Soils and waters with high levels of toxic metals such as cadmium, arsenic, lead, and mercury are detrimental to human and environmental health. Toxic heavy metals contaminate soils and waters in industrialized nations as well as in developing nations. The four heavy metals arsenic, lead, mercury, and cadmium have been identified as belonging to the five priority most hazardous substances found at toxic Superfund sites in the United States (1). Many human disorders have been attributed to ingestion of heavy metals including learning disabilities in children, dementia, and increased rates of cancer in response to cadmium (Cd) (2,3). Removal of heavy metals from highly contaminated soils and waters is therefore a very costly but necessary process that is currently being pursued at contaminated sites worldwide.
Remediation of soils containing high levels of toxic heavy metals is pursued by physical removal of metals because most of these metals cannot be degraded in the soil, in contrast to many organic contaminants (4). Current practical methods used to decontaminate such sites involve physical excavation of top soils, transport, and reburial elsewhere. However, these cleanup methods are feasible only for small soil areas and are very costly. For example, costs for cleaning 1 ha to a depth of 1 m have been reported between $600,000 and $3,000,000 (5). Furthermore, excavation strategies are not applicable to contaminated waters. Alternatively, metals can be immobilized in soils. This approach can carry the long-term risk of resolubi-lization because of chemical or biological changes (e.g., acidification) (6). Complementary approaches involving both removal and immobilization will be needed to remediate heavy metal-contaminated sites.
Research and applications indicate that uptake of heavy metals into plants via the root system could provide a cost-effective approach for toxic metal removal and remediation of heavy metal-laden soils and waters. Plant roots have been shown to remediate waters by removal of heavy metals (7). Furthermore, increasing the accumulation of valuable metals could eventually lead to "phytomining," as proposed, for instance, for gold (8).
On the other hand, uptake of toxic heavy metals in crop plants is known to cause health problems (see earlier). Understanding of the mechanisms underlying toxic metal uptake and sequestration could lead to engineering of crops that avoid toxic metal accumulation in edible parts of plants.
Toxic heavy metals are transported across the plasma membrane into plant root cells via physiological metal uptake transporters. However, for plants to accumulate large amounts of toxic metals for bioremediation purposes, many mechanisms and genes need to be identified and modified in plants. Several rate-limiting steps are critical for effective removal of heavy metals from soils. These include making the contaminants biologically accessible in the soil by chelation or external acidification and subsequent transport of metals or complexed metals across the plasma membrane of root cells. Upon uptake of heavy metals into plant cells, intracellular detoxification and transport through plant tissues are required.
Other metals, such as iron and zinc, are essential nutrients. Iron deficiency is the most widely spread micronutrient deficiency worldwide, affecting up to 3.7 billion people, particularly women (9), and zinc deficiency is a significant limiting factor for production and quality of cereals (10). Identification of molecular mechanisms that enhance accumulation of essential nutrients in plants could lead to development of crops that help to address major nutritional problems in humans and could also play an important role in plant nutrition and growth.
Phytoremediation involves the biotechnological exploitation of metal tolerance mechanisms. The same applies for the nutrition-related objectives of minimizing the concentration of toxic metals in edible parts and enriching crops for essential metals. In the present chapter we summarize the molecular understanding that has been gained to date and review the transgenic approaches that have been pursued to increase plant metal tolerance. This includes work that does not exploit plant tolerance mechanisms but instead utilizes bacterial or mammalian genes. The heavy metals mainly considered are cadmium, mercury, copper, zinc, iron, and nickel. Also, even though aluminum, strictly speaking, is not a heavy metal (because the specific weight is under 5 g/cm3), we include work on A1 tolerance because of its agronomic importance (11). Most of the fundamental insight into metal homeostasis in eukaryotes has come from studies involving Saccharomyces cerevisiae. For many of the molecular components identified in yeast, plant and mammalian homologues have been found, indicating a high degree of conservation. Thus, the work on S. cerevisiae is covered in some detail.
Was this article helpful?
Our internal organs, the colon, liver and intestines, help our bodies eliminate toxic and harmful matter from our bloodstreams and tissues. Often, our systems become overloaded with waste. The very air we breathe, and all of its pollutants, build up in our bodies. Today’s over processed foods and environmental pollutants can easily overwhelm our delicate systems and cause toxic matter to build up in our bodies.