Growth Regulators in Plants: Types, Functions, Examples and Exam Guide

Growth regulators in plants including auxins cytokinins gibberellins ABA and ethylene — SEO featured image: plant growth regulators including auxins, cytokinins, gibberellins, abscisic acid and ethylene.

Growth regulators are chemical substances that control plant growth and development at very low concentrations. In plants, naturally produced growth regulators are called plant hormones or phytohormones. These substances regulate cell division, cell elongation, root formation, shoot development, flowering, fruit ripening, seed dormancy, stress responses, and senescence.

The five classical plant growth regulators are auxins, cytokinins, gibberellins, abscisic acid and ethylene. Each hormone has a specific role, but plant development usually depends on the balance between different hormones rather than the action of one hormone alone.

This topic is extremely important for biology, botany, agriculture, plant physiology, biotechnology, horticulture, tissue culture and exam preparation. Questions about auxin, cytokinin, gibberellin, ABA and ethylene are common in board exams, university biology, NEET-style biology, MCAT-style biology, AP Biology, A-Level Biology and IB Biology.

For related study, read: Plant Hormones, Plant Physiology, Plant Tissue Culture, Plant Nutrition, and Seed Germination.

Table of ContentsGrowth Regulators at a Glance What Are Growth Regulators?Types of Plant Growth Regulators Auxins Cytokinins Gibberellins Abscisic Acid Ethylene Growth Regulators in Plant Tissue Culture Agricultural Importance Comparison Table Exam Importance 50 Exam-Style MCQs 20 Exam-Style FAQs

Growth Regulators at a Glance

Growth Regulator	Main Functions	Common Examples	Exam Keyword
Auxins	Cell elongation, root initiation, apical dominance, callus formation	IAA, IBA, NAA, 2,4-D	Rooting and apical dominance
Cytokinins	Cell division, shoot formation, bud growth, delay of senescence	Kinetin, Zeatin, BAP	Shoot induction
Gibberellins	Stem elongation, seed germination, bolting, enzyme production	GA3	Stem elongation
Abscisic Acid	Seed dormancy, stomatal closure, stress response	ABA	Stress hormone
Ethylene	Fruit ripening, abscission, senescence, stress response	Ethylene gas, ethephon	Fruit ripening

What Are Growth Regulators?

Growth regulators are organic substances that regulate plant growth and development in very small amounts. They may be naturally produced inside plants or applied externally as synthetic compounds. Unlike nutrients, growth regulators do not mainly provide energy or building material. Instead, they act as chemical signals that control physiological processes.

Plant growth regulators can stimulate, inhibit, or modify growth depending on their concentration, tissue type, plant species, developmental stage, and interaction with other hormones.

Types of Plant Growth Regulators

The five classical types of plant growth regulators are auxins, cytokinins, gibberellins, abscisic acid and ethylene. Modern plant physiology also recognizes other regulators such as brassinosteroids, jasmonates, salicylic acid and strigolactones, but the classical five remain most important for exams.

Auxins: Cell Elongation, Rooting and Apical Dominance

Auxins are plant growth regulators mainly involved in cell elongation, root initiation, apical dominance, vascular differentiation, tropic movements and callus formation. The most common natural auxin is indole-3-acetic acid, usually written as IAA.

Major Functions of Auxins

Promote cell elongation in shoots
Control apical dominance
Promote adventitious root formation
Control phototropism and gravitropism
Help in callus induction in tissue culture
Promote fruit development and parthenocarpy in some cases
Synthetic auxins such as 2,4-D can act as herbicides

Examples of Auxins

IAA: Indole-3-acetic acid
IBA: Indole-3-butyric acid
NAA: Naphthalene acetic acid
2,4-D: 2,4-dichlorophenoxyacetic acid

Cytokinins: Cell Division and Shoot Formation

Cytokinins are growth regulators that mainly promote cell division and shoot formation. They are often produced in roots and transported to shoots. Cytokinins also delay leaf senescence and help in chloroplast development.

Major Functions of Cytokinins

Promote cell division
Stimulate shoot initiation in tissue culture
Promote lateral bud growth
Delay leaf senescence
Support chloroplast development
Work with auxins to control organogenesis

Examples of Cytokinins

Kinetin
Zeatin
BAP or BA: Benzylaminopurine

Gibberellins: Stem Elongation and Seed Germination

Gibberellins are plant growth regulators best known for promoting stem elongation, seed germination, bolting and flowering in some plants. GA3 is one of the most commonly studied gibberellins.

Major Functions of Gibberellins

Promote stem elongation
Break seed dormancy
Stimulate seed germination
Promote bolting in rosette plants
Stimulate amylase production in cereal grains
Increase fruit size in some crops

Abscisic Acid: Stress Hormone and Dormancy Regulator

Abscisic acid, commonly called ABA, is known as a stress hormone. It helps plants survive unfavorable conditions such as drought. ABA promotes stomatal closure, seed dormancy and stress tolerance.

