IAA

• tropism

  • type of directional growth in response to stimuli
  • eg phototropism or geotropism
  • +=growth towards the stimuli
  • -=growth away from the stimuli
• plant growth factors

  • act in a similar way to hormones in animals
  • produced in growing parts of a plant before moving from growing regions to other tissues where they regulate cell growth
• plant hormones


  giberelins=stem elongation, flowering,seed germination

  cytokines=cell growth and division

  ADA=leaf loss, seed dormancy
• IAA

  • alters transcription of genes inside plant cells-genes that code for proteins involved in growth
  • IAA produced by cells in growing parts before it is redistributed to other tissues
  • IAA moves from cell to cell via diffusion/active transport
  • IAA moves via phloem
• IAA in plant shoots

  • phototropsim
  • when light shines on a stem from one side, IAA diffuses from the tip down to the shaded side
  • the higher concentration of IAA on the shaded side of shoot causes faster rate of cell elongation
  • lowering of the PH in the cellulose cell wall, causing it to loosen, which means they take in more water via osmosis and elongate/expand.
  • and shoot bends/grows towards the source of light
• IAA in roots

  • roots respond to gravity- geotropism
  • in roots, a higher concentration of IAA results in lower rate of cell elongation
  • AA is transported towards the lower side of plant roots
  • The resulting high concentration of auxin at the lower side of the root inhibits cell elongation
  • As a result, the lower side grows at a slower rate than the upper side of the root, causing the root to bend downwards