گیاهان ترانس ژنیک برای گیاه پالایی و بیگانه‌زیست های آلی و تجزیه بیولوژیکی ارتقا یافته

ABSTRACT Transgenic plants for enhanced biodegradation and phytoremediation of organic xenobiotics

Environmental pollution with organic xenobiotics (pesticides, pharmaceuticals, petroleum compounds, PAHs, PCBs etc.) is a global problem, and the development of inventive remediation technologies for the decontamination of impacted sites are therefore of paramount important. Physical, chemical and biological methods can all be used for the remediation of contaminated sites (Lee and Huffman, 1989; Felsot and Dzantor, 1995; Johnson et al., 1997; Hatakeda et al., 1999; Wirtz et al., 2000; Kummling et al., 2001; Perrin-Ganier et al., 2001; Matsunaga and Yashuhara, 2003; Gao et al., in press; Yang et al., 2007), however, phytoremediation has long been recognized as a cost effective method for the decontamination of soil and water resources (Salt et al., 1998; Macek et al., 2000; Meagher, 2000; Eapen and D’Souza, 2005). Further, a variety of pollutant attenuation mechanisms possessed by plants makes their use in remediating contaminated land and water more feasible than physical and chemical remediation (Glick, 2003; Huang et al., 2004, 2005; Greenberg, 2006; Gerhardt et al., 2009). As a result of their sedentary nature, plants have evolved diverse abilities for dealing with toxic compounds in their environment. Plants act as solar-driven pumping and filtering systems as they take up contaminants (mainly water soluble) through their roots and transport/translocate them through various plant tissues where they can be metabolized, sequestered, or volatilized (Cunningham et al., 1996; Greenberg et al., 2006; Abhilash, 2007; Doty et al., 2007).

When microorganisms are used for remediation of xenobiotics, both inoculation of microorganisms and nutrient application are essential for their maintenance at adequate levels over long periods (Eapen et al., 2007). Besides, the microbes which show highly efficient biodegradation capabilities under laboratory conditions may not perform equally well at actual contaminated sites (Goldstein et al., 1985; Macek et al., 2008). The potential of genetic engineering to accelerate the bioremediation of xenobiotics has been recognized since the early 1980s, with initial attempts being focused on microorganism (Rugh et al., 1998; Rosser et al., 2001; Sung et al., 2003; Rugh, 2004; Doty, 2008; Singleton, 2007). However, there are two main problems with the introduction of transgenic microorganisms: the bureaucratic barriers blocking their release into the environment and the poor survival rate of those engineered strains that have been introduced into the contaminated soil. Phytoremediation for removal of xenobiotics can be an alternate/supplementary method, since plants are robust in growth, are a renewable resource and can be used for in situ remediation (Cunningham and Berti, 1993; Cunningham et al.,1995,1996; Cunningham and Ow,1996; Suresh and Ravishankar, 2004; Parameswaran et al., 2007). It has almost certain public acceptance. Further, plants may survive higher concentrations of hazardous wastes than many micro-organisms used for bioremediation. Phytoremediation increases the amount of organic carbon in the soil, which can stimulate microbial activity and augment the rhizospheric degradation of the pollutants. Phytoremediation also yields other benefits including carbon sequestration, soil stabilization, and the possibility of biofuel or fiber production. The development of phytoremediation technologies for the plant-based clean-up of contaminated soils is therefore of significant interest (Hooker and Skeen, 1999; Dietz and Schnoor, 2001; Eapen and D’Souza, 2005).

With respect to their direct roles in remediation processes, plants use several different strategies for dealing with environmental chemicals: phytoextraction, phytodegradation, phytovolatilization, and rhizodegradation (Fig.1) (Schnoor,1997). Phytoextraction involves the removal and subsequent storage of contaminants by the plant and is often applied to the exclusion and storage of metals that may undergo speciation in plants, but cannot be metabolized. However, certain organic chemicals may also be treated in this manner due to inherent resistance to degradation. Conversely, phytodegradation describes processes in which plants metabolize the contaminants they take up.

Components of this mechanism are often utilized by plants exposed to herbicides and thus have been researched extensively. The metabolic processes involved in phytodegradation have strong similarities to those used by animals for modification and degradation of drugs and other toxins. This has given rise to a conceptual model for phytodegradation known as the “green liver” model (Sanderman, 1994). A further attenuation mechanism, referred to as phytovolatilization, involves the release of contaminants to the atmosphere following their uptake from the soil or water. This mechanism has been observed for both organic and heavy metal contaminants, including trichloroethylene (TCE), which has been observed in the off-gas from plant leaves in the laboratory and field (Compton et al., 1998), and in the production of volatile, elemental mercury by genetically-engineered Arabidopsis thaliana grown in the presence of ionic mercury (Rugh et al., 1996; Bizily et al., 1999). An indirect mechanism, rhizodegradation refers to the transformation of contaminants by resident microbes in the plant rhizosphere (i.e., the microbe-rich zone in intimate contact with the root vascular system). As mentioned above, the presence of plants on contaminated sites can drastically affect soil redox conditions and organic content (often through the secretion of organic acids from roots), as well as soil moisture. Rhizodegradation is the dominant mechanism in the removal of total petroleum hydrocarbons from soil by deep-rooted trees (Carman et al., 1998), as well as annual species (Schwab and Banks, 1994).

Although much research has been done to demonstrate the success of phytoremediation, resulting in its use on many contaminated sites, (Aprill and Sims, 1990; Gunther et al., 1996; Binet et al., 2000; Liste and Alexander, 2000; Mattina et al., 2000; White, 2000, 2001, 2002; Fismes et al., 2002; Li et al., 2002; Maila and Cloete, 2002; Yoon et al., 2002; Singh and Jain, 2003; Sung et al., 2003; Sunderberg et al., 2003; Trapp et al., 2003; White, 2003; Gao and Zhu, 2004; Ma et al., 2004; Mattina et al., 2004; Suresh et al., 2005; Parrish et al., 2006; Mills et al., 2006; Aslund et al., 2007; Kobayashi et al., in press) the method still lacks wide application. Further, detoxification of organic pollutants by plants is often slow, leading to the accumulation of toxic compounds in plants that could be later released into the environment (Aken, 2008). There are also concerns over the potential for introduction of contaminants into the food chain. The question of how to dispose of plants that accumulate organic pollutants is also a serious concern. A direct method for enhancing the effectiveness of phytoremediation is to overexpress in transgenic plants the genes involved in metabolism, uptake, or transport of specific pollutants (Shiota et al., 1994; Rugh, 2004; Cherian and Oliveira, 2005; Kramer, 2005; Eapen et al., 2007; Macek et al., 2008; Aken, 2008; Doty, 2008). If the plants are able to degrade the xenobiotics to non-toxic metabolites or completely mineralized into carbon dioxide, nitrate, chlorine etc., there is no apprehension over hazardous waste management strategies for disposing of harvested plants. The purpose of this review is to provide recent advances in development of transgenic plants overexpressing catabolic genes including bacterial and human cytochrome P450 for the enhanced degradation and mineralization of xenobiotic pollutants.