Plant Hormones and Tropisms
Plant Hormones and Tropisms
Definition: Plant hormones are chemical messengers, active in tiny concentrations, that coordinate growth and development throughout a plant, and tropisms are the directional growth responses, toward or away from a stimulus, that many of these hormones control.
How It Works
- Unlike animals, plants have no nervous system. Growth and response instead depend entirely on hormones produced in one part of the plant that travel to and act on another.
- Auxin, produced mainly at shoot tips, promotes cell elongation and is the primary driver behind phototropism (growth toward light) and gravitropism (growth relative to gravity).
- In phototropism, light causes auxin to shift toward the shaded side of a stem. Since auxin promotes elongation, cells on the shaded side grow longer than those on the lit side, physically bending the stem toward the light.
- In gravitropism, a root’s tip senses gravity and redistributes auxin to the lower side, but roots respond to extra auxin by slowing growth there, curving the root downward (positive gravitropism). Shoots do the opposite, growing away from gravity (negative gravitropism).
- Gibberellins promote stem elongation, seed germination, and flowering, discovered originally in a fungus that caused rice seedlings to grow abnormally tall and spindly.
- Cytokinins promote cell division and work opposite auxin in many contexts. Together the ratio of auxin to cytokinin controls whether a plant tissue grows roots, shoots, or stays undifferentiated, a balance exploited constantly in plant tissue culture.
- Ethylene, unusually a gas, triggers fruit ripening and leaf drop (abscission), which is why one ripening fruit can trigger nearby fruit to ripen faster (the “one bad apple” effect).
- Abscisic acid (ABA) generally suppresses growth, enforces seed and bud dormancy, and triggers stomata to close under drought stress, effectively the plant’s main stress-response hormone.
- Thigmotropism is growth in response to touch, best seen in climbing vine tendrils that coil tightly around anything they brush against.
- Nastic movements (like a Venus flytrap snapping shut, or a Mimosa plant folding its leaves when touched) look similar to tropisms but are not directional, they are a fixed response regardless of which direction the stimulus came from.
Illustration
Under the Hood
The classic experiment that first revealed auxin’s role, run by Charles and Francis Darwin and later refined by Frits Went in the 1920s:
Intact seedling tip, one-sided light → bends toward light
Tip covered with an opaque cap → no bending (tip must sense the light)
Tip removed entirely → no bending (tip must be the source of "something")
Tip removed, agar block (that had touched a tip) placed off-center on the stump → stump bends on its own, even in the dark- That final step proved a diffusible chemical, later identified and named auxin, was responsible, not any structural or nervous signal.
History
- Charles Darwin and his son Francis published early experiments on phototropism in “The Power of Movement in Plants” (1880), correctly inferring a signal traveled from the tip down the stem.
- Frits Went isolated the growth-promoting substance in 1926 using his now-classic agar-block experiment, and named it auxin shortly after.
- Gibberellins were identified by Japanese scientists studying “bakanae” (foolish seedling) disease in rice in the 1930s, though the work only became widely known outside Japan after World War II.
- Ethylene’s role as a plant hormone was recognized as early as the 1900s-1930s, after gas leaks from streetlamps were noticed to cause nearby trees to drop their leaves prematurely.
Why It Matters
- Synthetic auxins are widely used commercially: as rooting hormones to propagate cuttings, and, at much higher concentrations, as selective herbicides that overstimulate and kill broadleaf weeds.
- Controlling ethylene exposure is central to the produce industry: bananas and other fruit are often shipped unripe and ripened on demand with controlled ethylene gas exposure.
- Gibberellin treatments are used commercially to produce seedless grapes with larger fruit size.
- Understanding ABA’s role in stress response is a major target in breeding drought-resistant crop varieties for regions facing water scarcity.
- Tissue culture and micropropagation in horticulture rely directly on manipulating the auxin-to-cytokinin ratio to coax a small piece of plant tissue into growing roots, shoots, or staying as an undifferentiated mass.
Common Pitfalls
- Assuming a plant “chooses” to grow toward light the way an animal decides to move. It is an automatic, hormone-driven cell-elongation response, with no decision-making involved.
- Confusing tropisms (directional, stimulus-oriented) with nastic movements (non-directional, same response regardless of stimulus direction), like a Venus flytrap’s snap.
- Thinking one hormone does one job in isolation. Most plant hormones interact, and it is usually the ratio between two or more hormones, not the absolute level of just one, that determines the outcome.
- Believing gravitropism only matters for a seed planted the “wrong way.” It continuously corrects growth direction throughout a plant’s life, including after a storm knocks a plant over.
Comparison
| Hormone | Main Effect | Common Application |
|---|---|---|
| Auxin | Cell elongation, tropisms | Rooting powder, herbicides |
| Gibberellin | Stem elongation, germination | Seedless grape production |
| Cytokinin | Cell division | Tissue culture |
| Ethylene | Fruit ripening, leaf drop | Controlled fruit ripening |
| Abscisic acid | Dormancy, stress response | Drought-tolerance breeding |
More Tropisms and Movements
- Hydrotropism: roots bend and grow preferentially toward moisture in soil, a response that can override gravitropism when water is scarce in one direction but not another.
- Chemotropism: growth directed by a chemical gradient, most notably a pollen tube growing down the style toward chemical signals released by an ovule.
- Skototropism: some vine seedlings grow toward shade or darkness rather than light, a strategy for finding a solid trunk or structure to climb before redirecting growth toward the light once support is found.
- Seismonastic movement: the Mimosa pudica (“sensitive plant”) rapidly folds its leaflets when touched, driven by a sudden loss of water pressure in specialized cells at the leaf base, not by growth at all, which is what separates a nastic response from a true tropism.
FAQ
Why do houseplants on a windowsill lean toward the window? Phototropism: auxin builds up on the shaded side facing into the room, elongating those cells more and bending the stem toward the brighter window side, exactly the mechanism Darwin first investigated.
Can a plant have both positive and negative tropism to the same stimulus? Yes. Gravitropism is the clearest example: roots grow toward gravity (positive) while shoots grow away from it (negative), both driven by the same hormone responding oppositely in the two tissue types.
Example
A potted plant rotated 90 degrees will, within days, begin curving its stem back toward vertical growth and its roots back toward straight-down growth, entirely through hormone redistribution, with no repositioning of the pot required.
Related Terms
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