads to an increase of glutathione rather than phytochelatins in the cadmium hyperaccumulator Sedum alfredii

Sedum alfredii has been reported to be a cadmium (Cd) hyperaccumulator. Phytochelatins (PCs) and other thiol (SH)-containing compounds have been proposed to play an important role in the detoxification and tolerance of some heavy metals, but it is not clear whether PCs are responsible for Cd hyperaccumulation and tolerance in S. alfredii. In this study, two geographically isolated populations of S. alfredii were studied: one population grew on an old Pb/Zn mine site, while the other on a non-mine site. The mine population of this species exhibited a stronger heavy metal tolerance than in the other population. Root-to-shoot transport of Cd was higher in population located at the mine site than at the non-mine site. Considerable amounts of Cd were accumulated in leaves and stems of mine plants, while most Cd was distributed in roots of non-mine plants. Non-protein SH in plant tissues of two populations were further investigated by a HPLC pre-column derivatization system. Upon exposure to Cd, no PCs were detected in all tissues of mine population, while an appreciable amount of glutathione (GSH) was observed in the descending order of stem>root>leaf. The concentrations of GSH consistently increased with the increase of exogenous Cd concentrations and time. On the contrary, Cd exposure strongly induced the production of PCs (mainly PC(2) and PC(3)) and GSH in plant tissues of non-mine population, and the concentrations of GSH showed an initial drop over the duration of 7-d exposure. The present results provided strong evidence that PCs are not involved in Cd transport, hyperaccumulation and tolerance in mine population of S. alfredii.     

高等植物重金属耐性与超积累特性及其分子机理研究

由于重金属污染日益严重, 重金属在土壤_植物系统中的行为引起了人们的高度重视。高等植物对重金 属的耐性与积累性, 已经成为污染生态学研究的热点。近年来, 由于分子生态学等学科的发展, 有关植物对重金属的解毒和耐性机理、重金属离子富集机制的研究取得了较大进展。高等植物对重金属的耐性和积累在种间和基因型之间存在很大差异。根系是重金 属等土壤污染物进入植物的门户。根系分泌物改变重金属的生物有效性和毒性, 并在植物吸收重金属的过程中发挥重要作用。土壤中的大部分重金属离子都是通过金属转运蛋白进入根细胞, 并在植物体内进一步转运至液泡贮存。在重金属胁迫条件下植物螯合肽 (PC) 的合成是植物对胁迫的一种适应性反应。耐性基因型合成较多的PC, 谷胱甘肽 (GSH) 是合成PC的前体, 重金属与PC螯合并转移至液泡中贮存, 从而达到解毒效果。金属硫蛋白 (MTs) 与PC一样, 可以与重金属离子螯合, 从而降低重金属离子的毒性。该文从分子水平上论述了根系分泌物、金属转运蛋白、MTs、PC、GSH在重金属耐性及超积累性中的作用, 评述了近 10年来这方面的研究进展, 并在此基础上提出存在的问题和今后研究的重点。     

Function of phytochelatin synthase in catabolism of glutathione-conjugates

Detoxification of xenobiotic compounds and heavy metals is a pivotal capacity of organisms, in which glutathione (GSH) plays an important role. In plants, electrophilic herbicides are conjugated to the thiol group of GSH, and heavy metal ions form complexes as thiolates with GSH-derived phytochelatins (PCs). In both detoxification processes of plants, phytochelatin synthase (PCS) emerges as a key player. The enzyme is activated by heavy metal ions and catalyzes PC formation from GSH by transferring glutamylcysteinyl residues (gamma-EC) onto GSH. In this study with Arabidopsis, we show that PCS plays a role in the plant-specific catabolism of glutathione conjugates (GS-conjugates). In contrast to animals, breakdown of GS-conjugates in plants can be initiated by cleavage of the carboxyterminal glycine residue that leads to the generation of the corresponding gamma-EC-conjugate. We used the xenobiotic bimane in order to follow GS-conjugate turnover. Functional knockout of the two PCS of Arabidopsis, AtPCS1 and AtPCS2, revealed that AtPCS1 provides a major activity responsible for conversion of the fluorescent bimane-GS-conjugate (GS-bimane) into gamma-EC-bimane. AtPCS1 deficiency resulted in a gamma-EC-bimane deficiency. Transfection of PCS-deficient cells with AtPCS1 recovered gamma-EC-bimane levels. The level of the gamma-EC-bimane conjugate was enhanced several-fold in the presence of Cd2+ ions in the wild type, but not in the PCS-deficient double mutant, consistent with a PCS-catalyzed GS-conjugate turnover. Thus AtPCS1 has two cellular functions: mediating both heavy metal tolerance and GS-conjugate degradation.     

