重金属镉(Cd)在植物体内的转运途径及其调控机制

重金属镉(Cd)的毒害效应与其由土壤向植物地上部分运输有关,揭示Cd~(2+)转运途径及其调控机制可为提高植物抗镉性以及镉污染的植物修复提供依据。对Cd~(2+)在植物体内的转运途径,特别是限制Cd~(2+)移动的细胞结构和分子调控机制研究进展进行了回顾。Cd~(2+)通过共质体和质外体途径穿过根部皮层进入木质部的过程中,大部分在皮层细胞间沉积,少部分抵达中柱后转移到地上部分。为了免受Cd~(2+)的危害,植物体产生了多种限制Cd~(2+)吸收和转移的生理生化机制:1)环绕在内皮层径向壁和横向壁上的凯氏带阻止Cd~(2+)以质外体途径进入木质部;2)螯合剂与进入根的Cd~(2+)螯合形成稳定化合物并区隔在液泡中;3)通过H+/Cd~(2+)离子通道等将Cd~(2+)逆向转运出根部。植物共质体和质外体途径转运重金属镉的能力以及两条途径的串扰尚待进一步明晰和阐明。     

Responses of two kidney bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis>) cultivars to the combined stress of sulfur deficiency and cadmium toxicity

Plants suffer from combined stress of sulfur deficiency and cadmium toxicity in some agricultural lands. However, little is known about the reaction in plants, such as responses in antioxidant enzymes and non-protein thiol compounds, to such combined stress. Therefore, in this study, four treatments, S-sufficiency (T_S?Cd), S-deficiency (T_?S?Cd), Cd stress (T_S+Cd) and combined stress of S-deficiency and Cd stress (T_?S+Cd), were set up to investigate (1) the effects of sulfur deficiency or sulfur sufficiency on Cd toxicity to kidney bean cultivar seedlings and the related mechanisms, and (2) the responses of two kidney bean cultivars to combined stress of S-deficiency and Cd-tolerance. The results showed significant increases in hydrogen peroxide (H₂O₂) and malondialdehyde contents and significant increases in antioxidant enzyme (superoxide dismutase, catalase, peroxidase, and glutathione S-transferase) activities and non-protein thiol compounds (non-protein thiols, reduced glutathione, phytochelatins) synthesis in the plants in T_S+Cd and T_?S+Cd. On the tissue level, higher proportion of Cd was found to be immobilized/deposited in roots, while on the sub-cell level, higher proportion of Cd was located in cell walls and vacuole fractions with lower in cell organelles. Taken together, the results indicated that Cd detoxification was achieved by the two kidney bean cultivars through antioxidant enzyme activation, non-protein thiol compound synthesis and sub-cellular compartmentalization. In addition, the results indicated that sufficient S supply helped to relieve Cd toxicity, which is of special significance for remediation or utilization of Cd-contaminated soils as S is a plant essential nutrient.     

Fast cadmium inhibition of photosynthesis in cyanobacteria in vivo and in vitro studies using perturbed angular correlation of γ-rays

The effect of cadmium on the photosynthetic activity of Synechocystis PCC 6803 was monitored in this study. The oxygen evolving capacity of Synechocystis treated with 40 μM CdCl₂ was depressed to 10% of the maximum in 15 min, indicating that Cd²⁺ penetrated rapidly into the cells and blocked the photosynthetic activity. However, neither photosystem II (PSII) nor photosystem I (PSI) activity showed a significant short-term decrease which would explain this fast decrease in the whole-chain electron transport. Thermoluminescence measurements have shown that the charge separation and stabilization in PSII remains essentially unchanged during the first few hours following the Cd²⁺ treatment. The electron flow through PSI was monitored by following the redox changes of the P700 reaction centers of PSI. Alterations in the oxidation kinetics of P700 in the Cd²⁺-treated cells indicated that Cd²⁺ treatment might affect the available electron acceptor pool of P700, including the CO₂ reduction and accumulation in the cells. Perturbed angular correlation of γ-rays (PAC) using the radioactive ^111mCd isotope was used to follow the Cd²⁺ uptake at a molecular level. The most plausible interpretation of the PAC data is that Cd²⁺ is taken up by one or more Zn proteins replacing Zn²⁺ in Synechocystis PCC 6803. Using the radioactive ¹⁰⁹Cd isotope, a protein of approximately 30 kDa that binds Cd²⁺ could be observed in sodium dodecyl sulfate polyacrylamide gel electrophoresis. The results indicate that Cd²⁺ might inactivate different metal-containing enzymes, including carbonic anhydrase, by replacing the zinc ion, which would explain the rapid and almost full inhibition of the photosynthetic activity in cyanobacteria.     

