Plant Hormones (AQA GCSE Biology)

• Plants produce plant hormones called auxins to coordinate and control growth
• Plants need to be able to grow in response to light (phototropism) and gravity (gravitropism or geotropism)
  • The shoots must grow upwards, away from gravity and towards light, so that leaves are able to absorb sunlight – shoots show a positive phototropic response and a negative gravitropic response
  • Roots need to grow downwards into the soil, away from light and towards gravity, in order to anchor the plant and absorb water and minerals from the soil particles so roots show a negative phototropic response and a positive gravitropic response

Gravitropism and phototropism table

• Auxins are produced in the tips of the shoots and the roots; they diffuse to the cells behind the tips and have the following effects:
  • In the shoots auxins promote cell elongation (growth); more auxin = more cell elongation = more growth
  • In the roots auxins inhibit cell elongation (growth); less auxin = less cell elongation = less growth

• The distribution of auxin in the shoots is affected by light and gravity, whereas the distribution in the roots is primarily affected by gravity alone
  • If a shoot or root is placed on its side, auxins will accumulate along the lower side as a result of gravity; so the uppermost side has a lower auxin concentration
  • In the shoots, the lower side grows faster the upper side, so the shoot grows upwards
  • In the roots, the lower side grows slower than the upper side (as auxin inhibits cell elongation and growth in the roots), so the root grows downwards

• Unequal distributions of auxin cause unequal growth rates in plant roots and shoots

How auxins control growth in the shoots

• Auxin is mostly made in the tips of the growing shoots diffuses to the region behind the tip
• Auxin stimulates the cells behind the tip to elongate (get larger); the more auxin there is, the faster they will elongate and grow
  • This is an important point - only the region behind the tip of a shoot is able to contribute to growth by cell division and cell elongation

• If light shines all around the tip, auxin is distributed evenly throughout and the cells in shoot grow at the same rate - this is what normally happens with plants growing outside
• When light shines on the shoot predominantly from one side though, the auxin produced in the tip concentrates on the shaded side, making the cells on that side elongate and grow faster than the cells on the sunny side
• This unequal growth on either side of the shoot causes the shoot to bend and grow in the direction of the light

Positive phototropism in plant shoots is a result of auxin accumulating on the shaded side of a shoot

Higher tier only

• Auxins are just one example of hormones found in plants
• Gibberellins are important in initiating seed germination – the process that occurs when a seed starts to grow
• Gibberellins also have a role in inducing flowering and the growth of fruit
• Ethene is a gas released by plants which controls cell division and ripening of fruits
• Both gibberellins and ethene are used commercially – see Uses of Plant Hormones