Sulfur assimilation and glutathione metabolism under cadmium stress in yeast, protists and plants

Glutathione (gamma-glu-cys-gly; GSH) is usually present at high concentrations in most living cells, being the major reservoir of non-protein reduced sulfur. Because of its unique redox and nucleophilic properties, GSH serves in bio-reductive reactions as an important line of defense against reactive oxygen species, xenobiotics and heavy metals. GSH is synthesized from its constituent amino acids by two ATP-dependent reactions catalyzed by gamma-glutamylcysteine synthetase and glutathione synthetase. In yeast, these enzymes are found in the cytosol, whereas in plants they are located in the cytosol and chloroplast. In protists, their location is not well established. In turn, the sulfur assimilation pathway, which leads to cysteine biosynthesis, involves high and low affinity sulfate transporters, and the enzymes ATP sulfurylase, APS kinase, PAPS reductase or APS reductase, sulfite reductase, serine acetyl transferase, O-acetylserine/O-acetylhomoserine sulfhydrylase and, in some organisms, also cystathionine beta-synthase and cystathionine gamma-lyase. The biochemical and genetic regulation of these pathways is affected by oxidative stress, sulfur deficiency and heavy metal exposure. Cells cope with heavy metal stress using different mechanisms, such as complexation and compartmentation. One of these mechanisms in some yeast, plants and protists is the enhanced synthesis of the heavy metal-chelating molecules GSH and phytochelatins, which are formed from GSH by phytochelatin synthase (PCS) in a heavy metal-dependent reaction; Cd(2+) is the most potent activator of PCS. In this work, we review the biochemical and genetic mechanisms involved in the regulation of sulfate assimilation-reduction and GSH metabolism when yeast, plants and protists are challenged by Cd(2+).     

Biochemical changes and adaptive strategies of plants under heavy metal stress

Heavy metal contamination of soil, aqueous waste stream and ground water causes major environmental and human health problems. Heavy metals are major environmental pollutants when they are present in high concentration in soil and show potential toxic effects on growth and development in plants. Due to unabated, indiscriminate and uncontrolled discharge of hazardous chemicals including heavy metals into the environment, plant continuously have to face various environmental constraints. In plants, seed germination is the first exchange interface with the surrounding medium and has been considered as highly sensitive to environmental changes. One of the crucial events during seed germination entails mobilization of seed reserves which is indispensable for the growth of embryonic axis. But, metabolic alterations by heavy metal exposure are known to depress the mobilization and utilization of reserve food by affecting the activity of hydrolytic enzymes. Some plants possess a range of potential mechanisms that may be involved in the detoxification of heavy metals by which they manage to survive under metal stress. High tolerance to heavy metal toxicity could rely either on reduced uptake or increase planned internal sequestration which is manifested by an interaction between a genotype and its environment. Such mechanism involves the binding of heavy metals to cell wall, immobilization, exclusion of the plasma membrane, efflux of these toxic metal ions, reduction of heavy metal transport, compartmentalization and metal chelation by tonoplast located transporters and expression of more general stress response mechanisms such as stress proteins. It is important to understand the toxicity response of plant to heavy metals so that we can utilize appropriate plant species in the rehabilitation of contaminated areas. Therefore, in the present review attempts have been made to evaluate the effects of increasing level of heavy metal in soils on the key behavior of hydrolytic and nitrogen assimilation enzymes. Additionally, it also provides a broad overview of the strategies adopted by plants against heavy metal stress.     

