Cellular antioxidants including low molecular weight antioxidants and enzymes of the ROS-scavenging system. (Adapted from Pinto et al., 2003)
1. Introduction
Metals like zinc, iron and copper are essential micronutrients required for a wide range of physiological processes in all plant organs for the activities of various metal-dependent enzymes and proteins. However, they can also be toxic at elevated levels. Metals like arsenic, mercury, cadmium and lead are nonessential and potentially highly toxic. Once the cytosolic metal concentration in plant turns out of control, phytotoxicity of heavy metal inhibits transpiration and photosynthesis, disturbs carbohydrate metabolism, and drives the secondary stresses like nutrition stress and oxidative stress, which collectively affect the plant development and growth (Krämer & Clemens, 2005).
Plants have developed a complex network of highly effective homeostatic mechanisms that serve to control the uptake, accumulation, trafficking, and detoxification of metals. Components of this network have been identified continuously, including metal transporters in charge of metal uptake and vacuolar transport; chelators involved in metal detoxification via buffering the cytosolic metal concentrations; and chaperones helping delivery and trafficking of metal ions (Clemens, 2001).
This chapter summarizes heavy metal stress and detoxification in plant. Special focus is given to metallothionein, yet vacuolar metal transporters, phytochelatins as well as certain organic acids, amino acids, and chaperones are also addressed with recent advances. Besides, heavy metal-induced oxidative stress and tolerance as an example of abiotic stress cross-talk will be discussed.
1.1. The vacuolar compartmentation mediated by transporter families CDF and Nramp
A balanced cytosolic metal concentration has to be maintained all the time via strict compartmentation and chelation. The plant vacuole is a main storage compartment site for heavy metals present in excess (Ernst et al., 1992). Nickel-hyperaccumulator plant
The CDF (cation diffusion facilitator) transporters, once named as MTP for metal tolerance protein, are involved in mediating the cytoplasmic efflux of transition metal cations such as Zn2+, Cd2+, Co2+, Ni2+ or Mn2+. In
It’s interesting to note the other side, releasing metal ions from the vacuole into the cytosol if required by metabolism. Which transporter takes the challenge? The NRAMPs might be a possible candidate. The plant NRAMP (natural resistance associated macrophage protein) family transport divalent metal cations into the cytoplasm.
Progressive reports implicated that
1.2. Chelation of cadmium ions by phytochelatin
Chelation of metals in the cytosol is a very important mechanism of heavy metal detoxification and tolerance (Fig 1). The principal classes of known metal chelators in plant
are metallothioneins (MTs), phytochelatins (PCs), organic acids and amino acids (Clemens, 2001). Metallothionein and phytochelatin are proteins or peptides with low molecular weight, high cysteine content, and unique metal-binding capacity. In early reports lack of detail amino acid sequence data, metal-binding proteins in plants were generally assumed to be MTs, which in fact covered at least part of PCs. In an old classification system of three-class MT proteins, phytochelatins are somewhat confusingly described as enzymatically-synthesized class III whereas other two are gene-encoded class I and II (Cherian & Chan, 1993). Later the classification system has been improved and now it’s clear plants express both PCs and MTs, which play relatively independent roles in metal detoxification and/or metabolism (Cobbett & Goldsbrough, 2002).
Phytochelatins have been identified in many plants and photosynthetic organisms, ranging from algae, gymnosperms to monocots and dicots. Phytochelatins (PCs) are synthesized from glutathione (GSH) (in some cases, related compounds) by PC synthases (PCS), and play a role in the distribution and accumulation of Cd and some other highly toxic metals like Ag, Hg, As (Cobbett, 2000, Rauser, 1999). Modern techniques including X-ray absorption spectroscopy (XAS), high performance liquid chromatography-mass spectrometry (HPLC-MS), inductively coupled plasma optical emission spectrometer (ICP-OES) help to reveal that cadmium ions are generally bound to phytochelatins in plant. The percentage of Cd bound to PCs in Indian mustard seedlings increased from 34% after 6 hours of Cd exposure to 60% after 72 hours (Salt et al., 1997). In a Cd-hyperaccumulator desert plant tumbleweed (
There are two types of Cd-PC complexes produced during Cd sequesteration: low-molecular-weight (LMW) and high-molecular-weight (HMW). The LMW complex serves as the transient form for transporting Cd2+ from cytosol to vacuole where more Cd and sulfide are incorporated to produce the HMW complex, which turns the main storage form of Cd2+ (Rauser, 1995). The first molecular insight into transporting the PC-Cd complex comes from the
Besides the phytochelatin-Cd2+ complex transported by ABC transporters, cadmium ions can also reach the vacuole via a direct exchange with protons by tonoplast-localized cation/proton exchanger (CAX) transporters. In oat roots, the pH-dependent Cd2+ accumulation in vesicles was accompanied by efflux of protons, which offers the first clue of Cd2+/H+ antiport in plant (Salt & Wagner, 1993). Then several
1.3. Metallothionein: metal-binding protein and more
Metallothioneins (MTs) are ubiquitous low-molecular-weight, cysteine-rich proteins that can bind metals via mercaptide bonds. Since the first MT was characterized from horse kidneys as cadmium-binding proteins in 1957 (Margoshes & Vallee, 1957), plenty of MT genes have been identified in a wide variety of organisms including bacteria, fungi, and all eukaryotic animal and plant species (Robinson et al., 1993).
The spatial structures of MTs have been uncovered as a dumbbell-like shape with two separate domains, α and β, containing in their core clusters built up of several tetrahedral Metal-Cys units (Fig 2). The different metal reactivity and metal affinity of two domains prompt different functional roles of the two metal clusters, that is, N-terminal β domain is involved in the homeostasis of essential metal ions (Kagi & Schaffer, 1988, Willner et al., 1987), and C-terminal α domain, the tight binding sequestration of excess and/or toxic metal ions (Cherian et al., 1994, Wright et al., 1987). As for the spacer region linking the α and β domains, it may contribute to stability or subcellular localization of MT proteins (Domenech et al., 2005), and is necessary for MT metal detoxification function (Domenech et al., 2007, Zhou & Goldsbrough, 1994).