镉暴露对黑斑蛙精巢ROS的诱导及其蛋白质氧化损伤作用机理

重金属镉对精巢发育、呼吸及神经系统信号转导等途径均有不良影响,被认为是造成两栖动物种群数量急剧下降的重要原因之一。然而,有关镉对精巢损伤的分子机理还不清楚。通过对镉暴露后的黑斑蛙精巢活性氧自由基(ROS)、蛋白质羰基(PCO)以及DNA蛋白质交联(DPC)等指标的系统分析,探讨了镉对精巢毒害的分子作用机理。随镉浓度的增加,黑斑蛙精巢细胞线粒体ROS随镉暴露浓度的增加而升高,0.5、1.0 mg/L镉染毒组与对照组比较有显著性差异(P<0.05);精巢组织PCO和DPC也随镉暴露浓度的增加而逐渐上升,且均呈明显的浓度-效应关系。结果表明:镉诱导机体产生ROS,进而导致蛋白质氧化损伤以及DNA损伤,说明精巢组织ROS的产生是镉致雄性生殖毒效应机制的重要因素之一。     

Enhanced cadmium accumulation in maize roots—the impact of organic acids

Low molecular weight organic acids are important components of root exudates and therefore, knowledge regarding the mechanisms of cadmium (Cd) uptake and distribution within plants under the influence of organic acids, is necessary for a better understanding of Cd behavior in the plant–soil system. In this study, acetic and malic acids increased the uptake of Cd by maize (Zea mays L. cv. TY2) roots and enhanced Cd accumulation in shoots under hydroponic conditions. Concentration-dependent net Cd influx in the presence and absence of organic acids could be resolved into linear and saturable components. The saturable component followed Michaelis–Menten kinetics, which indicated that Cd uptake across the plasma membrane was transporter-mediated. While the K _m values were similar, the V _max values in the presence of acetic and malic acids were respectively 6.0 and 3.0 times that of the control. Zinc transporters were the most probable pathways for Cd accumulation. It was hypothesized that Cd(II)–organic acid complexes associated with the root zone, could decompose and liberate Cd²⁺ for subsequent absorption by maize roots; and that in the layer of the roots or within the root free space, depletion of Cd²⁺ was buffered by the presence of Cd(II)–organic acid complexes. Plant response to elevated Cd levels involved overproduction of organic acids in maize roots as a resistance mechanism to alleviate Cd toxicity.     

Exogenous Abscisic Acid Alleviates Cadmium Toxicity by Restricting Cd2+ Influx in Populus euphratica Cells

Abscisic acid (ABA), a widely known phytohormone involved in the plant response to abiotic stress, plays a vital role in mitigating Cd²⁺ toxicity in herbaceous species. However, the role of ABA in ameliorating Cd²⁺ toxicity in woody species is largely unknown. In the present study, we investigated ABA restriction on Cd²⁺ uptake and the relevance to Cd²⁺ stress alleviation in Cd²⁺-hypersensitive Populus euphratica. ABA (5 μM) markedly improved cell viability and growth but reduced membrane permeability in CdCl₂ (100 μM)-stressed P. euphratica cells. Moreover, ABA significantly increased the activity of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), and ascorbate peroxidase (APX), contributing to the scavenging of Cd²⁺-elicited H₂O₂ within P. euphratica cells during the period of CdCl₂ exposure (100 μM, 24–72 h). ABA alleviation of Cd²⁺ toxicity was mainly the result of ABA restriction of Cd²⁺ uptake under Cd²⁺ stress. Steady-state and transient flux recordings showed that ABA inhibited Cd²⁺ entry into Cd²⁺-shocked (100 μM, 30 min) and short-term-stressed P. euphratica cells (100 μM, 24–72 h). Non-invasive micro-test technique data showed that H₂O₂ (3 mM) stimulated the Cd²⁺-elicited Cd²⁺ influx but that the plasma membrane (PM) Ca²⁺ channel inhibitor LaCl₃ blocked it, suggesting that the Cd²⁺ influx was through PM Ca²⁺-permeable channels. These results suggested that ABA up-regulated antioxidant enzyme activity in Cd²⁺-stressed P. euphratica and that these enzymes scavenged the Cd²⁺-elicited H₂O₂ within cells. The entry of Cd²⁺ through the H₂O₂-mediated Ca²⁺-permeable channels was subsequently restricted; thus, Cd²⁺ buildup and toxicity were reduced in the Cd²⁺-hypersensitive species, P. euphratica.

