Heavy Metals

Distribution of Heavy Metals and Conjugating Ligands in Root

Besides bioavailability, uptake and translocation efficiencies determine metal accumulation and distribution in plants (Clemens 2006). Roots are the plant organs in closest contact with metal-contaminated soils therefore, they are the most affected by metals. Resistance to excess metals can be achieved by avoidance when the plant is able to restrict metal uptake into the cells, or tolerance when the plant is able to survive in the presence of excess metals inside. Having been taken up by the root and transported to various cells and tissues within the plant, heavy metals concentrate there to cause injury in a sensitive plant, or as an inactivated form in a tolerant plant.

Distribution of Heavy Metals and Conjugating Ligands in Shoots

An energy-dispersive X-ray (EDX) analysis system equipped to variable pressure scanning electron microscopy (VP-SEM) revealed that the tobacco trichomes exudates contain amounts of heavy metals. Overexpression of cysteine synthase confers Cd tolerance to tobacco, and the endogenous concentration of Cd was 20 less in transgenic plants than in wild-type plants. The numbers of both long and short trichomes in the transgenic plants were 25 higher than in that of wild-type plants, indicating the active excretion of Cd from trichomes in transgenic plants (Harada and Choi 2008). Upon Cd or Zn treatment, the number of trichomes was increased more than 2-fold (Choi et al. 2001 Sarret et al. 2006). Confocal laser scanning electron microscopy showed metal accumulation in the tip cells in trichomes. The chemical forms of the exudated grains were identified as metal-substituted calcite (calcium carbonate) by using synchrotron-based X-ray microanalyses (Sarret et al. 2006, 2007). Observation by...

Wholecellbased Biosensors For Detection Of Bioavailable Heavy Metals

The quantitation of bioavailable metal is difficult with traditional analytical methods. However, the bioavailability of metals is an important factor in the determination of metal toxicity and therefore the detection of bioavailable metals is of interest. The new concept of analyzing bioavailability of heavy metals by creating microbial strains capable of sensing the environment will rely largely upon the functional genomics of a metal resistance genetic system. The greatest advantage is the ability of biosensors to detect the bioavailable fraction of the contaminant, as opposed to the total concentration. Such whole-cell bacterial biosensors will create a clearer picture by providing physiologically relevant data in response to a contaminant. The essence of all metal resistance genetic systems is the specificity of genetic regulatory elements so that the corresponding metal controls the expression of the uptake or resistance gene products. For example, cobalt-zinc-cadmium resistance...

Role Of Pcs In Detoxification Of Heavy Metals Other Than Cadmium

As stated earlier, synthesis of PCs is induced by most heavy metals, including the multiatomic anions 74,76 , in most of the higher plants 73,83 . It has also been observed that the enzyme involved in its synthesis, PC synthetase, needs the presence of heavy metals for its activation a crude preparation of the enzyme from S. vulgaris was activated best by Cd2+, and by Ag+, Bi3+, Pb2+, Zn2+, Cu2+, and Au+ in decreasing order 95 . No activation of the enzyme was detected by the metals of the hard-acceptor category including Al3+, Ca2+, Fe3+, Mg2+, Mn2+, Na+, and K+. The trend of activation observed by Grill et al. 95 , however, was not observed for PC synthetase from tobacco cells, except that Cd was the most effective activator, followed by Ag+. The activation by Cu2+ was next to Ag+, and Pb2+, Zn2+, and Hg2+ produced only weak stimulation of the enzyme activity 96 . Thus, although the enzyme has a rather nonselective domain for binding with metals, it is mostly activated by heavy...

Metal Tolerance of ECM Associations 1131 Metal Toxicity

Metal toxicity can interfere with the essential physiological and reproductive processes in fungi (Gadd 1993 Amir and Pineau 1998). Metal toxicity-based antimycotica illustrates the relevance of fungal metal sensitivity. Cu(II)sulphate, a fungicide still used in viticulture, is effective in controlling plant parasitic fungi at concentrations which are not toxic to the host plant. Metal toxicity depends largely on speciation. Free metal ions, oxyanions and certain organic metal compounds such as methylated Hg are particularly toxic (Gadd 2007). Mechanisms of metal toxicity comprise the elicitation of oxidative stress (even in non-redox-active metals such as Cd), depletion of antioxidant pools, competitive inhibition of the uptake of essential elements, denaturation of proteins, interference with functional groups of proteins by displacement of essential cationic cofactors, precipitation of P inducing P deficiency and membrane disruption. Cytoplasmatic

Factors Affecting Uptake Of Heavy Metals By Trees

Phytoextraction is the subset of phytoremediation that aims to maximize the uptake of heavy metals from soil into the plant. Thus, measurements of metal concentrations in plant tissue have been commonly carried out in order to assess the effectiveness of various tree species. Much, but not all, of this work has focused on the growth of Salix spp. in SRC 18,31-34 . Populus, the other genus within the same family as willows (the Salicaceae), has also been studied 35-39 . Pine 40,41 , sycamore 24,26,42 , birch 43-48 , and oak 30 have also attracted interest. In Mediterranean climates, Acacia retinoides and Eucalyptus torquata growing on Cu mine wastes have been found to take up Cu and Pb from soil 48 . Species of Acacia and Leucaena leucocephala are potential candidates for tropical soils 49,50 . Field and pot experiments have been carried out on a wide range of contaminated substrates (Table 20.1). Variations in pH, redox potential, particle size, and organic matter content, as well as...

Interference of Heavy Metal Toxicity with Auxin Physiology

Abstract Auxins are important phytohormones involved in the coordination of plant growth and defence. In this chapter, we summarize auxin functions in plant biology and identify interactions with heavy metal toxicity. Cadmium induces the formation of reactive oxygen species, which in turn activate auxin oxidases. Auxin oxidases lead to an increased degradation of auxin and thereby are likely to decrease the activities of many genes involved in growth processes. Evidence supporting this hypothetic signalling cascade from heavy metals to eventual growth reductions comes from auxin feeding experiments that ameliorate Cd toxicity, positive effects of auxin producing endophytes on Cd tolerance and by monitoring endogenous auxin physiology with auxin reporter lines of Arabidopsis thaliana and Populus x canescens. Available data for essential micronutrients suggest that they interact with auxin physiology in a manner similar to Cd when present in excess.

Heavy Metal Toxicity Oxidative Stress Parameters and DNA Repair

Abstract Plant growth and productivity are adversely affected by frequent exposure to various abiotic and biotic stress factors, such as heavy metals. Heavy metals are elements with a relatively high density and are toxic or poisonous. Different molecular mechanisms for heavy metal toxicity have been described, and, among these, the production of reactive oxygen species (ROS) deserves special attention. ROS are highly reactive atoms or molecules naturally produced in plants and predominantly formed in the electron transport chain of cellular respiration (chloroplasts) and in photoreactivation. Oxidative attack on DNA generates both altered bases and damaged sugar residues that undergo fragmentation and lead to strand breaks. DNA damage caused by exposure to ROS is one of the primary causes of DNA decay in most organisms. The irreversible DNA damage can interfere with plant development and affect crop productivity. To protect the cells, a complex network of proteins is activated for...

Distribution of Heavy Metals in Soil

Distribution and accessibility of heavy metals to plants is important while assessing the environmental quality of an area. The levels of the heavy metals cadmium (Cd), copper (Cu), lead (Pb), manganese (Mn), nickel (Ni), and zinc (Zn) in the agricultural soils of the Bursa plain have been shown in the Table 20.1 to determine the degree of pollution and was found generally higher than the levels reported in literature for similar soils, suggesting some degree of pollution with heavy metals. The exchangeable forms of the heavy metals, though very low, indicates that the availability of heavy metals to plants is at a minimum (Aydinalp and Marinova 2003) .

Localization of Heavy Metals in Cells and Tissues of Different Plant Organs

Rejective Way Silicon Uptake

As shown in Fig. 2, general mechanisms for detoxification and accumulation of heavy metals in plants are the distribution of the metals to apoplastic compartments like cell walls or trichome, and the chelation of the metals by a ligand in cytoplasm, followed by the sequestration of the metal ligand complex into the vacuole, in the different organs such as roots, stems and leaves (Yang et al. 2005). Generally, the heavy metal contents in plant organs decrease in the following sequence root leaves stems inflorescence seeds. However, this order sometimes varies with plant species, especially in hyperaccumulators, of which the shoots have the highest heavy metal content. Roots usually manifest the maximum content of heavy metals. Leaves vary with age in their ability to accumulate heavy metals, some heavy metals accumulate preferentially in the youngest leaves of plants, whereas in others, the maximum content is found in senescing leaves. Preventing Cd ions from entering the cytosol by...

Plant Tolerance and Fatty Acid Profile in Responses to Heavy Metals

Heavy metals are more widespread around the world and dangerous for biosphere because they cannot be degraded or destroyed rather tend to be bioaccumulated. Plants can survive even in the extreme environmental conditions, but some environmental factors can affect its various growth aspects and hence the plant productivity. The problem of heavy metal toxicity is further aggravated by the persistence of the metals in the environment. Toxic heavy metals entering the plant tissues inhibit most physiological processes at all levels of metabolism. The extent of inhibition of photosynthesis, ion water uptake, and nitrate assimilation is greatly dependent on the concentration of the metal ions, sensitivity, and tolerance of the plant. There is, therefore, a pressing need to deal with the problem of excess metal already present in the soil and to prevent future contamination. Heavy metals Toxicity Contamination Tolerance Accumulation Fatty acids

Effect of Heavy Metals on Plants

Heavy metals decrease leaf expansion, resulting in a more compact leaf structure and increased stomatal resistance (Horvath et al. 1996). They may impair leaf transpiration and CO2 fixation by decreasing leaf conductance to CO2 diffusion as a result of stomatal closure (Barcelo et al. 1988). Heavy metals in growth media can function as stressors causing physiological constraints that suppress plant vigor and inhibit plant growth. Heavy metals can inhibit photosynthesis of intact plants at several physiological levels stomata, pigment synthesis, chloroplast structure and function, and indirectly by affecting various other metabolic pathways (Costa and Spitz 1997). Their treatments inhibit net photosynthesis in various crop plants such as corn and soybean (Bazzaz et al. 1974i tomato (Baszynski et al. Heavy metal exerted specific influence on the differentiation of various tissues in the root as well as stem. Elevated concentrations of these metals induced drastic anatomical changes....