MT proteins are generally classified into mammalian Class I and plant Class II, and plant MTs can be further subdivided into four types based on the number and arrangement of cysteine residues and the length of spacer region (Cobbett & Goldsbrough, 2002). These four-type plant MTs exhibited certain tissue-preferential expression patterns. Type 1 MTs
are expressed much higher in roots than in shoots (Hudspeth et al., 1996), whereas Type 2 MTs are found mainly in leaves (Hsieh et al., 1995, Zhou & Goldsbrough, 1994). Type 3 MTs are expressed abundantly in the ripe fruits (Clendennen & May, 1997, Ledger & Gardner, 1994, Reid & Ross, 1997), and expression of Type 4 MTs, also known as the Ec type, was only found in developing seeds so far (Chyan et al., 2005, Lane et al., 1987).
A vast number of stimuli have been demonstrated capable of inducing MT genes expression in plants, including natural senescence (Bhalerao et al., 2003), hormones like ABA (Reynolds & Crawford, 1996), ethylene (Coupe et al., 1995), wounding and virus infection (Choi et al., 1996), heat shock (Hsieh et al., 1995), sucrose starvation (Hsieh et al., 1996), UV-light (Foley & Singh, 1994), cold and salt stress (Reid & Ross, 1997), etc. Apparently, different types of MTs respond to different factors, which is especially true when treated with heavy metal stress under different concentrations. Copper increased
Ever since the first identification of MTs, its striking metal-binding property has been brought into sharp focus, which suggests MTs play the principal role in metal homeostasis and detoxification. In animals, MTs are well-known metal-binding proteins protecting against cadmium toxicity (Klaassen et al., 1999), while in plant PCs mainly take the charge of Cd detoxification (Zenk, 1996). MTs seem to have a broader spectrum of metal affinity than PCs, which points to more complicated functions. It’s proposed that MTs participate in maintaining the homeostasis of essential copper (Cu) or zinc (Zn) at micronutrient levels, and also in the detoxification of non-essential toxic metals such as cadmium (Cd) and arsenic (As) (Lee et al., 2004, Merrifield et al., 2004, Roosens et al., 2004).
Though modulation of metal concentrations has great impact on cellular redox balance (Bell & Vallee, 2009), MTs may just scavenge reactive oxygen species (ROS) directly. With a large quantity of nucleophilic sulphydryl groups in the structure, MTs provide a fine nucleophilic “sink” to trap electrophiles and free radicals, that is, the multiple cysteine residues can react with superoxide (•O2
–) and hydroxyl radicals (•OH) leading to their degradation (Klaassen & Cagen, 1981, Sato & Bremner, 1993). Moreover, MTs can be recycled via thiolate exchange with GST (Vasak et al., 1985). Now accumulating evidences support hypothesis that MTs function as an antioxidant in plants. In wild watermelon, drought-induced CLMT2 showed an extraordinarily high activity for detoxifying hydroxyl radicals
Thanks to dynamic instability of metal ions in clusters, MTs can exchange metal ions with other metalloproteins universally necessary for a life cycle. There’s zinc transfer between metallothionein and zinc transporter ZnT1 (Palmiter, 2004), chelator EDTA (Leszczyszyn & Blindauer, 2010), SOD (Koh & Kim, 2001, Suzuki & Kuroda, 1995), and other zinc proteins (Jacob et al., 1998). The metal-transfer mechanism should be a cornerstone for MTs' dual role abstracting the toxic metals arsenic (Ngu et al., 2010) or cadmium (Roesijadi, 2000), as well as donating the essential metals like zinc or copper (Liu et al., 2000). In this sense, metal-binding protein MTs are involved not merely in the coordination of metal concentraions, but contribute more to diverse physiological processes like development or senescence. The wheat type 4 MT Ec gene was specially expressed during pollen embryogenesis, and its accumulation correlates well with increase of the plant hormone abscisic acid (ABA). It’s suggested that induced by the ABA signal, this zinc-containing Ec may regulate certain gene expression via zinc trafficking with zinc-dependent DNA/RNA polymerase or zinc-finger proteins (Reynolds & Crawford, 1996). MTs have been implicated during senescence in many plants (Bhalerao et al., 2003, Breeze et al., 2004, Buchanan-Wollaston & Morris, 2000), and hypotheses for MT’s role in senescence primarily reckon on either MTs' chelating and detoxifying abilities which alleviate the senescence-induced metal ion disturbance and oxidative burst, or the release of necessary metal ions to required places for nutrient recycling.
The positive correlation between MT expression in diverse organisms and the environmental metal concentration suggests that MTs can be effective biomarkers of heavy metal pollution. Such monitoring programs have already gained great potential comprehensively in aquatic and terraneous invertebrates (Chu et al., 2006, Dallinger et al., 2004, Navarro et al., 2009). In plants, MTs are favorable candidates for phytoremediation of heavy metal contaminants, a low-cost, effective, and sustainable plant-based approach for environment governance (Eapen & D'Souza, 2005, Memon & Schroder, 2009). On the other side, biofortification of mineral micronutrients in food crops for the benefit of human health, is another application and extension for metal research in plants, and MTs could also be contributive. The
1.4. Organic acids, amino acids and chaperones
The reactive interactions between metal ions and S, N, and O made organic acids and amino acids potential ligands for metal chelation. Citrate has been proposed the major ligand for Cd2+ at low Cd concentration within cell (Wagner, 1993), and can form Nickel-citrate complex in Ni-hyperaccumulating plant
The coordination of nickel with histidine has been confirmed with analyses of Ni-hyperaccumulating and non-accumulating species. Upon Ni exposure, a large and proportional increase of free histidine was detected in xylem sap in Ni-hyperaccumulating
Copper chaperones are a novel class of proteins involved in intracellular trafficking and delivery of copper to copper-containing proteins such as copper-ATPases or copper/zinc superoxide dismutase.
2. Heavy metal-induced oxidative stress and stress tolerance: Cross-talk
Seen from a systemic view, different abiotic stresses may bring general effects on plant growth and development. For example, drought, salt, and cold stresses can all interrupt the cellular water balance leading to osmotic stress, and generate a phytohormone abscisic acid (ABA) for osmotic adjustment (Wang et al., 2003). ABA acts as a key endogenous messenger in stress response, and hence the ABA sigalling pathway is more or less involved during plant cross-adaptive processes (Tuteja, 2007). In addition, all abiotic stresses can accumulate excess ROS (reactive oxygen species) at certain stage of stress exposure leading to oxidative stress. However, ROS are not only toxic compounds, but sometimes play as important regulators for many biological processes in plants such as cell cycle, programmed cell death, hormone signaling, biotic and abiotic cell responses (Laloi et al., 2004). As common consequences of abiotic stresses, osmotic stress and the ubiquitous oxidative stress have been extensively studied and offer more and more evidences for cross-talk at various steps or levels in the complicated network of abiotic stress signalling pathways.