     

Effect of cadmium uptake and heat stress on root ultrastructure, membrane damage and antioxidative response in rice seedlings

Excess cadmium (Cd²⁺) in the soil environment is taken up by plants and can cause phytotoxicity. Elevated temperatures also lead to deleterious effects on plants. Plants are very often exposed to a combination of stresses rather than a single stress. The effect of Cd²⁺ and heat stress (HS) on the growth, root ultrastructure, lipid peroxidation (MDA), hydrogen peroxide accumulation and the activities of antioxidant enzymes peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) of rice roots from sensitive cv. DR-92 and tolerant cv. Bh-1 were investigated at 10 and 20 day of growth under controlled conditions. At day 10 under all Cd²⁺ treatments, the Cd²⁺ content between the two rice cultivars were almost similar. Application of 500 μM Cd²⁺ significantly increased metal concentrations at day 20 in the roots of rice seedlings resulting in a maximum accumulation of 44.25 μg Cd²⁺ g^-1 dry wt in cv. DR-92 and 30 μg Cd²⁺ g^-1 dry wt in cv. Bh-1 with a ~25 % decline in Relative Growth Index (RGI) in cv. DR-92. TEM studies revealed slight disorganization with cell wall ingrowths in root tissues from cv. DR-92 grown in 100 μM Cd²⁺ + HS. Uptake and accumulation of Cd²⁺ increased upon heat treatment in parenchyma, vacuoles and vascular cylinder of root tissues. Peroxidase primarily located in cell walls, the intensity being higher in sensitive cv. DR-92. Under Cd²⁺ stress alone, plants of sensitive cv. DR-92 significantly increased the H₂O₂ and MDA levels together with increased activities of the enzymes POD, CAT and APX at day 10 but remained almost stable at day 20. A strong increase in MDA levels was noted at day 20 in tolerant cv. Bh-1. Cd²⁺ + HS treatments in tolerant cv.Bh-1 led to a decreased H₂O₂ and MDA levels and decreased activities of the enzymes POD, CAT and APX. Results suggest stimulation of root antioxidant system under combination of two stresses and that heat stress seem to have a direct protective role by mitigating the effect of mild Cd²⁺ toxicity largely by enhanced Cd²⁺-MT formation contributing thereby towards the management of Cd²⁺ toxicity at cellular level that confers Cd²⁺ tolerance to rice cv. Bh-1.     

Effects of cadmium on respiratory burst,intracellular Ca2+ and DNA damage in the white shrimp Litopenaeus vannamei

Acute effects of heavy metal ions on shrimp have been an area of intense study worldwide. However, the molecular mechanism by which cadmium-induced injury occurs remains largely unclear, and methods for mitigating toxicity in vivo have rarely been reported. In this study, the changes in respiratory burst and intracellular free calcium in haemocytes of pacific white shrimp, Litopenaeus vannamei, after exposure to Cd²⁺ (CdCl₂) were examined using flow cytometry. Meanwhile, DNA damage and repair in haemocytes and hepatopancreas cells were studied using the comet assay. Respiratory burst generation, intracellular Ca²⁺ concentration ([Ca²⁺]i) and DNA damage in haemocytes and hepatopancreas cells all exhibited a dose-dependent increase and a time-dependent change after treatment with Cd²⁺ compared with controls. These results indicate that Cd can induce oxidative stress and DNA damage in the shrimp L. vannamei. Moreover, the results also demonstrate that these parameters can be used as sensitive indicators of exposure to this genotoxicant.     

