1. Definition
Plant hormones, also called phytohormones, are naturally occurring organic signaling molecules produced in very small amounts that regulate growth, development, metabolism and responses to environmental stimuli.
Unlike nutrients, hormones act primarily as signals, often at concentrations far below those required for structural or nutritional functions.
The major classical plant hormones are:
Auxins
Gibberellins (GAs)
Cytokinins
Abscisic acid (ABA)
Ethylene
Other important plant signaling regulators include brassinosteroids, jasmonates, salicylic acid, strigolactones and peptide hormones.
2. How Do Plant Hormones Work?
Plant hormones are produced in particular tissues and can act:
At the site of synthesis
In nearby tissues
At distant tissues after transport
Their effects depend on:
Hormone concentration
Tissue/cell type
Developmental stage
Interaction with other hormones
Environmental conditions
Important concept
A single hormone can produce different effects in different tissues.
For example, auxin promotes cell elongation in shoots but can inhibit elongation in roots at sufficiently high concentrations.
3. Auxin 🌱
Main hormone
Auxin is strongly associated with cell elongation, apical dominance, tropic responses, root initiation and vascular development.
The principal naturally occurring auxin is:
IAA — Indole-3-acetic acid
Auxin is synthesized prominently in:
Shoot apical meristems
Young leaves
Developing seeds and fruits
Major Functions of Auxin
1. Cell elongation
Auxin promotes cell elongation, particularly in shoots, through mechanisms involving changes in cell-wall properties.
2. Phototropism
Shoots bend toward light because of differential auxin distribution.
Simplified model:
Light from one side → Auxin redistributes toward shaded side → Greater shoot elongation on shaded side → Shoot bends toward light
3. Apical dominance
The shoot apex can suppress growth of lateral buds, with auxin being an important component of this regulatory system.
4. Root initiation
Auxins can stimulate adventitious root formation, particularly at appropriate concentrations.
5. Fruit development
Auxin contributes to fruit set and development.
6. Abscission
Auxin interacts with ethylene and other signals to regulate leaf and fruit abscission.
4. Gibberellins (GA) 🌾
Gibberellins are a large family of plant hormones.
One important example is:
GA₃ — Gibberellic acid
They are involved in:
Stem elongation
Seed germination
Bolting
Flowering in some species
Fruit growth
Mobilization of stored reserves
Gibberellin and Seed Germination
This is especially important in cereal grains.
Simplified pathway:
Embryo → GA → Aleurone → Hydrolytic enzymes → Starch breakdown → Sugars → Embryo growth
One important enzyme induced in germinating cereals is:
α-Amylase
It breaks down starch into smaller carbohydrates.
5. Major Functions of Gibberellins
🌱 Stem elongation
Gibberellins can stimulate internode elongation.
🌾 Seed germination
They promote processes involved in reserve mobilization in many seeds.
🌼 Bolting
They can promote rapid stem elongation before flowering in rosette plants.
🍇 Fruit growth
Gibberellins are used commercially in some crops to modify fruit growth and characteristics.
🌸 Flowering
Gibberellins can promote flowering in some species under particular environmental conditions.
6. Cytokinins 🌿
Cytokinins are hormones particularly associated with cell division and shoot development.
A naturally occurring cytokinin is:
Zeatin
Cytokinins are produced in:
Root apical regions
Developing seeds
Young tissues
They are transported through the plant and interact strongly with auxin.
7. Functions of Cytokinins
1. Cell division
They promote cell-cycle progression in cooperation with other signals.
2. Shoot formation
The relative balance of auxin and cytokinin is important in determining organ formation in tissue culture.
3. Delay of leaf senescence
Cytokinins can delay some aspects of leaf senescence.
4. Nutrient mobilization
They can influence the movement and utilization of nutrients in developing tissues.
5. Apical dominance
Cytokinins generally promote lateral bud growth, counteracting aspects of apical dominance.
8. Auxin–Cytokinin Balance
This is extremely important in plant tissue culture.
A simplified textbook model is:
| Hormonal balance | Typical response |
|---|---|
| High auxin : low cytokinin | Root formation |
| Low auxin : high cytokinin | Shoot formation |
| Intermediate/balanced ratio | Callus formation |
However, actual responses depend on species, genotype, tissue type, hormone identity and culture conditions, so the ratio is not an absolute rule.