Reactive oxygen species (ROS) such as •O2 –, H2O2 and •OH are unavoidable by-products of aerobic metabolism, and also commonly generated under various stress conditions. The unwelcome result of ROS overproduction is the oxidative stress, which can cause extensive cellular damages (Miller et al., 2008). Therefore, a delicate antioxidant system is indispensably required to supervise the cytotoxic effects of ROS tightly. The plant antioxidant system consists of ROS-scavenging enzymes, such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), as well as low-molecular-weight antioxidants like glutathione, ascorbate, carotenoids, metallothionein, etc (Table 1). Analysis with transgenic plants overexpressing these antioxidant genes revealed that maintenance of a high antioxidant capacity in cells is linked to increased tolerance against various adverse conditions (Guo et al., 2009, Jayaraj & Punja, 2008, Tseng et al., 2007, Wang et al., 2010).
Heavy metal stresses can shift the cellular balance of free radical homeostasis into terrible accumulation of H2O2. For those redox-active transition metals like copper or iron, autoxidation in Fenton reaction and Haber–Weiss reaction will convert H2O2 to the highly reactive •OH molecule in a metal-catalyzed way. Non-redox-active metals like cadmium or mercury can also result in H2O2 accumulation and an oxidative burst via depletion of the antioxidant glutathione (GSH) pool and inhibition of antioxidative enzymes, especially glutathione reductase (GR) (Mithofer et al., 2004, Schutzendubel & Polle, 2002). To cope with heavy metal stress and associated oxidative stress, metallothionein, a well-known metal chelator and also antioxidant would possibly be a good way out.
Low molecular weight antioxidants | |
Compounds | Target |
Ascorbate | O2 (1△g) , •OH, O2•, HO2• |
β-Carotene | O2 (1△g) , RO2• |
α-Tocopherol | RO2• |
Glutathione | Nonspecific |
Urate | O2 (1△g) , metal |
Metallothionein | •OH, metal |
Flavonoid | •OH and HOCl |
Phytochelatin | Metal |
Enzyme antioxidants | |
Enzyme | Reaction catalyzed |
Superoxide dismutase | 2O2•- + 2H+ → H2O2 + O2 |
Catalase | 2H2O2 → 2H2O + O2 |
Glutathione peroxidase | H2O2 or ROOH + 2GSH → 2H2O or ROH + GSSG |
Ascorbate peroxidase | H2O2 + Ascorbate → H2O + Monodehydroascorbate |
Thioredoxin | Prot-S2 + Prot’(SH)2 → Prot(SH)2 + Prot’-S2 |
Peroxiredoxin | ROOH + R’(SH)2 → ROH + R’S2 + H2O |
Glutathione reductase | GSSG + NAD(P)H + H+ → 2GSH + NAD(P)+ |
Take OsMT1a for example. Yang et al. reported functional characterization of a rice type 1 metallothionein, OsMT1a. A model has been proposed to elucidate how OsMT1a plays a role in drought tolerance in plant (Yang et al., 2009). On the one hand, OsMT1a can directly scavenge ROS via increasing activities of antioxidant enzymes CAT, POD and APX. On the other hand, OsMT1a lies upstream of some zinc finger transcription factors like Ossiz, and may tune up downstream defense genes in virtue of these transcription factors through Zn2+ trafficking. Additional data reveal that some zinc/cadmium transporter genes including forecasted vacuolar-membrane-localized ABC transporters
Despite their toxicity, ROS have been reevaluated in recent years as key signal molecules for regulating cell function and development (Rhee, 2006). In plants, the elaborate and efficient network of scavenging mechanisms allowed overcoming ROS toxicity and using some of these toxic molecules, mainly the hydrogen peroxide (H2O2) produced by cytosolic membrane-bound NADPH oxidases, as a signal in a wide range of abiotic stress responses (Bailey-Serres & Mittler, 2006, Mittler et al., 2004, Neill et al., 2002). For instance, in response to drought stress, ABA-induced H2O2 regulates the stomatal closing of
3. Outlook and challenges
As the global population and food demand keep increasing fast, and yet the environment has been endangered worse and worse by water deficit and soil salinization, abiotic stress becomes one of the most harmful factors that limit the growth and productivity of crops worldwide. Although we keep moving forward with the understanding of heavy metal stress and detoxification in plant, there are many components of the complex network yet to be identified. Especially much remains unknown about the signalling molecules of the metal-induced signal transduction, including sensing of the cellular metal change and subsequent transcription regulation of metal-responsive genes (DalCorso et al., 2008). In recent years, next-generation sequencing techniques emerge and develop fast, and the microarray-based analyses become available and efficient for transcriptome or proteome high-throughput screenings, which help to identify regulatory factors for the metal homeostasis and still more metal transporters, low-molecular-weight chelators, chaperones as well. In addition, some heavy metal responsive transcription factors can also be induced by other abiotic stresses such as cold, dehydration, Salicylic Acid (SA) and H2O2, suggesting cross-talk exists between heavy metal response and other abiotic stress defense signalling (Fusco et al., 2005, Singh et al., 2002, Suzuki et al., 2001, Weber et al., 2006). Nevertheless, determining the underlying regulatory and cross-talk mechanisms remain a future challenge.
Heavy metal hyperaccumulators are unique plants capable of accumulating high amounts of various toxic elements (Reeves & Baker, 2000), and the active hyperaccumulation is based on mechanisms of internal hypertolerance to cytotoxic metals and a powerful scavenging system compatible for efficient uptake of the pollutants (Salt, 2006). Therefore, comparative studies on hyperaccumulator and non-hyperaccumulator plants will provide us a good view of naturally selected metal hypertolerance and hyperaccumulation. The first core set of candidate genes with high expression in hyperaccumulators has been identified and will be analyzed at biochemical and genetic level (Krämer et al., 2007). Dissecting these genes opens up a wide avenue for understanding the plant metal homeostasis network, and also agricultural genetic engineering for crop tolerance and biofortification, as well as phytoremediation of environmental metal pollution.
