Environmental cleanup using plants: biotechnological advances and ecological considerations

inorganic and organic compounds, primarily as a result of human activities. While inorganic pollutants occur as natural elements in the Earth’s crust and atmosphere, human activities such as industry, mining, motorized traffic, agriculture, logging, and military actions promote their release and concentration in the environment, leading to toxicity (Nriagu 1979). Organic pollutants in the environment are mostly man-made and xenobiotic (ie not normally produced or expected to be present in organisms). Many of them are toxic and/or carcinogenic. Sources of organic pollutants in the environment include accidental releases (eg fuels, solvents), industrial activities (eg chemical, petrochemical), agriculture (eg pesticides, herbicides), and military activities (eg explosives, chemical weapons), among others. Moreover, polluted sites often contain a mixture of both organic and inorganic pollutants (Ensley 2000). Currently $6–8 billion a year is spent on environmental cleanup in the US, and $25–50 billion per year worldwide (Glass 1999; Tsao 2003). Most remediation activity still makes use of conventional methods such as excavation and reburial, capping, and soil washing and burning. However, newly emerging biological cleanup methods, such as phytoremediation, are often simpler in design and cheaper to implement. Phytoremediation incorporates a range of technologies that use plants to remove, reduce, degrade, or immobilize environmental pollutants from soil and water, thus restoring contaminated sites to a relatively clean, non-toxic environment. Phytoremediation depends on naturally occurring processes, in which plants detoxify inorganic and organic pollutants, via degradation, sequestration, or transformation. The different uses of plants and their associated microbes for environmental cleanup are summarized below (for reviews, see also Salt et al. 1998; Meagher 2000; Pilon-Smits 2005). Phytoextraction is the removal of pollutants by the roots of plants, followed by translocation to aboveground plant tissues, which are subsequently harvested. Continuous phytoextraction uses plants that accumulate high levels of pollutants over their entire lifetime. Induced phytoextraction enhances pollutant accumulation towards the end of the plant’s lifetime, when they attain their maxi203