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Molecular Plant Breeding 2012, Vol.3, No.1, 1
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2 The complex nature of drought tolerance
and conventional breeding
The drought tolerance is the ability of plant to survive
and reproduce in a limited water supply (Turner, 1979;
Ashley, 1993). Conventional breeding for drought
tolerance in rice has met with little success (Fukai and
Cooper, 1995). This is due to the polygenic nature of
the phenomenon with low heritability and high G × E
interaction (McWilliam, 1989; Ingrams and Bartels,
1996; Zhang, 2004). Furthermore drought stress is
accompanied by other biotic stresses like high
temperature, and nutrient deficiencies which further
complicate the breeding efforts (Fleury et al., 2010).
To over come this, breeder has to screen and select a
very large number of genotypes to get the desired ones.
This is itself a time consuming and labor intensive
operation involving difficulties in field operation and
unexpected rainfall.
Several morphological traits affect drought tolerance
in rice. Root characteristics such as thickness, depth of
rooting, root penetration ability etc., have been
associated with drought resistance in rice (Nguyen et
al., 1997; Price and Courtois, 1999). Osmotic
adjustment is also a reliable parameter making the
plant to sustain itself in limited water condition by
decreasing osmotic potential and maintaining turgor
(Blum, 2005). However, phenotypic selection for
these traits is expensive and labor intensive (Boopathi
et al., 2002). Furthermore, directly selecting rice
cultivars for grain yield under stressed conditions was
considered a relatively inefficient method (Fukai and
Cooper, 1995). Until recently, there are numerous
reports that direct selection for grain yield under stress
is effective without reducing the yield potential of rice
(as reviewed by Farooq et al., 2009)
3 Identification of drought related QTLs in
rice
Traits showing continuous variation are called
quantitative traits and genes which control these traits
are referred to as quantitative trait loci (QTLs). QTL
mapping in doubled haploid (DH) and recombinant
inbred lines (RIL) population is considered to be an
efficient methodology to identify genes of agronomic
characters (Doebley et al., 2006). The recent progress
made in the field of genomics offers new opportunities to
dissect the QTLs for drought tolerance. In rice
numerous QTLs associated with drought tolerance
have been identified and mapped. The cross between
upland rice (CT9993) and lowland (IR62266) cultivars
has been the center of focus for a number of breeders
as it revealed QTLs for morphological and physiological
traits. Babu et al (2003) used double haploids (DH)
derived from rice lines and subjected them to water
stress. The experiment resulted identification of 47
drought related QTLs and phenotypic variation ranged
from 5 to 59%. They also identified major QTLs on
chromosome 4 with pleiotropic effects on yield under
drought stress. Various QTLs for drought tolerance in
rice are summarized in table 2.
Courtois et al (2000) also identified 42 QTLs for
drought related traits in rice. A QTL for root length
and thickness has been mapped on chromosome 9
showing stable expression across different environments
(Steel et al., 2006). Obara et al (2010) mapped a major
QTL qRL6.1 for root length, on chromosome 6 in rice
seedlings grown under hydroponic conditions.
Main problem encountered in these studies were that
the QTLs were having a minor affect on the
phenotype posing a great challenge for the breeders to
discover major QTLs functioning independently to
their genetic background (Gowda et al., 2011).
Secondly, most of these QTL mapping studies in rice
have been conducted using progenies derived from
intra specific crosses. Much needed efforts are still
required to go for inter specific crosses to explore
novel alleles and with their effective incorporation in
to the breeding programs for drought tolerance in rice.
4 Marker assisted selection (MAS) for drought
tolerance in rice
Selection for drought tolerant rice cultivars is very
challenging and tedious operation. Traditionally this
selection is based on morphological features that
become increasingly difficult for polygenic characters.
The selection for polygenic traits can be hastened by
using linked DNA markers (William et al.,
2007).
These markers are very stable and powerful
considering the fact that they are
unaffected by the
external environmental conditions and
therefore
can be effectively utilized to tag QTLs related