Acknowledgments
This work was supported by grants from Chinese Academy of Sciences (grant numbers: KSCX2-YW-N-010 & KSCX2-YW-N-056), and National Science Foundation of China (grant numbers: 30825029 & 30621001).
References
- 1.
Akashi K. Nishimura N. Ishida Y. Yokota A. 2004 Potent hydroxyl radical-scavenging activity of drought-induced type-2 metallothionein in wild watermelon. , 323,72 78 ,0000-6291x - 2.
Andres-Colas N. Sancenon V. Rodriguez-Navarro S. Mayo S. Thiele D. J. Ecker J. R. Puig S. Penarrubia L. 2006 The Arabidopsis heavy metal P-type ATPase HMA5 interacts with metallochaperones and functions in copper detoxification of roots . , 45,225 236 ,0960-7412 - 3.
Bailey-Serres J. Mittler R. 2006 The roles of reactive oxygen species in plant cells . , 141,311 0032-0889 - 4.
Bell S. G. Vallee B. L. 2009 The metallothionein/thionein system: an oxidoreductive metabolic zinc link . , 10,55 62 ,1439-7633 - 5.
Bhalerao R. Keskitalo J. Sterky F. Erlandsson R. Bjorkbacka H. Birve S. J. Karlsson J. Gardestrom P. Gustafsson P. Lundeberg J. Jansson S. 2003 Gene expression in autumn leaves . , 131,430 442 ,0032-0889 - 6.
Blaudez D. Kohler A. Martin F. Sanders D. Chalot M. 2003 Poplar metal tolerance protein 1 confers zinc tolerance and is an oligomeric vacuolar zinc transporter with an essential leucine zipper motif . , 15,2911 2928 ,1040-4651 - 7.
Blindauer C. A. Harrison M. D. Parkinson J. A. Robinson A. K. Cavet J. S. Robinson N. J. Sadler P. J. 2001 A metallothionein containing a zinc finger within a four-metal cluster protects a bacterium from zinc toxicity . , 98,9593 9598 ,0027-8424 - 8.
Bovet L. Feller U. Martinoia E. 2005 Possible involvement of plant ABC transporters in cadmium detoxification: a cDNA sub-microarray approach. , 31,263 267 ,0160-4120 - 9.
Breeze E. Wagstaff C. Harrison E. Bramke I. Rogers H. Stead A. Thomas B. Buchanan-Wollaston V. 2004 Gene expression patterns to define stages of post-harvest senescence in Alstroemeria petals . , 2,155 168 ,1467-7652 - 10.
Brooks R. R. Reeves R. D. Morrison R. S. Malaisse F. 1980 Hyperaccumulation of Copper and Cobalt- a Review. , 113,166 172 ,0037-9557 - 11.
Brune A. Urbach W. Dietz K. J. 1995 Differential toxicity of heavy metals is partly related to a loss of preferential extraplasmic compartmentation- a comparison of Cd-stress, Mo-stress, Ni-stress and Zn-stress. , 129,403 409 ,0002-8646x - 12.
Buchanan-Wollaston V. Morris K. 2000 Senescence and cell death in Brassica napus and Arabidopsis. , 52,163 174 ,0081-1386 - 13.
Cailliatte R. Lapeyre B. Briat J. F. Mari S. Curie C. 2009 The NRAMP6 metal transporter contributes to cadmium toxicity. , 422,217 228 , ISSN 1470-8728 - 14.
Cherian G. M. Chan H. M. 1993 Biological functions of metallothioneins: a review. In: , K.T. Suzuki, Imura, N., Kimura, M., (Ed.),87 109 , Birkhäuser Verlag,978-3-76432-769-9 Basel. - 15.
Cherian M. G. Howell S. B. Imura N. Klaassen C. D. Koropatnick J. Lazo J. S. Waalkes M. P. 1994 Role of metallothionein in carcinogenesis. , 126,1 5 ,0004-1008x - 16.
Choi D. Kim H. M. Yun H. K. Park J. A. Kim W. T. Bok S. H. 1996 Molecular cloning of a metallothionein-like gene from Nicotiana glutinosa L. and its induction by wounding and tobacco mosaic virus infection. , 112,353 359 ,0032-0889 - 17.
Chu C. C. Lee W. C. Guo W. Y. Pan S. M. Chen L. J. Li H. M. Jinn T. L. 2005 A copper chaperone for superoxide dismutase that confers three types of copper/zinc superoxide dismutase activity in Arabidopsis . , 139,425 436 ,0032-0889 - 18.
Chu M. M. Guo Z. Q. Muto N. Itoh N. Tanaka K. Ren H. W. 2006 Development of ELISA for metallothionein-II allows determination of heavy metal pollution of fresh water . , 11,2113 2122 ,1093-4715 - 19.
Chyan C. L. Lee T. T. Liu C. P. Yang Y. C. Tzen J. T. Chou W. M. 2005 Cloning and expression of a seed-specific metallothionein-like protein from sesame. , 69,2319 2325 ,0916-8451 - 20.
Clemens S. 2001 Molecular mechanisms of plant metal tolerance and homeostasis. , 212,475 486 ,0032-0935 - 21.
Clendennen S. K. May G. D. 1997 Differential gene expression in ripening banana fruit. , 115,463 469 ,0032-0889 - 22.
Cobbett C. 2000 Phytochelatins and their roles in heavy metal detoxification . , 123,825 832 ,0032-0889 - 23.
Cobbett C. Goldsbrough P. 2002 Phytochelatins and metallothioneins: roles in heavy metal detoxification and homeostasis . , 53,159 182 ,1543-5008 - 24.
Collin-Hansen C. Pedersen S. A. Andersen R. A. Steinnes E. 2007 First report of phytochelatins in a mushroom: induction of phytochelatins by metal exposure in Boletus edulis. , 99,161 174 ,0027-5514 - 25.
Conklin D. S. Mc Master J. A. Culbertson M. R. Kung C. 1992 COT1, a gene involved in cobalt accumulation in Saccharomyces cerevisiae. , 12,3678 3688 ,0270-7306 - 26.
Coupe S. A. Taylor J. E. Roberts J. A. 1995 Characterisation of an mRNA encoding a metallothionein-like protein that accumulates during ethylene-promoted abscission of Sambucus nigra L. leaflets. , 197,442 447 ,0032-0943 - 27.
Dal Corso. G. Farinati S. Maistri S. Furini A. 2008 How plants cope with cadmium: Staking all on metabolism and gene expression. , 50,1268 1280 ,1672-9072 - 28.
