Auxin

• Auxin efflux pumps

  Can set up concentration gradients of auxin in plant tissue
• Auxins

  • Hormones produced by the tip of a shoot or root (i.e. apical meristems) that regulate plant growth
  • Auxin efflux pumps can control the direction of plant growth by determining which regions of plant tissue have high auxin levels
  • Auxin efflux pumps can change position within the membrane (due to fluidity) and be activated by various factors
• Auxin's mechanism of action in shoots

  Stimulates cell elongation and thus high concentrations of auxin promote growth (cells become larger)
• Auxin's mechanism of action in roots

  Inhibits cell elongation and thus high concentrations of auxin limit growth (cells become relatively smaller)
• Auxin's mechanism of action in plant shoots

  1. Auxin increases the flexibility of the cell wall to promote plant growth via cell elongation

  2. Auxin activates a proton pump in the plasma membrane which causes the secretion of H+ ions into the cell wall

  3. The resultant decrease in pH causes cellulose fibres within the cell wall to loosen (by breaking the bonds between them)

  4. Auxin upregulates expression of expansins, which similarly increases the elasticity of the cell wall

  5. With the cell wall now more flexible, an influx of water (to be stored in the vacuole) causes the cell to increase in size
• Auxin influences cell growth rates by changing the pattern of gene expression