Summary
Auxin, gibberellin, cytokinin, abscisic acid and ethylene regulate growth, development and responses.
In depth
Auxin: cell elongation, apical dominance. Gibberellins: stem growth, germination. Cytokinins: cell division, delay senescence. Abscisic acid: dormancy, stomatal closure. Ethylene: fruit ripening, leaf abscission.
Hormone effects depend on concentration, tissue sensitivity and interactions. Auxin:cytokinin ratios control root versus shoot formation in culture. Hormones are used in agriculture as herbicides (2,4-D), fruit ripening agents and rooting powders.
Remember: plant hormones are not produced in specialised glands.
Plant hormones act at low concentrations and often in combination. Ethylene is a gas controlling fruit ripening, leaf abscission and the triple response in seedlings; brassinosteroids promote growth; jasmonates mediate defence; strigolactones regulate branching and attract mycorrhizal fungi; salicylic acid signals against pathogens. Hormones are used commercially: rooting powders (auxin), seedless grapes (gibberellin), ripening of bananas (ethylene) and herbicides such as 2,4-D (synthetic auxin).
The same hormone can cause different effects depending on tissue, concentration and partners. Darwin's work on coleoptiles and Went's agar block experiments in 1928 led to the identification of auxin. Signalling uses receptors and often ubiquitin-mediated protein degradation.
Key idea: ratios of hormones, such as auxin to cytokinin, direct development.
Frequently asked questions
What is Plant Hormones?
Auxin, gibberellin, cytokinin, abscisic acid and ethylene regulate growth, development and responses.
Which area of biology covers Plant Hormones, and how advanced is it?
Plant Hormones is one of the 56 topics in Plant Biology. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Auxin and Phototropism, Apical Dominance, Plant Defence Responses.
Can you give an example of Plant Hormones?
Rooting powder contains auxin to encourage roots on cuttings.
What should I learn before Plant Hormones?
Work through the Beginner topics in Plant Biology first, such as Plant Kingdom, Photosynthesis, Chlorophyll, then return to Plant Hormones. Intermediate topics assume the Beginner vocabulary.
What topics are related to Plant Hormones?
The closest topics to Plant Hormones are Auxin and Phototropism, Apical Dominance, Plant Defence Responses, Mycorrhizal Networks. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Plant Hormones?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Plant Hormones. To practise the whole of Plant Biology, use the Plant Biology quiz.
How is Plant Hormones different from Auxin and Phototropism?
Plant Hormones: Auxin, gibberellin, cytokinin, abscisic acid and ethylene regulate growth, development and responses. Auxin and Phototropism: Auxin moves to the shaded side of a shoot, causing cells there to elongate so it bends toward light. Read both side by side and note what each one explains.
How should I study Plant Hormones step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Auxin and Phototropism and explain the difference aloud. Finally, take the Plant Biology quiz and come back to this page for any question you miss.