Agriculture Reference
In-Depth Information
to prevent P
i
toxicity via P
i
accumulation in the shoots (Bari et al.
2006
; Chitwood
and Timmermans
2007
). Five IPS genes have been found in the
Arabidopsis
genome (Franco-Zorrilla et al.
2007
), and two in rice, maize and barley (Hou
et al.
2005
). Promoters of the pho4-regulon in yeast have two
cis
-regulatory
elements, which were also found in
Arabidopsis
(At4/AtIPS4), tomato (TPSI1),
Medicago truncula
(Mt4) and rice (OsPI1) (Hammond et al.
2003
). At4 and AtIPS4
in
Arabidopsis
are involved in P
i
allocation between roots and shoot and enhance
lateral root development (Shin et al.
2006
; Franco-Zorrilla et al.
2007
).
The
at4
mutant exhibited P accumulation in shoots (Shin et al.
2006
), whereas
over-expression decreased P accumulation (Franco-Zorrilla et al.
2007
). AtIPS1
modulates PHR expression, a MYB-CC type transcription factor which is involved
in P starvation responses (Rubio et al.
2001
). PHR1 (phosphate starvation respon-
sive 1) plays a pivotal role in sensing P availability (Chiou and Lin
2011
) and has
been examined in detail. PHR1 is a member of the MYB-transcription factor family
(15 members) and seems to be a key regulator for downstream P responsive genes
through binding to a P1BS (
P
HR
1
specific
b
inding
s
equence)
cis
-element, which is
an imperfect palindromic sequence (GNATATNC) (Rubio et al.
2001
; Nilsson
et al.
2007
,
2010
; Bustos et al.
2010
). An important downstream target of
AtPHR1 and possible homologues is miRNA399, which is involved in the PHO2
regulation as previously mentioned (Miura et al.
2005
; Schachtman and Shin
2007
).
Over-expression of AthPHR1 increased the transcript level of miRNA399 and
decreased expression of PHO2, increased further the P
i
content and enhanced root
hair density in rice and
Arabidopsis
(Nilsson et al.
2007
; Zhou et al. 2008; Bustos
et al.
2010
). Promoters of several P starvation-induced and repressed genes, includ-
ing IPS and a high-affinity P-transporters, contain the P1BS
cis
-element (Oono
et al.
2011
; Hammond et al.
2003
; Rubio et al.
2001
; Sch¨nmann et al.
2004
; Guo
et al.
2013
; Bustos et al.
2010
). In wheat, over-expression of TaPHR1 did not
change transcript levels of TaPHF1, TaPHO2 or TaSPX3, whereas TaIPS and
TaPht1;2 exhibited increased expression levels in the transgenic lines suggesting
that transcriptional factors additional to TaPHR1 may be functional in the P
starvation signalling (Wang et al.
2013
). However, the
Athphr1
mutant impairs a
broad range of P starvation responses and shows impaired root growth and root hair
length (Rubio et al.
2001
; Bustos et al.
2010
; Nilsson et al.
2007
).
In rice, two homologues of AtPHR1, OsPHR1 and OsPHR2, are involved in P
starvation signalling (Zhou et al.
2008
; Wang et al.
2009a
,
b
). However only over-
expression of OsPHR2 resulted in increased shoot P
i
and altered root morphology
(Zhou et al.
2008
; Wu and Wang
2008
; Bustos et al.
2010
). OsPHR2 positively
regulated the low-affinity phosphate transporter OsPT2 in roots resulting in exces-
sive P
i
accumulation in the shoot tissue (Liu et al.
2010
). Further, a root-associated
purple acid phosphatase (10a) in rice, OsPAP10a, is controlled and induced by
OsPHR2 (Tian et al.
2012
).
SPX proteins (which contain a SPX domain,
S
YG1,
P
HO81,
X
PR1 at the
N-termini) are involved in the downstream responses of PHR1 in
Arabidopsis
(Duan et al.
2008
) and OsPHR2 and PHO2 in rice (Wang et al.
2009a
,
b
). Members
of the SPX protein family in rice (OsSPX3 and SPX1/2/6) have been shown to be
