Lecture Outline: Plant Responses To Internal And External Signals

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  1. Signal Transduction and Environmental Response
    1. Plants react to internal and external signals to adapt to their environment.
    2. Phototropism: The plant's growth response toward or away from light.
    3. General Signal Transduction Pathway
      1. Reception: A signal molecule (often a hormone) binds to a receptor.
      2. Transduction: The receptor activates a secondary messenger (e.g., $\text{Ca}^{2+}$ or cAMP).
      3. Response: A final action, such as enzyme activation or changes in gene expression.
  2. Major Plant Hormones and Their Roles
    1. Auxin (Indoleacetic Acid - IAA)
      1. The first plant hormone discovered.
      2. Promotes cell elongation, especially in the shoot.
      3. Its movement is influenced by light, causing the shady side of the stem to grow faster.
    2. Ethylene (Gaseous Hormone)
      1. Promotes fruit ripening.
      2. Promotes leaf loss (abscission).
      3. Responsible for the triple response (allowing seedlings to avoid obstacles).
    3. Abscisic Acid (ABA)
      1. Primary role is maintaining dormancy (e.g., in seeds).
      2. Causes stomata to close rapidly during drought or water stress.
  3. Plant Defenses Against Pathogens
    1. Hypersensitive Response
      1. Localized, intentional death of cells surrounding an infection site.
      2. The plant seals off the invasion, as pathogens cannot live on dead cells.
    2. Systemic Acquired Resistance (SAR)
      1. A hormone signal travels through the phloem from the infected site to healthy leaves.
      2. This signal provides the healthy cells with a generalized resistance or immunity, similar to an immune response in animals.