Plant growth and development are controlled by plant hormones (phytohormones)
- There are 5 main plant hormones that coordinate plant growth and development
Auxins:
- Promote primary growth (lengthening) by promoting cell elongation and increasing the rate of cell division
- Promote apical dominance – whereby the apex / tip of a plant grows while the lateral buds remain undeveloped
- Auxin concentrations may change in response to directional stimuli (i.e. play a key role in tropisms)
Cytokinins:
- Promote cell division (cytokinesis) and ensure roots and shoots grow at equal rates
- Promotes secondary growth (thickening) and help to control the rate of branching by a plant
- Cytokinins are also involved in stimulating the growth of fruit
Gibberellins:
- Triggers germination in dormant seeds (initiates plant growth)
- Gibberellin also causes stem elongation by promoting cell elongation and cell division
Ethylene:
- A gas which acts as a plant hormone and stimulates maturation and ageing (senescence)
- It is responsible for the ripening of certain fruit (auxins and gibberellins promote fruit growth but inhibit ripening)
- It also contributes to the loss of leaves (abscission) and the death of flowers
Abscisic Acid:
- Abscisic acid (ABA) principally functions to inhibit plant growth and development
- It promotes the death of leaves (abscission) and is responsible for seed dormancy
- It generally initiates stress responses in plants (like winter dormancy in deciduous plants)
- Abscisic acid controls the closing of stomata and hence regulates water loss in plants
Summary of the Main Classes of Plant Hormones
