Agriculture Reference
In-Depth Information
phosphate deficiency. Induction of SULTR1;3 and SULTR3;4 was repressed by
phosphite which is known to be involved in local phosphate sensing and long-
distance signalling (Varadarajan et al.
2002
). In contrast, SULTR2;1 was shown to
belong to the class of phosphate-inducible genes not responding to phosphite
(Rouached et al.
2011
). Additionally, they have shown that PHR1 plays an impor-
tant role in sulfate homeostasis in plants grown under phosphate deficient condi-
tions by stimulating the expression of SULTR1;3 whereas SULTR2;1 and
SULTR3;4 seem to be repressed by PHR1 in shoots only. These results suggest
an important effect of phosphate as a signaling factor in maintaining source-sink
sulfate distribution (Rouached et al.
2011
).
To maintain sulfur homeostasis, plants need to cope with rapid environmental
changes, which cause disturbances in levels of pathway intermediates and products.
Therefore, a tight regulation of sulfate uptake and assimilation is required
according to the demand for reduced sulfur. To ensure the precise modulation of
sulfur metabolism, sulfate uptake and assimilation have to be coordinated. Indeed,
sulfur limitation increases sulfate uptake and the expression and activity of key
enzymes of the pathway. Other enzymes such as SiR and SAT are also involved in
the control of sulfur metabolism, as disturbances in their activity cause growth
defects. Sulfur uptake and assimilation are regulated on transcription, post-
transcriptional and post-translational levels. However, some aspects of the control
are specific only for uptake, such as regulation of sulfate transporters by SLIM1,
and some only for assimilation. This suggests different mechanisms of achieving
efficient control and maintaining system homeostasis. However, there are still gaps
in the understanding of regulation of uptake, assimilation and distribution of sulfate
at the whole plant level, the examination of which is a great challenge for the future
research. Better understanding of molecular basis of control mechanisms will allow
the production of engineered plants with improved sulfur use efficiency, with
modulated amount of desirable sulfur-containing metabolites or with better resis-
tance to low sulfur availability, drought and many other stresses.
Dealing with Sulfur Deficiency
As it was mentioned at the beginning of this chapter the reduction of sulfur dioxide
emissions and changes in fertiliser practises have resulted in a widespread increase
in the occurrence of sulfur deficiency in crops (McGrath et al.
1996
). The applica-
tion of fertilisers may be helpful in dealing with deficiency in many instances.
However, the costs of fertilisers together with the possible negative effects of
improper use with respect to timing and type of sulfur application in terms of its
availability to the plant have led to the growing interest in looking for new
solutions. Very often a substantial seasonal variation in sulfur availability occurs.
Therefore, modern methods should be focused on uptake maximisation when sulfur
is abundant in order to increase the tolerance to low sulfur availability periods
(Hawkesford
2000
).
