Dr. Vibha Khanna discusses the importance of sulfur as an essential macronutrient for plants. Sulfur plays critical roles as a structural component in amino acids, peptides, vitamins, and secondary products. It is also important for its catalytic functions and participation in redox reactions via thiol groups. Glutathione plays an important role in stress mitigation through antioxidant activity and detoxification of heavy metals and toxins. Sulfur assimilation in plants is part of the global sulfur cycle, with plants fixing inorganic sulfur for use and as a source for animal nutrition.

1. DR. VIBHA KHANNA
Asso. Prof. (Botany)
SPC GOVERNMENT COLLEGE
AJMER (Rajasthan)

2. PLANT BIOCHEMISTRY
• BLOCK 2. : Sulphur metabolism
– PRESENTATION 1:
• Sulphur an essential macronutrient

3. WHY IS SULFUR IMPORTANT?
• Sulphur is among the most versatile elements in living
organism
• The versatility of sulphur derives in part from the
property that it shares with nitrogen – ‘multiple stable
oxidation states’
• It represents the ninth and least abundant essential
macronutrient in plants, preceded by carbon, oxygen,
hydrogen, nitrogen, potassium, calcium, magnesium,
and phosphorus.
• Sulphur plays critical roles in the catalytic or
electrochemical functions of the biomolecules in cells.

4. WHY IS SULFUR IMPORTANT?
BECAUSE SULFUR NOT ONLY SERVES AS A
STRUCTURAL COMPONENT BUT ALSO PLAYS
CHARACTERISTIC FUNCTIONS IN CELLS
• Sulphur is found in
– amino acids (Cys and Met),
– oligopeptides (glutathione [GSH] and phytochelatins),
– vitamins and cofactors (biotin, thiamine, CoA, andS-
adenosyl-Met), and
– a variety of secondary products (glucosinolates in
Cruciferae and allyl Cys sulfoxides in Allium).

5. IMPORTANCE OF SULPHUR
• The thiol (sulfhydryl) group of Cys in proteins maintains
protein structure by forming disulfide bonds between two
Cys residues via oxidation.
• The thiol of Cysteine (Cys) and glutathione (GSH) is often
involved in the redox cycle by two thiol 4 disulfide
conversions.
• This interchange is adaptable for redox control and
mitigation against oxidative stress in nearly all aerobic
organisms including plants
• The nucleophilicity of the thiol group, and in particular
GSH, plays a role in detoxification of xenobiotics by direct
conjugation with sulfhydryl group mediated by GSH S-
transferase.

6. IMPORTANCE OF SULPHUR
• Phytochelatins, a polymerized version of GSH, are
involved in detoxification of heavy metals by
serving as chelating ligands through thiol groups.
• Sulphur-containing secondary products often have
a characteristic smell and are regarded not only as
defense compounds against herbivores and
pathogenic organisms but also as signaling
molecules for fundamental cellular functions
• The catalytic sites for several enzymes and co-
enzymes, such as urease and co-enzyme-A ,
contain sulfur.
• Sulphur participates in electron transport through
iron-sulphur clusters.

7. GLUTATHIONE IS IMPORTANT IN STRESS
MITIGATION
• Because of its nucleophilic properties, glutathione serves as the first
line of defense against the products of oxygen metabolism, reactive
oxygen species, and other electrophilic compounds such as toxins
(herbicides), xenobiotics, and heavy metals.
• When plants encounter reactive oxygen species, glutathione is a
direct source of electrons for stress mitigation by the enzyme
glutathione peroxidase or an indirect means to maintain a reduced
pool of ascorbate, another antioxidant.
• Glutathione reacts directly with toxins in a reaction mediated by
glutathione S-transferase. In this way the toxins are inactivated and
tagged for transport into the vacuole and for degradation.
• In some plants heavy-metal detoxification is mediated by
glutathione derivatives called phytochelatins, which have the
general structure (γ-glutamylcysteine)nGly (n = 2–11), and by Cys-
rich proteins called metallothioneins. In both molecules thiol groups
serve as the metal ion ligand.

8. SULFUR NUTRITION IMPACTS AGRICULTURE, FOOD
QUALITY, AND NUTRA- AND PHARMACEUTICALS
• Sulphur assimilation in plants plays a key role in
the sulfur cycle in nature. The inorganic sulfur in
the environment, sulfate ion in soil and sulfur
dioxide gas in the air, is fixed into Cys by the
sulfur assimilation pathway in plants.
• Thereafter, Cys is converted to Met. Animals,
however, do not have the assimilatory
mechanisms for inorganic sulfur; they require
Met as an essential amino acid for their source of
sulfur nutrient.

9. Therapeutic Properties of S-compounds
• Antiseptic, ‘alliin’ in garlic {Alliin (S-allylcysteine sulfoxide) is the
most abundant organosulfur compound in the whole garlic bulbs
When fresh garlic is chopped or crushed, the enzyme alliinase
converts alliin into allicin which is antioxidant and anti-
inflammatory}
• Anticarcinogenic ‘sulphoraphane’ in broccoli {Sulforaphane (SFN),
an isothiocyanate, has the ability to modulate multiple cellular
targets involved in cancer development simultaneously, including:
(i) DNA protection by modulating carcinogen-metabolizing enzymes
and blocking the action of mutagens; (ii) inhibition of cell
proliferation and induction of apoptosis, thereby retarding or
eliminating clonal expansion of initiated, transformed, and/or
neoplastic cells; (iii) inhibition of neoangiogenesis, progression of
benign tumors to malignant tumors, and metastasis formation. SFN
is therefore able to prevent, delay, or reverse preneoplastic lesions,
as well as to act on cancer cells as a therapeutic agent.

11. Sulphur Cycle in Nature
• Sulphur is available to plants primarily in the
form of anionic sulphate (SO4
2−) present in
soil.
• It is actively transported into roots and then
distributed, mostly unmetabolized,
throughout the plant.
• SO4
2− is a major anionic component of
vacuolar sap; therefore, it does not necessarily
enter the assimilation stream.

12. Sulphur Cycle in Nature
• Sulphur assimilation in plants plays a key role in
the sulfur cycle in nature.
• The inorganic sulfur in the environment, sulfate
ion in soil and sulfur dioxide gas in the air, is fixed
into Cys by the sulfur assimilation pathway in
plants.
• Thereafter, Cys is converted to Met.
• Animals, however, do not have the assimilatory
mechanisms for inorganic sulfur; they require
Met as an essential amino acid for their source
of sulfur nutrient.

13. Sulphur deficiency in Plants
• Sulphur is not as mobile in plant tissue as
nitrogen. The result is that a deficiency affects
a plant in a different way. Both cause leaves to
turn yellow. The difference is that a nitrogen
deficiency first affects the older leaves and a
sulfur deficiency the younger leaves.
• A deficiency of sulfur is unusual but possible in
an acid soil with low organic content