Major Functions of Abscisic Acid

Promotes stomatal closure during water stress
Induces and maintains seed dormancy
Helps plants tolerate drought and other stresses
Opposes some effects of gibberellins during germination
Regulates stress-related gene expression

Ethylene: Fruit Ripening and Senescence Hormone

Ethylene is a gaseous plant hormone. It is best known for promoting fruit ripening, leaf abscission and senescence. Ethylene production increases during fruit ripening, mechanical injury, flooding and other stresses.

Major Functions of Ethylene

Promotes fruit ripening
Promotes leaf and fruit abscission
Promotes senescence
Helps plants respond to mechanical stress
Influences seedling growth responses

Role of Growth Regulators in Plant Tissue Culture

Growth regulators are extremely important in plant tissue culture. They control callus formation, shoot induction, root induction, organogenesis and somatic embryogenesis. The ratio of auxin and cytokinin is one of the most important concepts in tissue culture.

Hormone Balance	Typical Response
High auxin, low cytokinin	Root formation
High cytokinin, low auxin	Shoot formation
Balanced auxin and cytokinin	Callus formation
2,4-D rich medium	Callus induction and somatic embryogenesis in many systems
BAP or kinetin rich medium	Shoot multiplication
IBA or NAA rich medium	Root induction

Agricultural Importance of Growth Regulators

Plant growth regulators are widely used in agriculture, horticulture, nursery production and biotechnology. They help control rooting, flowering, fruit ripening, seed germination, plant height, fruit size and stress responses.

Effects of plant growth regulators on plant growth — Diagram: major effects of plant growth regulators on plant growth and development.

Important Uses in Agriculture

Auxins are used for rooting of cuttings.
2,4-D is used as a selective herbicide against broadleaf weeds.
Gibberellins are used to promote stem elongation and improve fruit size in some crops.
Ethylene or ethylene-releasing compounds are used for fruit ripening.
Cytokinins are used in tissue culture for shoot multiplication.
ABA-related responses help plants tolerate drought stress.

Comparison of Plant Growth Regulators

Hormone	Promotes	Inhibits / Regulates	Common Exam Point
Auxin	Cell elongation, rooting, apical dominance	Lateral bud growth under apical dominance	Phototropism and rooting
Cytokinin	Cell division, shoot formation	Leaf senescence	Shoot induction
Gibberellin	Stem elongation, seed germination	Seed dormancy	Bolting and amylase
ABA	Dormancy and stress tolerance	Stomatal opening, germination	Stress hormone
Ethylene	Fruit ripening and abscission	Some elongation growth	Gaseous hormone

Exam Importance of Growth Regulators

Important exam points: Auxin promotes cell elongation, rooting, apical dominance and tropisms. Cytokinin promotes cell division and shoot formation. Gibberellin promotes stem elongation, bolting and seed germination. ABA is a stress hormone that closes stomata and induces seed dormancy. Ethylene is a gaseous hormone responsible for fruit ripening, abscission and senescence. In tissue culture, high auxin promotes roots, high cytokinin promotes shoots, and balanced auxin-cytokinin ratio promotes callus.

50 Top Exam-Style MCQs on Growth Regulators

These MCQs are based on commonly repeated concepts from biology, botany, plant physiology, biotechnology, agriculture, tissue culture, NEET-style biology, MCAT-style biology, AP Biology, A-Level Biology, IB Biology and university exams.

1. Plant growth regulators are also called:
A. Plant hormones
B. Mineral salts
C. Enzymes only
D. Carbohydrates only
Answer: A

2. Which plant growth regulator mainly promotes cell elongation?
A. Auxin
B. Abscisic acid
C. Ethylene
D. Chlorine
Answer: A

3. IAA stands for:
A. Indole-3-acetic acid
B. Internal amino acid
C. Iron activated auxin
D. Induced abscisic acid
Answer: A

4. Auxin is mainly produced in:
A. Shoot tips and young leaves
B. Old bark only
C. Dead xylem only
D. Root cap only
Answer: A

5. Apical dominance is mainly caused by:
A. Auxin
B. Ethylene
C. ABA
D. Molybdenum
Answer: A

6. Which hormone promotes root initiation in cuttings?
A. Auxin
B. Cytokinin
C. Ethylene
D. Gibberellin only
Answer: A

7. Which synthetic auxin is commonly used in tissue culture?
A. 2,4-D
B. BAP
C. Kinetin
D. Zeatin
Answer: A

8. Which plant growth regulator promotes cell division?
A. Cytokinin
B. ABA
C. Ethylene
D. Calcium
Answer: A

9. Cytokinins are mainly associated with:
A. Shoot formation
B. Fruit ripening only
C. Stomatal closure only
D. Leaf fall only
Answer: A

