Agriculture Reference
In-Depth Information
performed to identify the genes controlling sulfate level, also revealed APR as a key
control step in sulfate reduction pathway. The analysis of Bay-0 x Sha recombinant
inbred lines (RILs) led to the identification of a single nucleotide polymorphism in
an APR2 isoform of APR. The substitution of alanine with glutamate in a conserved
domain of protein resulted in significant differences in enzyme activity leading to
sulfate accumulation (Loudet et al.
2007
). Additionally, reduction of APR activity
and mRNA accumulation in low nitrogen availability shown in this study confirmed
an interconnection of the two assimilatory pathways. Furthermore, APR over-
expression leads to accumulation of sulfite and thiosulfate which are toxic for
plants and strongly affect plant fitness (Martin et al.
2005
). Such an effect is not
observed when over-expressing other enzymes in the pathway suggesting that APR
regulation affects the entire pathway. It was also shown that APR activity increases
during the day and decreases during the night, which shows that it has a diurnal
rhythm (Kopriva et al.
1999
). A recently discovered transcription factor, Long
Hypocotyl 5 (HY5), seems to be responsible for APR regulation by light. In dark
adapted
Arabidopsis
seedlings, a rapid increase in the transcript levels of all three
APR isoforms was observed to different extents, the highest being for the
APR2
isoform where it reached a 12-fold higher level after 90 min of illumination,
compared to the control plants kept in the dark. However, in
hy5
mutant seedlings
no light induction was observed for
APR1
, and
APR2
induction was lower. Further
analysis revealed that HY5 is involved in APR regulation by OAS and nitrogen
deficiency which alter the demand for reduced sulfur (Lee et al.
2011
). Interest-
ingly, the analysis of the
sir1-1
mutant revealed the downregulation of ATPS4,
APR2, and SULTR2;1 in the mutant (Khan et al.
2010
). The most likely reason for
the downregulation of these genes, especially
ATPS4
and
APR2
, is to avoid the
accumulation of sulfite which cannot be incorporated into cysteine as a result of
reduced SiR activity in the mutant. These results suggest that SiR can contribute to
the control of sulfate reduction pathway (Khan et al.
2010
).
Regulation of Cysteine Synthesis - Protein-Protein
Interactions
Cysteine synthesis plays an important role in regulation of sulfate metabolism. The
regulation of SAT and OAS-TL, the main enzymes in cysteine biosynthesis, is
mainly due to a protein-protein interaction in the cysteine synthase complex (CSC;
Fig.
3.5
). As mentioned above, SAT is strongly activated by OAS-TL which is
inactive in the complex and has only a regulatory role (Droux et al.
1998
). Forma-
tion of the complex is strongly dependent on the availability of OAS and sulfide.
When SAT is bound to OAS-TL, the access of OAS to the complex is strongly
inhibited (Francois et al.
2006
). OAS, which cannot be bound to the complex, is
released and metabolised by free OAS-TL dimers. During sulfur deficiency there is
not enough sulfide for cysteine synthesis and therefore OAS accumulates in cells.
