Agriculture Reference
In-Depth Information
7 Development of Elite Cultivars Through
Plant Breeding
A significant genetic variation in rice has been reported, especially among rice cul-
tivars from Bangladesh with regard to arsenic accumulations. Further, a number of
local cultivars with low grain arsenic concentrations have been identified (Norton
et al.
2009
). These variations may form basis for selection as well as improvement
of rice cultivars with minimum arsenic accumulations through breeding practices.
Additionally, the 'red bran' character was found to be associated with arsenic toler-
ance (Norton et al.
2009
), which may aid in selection of arsenic-tolerant genotypes
provided that there is a strong linkage between the two traits.
Genetic mapping of arsenic tolerance has also been reported in rice (Dasgupta
et al.
2004
) which revealed a segregation of a single gene which was termed
AsTol
and mapped to a rice chromosome 6. However, further report showed presence of
three-gene model of arsenic tolerance (Norton et al.
2008b
). From the position of
these three major genes, it may be possible to produce a list of candidate genes, and,
by integrating microarray analysis (Norton et al.
2008b
), it was possible to narrow
these lists down further. Additionally, quantitative trait loci (QTL) have been pro-
posed for arsenic tolerance in rice (Zhang et al.
2008
).
There has been a negative correlation observed between arsenic accumulation
in straw and grain and root porosity and oxygen release likely through iron plaque
formation, arsenite oxidation and arsenate retention on the iron plaques. Selection
of genotypes with profound oxygen release characteristic may also help to reduce
arsenic concentrations in grains (Zhao et al.
2010
; Mei et al.
2009
). This informa-
tion may form a basis to initiate breeding programs for development of cultivars
with less arsenic and arsenic stress tolerance.
8 Genetic Engineering
Increase in synthesis of PC and GSH through overexpression of genes involved in
their biosynthesis has been used to enhance arsenic tolerance in plants (Pickering
et al.
2006
). Overexpression of
AtPCS1
led to increase in arsenic resistance and
greater biomass than that of wild type plants upon arsenic treatment. However, these
transgenic plants showed cadmium hypersensitivity (Li et al.
2004
). Chloroplast
targeting
AtPCS1
made transgenic plants sensitized to arsenic whereas cytosolic
targeting of the same resulted in conferring tolerance to plants (Li et al.
2004
).
These results suggest that mere increase in PCS may not be sufficient for tolerance
in plants rather the unwanted phenotype of overexpression was attributed partly
due to limiting metabolites such as cysteine and glutamylcystein and GSH which
are essential for the biosynthesis of PCs (Picault et al.
2006
). Overexpression of
other components of GSH biosynthesis such as γ-glutamylcystein synthase (γ-
EC
)
and glutathione synthase (
GS
) may prove to be a successful approach in enhancing
biosynthesis of PCs (Tripathi et al.
2007
). Enhanced production of γ-EC, GSH and
