M5:S4 Plant responses and hormones

• Plants, like animals, increase their chances of survival by responding to changes in their environment
• Plant responses to stimuli

  • Sensing the direction of light and growing towards it
  • Sensing gravity and growing roots and shoots in the right direction
  • Climbing plants having a sense of touch to find things to climb
• Plants are more likely to survive if they respond to herbivory-being eaten by animals (including insects)
• Chemical defences plants produce in response to herbivory

  • Alkaloids
  • Tannins
  • Pheromones
• Alkaloids


  Chemicals with bitter tastes, noxious smells or poisonous characteristics that deter or kill herbivores
• Tannins


  Taste bitter, and in some herbivores (e.g. cattle, sheep) they can bind to proteins in the gut, making the plant hard to digest
• Pheromones


  Signalling chemicals that produce a response in other organisms
• Plant responses to herbivory

  • Releasing alarm pheromones into the air
  • Producing pheromones that attract parasitic wasps
• Plant responses to being touched

  • Mimosa pudica folding up its leaves
• Plants are more likely to survive if they respond to abiotic stress (anything harmful that's natural but non-living, like a drought)
• Plant responses to abiotic stress

  • Carrots producing antifreeze proteins at low temperatures
• Tropism


  A plant's growth response to an external stimulus
• Tropisms


  • A positive tropism is growth towards the stimulus
  • A negative tropism is growth away from the stimulus
• Phototropism


  The growth of a plant in response to light
• Phototropism


  • Shoots are positively phototropic and grow towards light
  • Roots are negatively phototropic and grow away from light
• Geotropism


  The growth of a plant in response to gravity
• Geotropism


  • Shoots are negatively geotropic and grow upwards
  • Roots are positively geotropic and grow downwards
• Other tropisms


  • Hydrotropism-plant growth in response to water
  • Thermotropism-plant growth in response to temperature
  • Thigmotropism-plant growth in response to contact with an object
• Growth hormones


  Chemicals that speed up or slow down plant growth
• Gibberellin


  A growth hormone that stimulates seed germination, stem elongation, side shoot formation and flowering
• Auxins


  Growth hormones that stimulate the growth of shoots by cell elongation
• High concentrations of auxins inhibit growth in roots
• Indoleacetic acid (IAA)


  An important auxin
• IAA


  • Produced in the tips of shoots in flowering plants
  • Moves around the plant to control tropisms
  • Uneven distribution of IAA causes uneven growth
• Phototropism


  IAA moves to the more shaded parts of the shoots and roots, so there's uneven growth
• Geotropism


  IAA moves to the underside of shoots and roots, so there's uneven growth
• Removing the apical bud prevents auxin production, allowing side shoots to grow
• Replacing the apical bud with a source of auxin inhibits side shoot development
• Auxins become less effective at inhibiting side shoots as the plant matures
• Auxins


  Involved in Apical Dominance
• Apical bud


  • The shoot tip at the top of a flowering plant
• Apical dominance


  1. Auxins stimulate the growth of the apical bud

  2. Auxins inhibit the growth of side shoots from lateral buds
• Apical dominance


  • The apical bud is dominant over the lateral buds
  • Prevents side shoots from growing
  • Saves energy
  • Prevents side shoots from the same plant competing with the shoot tip for light
• Removing the apical bud

  The plant won't produce auxins, so the side shoots will start growing
• Replacing the tip with a source of auxin


  Side shoot development is inhibited
• Auxins become less concentrated as they move away from the apical bud to the rest of the plant
• If a plant grows very tall, the bottom of the plant will have a low auxin concentration so side shoots will start to grow near the bottom
• Investigating the role of auxins in apical dominance


  1. Plant 30 plants

  2. Count and record the number of side shoots

  3. Remove the tip of the shoot and apply a paste containing auxins

  4. Remove the tip of the shoot and apply a paste without auxins

  5. Leave 10 plants untreated as controls

  6. Let each group grow for six days

  7. After six days, count the number of side shoots
• The results suggest auxins inhibit the growth of side shoots, suggesting that auxins are involved in apical dominance
• Gibberellins


  Another type of plant hormone