Indoleacetic Acid (IAA) (AQA A Level Biology)

• Indoleacetic acid (IAA) is a specific growth factor found in plants
• 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

• IAA is involved in controlling growth by elongation

Controlling growth by elongation

• IAA molecules bind to a receptor protein on the cell surface membrane
• IAA 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) which is then stored in the vacuole
• 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 IAA in the elongation of cells

Phototropism

• Phototropism affects shoots and the top of a stem
• The concentration of IAA determines the rate of cell elongation within the region of elongation
• If the concentration of IAA is not uniform on either side of a root or shoot then uneven growth can occur
• When the shoots grow towards the light it is known as positive phototropism
  • It is described as positive because growth occurs towards the stimulus

• In shoots higher concentrations of IAA results in a greater rate of cell elongation
  • Experiments have shown that IAA moves from the illuminated side of a shoot to the shaded side
  • The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation
  • This causes the shoot to bend towards the light

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

Gravitropism

• Gravitropism affects roots
• When the roots grow towards gravity it is known as positive gravitropism
• In roots, higher concentrations of IAA results in a lower rate of cell elongation
• Some plants cells are able to detect gravity
  • Columellar cells near the root tip possess heavy organelles called amyloplasts
  • Amylopasts are densely packed with starch and so they sink to the bottom of the cell
  • When a root is moved from the vertical plane to the horizontal plane these organelles fall to where the bottom of the cell is

• IAA is actively transported to the region in the root tip where the amyloplasts have sunk
  • The larger concentration of IAA at the lower side of the root inhibits cell elongation
  • As a result, the lower side grows at a slower rate than the upper side of the root
  • This causes the root to bend downwards

IAA is transported towards the amyloplasts. It inhibits cell elongation in the roots causing the lower side to elongate at a slower rate than the upper side.

• In shoots higher concentrations of IAA results in a greater rate of cell elongation
  • Shoots show positive phototropism and negative gravitropism

• The opposite is true for roots, higher concentrations of IAA results in a lower rate of cell elongation
  • Roots show negative phototropism and positive gravitropism

• Experiments can be carried out to investigate the effect of IAA on shoot and root growth in seedlings

Method

• Use the scalpel to cut a 1cm section from the root tip of each seedling
• Mark the root tips at 2mm marks
• Divide the root tips into three groups and place them in test tubes of water
  • The water helps to keep the plant tissue alive

• Group A receives treatment 1
  • Remove the ends of the root tips using the scalpel
  • Transfer root cuttings with the end removed to an agar block
  • A uniform light source is present

• Group B receives treatment 2
  • Transfer intact root tips to an agar block
  • A light-proof container is placed over the seedlings to prevent light from entering

• Group C receives treatment 3
  • Transfer intact root tips to an agar block
  • Apply a directional light source to one side of the root tips

• Leave all the roots in their treatment conditions for 3 hours
• Use the 2mm marker lines to determine if growth has occurred
• Note if the growth has been even on both sides

Results and analysis

• In group A (tips removed) the roots grow evenly on both sides
  • IAA is synthesised in the root tips so removing them means that no IAA is produced
  • There is no inhibition of cell elongation

• In group B (no light) the roots grow slightly less than group A but evenly on both sides
  • There is an equal concentration of IAA on both sides of the root tip
  • The inhibition of cell elongation is equal on both sides of the root tip
  • The roots do not grow as long as those in group A due to the presence of IAA

• In group C (directional light) the cells on the illuminated side of the root grow longer than those on the shaded side
  • There is a greater concentration of IAA on the shaded side
  • This results in greater inhibition of cell elongation on the shaded side
  • So the illuminated side grows at a faster rate
  • The roots bend away from the light – negative phototropism

Limitations

• Although the same species of plant is being used the plants will still have differences
  • Certain genotypes may be more prone to bending or have slightly different sensitivities to IAA

• The 2mm marks used for measuring growth may get smudged or move
  • If the root is mishandled the marks can be altered, which will affect the results

• The evenness of growth can be hard to determine using the naked eye
  • Only general comments can be made about whether there has been even growth on both sides of the roots

The different treatments produce different levels of growth in the root tips. The IAA molecules inhibit cell elongation in roots.