text: Sulfate uptake first occurs in roots that have a high affinity for sulfur.  The maximal sulfate uptake rate is generally already reached at sulfate levels of 0.1 mM and lower. The uptake of sulfate by the roots and its transport to the shoot is strictly controlled and it appears to be 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 subcellular 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. 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. It is not yet fully understood whether sulfate and other metabolic products of sulfur assimilation (O-acetylserine, cysteine, glutathione) act as signals in the regulation of sulfate uptake and transport, or in the expression of the sulfate transporters involved.