Arsenic Toxicity and Tolerance Mechanisms in Plants: An Overview - Crop Improvement: New Approaches and Modern Techniques - page 361

toxicity. As is also responsible for oxidative stress in plants through the generation
of reactive oxygen species (ROS) which attack the biomolecules like, membranes
proteins, carbohydrates, nucleic acids etc. At the same time activity of enzymatic
and non-enzymatic antioxidants are increased which helped the plant to withstand
the toxicity of As. The present chapter throws light on the arsenic toxicity in plants
and their tolerance mechanism in plants. The chapter also highlights the generation
of reactive oxygen species and antioxidants during As stress.
Introduction
In human history, metal pollutants have been their part. However, due to the onset
of industrial revolution, the biosphere has been intensely polluted by toxic metals
leading to the major environmental and health problems (Bhattacharya et al. 2012 ;
Lui et al. 2012 ). Arsenic (As) is of major concern among all the metal pollutants
because it is a persistent bioaccumulative carcinogen (Kaur et al. 2011 ). The an-
thropogenic activities have increased many folds the level of arsenic as humans
constantly endeavored to improve quality of life when compared to the naturally
occurring elements occurring in the earth's crust (Moreno-Jimenez et al. 2012 ). The
global input of arsenic to soil by human activities was estimated between 52,000
and 1,12,000 t per year (Chandra and Srivastava 2003 ). Severe problems like vege-
tation loss, contamination of ground water, and toxicity of arsenic in animals, plants
and have been caused by arsenic contamination (Fowler 1983 ; Zhao et al 2010 ).
Although several studies have reported the detoxification of arsenic ions through
metal binding peptides (Schmoger et al. 2000 ; Bondada and Ma 2002 ), due to the
lack of literature regarding to the role of ascorbate -glutathione pathway in cellular
defense against arsenic in plants.
Heavy metals produce oxidative-stress possibly by free-radical generation and
active oxygen species (Hall 2002 ). The reaction of these oxyradicals with proteins,
lipids, pigments and nucleic acids causes lipid peroxidation, membrane damage
and inactivation of enzymes, thus affecting the cell viability. Two major roles are
played by active oxygen species: (1) exacerbating damage and (2) signaling the
activation of defence responses. Recently these two functions have been verified by
several abiotic stress responses (Dat et al. 2000 ). In higher vascular plants, change
in cellular metabolism is required to counteract with heavy metal stresses. Various
antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), ascorbate
peroxidase (APX) and glutathione reductase (GR) of the ascorbate glutathione path-
way is the main mechanism for ROS quenching under heavy metal stress. (Clijsters
et al. 1999 ). Apart from these enzymes, low molecular weight antioxidants metabo-
lites like ascorbate and reduced glutathione (GSH) also play an important role in
plants by protecting them from oxidative. Antioxidant responses to arsenic in higher
plants have not been studied so far.