10. Zeatin is a type of:
A. Cytokinin
B. Auxin
C. Gibberellin
D. ABA
Answer: A

11. BAP is commonly used for:
A. Shoot induction
B. Seed dormancy only
C. Fruit ripening only
D. Water loss only
Answer: A

12. High cytokinin to auxin ratio generally promotes:
A. Shoot formation
B. Root formation
C. No growth
D. Seed dormancy
Answer: A

13. High auxin to cytokinin ratio generally promotes:
A. Root formation
B. Shoot formation
C. Fruit ripening only
D. Leaf yellowing only
Answer: A

14. A balanced auxin and cytokinin ratio often promotes:
A. Callus formation
B. Fruit fall only
C. Seed drying only
D. Stomatal closure only
Answer: A

15. Gibberellins mainly promote:
A. Stem elongation
B. Fruit ripening only
C. Root death
D. Leaf abscission only
Answer: A

16. GA3 is a form of:
A. Gibberellin
B. Auxin
C. Cytokinin
D. Ethylene
Answer: A

17. Gibberellins help break:
A. Seed dormancy
B. Cell wall permanently
C. All roots
D. All flowers
Answer: A

18. Which hormone is known as a stress hormone?
A. Abscisic acid
B. Auxin
C. Cytokinin
D. Gibberellin
Answer: A

19. ABA causes:
A. Stomatal closure during water stress
B. Fruit ripening only
C. Stem elongation only
D. Shoot induction only
Answer: A

20. ABA promotes:
A. Seed dormancy
B. Rapid germination always
C. Fruit softening only
D. Pollen tube elongation only
Answer: A

21. Ethylene is a:
A. Gaseous plant hormone
B. Solid mineral
C. Protein enzyme
D. Soil salt
Answer: A

22. Ethylene mainly promotes:
A. Fruit ripening
B. Root hair death only
C. Stem photosynthesis only
D. Seed coat hardness only
Answer: A

23. Which hormone is associated with leaf abscission?
A. Ethylene
B. Magnesium
C. Zinc
D. Molybdenum
Answer: A

24. Which hormone can promote senescence?
A. Ethylene
B. Cytokinin only
C. Gibberellin only
D. Calcium only
Answer: A

25. Which growth regulator delays leaf senescence?
A. Cytokinin
B. Ethylene
C. ABA
D. 2,4-D only
Answer: A

26. Which hormone is useful in parthenocarpy?
A. Auxin or gibberellin
B. Ethylene only
C. ABA only
D. Nickel only
Answer: A

27. Which hormone is associated with bolting in rosette plants?
A. Gibberellin
B. ABA
C. Ethylene
D. Cytokinin only
Answer: A

28. Which plant hormone helps mobilize food reserves during seed germination?
A. Gibberellin
B. Ethylene
C. ABA
D. Auxin only
Answer: A

29. Amylase production in germinating cereal seeds is promoted by:
A. Gibberellin
B. ABA
C. Ethylene
D. Boron
Answer: A

30. Auxin movement is generally:
A. Polar
B. Random only
C. Absent
D. Only upward in xylem
Answer: A

31. Phototropism is mainly controlled by:
A. Auxin redistribution
B. Ethylene gas only
C. ABA only
D. Calcium deficiency
Answer: A

32. Geotropism/root gravitropism is influenced by:
A. Auxin
B. Ethylene only
C. Nickel
D. Molybdenum
Answer: A

33. The synthetic auxin used as a weed killer is:
A. 2,4-D
B. GA3
C. BAP
D. Zeatin
Answer: A

34. Kinetin belongs to:
A. Cytokinins
B. Auxins
C. Gibberellins
D. ABA group
Answer: A

35. NAA and IBA are commonly used for:
A. Root induction
B. Fruit ripening only
C. Seed dormancy only
D. Stomatal closure only
Answer: A

36. In plant tissue culture, growth regulators mainly control:
A. Organogenesis and differentiation
B. Only agar melting
C. Only medium color
D. Only sterilization
Answer: A

37. Shoot multiplication in tissue culture often uses:
A. Cytokinins
B. Ethylene gas only
C. ABA only
D. No hormone ever
Answer: A

38. Rooting medium often contains:
A. Auxins
B. Only cytokinins
C. Only ethylene
D. Only agar
Answer: A

39. Callus induction often requires:
A. Auxin such as 2,4-D
B. Ethylene only
C. Magnesium only
D. No nutrients
Answer: A

40. Which hormone generally opposes gibberellin in seed dormancy?
A. ABA
B. Cytokinin
C. Auxin
D. Ethylene
Answer: A

41. Fruit ripening in bananas is promoted by:
A. Ethylene
B. ABA only
C. Cytokinin only
D. Calcium only
Answer: A

42. The hormone used commercially for artificial fruit ripening is related to:
A. Ethylene
B. Auxin
C. Cytokinin
D. Gibberellin only
Answer: A

