Unit 7.3

• Plant Hormones

  Chemical signals that coordinate cell responses and enable plant cells to communicate
• 5 main types of plant hormones

  • Auxins
  • Gibberellins
  • Cytokinins
  • Abscisic Acid
  • Ethylene
• Auxins


  • Produced in shoot apical meristem
  • Found in young leaves, flowers, and fruits
  • Prevent growth of axillary buds (apical dominance)
  • Promote growth of roots and fruit
  • Prevent loss of leaves and fruit
  • Promote positive phototropism of stems

  
• Auxin-induced stem bending


  1. Auxin moves to shady side of stem

  2. Auxin binds to plasma membrane receptors

  3. Activates proton pump to pump H+ out of cell

  4. Cell wall loosens

  5. Turgor pressure increases due to water entry

  6. Cell enlarges
• Gibberellins


  • Growth-promoting hormones
  • Cause stem elongation
  • ~70 types, each differing slightly chemically
  • Most common is gibberellic acid
• Cytokinins

  • Promote cell division
  • Found in dividing tissues of roots, seeds, and fruits
  • Varying auxin:cytokinin concentrations cause tissue differentiation
• Abscisic acid (ABA)

  • Produced by any "green tissue"
  • Initiates and maintains bud dormancy
  • Causes stomatal closure
• Ethylene


  • A gas formed from the amino acid methionine
  • Stimulates enzymes that cause leaf, fruit, or flower drop (abscission)
  • Increases enzymes that soften and ripen fruits
  • Inhibits stem and root elongation

  
• Tropism

  Plant growth toward or away from a unidirectional stimulus
• Gravitropism


  • Upward growth of stem, downward growth of root in response to gravity
  • Depends on sensors called statoliths (starch grains in amyloplasts)

  
• Phototropism

  • Positive phototropism of stems due to auxin-induced elongation of shady side cells
  • Phototropin pigment in cell membrane absorbs blue light and initiates phototropism

  
• Thigmotropism

  • Unusual growth due to contact with solid objects, e.g. coiling of tendrils
  • Thigmomorphogenesis is the entire plant's response to environmental stimuli like wind/rain

  
• Nastic movements

  Do not involve growth and are not dependent on stimulus direction, involve changes in turgor pressure
• Sleep movements

  Occur daily in response to light and dark changes, driven by circadian rhythm and biological clock
• Photoperiodism

  Physiological response prompted by changes in day or night length, influences flowering
• Flowering plant groups based on day length

  • Short-day plants
  • Long-day plants
  • Day-neutral plants
• Plant defenses


  • Physical (cuticle, bark)
  • Chemical (tannins, alkaloids, cyanogenic glycosides)
  • Wound responses (proteinase inhibitors, systemin)
  • Hypersensitive response
  • Indirect defenses (prevent insect egg laying)
  • Mutualistic relationships (acacia tree and ant)