Let us make an in-depth study of the sulfur assimilation in higher plants.
Conversion of inorganic sulphur compounds such as SO42- into sulfur-containing organic compounds such as cysteine by plants is called as sulfur or sulfate assimilation. The first step in sulfate assimilation in plants is conversion of sulfate into cysteine.
Sulfur in sulfate is present in highly oxidised state with six positive charges while in cysteine it is present in reduced state with four negative charges. Therefore, conversion of sulfate into cysteine is a reduction process that is energy dependent and requires ATP. It entails transfer of 10 electrons which are provided by reduced glutathione (GSH), reduced ferredoxin (Fd. red) and O-Acetylserine.
APS (Adenosine – 5′- phosphosulfate), sulfite and sulfide (S2-) are important intermediates in sulfate assimilation to cysteine.
Reduction of sulfate to cysteine is a multistep process.
Various steps of this process which is also shown in Fig. 9.30 are as follow:
(i) In the first step, sulfate is activated by ATP in the presence of the enzyme ATP- sulfurylase to form APS (Adenosine-5′-phosphosulfate) and pyrophosphate (PPi). Mg++ ions are required in this reaction.
SO42- + Mg-ATP → APS + PPi
PPi so formed is quickly hydrolysed by the enzyme inorganic pyrophosphatase to yield two molecules of inorganic phosphaie (2Pi).
PPi + H2O → 2Pi
(ii) The APS is now reduced [Leustek et al (2000)]. The enzyme APS-reductase transfers two electrons from two molecules of reduced glutathione (2 GSH) to produce sulfite (SO32-). Glutathione is oxidised (GSSG).
APS + 2 GSH → SO32- + 2 H+ + GSSG + AMP
(iii) Sulfite (SO32-) is now reduced to form sulfide (S2-) in the presence of the enzyme sulfite reductase. This reduction requires six electrons which are provided by 6 mols of reduced ferredoxin (Fd.red).
SO32- + 6 Fd.red → S2- + 6 Fd.oxi
(iv) In the last step, sulfide reacts with O-acetylserine (OAS) under the influence of the enzyme OAS-thiolyase to form the amino acid cysteine and acetic acid. There is transfer of two electrons in this reduction, from OAS to sulfide.
OAS + S2- → cysteine + acetate
(The OAS is formed from serine and acetyl-CoA in the presence of the enzyme serine acetyl transferase.)
Sulfate (Sulfur) assimilation takes place chiefly in leaves in chloroplasts. The sulfate absorbed by roots from soil solution is translocated through xylem to shoots for assimilation. To some extent sulfate assimilation may also occur in roots in proplastids.
Formation of other Sulphur-containing Organic Compounds:
After the synthesis of cysteine, another sulfur-containing amino acid methionine is synthesized from it. Thereafter, other sulfur-containing organic compounds are synthesized from these two amino acids.
Synthesis of Methionine from Cysteine:
Various steps of synthesis of methionine from cysteine (Fig. 9.31) are as follow:
(i) In the first step, cysteine reacts with O-phosphohomoserine in the presence of the enzyme cystathionine 
-synthase to form cystathionine. Inorganic phosphate (Pi) is released in the reaction.
(ii) In the next step, the enzyme cystathionine β-Iyase splits cystathionine into homocysteine, NH3 and pyruvate.
(iii) Finally, homocysteine is methylated by the enzyme methionine synthase to form methionine.
(i) Synthesis of methionine from cysteine also occurs in chloroplasts in leaves.
(ii) The sulfur-containing organic compounds produced after sulfur (sulfate) assimilation are exported to other parts of the plant, such as root and shoot apices and fruits, through phloem chiefly as glutathione.
(iii) It is believed that glutathione may also coordinate absorption of sulfate by roots and its assimilation by shoots.