Dallinger R. Lagg B. Egg M. Schipflinger R. Chabicovsky M. 2004 Cd accumulation and Cd-metallothionein as a biomarker in Cepaea hortensis (Helicidae, Pulmonata) from laboratory exposure and metal-polluted habitats . , 13,757 772 ,0963-9292 - 29.
de la Rosa G. Peralta-Videa J. R. Montes M. Parsons J. G. Cano-Aguilera I. Gardea-Torresdey J. L. 2004 Cadmium uptake and translocation in tumbleweed (Salsola kali), a potential Cd-hyperaccumulator desert plant species: ICP/OES and XAS studies, ,55 1159 1168 ,0045-6535 - 30.
Delhaize E. Ryan P. R. 1995 Aluminum Toxicity and Tolerance in Plants. , 107,315 321 ,0032-0889 - 31.
Delhaize E. Kataoka T. Hebb D. M. White R. G. Ryan P. R. 2003 Genes encoding proteins of the cation diffusion facilitator family that confer manganese tolerance . , 15,1131 1142 ,1040-4651 - 32.
Domenech J. Mir G. Huguet G. Capdevila M. Molinas M. Atrian S. 2005 Plant metallothionein domains: functional insight into physiological metal binding and protein folding . , 88,583 593 ,0300-9084 - 33.
Domenech J. Orihuela R. Mir G. Molinas M. Atrian S. Capdevila M. 2007 The Cd(II)-binding abilities of recombinant Quercus suber metallothionein: bridging the gap between phytochelatins and metallothioneins. J Biol Inorg Chem, 12,867 882 ,0949-8257 - 34.
Eapen S. D’Souza S. F. 2005 Prospects of genetic engineering of plants for phytoremediation of toxic metals. , 23,97 114 ,0734-9750 - 35.
Edmond C. Shigaki T. Ewert S. Nelson M. D. Connorton J. M. Chalova V. Noordally Z. Pittman J. K. 2009 Comparative analysis of CAX2-like cation transporters indicates functional and regulatory diversity . , 418,145 154 ,1470-8728 - 36.
Ernst W. H. O. Verkleij J. A. C. Schat H. 1992 Metal tolerance in plants. , 41,229 248 ,0044-5983 - 37.
Foley R. C. Singh K. B. 1994 Isolation of a Vicia faba metallothionein-like gene: expression in foliar trichomes. , 26,435 444 ,0167-4412 - 38.
Foley R. C. Liang Z. M. Singh K. B. 1997 Analysis of type 1 metallothionein cDNAs in Vicia faba. , 33,583 591 ,0167-4412 - 39.
Fusco N. Micheletto L. Dal Corso. G. Borgato L. Furini A. 2005 Identification of cadmium-regulated genes by cDNA-AFLP in the heavy metal accumulator Brassica juncea L. , 56,3017 3027 ,0022-0957 - 40.
Garcia-Hernandez M. Murphy A. Taiz L. 1998 Metallothioneins 1 and 2 have distinct but overlapping expression patterns in Arabidopsis . , 118,387 397 ,0032-0889 - 41.
Guo X. Wu Y. Wang Y. Chen Y. Chu C. 2009 OsMSRA4.1 and OsMSRB1.1, two rice plastidial methionine sulfoxide reductases, are involved in abiotic stress responses . , 230,227 238 ,0032-0935 - 42.
Gustin J. L. Loureiro M. E. Kim D. Na G. Tikhonova M. Salt D. E. 2009 MTP1-dependent Zn sequestration into shoot vacuoles suggests dual roles in Zn tolerance and accumulation in Zn-hyperaccumulating plants . , 57,1116 1127 ,0136-5313x - 43.
Hell R. Stephan U. W. 2003 Iron uptake, trafficking and homeostasis in plants. , 216,541 551 ,0032-0935 - 44.
Herbik A. Giritch A. Horstmann C. Becker R. Balzer H. J. Baumlein H. Stephan U. W. 1996 Iron and copper nutrition-dependent changes in protein expression in a tomato wild type and the nicotianamine-free mutant chloronerva . , 111,533 540 ,0032-0889 - 45.
Himelblau E. Amasino R. M. 2000 Delivering copper within plant cells. , 3,205 210 ,1369-5266 - 46.
Hirschi K. D. Korenkov V. D. Wilganowski N. L. Wagner G. J. 2000 Expression of arabidopsis CAX2 in tobacco. Altered metal accumulation and increased manganese tolerance. , 124,125 133 ,0032-0889 - 47.
Hsieh H. M. Liu W. K. Huang P. C. 1995 A novel stress-inducible metallothionein-like gene from rice. , 28,381 389 ,0167-4412 - 48.
Hsieh H. M. Liu W. K. Chang A. Huang P. C. 1996 RNA expression patterns of a type 2 metallothionein-like gene from rice . 32,525 529 ,0167-4412 - 49.
Hudspeth R. L. Hobbs S. L. Anderson D. M. Rajasekaran K. Grula J. W. 1996 Characterization and expression of metallothionein-like genes in cotton. , 31,701 705 ,0167-4412 - 50.
Jacob C. Maret W. Vallee B. L. 1998 Control of zinc transfer between thionein, metallothionein, and zinc proteins . , 95,3489 3494 ,0027-8424 - 51.
Jayaraj J. Punja Z. K. 2008 Transgenic carrot plants accumulating ketocarotenoids show tolerance to UV and oxidative stresses . , 46,875 883 ,0981-9428 - 52.
Jiang X. Wang C. 2008 Zinc distribution and zinc-binding forms in Phragmites australis under zinc pollution . , 165,697 704 ,1618-1328 - 53.
Kagi J. H. Schaffer A. 1988 Biochemistry of metallothionein. , 27,8509 8515 ,0006-2960 - 54.
Kamizono A. Nishizawa M. Teranishi Y. Murata K. Kimura A. 1989 Identification of a gene conferring resistance to zinc and cadmium ions in the yeast Saccharomyces cerevisiae. , 219,161 167 ,0026-8925 - 55.
Kerkeb L. Kramer U. 2003 The role of free histidine in xylem loading of nickel in Alyssum lesbiacum and Brassica juncea . , 131,716 724 , ISSN0032-0889 - 56.
