What are plant hormones?
Plants make chemicals (plant hormones) that coordinate growth and responses to light, gravity, water and other stimuli. The main one on the spec is auxin, made in shoot and root tips.
Plants cannot move from place to place β but they can grow in a direction in response to their environment. The chemicals that control this growth are called plant hormones (sometimes 'plant growth regulators').
The main plant hormones you need on the AQA spec:
- Auxin β the central one. Controls growth responses to light and gravity.
- Gibberellins β promote seed germination.
- Ethene (also written 'ethylene') β promotes fruit ripening and is involved in cell division.
Where is auxin made?
- In the tips of shoots and roots (the meristem regions where cells are actively dividing).
What does auxin do?
- In shoots: auxin causes cells to elongate (get longer). High auxin = more elongation = faster growth on that side.
- In roots: the OPPOSITE β auxin INHIBITS cell elongation. High auxin = slower growth on that side.
This contrast is important because it explains why shoots and roots bend in opposite directions in response to the same stimulus.
Plant hormones coordinate growth in response to stimuli.
Auxin is the main one β made in shoot and root tips.
Auxin promotes elongation in shoots, inhibits it in roots.
Other hormones: gibberellins (germination), ethene (ripening).