Agriculture Reference
In-Depth Information
deficiency massively increased sulfate uptake (Clarkson et al.
1983
). SULTR1;2
mediates sulfate uptake in normal conditions and in sulfur deficiency, and its
expression is relatively independent of sulfate supply. In contrast, SULTR1;1 is
strongly inducible by sulfate limitation but almost absent in normal sulfur condi-
tions (Howarth et al.
2003
; Yoshimoto et al.
2002
). Moreover, another study
showed that in mutants deficient in SULTR1;2, the expression of SULTR1;1 is
slightly up-regulated. However, reduced growth suggests that SULTR1;1 is not able
to compensate for the missing SULTR1;2. This might indicate that SULTR1;2 is
the major component for sulfate acquisition (Maruyama-Nakashita et al.
2003
).
Sulfate absorbed into the epidermis needs to be transferred across the root cells
to the xylem. This step is necessary for delivery to the target cells in shoot organs
for reduction or storage into the vacuole. The horizontal sulfate transfer from the
epidermis to the central cylinder cells may occur via plasmodesmata. This is the
likely strategy to cross the barrier of the Casparian strip at the endodermal cell
layers. During this process sulfate may leak from the symplast to apoplast. This
mechanism seems to be passive but has not yet been identified (Takahashi
et al.
2011
). The efflux of sulfate into the xylem vessels is still unknown. There is
no evidence that sulfate transporters may act in the reverse direction (Buchner
et al.
2004b
). However, the expression patterns of
Arabidopsis
Group 2 low-affinity
sulfate transporters in the central cylinder cells suggest they may contribute to long
distance sulfate transport (Fig.
3.1
). In roots SULTR2;1 is expressed in the xylem
parenchyma and pericycle cells whereas SULTR2;2 is restricted to the root phloem.
In contrast, in leaves SULTR2;1 is expressed in xylem parenchyma and phloem
cells, and SULTR2;2 in the cells surrounding the xylem vessels (Takahashi
et al.
2000
). Additionally, SULTR2;1 was assumed to be involved in sulfate
transport into developing seeds (Awazuhara et al.
2005
). Localisation of
SULTR2;2 suggests a role in sulfate transport via the phloem. The efflux of sulfate
to the apoplast of the root vascular tissue leads to a high sulfate concentration.
SULTR2;1 expressed in the xylem parenchyma cells can reabsorb this sulfate, thus
regulating the amount of sulfate which is transported to the shoots. Induction of the
SULTR2;1 gene during sulfur starvation strongly supports this strategy. In the leaf
the expression of SULTR2;2 in the closest cells to the xylem vessels suggests its
role in sulfate uptake from the vessels, most likely at millimolar concentrations.
Subsequently sulfate is probably transferred to cells where it will be assimilated.
The expression of SULTR2;1 in the phloem suggests its role in sulfate transfer to
other organs, and in xylem parenchyma - reabsorption for further xylem transport
(Buchner et al.
2004b
). Moreover, it was shown that SULTR3;5 from the Group
3 sulfate transporters is co-expressed with SULTR2;1 and involved in the sulfate
influx to the xylem parenchyma cells in the roots. However, it does not work as a
sulfate transporter by itself. It has been suggested that formation of heterodimer is
required for the activity of SULTR3;5 and maximum activity of SULTR2;1
(Kataoka et al.
2004a
). Taken together, it seems that Group 2 sulfate transporters
are involved in the balancing of the sulfate flux through the plant in various sulfate
supply (Takahashi et al.
2000
). It was also suggested that not only low- but also
high-affinity sulfate transporters from Group 1 SULTR1;3 are involved in long
