--- title: "Plant Hormone Signalling and How a Plant Decides" description: "Plant hormones help coordinate growth, development and responses to changing conditions. Their effects depend on concentration, the responding tissue and interactions with other signals. Auxin and cyt" canonical: https://lightmysky.com/learn/science/plant-hormone-signalling-and-how-a-plant-decides-mt_Zg2xpsb-p_ source: https://lightmysky.com/learn/science/plant-hormone-signalling-and-how-a-plant-decides-mt_Zg2xpsb-p_.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Plant Hormone Signalling and How a Plant Decides Plant hormones help coordinate growth, development and responses to changing conditions. Their effects depend on concentration, the responding tissue and interactions with other signals. Auxin and cytokinin balance can influence organ formation in tissue culture. Receptor-mediated signalling can change gene expression. Subject: Science · Area: Organisms & Life Processes · Ages 19 to 20 Page: https://lightmysky.com/learn/science/plant-hormone-signalling-and-how-a-plant-decides-mt_Zg2xpsb-p_ ## Ready when they can - Name five traditionally studied plant hormone classes and describe an effect of each - Explain how auxin and cytokinin balance can influence tissue-culture responses, including the limits of a ratio-only prediction - Describe how receptor-mediated hormone signalling can change transcription-factor activity and target-gene transcription ## Lesson: How plants decide without nerves You have met auxin in tropisms. Plants lack nerves, but also use electrical and hydraulic signals. In a young shoot lit from one side, extra auxin on the shaded side promotes elongation, bending the shoot toward light. In a horizontal root, extra auxin below inhibits elongation, so the root bends down. Tropisms are growth movements, without locomotion. Five traditionally studied classes are auxins, cytokinins, gibberellins, abscisic acid (ABA), and ethylene. Gibberellic acid is a gibberellin. Cytokinins promote cell division. Gibberellins promote stem elongation and can favour germination, opposing ABA's dormancy effects. Seeds need water and suitable conditions; water alone may not break dormancy. ABA promotes stomatal closure during drought. Ethylene is a gas that promotes ripening in some fruits and can promote leaf senescence. Hormone effects depend on concentration, tissue and other signals. In many responsive tissue cultures, more auxin relative to cytokinin favours roots; more cytokinin relative to auxin favours shoots. This tendency is not universal. Absolute doses, species and culture conditions also matter. Equal ratios at different doses need not give equal responses. Auxin from the shoot tip travels down the stem and helps limit side-bud growth. This apical dominance involves other signals, including cytokinins and strigolactones. Auxin acts indirectly on buds; one hormone ratio does not explain the whole process. Removing the tip can release side buds to grow. Hormones act locally or reach other tissues, where receptors detect them. Binding changes intracellular signalling proteins. For gene-expression responses, this alters transcription-factor activity and target-gene transcription in the nucleus. The hormone need not enter the nucleus. Other responses are faster: ABA can change ion-channel activity during stomatal closure. *(drawing: This sequence shows a gene-expression response. Other hormone responses act through proteins already present.)* Hormone arrival does not guarantee a response. Cells need a working receptor and downstream pathway. Two tissues can respond differently to the same hormone concentration. If only one responds, receptor or pathway differences are possible explanations. Further tests are needed to identify the cause. **Recap.** Hormone responses depend on concentration, tissue and interacting signals. Receptors connect hormone detection to cellular responses, including changes in gene transcription. *Lesson, questions and flashcards written for this dataset. OpenStax plant sensory systems chapters were consulted for background facts.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Plant growth responses: tropisms and the hormones behind them](https://lightmysky.com/learn/science/plant-growth-responses-tropisms-and-the-hormones-behind-them-mt_q1N7OG2_6j) ## Optional connections - [G-Protein-Coupled Receptors and Second Messengers](https://lightmysky.com/learn/science/g-protein-coupled-receptors-and-second-messengers-mt_7ozcs_sUOJ)