Plants can’t move, but they respond to their environment with remarkable precision — bending toward light, growing taller, dropping leaves in autumn, ripening fruit in a coordinated wave. All of this is orchestrated by plant hormones (also called phytohormones): small chemical messengers produced in tiny amounts that regulate plant growth, development, and responses.
The five major plant hormones — auxins, gibberellins, cytokinins, abscisic acid (ABA), and ethylene — each have distinct roles. Understanding them means understanding not just botany, but food production, agriculture, and plant physiology at the molecular level.
Key Terms & Definitions
Phytohormone / Plant hormone: A chemical substance produced in small quantities in one part of a plant that regulates growth or other processes in another part of the plant.
Tropic movement: Growth movement in a specific direction in response to a directional stimulus (e.g., phototropism — toward light).
Apical dominance: The suppression of lateral bud growth by the actively growing shoot tip.
Senescence: The process of aging and eventual deterioration of plant cells/organs.
Abscission: The shedding of leaves, fruits, or flowers from the plant body.
Vernalisation: The process by which prolonged cold exposure promotes flowering.
The Five Major Plant Hormones
1. Auxins (IAA — Indole-3-acetic acid)
Produced in: Shoot apical meristem, young leaves, developing seeds.
Major effects:
- Cell elongation: Auxins stimulate cell wall loosening, allowing cells to take in more water and expand. This is how plants grow.
- Phototropism: Light causes uneven auxin distribution — more auxin accumulates on the shaded side, causing cells there to elongate more → shoot bends toward light.
- Geotropism: Gravity causes auxin to accumulate on the lower side of horizontal roots and shoots. In roots (which are highly sensitive), high auxin inhibits growth → root curves downward. In shoots (less sensitive), high auxin stimulates growth → shoot curves upward.
- Apical dominance: High auxin from the shoot tip inhibits lateral bud growth. Removing the tip (pruning) stimulates bushier growth.
- Root initiation: Auxins promote adventitious root formation — used in vegetative propagation (rooting powder contains synthetic auxins like IBA).
- Parthenocarpy: Application of auxins can cause fruit development without fertilisation (seedless fruits).
Synthetic auxins: 2,4-D (2,4-dichlorophenoxyacetic acid) is a selective herbicide that kills dicot weeds in cereal crops.
Auxin activates proton pumps → cell wall acidified → cell wall loosens → cell absorbs water → cell expands
This “acid growth hypothesis” explains how auxins cause rapid elongation.
2. Gibberellins (GAs)
Produced in: Meristems, developing seeds, fruits.
Major effects:
- Stem elongation: Gibberellins promote internodal elongation — they “stretch” the stem between nodes. Dwarf plants often have mutations in gibberellin synthesis or response.
- Breaking seed dormancy: Gibberellins promote germination of seeds that require cold or light treatment (overcome dormancy).
- Bolting: Application of GAs causes rapid elongation of shoot in rosette plants before flowering.
- Fruit development: GA can promote fruit growth and delay ripening.
- Malting: GA from germinating barley seeds activates amylase → starch digestion → important in brewing industry.
Historical context: Gibberellins were discovered from “bakanae” (foolish seedling) disease in rice — infected plants grew abnormally tall. The causative fungus Gibberella fujikuroi produces gibberellins.
3. Cytokinins
Produced in: Roots, actively dividing tissues, developing fruits and seeds.
Major effects:
- Cell division (cytokinesis): Cytokinins promote cell division — hence the name. They work with auxins; the auxin:cytokinin ratio determines differentiation.
- Delays senescence: Cytokinins delay aging of leaves. Cut flowers stay fresh longer if treated with cytokinins. “Richmond-Lang effect” — cytokinin-treated leaves stay green longer.
- Breaking apical dominance: Cytokinins from roots counter auxin from shoot tip → promote lateral bud growth.
- Nutrient mobilisation: Cytokinins attract nutrients toward the application site — useful in wound healing.
Auxin:Cytokinin ratio in tissue culture:
- High auxin, low cytokinin → root formation
- Low auxin, high cytokinin → shoot formation
- Equal ratio → callus (undifferentiated cells)
This principle is the foundation of plant tissue culture and micropropagation.
4. Abscisic Acid (ABA) — The Stress Hormone
Produced in: Leaves, stems, roots (especially under stress).
