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Molecular Plant Breeding 2012, Vol.3, No.1, 1
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to drought tolerance. Once these molecular markers
are identified to be tagged with QTLs, selection at
early segregating generation can be carried out. Close
linkage of marker with the QTL and efficient means of
screening are the essential requirements for effective
MAS (Mohan et al., 1997).
Courtois et al (2003) used MAS to transfer a number
of QTLs related to deep rooted character from the
japonica upland cultivar ‘‘Azucena’’ to the lowland
indica variety ‘‘IR64’’. MAS selected lines showed a
greater root mass and higher yield in drought stress.
Similarly, Steele et al (2006) used marker assisted
breeding program to improve some root traits related
to drought tolerance in an Indian rice cultivar Kalinga
Ⅲ
. Cultivar Azucena was used as a donor parent.
About 22 near-isogenic lines (NILs) were evaluated
and their performance in water limited conditions was
markedly improved due to a target segment on
chromosome 9 of Azucena. Bernier et al (2007) also
screened 436 F
3
lines by adopting the same
methodology of MAS. They identified a QTL on
chromosome 12 having a marked effect on the yield
under drought stress conditions.
The advent of molecular markers has revolutionized
the screening of complex traits like drought tolerance
in crop plants. Molecular markers such as restriction
fragment length polymorphism (RFLP) are very
reliable and have been extensively used in rice
(Mohan et al., 1997). The very first RFLP map for rice
was constructed by McCouch et al (1988). Microsatellite
markers, also called simple sequence repeats (SSRs),
have been widely applied for rice genome mapping for
abiotic stress tolerance (Temnykh et al., 2000). Recent
developments in DNA marker technology coupled
with MAS provide efficient means to plant breeders to
carry out selection of rice cultivars under drought
prone environments. The only prerequisite requirement
for effective MAS program is the stable and continued
expression of QTLs under different environments.
5 Transgenic approaches for generating drought
tolerant rice
A transgenic approach is one which involves structural
modification in traits by transferring desired genes
from one species to the other (Ashraf, 2010). This
relives the breeder from limitation of using same
species for gene transfer. Transgenic approaches are
being widely used through out the world for abiotic
and biotic stress tolerance in various crops (Ashraf,
2010). Some of the recent transgenic lines produced in
rice for drought stress tolerance are listed in table 3.
The important objective of genetic engineers is to
incorporate those genes that encode several transcription
factors, heat shock and late embryogenesis abundant
proteins (LEA), and compatible organic osmolytes.
Transcription factors are basically proteins that are
involved in gene regulation. These transcription
factors play very important role in almost every stress
response. Dehydration-responsive element-binding
factors (DREB) are especially important as these
regulate genes involved in drought, salinity and
freezing (Khan, 2011; Gosal et al., 2009). Much of the
work has been done in
Arabidopsis
with reference to
DREB transcription factors. According to Kasuga et al
(1999) in transgenic
Arabidopsis
plants, the over
expression of
CBF3/DREB 1A
when accompanied by
constitutive promoter CaMV 35S greatly improved
plant’s tolerance to drought, salinity and freezing
stresses. Similar DREB genes and promoters have
been identified in rice (Dubouzet et al., 2003). Oh et
al (2005) successfully engineered the rice with
transcription factor
CBF3/DREB 1A
from Arabidopsis.
The stress-responsive NAC (SNAC1) is another class
of transcription factors, originally identified as an
overexpressed gene induced by drought stress in
microarray analysis (Leung, 2008). Over-expression
of SNAC1 in transgenic rice showed improved drought
tolerance under field conditions (Hu et al., 2006).
Heat shock and late embryogenesis abundant proteins
(LEA) are among the class of those proteins that
accumulate when drought conditions ensue. Thus,
protecting the plant from adverse effects of drought
was urgent (Wang et al.,
2004; Gosal et al., 2009;
Hussain et al., 2011). Over expression of
OsWRKY11
allele under the control of the heat shock protein 101
(HSP101) promoter enhanced heat and drought
tolerance in rice (Wu et al., 2009). Working on the
same path, Tao et al (2011) identified two alleles
OsWRKY45-1
and
OsWRKY45-2
showing differential