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highly induced (preferentially) in rice roots and shoots where they are involved in
the regulation of PSI and OsIPS1 (Wang et al. 2009a , b ; Oono et al. 2011 ). OsSPX1
over-expression suppressed IPS gene induction, miRNA399 and phosphate trans-
porter Pht1 expression (Wang et al. 2009a , b ) and in yeast, an SPX domain limited
the phosphate uptake velocity (H¨rlimann et al. 2009 ). Similar results were
obtained with Arabidopsis mutants for AtSPX1-AtSPX4 affecting the expression
pattern of purple acid phosphatases genes (Duan et al. 2008 ). Furthermore, OsSPX1
is positively regulated by OsPHR2, involved in the feedback P i signalling network
in roots by suppressing OsPT2 and other PSI genes in the PHR2/Pho2 background
(Liu et al. 2010 ), and negatively regulates shoot P accumulation (Wang et al. 2009a ,
b ). OsSPX1 over-expression counteracted the effect of PHR2 over-expression in
rice, which mimics P starvation and induces PSI gene expression but not the
function of Ospho2 regulating OsPT2 expression (Liu et al. 2010 ). In summary,
SPX proteins seem to be essential players for maintaining P homeostasis and P
signalling in plants (Rouached et al. 2010 ; Nilsson et al. 2012 ; Secco et al. 2012 ).
The regulatory mechanism of P allocation among different organs during plant
development under P stress remains relatively elusive and investigations have been
mainly focused on screening Arabidopsis mutants with abnormal P distribution.
The Athpho1 mutant showed severe P deficiency in above-ground shoot tissues due
to influencing transfer of P i to the xylem vessels for subsequent transport to the
shoot and leaves (Poirier et al. 1991 ; Liu et al. 2012 ). PHO1 is a membrane-
spanning protein but there is no evidence that it is a transporter itself and it does
not have homology to any other previously known transporter (Hamburger
et al. 2002 ). Eleven members of the AthPHO1 transporter family are known
which share the same topology (Wang et al. 2004 ); a SPX tripartite domain in the
N-terminal ( S YG1/ P HO81/ X PR1) and an EXS domain at the C-terminal ( E RD1/
X PR1/ S YG1). AthPHO1 has been localised to the ER and the Golgi (Liu
et al. 2012 ). The EXS/SPX domains, and particularly the N-terminal region of
PHO1, have been identified in yeast as being involved in either phosphate transport
or in sorting proteins to endomembranes (Liu et al. 2012 ; Wang et al. 2004 ).
AtPHO1 seem to mediate P i efflux out of root stellar cells along its electrochemical
gradient (Hamburger et al. 2002 ) and AtPHO1;H1, seems to be regulated by PHR1
(Stefanovic et al. 2007 ). The roles of the other members, AtPHO1;H2 to AtPHO1;
H9, are unknown (Secco et al. 2010 ) but show a distinct expression pattern from
that of AtPHO1 and AtPHO1;H1 (Hamburger et al. 2002 ). The AthPHO1 family
clusters into two clades, which are expressed in a broad range of tissues, including
leaves and predominately in vascular tissues of roots, leaves, stems or flowers. Only
one clade which contains AtPHO1 and AtPHO1;H1 clusters with the three OsPHO1
proteins found in rice (Secco et al. 2010 ). OsPHO1;2 was mostly expressed in roots
and was relatively lowly and constantly expressed in other tissues. A mutation
affected root-to-shoot Pi transfer (Secco et al. 2012 ). OsPHO1;1 was predominately
expressed in flowers before and during pollination and OsPHO1;3 was the lowest
expressed, and higher in leaves and flowers (Secco et al. 2010 ). AthPHO1 is a
downstream component of the AthPHO2 regulatory pathway (Liu et al. 2012 ).
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