Agriculture Reference
In-Depth Information
for WRKY75 (Devaiah et al.
2007a
,
b
; Miao et al.
2009
), suggesting their embed-
ding in the cross-talk of P signalling during P starvation to maintain P homeostasis.
It may be concluded that the regulation of P
i
uptake and P
i
transport mediated by
phosphate transporters, for which multiple roles in P
i
acquisition and P
i
remobilisation have been suggested, are very complex. Roles of transporters in
the genetically diverse trait of tolerance to low P or in the PUE context will be
discussed further but remain elusive.
Morphological and Biochemical Adaptations
of Plants During P Limitation
When sensing nutrient depletion, plants have developed broad morphological and
biochemical strategies to deal with the heterogeneous availability of soil resources.
Root plasticity (morphology, topology and architecture) is a crucial but neglected
factor which is linked with immobile nutrients such as P (Lynch
1995
). Plant roots
typically respond to P deficiency through allocation of more carbohydrates towards
the roots, which enhances root growth to maximise the soil volume exploited and
increases root to shoot ratio (Hermans et al.
2006
; Hammond and White
2008
).
Root hair formation (number, length and surface area) is strongly related to P
depletion (Bates and Lynch
1996
; Gahoonia et al.
1997
; Jungk
2001
; Zhu
et al.
2005a
,
b
) emphasizing a strong role in P
i
acquisition from the soil (Gahoonia
and Nielsen
1998
; Gahoonia et al.
2001
). Topsoil foraging, which is characterised
by enhanced lateral root branching over primary root growth, contributes to effi-
cient P acquisition (Lynch and Brown
2001
; Williamson et al.
2001
;P
ยด
rez-Torres
et al.
2008
).
P acquisition is enhanced through symbioses with arbuscular mycorrhizal
(AM) fungi (Barber
1984
; Fitter
2006
), by substantially increasing the P absorbing
surface for P uptake (Jakobsen et al.
1992
) and the ability to access mineralised
organic P sources (Koide and Kabir
2000
) and increased expression and secretion
of plant acid phosphatases (Tarafdar and Marschner
1994
).
Strigolactones have a role in facilitating symbiosis formation (Gomez-Roldan
et al.
2008
), and also stimulate tiller formation (Hong et al.
2012
), which is an
important parameter for yield. As pH has a strong influence on the bioavailability of
soil P
i
(Barber
1984
; Hinsinger
2001
) root excreted protons tend to acidify the
rhizosphere and, along with organic acids like malic acid, citric acid or phenolic
compounds that also act as chelators (Raghothama
1999
; Vance et al.
2003
), will all
help to solubilise P
i
in the rhizosphere. Organic acids displace bound P
i
from Al
3+
-,
Fe
3+
- and Ca
2+
-phosphates (Dinkelacker et al.
1989
; Gerke et al.
1994
). In partic-
ular, cluster roots (brush-like root formations) are an adaptation strategy to low soil
P
i
availability of many plant species such as white lupin and other members of the
Proteaceae family and induce such chemical changes in the rhizosphere (Neumann
and Martinoia
2002
). Plants also respond to P deprivation through the induction of
