Biology Reference
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the presence of large genotypic variation for P-
deficiency tolerance in rice (Fageria et al. 1988;
Wissuwa and Ae 2001a) suggests that breed-
ing for P-deficiency tolerance in rice is feasible.
That most of the tolerant genotypes identified
were landraces or of traditional plant type further
suggests that such selection has been practiced
locally in the pre-green revolution era (Wissuwa
et al. 2009). The breeding target that we fol-
low in our own work is to combine the supe-
rior tolerance to P deficiency of older cultivars
and landraces with the responsiveness to P fer-
tilizer typically found in modern high yielding
varieties. This responsiveness would mainly be
a result of the higher yield potential of modern
varieties, whereas better P acquisition or higher
internal P utilization efficiency are expected to
drive the higher tolerance of P deficiency in older
cultivars.
Tolerance of P deficiency is a complex trait
with a multitude of plant physiological and
morphological adaptations that are of potential
importance for increased P uptake or internal P-
use efficiency (Lambers et al. 2006; Rose and
Wissuwa 2012). Furthermore, P deficiency itself
is not a very well-defined condition, as it may
refer to a situation in which no P fertilizer is
applied on poor soils with average yields of 1-2 t
ha
−
1
, or to a situation in which additional P fer-
tilizer application may further increase already
high rice yields of 5-6 t ha
−
1
. In each case, breed-
ing targets and traits for selection would likely
differ and it may therefore be of value to briefly
review the potential benefits of targeting P uptake
or internal P use:
fertilized soils over the years, and may
therefore be a crucial factor allowing a
reduction
of
maintenance-fertilizer
rates
without sacrificing yield.
(ii) Improving internal P-use efficiency (PUE)
through breeding has been suggested but
never really attempted, despite the obvious
advantages of reducing the amount of plant
P needed for producing equal biomass or
grain yield (Rose and Wissuwa 2012). The
benefits of enhancing PUE would theoret-
ically improve yield across environments,
irrespective of whether P was applied or
not. However, little is known about whether
different PUE mechanisms operate across
the spectrum of P deficiency.
(iii) Recently, selection for reduced grain-P con-
centration has been suggested as an alter-
native PUE mechanism beneficial under P
deficiency, and as a general way to reduce
removal of P from fields at harvest (Rose
et al. 2011). Since export of P from fields
with harvested grain is the main driver of
the P cycle in agriculture, cultivars with
lower grain-P concentrations would reduce
the need for continuously high P-fertilizer
application in high-input systems, or would
reduce P mining in low-input systems.
Given that only a fraction of total soil P is
readily plant-available in any agricultural soil, it
is not surprising that most approaches to improve
crop yields under P deficiency have targeted
P-acquisition efficiency. In following sections
we describe the most comprehensive approach
undertaken so far to develop rice with improved
tolerance of P deficiency, and we describe in
detail the identification and characterization of
Phosphorus uptake 1
(
Pup1
), a major quantita-
tive trait locus (QTL) for P uptake in rice.
(i) Benefits of improving P uptake (P-
acquisition efficiency, PAE) have typi-
cally been associated with tolerance of
more severe P-deficiency as encountered in
highly P-fixing soils where less than 1% of
total soil P may be plant-available. P solu-
bilization due to plant-induced rhizosphere
modifications would enhance P availabil-
ity. However, the same processes should
allow crops to access a bigger portion of
the fixed fertilizer P that has accumulated in
The
Pup1
QTL and Its Application
in Molecular Breeding
Based on a field screening of thirty diverse
rice varieties under P-deficient rain-fed condi-
tions, the
aus
-type rice variety Kasalath was
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