9. Abscisic Acid (ABA) 🍂
Abscisic acid (ABA) is a major plant hormone involved in:
Seed dormancy
Stomatal closure
Responses to drought and other stresses
Maturation of seeds
Regulation of growth
Despite its name, ABA is not simply a hormone that causes abscission.
10. ABA and Seed Dormancy
ABA promotes the maintenance of seed dormancy during appropriate developmental stages.
A simplified relationship is:
ABA ↑ → Dormancy maintained
GA ↑ → Germination-promoting processes
The balance between ABA and GA is particularly important in controlling the transition between dormancy and germination.
11. ABA and Stomatal Closure
During water stress, ABA accumulates in leaves and promotes stomatal closure.
This reduces water loss through transpiration.
Simplified pathway
Drought → ABA signaling → Ion efflux from guard cells → Water leaves guard cells → Guard cells lose turgor → Stomata close
12. Ethylene 🍎
Ethylene is a unique plant hormone because it is a gas.
Its molecular formula is:
C₂H₄
It is involved in:
Fruit ripening
Senescence
Abscission
Seedling responses
Responses to mechanical stress
Some flowering processes
13. Ethylene and Fruit Ripening
Ethylene is particularly important in climacteric fruits.
Examples
Banana
Mango
Apple
Tomato
Ethylene can promote:
Chlorophyll degradation
Fruit softening
Aroma development
Changes in sugar and acid metabolism
14. Ethylene and the Triple Response
Ethylene produces a characteristic triple response in young seedlings:
Reduced stem elongation
Increased radial swelling
Exaggerated horizontal growth/curvature
This response helps seedlings growing through soil or other mechanical obstacles.
15. Brassinosteroids 🌿
Brassinosteroids (BRs) are steroid hormones involved in:
Cell expansion
Cell division
Vascular development
Pollen development
Stress responses
Overall plant growth
They interact extensively with auxin, gibberellins and other hormonal pathways.
16. Jasmonates
Jasmonates, particularly jasmonic acid (JA) and its derivatives, are important signaling molecules involved in:
Herbivore defense
Wound responses
Responses to some pathogens
Reproductive development
Senescence
Simple concept
Herbivore/wounding → Jasmonate signaling → Defense responses
17. Salicylic Acid
Salicylic acid (SA) is particularly important in plant defense signaling.
It contributes to:
Defense against many biotrophic pathogens
Systemic acquired resistance
Regulation of defense-related genes
Simple concept
Pathogen recognition → SA signaling → Defense response
18. Strigolactones
Strigolactones are hormones/signaling molecules involved in:
Regulation of shoot branching
Root development
Responses to nutrient availability
Interactions with mycorrhizal fungi
They generally act as important regulators of shoot branching, often in interaction with auxin and cytokinin.
19. Comparison of Major Plant Hormones
| Hormone | Major functions | Easy keyword |
|---|---|---|
| Auxin | Elongation, phototropism, apical dominance, rooting | Elongation |
| Gibberellin | Stem elongation, germination, bolting | Growth |
| Cytokinin | Cell division, shoot growth, delayed senescence | Division |
| ABA | Dormancy, stress responses, stomatal closure | Stress/Dormancy |
| Ethylene | Ripening, senescence, abscission | Ripening |
| Brassinosteroids | Growth, cell expansion, vascular development | Expansion |
| Jasmonates | Wound/herbivore defense | Defense |
| Salicylic acid | Pathogen defense | Disease defense |
| Strigolactones | Shoot branching, nutrient signaling | Branching |
20. Plant Hormones in Seed Germination
Several hormones work together during germination.
Dormant seed
ABA activity → Dormancy
Germination-promoting conditions
GA signaling → Reserve mobilization + growth
The outcome depends on the balance and interaction of these pathways with environmental signals.
21. Plant Hormones in Fruit Development
Fruit development involves several hormones.
Before fertilization
Auxin and gibberellin signaling can contribute to ovary growth.
After fertilization
Developing seeds produce signals that influence surrounding fruit tissues.
Ripening
Ethylene is especially important in climacteric fruits.
Simplified sequence
Fertilization → Auxin/GA-related growth → Fruit development → Maturation → Ethylene-mediated ripening
22. Plant Hormones and Tropisms
Plant hormones help plants respond directionally to environmental stimuli.
Phototropism
Light → Auxin redistribution → Unequal growth → Bending toward light
Gravitropism
Auxin redistribution also contributes to differential growth responses to gravity.
Roots and shoots respond differently because their sensitivity to auxin differs.