43. Which of the following is not a classical plant hormone?
A. Chlorophyll
B. Auxin
C. Cytokinin
D. Ethylene
Answer: A

44. Plant responses to growth regulators depend on:
A. Concentration, tissue type, and hormone balance
B. Only leaf color
C. Only soil texture
D. Only air pressure
Answer: A

45. Low auxin concentration may promote:
A. Root growth
B. Fruit drying only
C. Leaf abscission only
D. Seed death only
Answer: A

46. Very high auxin concentration may:
A. Inhibit growth or act as herbicide
B. Always promote unlimited growth
C. Remove all roots
D. Stop photosynthesis instantly
Answer: A

47. Cytokinins are often synthesized in:
A. Roots
B. Dead bark only
C. Old xylem only
D. Dry seeds only
Answer: A

48. Ethylene production increases during:
A. Fruit ripening and stress
B. Only photosynthesis
C. Only seed storage
D. Only mineral absorption
Answer: A

49. ABA level commonly increases during:
A. Drought stress
B. Excess rainfall only
C. Fruit sweetness only
D. Shoot multiplication only
Answer: A

50. The best title for auxin, cytokinin, gibberellin, ABA and ethylene is:
A. Plant growth regulators
B. Mineral nutrients
C. Respiratory gases
D. Photosynthetic pigments
Answer: A

20 Exam-Style FAQs on Growth Regulators

1. What are growth regulators?

Growth regulators are natural or synthetic chemical substances that control plant growth and development at very low concentrations.

2. What are plant growth regulators also called?

Plant growth regulators are also called plant hormones or phytohormones when they are naturally produced by plants.

3. What are the main types of plant growth regulators?

The main types are auxins, cytokinins, gibberellins, abscisic acid, and ethylene.

4. What is the role of auxins?

Auxins promote cell elongation, root initiation, apical dominance, tropic movements, callus formation, and fruit development.

5. What is the role of cytokinins?

Cytokinins promote cell division, shoot formation, bud growth, chloroplast development, and delay leaf senescence.

6. What is the role of gibberellins?

Gibberellins promote stem elongation, seed germination, bolting, flowering in some plants, and mobilization of food reserves.

7. What is the role of abscisic acid?

Abscisic acid promotes seed dormancy, stomatal closure during water stress, and stress tolerance.

8. What is the role of ethylene?

Ethylene promotes fruit ripening, leaf abscission, senescence, and responses to stress.

9. Which hormone promotes root formation?

Auxins such as IBA, NAA, and IAA commonly promote root formation.

10. Which hormone promotes shoot formation?

Cytokinins such as BAP, kinetin, and zeatin commonly promote shoot formation.

11. What is apical dominance?

Apical dominance is the suppression of lateral bud growth by the shoot tip, mainly due to auxin.

12. What is the role of growth regulators in tissue culture?

Growth regulators control callus formation, shoot induction, root induction, organogenesis, and somatic embryogenesis in plant tissue culture.

13. What hormone ratio promotes root formation in tissue culture?

A high auxin-to-cytokinin ratio generally promotes root formation.

14. What hormone ratio promotes shoot formation in tissue culture?

A high cytokinin-to-auxin ratio generally promotes shoot formation.

15. What hormone ratio promotes callus formation?

A balanced auxin and cytokinin ratio often promotes callus formation.

16. Which hormone causes fruit ripening?

Ethylene is the main hormone responsible for fruit ripening.

17. Which hormone closes stomata during drought?

Abscisic acid closes stomata during drought stress to reduce water loss.

18. Which hormone breaks seed dormancy?

Gibberellins often help break seed dormancy and promote germination.

19. Which plant growth regulator is used as a weed killer?

2,4-D is a synthetic auxin used as a selective herbicide against broadleaf weeds.

20. Why are growth regulators important in agriculture?

They are important because they control rooting, flowering, fruit ripening, seed germination, stress responses, and tissue culture propagation.

Conclusion

Growth regulators are chemical messengers that control plant growth and development at very low concentrations. The major plant growth regulators are auxins, cytokinins, gibberellins, abscisic acid and ethylene. These hormones regulate cell elongation, cell division, root and shoot formation, seed germination, dormancy, fruit ripening, stress response and senescence.

In plant tissue culture, growth regulators are especially important because the ratio of auxin and cytokinin controls callus formation, root induction and shoot induction. In agriculture, growth regulators are used for rooting cuttings, fruit ripening, weed control, shoot multiplication and improving plant growth responses.

For exams, remember the key associations: auxin means elongation and roots, cytokinin means division and shoots, gibberellin means stem elongation and germination, ABA means stress and dormancy, and ethylene means fruit ripening and abscission.

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Growth Regulators in Plants: Types, Functions & Examples