Heavy metal tolerance to plants represents the ability of particular plants to thrive under conditions that are characterized by excess of metal ions, which have toxic effect for other plants (Macnair et al. 2000) . The first research on the plants' tolerance toward HM ever made dates back to the beginning of the twentieth century, when it was established that two populations of the species Silene dioica have different tolerance toward the excess of Cu (Ernst et al. 1992).
Popova 2003). The sustainable use of these soils can be achieved by developing various remediation phytotechnologies as well as adaptive agriculture practices (Vassilev et al. 2005) .
Tolerance is based on two different strategies (1) to avoid the entry of excess HM into the plants and (2) to achieve effective intracellular detoxification. The main tolerance-related mechanisms are already wellknown, the most important of which include (1) the reduced uptake and/or accelerated excretion of HM by the cells, (2) the metal detoxification and compartmentalization, (3) the control of the metal induced oxidative stress, etc. (Vassilev and Nikolova 2010). The scientific interest with respect to the plant tolerance toward HM has become considerably larger in recent years. On the one hand, this is due to possible usage of tolerant plants for phytoreme-diation of soils contaminated by HM (Kulakov et al. 2009), and on the other hand, the interest is a result of the possible wider usage of the plants as model objects for ecotoxicological studies (Hock and Elstner 2005). The number of research papers related to the identification of plants that have high tolerance and hyperaccumulative abilities toward HM are constantly increasing (Schulze et al. 2005) .
Heavy metal hyperaccumulator plant eco-types, so-called metallophytes, distinguish from others by their evolving genetically based metal resistance due to their specific detoxification mechanisms. However, there is not a common exact opinion on these mechanisms to date. Several mechanisms of detoxification and inacti-vation of metals taken up have been proposed (Ernst 2005) : compartmentation in physiologically low active compartments of cells and organs (Brooks 1998); binding to the cell wall and sequestration by phytochelatins (Keltjens and Van Beusichem 1998; Seregin and Kojevnikova 2006) , metallothioneins (Burdin and Polyakova 1987), intracellular molecules (Seregin and Ivanov, 2001), and low-molecular-weight organic acids (Salt et al. 1999), and their precipitation in the vacuoles (Van Steveninck et al. 1990). The increase of some antioxidant enzyme activities (peroxidase, SOD, catalase) (Schickler and Caspi 1999; Seregin and Ivanov 2001; Guo et al. 2004)
and synthesis of osmolytes (Seregin and Kojevnikova 2006) have been proposed to play a role in the resistance mechanisms of tolerant plants. It is significant to note that mechanisms of both a metal toxic action and their detoxification are complex processes and do not become formed by only one mechanism. Plants respond to heavy metal by a number of parallel and/or consecutive processes at molecular, physiological, and morphological levels.
Methods are being employed for the remediation of mercury polluted soils. It involves the use of transgenic plants encoding the bacterial mercury ion reductase (merA) gene. These plants have been shown to grow in and volatilize mercury from soils (Meagher et al. 2000). Microorganisms are manipulated genetically to remove not only mercury but also other toxic elements from the environment (Wood and Wang 1983). It also involves the use of sulfur-containing solutions, as ammonium-thiosulfate, to induce mercury accumulation into aboveground tissues of high-biomass plant species (Moreno et al. 2004). In the latter system, mercury accumulates in the plant causing the plant to die. However, the mercury-laden plant, including roots, can be removed from the soil, thereby allowing mercury removal from the polluted soil. Root mercury accumulation and root area and length are related (Cocking et al. 1995; Heeraman et al. 2001). Plants with large root system, therefore are desirable for removal of mercury in contaminated soils. When the nutrient availability is less than that required for the optimum growth conditions plants suffer nutrient deficiency stress. This results in an inherently low nutrient status of the soil and low mobility of nutrients within the soil. The mobility of nutrients within the soil is governed by a number of factors including mass flow of water, adsorption capacity of the soil, and soil pH. The chemical form of nutrients within the soil also determines the extent of availability. In this way plants have adopted to nutrient deficiency stress, some of which are morphological adaptation to increase the ability of the plant to take up nutrients, such as cluster roots. Plants may also release chemical compounds into the soil environment to increase the efficiency at which nutrients are taken up or increase the number of soil nutrient pools available for uptake. Plants have evolved a mechanism to alleviate nutritional stress by symbiotically associating with microorganisms, such as legumes with Rhizobium and most terrestrial plants with myc-orrhizal fungi.