Combined effects of NaCl and Cd2+ stress on the photosynthetic apparatus of Thellungiella salsuginea

Plants show complex responses to abiotic stress while, the effect of the stress combinations can be different to those seen when each stress is applied individually. Here, we report on the effects of salt and/or cadmium on photosynthetic apparatus of Thellungiella salsuginea. Our results showed a considerable reduction of plant growth with some symptoms of toxicity, especially with cadmium treatment. The structural integrity of both photosystems (PSI and PSII) was mostly maintained under salt stress. Cadmium induced a considerable decrease of both PSI and PSII quantum yields and the electron transport rate ETR(I) and ETR(II) paralleled by an increase of non-photochemical quenching (NPQ). In addition, cadmium alone affects the rate of primary photochemistry by an increase of fluorescence at O-J phase and also the photo-electrochemical quenching at J-I phase. A positive L-band appeared with (Cd) treatment as an indicator of lower PSII connectivity, and a positive K-band reflecting the imbalance in number of electrons at donor and acceptor side. In continuity to our previous studies which showed that NaCl supply reduced Cd²⁺ uptake and limited its accumulation in shoot of divers halophyte species, here as a consequence, we demonstrated the NaCl-induced enhancement effect of Cd²⁺ toxicity on the PSII activity by maintaining the photosynthetic electron transport chain as evidenced by the differences in ψ_O, φ_Eo, ABS/RC and TR₀/RC and by improvement of performance index PI_(ABS), especially after short time of treatment. A significant decrease of LHCII, D1 and CP47 amounts was detected under (Cd) treatment. However, NaCl supply alleviates the Cd²⁺ effect on protein abundance including LHCII and PSII core complex (D1 and CP47).     

Voltage-Induced Ca2+ Release in Postganglionic Sympathetic Neurons in Adult Mice

Recent studies have provided evidence that depolarization in the absence of extracellular Ca²⁺ can trigger Ca²⁺ release from internal stores in a variety of neuron subtypes. Here we examine whether postganglionic sympathetic neurons are able to mobilize Ca²⁺ from intracellular stores in response to depolarization, independent of Ca²⁺ influx. We measured changes in cytosolic ΔF/F₀ in individual fluo-4 –loaded sympathetic ganglion neurons in response to maintained K⁺ depolarization in the presence (2 mM) and absence of extracellular Ca²⁺ ([Ca²⁺]_e). Progressive elevations in extracellular [K⁺]_e caused increasing membrane depolarizations that were of similar magnitude in 0 and 2 mM [Ca²⁺]_e. Peak amplitude of ΔF/F₀ transients in 2 mM [Ca²⁺]_e increased in a linear fashion as the membrane become more depolarized. Peak elevations of ΔF/F₀ in 0 mM [Ca²⁺]_e were ~5–10% of those evoked at the same membrane potential in 2 mM [Ca²⁺]_e and exhibited an inverse U-shaped dependence on voltage. Both the rise and decay of ΔF/F₀ transients in 0 mM [Ca²⁺]_e were slower than those of ΔF/F₀ transients evoked in 2 mM [Ca²⁺]_e. Rises in ΔF/F₀ evoked by high [K⁺]_e in the absence of extracellular Ca²⁺ were blocked by thapsigargin, an inhibitor of endoplasmic reticulum Ca²⁺ ATPase, or the inositol 1,4,5-triphosphate (IP₃) receptor antagonists 2-aminoethoxydiphenyl borate and xestospongin C, but not by extracellular Cd²⁺, the dihydropyridine antagonist nifedipine, or by ryanodine at concentrations that caused depletion of ryanodine-sensitive Ca²⁺ stores. These results support the notion that postganglionic sympathetic neurons possess the ability to release Ca²⁺ from IP₃-sensitive internal stores in response to membrane depolarization, independent of Ca²⁺ influx.     

A Functional Putative Phytochelatin Synthase from the Primitive Red Alga Cyanidioschyzon merolae

Phytochelatin synthase (PCS) catalyzes the synthesis of phytochelatins (PCs), which play a detoxification role in higher plants. Heterologous expression of CmPCS, a product of a PCS-like gene from the genomic DNA of the red alga Cyanidioschyzon merolae, rescued Cd²⁺-sensitive yeast from Cd²⁺ toxicity. The fact that these transformed cells synthesized PCs demonstrates that CmPCS is functional.     