Heavy Metal Detoxification Mechanisms In Plants

Cunninghum and Ow 36 envisioned the working of phytoremediation as follows By growing plants over a number of years the aim is to either remove the pollutants from the contaminated matrix or to alter the chemical and physical nature of the contaminants within the soil so that it no longer presents a risk to human health and the environment. Thus, plants resistant to heavy metals can be used under the concept of phytoremediation in one or more of the following ways It is explicit that the plants to be used under the first category, i.e., for the removal of metals from soil, should be hyperaccumulators of the heavy metals contaminating the land to be used under the second category, plants may or may not be hyperaccumulators, but should be resistant to the metals present in the soil and able to grow well, with good rooting systems. For plants to be used under the third category, in addition to being resistant to the metals contaminating the soil, it is necessary that they do not take up...

Heavy Metals in Different Environmental Matrices

The problem of such heavy metals and metalloids as cadmium, mercury, nickel, lead or chromium accumulation in various ecosystems is not new and is increasing significantly because of the improvement of the level of life, population expansion and heavy industry development. High natural levels of the pollutants in environmental matrices are the result of agricultural and semi-industrial activities, energy supply, mining or waste disposal. Heavy metal ions are ubiquitous in water (ground or surface water), plants, animals, soil as sediments and sewage sludge. They are present in all environmental matrices but in diverse concentration levels, depending on the metal, the matrix and the distance from pollutant sources, which - considering their toxicity and easy translocation in living organisms - causes the real threats. and accumulating in the food chain - fruits, vegetables, crops, plant tissue, etc). Uptake of heavy metal ions by plants and or animals is one of the main causes of their...

Diversity and Structure of ECM Communities Exposed to Metal Toxicity

Are there fungal species, genera or larger phylogenetic groups that are excluded by metal toxicity The representation of major phylogenetic groups in ECM communities can be very different in similar edaphic conditions. Urban et al. (2008) found most major ECMF orders associated with pines and oaks on serpentine, except Pezizales and Gomphales. Moser et al. (2008) found several pezizalean ECMF associated with oaks on serpentine. Possibly, the potential of various species to colonize metalliferous soils is conditioned by the local evolutionary history rather than by genetic predispositions. At present, our knowledge about constitutive and adaptive mechanisms of tolerance is limited to a few well-studied model organisms. It is unknown whether different ECMF species use similar or diverse tolerance mechanisms, and if their population structures are similar. It appears that metal-tolerant ECMF genotypes are typically derived from local species pools (Staudenrausch et al. 2005 Colpaert...

Ectomycoremediation of Heavy Metals

Metals mainly in their shoot (phytoextraction). However, plants often accumulate heavy metals in their root system to protect the shoot from toxic heavy metal concentrations (Bucking and Heyser 1994). This is the reason why in the past, studies on the phytoremediation of heavy metal-contaminated soils were mainly conducted with hyperaccumulating plants, such as Thlaspi caerulescens, which are able to tolerate and to concentrate high heavy metal concentrations in the shoot (Robinson et al. 1998). However, the small biomass development of these species significantly reduces their potential to extract significant amounts of heavy metals from contaminated soils. Fast growing trees, such as Populus and Salix, could potentially be used for the phytoremediation of these sites, because (1) both are known to naturally colonize areas with high metal soil concentrations such as active and inactive smelter sites (Cripps 2003), (2) they are genetically transformable (Doty 2008), and (3) the high...

Heavy Metals

Aluminum, arsenic, cadmium, lead, mercury, nickel, and other heavy metals abound in the human environment, where they are found in pesticides, cooking utensils, paint, tin cans, solder, cigarettes, dental fillings, contaminated fish, some cosmetics and antacids, and industrial products and by-products. Battery makers, gas station attendants, agricultural workers, printers, jewelers, and dentists, for example, face increased heavy-metal exposure risks.

Heavy Metal Bindings and Their Interactions with Thiol Peptides and Other Biological Ligands in Plant Cells

Abstract Plants have developed their potentials for uptake, transport and accumulation of terrestrial elements in order to coordinate their developmental and life-cycle performance. The utilization and toxicity of the metallic elements in plants are principally based on their own chemical properties in water and the interaction with their counterpart anions and cooperative molecules. Biochemical partners of the metals are various organic ligands composed of C, H, O, N, P, or S. Their roles are shared by two cell sites - the outside apoplast and the inside symplast. The apoplast equips the polymeric ligands of polysaccharides, phenolics, and proteins with carboxylic and some other functional groups capable of conjugating metals in the cell surfaces, but excess heavy metals in the primary cell wall are toxic to plants. Mobile organics in the apoplast have another function in xylem transport or biological interactions in the rhizosphere underground. The symplast (and vacuole) contains a...

Origination Composition and Metal Chelating Properties

Sludge-borne organic matter will improve physical and chemical soil conditions. In physical terms, organic matter improves the stability of soil aggregates, thus resulting in better aeration, and increases water retention capacity of the soil. In chemical terms, soil solid phase per unit mass increases significantly, due to the large specific surface of the humic substances. Thus, it contributes to the increase of the adsorption sites, improves the nutrient retention of soil, and reduces the losses by leaching. Further decomposition of humic substances will produce low molecular weight organic substances (LMWOS) or dissolved organic carbon (DOC) these are highly active in the soil environment and bear significant binding abilities for heavy metals. These LMWOS comprise free amino acids, sugars, peptides, aliphatic acids, and their polymers, with molecular weights usually not exceeding 5000 to 10,000 g mol-1 6 . Heavy metals added in soil with sewage sludge are greatly influenced by...

Plant Roots and the Rhizosphere

Organisms colonise roots of higher plants and form symbiotic relationships. In this way the availability of nutrients, especially P, can be greatly increased (Bolan, 1991). Jenny and Grossenbacher (1963) were the first to draw attention to the possibility of an intimate contact between roots and minerals when they published electron micrographs which showed that the root can be surrounded by a mucigel which fills the space between the cell wall and a soil particle. Morel et al. (1986) have discussed the importance of this mucilagenous layer in retaining heavy metals in the rhizosphere. The flux of Cu and Pb to the root can be reduced whereas Cd is more easily transported.

Heavy Metal Perception in a Microscale Environment A Model System Using High Doses of Pollutants

Abstract The characterization of the mechanisms of heavy metal detoxification has been undertaken through several experimental approaches, where high metal concentrations have been frequently used. A microscale hydroponic system was used to discriminate between the direct and indirect phytotoxic effects that may occur under heavy metal stress at short exposure times. Induction of oxidative stress and generation of stress signaling molecules are some of the physiological responses triggered soon after the exposure of plant cells to heavy metals, which might be part of stress perception mechanisms. The generation of reactive oxygen species, in particular H2O2, ethylene or jasmonate are envisaged as messengers in signaling pathways that may result ultimately in cell senescence and growth inhibition. D.K. Gupta and L.M. Sandalio (eds.), Metal Toxicity in Plants Perception, Signaling and Remediation, DOI 10.1007 978-3-642-22081-4_2, Springer-Verlag Berlin Heidelberg 2012

Heavy Metal Loadings in Sewage Sludge and in Soils where Sludge Is Added

Heavy metals occur in sewage sludge through domestic, run-off, and industrial inputs, with the industrial input contributing only a small percentage of the total in specific elements, such as Pb and Zn 1 . Nowadays, heavy metal loads in sewage sludge have been significantly reduced as a result of improved effluent control and the use of cleaner technologies from industries due to tighter state legislations. As a result, the metal loadings in sewage sludge and top soils are being reduced (e.g., reports by Chaudri et al. 26 for England and Wales concerning Cd Barbarick et al. 27 , reporting a period from 1982 to 1992 Sloan et al. 28 and Berti and Jacobs 29 ).

Typical Concentrations of Nutrients and Organic Carbon in Sewage Sludges

Nevertheless, it is inevitable that sewage sludge application to soil will increase heavy metal concentrations well beyond their background concentrations. This is the reason that governments deal with this prospect, setting upper limits of heavy metals in soils and sludges to be added to soils. Heavy metal concentration limits in sewage sludge for the U.S. and the E.U. are shown in Table 3.4. Background metal concentrations in soils and maximum permissible loading rates are shown in Table 3.5. It is evident that metal-loading standards differ greatly between the U.S. and the E.U., and this stresses that sludge-induced metal contamination is still quite controversial and a matter not yet fully resolved 20 .

ROS Signaling and Antioxidant Responses

An increase in H2O2 production has been reported in plant cells treated with several heavy metals, even those that have no direct or very little redox activity such as Cd or Hg. Cadmium is one of the heavy metals most widely studied, and relevant information has been provided by Olmos et al. (2003), Garnier et al. (2006), Cho and Seo (2005), and Romero-Puertas et al. (2002, 2004). Mercury is also a potent H2O2 inductor, as shown by Cho and Park (2000) and Ortega-Villasante et al. (2007). Similar responses were found for Cu (Xiang and Oliver 1998 Maksymiec and Krupa 2006) and Mn (Demirevska-Kepova et al. 2004), although at much higher doses due to their essential nature, which depends on the threshold of toxicity for each plant species. This production of ROS usually leads to damage in several cellular components, causing membrane lipid peroxidation (Lozano-Rodriguez et al. 1997), alteration of nucleic acids structure (Fojtova et al. 2002), or oxidation of proteins (Romero-Puertas et...

Elemental Interactions and Comparisons

Tinker (1981) compared uptake rates of trace metals with those of major elements and suggested that the absorbing power for trace metals is relatively low. Bowie and Thornton (1985) have summarised the effect of elemental interactions on their uptake. The uptake of trace metal ions is inhibited by the major cations, especially by Ca. Interaction of Cu and Zn is such that both ions reduce the uptake of the other. Zn is also known to affect Fe levels in plants. Interactions between major elements and trace elements have been listed by Kabata-Pendias and Pendias (1984) where Ca and Mg are seen to be mainly antagonistic to the absorption and metabolism of several trace elements. A list of some of the interactions for the heavy metals has been given by Fergusson (1990). Some elements can be both antagonistic and synergistic to another element, presumably in different biochemical processes. Fergusson (1990) has also listed the relative uptake of some heavy elements by selected plants....

