Arsenic Stress in Plants: An Inside Story - Crop Improvement: New Approaches and Modern Techniques - page 390

when exposed to high concentrations of As(V), probably because of the negative
effects of As(V) on phosphate metabolism. By contrast, Bleeker et al. ( 2006 ) found
that in Arabidopsis (equivalent to AtACR2) loss of function mutants are more As
sensitive than WT plants, even to low levels of As(V) and when they over expressed
AtASR the plant showed enhance tolerance to mildly toxic levels of As(V) to more
toxic As(III). Recently two arsenate reductase genes have been identified in rice
(OsArs 1, Oslog39860 and OsArs2, Os03g0 1770) (Duan et al. 2007 ). But interest-
ingly, besides their major role in arsenic detoxification these genes are not differ-
entially regulated on exposure to arsenic in the whole rice genome study (Norton
et al. 2008 ).
Further arsenite binds with phytochelatins as described earlier in respect to re-
ducing arsenic toxicity. Overexpression of genes involving in phytochelatins syn-
thesis, like phytochelatins synthatase, γ-glutamylcystein synthatase and glutathione
synthetase provides tolerance to arsenic toxicity. Li et al. ( 2004 ) overexpressed the
AtPCSI resulted in a substantial increase in arsenic resistance, with a 20-100 times
greater biomass in transgenic plants after exposure to arsenic, but led to Cd hy-
persensitivity. In contrasts in the study of Picault et al. ( 2006 ) overexpression of
cytoplasmic AtPCS 1 markedly increased tolerance in transgenic plants to arsenic,
whereas chloroplast-targeted overexpression of the same gene resulted in decreased
tolerance of transgenic plants to arsenic. This is may be due to the limiting supply of
essential metabolites such as cystein, γ-glutamylcystein and glutathione, which are
needed for the production of phytochelatins. Because of phytochelatins production
on exposure to arsenic leads to depletion of GSH, overexpression of components
involved in GSH biosynthesis, such as γ-glutamylcystein synthetase (γ ECS) and
glutathione synthetase (GS), will lead to increased tolerance to arsenic. Li et al.
( 2005 ) overexpressed γ ECS in Arabidopsis thaliana and found a 3-20 fold greater
production of γ-glutamylcystein, glutathione and phytochelatins in plants exposed
to arsenic. These studies shows that the expression of all these genes play important
role in balance between toxicity and resistance to arsenic.
As part of the detoxification mechanism, Arsenic can be effluxed from the cy-
toplasm through As(III)-efflux transporters and can be sequestered in vacuoles by
ABC type transporters. The genes for these transporters are not well identified in
plants till now but it has homology to yeast. In yeast in ACR gene cluster, ACR3
gene encoded a plasma membrane As(III)-efflux transporter, for the removal of
cytosolic arsenic and an ABC type transporter, yeast cadmium factor (YCF I) is
located at the vacuolar membrane by which arsenic sequester in vacuoles (Rosen
2002 ). Ali et al. ( 2012 ) reported that heterologous expression of the yeast arsenite
efflux system ACR3 improves Arabidopsis thaliana tolerance to arsenic stress.
In general, the expression of genes that encode various transporters is higher in
hyperaccumulater plants than non-accumulators (Krammer 2005 ). By some stud-
ies it is confirm that by increase in the over expression of these transporters can
increase the tolerance of arsenic. Ellis et al. ( 2006 ) isolate and characterized arse-
nate reductase (PvACR2) and arsenite transporter (PvACR3) from pteris vittata that