Biology Reference
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balances, based on figures of global P fertil-
izer/manure application and removal of P with
crop harvest, suggests that negative P balances
occur on 29% of the world's crop land with the
most severe deficits in Eastern Europe, Africa,
and Southeast Asia (MacDonald et al. 2011). A
global assessment of soils with P-retention prop-
erties further showed that vast areas in South
America, West and Central Africa, and South-
East Asia have low plant available P as a result
of unfavorable soil properties, such as low pH
or high concentrations of aluminum and iron
(Batjes, 2011). On those soils, P would be suf-
ficient to support high crop yields for proba-
ble decades if it were not “locked up” in soil-P
pools of very low plant availability. Even where
plant-available soil P was sufficient initially, the
continuous removal (mining) of soil P in har-
vested grain will eventually induce P deficiency
unless P mining is balanced by sufficient P fer-
tilizer application (Rose et al. 2010). The extent
to which neutral P balances can be maintained in
high-input agriculture or achieved in less inten-
sive systems will, to a large extent, depend on the
cost of P fertilizers. Given that high-quality phos-
phate rock, the source of P fertilizers, is a non-
renewable and increasingly limited resource, and
that production and transportation costs strongly
depend on the price of oil, it is foreseeable
that P-fertilizer prices will continue to increase
(Cordell et al. 2009; Van Kauwenbergh 2010;
Vance and Chiou 2011). Such price increases
will place a considerable burden on many of the
resource-poor farmers in developing countries. It
has therefore been suggested to increase efforts
to develop crop cultivars with enhanced P effi-
ciency (Ismail et al. 2007; Wissuwa et al. 2009;
Richardson et al. 2011).
ribonucleases, resulting in enhanced P uptake
(Lambers et al. 2006; MacIntosh 2011). Mod-
ifications in root architecture and morphology
facilitate exploration beyond the root-soil inter-
face to forage for P. Root structural changes
include increased root hair growth and length,
bias toward lateral root growth over primary root
growth, and, in the case of lupin plants, formation
of cluster roots (Peret et al. 2011; Sato and Miura
2011). To further increase the soil area foraged
and P acquisition, plants form symbiotic associ-
ations with mycorrhizal fungi in the root cortex,
which is referred to as arbuscular-mycorrhizal
(AM) symbiosis (for review, see Bucher 2007;
Sawers et al. 2008).
Within the plant, internal P is remobilized
from RNA molecules by intracellular ribonucle-
ases (MacIntosh 2011) and from phospholipids
in membranes that are then replaced with
galacto- and sulfolipids (Nussaume et al. 2011).
Under P-deficient conditions, intracellular
ATP/ADP and P
i
become low, whereas the level
of pyrophosphate (PP
i
) remains unchanged
(Plaxton and Podesta 2006). Hence, stressed
plants use bypass enzymes involved in PP
i
-
dependent
metabolic
reactions
(Plaxton
and
Tran 2011).
P is taken up by plants as inorganic H
2
PO
4
−
and HPO
4
2
−
phosphate ions wherein the for-
mer form is prevalent in soils with pH 4.5-5.0
(Raghothama 1999). P uptake through the root
requires P transporters located in the plasma
membrane. In the rice reference genome, thir-
teen P transporters (PT) have been identified, of
which OsPT2 has been classified as a low-affinity
transporter, whereas OsPT1 and OsPT6 are high-
affinity transporters (Seo et al. 2008; Ai et al.
2009). Analysis of transgenic plants showed that
OsPT8 regulates P homeostasis as overexpres-
sion, and silencing of this gene resulted in the
expected increase and decrease, respectively, of
P uptake, but plants showed aberrant P accumula-
tion and plant growth (Jia et al. 2011). Expression
of the P transporter OsPT11 is closely associated
with AM symbiosis as its expression is specific
to roots colonized with mycorrhiza (Paszkowski
Plant Responses to P Deficiency
Plants have developed adaptive mechanisms to
access P beyond the rhizosphere when bioavail-
able P is limited. P-starved plants mobilize
the Al-, Fe-, or Ca-bound P through secre-
tion of organic acids, acid phosphatases, and
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