Defining Metal Tolerance

Interaction of resistance to metals and resistance to other stresses has not been thoroughly explored however, in the past, many studies on heat shock and drought stress did consider possible interactions with the response to heavy metals (see, for example, Gulli et al. 2005). Zhang et al. (2008) identified a correlation between resistance to water deficit, oxidative stress and metals by using transgenic plants overexpressing aquaporin PIP1. The authors postulated that the improved water status of the plant can explain the pleiotropic effect. The possible interactions between metals and organic pollutants in plants has been highlighted in a review (Verkleij et al. 2009), and metal-binding molecules such as glutathione and other antioxidative defences can act simultaneously on both types of pollutants (Rausch et al. 2007).

Calcium and Magnesium

It has long been known that the nature and extent of expression of environmental stresses, particularly ion stresses, on a whole plant depend on Ca. Ca ions alleviate toxic effects of Al (Foy, 1988 Brady et al., 1993), Na (Cramer et al., 1986) and various heavy metals (Baker and Proctor, 1990). It has been suggested that cytoplasmic levels of Ca2+ and Ca-related processes may represent a signal for metal toxicity and tolerance (Leonard and Hepler, 1990). Ca, or Ca bound to Ca2+ binding proteins (including calmodulin) is involved in the activity of a number of key enzymes, thus toxic metals may have an impact on normal Ca2+ fluxes and cytoplasmic concentrations, and thus alter cell metabolism (Siegel and Haug, 1983 Cheung, 1984). Calcium cycling amongst the various pools within the plant have been discussed by Cumming and Tomsett (1992).

Kenneth S Sajwan S Paramasivam AK Alva and J Afolabi

Furthermore, wastewater treatment plants in cities around the world still encounter serious problems in disposing of SS because of odor, high acidity, and levels of some heavy metals in excess of critical limits, public acceptance is lacking. This has caused exploration of alternative disposal methods. These endeavors have resulted in production of ash by incinerating dewatered activated sludge (ISS) and weathered ash (WISS) by dissolving and storing the incinerated byproduct in ash ponds by wastewater treatment plants. Despite all this, the beneficial effects of amending soils with various organic amendments, including the use of sewage sludge, are still dramatically on the rise.

Transport of Ions to the Aerial Parts of the Plant

There is a close relationship between the metabolism of the shoot and the root. It is generally accepted that the xylem forms the main path for upward movement of water and ions from the roots to the leaves. Most of the essential major elements are transported in the xylem as inorganic ions. Nitrogen may be transported along the xylem as N03 if it is present in the external solution as nitrate. However, the plant sap may also contain organic nitrogen compounds such as amino acids. In the xylem, heavy metals will usually only be transported if special chelates are formed, eg, by citrate (Streit and Stumm, 1993). Iron is taken up and transported more readily when supplied as a chelated complex, such as ferric ethylenediamine tetraacetate (FeEDTA) or as ferric diethylenetriamine pentaacetate (FeDTPA) (Wallace and North, 1953). Calcium may also be transported in a chelated form (Jacoby, 1966).

Mechanisms of Tolerance

To illustrate the integration of physiological and biochemical mechanisms of metal tolerance, we have selected tolerance to copper as an example. There has been a great deal of work on the mechanisms of copper tolerance in plants, but as yet we are still very far from resolving the problem. An early paper by Wainwright and Woolhouse (1977) pointed out that copper appeared to damage the cell membranes of nontolerant plants, and produced evidence that cell-wall bound acid-phosphatases of tolerant and non-tolerant plants might differ in their sensitivity to copper. In 1980, Rauser and Curvetto (1980) published the first paper suggesting that copper-tolerant plants of A. gigantea might possess a metallothionein which was involved in tolerance. Due to the known importance of metallothioneins in producing metal tolerance in animals, this discovery opened up considerable possibilities in plants, and substantial research effort was devoted to the study of these interesting compounds. A decade...

Development of an LacZ Based Arsenic Biosensor [344

These two methods are apparently suitable for environmental samples of low bioavailable arsenic content however, both require relatively costly instruments and may not be suitable in a field study. There are few reports of successful gene-fusion biosensors in the monitoring of a metal toxicity in field application. For instance, a luminescent bacterial biosensor was shown to be effective in the evaluation of arsenic bioavailability of chromated copper arsenate contamination 343 . Detection limit of arsenite by arsenic hypersensitive clone pASH3 was comparable with the earlier claims the methodology is relatively simple as well. Moreover, the process can easily be improvised to an acceptable arsenic assay kit with a low-cost investment for monitoring a large number of samples for on-site analysis.

Hyperaccumulators of Copper

In any discussion of hyperaccumulation of heavy metals, the question immediately arises as to whether these taxa are obligate or facultative metallophytes ie, requiring, or not requiring, the accumulated element for plant growth or development. From the many studies that have been carried out on these plants (summarised by Baker and Brooks, 1989), it is clear that all the species tested in the laboratory will grow quite well in soils containing 'normal' levels of heavy metals (eg, Haumaniastrum robertii and H. katangense). Nevertheless, in the course of laboratory experiments at this university, we have noticed that all the metallophytes tested, were very sensitive to fungal attack, and it is probably this factor, as well as an inability to withstand competitive pressure from other species, that restricts them to mineralised soils. To that extent they may be considered to have obligate characteristics, albeit related to be low abundance of fungi in their growth substrates.

ROS Metabolism in Response to Cadmium 51 ROS Production Under Cd Stress

Different enzymes including NADPH oxidases, peroxidases and different sources from organelles and the responsibility for the metal toxicity signalling depends on the tissue, timing and plant conditions (Romero-Puertas et al. 2004 Garnier et al. 2006 Horemans et al. 2007 Cuypers et al. 2011). The Cd-induced oxidative burst follows a signalling cascade that is dependent on calmodulin, intracellular Ca2+ mobilization, entry and competition with Cd. H2O2 production is also dependent on kinases and protein phosphatases and, in part, on ADPR cyclase and guanylate cyclase (Olmos et al. 2003 Romero-Puertas et al. 2004 Garnier et al. 2006 Rodr guez-Serrano et al. 2009a). Additionally, in Arabidopsis suspension cells exposed to 100 and 150 p.M of Cd that undergo PCD, H2O2 production is dependent on a previous NO release (De Michele et al. 2009).

Overexpression Of Mts As A Means To Increase Cadmium Tolerance

An important pathway by which plants detoxify heavy metals is through sequestration with heavy metal-binding peptides called phytochelatins or their precursor, glutathione. To identify limiting factors for heavy metal accumulation and tolerance and to develop transgenic plants with an increased capacity to accumulate and or tolerate heavy metals, the Escherichia coli gshII gene encoding glutathione synthetase (GS) was overexpressed in the cytosol of Indian mustard (Brassica juncea). The transgenic GS plants accumulated significantly more Cd than the wild type shoot Cd concentrations were up to 25 higher and total Cd accumulation per shoot was up to threefold higher. Moreover, the GS plants showed enhanced tolerance to Cd at the seedling and mature plant stages. Cd accumulation and tolerance were correlated with the gshII expression level. Cd-treated GS plants had higher concentrations of glutathione, phytochelatin, thiol, S, and Ca than wild-type plants. The conclusion was that, in...

Heavy Metal Contamination Of Soil And Associated Agricultural And Environmental Problems

Table 16.1 shows the yearly figures of production of heavy metals. Although the figures are of much environmental concern, these are of little importance as far as contamination of soil is concerned. This is because the use of heavy metals as industrial produce by mankind remains confined to cities and suburban areas, which may constitute less than 10 to 15 of the total inhabitable land mass. More importantly, the heavy metals used as industrial produce mostly find their way into aquatic environments through the drainage system and run-off water during the rainy season from there, their return to the atmosphere and landmass through biogeochemical cycling is very slow 4 . Furthermore, it may also be noted that the use of heavy metals like Hg and As as components of pesticides in agriculture has been nearly discontinued, and the contamination of the land mass by these through agricultural practices is now only history. Also, although the use of fertilizers may result in contamination of...

Organelles Involvement in Cd Stress

ROS production after cadmium stress differs in the nature of the source and localization in the cell. It seems that NADPH oxidase from plasma membrane is one of the first enzymes involved in ROS production in response to heavy metal (Olmos et al. 2003 Garnier et al. 2006). Other organelles, however, such as mitochondria (Garnier et al. 2006), peroxisomes (Romero-Puertas et al. 1999) and chloroplasts (Bi et al. 2009), are also involved and play a key role in ROS release and or detoxification in response to Cd. Several studies have shown that the vacuole is a site of accumulation of different heavy metals (Cobbett 2000 Verbruggen et al. 2009). Actually, chelating complexes formed by Cd can be transported into the vacuoles, preventing the free circulation of Cd ions and placing them into a limited area (Sanita di Toppi and Gabrielle 1999). On the other hand, it has been reported that Cd2+ increase H2O2 and O2 - accumulation in mitochondria (Heyno et al. 2008 Garnier et al. 2006)....

Cellular Compartmentalization Of Pcmetal Complexes And Metal Tolerance

Another important aspect of PC-mediated tolerance of plants to heavy metals is probably the effective transportation of the metal to vacuoles for storage in which they could be playing an important role. Arguments in favor of this come from several observations. Vogeli-Lange and Wagner 124 isolated mesophyll protoplast from tobacco exposed to Cd and showed that the vacuoles contained 110 8 of the protoplast Cd and 104 8 of the protoplast PCs. These workers envisioned the synthesis of PCs in cytosol and transfer of Cd and the peptides, perhaps as complex, across the tonoplast into the vacuole, where the metal is chelated by the peptides and organic acids. Once inside the vacuole, more Cd, transported by Cd2+ H+ antiporter, is added to the LMW, along with Apo-PCs and sulfide complexes, to produce HMW complexes. Genetic and biochemical analyses suggest that the formation of sulfide moiety in the HMW PC-Cd-S2- complex involves purine metabolism, which serves as the source of sulfide...