Cadmium lets increase the glutathione pool in bryophytes

Glutathione (GSH) plays an important role in protecting plants from environmental stresses like oxidative stress and xenobiotics. Glutathione-derived peptides are involved in heavy metal detoxification in plants and fungi. Terrestrial and aquatic bryophytes were investigated for their biochemical response to heavy metals. The GSH pool increased significantly in the first two days after supply of 100 μmol/L Cd(II). PCs were not detected. Cd(II) also induced the enhancement of the GSH pool in the water moss Fontinalis antipyretica. Cysteine and γ-glutamyl-cysteine also increased during Cd(II) treatment, but remained on a lower level. Uptake experiments with Cd(II) showed a fast regulation of equilibrium between the Cd(II) content of the medium and the plant surface, followed by a slow migration of Cd(II) to intracellular sites. The main storage compartment of heavy metals in Fontinalis are the vacuoles, where they are precipitated as phosphates. In the cytoplasm, the S-content increased during Cd(II) exposition. EEL-spectra indicate that in the cytoplasm, Cd(II) is chelated by SH-groups. All findings support the idea that in the investigated moss species, GSH plays an essential role in heavy metal detoxification during the transport of the metals through the cytoplasm.     

Identification of high levels of phytochelatins, glutathione and cadmium in the phloem sap of Brassica napus. A role for thiol-peptides in the long-distance transport of cadmium and the effect of cadmium on iron translocation

Phytochelatins (PCs) are glutathione-derived peptides that function in heavy metal detoxification in plants and certain fungi. Recent research in Arabidopsis has shown that PCs undergo long-distance transport between roots and shoots. However, it remains unknown which tissues or vascular systems, xylem or phloem, mediate PC translocation and whether PC transport contributes to physiologically relevant long-distance transport of cadmium (Cd) between shoots and roots. To address these questions, xylem and phloem sap were obtained from Brassica napus to quantitatively analyze which thiol species are present in response to Cd exposure. High levels of PCs were identified in the phloem sap within 24 h of Cd exposure using combined mass spectrometry and fluorescence HPLC analyses. Unexpectedly, the concentration of Cd was more than four-fold higher in phloem sap compared to xylem sap. Cadmium exposure dramatically decreased iron levels in xylem and phloem sap whereas other essential heavy metals such as zinc and manganese remained unchanged. Data suggest that Cd inhibits vascular loading of iron but not nicotianamine. The high ratios [PCs]/[Cd] and [glutathione]/[Cd] in the phloem sap suggest that PCs and glutathione (GSH) can function as long-distance carriers of Cd. In contrast, only traces of PCs were detected in xylem sap. Our results suggest that, in addition to directional xylem Cd transport, the phloem is a major vascular system for long-distance source to sink transport of Cd as PC–Cd and glutathione–Cd complexes.     

Enhanced toxic metal accumulation in engineered bacterial cells expressing Arabidopsis thaliana phytochelatin synthase

Phytochelatins (PCs) are metal-binding cysteine-rich peptides, enzymatically synthesized in plants and yeasts from glutathione in response to heavy metal stress by PC synthase (EC 2.3.2.15). In an attempt to increase the ability of bacterial cells to accumulate heavy metals, the Arabidopsis thaliana gene encoding PC synthase (AtPCS) was expressed in Escherichia coli. A marked accumulation of PCs was observed in vivo together with a decrease in the glutathione cellular content. When bacterial cells expressing AtPCS were placed in the presence of heavy metals such as cadmium or the metalloid arsenic, cellular metal contents were increased 20- and 50-fold, respectively. We discuss the possibility of using genes of the PC biosynthetic pathway to design bacterial strains or higher plants with increased abilities to accumulate toxic metals, and also arsenic, for use in bioremediation and/or phytoremediation processes.     

PRODUCTION OF PHYTOCHELATINS AND GLUTATHIONE BY MARINE PHYTOPLANKTON IN RESPONSE TO METAL STRESS1

Phytoplankton deal with metal toxicity using a variety of biochemical strategies. One of the strategies involves glutathione (GSH) and phytochelatins (PCs), which are metal‐binding thiol peptides produced by eukaryotes and these compounds have been related to several intracellular functions, including metal detoxification, homeostasis, metal resistance and protection against oxidative stress. This paper assesses our state of knowledge on the production of PCs and GSH by marine phytoplankton in laboratory and field conditions and the possible applications of PCs for environmental purposes. Good relationships have been observed between metal exposure and PC production in phytoplankton in the laboratory with Cd, Pb, and Zn showing the greatest efficacy, thereby indicating that PCs have a potential for application as a biomarker. Fewer studies on PC distributions in particulate material have been undertaken in the field. These studies show that free Cu has a strong relationship with the levels of PC in the particulate material. The reason for this could be because Cu is a common contaminant in coastal waters. However it could also be due to the lack of measurements of other metals and their speciation. GSH shows a more complex relationship to metal levels both in the laboratory and in the field. This is most likely due to its multifunctionality. However, there is evidence that phytoplankton act as an important source of dissolved GSH in marine waters, which may form part of the strong organic ligands that control metal speciation, and hence metal toxicity.     