Major effects (most are inhibitory):
- Stomatal closure: ABA causes guard cells to lose water → stomata close → reduces water loss during drought. This is the fastest, most important ABA response.
- Seed dormancy: ABA promotes seed dormancy — seeds don’t germinate at unfavourable times. Germination occurs when ABA breaks down.
- Fruit abscission: ABA promotes leaf and fruit drop in autumn.
- Stress responses: ABA levels rise dramatically under water stress, salt stress, cold — “stress hormone.”
- Inhibits seed germination: Opposes gibberellins.
ABA is the ONLY primarily inhibitory plant hormone among the five major ones. All others (auxins, gibberellins, cytokinins, ethylene) have primarily stimulatory roles in their key functions, although each has some inhibitory effects in specific contexts.
5. Ethylene — The Gaseous Hormone
Produced in: Ripening fruits, aging tissues, damaged or stressed tissues.
Unique feature: Ethylene is the ONLY gaseous plant hormone. It diffuses through the air and can affect neighbouring plants.
Major effects:
- Fruit ripening: Ethylene triggers the climacteric rise — rapid increase in respiration during fruit ripening. Applying ethylene (or its precursor) accelerates ripening. Storing fruits with ethylene-absorbing materials (KMnO₄) delays ripening.
- Senescence and abscission: Promotes leaf drop, petal drop.
- Breaking seed dormancy: Some seeds.
- Epinasty: Downward bending of leaves.
- Inhibits stem elongation: Ethylene has “triple response” in seedlings: inhibit elongation, increase radial expansion, cause horizontal growth (detected by feel).
Agricultural applications:
- Ripen bananas during transport (ethylene generators used in ripening chambers)
- Synchronise flowering in pineapple fields (using ethephon, which releases ethylene)
- Defoliation in cotton before mechanical harvesting
Solved Examples
Easy — CBSE Level
Q: Why do plants bend toward light?
This is phototropism, mediated by auxins. Light causes lateral transport of auxin away from the light side. More auxin accumulates on the shaded side → cells on shaded side elongate more → plant bends toward the light source.
Medium — NEET Level
Q: A student applies gibberellin to a dwarf pea plant. What happens and why?
The dwarf pea plant grows taller. Dwarf plants often have defective gibberellin synthesis (like Le gene mutants in peas, which Mendel studied!). Applying exogenous GA restores normal internodal elongation → plant reaches normal height. This is why GA is used to identify whether dwarfism is GA-related.
Hard — NEET Advanced Level
Q: How does the auxin:cytokinin ratio determine plant organ differentiation in tissue culture?
In Skoog and Miller’s experiments (1957), the ratio of auxin to cytokinin in tissue culture medium determines what the callus differentiates into:
- High auxin/cytokinin → roots formed (auxin promotes root initiation)
- High cytokinin/auxin → shoots formed (cytokinin promotes shoot development)
- Balanced ratio → callus continues to grow undifferentiated
This is exploited in micropropagation: by controlling hormone ratios, we can produce millions of identical plants from a small explant.
Exam-Specific Tips
NEET: Plant hormones typically contribute 2–3 questions per year. Know the specific effects: ABA closes stomata (not opens), ethylene is gaseous, cytokinin delays senescence, gibberellin breaks dormancy. The 5-mark “describe plant hormones” question in CBSE Class 12 requires effects AND examples for each hormone.
CBSE Class 10: At Class 10 level, only auxins (phototropism, geotropism) are taught in detail. Class 12 introduces all five hormones. The “discovery of gibberellin from foolish seedling disease” is a commonly tested historical context.
Common Mistakes to Avoid
Mistake 1: Writing that auxins always inhibit growth. Auxins STIMULATE elongation at optimal concentrations in shoots. They INHIBIT growth in roots at the same concentrations (roots are more sensitive — supra-optimal auxin inhibits). This concentration-dependent, tissue-dependent action confuses many students.
Mistake 2: Confusing ABA with a hormone that opens stomata. ABA closes stomata. This is its most tested function. Stomata opening is not primarily hormone-mediated — it depends on K⁺ ions and light.
Mistake 3: Saying ethylene “prevents ripening.” Ethylene promotes ripening. To delay ripening (for storage), you remove ethylene or block its action (e.g., 1-MCP, which blocks ethylene receptors, is used commercially to keep apples fresh).
