User:JCLuthier1993/sandbox

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Sulfur assimilation is the process by which living organisms incorporate sulfur into their biological molecules. In plants, sulfate is absorbed by the roots and then transported to the chloroplasts by the transpiration stream where the sulfur are reduced to sulfide with the help of a series of enzymatic reactions. Furthermore, the reduced sulfur is incorporated into cysteine, an amino acid that is a precursor to many other sulfur-containing compounds. In animals, sulfur assimilation occurs primarily through the diet, as animals cannot produce sulfur-containing compounds directly. Sulfur is incorporated into amino acids such as cysteine and methionine, which are used to build proteins and other important molecules. Besides, With the rapid development of economy, the increase emission of sulfur results in environmental issues, such as acid rain and hydrogen sulfilde.

Sulfate uptake by plants
Sulfate uptake first occurs in roots that have a high affinity for sulfur.

Sulfate is taken up by the roots that have high affinity. The maximal sulfate uptake rate is reached at sulfate levels of 0.1 mM and lower. Uptake of sulfate by the roots and its transport to the shoot i one of the primary regulatory sites of sulfur assimilation.

Sulfate is actively taken up across the plasma membrane of the root cells, subsequently loaded into the xylem vessels and transported to the shoot by the transpiration stream. The uptake and transport of sulfate is energy dependent (driven by a proton gradient generated by ATPases) through a proton/sulfate co-transport. In the shoot, the sulfate is unloaded and transported to the chloroplasts where it is reduced. The remaining sulfate in plant tissue is predominantly present in the vacuole, since the concentration of sulfate in the cytoplasm is kept rather constant.

Distinct sulfate transporter proteins mediate the uptake, transport, and sub-cellular distribution of sulfate. The sulfate transporters gene family has been classified in up to 5 different groups according to their cellular and sub-cellular gene expression, and possible functioning.

Some groups are expressed exclusively in the roots or shoots or expressed both in the roots and shoots.

--> Each group of transporter proteins may be expressed exclusively in the roots or shoots of the plant, or both.


 * Group 1 are 'high affinity sulfate transporters', which are involved in the uptake of sulfate by the roots.
 * Group 2 are vascular transporters and are 'low affinity sulfate transporters'.
 * Group 3 is the so-called 'leaf group', however, still little is known about the characteristics of this group.
 * Group 4 transporters are involved in the efflux of sulfate from the vacuoles, whereas the function of Group 5 sulfate transporters is not known yet, and likely function only as molybdate transporters.

Regulation and expression of the majority of sulfate transporters are controlled by the sulfur nutritional status of the plants. Upon sulfate deprivation, the rapid decrease in root sulfate is regularly accompanied by a strongly enhanced expression of most sulfate transporter genes (up to 100-fold), accompanied by a substantially enhanced sulfate uptake capacity.

Edit:

"The nature of these transporters is not yet fully solved, whether sulfate itself or metabolic products of the sulfur assimilation (O-acetylserine, cysteine, glutathione) act as signals in the regulation of sulfate uptake by the root and its transport to the shoot, and in the expression of the sulfate transporters involved." --> "It is not yet fully understood whether sulfate itself or metabolic products of the sulfur assimilation (O-acetylserine, cysteine, glutathione) act as signals in the regulation of sulfate uptake and transport from the root to the shoot, or in the expression of the sulfate transporters involved."