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shown to play a role in sulfate translocation from old to young leaves by targeting
SULTR2;1 (Liang et al.
2010
). Subsequently Kawashima et al. (
2011
) in experi-
ments on SLIM1 and miR395-dependent regulation, reported an increase in
SULTR2;1
mRNA levels, a decrease in
ATPS4
levels and no changes in
ATPS1
mRNA levels in sulfate-limited conditions, suggesting three different mechanisms
of miR395-mediated regulation. The increased expression of
SULTR2;1
in roots
during sulfate starvation is limited to xylem parenchyma cells (Kawashima
et al.
2009
). The decrease in mRNA levels of
ATPS4
following induction of
miR395 suggests a canonical regulation of
ATPS4
by miR395. The lack of response
in
ATPS1
transcripts is in contrast to results obtained by Jones-Rhoades and Bartel
(
2004
) who observed a decrease in
ATPS1
mRNA, however, these differences may
be caused by different experimental setups. The increase in
SULTR2;1
expression
in the xylem and reduction of flux through sulfate assimilation in the roots caused
by miR395 indicate the importance of the SLIM-1-dependent induction of miR395
for the increased translocation of sulfate to the shoots when sulfate is limited. This
results in an increase in the efficiency of sulfate assimilation in leaves (Kawashima
et al.
2011
). Very recently Matthewman and co-workers (
2012
) have shown that the
complex regulation of miR395 is linked not only to SLIM1-dependent regulation
during sulfate starvation but also to GHS and more generally thiol levels and/or cell
redox state. Increased expression of miR395 in
fou8
and
sultr1;2
mutants affected
in sulfate accumulation
,
suggests that miRNA395 is regulated by the internal
sulfate level irrespective of external sulfate availability. All these results confirm
that miR395 is an integral component of the sulfate assimilation regulatory network
in a complex regulation mechanism.
Regulation of Sulfate Assimilation
Sulfate assimilation is tightly regulated in response to sulfate demand and environ-
mental changes. The control mechanisms appear on different steps of the pathway
and involve regulation of specific enzymes. Additionally, the whole pathway may
be controlled as a process. Experiments focused on the first step of the pathway
revealed the regulation of ATPS by sulfate availability (Logan et al.
1996
). It was
also shown that ATPS activity is inhibited by GSH. However, a number of studies
revealed that the key regulatory step is sulfate reduction by APR (Fig.
3.5
), as it is
strongly affected by various treatments and environmental changes (Hesse
et al.
2003
; Koprivova et al.
2008
; Vauclare et al.
2002
). Vauclare et al. (
2002
)in
the GSH feeding experiment showed that an increase of GSH concentration in the
medium decreases APR expression and activity, indicating its regulation by GSH
rather than cysteine. They also estimated the flux control coefficient for APR as
0.57, considering that the coefficient of all enzymes in the sulfate uptake and
reduction pathway is up to 1, confirming the importance of APR in control of flux
through sulfate assimilation pathway. The analysis of natural variation in sulfate
content between two wild
Arabidopsis
accessions Bay-0 and Shahdara (Sha),