Klaassen C. D. Cagen S. Z. 1981 Metallothionein as a trap for reactive organic intermediates. , 136 Pt A,633 646 ,0065-2598 - 57.
Klaassen C. D. Liu J. Choudhuri S. 1999 Metallothionein: An intracellular protein to protect against cadmium toxicity. , 39,267 294 ,0362-1642 - 58.
Kobae Y. Uemura T. Sato M. H. Ohnishi M. Mimura T. Nakagawa T. Maeshima M. 2004 Zinc transporter of Arabidopsis thaliana AtMTP1 is localized to vacuolar membranes and implicated in zinc homeostasis. , 45,1749 1758 ,0032-0781 - 59.
Koh M. Kim H. J. 2001 The effect of metallothionein on the activity of enzymes involved in removal of reactive oxygen species. , 22,362 366 ,0253-2964 - 60.
Korenkov V. Park S. Cheng N. H. Sreevidya C. Lachmansingh J. Morris J. Hirschi K. Wagner G. J. 2007 Enhanced Cd2+-selective root-tonoplast-transport in tobaccos expressing Arabidopsis cation exchangers. , 225,403 411 ,0032-0935 - 61.
Krämer U. Cotter Howells. J. D. Charnock J. M. Baker A. J. M. Smith J. A. C. 1996 Free histidine as a metal chelator in plants that accumulate nickel . , 379,635 638 ,0028-0836 - 62.
Krämer U. Clemens S. 2005 Molecular Biology of Metal Homeostasis and Detoxification. In: , M. Tamäs, Martinoia, E., (Ed.),216 271 , Springer Verlag,978-3-64206-062-5 New York. - 63.
Krämer U. Talke I. N. Hanikenne M. 2007 Transition metal transport . , 581,2263 2272 ,0014-5793 - 64.
Laloi C. Apel K. Danon A. 2004 Reactive oxygen signalling: the latest news. , 7,323 328 ,1369-5266 - 65.
Lane B. G. Kajioka R. Kennedy T. D. 1987 The wheat germ Ec protein is a zinc-containing metallothionein. , 65,1001 1005 ,0829-8211 - 66.
Lanquar V. Lelievre F. Bolte S. Hames C. Alcon C. Neumann D. Vansuyt G. Curie C. Schroder A. Kramer U. Barbier-Brygoo H. Thomine S. 2005 Mobilization of vacuolar iron by AtNRAMP3 and AtNRAMP4 is essential for seed germination on low iron . , 24,4041 4051 ,0261-4189 - 67.
Lanquar V. Ramos M. S. Lelievre F. Barbier-Brygoo H. Krieger-Liszkay A. Kramer U. Thomine S. 2010 Export of vacuolar manganese by AtNRAMP3 and AtNRAMP4 is required for optimal photosynthesis and growth under manganese deficiency . , 152,1986 1999 ,1532-2548 - 68.
Ledger S. E. Gardner R. C. 1994 Cloning and characterization of five cDNAs for genes differentially expressed during fruit development of kiwifruit. , 25,877 886 ,0167-4412 - 69.
Lee J. Shim D. Song W. Y. Hwang I. Lee Y. 2004 Arabidopsis metallothioneins 2a and 3 enhance resistance to cadmium when expressed in Vicia faba guard cells . 54,805 815 ,0167-4412 - 70.
Leszczyszyn O. I. Blindauer C. A. 2010 Zinc transfer from the embryo-specific metallothionein EC from wheat: a case study. Phys Chem Phys, 12,13408 13418 ,1463-9084 - 71.
Li L. Kaplan J. 1998 Defects in the yeast high affinity iron transport system result in increased metal sensitivity because of the increased expression of transporters with a broad transition metal specificity. , 273,22181 22187 , ISSN0021-9258 - 72.
Liu S. X. Fabisiak J. P. Tyurin V. A. Borisenko G. G. Pitt B. R. Lazo J. S. Kagan V. E. 2000 Reconstitution of apo-superoxide dismutase by nitric oxide-induced copper transfer from metallothioneins. , 13,922 931 ,0089-3228X - 73.
Lucca P. Hurrell R. Potrykus I. 2002 Fighting iron deficiency anemia with iron-rich rice. , 21,184 190 ,0731-5724 - 74.
Marentes E. Rauser W. E. 2007 Different proportions of cadmium occur as Cd-binding phytochelatin complexes in plants . , 131,291 301 ,1399-3054 - 75.
Margoshes M. Vallee B. L. 1957 A cadmium protein from equine kidney cortex. , 794813 4819 ,0022-4936 - 76.
Mei H. Cheng N. H. Zhao J. Park S. Escareno R. A. Pittman J. K. Hirschi K. D. 2009 Root development under metal stress in Arabidopsis thaliana requires the H+/cation antiporter CAX4. New Phyto l, 183,95 105 ,1469-8137 - 77.
Memon A. R. Schroder P. 2009 Implications of metal accumulation mechanisms to phytoremediation . , 16,162 175 ,0944-1344 - 78.
Merrifield M. E. Ngu T. Stillman M. J. 2004 Arsenic binding to Fucus vesiculosus metallothionein. , 324,127 132 ,0000-6291X - 79.
Miller G. Shulaev V. Mittler R. 2008 Reactive oxygen signaling and abiotic stress. , 133,481 489 ,1399-3054 - 80.
Mittler R. Vanderauwera S. Gollery M. Van Breusegem F. 2004 Reactive oxygen gene network of plants , , 9,490 498 ,1360-1385 - 81.
Mithofer A. Schulze B. Boland W. 2004 Biotic and heavy metal stress response in plants: evidence for common signals. , 566,1 5 ,0014-5793 - 82.
Navarro A. Quiros L. Casado M. Faria M. Carrasco L. Benejam L. Benito J. Diez S. Raldua D. Barata C. Bayona J. M. Pina B. 2009 Physiological responses to mercury in feral carp populations inhabiting the low Ebro River (NE Spain), a historically contaminated site . , 93,150 157 ,1879-1514 - 83.
Neill S. J. Desikan R. Clarke A. Hurst R. D. Hancock J. T. 2002 Hydrogen peroxide and nitric oxide as signalling molecules in plants. , 53,1237 1247 ,0022-0957 - 84.
Ngu T. T. Dryden M. D. Stillman M. J. 2010 Arsenic transfer between metallothionein proteins at physiological pH . , 401,69 74 ,0000-6291 X - 85.
