Agriculture Reference
In-Depth Information
nitrogen deficiency affects the mRNA accumulation of several other sulfate metab-
olism genes. Interestingly, the concentration of cysteine and GSH in both roots and
leaves does not change significantly. Addition of ammonia or glutamate increased
the sulfate flux through the
Arabidopsis
plants (Koprivova et al.
2000
). Analysis of
the
hy5
mutant revealed that the decrease in APR activity in nitrate deficiency was
much more rapid in Col-0 than in the mutant suggesting HY5 involvement in
regulation of APR in nitrate deficiency (Lee et al.
2011
). Although this regulatory
mechanism seems to play an important role in sulfate homeostasis, the molecular
basis still requires elucidation. Koprivova and co-workers (
2000
) used feeding
experiments to demonstrate a strong positive effect of nitrogen-containing com-
pounds on the flux of sulfate through the pathway. Moreover, the closer the
metabolic relationship between nitrogen source and OAS, the higher was the
incorporation of
35
S into proteins. OAS also caused the accumulation of APR
mRNA level and proteins (Koprivova et al.
2000
). Therefore, it was proposed to
play a signalling role in the co-ordination of nitrate and sulfate metabolic pathways.
Other suggested signalling molecules are cytokinins: biosynthesis of cytokinins
seems to be related to nitrate availability and they are known to affect nitrogen
nutrition. As mentioned above, Ohkama et al. (
2002
) have shown that the
APR1
and
SULTR2;2
were up-regulated by cytokinins. Additionally, Wang et al. (
2003
) dem-
onstrated that adding nitrate to plants grown on ammonium as the main source of
nitrogen increased the accumulation of mRNA for high-affinity transporters and
APR. Therefore, the nitrate effect on sulfate metabolism genes could be mediated
by cytokinins (Kopriva and Rennenberg
2004
). On the other hand, it was shown that
sulfate deficiency decreased nitrate uptake and the activity of nitrate reductase and
amino acid accumulation (Prosser et al.
2001
). Abdallah et al. (
2010
) have shown
that oilseed rape, which is known to have a high demand for sulfate, is able to
maintain growth during sulfate deficiency by an optimisation of nitrogen uptake
and remobilising sulfate from internal reservoirs. In field experiments with oilseed
rape it was demonstrated that sulfur deficiency reduces nitrogen use efficiency and
nitrogen deficiency reduces sulfate use efficiency (Fismes et al.
2000
). This sug-
gests the importance of further investigation of regulatory processes of nutrient
metabolism in plants in order to improve crops yield and quality.
Interaction of Sulfate Assimilation with Carbon Metabolism
Carbohydrates provide the acceptor of sulfide for cysteine biosynthesis and serve as
a source of reductants for sulfate reduction in non-photosynthetic tissues (Kopriva
and Rennenberg
2004
). However, despite this crucial role very little is known about
regulation of sulfate assimilation by carbon. Kopriva et al. (
1999
) have shown that
APR can be induced by light, suggesting a diurnal rhythm of regulation. However,
when plants were subjected to continuous light or darkness the diurnal rhythm
disappeared. This indicates that the light is not the direct signal for the changes in
APR expression and the enzyme undergoes
regulation by carbohydrates