Heavy metal effects on lipid peroxidation in the tissues of mytilus gallopro vincialis lam.

1. The effects of heavy metals on lipid peroxidation in the gills and digestive gland of mussels exposed for six days to Cu²⁺, Cd²⁺ or Zn²⁺ (40 μg/l/animal) were investigated.2. In the tissues of Cu-exposed mussels a significant increase of the level of malondialdehyde (MDA), which is indicative of the peroxidative process, and a decrease of the concentration of glutathione were observed.3. Moreover, in the digestive gland of mussels, copper exposure results in an increase of other carbonyl compounds and in the lysosomal accumulation of lipofuscin granules.4. The exposure of mussels to Zn or to Cd did not elicit any of the above effects.5. The results are discussed in relation to the possible role that Cu-induced lysosomal lipofuscin accumulation may play in heavy metal detoxification.     

Antioxidant enzyme activities and hormonal status in response to Cd stress in the wetland halophyte Kosteletzkya virginica under saline conditions

Salt marshes constitute major sinks for heavy metal accumulation but the precise impact of salinity on heavy metal toxicity for halophyte plant species remains largely unknown. Young seedlings of Kosteletzkya virginica were exposed during 3 weeks in nutrient solution to Cd 5 µM in the presence or absence of 50 mM NaCl. Cadmium (Cd) reduced growth and shoot water content and had major detrimental effect on maximum quantum efficiency (F_v/F_m), effective quantum yield of photosystem II (Y(II)) and electron transport rates (ETRs). Cd induced an oxidative stress in relation to an increase in O₂^?? and H₂O₂ concentration and lead to a decrease in endogenous glutathione (GSH) and α‐tocopherol in the leaves. Cd not only increased leaf zeatin and zeatin riboside concentration but also increased the senescing compounds 1‐aminocyclopropane‐1‐carboxylic acid (ACC) and abscisic acid (ABA). Salinity reduced Cd accumulation already after 1 week of stress but was unable to restore shoot growth and thus did not induce any dilution effect. Salinity delayed the Cd‐induced leaf senescence: NaCl reduced the deleterious impact of Cd on photosynthesis apparatus through an improvement of F_v/F_m, Y(II) and ETR. Salt reduced oxidative stress in Cd‐treated plants through an increase in GSH, α‐tocopherol and ascorbic acid synthesis and an increase in glutathione reductase (EC 1.6.4.2) activity. Additional salt reduced ACC and ABA accumulation in Cd+NaCl‐treated leaves comparing to Cd alone. It is concluded that salinity affords efficient protection against Cd to the halophyte species K. virginica, in relation to an improved management of oxidative stress and hormonal status.     

Net cadmium flux and accumulation reveal tissue-specific oxidative stress and detoxification in Populus × canescens

To characterize the dynamics of Cd²⁺ flux in the rhizosphere and to study cadmium (Cd) plant‐internal partitioning in roots, wood, bark and leaves in relation to energy metabolism, reactive oxygen species (ROS) formation and antioxidants, Populus × canescens plantlets were exposed to either 0 or 50 µM CdSO₄ for up to 20 days in the nutrient solution. A strong net Cd²⁺ influx in root apex was observed after Cd exposure for 24 h, even if net Cd²⁺ influx decreased gradually in roots. A large amount of Cd was accumulated in roots. Cd ions were uploaded via the xylem to leaves and further transported to the phloem where significant accumulation was detected. Cd accumulation led to decreased photosynthetic carbon assimilation but not to the depletion in soluble carbohydrates. Increased levels of ROS were present in all tissues, except the bark of Cd‐exposed poplars. To combat Cd‐induced superoxide and hydrogen peroxide, P.×canescens appeared to rely mainly on the formation of soluble phenolics as these compounds showed the highest accumulation in the bark and the lowest in wood. Other potential radical scavengers such as proline, sugar alcohols and antioxidant enzymes showed tissue‐ and exposure time‐specific responses to Cd. These results indicate a complex pattern of internal Cd allocation in P.×canescens resulting in higher ROS stress in wood than in bark and intermediate responses in roots and leaves, probably because of differential capacities of these tissues for the production of protective phenolic compounds.     