Electrodialytic Soil Remediation

An anion exchange membrane, AN, interposed between the anode compartment and the soil, will prevent cations from passing from this electrode compartment into the soil (apart from H+, which can be an exception) and will allow anions to pass from the soil into this compartment. Similarly, a cation exchange membrane, CAT, interposed between the soil and the cathode compartment, will prevent the passage of negatively charged ions into the soil and will permit the cations (e.g., heavy metals and natural cations present in the soil) to pass from the soil into the cathode compartment (Figure 18.3).

Results And Discussion

The data from the adsorption experiments (Figure 19.7 and Table 19.2) indicated that thiol-SAMMS adsorbed the heavy metals with significant affinity. The predicted adsorption maxima were 0.56, 0.72, 1.27, 4.11, and 6.37 meq g for Cu, Pb, Cd, Ag, and Hg, respectively. The calculated distribution coefficients were 4.6 x 101 to 1.8 x 105 2.2 x 102 to 8.6 x 103 2.2 x 102 to 1.9 x 104 1.2 x 103 to 8.7 x 105 and 1 x 103 to 3.5 x 108 ml g for Cu, Pb, Cd, Ag, and Hg, respectively. Such selectivity and affinity in binding these heavy metals by thiol-SAMMS can be explained on the basis of the hard and soft acid base principle (HSAB) 8-10 , which predicts that the degree of cation softness directly correlates with the observed strength of interaction with soft base

Oxidative Stress and Cell Defenses

Plants have developed a variety of strategies to prevent excessive accumulation of nonessential metals within cells and or transform these metals into less toxic forms (Cobbett 2000). Some plants produce metabolites that bind to heavy metals in the cytosol, such as glutathione (GSH), polypeptides and proteins (e.g., metallothioneins and phytochelatins) and proline (Hall 2002). However, when these defense mechanisms are not sufficient, ROS overproduction occurs, which causes oxidative stress and activates other mechanisms (Patra et al. 2004). The defense system includes enzymatic and nonenzymatic antioxidants. Representatives of the enzymatic antioxidant defense system include superoxide dismutase, ascor-bate peroxidase, and catalase for ROS removal from cells (Halliwell and Gutteridge 1989). The main nonenzymatic water-soluble defenses include ascorbic acid (AA), thiols (predominantly glutathione), a-tocopherol, carotenoids, flavonoids, and polyamines, which have also been shown to...

Functions Of Aquatic Plants

Submerged rooted plants had some potential for the extraction of metals from water as well as sediments rootless plants extracted metals rapidly only from water 22 . In submerged plants, leaves are the site of mineral uptake 23 . The foliar absorption of heavy metals is by passive movement through the cuticle, where the negative charges of the pectin and cutin polymers of the thin cuticle and the polygalacturonic acids of the cell walls create a suck inwards. Due to the increase in the charge density inwards, transport of positive metal ions takes place 23,24 . No ions enter stomata and, in submerged plant leaves, no stomata are present.

Effects of Contamination on Plants

The most widely described effects of metal toxicity in plants are inhibited root growth, depressed shoot and leaf growth, and general chlorosis of the younger leaves (Baker and Walker, 1989). The index of metal tolerance is usually measured by the root elongation method (Wilkins, 1978). Young plants or tillers are grown both in control solution and in solutions containing known concentrations of the metal under test. The growth in the toxic solution is expressed as a percentage of that in the control solutions. Using such methods the tolerance of various species has been demonstrated. Other methods to measure toxicity and tolerance have been suggested. Wong and Bradshaw (1982) have described the ED50 method. The ED50 is defined as the effective dose of metal that produces 50 of normal root growth (in the control solution) and this method has been used to test toxicities of a number of metals, for example Wong and Bradshaw (1982) found that the order of increasing toxicities (from root...

Plants and Atmospheric Pollution

Some grasses growing on contaminated sites have been shown to have tolerant ecotypes (Gregory and Bradshaw, 1965), for example, Holcus lanatus has been shown to be Cd-tolerant (Coughtrey and Martin, 1977) Agrostisgigantea has been described as metal tolerant (Hogan et al., 1977 Hogan and Rauser, 1979) metal co-tolerances have been described in Deschampsia caespitosa (Cox and Hutchinson, 1979) Kovacs et al. (1993) have shown that the roots of Lolium perenne are effective accumulators of a number of heavy metals. The use of lower plants as biomonitors has recently been detailed. Garty (1993) has extensively reviewed the use of lichens as biomonitors the use of fungi has been reviewed by Mejsth'k and Lepsov (1993) and by Wondratschek and R der (1993) and mosses as biomonitors have been discussed by Steinnes (1993) and by Br ning and Kreeb (1993) the effects of heavy metals on bryophytes has been reviewed by Tyler (1990).

Role of 20S Proteasome

With low metal concentrations together with MG132 or higher cadmium concentrations decreased 20S proteasome activity with a concomitant accumulation of oxidized and ubiquitinated proteins. Impairment of proteasome functionality under high Cd concentration was associated with severe oxidation of 20S protein (Pena et al. 2007). Similar observations were described for proteasome isolated from maize roots submitted to a mild oxidative treatment through MCO system in vitro (Basset et al. 2002). Polge et al. (2009) demonstrated that RNAs encoding subunits of the 20S proteasome were up-regulated in response to cadmium in the leaves of A. thaliana. The increase in proteasome structural and catalytic subunit transcripts was followed by increases in proteasome quantity and chymotrypsin-like activity. Further, mutations of one of the subunits of the 19S regulatory particle resulted in an enhanced accumulation of the 20S versus 26S proteasome and a higher tolerance to oxidative stress in...

Interest in Phytoremediation

HMA4 under the control of strong promoters, as discussed above for Zn2+, in plants producing high biomass such as tobacco or poplar could result in lines specifically dedicated to phytoextraction of heavy metals, also able to decrease the length of the decontamination treatment which is the major drawback. Additionally, a coexpression of HMA3 could result in a better tolerance of these plants (Morel et al. 2009). Cys and His residues in the soluble C-termini of AtHMA2 and AtHMA4 have been identified as metal-binding residues (Wong et al. 2009b Baekgaard et al. 2010). Interestingly, expression of the unique soluble C-terminus of AtHMA4 or TcHMA4 was sufficient to rescue Cd2+ Zn2+-sensitive yeast mutants (Bernard et al. 2004 Papoyan and Kochian 2004). In plants, overexpression of the AtHMA4 C-terminus in tobacco increased cadmium and zinc concentrations in roots and shoots up to fourfold (Siemianowski et al. 2011). This last result shows that expression of chelating peptides in plants...

Sludge Usage International Regulations

The residual product generated from sewage treatment is termed sludge. Sewage sludge contains heavy metals, organic compounds, and pathogens, in addition to substantial amounts of nutrients. Sludge may be disposed of by depositing, burning, or dumping into the sea or it may be used in forestry and agriculture. On the one hand, spreading sludge onto the land is desirable because the nutrients of sludge would participate in the biogeochemical cycle in ecosystems. However, due to the inherent nature of its composition, sludge involves the risk of harming the environment because the substances may be accumulated in the soil or damage the ecosystem. Thus, health considerations must be considered by environmental regulatory agencies with respect to the use of sludge because it contains pathogens in the form of bacteria, viruses, and parasite eggs. This content can be reduced by stabilization or disinfection of the sludge. To ensure that the environment and man are fully safeguarded against...

Bioaccumulation of Trace Elements in Plants

Soil microorganisms affect trace element mobility and availability to plants by production of metal chelators, siderophores, alteration of soil pH, and solubilization of metal phosphates they also influence root parameters such as root morphology, growth, and root exudation, leading to increased uptake trace metals 32 . Abou-Shanab et al. 32 showed that rhizobacteria play an important role in increasing the Ni availability in soil by releasing Ni from the nonlabile phase in the soil, thus enhancing the availability to the plants. In contrast, vesicular-arbuscular fungi in the roots of the plants reduced the uptake of trace elements, leading to increased plant biomass due to decreased uptake of heavy metals 33-35 . Furthermore, Leyval and Joner 36 also concluded that ecto- and AM-mycorrhizae tended to reduce metal concentration in the shoots of nonhyperaccu-mulator plants. Certain plant growth-promoting bacteria also reduce the metal stress on plants. Burd et al. 37 found that Kluyvera...

Immobilization Using Chemical Amendments

Many additives have been screened for their potential to immobilize heavy metals in soils. Many of these additives are alkaline materials such as lime 96,97 , zeolites 80 , incinerator ashes 98 , Fe-rich byproducts from TiO2 pigment production 99 , and hydroxyapatite 100 . In addition to the amendment fixing the metal, additions of alkaline amendments will increase soil pH values, thus causing other exchange sites (present on clay surfaces, iron oxides, and organic matter) to be more reactive to metal binding 80 . Applications of chemical amendments have been shown to bind significant amounts of Co, Cu, and Zn in contaminated soils and sediments. For example, apatite was effective at binding Co, Cu, and Zn in contaminated media 92,93 . Additionally, significant binding of Zn in contaminated materials by Fe-oxides 99 , and by zeolites 80,92 has been reported.

Molecular Genetic And Transgenic Strategies For Phytoremediation Hyperacumulation

Transgenic plants capable of tolerating high levels of accumulated cadmium and lead have been developed recently 128 . These plants take up heavy metals more rapidly than traditional biore-mediation plants do, thus making them potential hyperaccumulators with application for phytoex-traction and rhizofiltration in the field 129-131 . Observing that certain Saccharomyces cerevisiae, which possess the YCF1, or yeast cadmium factor 1 protein, are known to pump cadmium Cd(II) into vacuoles, Li et al. 129 tested whether YCF1 would also confer resistance to lead Pb(II) . Also known as vacuolar glutathione S-conjugate transporter, YCF1 belongs to the ATP-binding cassette superfamily 2,3 . Li's team confirmed that YCF1 gene expression permitted S. cerevisiae to withstand the toxic effects of 3 mM lead (Pb II) and 0.1 mM cadmium (Cd II) concentrations in growth media. This protection against lead and cadmium toxicity was due to the uptake and storage of the heavy metals in yeast vacuoles....

