Agriculture Reference
In-Depth Information
synthesised during photosynthesis. Furthermore, light induction of APR expression
could be mimicked by addition of sucrose during the dark period (Kopriva
et al.
1999
). Subsequently, Hesse and co-workers (
2003
) have confirmed similar
effect of glucose on APR activity. In addition, they have shown that sorbitol,
mannitol and 2-deoxyglucose do not affect APR activity. Sugars seem to act in
the same way as OAS: feeding plant with glucose and OAS simultaneously resulted
in greater induction of APR activity than the sum of levels induced by individual
treatments. Additionally, APR was also induced by glucose in plants subjected to
nitrogen deficiency. Hesse et al. (
2003
) concluded that signals from nitrogen and
carbon metabolism act synergistically on sulfate assimilation. Moreover, positive
signals from sugars can exceed negative signals from nitrate assimilation. The
reduction in APR mRNA level and activity was also observed in a study on
Lemna mino
r grown in an atmosphere without CO
2
(Kopriva et al.
2002
). The
addition of sucrose prevented the decrease in APR activity indicating that this
regulation is not dependent on CO
2
fixation. Additionally, it was shown that
incubation in an atmosphere without CO
2
strongly inhibited sulfate uptake and
the flux through the pathway, which also could be restored by addition of sucrose.
However, sucrose was not as effective in restoring enzyme activities as normal air,
suggesting that although it is the final product of carbon assimilation, it probably is
not the molecular signal connecting the sulfate and carbon metabolism (Kopriva
et al.
2002
).
Coordination of Sulfate and Phosphate Homeostasis
Plants have evolved coordinated mechanisms to maintain the homeostasis of sulfur
and phosphorus in response to changing environmental conditions, which confirms
the importance of these two macroelements. It was shown that during sulfate
limitation, sulfolipids are rapidly replaced by phospholipids and vice versa
(Essigmann et al.
1998
). Additionally, phosphate deficiency leads to an increase
of GSH levels (Kandlbinder et al.
2004
). It is interesting to note the similarity
between the topology and regulatory mechanisms in the two pathways (Rouached
2011
). In addition to the similar expression patterns of the main uptake transporters
and the regulation of both of the pathways by cytokinins, the involvement of
microRNAs in the regulation of both pathways seems to be particularly noteworthy
in terms of cross-talk. As mentioned above, sulfate assimilation is regulated by
miR395, whereas miR399 is known to be involved in regulation of phosphate
uptake and translocation (Chiou et al.
2006
; Fujii et al.
2005
; Lin et al.
2008
). It
has been shown that miRNAs and the Phosphate Response 1 (PHR1) transcription
factor are involved in the interconnection of sulfate and phosphate assimilation
pathways (Rouached
2011
). Additionally, it was suggested that miR395 might be
suppressed in phosphate deficient plants (Hsieh et al.
2009
), however, this obser-
vation requires further investigation. Recently Rouached et al. (
2011
) have shown
that SULTR1;3, SULTR2;1 and SULTR3;4 sulfate transporters are up-regulated by