Agriculture Reference
In-Depth Information
PHO1 degradation or down-regulation is not P
i
and PHO2 dependent and was
suggested to occur at the post-translational level (Liu et al.
2012
; Aung et al.
2006
).
The
Arabidopsis pho2
mutant exhibits excessively high P concentrations in the
shoots, displaying symptoms of toxicity as a result of enhanced uptake and root-to-
shoot translocation (Delhaize and Randall
1995
; Liu et al.
2012
). PHO2 is predom-
inantly expressed in the root (Chiou et al.
2006
) and its localisation within the cell
has been determined as being with the ER and Golgi (Liu et al.
2012
). PHO2 is a
member of the E2 ubiquitin conjugase family (UBC24) (Bari et al.
2006
) and is a
target for miRNA399, which suppresses PHO2 expression (Aung et al.
2006
; Chiou
et al.
2006
; Fujii et al.
2005
). Over-expression of miRNA399 or loss of function of
UBC24 resulted both in enhanced P
i
accumulation and impairment of P
i
remobilisation from old to young leaves in
Arabidopsis
(Chiou et al.
2006
). Expres-
sion of miRNA399 is an early response to P deficiency and a systemic signal of P
i
deficiency derived from P deplete root and shoot tissues (Lin et al.
2009
; Bari
et al.
2006
; Aung et al.
2006
; Chiou et al.
2006
). Its expression is regulated by
PHR1 (Bari et al.
2006
; Aung et al.
2006
) in a mechanism called “target mimicry”
(Franco-Zorrilla et al.
2007
) as previously described. In addition to miRNA399,
various other miRNAs could be identified by deep sequencing in
Arabidopsis
(Hsieh et al.
2009
; Lundmark et al.
2010
) and other plant species (Chiou and Lin
2011
), including white lupin (Zhu et al.
2010
), soybean (Zeng et al.
2010
), tomato
(Gu et al.
2010
) and wheat (Zhao et al.
2013
). It has been assumed that high-affinity
phosphate transporters, AthPht1;8 and AthPht1 cause the phenotype of the
pho2
mutant and miRNA399 over-expressors because they are up-regulated in these
mutants (Aung et al.
2006
; Bari et al
2006
). In rice shoots, Os
pho2
mediated P
accumulation was assumed being the result of induced expression of high- and
low-affinity phosphate transporters OsPht1;2, OsPht1;9 and OsPht1;10 (Liu
et al.
2010
). It has been suggested that PHO2 encodes an additional regulator for
the low-affinity P
i
translocator protein, AthPT2;1, in green shoot tissues of
Arabidopsis
(Daram et al.
1999
). Nonetheless, downstream responses of PHO2
are still not completely understood (Liu et al.
2012
). PHO1 and PHO2 are both
expressed in vascular root tissues (Hamburger et al.
2002
) as is miRNA399 (Aung
et al.
2006
).
There is evidence that sucrose is a global regulator of plant P starvation
responses, interacting with P starvation signals (Lloyd and Zakhleniuk
2004
;
Karthikeyan et al.
2007
; Lei et al.
2011
) and root architecture alterations (Niu
et al.
2012
). Sucrose phosphate synthase (SPS) has been reported to be more
abundant in P deficiency tolerant cultivars of
Brassica napus
(Yao et al.
2011
).
Exogenously applied sugars or sucrose-enriched growing media could stimulate P
starvation inducible genes such as acid phosphatases (APase), AtIPS1 (M¨ ller
et al.
2005
) or an APGase subunit in tobacco (Nielsen et al.
1998
).
Phosphate transporters, for instance TaPht1;2 (Miao et al.
2009
), seem to belong
to the group of sugar-modulated genes under P-starvation (Jain et al.
2007
;
Karthikeyan et al.
2007
; Hammond and White
2008
).
The
Arabidopsis pho3
mutant exhibits a restricted sucrose translocation from
root to shoot caused by a defective sucrose transporter, SUC2, which is involved in
