85 Soil Remediation and Plants. http://dx.doi.org/10.1016/B978-0-12-799937-1.00004-8 Copyright © 2015 Elsevier Inc. All rights reserved. Phytoremediation: Mechanisms and Adaptations Muhammad Sabir,* ,† Ejaz Ahmad Waraich, ‡ Khalid Rehman Hakeem, § Münir Öztürk, ¶ Hamaad Raza Ahmad* and Muhammad Shahid ‡‡ *Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan, † School of Plant Biology, University of Western Australia, Crawley, Australia, ‡ Department of Crop Physiology, University of Agriculture, Faisalabad, Pakistan, § Faculty of Forestry, Universiti Putra Malaysia, Serdang, Selangor, Malaysia, ¶ Department of Botany, Ege University, Izmir, Turkey, ‡‡ Department of Environmental Sciences, COMSATS Institute of Information Technology, Vehari, Pakistan INTRODUCTION Metal contamination of soils is ubiquitous around the globe. Metals enter the soil due to anthropogenic activities such as the use of sewage sludge, urban composts, fertilizers, pesticides, sewage irrigation, incineration of municipal waste, auto- vehicle exhausts, industrial emissions and metal mining and smelting (Hussain et al., 2006; McGrath et al., 2001; Murtaza et al., 2010). These metals include iron (Fe), copper (Cu), manganese (Mn), zinc (Zn), cadmium (Cd), lead (Pb), chro- mium (Cr), mercury (Hg) and nickel (Ni) (McIntyre, 2003). Metals accumulate in the soil to toxic levels that may lead to accumulation of metals in plants to unac- ceptable levels. Metal accumulation is a subject of serious concern due to threat to plant growth, soil quality, animal and human health (McGrath et al., 2001). Clean- ing up soils to remove metals is a sign of the times, but it is a challenging task. Different technologies being used nowadays are ex situ which lead to destruction of soil structure, thus leaving it unusable with poor vegetative cover (He and Yang, 2007). Growing plants to clean up the soils is a cost-effective and environmen- tally friendly alternative (Yang et al., 2005). Phytoremediation seems attractive due to non-invasive and non-destructive technologies which leave the soil intact and biologically productive (Wenzel, 2009). Plants respond differentially to metal contamination in soils and can be classified into different categories, depending upon their responses to metal contamination in their rooting medium. Plants can be classified into accumulators, indicators or excluders depending upon absorp- tion and translocation of metals by the plants to above-ground parts (Baker, 1981). Accumulators can survive by maintaining high concentration of metals in their Chapter 4