The Role of Auxin in Elongation Growth (Cambridge International A-Level Biology)

Spec point — 15.2.2 · explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls

The role of auxin in elongation growth

• Plant hormones (also known as plant growth regulators) are responsible for most communication within plants

• Auxins are a type of plant growth regulator that influence many aspects of growth, including elongation growth which determines the overall length of roots and shoots

• The principle chemical in the group of auxins made by plants is IAA (indole 3-acetic acid) and this chemical is often simply referred to as ‘auxin’

  • Auxins are a class of plant hormones that regulate growth in plants

• Auxin (IAA) is synthesised in the growing tips of roots and shoots (ie. in the meristems, where cells are dividing)

• Growth in these meristems occurs in three stages:

  • Cell division by mitosis
  • Cell elongation by absorption of water
  • Cell differentiation

• Auxin (IAA) is involved in controlling growth by elongation

Controlling growth by elongation

• Auxin molecules bind to a receptor protein on the cell surface membrane
• Auxin stimulates ATPase proton pumps to pump hydrogen ions from the cytoplasm **into the cell wall **(across the cell surface membrane)
• This acidifies the cell wall (lowers the pH of the cell wall)
• This activates proteins known as expansins, which** loosen the bonds between cellulose microfibrils**
• At the same time, potassium ion channels are stimulated to open
• This leads to an increase in potassium ion concentration in the cytoplasm, which decreases the water potential of the cytoplasm
• This causes the cell to absorb water by osmosis (water enters the cell through aquaporins)
• This increases the internal pressure of the cell, causing the cell wall to stretch (made possible by expansin proteins)
• The cell elongates

The role of auxin (IAA) in controlling growth by elongation