plant hormone

Plant Hormone Classes

Auxins

• Example: Indoleacetic acid (IAA)
    - Main Functions:
      - Growth promotion
      - Cell elongation
      - Cell division
      - Root initiation
      - Stimulates fruit development
      - Inhibits leaf abscission

• Production & Transport:
      - Produced in the apical meristem
      - Transported in parenchyma cells

• Light Effect:
      - Light illumination affects concentration
      - Illuminated side of coleoptile shows more auxin

Cytokinins

• Example: Zeatin
    - Main Functions:
      - Promotes cell division
      - Promotes chloroplast development
      - Stimulates lateral buds
      - Inhibits leaf abscission

• Mechanism:
      - Cytokinins from roots counteract auxin due to the direct inhibition hypothesis
      - Travels upward in xylem

Gibberellins

• Example: Gibberellic acid (GA)
    - Discovery: Isolated by Japanese scientist from the fungus Gibberella fujikuroi in the 1930s.
    - Production Sites:
      - Produced in young leaves, roots, and shoot apical meristem.
    - Main Functions:
      - Seed germination
      - Cell division and elongation
      - Stimulates flowering, especially under long day conditions
      - Breaks seed and winter dormancy

Brassinosteroids

• Example: Brassinolide
    - Main Functions:
      - Stimulates cell elongation and division
      - Retards leaf abscission
      - Promotes xylem differentiation
      - Triggers overall growth spurts and gravitropism

Abscisic Acid (ABA)

• Example: Abscisin II (ABA)
    - Discovery: Isolated in the 1960s
    - Main Functions:
      - Closes stomata
      - Inhibits shoot growth
      - Inhibits seed germination
      - Promotes winter and seed dormancy
    - Transport: Travels through vascular tissue

Ethylene

• Functions:
      - Acts as a stress hormone
      - Promotes apoptosis and abscission of leaves
      - Promotes ripening of fruits
      - Involved in the triple response in seedlings:
          - Slow stem elongation
          - Thickening of the stem
          - Curving

• Mechanism:
      - Enzymes hydrolyze cell walls to facilitate abscission of leaves
      - Forms a protective layer of cork cells on the stem side

Plant Behavior

Tropisms

• Definition: Response of plants to directional stimuli

• Concepts:
      - Taxis vs. Tropism: Taxis refers to movement toward or away from a stimulus, whereas tropism denotes growth toward or away from a stimulus.
  

Photomorphogenesis

• Phototropism: Growth response to light
      - Blue light accelerates hypocotyl elongation and stomatal opening

• Blue-light Photoreceptors:
      - Phototropin
      - Zeaxanthin
      - Cryptochromes

Phytochromes

• Function: Involved in the detection of light and germination of seeds
      - Types:
          - Pr: absorbs red light at 660 nm
          - Pfr: absorbs far-red light at 730 nm
      - Photoreversibility: Conversion of Pr to Pfr and its role in the photoperiodism

• Photoperiodism Effects:
      - Short-Day Plants: Require a minimum critical night length; Pfr inhibits flowering.
      - Long-Day Plants: Require maximum critical night length; Pfr induces flowering.
      - Day-Neutral Plants: Not dependent on day length for flowering.

Gravitropism

• Mechanism: Auxins promote cell elongation in shoots while inhibiting it in roots, facilitated by statoliths, which lead to the accumulation of auxin.
      - Statoliths: Small starch-filled organelles that assist with gravity sensing.
      - Example Observation: (a) Root gravitropic bending due to statolith settling.

Thigmomorphogenesis

• Definition: Growth response to mechanical stress, causing plants to grow shorter and stockier.

• Thigmotropism/Thigmonasty: Movement or growth in response to being touched, which can influence plant morphology and ecology.