PH Changes in the Rhizosphere and Bioavailability of Trace Elements

Bernal et al. 32 compared redox potential and pH changes in the rhizosphere of the Ni hyperaccumulator Alyssum murale and the nonhyperaccumulator Raphanus sativus. These workers concluded that the form of N taken up by the plants was the main factor responsible for pH changes and that the plants were able to reduce system more effectively than the hyperaccumulator. These results indicate that the hyperaccumulator mechanisms may be due to other rhizosphere processes, such as the release of chelating agents, or to differences in the number and affinity of metal root transporters. McGrath et al. 33 studied the heavy metals uptake and chemical changes in the rhizosphere of Thlaspi caerulescens and Thlaspi ochroleucum grown in contaminated soils. Knight et al. 34 investigated the Zn and Cd uptake by the hyperaccumulator Thlaspi caerulescens in contaminated soils and its effects on the concentration and chemical speciation of metals in soil solution. They found that the decrease in the...

Hmw Pc And Metal Tolerance

Although the accumulation of PCs could be a major component of the heavy metal detoxification process, the increased tolerance to metals may involve other aspects of PC function. The first argument in favor of this came from Delhaize et al. 87 , who observed that, although Cd-sensitive and Cd-tolerant cells of Datura innoxia synthesized the same amount of PCs during the initial 24-h exposure to 250 M Cd, the concentration was toxic to the Cd-sensitive cells only, as revealed by a cell viability study. However, they differed in their ability to form PC-Cd complexes the sensitive cells formed complexes later than the tolerant cells. In addition, the complexes formed by the sensitive cells were of lower molecular weight than those of tolerant cells and did not bind all the Cd, unlike in the tolerant cells. Thus, the rapid formation of PC-Cd complexes sequestering most of the Cd within a short period could be a necessity for plants or cells showing tolerance to heavy metals. Evidence in...

Phytoremediation and the beginning of interest in mycorrhiza

First, because heavy metals are taken up and transported in water solution, increased plant transpiration would increase metal translocation to the shoot. There is no doubt that mycorrhizal colonization affects the water relations of plants (Smith and Read 2 and references therein). A number of papers indicate that transpiration rates of mycorrhizal plants are significantly higher than those observed in nonmycorrhizal ones 34-39 . Mycorrhizal root systems are usually more branched 40 and therefore they present a larger absorbing surface even in the absence of changes in root biomass 34 . Also, the increased leaf area can be an important factor leading to increased transcription 41 however, even comparing plants of the same size and root system length, the transpiration rates of mycorrhizal plants remain superior, due to the reduced stomatal resistance 38 . In the second place, roots and hyphae of mycorrhizal root systems explore an incredibly larger volume of soil in comparison with...

Phytohormone Signaling Pathways

The accumulation of H2O2 driven by plasma membrane NADPH-oxidases are thought to induce the formation of lipid peroxides (i.e., fatty acid radicals orFA-OO Neil et al. 2002). This oxidative process, together with the activity of several classes of lipoxigenases, generate oxylipins, which are precursors of jasmonate (JA Turner et al. 2002 Foreman et al. 2003). Several short-term experiments have shown that heavy metals can induce the overexpression of several genes encoding LOXs. Thus, the exposure to p.M levels of Cd and Cu for 24 h caused the overexpression of LOX in Arabidopsis (Remans et al. 2010). Similar results were observed by Maksymiec et al. (2005) in Arabidopsis treated with excess of Cd or Cu, seedlings that accumulated JA after just 6-7 h of incubation. In addition, Rakwal et al. (1996) showed that Cu induced a rapid increase of JA content in Oryza sativa excised leaves. A biphasic JA accumulation was found in Cu or Cd stressed plants, e.g., a rapid one within a few hours,...

Strategies For Enhanced Uptake Of Trace Elements To Facilitate Phytoextraction

Heavy metal accumulation in soils is highly dependant on the availability of metals for plant uptake. Soils consist of a heterogeneous mixture of different minerals (primary minerals, clay minerals, and hydrous oxides of Al, Fe, and Mn) organic and organo-mineral substances, and other solid components. The binding mechanisms for heavy metals are therefore complex and vary with the composition of the soil, soil acidity, and redox conditions. Heavy metal behavior (e.g., mobility, bioavailability) depends upon several factors (Table 27.2), which can be classified as 31 Soil pH. The lower soil pH increases concentration of heavy metals in solution via decreasing their adsorption. Soil pH was adjusted using HNO3 and CaCo3 to provide a range of pH before planting. Acidified treatments were leached to remove excess nitrate before fertilizers were added. Chaney et al. 97 pointed out that, because soil pH is known to affect plant uptake of most heavy metals from soils, studies needed to be The...

Organic Acids Nicotianamine Amino Acids and Phytates

Studies have demonstrated that the primary constituents of root exudates are low-molecular weight organic acids (LMWOAs) that play essential roles in making sparingly soluble soil Fe, P, and other metals available to growing plants (Romheld and Awad 2000). Acetic, lactic, glycolic, malic, maleic, and succinic acids were found in rhizosphere soils of tobacco and sunflower (Chiang et al. 2006). Concentrations of these LMWOAs exudates increased with increasing amendment of Cd concentrations in the rhizosphere soils. After the loss of H+, each acid contains a COO- group, which binds to the cations. Correlation coefficients between concentrations of Cd amendment versus LMWOAs exudates of tobacco and sunflower were 0.85 and 0.98, respectively (Chiang et al. 2006). Positive correlations have been found between external Zn and organic acid concentrations in the roots of hyperaccumulator plants A. halleri (Zhao et al. 2000). These results suggest that the different levels of LMWOAs present in...

Metal Tolerant Plants And Chelators Might Promote Phytoremediation Technology

Use of soil amendments such as synthetics (ammonium thiocyanate) and natural zeolites has yielded promising results 62-66 . EDTA, NTA, citrate, oxalate, malate, succinate, tartrate, phthalate, salicylate, acetate, etc. have been used as chelators for rapid mobility and uptake of metals from contaminated soils by plants. Use of synthetic chelators significantly increased Pb and Cd uptake and translocation from roots to shoots, facilitating phytoextraction of the metals from low-grade ores. Synthetic cross-linked polyacrylates, hydrogels have protected plant roots from heavy metal toxicity and prevented the entry of toxic metals into roots. Application of low-cost synthetics and natural zeolites on a large scale are applied to the soil through irrigation at specific stages of plant growth this might be beneficial to accelerate metal accumulation 67 .

Complex Formation With Organic Acids

Organic acids are the other group of biomolecules that can function as chelators of heavy metals inside the cell, converting the metals to almost inactive and nontoxic forms. With regard to Al, at least two organic acids are known to function as chelators. One is citric acid 138 nearly two-thirds of Al in hydrangea leaves remain present in the cell sap in soluble form as Al-citrate complex at a 1 1 molar ratio of Al to citrate, a nontoxic form of Al. Among the heavy metals reported to be chelated by organic acids inside the cells are Zn and Ni. After exposure to high concentrations of various heavy metals, vacuoles of the Zn- and Ni-tolerant plants, as well as those of the nontolerant plants, often contain high concentrations of zinc and nickel 142,143 , as well as some Cu and Pb 144 and Cd 145-147 . The results of the studies

Bioaccumulation of Trace Elements in Animals and Trophic Transfer

Exposure to trace elements such as Cd and Pb in free ranging wildlife like small mammals and birds depends on the dietary preferences of the animal species. Metcheva et al. 62 estimated the metal content in tissues of six species of small mammals from different habitats and found that the heavy metal loading in small mammals is not only due to specific accumulation features of physiological origins or preferable bioaccumulation of elements in respective organs and tissues, but also due to ecological and food characteristics. Hunter and Johnson 63 analyzed the total body burden and tissue distribution of heavy metals in small mammals. They found that Apodemus sylvaticus, which fed on fruits and seeds of ground cover vegetation and canopy species, had the lowest metal burden due to minimal translocation of metals to the reproductive structures of plants parts. On the other hand, Sorex araneus fed predominantly on detritivorous soil invertebrates and had the highest concentration of...

Auxin and Heavy Metal Stress

Among the nonessential heavy metals, cadmium (Cd) is the most well studied compound. It is mainly released into the environment by anthropogenic activities and its emissions have been estimated to be in the range of 30,000 t per annum (Sanita di Toppi and Gabrielli 1999). As Cd is highly toxic its over-accumulation in plants results in growth reductions and death. Environmental pollution with Cd is of great concern since its accumulation in the food chain may eventually affect human health. Although other heavy metals such as Hg and Pb have similar sources and consequences, we will mainly focus on Cd because of the wealth of information available on plant responses to the latter element. Further support for the hypothesis that growth reductions caused by Cd may at least partially be caused by depletion of active auxin comes from several auxin feeding experiments. For example, in Zea mays uptake of Cd and Ni increased concurrently with growth reduction however, exogenous application of...

Proteomics as a Toolbox to Study the Metabolic Adjustment of Trees During Exposure to Metal Trace Elements

Although often regarded as a recent, industry-related problem, environmental pollution with heavy metals and its link with human activity predate the industrial age (Cooke et al. 2009). Nonetheless, the increasing pressure contributed by the growing world population makes environmental pollution, which results in the deterioration of available natural resources and or threatening the natural habitat of organisms, a major concern for current and future generations. Although everyone has their own clear personal idea of what constitutes pollution, it is particularly difficult to define. While a streetlight for most of us is an aid in helping to get home safely, the amateur astronomer will consider it as light pollution. Therefore, thresholds, i.e., values above which the concentration of a specific component becomes a problem in specific conditions, have been established by national or supranational legislation. Some metal trace elements, such as Zn, Fe, Cu, Mn, Mb and Co, are essential...

