Biology Reference
In-Depth Information
correlation between predicted and true breed-
ing values has been reported (Meuwissen et al.
2001). Both these techniques can play an impor-
tant role in the rapid development of breeding
populations rich in alleles for a wide range of
biotic and abiotic stresses. Such populations also
form a strong base for a study of the inter-
action of alleles across the genome for com-
plex quantitative traits such as grain yield under
drought.
etative, or reproductive stage - would provide the
opportunity to combine such QTLs and develop
rice lines tolerant of drought at all stages.
The most direct role of physiology in breed-
ing for drought tolerance in rice has traditionally
been to recommend traits for selection (Courtois
and Lafitte 1999). While the identified traits such
as root growth at depth were determined based
on many empirical studies relating genetic vari-
ation for root growth at depth with physiological
response to drought, the approach of selecting for
specific traits has relied on several assumptions.
The trait selection approach depends on scien-
tists' interpretation of plant function and drought
response, the prediction that a similar response
will be observed when the underlying QTL is
introgressed into a susceptible variety, and the
assumption that the trait and resulting improve-
ment in drought response (i.e., leaf water status)
will be tied to improved yield under drought.
The limitation of this strategy is that if there is
some aspect of the response of the trait/QTL to
environmental or genetic factors that is not fully
understood, the trait/QTL might not be effec-
tive when introgressed into drought-susceptible
backgrounds.
However, recent advances in plant physiol-
ogy, particularly in root biology, have identi-
fied several traits showing direct correlations
between their expression and yield via nutri-
ent and water uptake (Lynch 2007), making
them strong candidates for selection for drought
improvement. One recent case of a success-
ful introgression of a root QTL for improved
yield under drought in rice is that of the QTL
Dro1
(Uga et al. 2011), which causes deeper
root growth angle, resulting in greater yield
under drought. Another case is rice ARB lines,
which were developed with successive selections
for increased maximum root depth (Shashid-
har 2008) and have shown promising drought
responses in terms of yield (Verulkar et al.
2010). Therefore, although direct selection for
yield is currently the most effective approach
for improving rice yield under drought, selection
for other traits may also become more effective
Collaborative Strategies of Breeding
and Physiology for Improvement of
Drought Tolerance in Rice
Successful breeding for drought tolerance
requires knowing the precise mechanisms of
drought tolerance operational in the donor and
the combining ability of the donor. Some donors,
such as Moroberekan, do not possess good com-
bining ability and have not been used exten-
sively in breeding programs or for mapping stud-
ies. On the other hand, donors such as N22
and Aday Sel have been used extensively in
mapping and candidate gene studies and have
been the source of major QTLs for grain yield
under drought. A clear-cut understanding of the
physiological mechanisms of drought tolerance
of different QTLs provides opportunities for
combining QTLs that contribute to the increase
in grain yield under drought through different
mechanisms that either add to or complement
effects of each trait. For example, a QTL that
enhances water uptake, if combined with QTLs
that increase yield through efficient transloca-
tion of resources from shoots to panicles during
grain filling or QTLs related to transpiration effi-
ciency, will result in the development of an effi-
cient drought-tolerant plant type. QTL combina-
tions pyramiding complementary traits (such as
a QTL conferring improved nutrient uptake with
a QTL conferring improved water uptake) may
confer a synergistic interaction resulting in yield
advantages beyond the sum of the effects of the
two individual QTLs. Further, QTLs with effects
at different stages of rice growth - seedling, veg-
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