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Secondly, cell-specific expression of some transporters from Groups 1, 2, and 4 is
modulated by sulfur limitation. Finally, cell/tissue-specific, sulfur deficiency-
related transporters from Groups 1 and 2 are derepressed (Buchner et al.
2004b
).
The question of how plants sense sulfate deficiency and how the signal is trans-
duced to the transporter gene promoters still needs to be answered. Lejay
et al. (
2003
) studying the regulation of ion transporter in roots by photosynthesis,
concluded that some sulfate transporters (but also nitrate, phosphate, potassium and
other metal transporters) show diurnal changes in expression and might be induced
by sucrose, linking thus sulfate uptake with general primary metabolism.
In conclusion, plants are able to adjust sulfate transport to environmental
changes and to the availability of the other nutrients. It is worth noting that these
adaptations vary between species similarly to demands in sulfate metabolism.
Different molecular regulation mechanisms are described in the following sections
of this chapter.
Sulfate Assimilation and Metabolism in Arabidopsis thaliana
In nature, sulfur occurs in many oxidation states in inorganic, organic and
bioorganic compounds. Various organisms such as algae, bacteria, plants and
fungi are able to reduce sulfate and incorporate it into amino acids in the assimi-
latory sulfate reduction pathway. This process is very well described on both
biochemical and molecular levels. In photosynthetic organisms it occurs in plastids
(Brunold and Suter
1989
). The only exception is
Euglena gracilis
where sulfate
reduction takes place in mitochondria (Brunold and Schiff
1976
). Since sulfate is
chemically very stable, it requires activation before reduction. This occurs by
adenylation to adenosine 5
0
-phosphosulfate (APS), which is catalysed by ATP
sulfurylase (ATPS; EC: 2.7.7.4). ATPS in plants appears to be a homotetramer
composed from 52 to 54 kDa polypeptides (Murillo and Leustek
1995
). Most of
total ATPS is plastid localised where it is responsible for sulfate activation for
further reduction. However, activity was also detected in the cytosol (Lunn
et al.
1990
; Renosto et al.
1993
; Rotte and Leustek
2000
). During plant growth
activity in chloroplasts declines, whereas it increases in the cytosol. ATPS is
encoded by a small multigene family. Most plant species possess two ATPS
isoforms. However, four different isoforms were isolated from
Arabidopsis
,
which may indicate some level of genetic redundancy (Kopriva et al.
2009
). Sur-
prisingly all contain a chloroplast transit peptide (Hatzfeld et al.
2000a
; Murillo and
Leustek
1995
). The most likely explanation for the existence of cytosolic ATPS
isoforms is the use of a different translational start codons (Hatzfeld et al.
2000a
).
However, this hypothesis remains to be investigated.
APS forms a branching point in the sulfate reduction pathway (Fig.
3.3
). APS
can be directly reduced to sulfite by APS reductase (APR; EC: 1.8.99.2) or
phosphorylated by APS kinase (APK; EC: 2.7.1.25) to form 3
0
-phosphoadenosine