Trace Element Solubility In The Rhizosphere

Trace elements may be displaced from exchange sites by other ions attracted from the soil solution. The extent of this competition for binding sites depends on the type and concentration of the trace element as well as that of the competing ion. As a general rule, trace elements such as Cd2+, which has an atomic radius (r) of 0.97A, can be displaced by other ions of a similar size and charge in soil solution, such as Ca2+ (r 0.99A). Therefore, soil amendments such as phosphates that are designed to immobilize heavy metals may actually promote the solubility of some co-contaminants such as As.

Antioxidative System In Metal Tolerance

It is generally considered that virtually all the biochemical effects of heavy metals may ultimately lead to damage of cells and tissues 1 . Thus, arguments are made that heavy metal tolerance could also be linked, to some extent, with reactive oxygen scavenging capability of a plant species 154 . However, little direct evidence supports this hypothesis, although indirect evidence does suggest such a relationship. pAL20l gene and opined the possibility of the isoenzyme having some function in Al resistance. Significant enhancement in the activity of peroxidase in response to heavy metals has also been reported 158-162 . Hendry et al. 158 observed enhancement in the activity of peroxidase in the Cd-sensitive, but not in the Cd-tolerant, plants of Holcus lanatus in response to Cd. The activity of ascorbate peroxidase has also been reported to increase in plants in response to heavy metals 161-163 . Working on Phaseolus aureus, Shaw and Rout 161 observed metal-specific (Hg and Cd)...

Bioavailability of Trace Elements as Related to Mycorrhizal Fungi

Gast et al. 71 studied the heavy metals in mushrooms and their relationship with soil characteristics and they found large differences between metals with very high accumulation for Cd, exclusion for Pb, and a narrower range of concentrations for Zn and Cu. These workers suggested a regulation of uptake for essential elements and concluded that species differences not soil factors are the primary determinants of metal levels in fungi. Ectomycorrhizal fungi can increase the bioavailability of heavy metals in the rhizosphere by solubilizing minerals containing metals such as rock phosphates 72 . Mycorrhizal fungi associated with plant roots in symbiosis affect plant root exudation quantitatively and modify the composition of root exudates containing carbohydrate, amino acids, and aliphatic acids 73 . Leyval and Ber-thelin 74 proposed that the modification of the composition of root exudates by mycorrhizal fungi influence the bioweathering of minerals in the rhizosphere and the...

Metal Uptake By Cereal Crops

Wheat cultivars vary considerably in their ability to grow and yield well in contaminated soils. An ability of wheat to accumulate different metals has been demonstrated in many publications 21-25 . According to recent literature data, different cultivars of wheat may differ in the ability to take up heavy metals 26-28 . Moreover, the experimental data may be very contradictory. It was reported 29 that whole-plant Cd accumulation and Cd translocation to shoots is greater in the bread wheat cultivar than in the durum cultivar. On the other hand, Li et al. 30 found that many durum wheat cultivars accumulate two to three times as much Cd in grain as does bread wheat. Greger and Lofstedt 27 showed that different Cd accumulation in wheat grains grown in nutrient solution was related to variations in the translocation from root to shoot and to the Cd concentration in shoot, flag leaf, and grain coats, but not to the uptake of Cd by roots.

Incorporation Of Metals In Bioactive Molecules In The Process Of Evolution

11.5.1 Concepts of Heavy Metal Toxicity, Tolerance, and Resistance Heavy metal cations may bind to glutathione the resulting bis-glutathione complexes tend to react with molecular oxygen to oxidized bis-glutathione GS-SG 17 , the metal cation and H2O2. Because the oxidized bis-glutathione must be reduced again in an NADPH-dependent reaction and the metal cations immediately catch another two glutathione molecules, heavy metal cations cause considerable oxidative stress. Finally, heavy metal oxyanions interfere with the metabolism of the structurally related nonmetal (chromate with sulfate arsenate with phosphate) and reduction of heavy metal oxyanion leads to the production of radicals, e.g., in the case of chromate. Therefore, the gate that was always open for Mg2+ or phosphate uptake turns out to be responsible for the heavy metal toxicity. fit than the wild type in the medium without toxic heavy metal ion and are thus rapidly overgrown by the revertant strain.

Luminescencebased biosensors

There are few successful reports of successful use of gene-fusion biosensor in the monitoring of a metal toxicity in field application. These strains express a sensitive reporter gene, luciferase, connected to a promoter element responding specifically to various heavy metals. Inserting a heavy metal responsive element directionally cloned in a suitable vector in front of the luciferase gene by using standard recombinant-DNA techniques created all the sensor plasmids. Using a nucleotide database, one can isolate a promoter operator element generated by a polymerase chain reaction using specific oligonucleotide primers. The plasmids can then be expressed in different hosts, in order to obtain maximal and specific response to each of the metal tested. For instance, Biomet sensors (patented and recognized by OVAM) used a relevant soil bacterium, Ralstonia metallidu-rans, to modify it to deliver a light reaction when specific heavy metals go into the cell. The quantification of light...

Heavy Metal Chemistry in Soils and Heavy Metal Properties after Termination of Sewage Sludge Application

After sewage sludge-borne heavy metals are introduced into the soil, a series of mechanisms tend to bind metals to the organic (soil and sludge-borne) and the inorganic constituents. A percentage of the metals will be reversibly adsorbed onto clay surfaces some will precipitate out of the solution with other inorganic phases (carbonates, phosphates, etc.), especially at alkaline pH values. Some will be specifically adsorbed onto solid surfaces, and the organic colloids (fulvic and humic acids, as discussed earlier) compete for metal binding effectively. A small portion of the added metals will be soluble into the soil solution in pure ionic form and thus readily available for plant uptake or easily leached out of the root zone and probably into the groundwater. These processes are usually pH dependent, with the metals bound more strongly onto the inorganic phases with an increase in pH value and heavy metals becoming more available in acidic pHs. In his early work, Harmsen 30...

Summary And Conclusions

A new class of hybrid nanoporous materials has been developed for removing toxic heavy metals, oxyanions, and radionuclides from aqueous waste streams. Tests showed that thiol-SAMMS designed for heavy metal adsorption showed significant loading (0.56 to 6.37 meq g) and high selectivity (Kd 4.6 x 101 to 3.5 x 108 ml g) for contaminants such as Cu, Pb, Cd, Ag, and Hg.

Mycorrhiza and its role in the environment

Molecular and paleobotanical studies seem to support the hypothesis of a close relationship of the AMF with plants since they appeared on land 11,12 . This mycorrhiza plays a key role in the productivity, stability, and diversity of natural ecosystems. Natural soils with low levels or completely devoid of AMF propagules are rare. Several factors can influence the quantity (i.e., the number of propagules) and the quality (i.e., the composition in species) of AM fungi in the soil. The presence of heavy metals and or other pollutants, the use of amendments to remediate pollution, and the kind of vegetation heavily affect the composition and abundance of the Glomalean fungi 13-15 . The disappearance of the propagules leads to serious consequences, such as the degradation of plant communities decreased availability of essential elements and loss of ecosystem stability. Among examples in which it is necessary to introduce AMF propagules during creation and rebuilding of plant communities...

Physicochemical Restriction in Detoxification Process visvis Choice for Getting Rid of Excess Heavy Metal Ions

Because reduction is not possible or may not be sensible as a sole mechanism of detoxification, heavy metal ions must be detoxified by efflux, alone or in combination, in any organism growing rapidly in an environment contaminated with high concentrations of heavy metals. Heavy metal metabolism is therefore transport metabolism 11 and the protein families involved in heavy metal transport will be examined.

Biogeogenic Cycling Of Metals

Some aquatic plants are used as food and feed. Water spinach, Ipomea aquatica, is commonly used as a vegetable and pig food in Thailand. It is very easy to grow, grows fast, and is present in cultivated water, as well as in industrial areas and big cities. It takes up and accumulates heavy metals to such an extent that a threat to human health has been discussed 31,205 . Recent studies have shown that plants collected in Thailand, near Bangkok, contained up to 530, 350, and 123 g g DW-1 of Pb, Hg, and Cd, respectively 205 . The biggest problem seems to be Hg. According to FAO, the weekly intake of Hg should not exceed 43 g per week, which means not more than 250 g of plant per day can be eaten. However, the real problem seems to be the property of this plant to accumulate high levels of CH3-Hg 205 . This plant has also been tested for removal of metals from wastewaters 37 .

Trace Element Toxicity

Despite these positive results of soil amendment with biosolids, the dark side of the sludge is evident in the health risks. Sewage sludges also contain components considered to be harmful to the environment. Sewage sludge contains three constituents of environmental concern (1) heavy metals (2) organic pollution (c) pathogenic organism 24 . Generally, sludges contain heavy trace elements such as Cu, As, Cd, Ni, Zn, and Pb in diverse concentration these originate from a number of different sources, such as industry, commerce, business, domestic household waste, corroding pipes, and runoff from roads and roofs 5 .

Factors Affecting Metal Offtake During Harvest

Tree foliage can contain high concentrations of heavy metals, especially Cd and Zn. On highly contaminated sites, these may be at their peak values just before leaf senescence, appearing not to be translocated and redistributed within the plant prior to leaf fall. If these metals return to the rhizosphere, they represent a significant pool of potentially bioavailable metal. Under such circumstances, phytoremediation will fail in its aim of cleaning up the soil. It does, however, raise the question of improving offtake of metal by harvesting the trees prior to leaf fall. This is not normal practice in SRC systems in which the stems without leaves are harvested over the winter period. With long-term tree covers that are not regularly harvested (and in the years intervening in the usual 3-year SRC harvest), this recycling process of metals to surface soil may be significant. Roots usually contain the highest concentrations of all metals in trees depending on the size of the root biomass,...

Metallomics And Metallomes

These peptides are synthesized from glutathione by means of a-glutamylcysteine transferase enzyme (EC, which is also called phytochelatin synthase (PCS), catalyzing transfer reaction of (a-Glu-Cys) group from a glutathione donor molecule to glutathione, an acceptor molecule. PCS is a cytosolic, constitutive enzyme and is activated by metal ions, namely, Cd2+, Pb2+, Ag1+, Bi3+, Zn2+, Cu2+, Hg2+, and Au2+. PCs thus synthesized chelate heavy metals and form complexes that are transported through cytosol in an ATP-dependent manner through tonoplast into vacuole. Thus, the toxic metals are swept away from cytosol. Some high molecular weight complexes (HMW) with S-2 can also be formed from these LMW complexes in vacuole 7 .

