Plant Hormones: Auxins, Gibberellins, and Ethylene

Plant hormones (plant growth regulators) are chemical messengers produced in small amounts that coordinate growth, development and responses to the environment; the five classical groups are auxins, gibberellins, cytokinins, abscisic acid and ethylene. In brief: auxins promote cell elongation and are responsible for phototropism, gibberellins promote stem elongation and seed germination, cytokinins promote cell division, abscisic acid (ABA) causes dormancy and stomatal closure, and ethylene, a gas, ripens fruit and causes leaf fall. Hormones often act in balance, and the same hormone can have different effects at different concentrations or in different tissues.

Unlike animals, plants have no nervous system and no glands. They depend on hormones and on chemical gradients. This guide describes each major hormone, its effects, and its practical uses, explains how the hormones interact, compares them in a table, works through the example of the auxin-cytokinin balance in tissue culture and corrects common mistakes.

What are plant hormones?

A plant hormone is an organic compound that is made in one part of the plant and, at very low concentrations (micromolar or less), changes the growth or the activity of target cells, which may be in the same place or in another. They bind to specific receptors on or in the target cells and trigger changes in gene expression or in enzyme activity. They are not made in specialized glands, and in many cases the hormone acts in the tissue where it is made. Besides the five classical groups, other signaling molecules include brassinosteroids, jasmonates, salicylic acid and strigolactones.

What do auxins do?

The main natural auxin is indole-3-acetic acid (IAA). It is produced in shoot tips and young leaves and moves downward through the plant in a one-way manner, by active transport carried out by protein carriers in the cell membranes (polar transport).

• Cell elongation: auxin acidifies the cell wall (by promoting the pumping of H⁺ out of the cell), which loosens cellulose fibers and allows the cell to elongate as water enters. This is the acid growth hypothesis.
• Phototropism and gravitropism: uneven distribution of auxin causes bending.
• Apical dominance: auxin from the apical bud inhibits the growth of side buds. Cutting off the tip (pruning) removes this inhibition and makes the plant bushier.
• Root formation: auxin stimulates the development of roots on cuttings (rooting powder contains synthetic auxins such as IBA).
• Fruit development: auxin from developing seeds stimulates the growth of the fruit, and prevents fruit drop.
• Herbicides: synthetic auxins such as 2,4-D act as selective weedkillers on broad-leaved plants, since high concentrations cause uncontrolled growth and death.

What do gibberellins do?

Gibberellins (GAs) are a large family of compounds, with more than 100 known. They were first isolated from a fungus that causes “foolish seedling” disease in rice, in which seedlings grow abnormally tall.

• Stem elongation: they promote the elongation of stems, particularly in dwarf plants. Dwarf varieties of pea and wheat have a defect in gibberellin synthesis or response. The “Green Revolution” dwarf wheat varieties had altered gibberellin response.
• Seed germination: in cereal seeds, the embryo releases gibberellin when the seed takes up water. This signals the aleurone layer to make amylase, which breaks down starch in the endosperm into sugars to feed the embryo. Brewers use this in malting.
• Breaking dormancy and flowering: in some species, gibberellins end seed dormancy or induce flowering.
• Fruit size: sprayed on seedless grapes to make them larger.

What do cytokinins do?

Cytokinins, such as zeatin, are made mainly in the roots and are transported in the xylem. They stimulate cell division (cytokinesis) in the presence of auxin. They also delay leaf senescence (aging), which is why cut herbs stay green longer when sprayed with them, and they counteract apical dominance by promoting the growth of side buds. The relative amounts of auxin and cytokinin direct development in tissue culture.

What does abscisic acid do?

Abscisic acid (ABA) is often called the stress hormone. It is made in roots and leaves, particularly during water stress.

• Stomatal closure: under drought, ABA causes guard cells to lose K⁺, so the stomata close and water is saved.
• Seed dormancy: ABA maintains dormancy, and prevents seeds from germinating at the wrong time, such as in the fall. Dormancy ends when ABA falls or gibberellin rises, for example after a cold period.
• Bud dormancy and stress tolerance: it prepares buds for winter and helps cells to survive drought and cold.

ABA and gibberellin often have opposite effects on dormancy, and the ratio between them determines whether a seed germinates.

What does ethylene do?

Ethylene (C₂H₄) is a simple gas that is made by most tissues, particularly ripening fruit, aging leaves and wounded tissue, and it diffuses through the air and between plants.

• Fruit ripening: ethylene triggers the breakdown of cell walls (softening), the conversion of starch to sugars, the loss of green color and the development of aroma. It is autocatalytic: ethylene stimulates more ethylene production, so ripening spreads quickly. A ripe banana in a bag with unripe fruit speeds their ripening. Fruit is transported unripe and ripened with ethylene gas at the destination. Storing fruit in a cold, low-oxygen atmosphere slows ripening.
• Leaf and fruit abscission: ethylene stimulates the formation of the abscission layer, so leaves fall in autumn and fruit drops.
• Stress responses: the “triple response” of seedlings (a short, thick stem and horizontal growth) when they meet an obstacle in the soil.

Plant hormones compared

Worked example: hormone balance in tissue culture

In plant tissue culture, pieces of tissue are grown on a sterile medium containing auxin and cytokinin. In classic experiments with tobacco tissue, Folke Skoog and Carlos Miller showed that the ratio of the two hormones controls development. Interpret the following results for a medium with the same total amount of hormones.

1. High cytokinin relative to auxin (ratio 0.1:1): cell division and shoot development are favored; shoots form from the tissue.
2. Equal amounts (1:1): cells divide, but do not specialize, so a mass of undifferentiated cells (callus) forms.
3. High auxin relative to cytokinin (10:1): roots develop.

The results show that hormones act by their ratio. Growers use this to produce thousands of identical plants (clones) from a single piece of tissue (micropropagation), as with orchids, bananas and potatoes. The principle is the same as in apical dominance, in which auxin from the shoot tip suppresses the side buds and cytokinin from the roots promotes them, and the balance between the two influences the shape of the plant.

How were plant hormones discovered?

Charles Darwin and his son Francis observed in 1880 that the tip of a grass seedling senses light. Frits Went isolated the growth-promoting substance in 1928 by placing agar blocks under severed oat coleoptile tips and then onto decapitated seedlings, and called it auxin. Eiichi Kurosawa discovered gibberellins in 1926 in studies of rice disease, and Folke Skoog and Carlos Miller discovered cytokinins in 1955. Richard Gane showed in 1934 that plants produce ethylene, and ABA was identified in the early 1960s.

Which mistakes are common?

Mistake 1: Saying that hormones have one fixed effect

An effect depends on the concentration, the tissue and the other hormones present.

Mistake 2: Assuming plant hormones are made by glands

Plants have no endocrine glands. Hormones are made in various tissues.

Mistake 3: Mixing up gibberellin and auxin

Both promote growth, but auxin mainly causes cell elongation in bending responses, while gibberellin mainly promotes stem elongation and germination.

Mistake 4: Forgetting that ethylene is a gas

It spreads through air, which is why one rotting fruit can affect others stored nearby.