Applications of Cd NMR to polycadmium complexes. II. [Cdx(SPh)y]2x−y species in solution in relation to metallothionein aggregates

¹¹³Cd NMR spectra of mixtures of (Me₄N)₂ [Cd- (SPh)₄] and (Me₄N)₂[Cd₄(SPh)₁₀] in DMF reveal the formation of one major and two very minor new cadmium thiolate species in labile equilibria. Resonance broadening and intensities indicate that the dominant species is monocyclic, probably [(μ- SPh)₃{Cd(SPh)₂}₃]^3?, and thus a model for the proposed Cd₃(S-cys)₉ site of metallothioneins.     

Effect of Cadmium on gamma-Glutamylcysteine Synthesis in Maize Seedlings

Cysteine, γ-glutamylcysteine, and glutathione and the extractable activity of the enzymes of glutathione biosynthesis, γ-glutamylcysteine synthetase (EC 6.3.2.2) and glutathione synthetase (EC 6.3.2.3), were measured in roots and leaves of maize seedlings (Zea mays L. cv LG 9) exposed to CdCl₂ concentrations up to 200 micromolar. At 50 micromolar Cd²⁺, γ-glutamylcysteine contents increased continuously during 4 days up to 21-fold and eightfold of the control in roots and leaves, respectively. Even at 0.5 micromolar Cd²⁺, the concentration of γ-glutamylcysteine in the roots was significantly higher than in the control. At 5 micromolar and higher Cd²⁺ concentrations, a significant increase in γ-glutamylcysteine synthetase activity was measured in the roots, whereas in the leaves this enzyme activity was enhanced only at 200 micromolar Cd²⁺. Labeling of isolated roots with [³⁵S]sulfate showed that both sulfate assimilation and glutathione synthesis were increased by Cd. The accumulation of γ-glutamylcysteine in the roots did not affect the root exudation rate of this compound. Our results indicate that maize roots are at least in part autonomous in providing the additional thiols required for phytochelatin synthesis induced by Cd.     

Solid phase speciation of Zn and Cd in zinc smelter effluent-irrigated soils

Solubility of metal in contaminated soils is a key factor which controls the phytoavailability and toxic effects of metals on soil environment. The chemical equilibria of metal ions between soil solution and solid phases govern the solubility of metals in soil. Hence, an attempt was made to identify the probable solid phases (minerals), which govern the solubility of Zn²⁺ and Cd²⁺ in zinc smelter effluent-irrigated soils. Estimation of free ion activities of Zn²⁺ (pZn²⁺) and Cd²⁺ (pCd²⁺) by Baker soil test indicated that metal ion activities were higher in smelter effluent-irrigated soils as compared to that in tubewell water-irrigated soils. Identification of solid phases further reveals that free ion activity of Zn²⁺ and Cd²⁺ in soil highly contaminated with Zn and Cd due to long-term irrigation with zinc smelter effluent is limited by the solubility of willemite (Zn₂SiO₄) in equilibrium with quartz and octavite (CdCO₃), respectively. However, in case of tubewell water-irrigated soil, franklinite (ZnFe₂O₄) in equilibrium with soil-Fe and exchangeable Cd are likely to govern the activity of Zn²⁺ and Cd²⁺ in soil solution, respectively. Formation of highly soluble minerals namely, willemite and octavite indicates the potential ecological risk of Zn and Cd, respectively in smelter effluent irrigated soil.     

Effects of Cadmium and Gibberellin on Growth and Photosynthesis of Glycine Max

The effects of 0, 2.5, 5.0, and 10.0 mg(Cd²⁺) m^-3 [Cd(NO₃)2×4 H₂O] and 0 and 10.0 mg m^-3 gibberellin on certain parameters of photosynthesis and growth in soybean (Glycine max L. cv. Pershing) plants were studied. With increasing Cd²⁺ concentration in the Hoagland nutrient solution, the contents of chlorophyll and CO₂ compensation concentration decreased. The addition of 10 mg m^-3 gibberellin reduced the negative effects of Cd²⁺ in shoot and root growth. With increasing of Cd²⁺ concentration in the culture medium, the dry matter production in both the roots and shoots decreased as shown by the decline in growth rate (PGR), net assimilation rate (NAR), and leaf area ratio. The addition of gibberellin caused a partial elimination of the Cd effects on the roots and shoots and the PGR and NAR and it increased leaf area and length of stem.