A phytohormone (plant hormone) is a chemical substance, made in one part of a plant, that regulates growth and development. Unlike animal hormones, phytohormones are not made in special glands; they are produced in growing regions such as shoot tips, root tips and young leaves, and they act at very low concentrations.
Major groups
- Auxin — the best-known phytohormone; it promotes cell elongation in shoots and controls tropisms.
- Gibberellin — stimulates stem elongation, breaks seed and bud dormancy, and promotes germination and flowering.
- Cytokinin — promotes cell division and delays the ageing (senescence) of leaves.
- Ethylene — a gas that promotes the ripening of fruits and the falling of leaves and fruits (abscission).
- Abscisic acid (ABA) — an inhibitor; it maintains seed and bud dormancy and causes stomata to close during water stress.
How auxin causes tropisms
Auxin is made at the shoot tip and moves downwards. When light shines from one side, auxin becomes more concentrated on the shaded side of the shoot. The higher auxin concentration makes cells on the shaded side elongate faster, so the shoot bends towards the light — a positive phototropic response.
Key idea
In shoots, auxin promotes elongation, so more auxin means faster growth. In roots, a high auxin concentration inhibits elongation, which is why the same uneven distribution bends shoots and roots in opposite directions.
Example
A shoot laid on its side: auxin gathers on the lower side. In the shoot the lower side elongates faster, so the shoot bends upwards (negative geotropism). In the root the same high auxin on the lower side slows growth there, so the root bends downwards (positive geotropism).
Interaction of hormones
Plant growth is controlled by the balance between hormones rather than by one hormone alone. For example, the ratio of auxin to cytokinin decides whether a plant tissue forms roots or shoots, a principle used in tissue culture.
Remember
Auxin = elongation and tropisms; Gibberellin = germination and stem growth; Cytokinin = cell division; Ethylene = ripening; ABA = inhibition and stomatal closure.