Indoleacetic Acid (IAA) (AQA A Level Biology): Revision Note

Indoleacetic acid (IAA)

• Indoleacetic acid (IAA) is a plant growth factor

  • IAA belongs to a group of growth factors known as auxins

• IAA is synthesised in the growing tips of roots and shoots, after which it moves to different regions of the plant

  • It can move from cell to cell by diffusion and active transport, and over longer distances via the phloem

• IAA influences plant growth by controlling elongation of plant cells

  • The concentration of IAA determines the rate of cell elongation

  • An uneven concentration of IAA results in uneven growth

IAA and gravitropism

• Gravitropism is a directional response to gravity

  • Growth towards gravity is positive gravitropism

• IAA causes positive gravitropism in roots as follows:

  1. IAA is produced in the root tip, from where it is transported throughout the root

  2. IAA moves toward the lower side of the root, setting up an IAA gradient across the root

     • The lower side of the root is indicated by the presence of amyloplasts, which gather at the bottom of cells due to their density

  3. IAA in roots inhibits cell elongation, so the cells on the lower side of a root elongate more slowly than those on the upper side

  4. the root bends towards gravity as it grows

IAA and phototropism

• Phototropism is a directional response to light

  • Growth towards light is positive phototropism

• IAA causes positive phototropism in shoots as follows:

  1. IAA is produced in the shoot tip, from where it is transported throughout the shoot

  2. IAA moves toward the shaded side of the root, setting up an IAA gradient across the shoot

  3. IAA in shoots causes cell elongation, so the cells on the shaded side of a shoot elongate faster than those on the light side

  4. the shoot bends towards light as it grows

Investigating the effect of IAA on root growth

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

Apparatus

• Seedlings (of the same age and plant species)

• Cutting tile

• Scalpel

• Light source

• Lightproof container

• Blocks of agar

• Marker/pen

• Test tubes

• Water

Method

1. Use the scalpel to cut a 1 cm section from the root tip of each seedling

2. Mark the root tips at 2 mm intervals

3. Divide the root tips into three groups and place them in test tubes of water

   • The water helps to keep the plant tissue alive

4. 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

   • Illuminate the seedlings equally on all sides

5. Group B receives treatment 2:

   • Transfer intact root tips to an agar block

   • Place a light-proof container over the seedlings

6. 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

7. Leave all the roots in their treatment conditions for 3 hours

8. Use the 2 mm marker lines to determine whether growth has occurred

   • Measure growth

   • Observe whether the growth has been even on both sides

Results and analysis

• In group A (tips removed) we would expect the roots to grow evenly

  • 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) we would expect the roots to grow less than group A, but still to grow evenly on both sides

  • The roots do not grow as long as those in group A due to the presence of IAA, which inhibits elongation

  • IAA does not redistribute in response to light so there is an equal concentration of IAA on both sides of the root tip

• In group C (directional light) we would expect the root to bend away from light

  • There is a greater concentration of IAA on the shaded side, so there is inhibition of cell elongation

  • The illuminated side grows at a faster rate

  • The root bends away from the light; this is negative phototropism