Sources Ubiquity and Diversity of Environmentally Significant Radionuclides

(v) The heavy metals comprise a group which contains a mixture of elements with a complex and varied chemistry. Elements with important radionuclides include manganese (54Mn), iron (55Fe and 59Fe), cobalt (57Co and 60Co), technetium ( Tc and 99mTc), ruthenium (103Ru and 106Ru), lead (210Pb)

Heavy Metal Hyperaccumulator Phenotypes in Plants

Some plants, in the same way as other organisms, have developed complex homeostasis mechanisms to minimize the deleterious effects of heavy metals by controlling their absorption, accumulation, and translocation. These mechanisms protect the cell by avoiding the accumulation of excess free ions in the cytosol, which result in various degrees of tolerance to certain heavy metals. Some plants not only tolerate high concentrations of heavy metals, but also hyperaccumulate the metals. The ability to acquire a tolerance against heavy metals and to accumulate metals to very high concentrations has evolved both independently and together in a number of different plant species (Memon and Schroder 2009). Approximately 400 plant species are described as hyperaccumulators of heavy metals, which are defined as plants that can accumulate more than 0.1 of its dry weight as Ni, Co, and Pb, more than 1 Zn and 0.01 of its dry weight in Cd (Baker and Brooks 1989). Plants have different tolerance...

Auxin A Mediator Between Growth and Stress Adaptation

(2006) found that a strong decline of radial growth in the xylem of salt-stressed poplars corresponds to a strong decrease in auxin levels, whereas in salt-tolerant poplars the concentrations of auxin were maintained. They identified an auxin isoleucine amido hydrolase (ILL3) that was able to release auxin from its amino acid conjugate, thereby maintaining homeostasis under stress and decreasing salt-induced growth reductions. Application of exogenous auxin rescued biomass production in stressed salt-sensitive poplars (Popko et al. 2010). While these examples show that the phytohormone auxin is a central regulator of growth and adaptation to stress, knowledge on how auxin physiology is affected by metal toxicity is scattered. Plant defence reactions against metal toxicity and their implications for plant growth have been extensively investigated. Heavy metals are defined as metals with a density above 5 g cm 3 and encompass compounds such as Fe, Mn, Zn, Ni, and Cu, which are required...

The Essential Elements

Both the micronutrients and most of the non-essential trace elements are known to have undesirable effects on plant and animal growth if present in excess concentration in their tissues. Plants commonly have their growth affected by excess manganese and aluminium in acid soils, and by nickel, cobalt and chromium on acid soils derived from ultramafic igneous rocks. In addition there is growing concern that several toxic elements are accumulating in some soils as a consequence of man's industrial or urban activities which lead to soil pollution or because of the use of metal-contaminated sewage sludge as a substitute for farm yard manures. Whenever an element or its compounds are heated, dissolved or pulverised it becomes environmentally labile and may escape from the working environment and ultimately accumulate in soils or sediments. Much research has been carried out on the 'heavy' metals, especially cadmium, copper, lead, mercury and zinc. The qualifier 'heavy' is not rigorously...

Soluble Phenolics

Perform as metal-accumulating polymeric ligands. Tong et al. (2004) have reported that compartmentation and the formation of complexes with phenol derivatives in the vacuole may be another example of the mechanisms of resistance to heavy metals. Precipitation of phenolics generally revealed a significant higher electron-opacity over all protoplasm in bilberry leaves collected in a polluted forest in comparison to leaves from an unpolluted locality (Bialonska et al. 2007). These results indicate that the distribution and properties of phenolics depend on the level of heavy metals accumulated in the cell and the phenolics accumulated in vacuoles and apoplasts may play a significant role in scavenging of free radicals produced in plant cells (Bialonska et al. 2007). In a herbaceous plant chamomile (Matricaria chamomilla), soluble phenolics in the root and leaf rosettes were elevated by high doses of Cu and Cd, whereby Cu had a more expressive effect in roots and Cd in leaf rosettes,...


The physiology of metal toxicity in plants has been reviewed by Foy et al. (1978). The most widely described effects of metal toxicity in plants are inhibited root growth, depressed shoot and leaf growth, and general chlorosis of the younger leaves (Brad-shaw and McNeilly, 1981 Baker and Walker, 1989). Literature on the relative toxicities of different metals to different species is limited to a few, mostly species-specific studies.


Might provide new data, as it is possible to study the changes that occur at the cellular level. Future work should be directed to describe in detail the components that permit the perception of the stress induced by heavy metals, and how different phytohormones and signaling components interact. Functional tools available (i.e. mutants) from the studies of biotic interactions could be used to the characterization of metal responsive components, due to the common responses found.

Concluding Remarks

Enzyme induction is an indirect effect of metal toxicity. In consequence, it only appears after in-vivo metal application. Increase in capacity in the presence of toxic concentrations of metals implies that the enzyme involved is insensitive to or well masked from direct metal action. In Silene cucubalus POD and to a lesser extent ICDH were shown to be very tolerant to zinc, copper and cadmium applied in-vitro (Mathys, 1975). MDH extracted from roots of both copper tolerant and non-tolerant clones of Agrostis stolonifera was found to be insensitive in-vitro to copper concentrations up to 180 im. However, in the literature, conflicting results are reported about the effects of in-vitro and in-vivo application of metals on enzymes. In Phaseolus vulgaris, GDH was inhibited by cadmium in-vitro, while an induction was found after in-vivo application of a toxic dose of the same metal (Weigel and J ger, 1980b).

Trace Element Uptake

Macrophytes take up heavy metals via roots from the sediment and via shoots directly from the water. Therefore, the integrated amounts of available metals in water and sediment can be indicated by using macrophytes. Plants can also evolve ecotypes fairly soon and thereby be used in unfavorable conditions. Plants are also stationary and long lived and accumulate metals therefore, they are suitable in monitoring of polluted sites. Metal concentration in plants must be related to the time of the year because the metal concentrations vary by season 35 . Aquatic plants also release metals through their leaves. Plants used as bioindicators must retain the metals in their plant body. Ceratophyllum demersum, Myriophyllum spicatum, Potamogeton pectinarus, P. perfoliatus, and Zannichellia palustris are proposed as bioindicators 36-42 . Aquatic macrophytes such as water hyacinth Eichhornia crassipes and several duckweeds have attracted the attention of scientists for their ability to accumulate...


Studies provide evidence that heavy metals induce oxidative stress, which in turn might activate auxin oxidases. These enzymes inactivate auxin and thereby might play roles in growth regulation. Therefore, we propose that reductions in plant productivity under the influence of (moderate) metal stress may initially not be caused by photosynthetic reduction and energy limitation but may be the result of interference of heavy metals with auxin metabolism. This would explain why exogenous addition of auxin or stimulation of endogenous auxin levels prevents growth inhibition and increase heavy metal tolerance. Metal transporters and GH3-regulated genes are involved in these processes but the mechanisms are still elusive. A better understanding of these basic processes will not only increase knowledge on the regulation of plant metal homeostasis but may also be useful to improve plants for phytoremediation.


To protect themselves from the toxicity of metal ions, plant cells have developed a mechanism to inactivate metal ions thus preventing enzymatic and structural proteins (Kneer and Zenk 1992). This mechanism consists of the biosynthesis of a set of iso-peptides PCs with varying chain lengths such as (y-Glu-Cys)n-Gly where n 2-11 (Fig. 1). PCs (or cadystins) were first discovered in fission yeast Schizosaccharomyces pombe exposed to Cd (Murasugi et al. 1981) and then in many plants (Grill et al. 1989 Rauser 1995). PCs are formed directly from glutathione (GSH, a reduced form) by the activity of PC synthase (g-Glu-Cys dipeptidyl transpeptidase EC, in the last step of the following metabolic sequence Glu + Cys g-Glu-Cys (gEC peptide) g-Glu-Cys-Gly (GSH) PCs. The first and second steps of this sequence are mediated by gEC synthetase (EC and GSH synthetase (EC, respectively. PC synthase (PCS) consists of 95,000 Mr tetramers of protein subunits and has a Km of 6.7...


The organization of these genes in Pseudomonas putida is different from that in E. coli but the same as in Brucella suis, where nikR is upstream of and transcribed divergently from nikABCDE 9 . Nickel detoxification may be done by sequestration and or transport, as with most heavy metal cations. Metal resistance based on extracellular sequestration has been hypothesized only in bacteria, but has been found in several species of yeast and fungi 255-257 . Nickel uptake in Saccharomyces cerevisiae may be reduced by excreting large amounts of glutathione, which binds with great affinity to heavy metals 258 . Nickel is bound to polyphosphate in Staphyloccus aureus 259 and to free histidine in nickel hyperaccumulating plants 260 . In S. cerevisiae, nickel is disposed off into the vacuole and probably bound to histidine in there 256 . The transport into the vacuoles requires a proton-pumping ATPase 261 thus, this kind of nickel transport may be also driven by a chemi-osomotic gradient. Other...


Selenium (Se) is a nonmetallic element that resembles sulfur (S) and occurs naturally as a trace element in most soils, rocks, and water (Djanaguiraman et al. 2010). Selenium has long been recognized as an essential micronutrient for animal and human nutrition because it is an integral part of the enzyme glutathione peroxidase, a selenoenzyme that prevents oxidative damage to body tissues (Akbulut and Cakir 2010). Recent studies have shown that Se, at low concentrations, can protect plants from several types of abiotic stresses (Hawrylak-Nowak et al. 2010). Many kinds of biotic and abiotic stresses, such as drought, extreme temperature, soil salinity, and heavy metals, are known to cause oxidative damage to plants, either directly or indirectly, by triggering an increase of ROS. To resist oxidation damage, antioxidant enzymes and certain metabolites play important roles, leading to adaptation and ultimately the survival of plants during periods of stress (Hartikainen et al. 2000)....