23. Plant Hormones and Senescence
Senescence is the genetically regulated deterioration of tissues as they age.
Hormonal regulation involves several hormones.
Cytokinins
Generally delay aspects of senescence.
Ethylene
Promotes senescence in many tissues.
ABA
Can contribute to senescence and stress responses.
Thus, senescence is not controlled by a single hormone.
24. Hormonal Interaction
One of the most important concepts in modern plant physiology is:
Plant hormones rarely act alone.
For example:
Auxin + Cytokinin
Regulate organ formation and meristem activity.
ABA + GA
Regulate the dormancy–germination transition.
Auxin + Ethylene
Interact in root growth and abscission.
Jasmonate + Salicylic acid
Interact in plant immune responses.
Auxin + Strigolactone
Interact in regulation of shoot branching.
25. Natural Hormones vs Plant Growth Regulators
Plant hormones are naturally occurring signaling compounds.
Plant growth regulators (PGRs) is a broader term that includes natural hormones and synthetic compounds used to modify plant growth.
Examples of synthetic PGRs
2,4-D — synthetic auxin
NAA — synthetic auxin
GA₃ — gibberellin used commercially
Ethephon — releases ethylene
BAP — synthetic cytokinin commonly used in tissue culture
26. Agricultural Applications 🌾
Plant hormones and growth regulators have numerous agricultural uses.
Auxins
Used for:
Rooting of cuttings
Fruit set in certain crops
Selective weed control using synthetic auxins such as 2,4-D
Gibberellins
Used for:
Fruit growth
Increasing size of some fruits
Modifying flowering or bolting in certain crops
Cytokinins
Used in:
Tissue culture
Shoot multiplication
Ethylene-related regulators
Used for:
Fruit ripening
Fruit maturation management
Growth retardants
Some synthetic compounds suppress excessive vegetative growth and are used in horticulture.
27. ⭐ High-Yield Exam Table
| Question | Answer |
|---|---|
| Main natural auxin | IAA |
| Auxin commonly associated with | Cell elongation |
| Phototropism | Auxin redistribution |
| Apical dominance | Auxin |
| Major gibberellin example | GA₃ |
| Gibberellin in cereal germination | Induces hydrolytic enzyme production |
| Important starch-digesting enzyme | α-Amylase |
| Major cytokinin example | Zeatin |
| Cytokinin | Cell division |
| ABA | Dormancy and stress responses |
| ABA during drought | Promotes stomatal closure |
| Ethylene | Gaseous hormone |
| Formula of ethylene | C₂H₄ |
| Ethylene | Fruit ripening |
| Brassinosteroids | Growth and cell expansion |
| Jasmonates | Wound/herbivore defense |
| Salicylic acid | Pathogen defense |
| Strigolactones | Shoot branching regulation |
🧠 28. Easy Memory Trick
A G C A E
A — Auxin → Apical dominance
G — Gibberellin → Growth
C — Cytokinin → Cell division
A — ABA → Avoids germination / stress
E — Ethylene → Edible fruit ripening
For the newer signaling hormones:
B → Brassinosteroids → Body/plant growth
J → Jasmonate → Injury defense
S → Salicylic acid → Systemic defense
S → Strigolactone → Shoot branching
🌿 29. One-Minute Revision
PLANT HORMONES
│
┌──────────────────┼──────────────────┐
│ │ │
GROWTH STRESS RIPENING
│ │ │
┌───┼────┐ ABA ETHYLENE
│ │ │ │ │
Auxin GA Cytokinin Dormancy Fruit ripening
│ │ │ Stomatal Senescence
│ │ │ closure Abscission
│ │ │
Elongation Germination Cell division
Tropism Bolting Shoot growth
Rooting Growth
OTHER IMPORTANT SIGNALS
│
┌───────────┼────────────┐
│ │ │
Brassinosteroids Jasmonates Salicylic acid
Growth Wound Pathogen defense
defense
│
Strigolactones
Shoot branching🌟 Final Concept
Plant hormones are signaling molecules that coordinate growth, development and environmental responses. The classical five are auxins, gibberellins, cytokinins, ABA and ethylene, but modern plant biology recognizes several additional hormone/signaling classes. Their effects depend strongly on concentration, tissue, developmental stage and interaction with other hormones.
🔑 The five most important associations
Auxin → Elongation & tropism
Gibberellin → Growth & germination
Cytokinin → Cell division
ABA → Dormancy & drought response
Ethylene → Ripening & senescence
