Plants cannot move from place to place, yet they respond to their environment with precision — growing toward light, pushing roots downward through soil, and timing their flowering to the season. These responses are controlled by chemical messengers called plant hormones, which travel through plant tissue to coordinate growth at the cellular level.
What are plant hormones and where are they made?
A plant hormone (or plant growth regulator) is a chemical produced in one part of a plant that moves to another part and affects its growth or development. Unlike animal hormones, plant hormones are not made by specific glands — they are synthesised in many different tissues.
The three main plant hormones at GCSE are:
| Hormone | Produced mainly in | Key roles |
|---|---|---|
| Auxin (IAA) | Shoot and root tips | Controls cell elongation; phototropism; gravitropism; apical dominance |
| Gibberellins | Young leaves, roots, seeds | Promotes stem elongation; seed germination; fruit development |
| Ethylene (ethene) | Ripening fruit; damaged tissue | Promotes fruit ripening; triggers leaf and fruit drop (abscission) |
What is phototropism and how do auxins cause it?
Phototropism is the growth of a plant part toward (positive) or away from (negative) a light source. Shoots are positively phototropic; roots are negatively phototropic.
Mechanism in shoots:
- Light comes from one side (e.g. a window).
- Auxin (IAA) produced at the shoot tip moves away from the light to the shaded side, accumulating there.
- Higher auxin concentration on the shaded side causes those cells to elongate more than cells on the lit side.
- The shoot curves toward the light because the shaded side grows faster.
The key point is that auxin promotes elongation in shoot cells. In root cells, the opposite is true — high auxin concentration inhibits elongation (roots are more sensitive to auxin).
What is gravitropism and how is it controlled?
Gravitropism (formerly called geotropism) is growth in response to gravity.
- Shoots are negatively gravitropic — they grow upward, away from gravity.
- Roots are positively gravitropic — they grow downward, toward gravity.
In a horizontally placed seedling, gravity causes auxin to accumulate on the lower side of both root and shoot:
- Lower side of shoot — higher auxin → cells elongate more → shoot curves upward (away from gravity). ✓
- Lower side of root — higher auxin → cells elongate less (roots are inhibited by high auxin) → root curves downward (toward gravity). ✓
This elegant mechanism uses the same hormone but produces opposite responses because root cells are far more sensitive to auxin than shoot cells are.
What are the commercial uses of plant hormones?
Plant hormones are extensively used in agriculture and horticulture:
- Rooting powder (auxin) — cuttings dipped in auxin powder develop roots faster, improving propagation success.
- Selective weedkillers (synthetic auxins) — compounds such as 2,4-D mimic auxin. At high concentrations they cause broadleaved plants to grow so rapidly they outpace their own resources and die, while narrow-leaved grasses are unaffected. This allows lawns and cereal crops to be treated without damaging the grass.
- Controlling fruit ripening (ethylene) — bananas are harvested unripe and transported in ethylene-free conditions; ethylene gas is then applied to ripen them uniformly before sale.
- Preventing early fruit drop (auxin sprays) — spraying orchards with auxin keeps fruit attached to trees until harvest.
- Seed germination (gibberellins) — gibberellins are used in the brewing industry to speed up the germination of barley seeds (malting), increasing maltose production.
What is apical dominance?
Apical dominance is the suppression of lateral (side) bud growth by the growing tip (apex) of the main shoot. The apex produces auxin, which moves down the stem. High auxin concentration near the apex inhibits the lateral buds. If the tip is removed (pruning), auxin levels fall, lateral buds grow, and the plant becomes bushier. Gardeners use this principle when they "pinch out" bedding plants to encourage branching.
How do gibberellins and ethylene differ from auxins?
Gibberellins promote stem elongation by stimulating cell division and elongation in internodal regions. Plants that lack gibberellins (dwarf varieties) can be made to grow tall by applying gibberellin solution. Gibberellins also break seed dormancy by stimulating amylase production to digest the seed's starch reserves.
Ethylene is unusual because it is a gas (not dissolved in water like auxin and gibberellins). It ripens fruit by stimulating the breakdown of cell walls and the conversion of starch to sugars. It also signals the formation of the abscission layer at the base of leaves and fruits, causing them to fall in autumn.
Frequently asked questions
Why does a plant on a windowsill lean toward the light?
Auxin produced at the shoot tip migrates away from the light, accumulating on the shaded side of the stem. Cells on the shaded side elongate more, causing the shoot to bend toward the light source. This is an adaptive response that maximises light interception for photosynthesis. If you rotate the pot regularly, the stem will grow straight because auxin is distributed evenly.
How does auxin have opposite effects in roots and shoots?
Auxin promotes cell elongation in shoots at moderate concentrations but inhibits elongation in roots at those same concentrations, because root cells are much more sensitive. This means that the same auxin concentration that makes shoot cells grow longer actually slows down root cell elongation. The result is that auxin accumulation on the lower side of a horizontal seedling makes the shoot curve up while simultaneously making the root curve down.
Are plant hormones the same as animal hormones?
Both plant hormones and animal hormones are chemical messengers that travel from the site of production to a target site and cause a response. However, plant hormones are typically small organic molecules produced in many tissues (not in dedicated glands), they move more slowly (often by diffusion through cells rather than through a bloodstream), and their effects are usually growth-related rather than regulatory of organ function. The underlying principle — chemical signalling over distance — is the same.
Why do bananas ripen faster when placed in a bag?
Bananas (and many other ripening fruits) release ethylene gas as they ripen. Placing them in a sealed bag traps the ethylene, increasing its concentration around the fruit. Higher ethylene concentration accelerates ripening in the same fruit and triggers ripening in any unripe fruit nearby. This is why placing an unripe avocado in a bag with a ripe banana speeds up softening.
For Socratic GCSE biology with Professor Darwin — tracing hormone signals from shoot tip to bending stem — visit aitutors.me.