Agriculture Reference
In-Depth Information
et al.
2008
; Tomatsu et al.
2007
) and renamed the MOT1 transporter. It is clear that
MOT1 is essential for molybdenum accumulation. However, the exact subcellular
localisation of this protein remains unclear. Tomatsu et al. (
2007
) localised this
transporter to endomembrane system and plasma membrane whereas Baxter
et al. (
2008
) suggested its localisation in the mitochondrial membrane. The expla-
nation for these two predicted locations may be found in the different sites of GFP
fusion, either to the N-terminal end of
MOT1
(Tomatsu et al.
2007
) or to its
C-terminus (Baxter et al.
2008
). Nevertheless, the exact location of MOT1 requires
further investigation.
SULTR5;1
was shown to be expressed in most plant tissues
but it was not affected by sulfate supply (Shinmachi et al.
2010
). Only recently its
function as a vacuolar molybdate export protein was shown and it was renamed for
MOT2 (Gasber et al.
2011
). As there are no reports indicating a sulfate transport
function for these two transporters and because of the absence of the STAS domain,
which is present in all other sulfate transporters, these two genes could reasonably
be excluded from the sulfate transporter family.
Regulation of Sulfate Transport at the Whole Plant Level
Growing plants require sulfate to synthesise amino acids, sulfolipids, thiols and
many other sulfur-containing compounds. The demand for sulfur differs depending
on the tissues, organs and developmental stage. Sulfate uptake and distribution is
regulated in response to plant demand and changing environment. Specific patterns
of expression of sulfate transporters genes in particular cells indicate the impor-
tance of proper sulfate distribution. Additionally, sulfate may be redistributed
during development from mature leaves to roots, younger leaves or seeds. Redis-
tribution requires changes in expression patterns. For example the up-regulation of
SULTR2;2 and SULTR1;3 in leaves during sulfate starvation indicates that these
transporters play an important role in sulfate allocation to other tissues (Yoshimoto
et al.
2003
). It is an important process that allows interconnection of different
nutrients and that keeps a balanced system even during large environmental fluc-
tuations. The main reservoir of sulfate is in the vacuoles of mature leaves. Redis-
tribution of sulfate from vacuoles is particularly important during sulfur limitation.
Accumulation of high levels of sulfate metabolism products such as cysteine and
GHS decreases sulfate uptake and transport whereas sulfur stress conditions
increase expression of sulfate transporters and activities of key enzymes in the
sulfate metabolism pathway. Regulation of sulfate transport is sulfate nutrition-
dependent. Many studies confirmed that the regulation occurs primarily at the level
of mRNA (Smith et al.
1997
; Takahashi et al.
2000
; Yoshimoto et al.
2002
). In
addition, post-transcriptional regulation and protein-protein interactions of sulfate
transporters with
O
-acetylserine (thiol)lyase were described (Shibagaki and
Grossman
2010
; Yoshimoto et al.
2007
). Their contribution to the overall control
of sulfate uptake is, however, not clarified yet. Changes in expression patterns of
sulfate transporter genes follow complex regulation responses. Firstly, tissue/organ-
specific expression of transporters from Group 3 is not regulated by sulfur nutrition.
