• 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