Plant Hormones and Growth Responses

Today’s Important Focus Points

  • Explain how hormones affect plant growth
  • Describe the multiple roles of ethylene in plants
  • Differentiate positive and negative tropisms

Plant Hormones and Tropisms

  • Tropism: Growth response of plant organs curving toward or away from stimuli.
    • Phototropism: Growth of a shoot in response to light; cells on the dark side elongate faster than on the light side.
    • Gravitropism: Response of plants to gravity.
    • Thigmotropism: Response to touch.

Plant Hormones

  • Hormones: Chemical signals produced in one part, acting on target cells to change their functioning.
    • Pioneering experiments by Charles Darwin showed that shoot tips control growth toward light by hormone production.
    • Various responses are mediated through signal transduction pathways when hormones bind to receptors.

Major Plant Hormones

  1. Auxins

    • Primarily indoleacetic acid (IAA), promotes seedling elongation.
    • Produced in apical meristems, enables apical dominance.
    • Stimulates cell membrane proteins to manage proton pumping, facilitating cell wall extension and elongation when water enters.
      • Affects shoot/root elongation based on concentration levels.

  2. Cytokinins

    • Promote cell division (cytokinesis) and are produced in roots and embryos.
    • Balance auxin effects, promoting lower buds' development into branches when no apical dominance is present.

  3. Gibberellins

    • Promote stem elongation, cell division in stems and leaves.
    • Named for fungi causing “foolish seedling” disease; enhance fruit development and seed germination.
    • High gibberellin levels can cause bolting in plants.

  4. Abscisic Acid (ABA)

    • Inhibits growth, acts as a stress hormone.
    • Closes stomata when dehydrated, promotes seed dormancy; high ABA must be removed for germination.
    • Ratio of ABA to gibberellins regulates germination.

  5. Ethylene

    • Gaseous hormone important for fruit ripening, produced in response to stress.
    • Alters auxin:ethylene ratios corresponding with environmental changes (e.g., shorter days lead to leaf abscission).

Mechanisms of Responses

  • Circadian Rhythms: Internal biological cycles around 24 hours, influenced by light and controlling seasonal developments including flowering and dormancy.
  • Photoperiod: Relative lengths of day and night, used by plants to determine seasonal changes.
    • Short-day plants flower with longer dark periods.
    • Long-day plants flower with shorter dark periods.

Plant Defense Mechanisms

  • Against Herbivores:

    • Use of physical (thorns) and chemical (toxic compounds) defenses against herbivores.
    • Damage initiates signal transduction, attracting parasitic wasps that kill pests.

  • Against Pathogens:

    • Physical barriers (epidermis) and localized defense responses (plant seals off infected areas and releases killing chemicals).
    • Systemic acquired resistance triggered by hormones for defenses across other organs.