Agriculture Reference
In-Depth Information
elongation (Ticconi et al.
2004
;S
´
nchez-Calder
´
n et al.
2005
; Franco-Zorrilla
et al.
2007
; Svistoonoff et al.
2007
). Phytohormone-related genes have been
reported to mediate root architectural changes under low P growing environments,
particular auxin-responsive genes (Bates and Lynch
1996
; Hammond et al.
2004
;
P
´
rez-Torres et al.
2008
; Jain et al.
2007
; Miura et al.
2011
). Expansins are involved
in cell wall extension (Zhao et al.
2012
), including root-hair formation
(Yu et al.
2011
) and are stimulated by indole-3-acetic acid and abscisic acid
under abiotic stress (Zhao et al.
2012
).
For instance, Miura et al. (
2011
) assumed that genes coding for expansin
17, glycosyl hydrolase 19 and UDP-glycosyltransferase are involved in the regula-
tion of cell wall-loosening and elongation in response to P starvation in
Arabidopsis
. Distinct responsiveness to P
i
availability among
Arabidopsis
ecotypes
(Chevalier and Rossignol
2011
) and the identification of quantitative trait loci
(QTLs) (Reymond et al.
2006
) suggests a genetically determined control of the
root growth response to low P
i
. Genetic factors controlling root plasticity have been
investigated using
low phosphorus insensitive
(
lip
) mutants (S´nchez-Calder´n
et al.
2006
). The mutation disrupted not only the root developmental response but
also altered the induction of P deprivation responsive genes, which are relevant for
adaptation to low-P
i
, including acid phosphatases (AtPAP1, AtACP5) and phos-
phate transporters (AtPT1, AtPT2) (S´nchez-Calder´n et al.
2006
). Furthermore,
their findings suggest that the root architectural response is mediated by a specific
nutrient (P) sensing signalling network (S
´
nchez-Calder
´
n et al.
2006
). For
instance, WRKY75 (Devaiah et al.
2007a
) and ZAT6 (Devaiah et al.
2007b
) are
among several transcription factors, which have a regulatory effect on root archi-
tecture of
Arabidopsis
and were suggested to have an impact on P starvation
responses. However, genetic selection based on root parameters has been difficult
due to their multigenic nature and the lack of appropriate evaluation methods
(Vance et al.
2003
).
P Starvation Signaling
Elicited responses to internal and external nutritional status involve local and
systemic signalling (Chiou and Lin
2011
); signalling molecules, their mode-of-
action and interacting pathways are summarised below.
A strong increase of Induced by P starvation gene transcripts (IPS) under P
i
starvation has been reported in
Arabidopsis
and rice (Rubio et al.
2001
; Oono
et al.
2011
) and members of the IPS gene family have been widely used as
molecular markers of plant P
i
nutritional status (Zhou et al. 2008; Tian
et al.
2012
; Wang et al.
2013
). IPS genes are involved in the miR399-PHO2
regulatory loop as ribo-regulators (Doerner
2008
) and function as miRNA399
antagonists, which negatively alter PHO2 expression at the post-transcriptional
level; a regulatory process which is described as “target mimicry” (Franco-Zorrilla
et al.
2007
). It seems likely that they stabilise the initial decrease of PHO2 transcript