Mechanisms of cadmium detoxification in cattail (Typha angustifolia L.)

Cadmium (Cd) is a widespread heavy metal pollutant and environmental and human health hazard, which may be partially resolved using green and cost-effective phytoremediation techniques. However, the efficiency of phytoremediation is often limited by the small biomass of Cd-hyperaccumulator plants. Although cattail (Typha angustifolia L.) is tolerant of heavy metals and has a high biomass, there is little information available on its detoxification mechanisms for heavy metals, especially Cd. In the present study we investigated the tolerance of cattail to Cd and mechanisms involved in its Cd detoxification. Our results show that: (a) cattail is tolerant of Cd; (b) the root Casparian band, cell wall, vacuole, glutathione (GSH), and glutathione peroxidase (GPX) play important roles in Cd detoxification; and (c) mechanisms of Cd detoxification differ in leaf cell cytoplasm (mainly a GSH-related antioxidant defense system) and root cell cytoplasm (mainly a GSH-related chelation system). In summary, cattail possesses multiple detoxification mechanisms for Cd and is a promising species for phytoremediation of Cd-polluted environments.     

Enhanced Toxic Metal Accumulation in Engineered Bacterial Cells Expressing Arabidopsis thaliana Phytochelatin Synthase

Phytochelatins (PCs) are metal-binding cysteine-rich peptides, enzymatically synthesized in plants and yeasts from glutathione in response to heavy metal stress by PC synthase (EC 2.3.2.15). In an attempt to increase the ability of bacterial cells to accumulate heavy metals, the Arabidopsis thaliana gene encoding PC synthase (AtPCS) was expressed in Escherichia coli. A marked accumulation of PCs was observed in vivo together with a decrease in the glutathione cellular content. When bacterial cells expressing AtPCS were placed in the presence of heavy metals such as cadmium or the metalloid arsenic, cellular metal contents were increased 20- and 50-fold, respectively. We discuss the possibility of using genes of the PC biosynthetic pathway to design bacterial strains or higher plants with increased abilities to accumulate toxic metals, and also arsenic, for use in bioremediation and/or phytoremediation processes.     

铜、镉胁迫下外源NO介导的番茄解毒途径

一氧化氮(NO)作为信号分子,在抵御重金属胁迫中起重要作用,但对不同离子胁迫下的解毒机制尚缺乏研究.本研究采用营养液培养法,研究了铜(Cu)、镉(Cd)单一或复合胁迫下,番茄幼苗对Cu、Cd的吸收转运特性及对外源NO的响应机制.结果表明: 50 μmol·L^-1的Cu²⁺、Cd²⁺均显著抑制番茄植株的生长,其中Cd胁迫对生长的抑制效应远高于Cu胁迫.Cu、Cd单一或复合胁迫均使番茄根系Cu、Cd含量显著升高,但根系对Cu、Cd吸收存在严格选择性.根细胞对必需元素Cu表现出“奢侈吸收”的现象,而对毒性较强的Cd则吸收相对较少,胞内Cd浓度仅为Cu的1/10左右.外源NO处理可不同程度地缓解Cu、Cd胁迫,其中缓解Cd胁迫的效能更强.番茄对被动进入细胞的Cu、Cd具有相似的解毒机制:一方面,Cu、Cd胁迫诱导细胞质中产生谷胱甘肽(GSH)、植物螯合肽(PCs)和金属硫蛋白(MTs),络合过多的Cu、Cd离子,降低其生物毒性;另一方面,过多的Cu、Cd离子或螯合物被转运至液泡区隔化.外源NO通过调控GSH-GSSG(氧化型谷胱甘肽)氧化还原状态及GSH-PCs代谢方向的改变,促进Cu、Cd离子转运至液泡区隔化来缓解胁迫抑制;NO还可诱导植株叶片或根系表达更多的金属硫蛋白、GSH和PCs,而且上述响应普遍存在叠加效应.这可能是NO介导番茄对Cu、Cd胁迫的另一主要解毒途径.