A Dictionary of Biology (Oxford University Press 2004):
“Growth substance” (phytohormone; plant hormone): Any of a number of organic chemicals that are synthesized by plants and regulate growth and development. They are usually made in a particular region, such as the shoot tip, and transported to other regions, where they take effect. See abscisic acid; auxin; cytokinin; ethylene; gibberellin.
Produced in the tips, or apical meristems’ of shoots and roots (‘coleoptiles’)
- Increase the ‘plasticity’ of cell walls
- Redistribution in the phloem away from light causes phototropism
- Promote growth by cell elongation in shoots while having the opposite effect in roots
- Apical dominance: By inhibiting the development of lateral bud, auxins promote tall, narrow growth preventing ‘bushiness’.
- Formation of lateral roots: Primary growth is inhibited in roots by auxins while the production of adventitious (lateral) roots is promoted
- Abscission: Probably by causing the secretion of ethene, auxin promotes the later stages of leaf and fruit fall
- Fruit development: Produced in the developing seed, auxins stimulate the development of fruits
GA is produced in the tips of leaves and also in roots, buds and seeds.
- breaks seed dormancy by promoting germination
- stimulates the secretion of hydrolytic enzymes in the seed which causes the liberation of stored food molecules
- stimulates fruit development
- causes stem elongation (by increasing internodal distance)
- can mimic the effects of long days
Remember: cytokinesis = cell division
Produced in the meristems, cytokinins promote cell division.
- They promote growth of lateral buds
- They delay senescence
- Help break seed dormancy
Found throughout the plant.
- promotes the abscission of leaves, fruits and flowers
- promotes seed dormancy
- causes stomatal closure during drought stress
- promotes senescence in fruit
The only gaseous hormone, it is moved by diffusion around the plant rather than translocation, Ethene is produced throughout the plant
- Stimulates the final stages of fruit development = ripening
- It also stimulates fruit and flower fall.
Other identified plant growth regulators include:
- Brassinosteroids, are a class of polyhydroxysteroids, a group of plant growth regulators. Brassinosteroids have been recognized as a sixth class of plant hormones which stimulate cell elongation and division, gravitropism, resistance to stress and xylem differentiation. They inhibit root growth and leaf abscission. Brassinolide was the first identified brassinosteroid and was isolated from organic extracts of rapeseed (Brassica napus) pollen in 1970.
- Salicylic acid – activates genes in some plants that produce chemicals that aid in the defense against pathogenic invaders.
- Jasmonates – are produced from fatty acids and seem to promote the production of defense proteins that are used to fend off invading organisms. They are believed to also have a role in seed germination, and affect the storage of protein in seeds, and seem to affect root growth.
- Plant peptide hormones – encompasses all small secreted peptides that are involved in cell-to-cell signaling. These small peptide hormones play crucial roles in plant growth and development, including defense mechanisms, the control of cell division and expansion, and pollen self-incompatibility.
- Polyamines – are strongly basic molecules with low molecular weight that have been found in all organisms studied thus far. They are essential for plant growth and development and affect the process of mitosis and meiosis.
- Nitric oxide (NO) – serves as signal in hormonal and defense responses.
- Strigolactones, implicated in the inhibition of shoot branching.
- Karrikins, a group of plant growth regulators found in the smoke of burning plant material that have the ability to stimulate the germination of seeds