Plant Responses II

Reading: Chapter 11, pp. 194-203 in Biology
Objectives: At the end of this lecture, you should be able to:
1. Define the terms listed below.
2. Describe experiments which demonstrate that the apical region of the coleoptile is involved in phototropism.
3. Describe what apical dominance is and how auxin is involved in this process.
4. Describe an experiment which demonstrates a role for auxin in apical dominance.
5. Describe the role of gibberellins in stem internodal elongation.
6. Discuss how cytokinin and auxin levels in tissue culture control morphogenesis leading to growth of either callus, roots or shoots.
7. Describe the role of ethylene in fruit ripening.
8. Understand the role of abscisic acid in stomatal responses to low water stress and seed dormancy.
9. Describe an experiment demonstrating that the hypothetical plant hormone "florigen" controls flowering.
Terms:
auxin
 florigen leaf senescence
gibberellin indole acetic acid (IAA) low-water stress
cytokinin polar auxin transport seed dormancy
abscisic acid basipetal morphogenesis
ethylene apical dominance fruit ripening


Lecture Outline:

I. Many aspects of plant growth and development are controlled by plant hormones.

  1. Plant hormones are chemicals produced by plants that affect growth and development at very low concentrations.
  2. At present 5 types of plant hormones have been identified and these are auxins, gibberellins, cytokinins, abscisic acid and ethylene.

II. Auxins


  1. The identification and study of auxin began with observations regarding its role in tropisms.
    1. Phototropism studies with coleoptiles demonstrated that growth towards the light depended upon the coleoptile tip-- (cut off the tip or cover the tip and no phototropism occurs).
    2. Studies indicated that the coleoptile tip produced that was responsible for phototropism.
      1. chemical could move through agar placed between tip and the remainder of the coleoptile.
      2. agar block absorbing a chemical from the tip could differential growth when applied to a "decapitated" coleoptile.
      3. chemical identified as indole acetic acid (IAA).
    3. Mechanism of phototropism depends upon polar auxin transport.
      1. auxin causes cell expansion (elongation).
      2. auxin (IAA) is transported in a basipetal manner--from the tip of shoots to the base of the plant.
      3. differential illumination causes a re-distribution of auxin to the side away from the light.
      4. such a differential auxin distribution leads to growth towards the light by a greater expansion of cells on the "dark" side of the stem (or coleoptile).
  2. Auxin is also involved in Apical Dominance.
    1. basipetal movement of auxin in the main stem inhibits branching near the apical tip by blocking development of lateral buds.
    2. if apical tip on main stem is removed, this promotes lateral branching.
    3. auxin artificially supplied to "decapitated" stem restores apical dominance.

III. Gibberellins


  1. discovered in studies on the "foolish seedling disease" which causes plants to grow tall and spindlely (they fall over).
  2. gibberellins were found to be the chemical responsible for the disease and produced by the fungal pathogen that caused the disease.
  3. subsequent work demonstrated that gibberellins were produced by the plant as a plant hormone.
  4. gibberellins have a major role in controlling elongation of the internode region of stems.
  5. gibberellins also involved in processes associated with seed germination--ex. activating enzymes from barley aleurone layer.

IV. Cytokinins

  1. cytokinins discovered in early studies on tissue culture as a chemical factor that promoted cell division.
  2. cytokinins also interact with auxins in tissue culture to determine whether growth of callus, roots or shoots occurs. ("kinetin" is the cytokinin used in the example below)
  3. cytokinins also control leaf senescence.
    1. leaf senescence is part of the natural ageing of the leaf that occurs in fall when leaves begin to fall off plants.
    2. during leaf senescence, important metabolites will be out of the leaf and re-used by the plant.
    3. application of cytokinin to the leaf can prevent leaf senescence.

V. Ethylene

  1. Gasous hormone produced by the plant which is involved in fruit ripening, responses to wounding and leaf senescence.
  2. ethylene release can lead to ripening of fruit in other plants or can promote leaf senescence.

VI. Abscisic acid

  1. initially thought to be involved in leaf abscission (falling of leaves).
  2. now known to be involved in:
    1. responses to low water stress--closes stomates during low water stress.
    2. seed dormancy--abscisic acid in seed coat may inhibit germination until a sufficient amount is washed out by water.

VII. Florigen is a hypothetical hormone implicated in photoperiodic flower induction.

  1. exposure of leaves to appropriate daylength appears to induce flowing via production of a chemical that can even induce flowering in an adjacent plant via grafting.
  2. florigen has never been identified.



For Further Exploration: