Arsenic Stress in Plants: An Inside Story - Crop Improvement: New Approaches and Modern Techniques - page 389

glutathione which is precursor of phytochelatins because chelation capacity of a
plant is increased under metal toxicity (Cherian and Oliveira 2005 ). As previously
describes that As(III) is taken up by aquaporins, two genes for these aquaporins in
rice (Os05g14240 and Os12g10280) are also down regulated in the study of Norton
et al. ( 2008 ). It is also reported that lower expression of genes expression of PHT1
encoding phosphate transporters contributes to arsenic tolerance and accumulation
in shrub willow (Puckett et al. 2011 ). The differential expression pattern of sul-
phate transporters were observed after As(V) exposure (Kumar et al. 2011 ). Three
members of rice PIP subfamily of aquaporins have been recently reported to medi-
ate As(III) transport (Mosa et al. 2012 ). Overexpression of OsPIP2;4, OsPIP2;6,
and OsPIP2;7 proteins in Arabidopsis resulted in increased As(III) resistance (Ma-
ciaszczyk-Dziubinska et al. 2012 ).
By analyzing maize root proteome Requejo and Tena ( 2005 ) revealed that oxi-
dative stress is the main contributing factor to plant arsenic toxicity. They reported
that three superoxide dismutase, two glutathione peroxidase, one peroxidation and
one p-benzoquinone are up regulated and these are involved in cellular homeosta-
sis for redox perturbation. Conversely, recently by the study of rice genome very
surprising results came in which enzymes involved in detoxifying various reactive
oxygen species and free radicals, gives no response to low concentration of arsenic
for long term exposure (Norton et al. 2008 ). Only some of the Tau classes of GSTs
(Glutathione-S-transferase) showed remarkable changes in expression which was
only agreement with Mylona et al. ( 1998 ).
As part of arsenic detoxification, the majority of arsenate is reduced to arsenite
by the enzyme arsenate reductase (AR). Recently, AR genes have been identified
in plants, including Arabidopsis thaliana (AtAsr/AtACR2), Holcus lanatus (HI-
Asr) and Pteris vittata (PvACR2) (Dhankher et al. 2006 ; Bleeker et al. 2006 ; Ellis
et al. 2006 ). The Arabidopsis, fungal, protist ACR2 sequences show homology a
region within the CDC25 super family of protein tyrosine phosphatases (PTPase)
that also contains the conserved HCX s -R motif and also has similar catalytic ac-
tivity like arsenate reductase. The arsenic detoxification gene is well studied in
yeast in which three genes in cluster ACR1, ACR2, ACR3 are present for As toler-
ance. ACRI encodes a transcription factor, ACR2 encodes an arsenate reductase and
ACR3 encodes a plasma membrane As(III) efflux transporter (Ghosh et al. 1999 ;
Rosen 2002 ).
Many studies showed that alteration in the expression of arsenate reductase
genes leads to the more arsenic tolerance in plants. Dhankher et al. ( 2006 ) cloned
an Arsenate reductase gene from Arabidopsis thaliana (At ACR2) and silenced its
expression in root (because of it appears that in most plants, arsenite is sequestered
in roots, preventing it from moving up into stems, leaves and reproductive organs
but to enhance phytoremediation it should be stored in aboveground tissues) and
obtaining RNAi transgenics that accumulated 10-16 fold more arsenic in the shoots
and retained less root As compared with WT plants when grown in the presence of
low levels of As(V). However transgenic lines are more sensitive than WT plants