Agriculture Reference
In-Depth Information
et al.
2003
; Nikiforova et al.
2003
). JA was also shown to induce the expression of
the genes involved in glucosinolate biosynthesis (Brader et al.
2001
; Doughty
et al.
1995
). JA-deficient mutants showed normal responses to sulfur limitation,
similar to those when sulfate transporters and APR are induced and glucosinolate
biosynthesis genes are repressed under sulfur starvation (Takahashi and Saito
2008
). JA significantly induced expression and activity of APR, mainly isoforms
1 and 3 (Koprivova et al.
2008
).
To summarise, JA acts positively for synthesis of both antioxidants and
glucosinolates and has an important general role in plant pathogen defence and
detoxification. It also co-ordinately induces multiple genes of sulfate assimilation
suggesting its positive effect on sulfur homeostasis in plants (Jost et al.
2005
).
Interconnection of Sulfate Assimilation with Other Nutrients
The sulfate assimilation pathway provides a range of sulfur-containing metabolites
which play essential roles in a number of cellular processes. Many reports have
shown that this pathway is very well co-ordinated with the assimilation of other
nutrients such as nitrate, phosphate or carbon. This interconnection is crucial in
dealing with the deficiency of one or more nutrients. It leads to activation or
deactivation of important regulatory components. Such cross-talks provide flexi-
bility in the adaptation to fluctuating environmental conditions, which helps plants
to maintain nutrient homeostasis and thus complete the life cycle.
Regulation of Sulfate Assimilation by Nitrogen
The regulatory interactions between sulfate and nitrate assimilation in plants have
been confirmed in a number of studies (Kim et al.
1999
; Koprivova et al.
2000
;
Takahashi and Saito
1996
). These two pathways are coordinated in the way that
deficiency of one element represses the assimilation of the other. Studies on
Lemna
minor
revealed that the activities of ATPS, APR and OAS-TL decreased during
nitrogen deficiency (Brunold and Suter
1984
). Addition of nitrate or ammonia to
nitrate-deficient medium restored the activity of these enzymes very quickly.
Additionally, when a normal nutrient solution was supplemented with ammonia
or amino acids such as arginine, glutamate or aspartate, APR activity increased
about 50-110 %. These nitrogen compounds also increased the flux through the
sulfate assimilation pathway (Brunold and Suter
1984
). Koprivova et al. (
2000
)
have shown that after 72 h of nitrate deficiency in
Arabidopsis
, the activity of APR
decreased about 30 % in leaves and 50 % in roots. This is correlated with the
decrease of APR mRNA and protein accumulation, which suggests that nitrogen
availability regulates sulfate assimilation on the transcriptional level. This finding is
supported by the results of Yamaguchi and co-workers (
1999
) who have shown that
