Agriculture Reference
In-Depth Information
There has been substantial progress in our understanding of how arsenic is taken
up by the plants and how it is being metabolized inside the plants (Zhao et al.
2009
,
2006
; Mendoza-Cózatl et al.
2011
). These works have helped in devising strate-
gies to counter arsenic stress. Discovery of brake fern (
Pteris vittata
) as hyper ac-
cumulator of arsenic by Ma et al. (
2001
) is one of the significant achievement in
this regard. After this discovery, 12 species of fern have been identified as hyper
accumulator of arsenic (Zhao et al.
2009
). Among the crop plants, interestingly
rice (
Oryza sativa
) is much more efficient in arsenic accumulation compared to
wheat and barley (Williams et al.
2007
; Su et al.
2010
). The reason for rice being
efficient arsenic accumulator is the bioavailability of arsenic since rice grows under
anaerobic condition and that the highly efficient silicon pathway in rice also helps
in uptake and transport of arsenic.
In this chapter, we will focus on different mechanisms involved in arsenic up-
take, transport and its metabolism in the plants. Finally, the different approaches
being used to develop stress resistance against arsenic in plants will be discussed.
2 Arsenic Toxicity in Plants
The nature of arsenic toxicity varies with different arsenic species. Since arsenate
is an analogue of phosphate, it interferes with essential cellular processes such as
phosphorylation and ATP synthesis, whereas arsenite, the reduced form of arsenate,
binds with vicinal sulphyhydryl groups of proteins resulting in alteration in their
structure and catalytic properties, and thus deleterious effects on protein functioning
(Hughes
2002
). The toxic effects of arsenate are largely attributed to arsenite since
the former is radially reduced to the latter (Hughes
2002
). Arsenate stress results in
generation of reactive oxygen species (ROS) in different plants viz., rice, maize and
Holcus lanatus
and thus induces oxidative stress such as lipid peroxidation (Rao
et al.
2011
; Ahsan et al.
2008
; Hartly-Whitakar et al.
2001
; Mylona et al.
1998
;
Requejo and Tena
2005
). Activity of several antioxidant enzymes like superoxide
dismutase, catalase, etc., and mRNA transcripts of genes encoding these enzymes
are also up-regulated in response to arsenic stress (Mylona et al.
1998
; Requejo and
Tena
2005
). In maize, these responses are found to be tissue as well as developmen-
tal stage specific. Depletion of cellular reduced GSH is considered to be the cause of
arsenic-induced oxidative stress (Mylona et al.
1998
). The toxicity level of arsenic
in shoots varies depending upon plant species, whether it is hyperaccumulator or
nonaccumulator. As hyperaccumulator
P. vittata
withstands 5,000-10,000 mg/kg
−1
of arsenic in frond tissue without any detectable effects of toxicity, whereas non-
hyperaccumulator plants show toxicity effects even with arsenic concentration
ranging from 1100 mg/kg
−1
(Kabata-Pendias and Pendias
1992
; Lombi et al.
2002
;
Tu and Ma
2002
). Rice seedlings grown in hydroponics with medium containing
arsenic concentration beyond 10 µM show roots as well as shoots growth retarda-
tion accompanied by decreased photosynthetic yields with other toxic effects
