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distance sulfate transport. SULTR1;3 is expressed in phloem in all the
Arabidopsis
organs tested (Yoshimoto et al.
2003
). Additionally, the analysis of a tomato
homologue of this transporter showed that it is expressed under sulfur stress
conditions (Howarth et al.
2003
). These results indicate that plants are able to
maintain balanced distribution of sulfate during limitation by inducing additional
genes (Yoshimoto et al.
2003
).
Plastids are the final destination for sulfate where it is assimilated. Alternatively,
it can also be transported to the vacuoles, which play the role of sulfate reservoir in
cells. It was shown that vacuoles isolated from the
Arabidopsis sultr4;1/sultr4;2
double mutant contain more sulfate than the wild type, suggesting that the efflux of
sulfate from the vacuole is mediated by Group 4 sulfate transporters (Fig.
3.1
).
Moreover, enhanced expression of these two genes under sulfur stress conditions
indicates that sulfur is released from the vacuole in response to sulfur demands in
the cells (Kataoka et al.
2004b
). The vacuole influx transporters have not yet been
identified. A chloroplast sulfate transporter SULTR3;1 has been characterised only
recently (Cao et al.
2013
); chloroplast subcellular localisation was confirmed with
the analysis of
SULTR3;1
-GFP constructs and the sulfate transport functionality
with a validated
in organello
assay. However, the affinity of chloroplast for sulfate
did not change despite the disruption of
SULTR3;1
suggesting the existence of other
chloroplast sulfate transporters. The analysis of other members of the SULTR3
subfamily revealed that
SULTR3;2, SULTR3;3, SULTR3;4
but not
SULTR3;5
might
be also chloroplast sulfate transporters (Cao et al.
2013
). These results are consis-
tent with those of Kataoka et al. (
2004a
) who demonstrated an essential role for
SULTR3;5 in the vascular root-to-shoot transport.
As mentioned above, the members of Group 5 differ significantly from the other
groups (Fig.
3.2
). This group contains two isoforms, which are also dissimilar to
each other. SULTR5;2 was described as involved in molybdenum transport (Baxter
0.2
Fig. 3.2 Unrooted phylogenetic tree of the
Arabidopsis thaliana
members of the sulfate trans-
porter family.
Different colours
represent different subfamilies. The tree was drawn using
MEGA5.1 software
