Plant Hormones: Auxin, Gibberellin, Cytokinin, Abscisic Acid and Ethylene

Plants have messages too

Why does a plant on a windowsill lean toward the glass? Why does one ripe banana make the others in the bunch ripen faster? Why does pinching the top off a basil plant make it bushier? Plants have no nervous system, but they do send chemical messages: plant hormones, also called plant growth regulators. Tiny amounts of these chemicals control almost every stage of a plant's life, from germination to leaf fall.

The five major plant hormones

1. Auxin (the main natural one is indole-3-acetic acid, IAA)

• Made in shoot tips and young leaves; moves down the stem.
• Makes cells elongate by loosening their cell walls, so the cell can stretch as it takes in water.
• Controls tropisms (bending toward light or away from gravity: Lesson 13).
• Causes apical dominance (below).
• Promotes root formation on cuttings (rooting powder is synthetic auxin) and fruit growth.
• High doses of synthetic auxins such as 2,4-D are used as herbicides: they make broad-leaved weeds (dicots like dandelions) grow uncontrollably and die, while lawn grasses (monocots) are much less affected.

2. Gibberellins (there are many forms)

• Made in young leaves, shoot tips and seeds.
• Promote stem elongation: dwarf varieties of peas are often short because they make little active gibberellin.
• Trigger seed germination (Lesson 10) and bolting (the sudden growth of a flower stalk in lettuce or cabbage).
• Used by growers to make seedless grapes larger.
• Discovered through "foolish seedling disease" in rice, where a fungus making gibberellin caused seedlings to grow so tall and spindly that they fell over.

3. Cytokinins

• Made mainly in roots, carried up in the xylem.
• Promote cell division ("cytokinesis" gives them their name).
• Encourage lateral buds to grow, working against auxin.
• Delay aging (senescence) of leaves, so cut greens stay green longer.
• In tissue culture, the ratio of cytokinin to auxin decides whether shoots or roots form (Lesson 11).

4. Abscisic acid (ABA)

• The stress hormone. Made in leaves (and roots) when water is short.
• Makes guard cells close the stomata to save water (Lesson 4).
• Keeps seeds and buds dormant through winter.

5. Ethylene

• A gas (C₂H₄) made by ripening fruit, aging flowers and stressed tissues.
• Triggers fruit ripening: softening, sweetening and colour change in fruits such as bananas, apples, tomatoes and avocados.
• Promotes abscission: the fall of leaves, flowers and fruit.
• Because it is a gas, it spreads from fruit to fruit: one ripe fruit speeds up ripening in its neighbours.

Apical dominance: auxin versus cytokinin

The terminal bud at the top of a shoot makes auxin, which flows down the stem and suppresses the growth of lateral buds below it. Cytokinin coming up from the roots does the opposite: it promotes lateral bud growth. Near the tip, auxin wins and side buds stay small; far from the tip, cytokinin has more effect, so lower branches are longer. This gives many conifers their cone shape.

If the tip is removed, the auxin supply drops and the lateral buds grow into branches. That's why gardeners pinch the tips of basil, coleus or chrysanthemums to make bushier plants, and why trimming a hedge makes it thicker.

Hormones in farming and food

• Rooting powder (auxin) for cuttings.
• Herbicides such as 2,4-D (synthetic auxin) for broad-leaved weeds in lawns and grain fields.
• Ripening rooms: bananas are shipped green and hard (so they don't bruise) and then ripened with ethylene gas near where they will be sold.
• Apple storage: apples are kept cold in rooms with low oxygen and high carbon dioxide, which slows their ripening and their own ethylene production, so Ontario apples picked in October can still be crisp in spring.
• Gibberellin sprays to enlarge seedless grapes and to help some crops grow taller or germinate evenly.
• Cut flowers last longer when ethylene is kept away from them (florists keep them away from ripening fruit).