Agriculture Reference
In-Depth Information
regulation of sulfate uptake and metabolism (Maruyama-Nakashita et al.
2006
).
EIL3 was the only protein from the EIL-family able to restore the wild type
phenotype of
slim1
mutants and was therefore renamed SLIM1. SLIM1 regulates
the expression of the majority of sulfate-limitation responsive genes in the pathway,
which suggests a hub-like function in the regulation system. Interestingly, APR, the
key enzyme of sulfur metabolism, seems not to be subjected to the control of
SLIM1, which suggests that it is not the only factor involved in the regulation
process.
Most sulfur responsive genes are co-ordinately regulated for metabolism and
stress reduction. A sulfur-responsive cis-acting element (SURE) was identified first
in the sequence of the promoter of
SULTR1;1
sulfate transporter (Maruyama-
Nakashita et al.
2005
). SURE is a 7 nucleotide long specific sequence localised in
the 5
0
-region of
SULTR1;1
. This sequence contains the core sequence of the auxin
response factor (ARF) (Hagen and Guilfoyle
2002
). However, SURE is sulfur-
specific and is not relevant to the auxin response. The analysis of sulfur limitation
inducible expression of
SULTR1;1
revealed that the SURE element was an essential
target for the sulfur limitation response in
Arabidopsis
roots. Interestingly, SURE
was identified in
SULTR1;1
but not in
SULTR1;2
, suggesting different mechanisms
of regulation of these two transporters (Maruyama-Nakashita et al.
2005
). Indeed,
SULTR1;1
is known to be controlled more specifically by sulfur limitation whereas
the control of
SULTR1;2
is more dependent on metabolic demand and cellular
status (Rouached et al.
2008
). The
in silico
promoter analysis with GeneChip
microarrays of 15 genes, which are known to be up-regulated by sulfur limitation,
revealed the presence of a SURE core sequence (GAGAC or GTCTC) in the
promoters of all of these genes. Similar sequences are present in the NIT3 nitrilase
(Kutz et al.
2002
) and
-subunit (Awazuhara et al.
2002
). Therefore it
was concluded that the SURE core sequences are conserved in sulfur-limiting
inducible promoters and may play a key role in sulfur limitation induction
(Maruyama-Nakashita et al.
2005
). However, a number of genes regulated by sulfur
starvation do not have a SURE element. Despite the important role of SURE
elements in the sulfur limitation-inducible response, there are still many gaps in
our knowledge. Many important questions, such as specific SURE-binding candi-
dates, still require further investigation.
In addition to transcriptional regulation, sulfur metabolism genes are also con-
trolled post-transcriptionally. The main player in post-transcriptional regulation is
microRNA395 (miR395; Fig.
3.5
). MiRNAs are a group of small RNAs, which are
formed from noncoding double-stranded RNA precursors. They are able to nega-
tively regulate their target genes by cleavage or by binding to the complementary
sequences in the target genes and repressing the translation (Bartel
2004
). Analysis
of the
Arabidopsis
genome revealed the low-affinity sulfate transporter SULTR2;1
and ATPS1 and ATPS4 isoforms of ATPS as target genes for miR395 (Jones-
Rhoades and Bartel
2004
). The accumulation of miR395 increases during sulfate
starvation and this process is dependent on SLIM1 (Kawashima et al.
2009
).
Further studies revealed that miR395 regulates sulfate accumulation in shoots by
cleavage of ATPS isoforms 1 and 4 during sulfate starvation. MiR395 was also
ʲ
-conglycinin
ʲ