Mistake 4: Thinking cytokinins are produced only in leaves. They’re produced mainly in roots and transported upward. This is why root health directly influences leaf senescence — a healthy root system continuously provides cytokinins that keep leaves green.
Mistake 5: Confusing “weed killer” (2,4-D) with the hormone it mimics. 2,4-D is a synthetic auxin that kills dicot weeds by causing uncontrolled growth. It doesn’t harm monocots (cereals) because they metabolise it differently.
Practice Questions
Q1. Which hormone is responsible for fruit ripening and why is it called a gaseous hormone?
Ethylene is responsible for fruit ripening. It’s called a gaseous hormone because it exists as a gas at physiological temperatures and concentrations, and can diffuse through the air. This is why keeping a ripe apple with unripe bananas accelerates banana ripening — ethylene from the apple acts on the bananas.
Q2. What would happen if you applied high concentrations of auxin to roots?
Roots are much more sensitive to auxin than shoots. At concentrations that stimulate shoot growth, auxin inhibits root growth. High concentrations would inhibit root elongation. This is why herbicides (like 2,4-D) that mimic auxin kill plants — they cause uncontrolled, chaotic growth and disrupt the normal auxin balance.
Q3. Explain why pruning a plant makes it bushier.
The apical bud produces auxin, which inhibits growth of lateral buds (apical dominance). When you prune (remove the apical bud), auxin source is removed. Lateral buds are no longer suppressed and grow actively, producing multiple branches — the plant becomes bushier. Cytokinins from roots further promote lateral bud growth.
Q4. A plant wilts during drought. Which hormone helps it respond and how?
Abscisic acid (ABA) is the drought stress hormone. Under water stress, ABA levels rise sharply. ABA causes guard cells to lose K⁺ ions → water leaves guard cells by osmosis → stomata close. This reduces transpiration and conserves water. ABA also signals the plant to reduce overall growth and increase root growth to access deeper water.
Q5. Why are gibberellins important in the brewing industry?
Germinating barley seeds produce gibberellins that activate amylase enzymes. Amylase breaks down starch in the seed into sugars — “malting.” These sugars are then fermented by yeast into alcohol. In brewing, commercial GA is sometimes applied to accelerate and enhance this malting process.
Q6. What is parthenocarpy and which hormone induces it?
Parthenocarpy is fruit development without fertilisation — producing seedless fruits. Auxins (and sometimes gibberellins) can induce parthenocarpy when applied to unpollinated flowers. Examples: seedless tomatoes, seedless grapes. Commercially important for producing seedless varieties.
FAQs
Q: Do plants have a hormone system like animals?
Yes and no. Plants have chemical messengers (hormones) that regulate growth and development, but they lack the specialised endocrine glands and rapid nervous system that animals have. Plant hormones are produced by specific tissues and transported through xylem, phloem, or diffusion. The responses are slower but highly coordinated.
Q: What is the difference between a plant hormone and a synthetic plant growth regulator?
A plant hormone (phytohormone) is naturally produced by the plant. A synthetic plant growth regulator (PGR) mimics the action of a natural hormone but is made synthetically — e.g., 2,4-D (synthetic auxin), ethephon (releases ethylene), paclobutrazol (gibberellin inhibitor). Both natural and synthetic can regulate plant growth.
Q: Why does fruit ripen faster when stored near other ripe fruits?
Because ripe fruits release ethylene gas. Ethylene diffuses to adjacent unripe fruits and triggers their ripening enzymes. This is the “one rotten apple spoils the barrel” phenomenon — explained entirely by ethylene diffusion.
Q: Is abscisic acid always inhibitory?
Primarily, yes. ABA inhibits germination, growth, and promotes stomatal closure. But it also promotes seed storage protein synthesis and is essential for normal seed development. In seeds, ABA is part of normal developmental programming — it’s not always “stress-induced” in seeds.
Q: What controls auxin transport direction?
Auxin transport is polar — it moves in one direction (from shoot tip toward roots), not by simple diffusion. This polar auxin transport is mediated by specific carrier proteins: PIN proteins (efflux carriers) on the basal/lower side of cells pump auxin downward. Changing PIN protein location redistributes auxin, causing differential growth (as in phototropism and gravitropism).