Agriculture Reference
In-Depth Information
Phosphate Transporters: Uptake and Translocation
Plants acquire P predominately as orthophosphate (H
2
PO
4
/HPO
4
2
;P
i
) from the
soil solution (Bieleski
1973
; Holford
1997
; Schachtman et al.
1998
; Marschner
2012
). This process is mediated by plasma membrane-localised phosphate trans-
porters which have been suggested to operate as H
+
co-transporters (Daram
et al.
1998
; Smith et al.
1999
; Mimura
2001
; Rae et al.
2003
; Raghothama
2005
).
Consistent with the physiological pH in many agricultural soils, maximal uptake
rates occur in a pH range 5-6 (Ullrich-Eberius et al.
1981
; Furihata et al.
1992
; Rae
et al.
2003
). A constitutively expressed low-affinity uptake system with a K
m
of 50-
300
M and a high-affinity uptake system, which is regulated by P
i
availability with
K
m
of 3-7
μ
M have been proposed (Ullrich-Eberius et al.
1981
; Furihata et al.
1992
;
Preuss et al.
2010
). Phosphate transporters are classified into distinct families: Pht1,
Pht2, Pht3 and Pht4 (Bucher et al.
2001
; Liu et al.
2011
).
Pht1 are high-affinity transporters homologous to the yeast PHO84 P
i
transporter
and other fungal high-affinity P
i
transporters (Pao et al.
1998
). However, the
functional characterisation of the Pht1 transporter family in rice and barley (Rae
et al.
2003
; Ai et al.
2009
) revealed kinetic properties, which are within both high-
and low-affinity ranges. Pht1 transporters belong to the distinct phosphate:H
+
symporter (PHS) family which is a member of the major facilitator superfamily
(MFS) of proteins (Pao et al.
1998
). All these transporters exhibit high sequence
similarity with each other, being similar in size and having 12 predicted transmem-
brane domains (TMs) with a large hydrophilic loop between TM6 and TM7 that
results in a 6 + 6 configuration (Liu et al.
2011
). The N- and C-termini are oriented
towards the inside of the cell and they contain potential sites for phosphorylation
and N-glycosylation (Smith et al.
1999
).
Transporters of the Pht2 family have been cloned in
Arabidopsis
, and have been
suggested to have roles as constitutively expressed low-affinity proton symporters
(H
+
/P
i
cotransporter) for P
i
loading in green shoot organs and predominantly leaf
tissues (Daram et al.
1999
; Versaw and Harrison
2002
). The Pht2 protein is
structurally similar to the Pht1 members, but is more closely related to the putative
P
i
transporters from bacteria and mammalian Na
+
/P
i
transporters. Pht2 amino acid
sequences are distinct from Pht1 transporters and have a large hydrophilic loop
between TM8 and TM9 (Daram et al.
1999
). In wheat, expression analysis of
TaPht2;1 revealed a predominant expression in a photoperiod-dependent manner
in the leaves, which was significantly enhanced during P starvation (Guo
et al.
2013
). GFP fusion studies localised TaPht2;1 to the chloroplast envelope,
suggesting a regulatory role mediating Pi translocation from the cytosol to the
chloroplast as low-affinity transporter with a K
m
of 225
μ
MP
i
(Guo et al.
2013
).
The Pht3 transporters belong to the mitochondrial transporter family (Rausch
and Bucher
2002
) and the Pht4 family has been suggested to play a role in P
i
translocation between the cytosol, chloroplast, plastids and the Golgi apparatus
(Guo et al.
2008
; Liu et al.
2011
).
μ
