Sulfur, Phosphorus, and Iron Metabolism in Plants

Abstract

Sulfur is an essential element for plant growth. Though it constitutes only 0.1% of the dry weight of a plant, its requirement for the plant is crucial. It is a constituent of sulfur-containing amino acids, cysteine, and methionine which are integral to the protein structure. Cysteine residues are responsible for holding proteins in proper conformation because of disulfide linkages (-S-S-) between two –SH containing amino acids. Iron-sulfur (4Fe-4S) clusters present in various proteins are engaged in electron transport reactions. Sulfur is also a constituent of a number of molecules such as lipoic acid, thiamin, biotin, ACP, and coenzyme A, which are required as cofactors by various enzymes. Sulfur-containing lipids, sulfoquinovosyldiacylglycerol, are structural constituents of thylakoids. Various secondary metabolites produced from cysteine and methionine have diverse roles in plants. Many molecules synthesized by the plants in response to abiotic and biotic stress contain sulfur. These include phytoalexins, thioredoxin, alliins, glucosinolates, etc. Alliins are found in onion and garlic, while glucosinolates are found in members of family Brassicaceae and are responsible for their flavor and smell. In some plants, elemental sulfur is deposited which functions as a potent fungicide (Fig. 12.1).

Multiple-Choice Questions

• 1. Phytoplanktons produce:

  • (a) Dimethyl sulfide

  • (b) Sulfur dioxide

  • (c) Dimethylsulfoniopropionate

  • (d) Dimethyl sulfoxide

• 2. SO₄ ⁻ uptake in root hair cell is facilitated by the plasma membrane localized:

  • (a) 3 H⁺/SO₄ ⁻ symporter

  • (b) H⁺/SO₄ ⁻ symporter

  • (c) 3 H⁺/SO₄ ⁻ antiporter

  • (d) H⁺/SO₄ ⁻ antiporter

• 3. The enzyme ATP sulfurylase catalyzes synthesis of:

  • (a) Cysteine

  • (b) Disulfide

  • (c) Glutathione

  • (d) Adenosine 5′-phosphosulfur

• 4. Which of the following statements is not correct?

  • (a) Reduction of SO₃ ⁻ to S²⁻ involves transfer of 6 electrons.

  • (b) Sulfite reductase draws electrons from reduced ferredoxin for reduction of SO₃ ⁻.

  • (c) Sulfite reductase draws electrons from NADH for reduction of SO₃ ⁻.

  • (d) Both nitrite reductase and sulfite reductase are similar since these consist of Fe₄-S₄ and siroheme.

• 5. Which of the following statements is correct?

  • (a) Methionine is the precursor of cysteine.

  • (b) Methionine is synthesized from cysteine.

  • (c) Cysteine biosynthesis occurs in the plastids.

  • (d) Glutathione acts as buffer for the changes in cytosolic pH.

• 6. Rieske iron-sulfur proteins are:

  • (a) Fe₄S₄ types in which the iron atoms are coordinated with four cysteine residues of the apoprotein.

  • (b) Fe₂S₂ types in which the iron atoms are coordinated with the cysteine residues of the apoprotein.

  • (c) Fe₃S₃ types in which all iron are coordinated with histidine of the apoprotein.

  • (d) Fe₂S₂ types in which one iron is coordinated with two histidine residues while another Fe is ordinated with two cysteine residues of the apoprotein.

• 7. The enzyme, ferredoxin reductase/oxidase (FRO), which is localized in plasma membrane of root hair epidermis facilitates Fe absorption because:

  • (a) It is transporter of the iron phytosiderophore complex.

  • (b) It facilitates reduction of Fe³⁺ to Fe²⁺ which can pass through the transporters of the membrane.

  • (c) It pumps protons out of the cell to make the soil pH acidic which results in release of Fe from the complex with organic compounds.

  • (d) It oxidizes Fe²⁺ to Fe³⁺ which makes the iron soluble and it can be absorbed.

• 8. The iron-storage protein is:

  • (a) Ferretin in chloroplasts

  • (b) Ferretin in mitochondria

  • (c) Frataxin in mitochondria

  • (d) Frataxin in chloroplasts

Answers

1.c	2. a	3. d	4. c	5. b	6. d	7. b
8. a