ABSTRACT
Arsenic (As) is a ubiquitous element present in the environment. Although metalloid, it is often grouped among the toxic metals. It is the 20th most abundant element in the Earth’s crust, with an average concentration ranging from 1.5 to 5 mg/kg (Cullen and Reimer, 1989). In noncontaminated soils, arsenic concentrations typically range from 0.2 to 40 mg/kg (World Health Organization, 1981), whereas in contaminated soils, arsenic concentrations as high as 100-2500 mg/kg have been reported (World Health Organization, 1981; Diaz-Barriga et al., 1993; Vaughan, 1993). Arsenic can enter soil through both natural processes such as weathering and erosion of arsenic-bearing rocks (Alloway, 1990; Yan-Chu, 1994) and anthropological activities such as mining, smelting of ores, coal combustion, release of arsenic-laden liquid and solid wastes from industrial plants, irrigation with arsenic-contaminated water, and use of arsenic-based pesticides, herbicides, and fertilizers
34.1 Introduction .......................................................................................................................... 733 34.2 Arsenic as Toxic Metal ......................................................................................................... 735
34.2.1 Toxic Species of Arsenic .......................................................................................... 735 34.2.2 Sources of Arsenic to the Soil .................................................................................. 737 34.2.3 Uptake of Arsenic by Plants ..................................................................................... 739
34.2.3.1 Uptake of Arsenite ..................................................................................... 741 34.2.3.2 Uptake of Arsenate .................................................................................... 742
34.2.4 Symptoms of Arsenic Toxicity ................................................................................. 743 34.3 Metabolic Alterations in Arsenic-Stressed Plants ................................................................ 745 34.4 Oxidative Stress and Antioxidative Defense under Arsenic Toxicity .................................. 749
34.4.1 Nonenzymatic Antioxidants ..................................................................................... 752 34.4.2 Enzymatic Antioxidants ........................................................................................... 753
34.5 Arsenic Tolerance Mechanisms in Plants ............................................................................. 756 34.5.1 Suppression of High-Afnity Phosphate/Arsenate Transport .................................. 758 34.5.2 Reduction of Arsenate to Arsenite ........................................................................... 758 34.5.3 Increased Synthesis of Glutathione and Phytochelatins ........................................... 759 34.5.4 Arsenic Sequestration in the Vacuoles ..................................................................... 760 34.5.5 Arsenic Efux ........................................................................................................... 760 34.5.6 Methylation and Volatilization ................................................................................. 761
34.6 Strategies for Developing Arsenic Tolerance in Plants ........................................................ 762 34.7 Phytoremediation of Arsenic-Polluted Soil .......................................................................... 765 34.8 Conclusions and Future Prospects ........................................................................................ 768 References ...................................................................................................................................... 768
(reviewed by Smith et al., 1998; Mahimairaja et al., 2005). Arsenic is a signicant contaminant of soils and groundwater in many regions of the world including Argentina, Bangladesh, Chile, Mexico, China, Hungary, India, and Vietnam (Mahimairaja et al., 2005 and references therein), and the situation is worst in the densely populated oodplains and river deltas of South and Southeast Asia (Nordstrom, 2002; Brammer and Ravenscroft, 2009). Arsenic leads to stimulation of plant growth at low concentrations (Woolson et al., 1971; Carbonell-Barrachina et al., 1997; Miteva, 2002; Garg and Singla, 2011); however, at high concentrations, it not only exerts toxic effects on plants and animals but may pose severe health complications for humans and animals.