O’Halloran T. V. Culotta V. C. 2000 Metallochaperones, an intracellular shuttle service for metal ions. , 275,25057 25060 ,0021-9258 - 86.
Oomen R. J. Wu J. Lelievre F. Blanchet S. Richaud P. Barbier-Brygoo H. Aarts M. G. Thomine S. 2009 Functional characterization of NRAMP3 and NRAMP4 from the metal hyperaccumulator Thlaspi caerulescens . , 181,637 650 ,1469-8137 - 87.
Ortiz D. F. Kreppel L. Speiser D. M. Scheel G. Mc Donald G. Ow D. W. 1992 Heavy metal tolerance in the fission yeast requires an ATP-binding cassette-type vacuolar membrane transporter, , 11,3491 3499 ,0261-4189 - 88.
Ortiz D. F. Ruscitti T. Mc Cue K. F. Ow D. W. 1995 Transport of metal-binding peptides by HMT1, a fission yeast ABC-type vacuolar membrane protein, , 270,4721 4728 ,0021-9258 - 89.
Palmiter R. D. 2004 Protection against zinc toxicity by metallothionein and zinc transporter 1 . , 101,4918 4923 ,0027-8424 - 90.
Park K. Y. Jung J. Y. Park J. Hwang J. U. Kim Y. W. Hwang I. Lee Y. 2003 A role for phosphatidylinositol 3-phosphate in abscisic acid-induced reactive oxygen species generation in guard cells . , 132,92 98 ,0032-0889 - 91.
Pei Z. M. Murata Y. Benning G. Thomine S. Klusener B. Allen G. J. Grill E. Schroeder J. I. 2000 Calcium channels activated by hydrogen peroxide mediate abscisic acid signalling in guard cells. , 406,731 734 ,0028-0836 - 92.
Persans M. W. Nieman K. Salt D. E. 2001 Functional activity and role of cation-efflux family members in Ni hyperaccumulation in Thlaspi goesingense . , 98,9995 10000 ,0027-8424 - 93.
Pich A. Manteuffel R. Hillmer S. Scholz G. Schmidt W. 2001 Fe homeostasis in plant cells: does nicotianamine play multiple roles in the regulation of cytoplasmic Fe concentration? , 213,967 976 ,0032-0935 - 94.
Pinto E. Sigaud-Kutner T. C. S. Leitao M. A. S. Okamoto O. K. Morse D. Colepicolo P. 2003 Heavy metal-induced oxidative stress in algae . , 39,1008 1018 ,0022-3646 - 95.
Puig S. Mira H. Dorcey E. Sancenon V. Andres-Colas N. Garcia-Molina A. Burkhead J. L. Gogolin K. A. Abdel-Ghany S. E. Thiele D. J. Ecker J. R. Pilon M. Penarrubia L. 2007 Higher plants possess two different types of ATX1-like copper chaperones . , 354,385 390 ,0000-6291 X - 96.
Rauser W. E. 1999 Structure and function of metal chelators produced by plants: the case for organic acids, amino acids, phytin, and metallothioneins. , 31,19 48 ,1085-9195 - 97.
Rea P. A. 2007 Plant ATP-Binding cassette transporters. , 58,347 375 ,1040-2519 - 98.
Reeves R. D. Baker A. J. M. 2000 Metal-accumulating plants. In:Phytoremediation of Toxic Metals: Using Plants to Clean Up the Environment, I. Raskin, Ensley, B.D., (Ed.),193 229 , John Wiley & Sons Inc,978-0-47119-254-1 New York. - 99.
Reid S. J. Ross G. S. 1997 Up-regulation of two cDNA clones encoding metallothionein-like proteins in apple fruit during cool storage . , 100,183 189 ,1399-3054 - 100.
Reynolds T. L. Crawford R. L. 1996 Changes in abundance of an abscisic acid-responsive, early cysteine-labeled metallothionein transcript during pollen embryogenesis in bread wheat (Triticum aestivum). , 32,823 829 ,0167-4412 - 101.
Rhee S. G. 2006 H2O2, a necessary evil forcell signaling. Science, 312, 1882 1883 ,1095-9203 - 102.
Robinson N. J. Tommey A. M. Kuske C. Jackson P. J. 1993 Plant metallothioneins. , 295,1 10 ,0264-6021 - 103.
Robinson N. J. Winge D. R. 2010 Copper metallochaperones . , 79,537 562 ,0066-4154 - 104.
Roesijadi G. 2000 Metal transfer as a mechanism for metallothionein-mediated metal detoxification. , 46,393 405 ,0145-5680 - 105.
Roosens N. H. Bernard C. Leplae R. Verbruggen N. 2004 Evidence for copper homeostasis function of metallothionein (MT3) in the hyperaccumulator Thlaspi caerulescens. , 577,9 16 ,0014-5793 - 106.
Sagner S. Kneer R. Wanner G. Cosson J. P. Deus-Neumann B. Zenk M. H. 1998 Hyperaccumulation, complexation and distribution of nickel in Sebertia acuminata. , 47,339 347 ,0031-9422 - 107.
Salt D. E. Wagner G. J. 1993 Cadmium transport across tonoplast of vesicles from oat roots. Evidence for a Cd2+/H+ antiport activity. , 268,12297 12302 ,0021-9258 - 108.
Salt D. E. Rauser W. E. 1995 MgATP-Dependent Transport of Phytochelatins Across the Tonoplast of Oat Roots , , 107,1293 1301 ,1532-2548 - 109.
Salt D. E. Pickering I. J. Prince R. C. Gleba D. Dushenkov S. Smith R. D. Raskin I. 1997 Metal accumulation by aquacultured seedlings of Indian mustard . , 31,1636 1644 ,0001-3936 X - 110.
Salt D. E. 2006 An extreme plant lifestyle: metal hyperaccumulation. In: , L. Taiz, Zeiger, E., (Ed.), Sinauer Associates, Inc.,978-0-87893-511-6 Sunderland, MA. - 111.
Sato M. Bremner I. 1993 Oxygen free radicals and metallothionein. , 14,325 337 ,0891-5849 - 112.
Satofuka H. Fukui T. Takagi M. Atomi H. Imanaka T. 2001 Metal-binding properties of phytochelatin-related peptides . , 86,595 602 ,0162-0134 - 113.
