Match List I with List II
LIST I                                                                LIST II
A. Gibberelins                                                 I. Phytoene
B. Cytokinins                                                  II. Squalene
C. Abscisic                                                      III. Isopentenyl diphosphate
D. Brassinosteroids                                    IV. Geranylgeranyl diphosphate
Choose the correct answer from the options given below:
1. A-III, B-IV, C-II and D-I
2. A-IV, B-II, C-I and D-III
3. A-III, B-IV, C-I and D-II
4. A-II, B-I, C-III and D-IV

Introduction to Plant Hormones and Their Biosynthesis

Plant hormones, also known as phytohormones, are organic compounds produced in plants that regulate various physiological processes, including growth, development, and stress responses. They play a vital role in controlling cell division, elongation, flowering, fruit ripening, and responses to environmental stimuli.

Understanding the biosynthetic pathways of plant hormones is essential for advanced studies in plant physiology, molecular biology, and biotechnology. This knowledge is particularly valuable for aspirants preparing for competitive exams like CSIR NET Life Science, DBT BET JRF, GATE Biotechnology, and IIT JAM Life Science.

Key Phrase: Plant Hormones and Their Biosynthetic Precursors

Question and Answer

Question:
Match List I with List II:

Options:

1. A-III, B-IV, C-II, and D-I
2. A-IV, B-II, C-I, and D-III
3. A-III, B-IV, C-I, and D-II ✅
4. A-II, B-I, C-III, and D-IV

Correct Answer: ✔️ Option 3 – A-III, B-IV, C-I, and D-II

Explanation of the Correct Answer

1. Gibberellins – Isopentenyl diphosphate (III)

• Gibberellins are diterpenoid compounds derived from the terpenoid biosynthetic pathway.
• The precursor molecule is isopentenyl diphosphate (IPP), which forms through the mevalonate pathway in the cytosol or the non-mevalonate pathway in plastids.
• Gibberellins promote stem elongation, seed germination, and flowering.

2. Cytokinins – Geranylgeranyl diphosphate (IV)

• Cytokinins are adenine derivatives synthesized from geranylgeranyl diphosphate (GGPP).
• The enzyme isopentenyl transferase (IPT) catalyzes the formation of cytokinins from GGPP.
• Cytokinins regulate cell division, leaf senescence, and shoot formation.

3. Abscisic Acid – Phytoene (I)

• Abscisic acid (ABA) is synthesized through the carotenoid pathway.
• Phytoene is a key intermediate in carotenoid biosynthesis, which eventually leads to the formation of abscisic acid.
• ABA regulates seed dormancy, stomatal closure, and stress responses.

4. Brassinosteroids – Squalene (II)

• Brassinosteroids are steroidal hormones synthesized from squalene, a precursor in the mevalonate pathway.
• They are involved in cell elongation, vascular differentiation, and stress tolerance.

Detailed Overview of Plant Hormones

1. Gibberellins

Gibberellins are diterpenoid acids involved in various aspects of plant growth and development.

• Function:
  • Stimulate stem elongation
  • Break seed dormancy
  • Promote flowering and fruit development
• Biosynthetic Pathway:
  • Mevalonate pathway → Isopentenyl diphosphate → Gibberellins
• Example:
  • Gibberellic acid (GA3) – Used to enhance grape size and induce flowering in certain crops

2. Cytokinins

Cytokinins are adenine-derived hormones that regulate cell division and differentiation.

• Function:
  • Delay leaf senescence
  • Promote lateral bud growth
  • Stimulate shoot formation in tissue culture
• Biosynthetic Pathway:
  • Geranylgeranyl diphosphate → Cytokinins
• Example:
  • Zeatin – Found in corn and other plants

3. Abscisic Acid (ABA)

Abscisic acid is a sesquiterpenoid that regulates stress responses and developmental processes.

• Function:
  • Promotes seed dormancy
  • Controls stomatal closure under drought stress
  • Regulates plant response to cold and salt stress
• Biosynthetic Pathway:
  • Carotenoid pathway → Phytoene → Abscisic acid
• Example:
  • ABA is involved in leaf drop during autumn.

4. Brassinosteroids

Brassinosteroids are polyhydroxysteroids essential for plant growth and development.

• Function:
  • Promote cell elongation
  • Stimulate root growth
  • Increase stress tolerance
• Biosynthetic Pathway:
  • Mevalonate pathway → Squalene → Brassinosteroids
• Example:
  • Brassinolide – First brassinosteroid discovered in mustard plants

Biosynthesis of Plant Hormones – Pathway Summary

Importance of Plant Hormones in Biotechnology

1. Agriculture – Gibberellins and cytokinins are used in agriculture to increase crop yield and improve plant growth.
2. Tissue Culture – Cytokinins and auxins regulate shoot and root formation in tissue culture.
3. Stress Resistance – Abscisic acid improves drought resistance in genetically modified crops.
4. Growth Regulation – Brassinosteroids are applied to enhance plant height and biomass production.

Challenges in Plant Hormone Research

• Complexity of biosynthetic pathways
• Hormone crosstalk complicates targeted regulation
• Environmental factors influence hormone signaling

Summary of Key Points

Plant hormones are essential for growth, development, and stress responses.
 Gibberellins, cytokinins, abscisic acid, and brassinosteroids are derived from different biosynthetic precursors.
Understanding biosynthetic pathways helps in improving crop performance and stress tolerance.