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et al. 2002). These functional analyses of rice P
transporters clearly show the significant role of
P transporters in P uptake and homeostasis.
P homeostasis and the associated complex
regulatory network has begun to be understood in
Arabidopsis
and, for some of the identified genes,
rice orthologs are known. One of the important
genes for P homeostasis, the
Arabidopsis PHO2
gene or the rice ortholog
Leaf Tip Necrosis 1
(
LTN1
), encodes a member of the E2 ubiquitin
conjugase family (Kraft et al. 2005; Aung et al.
2006; Bari et al. 2006; Hu et al. 2011). Mutation
of
PHO2/LTN1
leads to the overaccumulation of
P in leaves, a result of the deregulation of P trans-
fer from roots to shoots via the phloem (Delhazie
and Randall 1995; Dong et al. 1998; Hu et al.
2011). The expression of both genes is regulated
by microRNA
miR399
/
OsmiR399
as shown by
the down-regulation of
PHO2
/
LTN1
transcripts
(Fujii et al. 2005; Bari et al. 2006; Chiou et al.
2006; Hu et al. 2011), and
miR399
overexpres-
sion plants phenocopy the
pho2
mutant (Aung
et al. 2006). Regulation of
PHO2
by
miR399
is part of the systemic control in response to
P starvation and maintenance of P homeosta-
sis, as demonstrated by the long-distance sig-
naling of mobile
miR399
from shoots to roots
through the phloem (Lin et al. 2008; Lin and
Chiou 2008; Pant et al. 2008). In turn,
miR399
activity is inhibited by
AtIPS1
and
At4,
which are
non-protein-coding genes highly induced under
P deficiency (Bari et al. 2006; Aung et al. 2006).
Interestingly, it has been shown that
AtIPS1
contains a short sequence complementary to
miR399.
However, the presence of a mismatch
at the miRNA cleavage site prevents cleavage of
AtIPS
and “sequesters”
miR399
(Franco-Zorrilla
et al. 2007). Hence,
miR399
is a negative reg-
ulator of
PHO2
while
AtIPS1
acts as a positive
regulator, thereby fine-tuning transfer of P from
roots to shoots.
The orthologous genes
AtPHR1
and
OsPHR2
encode MYB-type transcription factors, and
their overexpression resulted in increased P con-
tent, while the
phr1
mutant displayed altered P
allocation between shoot and root (Rubio et al.
2001; Nilsson et al. 2007; Zhou et al. 2008). As
part of the regulation of P-starvation responses,
AtPHR1
is post-translationally modified by
SIZ1
,
a SUMO E3 ligase (Miura et al. 2005). It was
further shown that, in
phr1
and
siz1
mutants,
expression of four SPX-domain proteins was
reduced (Duan et al. 2008). Moreover,
AtSPX1
positively regulates a set of
P-starvation-induced
(PSI) genes, while
AtSPX3
modulates expression
of
AtSPX1
and PSI genes through negative feed-
back mechanisms (Duan et al. 2008). In rice, it
was shown that
OsPHR2
expression was highly
induced in
OsSPX1
RNAi plants as well as in
pho2
mutants and that the displayed phenotype
was similar to that of
OsPHR2
overexpressing
plants, suggesting that
OsSPX1
contributes to P
homeostasis as a negative regulator (Wang et al.
2009).
Increased understanding of the components
involved in plant P homeostasis has begun
to reveal a regulatory network that appears
to be more complex than previously thought.
Undoubtedly the information has broadened our
scientific understanding of P-response processes
and highlights the importance and dynamic of
P in plant growth and development. Unfortu-
nately, the identified regulatory factors and path-
ways have not yet been systematically assessed
for a possible role in stress tolerance, which
might employ mechanisms distinct from P-
starvation responses. In fact, a recent study in
Arabidopsis
showed suppression of P-starvation
responses in tolerant genotypes (Rouached et al.
2011) and, similarly, P-response genes were
not
differentially
expressed
in
tolerant
rice
(see below).
Phosphorus in Rice Cropping
Systems
Every year about 2.1 million t of fertilizer phos-
phorus (P) is applied to rice at a cost exceeding
US$11 billion (Rose et al. 2010). Within Asia,
negative P balances have been reported mainly
for Southeast Asian countries such as Burma,
Cambodia,
Indonesia,
the
Philippines,
and
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