Plant growth and hormones

1. Auxins

• Discovery: Went (1928)
• Natural Auxin: Indole-3-acetic acid (IAA)
• Functions:
  • Promotes cell elongation in stems.
  • Induces apical dominance (inhibits lateral bud growth).
  • Stimulates root initiation and adventitious roots.
  • Promotes fruit development (parthenocarpy).
  • Influences vascular differentiation.

Key Point: High auxin → apical dominance; removal of apical bud → lateral growth.

2. Gibberellins (GAs)

• Discovery: Kurosawa, from Gibberella fujikuroi
• Functions:
  • Stimulates stem elongation → bolting in rosette plants (e.g., cabbage, lettuce).
  • Breaks dormancy in seeds and buds.
  • Promotes flowering in long-day plants.
  • Stimulates enzyme production (α-amylase) in germinating seeds.

Key Point: Responsible for “foolish seedling disease” in rice (caused by fungus producing GAs).

3. Cytokinins

• Discovery: Miller
• Natural Cytokinin: Zeatin
• Functions:
  • Promotes cell division (cytokinesis).
  • Delays leaf senescence (anti-aging).
  • Promotes lateral bud growth (opposes apical dominance).
  • Helps in chloroplast development.

Key Point: Work synergistically with auxins in tissue culture for organogenesis.

4. Abscisic Acid (ABA)

• Functions:
  • Induces dormancy in seeds and buds.
  • Closes stomata during water stress → reduces transpiration.
  • Acts as stress hormone → drought, salinity, cold tolerance.
  • Inhibits growth and germination.

Key Point: ABA is also called “stress hormone”.

5. Ethylene

• Discovery: Cousins
• Functions:
  • Promotes fruit ripening (e.g., banana, tomato).
  • Induces leaf abscission and flower senescence.
  • Stimulates triple response in seedlings: inhibition of stem elongation, thickening, and horizontal growth.

Key Point: Ethylene is a gaseous hormone; used commercially to ripen fruits.

6. Brassinosteroids

• Promote cell expansion and elongation.
• Stimulate vascular differentiation and xylem formation.
• Enhance stress tolerance in plants.

7. Jasmonates (JAs) & Salicylic Acid (SA)

• Functions:
  • Regulate plant defense responses against pathogens and herbivores.
  • Involved in wound response, secondary metabolite synthesis.
  • Jasmonates → inhibit growth but enhance defense.

• Important Terms
• Apical dominance: Suppression of lateral buds by apical bud due to auxin.
• Bolting: Rapid stem elongation in rosette plants caused by gibberellins.

• Quick Comparison Table for Exams