Agriculture Reference
In-Depth Information
In plants, arsenic induces phytochelatin (PC) biosynthesis (Qin et al.
2009
;
Sneller et al.
1999
). Arsenite has high affinity to the sulphhydryl (-SH) groups of
peptides viz. GSH and PCs. GSH and arsenite also form (GS)
3
-arsenite complex in
vitro (Delnomdedieu et al.
1994
). The toxicity of arsenite is also due to binding of
arsenite to the -SH groups of proteins with deleterious effect on their structure and
activity (Meharg and Hartley-Whitaker
2002
). Dimethylarsinothioyl glutathione
(DMAS-GS) complex have also been identified from
Brassica oleracea
, a sulphur
rich plant (Raab et al.
2007b
). Furthermore, blocking of PC synthesis with
L
-buthio-
nine-sulfoxime results in hypersensitivity to arsenic (Bleeker et al.
2006
; Schat et al.
2002
; Schmöger et al.
2000
). More profound evidence comes from
Arabidopsis
mu-
tant
cad1-3
(cadmium sensitive), a PC deficient mutant, which showed 10-20 times
more sensitivity to arsenate compared to wild type plants (Ha et al.
1999
). Further,
overexpression of bacterial γ-glutamylcystein synthase gene (γ-
ECS
),
Arabidopsis
PC synthase (
AtPCS1
) also led to more arsenic-tolerant phenotype to transgenics
(Dhankher et al.
2002
; Li et al.
2004
). These studies provide unequivocal evidence
that thiols and PCs play significant role in arsenic detoxification in nonaccumulator
plants. Interestingly, hyper accumulator,
P. vittata
and
P. cretica
do not follow PC-
based mechanism to detoxify arsenic (Zhao et al.
2009
).
Like other heavy metal (Cd) and its complex with proteins localized in vacuoles
(Vögeli-Lange and Wagner
1990
), it is believed that PC-arsenite complexes are also
stored in vacuoles, where acidic pH (~5.5) helps in maintaining stability of com-
plexes (Zhao et al.
2009
). The yeast vacuolar transporter Ycf1p (yeast cadmium fac-
tor 1p), a member of ATP binding cassette (ABC) superfamily confers arsenite re-
sistance by transporting the glutathione-
S
-conjugated arsenite into vacuole (Ghosh
et al.
1999
). The ABC proteins are also likely transport PC-arsenite complexes;
however such proteins have not yet been identified (Zhao et al.
2010
). In support of
the ABC mediated transport, it has been demonstrated that As(III)-(GS)
3
complex
was transported into the tonoplast vesicles prepared from
H. lanatus
in a MgATP-
dependent manner (Bleeker et al.
2006
). Inorganic arsenic species is also known
to be stored in vacuoles in
P. vittata
(Lombi et al.
2002
). Owing to the possible
large concentration gradient from the cytoplasm to the vacuole, transport of arse-
nite across the tonoplast probably involves an energy-dependent active mechanism.
A transporter(s) responsible for arsenite uptake into the vacuoles is not yet known
but may be the key determinant of the hypertolerance phenotype in
P. vittata
and
other hyperaccumulator plants (Zhao et al.
2010
).
6 Strategies for Stress Resistance
There are three broad objectives before plant scientists with respect to arsenic-plant
interaction, enhancing tolerance of crop plants to withstand high arsenic concentra-
tions in soils, reducing accumulation of arsenic in edible plant part so as to reduce
arsenic toxicity for humans; and development of plants with increasing arsenic
