CIE A Level Biology复习笔记15.2.2 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’
• 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