Recently Chaney and Ryan (1992) and Henry and Harrison (1992) have reviewed the effects of heavy metals in sewage sludges and MSW-composts (municipal solid waste) used as soil amendments. Chaney and Ryan found that in contrast to sewage sludge, MSW-composts contain phytotoxic levels of boron. Application can also raise the pH of the soil-compost mixture, which can result in compost-induced Mn-defi-ciency. Chaney and Ryan conclude that uncontaminated sludges and MSW-composts comprise no risks in relation to Cd uptake by crops and vegetables, thus there is no food chain Cd risk to humans consuming western diets. This largely comes about because Zn is a natural limiting factor in the following way either soil pH is kept at a reasonable level for crop production limiting Cd uptake or if the soil pH drops enough to allow Cd uptake, then Zn-phytotoxicity reduces the yield and the Cd risk.


Induction of POD capacity is not restricted to heavy metals only but was found for a variety of other stress factors chilling (Levitt, 1972 Kacperska-Palacz and Uliasz, 1974), wounding (Gaspar et al., 1982) or pathogenic infection (Van Loon, 1986), air pollution by S02, ozone, NOx or fluoride (Horsman and Wellburn, 1976 Rabe and Kreeb, 1979 Endress et al., 1980 Khan and Malhotra, 1982 Decleire et al., 1984). POD-capacity of tree leaves was even utilised as a parameter for monitoring and mapping air pollution (Keller, 1974).

Preface to the Series

The assessment of safe to use starts with the harvested plant material, which has to comply with an official monograph. This may require absence of, or prescribed limits of, radioactive material, heavy metals, aflatoxin, pesticide residue, as well as the required level of active principle. This analytical control is costly and tends to exclude small batches of plant material. Large-scale, contracted, mechanized cultivation with designated seed or plantlets is now preferable.

Antioxidant Defence for Abiotic Stress Tolerance

Reactive Oxygen Species (ROS) such as singlet oxygen, hydrogen peroxide molecules, superoxide and hydroxyl radicals are constantly produced in chloroplasts, mitochondria and peroxisomes by aerobic processes (Apel and Hirt 2004) . Thought to be integral to downstream defense tolerance responses, the elevated levels of ROS are often associated with exposure to biotic (e.g. pathogens or pests) and abiotic (e.g. high light, UV radiation, temperature extremes, heavy metals, air pollutants, drought stress, salt stress, mechanical physical stress) factors (Neill et al. 2002 Imlay 2003 Einset et al. 2007). Overproduction of ROS leads to oxidative damage such as lipid peroxidation of cell membranes (Imlay 2003) or even cell death (Jones 2000) . In order to control ROS levels and protect cells from oxidative injury, plants possess both enzymes and non-enzymatic metabolites that may play a significant role in ROS signalling in plants (Vranova et al. 2002).

Farming Systems to Enhance General Suppressiveness

Although amending soil with high rates of organic matter can generate suppressive-ness to nematodes and other soilborne pathogens and maintain it for some time after the amendment is applied, it is important to recognise that this approach to disease control is likely to be most useful in high value horticultural production systems. Nurseries where plants are grown in containers, glasshouses producing vegetable or ornamental crops and intensive in-field production of crops with a high monetary value are perhaps the only situations where it is realistic to use amendments in this way to manage nematodes. In all other agricultural systems, applying organic matter at rates of 10-100 t ha annum is never likely to be economically feasible. Importation of organic matter will generally be expensive relative to the income derived from most crops, largely because transportation costs are high and non-agricultural markets compete for the resource. Since high application rates are required to...

Synthetic iron chelates

Iron chelates are usually effective but do not represent a sustainable approach to prevent or cure Fe deficiency (Tagliavini and Rombola, 2001). Soil applied Fe chelates are water soluble and easily leached out of the root zone if excessive irrigation regimes are applied or during the autumn-winter (Rombola et al., 2002b). A likely underestimated problem related to synthetic chelates is the potential of some of them to bind heavy metals (Grcman et al., 2001). Some iron chelates have a scarce degradability in the soil (Nortemann, 1999) and may cause toxic effects on soil microorganisms and mycorrhizae (Grcman et al., 2001). Moreover, it has been reported that plants may take up Fe-chelates directly and EDDHA traces could reach the fruits (Bienfait et al., 2004).

Or Resistance Productivity and Prospects

The target is to achieve Na+ excretion from the root, or its storage in the vacuole, so overexpression of a gene that encodes a vacuolar Na+ H+ antiport pump could be one solution (Apse and Blumwald 2002) . Such a pump would allow for more effective removal of salt from the cytoplasm and its transfer to the vacuole. These results have been obtained in GM tomato plants, which showed a higher tolerance to salt concentrations than did nontransformed individuals, and survived better in areas that were previously considered useless for agriculture. Furthermore, the fruit does not accumulate salt, and is edible. This effort to produce food in large areas, that are presently impractical for farming, has also been extended to problems of soil contamination by heavy metals (Shewry et al. 2008) .

Am Fungi And Alleviation Of Soil Heavy Metal Stress

Some heavy metal elements such as Cu, Fe, Mn, Ni and Zn are essential for normal growth and development of plants. These metals are required in numerous enzyme-catalyzed or redox reactions, in electron transfer, and have structural function in nucleic acid metabolism (Gohre and Paszkowski, 2006). In contrast, metals like Cd, Pb, Hg, and As are not essential (Mertz, 1981) and may be toxic to plants at very low concentrations in soils. Heavy metals occur in terrestrial and aquatic ecosystems from both natural and anthropogenic sources, and are also emitted into the atmosphere. The roots of terrestrial plants are in immediate contact with soil metal ions. Essential heavy metals are transferred into the root by specific uptake systems, but at high concentrations they also enter the cell via nonspecific transporters. At high concentrations heavy metals interfere with essential enzymatic activities by modifying protein structure or by replacing an essential element, resulting in deficiency...

Stress Responses in Plants 321 Short Term Responses

Gene expression is controlled at different levels which operate through changes in DNA methylation, histone modification and chromatin remodelling. As indicated above, stress can modify gene expression. It has been reported that stress can alter the pattern of gene methylation thus, those genes activated under stress conditions showed a modification in their methylation status. Choi and Sano (2007) reported that some genes were selectively demethylated and, subsequently, transcribed under stress conditions. They proposed that environmental responses of plants are partially mediated through active alteration of the DNA methylation status. In other cases, the modifications were distributed along the genome but not randomly, involving precise sites. This can be illustrated by changes in methylation patterns induced by water stress in pea (Pisum sativum Labra et al. 2002) or by heavy metals (Ni2+, Cd2+ and Cr6+) in white clover (Trifolium repens Aina et al. 2004), which involved specific...

Heavy metal accumulation in plant tissues

Heavy metals contaminating soils in elevated concentrations can be essential (Mn, Zn, Cu and Ni) or non-essential (Cd, Pb, Hg, Cr, etc.) for plants. As a first step for ion uptake, all these metals are adsorbed in the root apoplasm as divalent or trivalent cationic forms, replacing single charged cations and Ca2+ at cell wall binding sites. Heavy metal concentrations tend to increase in the apoplasmic space as compared to the average concentration in the soil or the rhizosphere, usually inducing high root metal concentrations, as it has been reported in many studies. Another possibility is that certain heavy metals may precipitate on root adsorbing surfaces. For example, in ragweed (Ambrosia artemisiifolia) Pb was suggested to accumulate in the apoplast as Pb-P (Huang and Cunningham, 1996). The physiology behind the accumulation of heavy metals in the shoot involves xylem loading and translocation, leaf cell uptake and storage in high concentrations. The possible interactions of heavy...

Differential Expression Proteomics

Plants are sessile organisms unable to escape environmental pressures. As a result, they have evolved an array of mechanisms and strategies to tolerate environmental conditions. How plants respond to stresses such as drought, high or low temperatures, high light, nutrition deficiencies, pathogen attacks, heavy metals, or other pollutants is crucial to improve crop productivity or plant's capacity to resist these aggressions. A number of proteomics studies have been conducted to analyze the impact of such environmental changes at the protein level. These proteomics investigations are generally based on a 2-DGE analysis of proteins extracted from treated or untreated plants, followed by gel image analyses to quantify and compare protein levels. Then, proteins up- and down-regulated in response to the treatment are identified by MS (MALDI and or ESI-MS MS). Table 10.2 summarizes the differential expression studies concerning abiotic and biotic stresses on...

Rhizospheric and Microbial Mobilization

On the contrary, the release of root exudates by some hyperaccumulators can bind and sequester heavy metals in soil (Schwab et al. 2005). This mechanism could protect roots from the toxic effects of heavy metals (Liao and Xie 2004) and may also decrease Cd uptake by plants (Sarwar et al. 2010). Microbes are ubiquitous even in soils with high heavy metal concentrations. Rhizosphere microorganisms have also been shown to influence metal availability and mobility by reducing soil pH or by producing chelators and siderophores (Abou-Shanab et al. 2003 Wenzel et al. 2003 Whiting et al. 2001a, b). The existence of active rhizospheric and microbial metal-mobilizing mechanisms has not conclusively been demonstrated.

Immediate short and mediumterm effects of elevated air CO2

Growing at elevated CO2 (700 ppm) and present-day CO2 (350 ppm) concentrations in OTCs (open top chambers) for one month. Photosynthetic quantum yield of PSII (Fv Fm) decreased in reponse to both heavy metals. The effect of the Cd treatment was statistically significant only at the high CO2 level. Chlorophyll yield values decreased in reponse to Pb and mixed (Cd, Pb) treatment, and reached zero in the Cd treatment. There were no significant differences between CO2 treatments in any of the fluorescence parameters. CO2 gas exchange measurements revealed that the elimination of photosynthetic activity due to heavy metal treatment was independent of the CO2 level. According to Takacs et al. (2004), future high CO2 concentrations might be beneficial for CO2 assimilation by the desiccation-tolerant mosses with physiological levels of heavy metals by improving their carbon balance. Elevated CO2 concentrations could partly ameliorate the deleterious effects of heavy metal stress as well, but...