Schutzendubel A. Polle A. 2002 Plant responses to abiotic stress: heavy metal induced oxidative stress and protection by mycorrhization. J Exp Bot, 53,1351 1365 ,0022-0957 - 114.
Sharma S. S. Dietz K. J. 2006 The significance of amino acids and amino acid-derived molecules in plant responses and adaptation to heavy metal stress . , 57,711 726 ,0022-0957 - 115.
Singh K. Foley R. C. Onate-Sanchez L. 2002 Transcription factors in plant defense and stress responses. , 5,430 436 ,0000-1369- 5266 - 116.
Song W. Y. Park J. Mendoza-Cozatl D. G. Suter-Grotemeyer M. Shim D. Hortensteiner S. Geisler M. Weder B. Rea P. A. Rentsch D. Schroeder J. I. Lee Y. Martinoia E. 2010 Arsenic tolerance in Arabidopsis is mediated by two ABCC-type phytochelatin transporters . , 107,21187 21192 ,1091-6490 - 117.
Suzuki K. T. Kuroda T. 1995 Transfer of copper and zinc from ionic and metallothionein-bound forms to Cu, Zn--superoxide dismutase. , 87,287 296 ,1078-0297 - 118.
Suzuki N. Koizumi N. Sano H. 2001 Screening of cadmium-responsive genes in Arabidopsis thaliana. , 24,1177 1188 ,0140-7791 - 119.
Thomine S. Wang R. Ward J. M. Crawford N. M. Schroeder J. I. 2000 Cadmium and iron transport by members of a plant metal transporter family in Arabidopsis with homology to Nramp genes . , 97,4991 4996 , ISSN0027-8424 - 120.
Thomine S. Lelievre F. Debarbieux E. Schroeder J. I. Barbier-Brygoo H. 2003 AtNRAMP3, a multispecific vacuolar metal transporter involved in plant responses to iron deficiency. , 34,685 695 ,0960-7412 - 121.
Tseng M. J. Liu C. W. Yiu J. C. 2007 Enhanced tolerance to sulfur dioxide and salt stress of transgenic Chinese cabbage plants expressing both superoxide dismutase and catalase in chloroplasts . , 45,822 833 ,0981-9428 - 122.
Tuteja N. 2007 Abscisic Acid and abiotic stress signaling . , 2,135 138 ,1559-2316 - 123.
van der Zaal B. J. Neuteboom L. W. Pinas J. E. Chardonnens A. N. Schat H. Verkleij J. A. Hooykaas P. J. 1999 Overexpression of a novel Arabidopsis gene related to putative zinc-transporter genes from animals can lead to enhanced zinc resistance and accumulation . , 119,1047 1055 ,0032-0889 - 124.
Vasak M. Hawkes G. E. Nicholson J. K. Sadler P. J. 1985 113Cd NMR studies of reconstituted seven-cadmium metallothionein: evidence for structural flexibility. , 24,740 747 ,0006-2960 - 125.
Wagner G. J. 1993 Accumulation of Cadmium in Crop Plants And Its Consequences to Human Health , 51,173 212 ,0065-2113 - 126.
Wang T. Wu M. 2006 An ATP-binding cassette transporter related to yeast vacuolar ScYCF1 is important for Cd sequestration in Chlamydomonas reinhardtii . , 29,1901 1912 ,0140-7791 - 127.
Wang W. Vinocur B. Altman A. 2003 Plant responses to drought, salinity and extreme temperatures: towards genetic engineering for stress tolerance. , 218,1 14 ,0032-0935 - 128.
Wang Y. C. Qu G. Z. Li H. Y. Wu Y. J. Wang C. Liu G. F. Yang C. P. 2010 Enhanced salt tolerance of transgenic poplar plants expressing a manganese superoxide dismutase from Tamarix androssowii . , 37,1119 1124 ,0301-4851 - 129.
Weber M. Harada E. Vess C. Roepenack-Lahaye E. Clemens S. 2004 Comparative microarray analysis of Arabidopsis thaliana and Arabidopsis halleri roots identifies nicotianamine synthase, a ZIP transporter and other genes as potential metal hyperaccumulation factors . , 37,269 281 ,0960-7412 - 130.
Weber M. Trampczynska A. Clemens S. 2006 Comparative transcriptome analysis of toxic metal responses in Arabidopsis thaliana and the Cd(2+)-hypertolerant facultative metallophyte Arabidopsis halleri. , 29,950 963 ,0140-7791 - 131.
Wei W. Chai T. Zhang Y. Han L. Xu J. Guan Z. 2009 The Thlaspi caerulescens NRAMP homologue TcNRAMP3 is capable of divalent cation transport . , 41,15 21 ,1073-6085 - 132.
Willner H. Vasak M. Kagi J. H. 1987 Cadmium-thiolate clusters in metallothionein: spectrophotometric and spectropolarimetric features. , 26,6287 6292 ,0006-2960 - 133.
Wong H. L. Sakamoto T. Kawasaki T. Umemura K. Shimamoto K. 2004 Down-regulation of metallothionein, a reactive oxygen scavenger, by the small GTPase OsRac1 in rice . , 135,1447 1456 ,0032-0889 - 134.
Wright C. F. Mckenney K. Hamer D. H. Byrd J. Winge D. R. 1987 Structural and Functional-Studies of the Amino Terminus of Yeast Metallothionein. , 262,12912 12919 ,0021-9258 - 135.
Xue T. T. Li X. Z. Zhu W. Wu C. G. Yang G. G. Zheng C. C. 2009 Cotton metallothionein GhMT3a, a reactive oxygen species scavenger, increased tolerance against abiotic stress in transgenic tobacco and yeast . , 60,339 349 ,0022-0957 - 136.
Yang Z. Wu Y. R. Li Y. Ling H. Q. Chu C. C. 2009 OsMT1a, a type 1 metallothionein, plays the pivotal role in zinc homeostasis and drought tolerance in rice . , 70,219 229 ,0167-4412 - 137.
Zenk M. H. 1996 Heavy metal detoxification in higher plants:a review. , 179,21 30 ,0378-1119 - 138.
Zhou J. Goldsbrough P. B. 1994 Functional homologs of fungal metallothionein genes from Arabidopsis . , 6,875 884 ,1040-4651 - 139.
Zhu J. Zhang Q. Wu R. Zhang Z. 2010 HbMT2, an ethephon-induced metallothionein gene from Hevea brasiliensis responds to H2O2 stress. , 48,710 715 ,1873-2690