Agriculture Reference
In-Depth Information
genes has been reported in tobacco cell cultures (Mitsukawa et al.
1997
) but could
not be confirmed at the plant level (Rae et al.
2004
). Promoter fusion to fluorescent
proteins showed that high expression levels of root expressed phosphate transporter
occur in trichoblast cells (Daram et al.
1998
; Mudge et al.
2002
; Sch¨nmann
et al.
2004
) and remarkably in root tips and root hairs under low P conditions
(S
´
nchez-Calder
´
n et al.
2006
). The increasing rate of P
i
uptake during P depriva-
tion via enhanced phosphate transporter expression presumably occurs as a result of
increased V
max
, rather than increased affinity (K
m
), implying increasing high-
affinity P transporter synthesis with similar kinetic properties (Raghothama
2005
). As variability in P depletion profiles in the rhizosphere of wheat genotypes
suggest genetic variability in root hair formation (Gahoonia et al.
1996
;
1997
),
varietal expression differences with respect to preferentially root expressed Pht1 are
very likely. Until now, there has been no evidence that genotypic variation exists
that could be exploited in breeding (Rose and Wissuwa
2012
): this should clearly be
area for future research.
P Partitioning and Re-translocation Within the Crop
Preferential re-translocation of P to the roots was characteristic for more P-efficient
rice genotypes (Wissuwa et al.
2005
) or better remobilisation of P from shoot to
root in oilseed rape (Hammond et al.
2009
; Akhtar et al.
2008
). Two rice genotypes,
distinguishable in their ability to tolerate a P-limited environment, showed that
genes like PEPC (phosphoenolpyruvate carboxylase), which are involved in the
modified glycolysis bypassing ATP-requiring reactions, were up-regulated in the
more tolerant but down-regulated in the more susceptible genotype (Li et al.
2010
).
Interestingly, over-expression of the bHLH transcription factor OsPTF1 enhanced
low P tolerance and resulted in increasing expression of Glu-6-P translocator, H
+
-
ATPase, but also PEP carboxykinase in the shoot (Yi et al.
2005
). A proteome study
showed that among the higher over-accumulated proteins in the more P starvation
tolerant genotype was an important enzyme of the pentose phosphate pathway,
6-phosphogluconate dehydrogenase (Li et al.
2008a
).
The authors assumed that requirements of the sugar metabolism could be
fulfilled more satisfyingly in the low P-tolerant genotype which was also
represented by the larger proportion of sucrose in the total soluble sugar fraction
(Li et al.
2008a
). More abundant pyruvate phosphate dikinase, pyruvate kinase-like
proteins and UDP-glucose pyrophosphorylase, which utilise PP
i
(pyrophosphate) to
produce ATP or UTP, in the low P-tolerant maize line could be assigned to a higher
in vivo
PUE (Li et al.
2008a
,
b
). Another fundamental issue would be assessing the
role or phosphate transporters involved in P loading into the grain (Rose and
Wissuwa
2012
).
