Plant Hormones (HL) (HL IB Biology)

• Phytohormones are plant hormones that regulate their growth, development, reproductive processes, longevity, and even death
• There are many chemicals which act as phyohormones in plants, some examples include:
  • Auxins which result in cell elongation
  • Abscisic acid which suppresses the growth of plants
  • Cytokinins which increase the rate of cell division
  • Ethylene which promotes fruit ripening
  • Gibberellin which control cell elongation, seed germination, flowering and dormancy
  • Brassinosteroids which regulate growth, development, and responses to stresses
• Plant hormones are sometimes referred to as plant growth regulators

• Auxins are a group of plant hormones that influence many aspects of plant growth
  • A common auxin is known as IAA (indole-3-acetic acid)
• In shoots, auxin is produced in cells at the growing tip before moving away into the surrounding tissues
• Auxin has an important role in regulating shoot growth
  • In shoots, auxin causes cells to elongate, leading to stem growth
    • Note that in roots, auxin inhibits cell growth; the opposite effect to that in shoot cells
    • Note that at very high concentrations, auxin can also inhibit shoot growth

Auxin efflux carriers

• Auxin enters cells by simple diffusion, however, to exit the cell (and therefore move to the next cell), it requires membrane proteins called auxin efflux carriers to exit the cell
  • The term 'efflux' refers to an outward flow of a substance; in this case auxin is pumped out of one cell and into another
  • Efflux carriers are a type of protein called PIN3 proteins
• Plant cells can distribute auxin efflux carriers on one side of the cell to encourage one way movement of auxin
• The process requires ATP so is a type of active transport
• These efflux carriers or pumps are important in establishing an auxin gradient across a stem or root in response to a stimulus such as light or gravity
  • E.g. Light is thought to affect the expression of genes that code for the PIN3 protein efflux pumps; light shining on one side of a stem more than the other can therefore lead to an uneven distribution of efflux pumps, creating an auxin gradient

• Light affects the growth of plant shoots in a response known as phototropism
• The concentration of auxin determines the rate of cell elongation within the stem
  • A higher concentration of auxin causes an increase in the rate of cell elongation 
  • If the concentration of auxin is not uniform across the stem then uneven cell growth can occur
• When light shines on a stem from one side, auxin is transported, by PIN3 proteins, from the illuminated side of a shoot to the shaded side
• An auxin gradient is established, with more auxin on the shaded side and less on the illuminated side
• The higher concentration of auxin on the shaded side of the shoot causes a faster rate of cell elongation, and the shoot bends towards the source of light

Effect of Auxin Diagram

Higher concentrations of auxin on the shaded side of a stem increases the rate of cell elongation so that the shaded side grows faster than the illuminated side

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

Cell growth by auxin diagram

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