Plant Hormones and Growth Regulation

Chemical messengers in plants

A plant hormone (or plant growth regulator) is a chemical made in small amounts in one part of a plant that changes growth or development, either in the same tissue or somewhere else. Unlike animals, plants have no glands: almost any living cell can make hormones, and they travel from cell to cell, in the xylem and phloem, or as a gas through the air. Each hormone has many effects, and the response depends on the tissue, the concentration and the other hormones present.

The five main hormone groups

Other signals, such as brassinosteroids, jasmonates, salicylic acid and strigolactones, are also important. Auxin’s role in bending towards light and gravity is explored in the Tropisms simulator.

Auxin and apical dominance

The growing shoot tip makes auxin, which is pumped cell by cell down the stem (polar transport). As it passes each node it keeps the side (axillary) buds dormant. This is apical dominance, and it is why many plants grow one main stem. Cut off the tip and the auxin supply stops: the buds below are released and grow into side shoots. In 1933 Kenneth Thimann and Folke Skoog showed that putting auxin on the cut stump keeps the buds dormant, so auxin really does stand in for the missing tip. Cytokinin from the roots works the other way, promoting bud growth: the balance between the two decides which buds grow.

Gibberellins

Gibberellins make stems grow taller, mainly by making cells elongate (and, in some tissues, divide). They were discovered in Japan through the “foolish seedling” disease, in which a fungus that makes gibberellin causes rice to grow tall and weak. Plants that make little gibberellin are dwarfs; Mendel’s short peas had a mutation in a gibberellin-making enzyme, and the “Green Revolution” semi-dwarf wheats respond less to gibberellin, so they put more into grain and less into straw. In a germinating cereal grain, the embryo releases gibberellin, which makes thealeurone layer produce amylase to break down stored starch into sugar for the seedling. TheSeed Germination simulator looks at the conditions a seed needs.

Cytokinins

Cytokinins, made mainly in root tips and carried up in the xylem, promote cell division together with auxin. They encourage buds and shoots to grow and slow the ageing of leaves, keeping them green. In 1957 Folke Skoog and Carlos Miller grew tobacco tissue on a jelly containing both hormones: a high auxin-to-cytokinin ratio made the tissue form roots, a low ratio made shoots, and a balance gave an undifferentiated lump called acallus. The same idea is used today to clone plants in the laboratory.

Abscisic acid and drought

When roots sense drying soil, plants make more abscisic acid in their roots and leaves. In the leaves, ABA makes the guard cells lose potassium ions and water, so they go limp and the stomata close, saving water within minutes. ABA also slows shoot growth during stress, and it keeps seeds and budsdormant until conditions are right. Gibberellin and ABA act against each other in seeds: the balance between them decides whether a seed stays dormant or germinates.

Ethylene and ripening

Ethylene (C₂H₄) is a simple gas. In fruits such as tomatoes, bananas and apples, ripening is triggered by a burst of ethylene, and ethylene makes the fruit produce even more ethylene. That positive feedback is why one ripe fruit can ripen others nearby, and why “one bad apple spoils the barrel”. Ethylene also promotes leaf and fruit fall (abscission) and ageing, and slows stem elongation. Blocking ethylene, for example with the gas 1-methylcyclopropene, keeps fruit fresh for longer in storage.

Hormones work together

• Auxin vs cytokinin: auxin from the tip keeps buds dormant, cytokinin from the roots releases them; their ratio also decides roots or shoots in tissue culture.
• Gibberellin vs ABA: gibberellin promotes germination and growth, ABA promotes dormancy and stress responses.
• Auxin and gibberellin work together to make stem cells elongate.
• Auxin triggers ethylene: very high auxin makes tissues produce ethylene, which then slows elongation.

Growth is regulated by this network of interactions, not by any single hormone acting alone.

Dose, tissue and timing

Hormones are not on/off switches. The same concentration of auxin that makes a stem grow faster strongly inhibits root elongation, because roots are far more sensitive: their best concentration is roughly a ten-thousandth of the stem’s. The response also depends on the age of the tissue and on the environment: ABA only rises during drought, and ripening needs a fruit that is mature enough to respond to ethylene.

Why it matters

• Seedless grapes are sprayed with gibberellin to make larger, looser bunches.
• Brewing: gibberellin speeds up the malting of barley, when starch is turned into sugar.
• Fruit supply: bananas are picked green, shipped, then ripened with ethylene; apples are stored with ethylene blockers.
• Pruning: pinching out shoot tips makes plants such as basil and fuchsias bushy.
• Micropropagation: orchids, bananas and potatoes are cloned in tissue culture using auxin and cytokinin.
• Crop yields: semi-dwarf cereals with altered gibberellin responses stand up better and produce more grain.

Common misconceptions

• “Each hormone has one job.” Every hormone affects many processes, and most processes are controlled by several hormones.
• “More hormone always means more growth.” Many responses have an optimum; too much auxin inhibits roots and triggers ethylene.
• “Plant hormones are made in glands.” Plants have no glands; many tissues make hormones.
• “Ethylene is a liquid sprayed on fruit.” Ethylene is a gas that fruit makes itself.
• “Cutting the tip kills the plant’s growth.” It removes apical dominance, so side shoots grow instead.

What the simulator simplifies

The plant is a generic, two-dimensional herb that grows two days every second, and each hormone is described by one number relative to normal. Responses follow simple saturating or optimum-shaped curves chosen to give realistic directions, not measured values. Real hormones are made, moved, stored and broken down in ways the model ignores, and hormones act through receptors and signalling chains inside cells. Drought is modelled only through ABA, cytokinin and reduced growth. The auxin chart shows the classic textbook curves, and the tissue culture follows the auxin-to-cytokinin rule of Skoog and Miller; real results vary between species.

Key takeaways

• Five main hormone groups (auxin, gibberellins, cytokinins, ABA and ethylene) coordinate plant growth and development.
• Auxin from the shoot tip causes apical dominance; cytokinin from the roots counteracts it.
• Gibberellins lengthen stems and mobilise food stores in seeds; ABA closes stomata and keeps seeds dormant.
• Ethylene is a gas that ripens fruit through positive feedback and promotes leaf fall.
• Responses depend on concentration, tissue, developmental stage, environment and the balance with other hormones.