sulfate assimilation in plants

1. Sulphate Assimilation
Jasmine Juliet .R
Teaching Assistant
Biotechnology Dept
AC&RI, Madurai.

2. Sulphate Assimilation - Introduction
• Sulphur is an essential macronutrient required for growth and
physiological functioning of plants.
• ‘S’ is essential for synthesis of some aminoacids (Cysteine,
Methionine), Cofactors, Prosthetic groups, 1°, 2° metabolites.
• Conversion of inorganic sulphur compounds such as Sulphate
(SO4
2) into sulfur-containing organic compounds such as cysteine
by plants is called as Sulfate assimilation.
• Sulphate is available to plants in the form of Anionic sulphate
present in soil.

3. Sulphate Assimilation - Introduction
• Sulphate is the absorbed form of sulphur in plants.
• Anionic sulphate is transported into roots and then distributed.
• Mostly unmetabolized, throughout the plant and stored in
Vacuole.
• Chloroplasts are the primary site for Cysteine synthesis.
• Sulphate assimilation requires the reduction of sulphate to
cysteine.

4. Sulphate Assimilation

5. Sulphate Assimilation - Introduction
• Sulphate is assimilated into organic molecules in two different ways.
 Reductive Pathway
 Sulphation
• Even though root plastids contain all sulfate reduction enzymes, reduction
predominantly takes place in the chloroplasts of the shoot.
• Sulfate is reduced to sulfite and then incorporated to Cysteine, which
functions as a sulphate donor in many reactions. This pathway is referred to
as the reductive sulphur assimilation pathway.
• Another sulphur assimilation called sulphation (Sulfite to organic
compounds).

6. I.Reductive sulfur assimilation pathway
• The reduction of sulfate to sulfide occurs in three steps.
Activation
Reduction to sulfide
Incorporation of sulfide into Cysteine
• Sulfate is activated to adenosine 5'-phosphosulfate (APS) prior to its reduction,
a reaction catalyzed by ATP sulfurylase. (Activation)
• Second, the activated sulfate (APS) is reduced by Glutathione dependent APS
reductase to sulfite, a reaction where glutathione most likely functions as
reductant. Sulfite is reduced to sulfide by Ferridoxin dependent Sulfite
reductase with reduced ferredoxin as reductant. (Reduction to sulfide)

8. Reductive sulfur assimilation pathway
• Sulfide obtained by the reduction of sulfate does not
accmulate, but is rapidly converted into organic sulfur
compounds.
• Cysteine is formed by condensation of O-acetylserine (OAS)
and sulfide catalysed by OAS (thiol) lyase.
• OAS is formed from serine and acetyl-CoA which is catalyzed
by serine acetyl transferase. (Incorporation of sulfur into
aminoacids).

10. Reductive sulfur assimilation pathway
• Cysteine is used as the reduced sulfur donor for the synthesis of
methionine.
• Most of the plant’s sulfur (90%) is in cysteine and methionine of
proteins. Cysteine is incorporated into CoA and traces of methionine
are used to form S-adenosyl methionine (SAM).
• SAM is used to transfer methyl group to help formation of lignins
and pectins of cell walls, flavonoids and chlorophylls.
• SAM is the precursor for the biosynthesis of plant hormone,
ethylene..

11. Reductive sulfur assimilation pathway
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.
• Cysteine is utilized for the synthesis of glutathione.
• Glutathione and its derivatives are involved in the storage and long distance
transport of reduced sulfur, in signal transduction pathways, in scavenging
hydrogen peroxide and other reactive oxygen species, in detoxifying
xenobiotics, etc.,

12. Reductive sulfur assimilation pathway
• Synthesis of Methionine from Cysteine:
• (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.

14. II. Sulfation Pathway
• In sulfation pathway, APS is phosphorylated by APS kinase to
form 3’phosphoadenosine-5’phosphosulphate (PAPS).
• Sulfate bound to PAPS can be added to a hydroxyl group of an
organic molecule. The reaction catalysed by
Sulfotransferases.

15. II. Sulfation pathway

16. Sulfate assimilation - Conclusion
• 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.