Role of auxins in plant responses

• what are meristems?

  tissue found in regions of growth in plants, which contain stem cells
• what are apical meristems?

  the main growing shoot at the top of plant
• what is the lateral bud meristems?

  dormant buds that can grow to form side shoots
• what is the most common form of auxin?

  • indole-3-acetic acid (IAA)
  • produced in apical meristem
  • causes cell elongation - diffuses or actively transported down shoot to zone of elongation, causing shoot to grow
• how does auxins mode of action work?

  • cell elongation is proportional to auxin concentration
  • auxin bind to receptors on plant cell membrane - causes H+ to be actively transported into cell wall by proton pump located in plasma membrane
  • low pH is optimum for enzymes called expansions - wall loosening enzymes which hydrolyse bonds in cellulose, low pH disrupts H bonds in cellulose
  • cellulose fibres weakened but not destroyed, cell walls now less rigid, cells can elongate when water moves in by osmosis
• what is the response of unidirectional light on a shoot?

  • dark side - less phototropins stimulated, less light absorbed
  • light side - more phototropins stimulated, more light absorbed
  • auxins redistributed over to shadier side, causing elongation of shadier side towards light
  • uneven growth of shoot
• how does auxin coordinate plant responses to gravity?

  • root placed horizontally
  • bottom side contains more auxin and grows less, causing root to grow in direction of force of gravity - positive geotropism
  • opposite in stem
• what is apical dominance?

  • when the apical shoots inhibit growth of lateral buds
  • in other words, main central stem of plant is dominant over and grows more strongly than other side of stems
• what are coppicing and pollarding?

  • consequences of removing apical dominance
  • coppicing - want many new branches from stump
  • pollarding - don't want young stems eaten
• • apical buds need higher concentration of auxin to grow than roots and lateral buds
  • small stimulation, low concentration of auxin - root growth
  • higher stimulation - apical bud growth, inhibition of lateral bud