**4. Phytoremediation**

Some workers quantified and compared the responses of soil microbial communities during the phytoremediation of PAHs in a laboratory trial [15]. A recent publication of some workers describes the development of transgenic poplars (*Populus* sp.) over expressing a mammalian cytochrome P450, a family of enzymes commonly involved in the metabolism of toxic com‐ pounds. The engineered plants showed enhanced performance about the metabolism of trichloroethylene and the removal of a range of other toxic volatile organic pollutants, including vinyl chloride, carbon tetrachloride, chloroform and benzene. Some workers suggested that transgenic plants might be able to contribute to the wider and safer application of phytoremediation [58]. Widespread phytoremediation field trials research was performed *in vitro* condition and many of the works explored the effects of plants on removal of contam‐ inants from spiked soil and soil excavated from contaminated sites [7] and most of these experiments provided valuable insights into the specific mechanisms of phytoremediation of organic contaminants [29]. Previously, numerous organic pollutants such as TCE (trichloro‐ ethylene), herbicides such as atrazine, explosives such as TNT (trinitrotoluene), PHC, BTEX (mono aromatic hydrocarbons) and PAHs, the fuel additive MTBE (methyl tertiary butyl ether), and PCBs (polychlorinated biphenyls) have been successfully phytoremediated. [59] Major advantages of phytoremediation *viz*., cost of the phytoremediation is lower than that of traditional processes both *in situ* and *ex situ*, plants can be easily monitored, possibility of the recovery and re-use of valuable products, use of naturally occurring organisms and preser‐ vation the natural state of the environment, low cost of phytoremediation (up to 1000 times cheaper than excavation and reburial) [60].
