Sulfate acquired from the soil solution by plant roots can be stored in vacuoles or enter the assimilatory pathway. Removal of S supply cause an increase in the protein activity or mRNA pools of some enzymes responsible for the uptake and the assimilatory pathway, such increase is observed after several days in whole plants (Buchner et al. 2004, Hell et al. 1997) or several hours in cell suspensions (Hatzfeld et al. 1998). Following resupply of SO42- all fall in parallel with cysteine and GSH increase. In most plant systems analyzed so far, sulfate, cysteine, and GSH are described as negative regulators and OAS as positive regulator of sulfur genome (Droux 2004). GSH-APS reductase is thought to be a prime regulation point of the pathway (Vauclare et al. 2002), since its activity and RNA increase concomitantly with S starvation and with stresses that increase the demand for GSH and then for cysteine. The control of gene expression limits excess uptake and assimilation with negative regulation by downstream S-compounds and a positive regulation by the serine derivative OAS. Therefore the efficient use of N in plant growth depends on the absorption of appropriate amounts of S.
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