Heavy metals can be removed from polluted sites by phytoextraction, which is a method of phytoremediation and involves the accumulation of pollutants in plant biomass (Zayed et al. 1998). As a result hyperaccumulators (plant species that accumulate extremely high concentrations of heavy metals in their shoots) become particularly useful. In addition, one can genetically engineer these species to improve their metal tolerance and metal-accumulating capacity. A suitable target species for this strategy is Indian mustard (Brassica juncea), which has a large biomass production and a relatively high trace element accumulation capacity. Most importantly, it can easily be genetically engineered (Zhu et al. 1999).
Most plants fail to maintain metal homeostasis and develop stress symptoms, when exposed to elevated concentration of micronutrient metals or heavy metals without nutritional functions. Metal-tolerant plants however are adapted to elevated heavy metal concentrations in their growth media. De Vos et al. (1991) observed that metal-tolerant plants do not possess an enhanced tolerance to free radicals (FR) and reactive oxygen species (ROS). By contrast, metal tolerance appears to arise from the prevention of metal-induced oxidative stress through an enhanced capacity and efficiency of metal-ion homeostasis mechanisms (De Vos et al. 1991, 1992, 1993).
Among the metal tolerance mechanisms in plants, metal sequestration has been most extensively documented. Copper has been shown to accumulate in the leaf vacuoles of a Cu-tolerant ecotype of Armeria maritime (Lichtenberger and Neumann 1997). An involvement of vacuolar metal sequestration has been proposed in Zn tolerance of Silene vulgaris and in metal tolerance of the Zn and Ni hyperaccumulators Thlaspi caer-ulescens and T. goesingense (Vazquez et al. 1994). In the Ni-tolerant hyperaccumulator Alyssum lesbiacum, and in a heavy metal tolerant ecotype of Armeria maritime, metals are accumulated in the epidermis and particularly in leaf trichomes (Neumann et al. 1995; Kramer et al. 1997).
Metal tolerance may be achieved by metal chelation with high-affinity ligands inside and outside the cytoplasm. In Alyssum lesbiacum, Ni accumulation was associated with a large and proportional increase in xylem concentrations of the free amino-acid histidine, which was proposed to reflect an increase of intracellular, most probably cytoplasmic histidine concentrations. Furthermore, it was demonstrated that histidine binds Ni in vivo and that exogenous application of histidine reduced Ni toxicity in the nontolerant species Alyssum montanum (Kramer et al. 1996). However, the histidine response is unlikely to provide oxidative protection, since histidine com-plexation of Ni is known to enhance oxidative DNA damage induced by the metal in vitro (Datta et al. 1992, 1993 ; Misra et al. 1993). This suggests that once the metal has entered the plant cell, high affinity chelation, targeting, and sequestration of the metal chelate are all vital for metal tolerance. Accumulation of Cd in the vacuole is related to the Cd tolerance of plants. Uptake rates depend on the cadmium concentration in the growth medium. Tolerance which is also related to phytochelatins, are a major class of heavy metal chelating peptides that exist in plants (Buchanan et al. 2000). They are low-molecular-weight, enzymatically synthesized cysteine-rich peptides known to bind cadmium and are important for cadmium detoxification (Buchanan et al. 2000; Salt et al. 1995a). A number of metal-binding compounds found in tolerant plants could function as antioxidants, but also have a high affinity for binding metals. So far, evidence has only been provided for metal binding by these compounds, for example phenolic compounds (Lichtenberger and Neumann 1997). Efficient metal binding may be sufficient to prevent FR and ROS formation. However, the possibility of an antioxidant role of such molecules in plants should be investigated.
Higher capacity to accumulate and store HM for a long time have been observed in Lichens because of their morphological and ecological peculiarities and are widely used as plant material to investigate or biomonitor airborne HM. The Artemisia species and lichens (Xantoria parietna, Physcia adscen-dens) growing on the different substrates at different distances from a Hydroelectric Power Station were compared for their HM accumulation capacity indicated higher accumulation of HM's in the thalluses of both lichen species than A. fragrans situated in the vicinity, which was assumed to be a tolerant/excluder plant. The HM concentrations found in Xantoria and Physcia considerably exceed the values reported for other species of lichens (Lavrinenko and Lavrinenko 1999) indicating close correlation between the levels of HM in the lichens and atmospheric deposition (Kobayashi et al. 1986). Hence, significantly high HM contents in lichen species consequently provide evidence of a high level of air contamination.
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