Agriculture Reference
In-Depth Information
PCs have been observed upon exposure to arsenic in
Arabidopsis
overexpressing
γ-ECS
conferring significant resistance to arsenic (Li et al.
2005
,
2006
). These stud-
ies suggest that multigenic approach might lead to higher tolerance in plants against
arsenic stress (Cherian and Oliveira
2005
). Overexpression of bacterial arsenic re-
ductase gene in leaves (
arsC
) under control of light inducible RuBisco promoter
and constitutive actin promoter driven
γECS
in roots as well as shoots have mark-
edly increased tolerance and accumulation of arsenic in shoots (Dhankher et al.
2002
). The leaf-specific expression of
arsC
possibly involved in arsenate reduction,
in spite of high endogenous activity of arsenate reduction in the wild type plants,
whereas
γ-ECS
overexpression might boost the biosynthesis of thiol rich peptides
for arsenite complexation. These results imply that enhanced shoot tolerance has
the effect of driving more arsenic accumulation in shoots. In future, it may be pos-
sible to engineer high-biomass plants for arsenic phytoextraction using genes from
P. vittata
, specifically those responsible for efficient xylem loading of arsenic and
detoxification in fronds, although the molecular mechanisms for arsenic hyperac-
cumulation are obscure at present (Zhao et al.
2010
). Another strategy could be an
increased biosynthesis of PCs in roots so that the As-PC complexes are sequestered
in vacuoles in roots thus restricting translocation of arsenic to shoots (Zhao et al.
2009
,
2010
).
Since arsenate is taken up by plant roots via phosphate transporters and different
phosphate transporters may vary in their affinity for arsenate, it may be possible to
identify variant of phosphate transporters which are more discriminatory against
arsenic. Similarly, variants of NIP aquaporins or Lsi2-like carrier proteins may also
help in reducing arsenic in shoots, thus increasing tolerance against arsenic materi-
als (Zhao et al.
2010
).
Arsenic specific methyltransferase, if identified, may be a good target for ma-
nipulation since methylated species of arsenic are less toxic than the inorganic form.
arsM
genes of microbial or algal origin may also be used for overexpression to
achieve the conversion of inorganic to methylated or even volatilizable species of
arsenic (Qin et al.
2006
,
2009
). Members miotgen activated protein kinase (MAPK)
cascade (OsMPK3, OsMPK4 and OsMKK4 in rice) which are activated upon arse-
nic stress (Rao et al.
2011
) might be an additional target provided that their in-planta
role in arsenite stress tolerance is established. Acitivities of MAPK were also shown
to be upregulated upon arsenic stimulation in
Brassica juncea
(Gupta et al.
2009
).
9 Future Perspective
Our knowledge of precise mechanism of tolerance in case of hyperaccumulator
plants is very limited. Similarly, genes and enzymes involved in arsenic metabo-
lism in nonaccumulator plants need further elucidation. Further research on me-
tabolism, root-to-shoot translocation and sequestration of arsenic would be needed.
In plant arsenate reduction which involves multiple pathways and enzymes, only
one enzyme has been identified until now (Zhao et al.
2009
). Since plants and seeds
