CUET UG Biology: Plant Physiology - Plant Growth Regulators, Auxin, Gibberellin, Cytokinin, ABA, Ethylene

Must-Know (15–20 detailed bullets)

• Auxin was first isolated from human urine by F.W. Went, who also developed the Avena curvature test to measure auxin activity.
• Indole-3-acetic acid (IAA) is the most common naturally occurring auxin in plants.
• Auxins promote apical dominance; removal of the apical bud leads to lateral bud growth due to reduced auxin concentration.
• Gibberellins were discovered from Gibberella fujikuroi, a fungus causing bakanae disease in rice.
• Gibberellic acid (GA?) induces internode elongation in dwarf pea plants, making them reach normal height.
• Cytokinins are synthesized primarily in roots and promote cell division; zeatin is a naturally occurring cytokinin isolated from corn kernels.
• The ratio of auxin to cytokinin determines organogenesis: high auxin favors root formation, high cytokinin favors shoot formation.
• Abscisic acid (ABA) is a growth inhibitor involved in stomatal closure during water stress; it accumulates in leaves under drought conditions.
• ABA plays a key role in seed dormancy; it inhibits germination and is degraded when conditions become favorable.
• Ethylene is a gaseous plant hormone that promotes fruit ripening; it is used commercially to ripen climacteric fruits like bananas and tomatoes.
• Ethylene is synthesized from methionine via the Yang cycle; ACC (1-aminocyclopropane-1-carboxylic acid) is a key intermediate.
• Auxins induce parthenocarpy (fruit development without fertilization); e.g., application of 2,4-D in tomatoes produces seedless fruits.
• Gibberellins break seed dormancy by inducing the synthesis of hydrolytic enzymes like ?-amylase in cereal grains (e.g., barley endosperm).
• Cytokinins delay senescence in leaves, a phenomenon known as the Richmond–Lang effect.
• ABA and gibberellins have antagonistic effects on seed germination: GA promotes it, ABA suppresses it.
• Ethylene inhibits longitudinal growth and promotes radial expansion, leading to thick, short stems (epinasty).
• Photoperiodic flowering is regulated by phytochrome, but gibberellins can induce flowering in some long-day plants under non-inductive short-day conditions.
• 2,4-D is a synthetic auxin used as a selective herbicide; it kills dicot weeds without affecting monocot crops.
• Coconut water contains cytokinins, which contribute to its ability to promote cell division.
• Five major classes of plant growth regulators recognized in NCERT: Auxins, Gibberellins, Cytokinins, Abscisic acid, Ethylene — no sixth class is mentioned.

Difficulty Level

Intermediate — requires understanding of hormone functions, interactions, and experimental evidence, but no complex calculations or derivations.

Common CUET Traps (3 bullets)

• Trap: Assuming abscisic acid (ABA) causes abscission; it does not. Avoid: Ethylene and auxin gradients regulate abscission; ABA is for abscission only in name, not function.
• Trap: Believing cytokinins are synthesized in shoots; they are primarily produced in root tips. Avoid: Recall that root-derived cytokinins are transported via xylem to aerial parts.
• Trap: Thinking all fruits produce ethylene equally. Avoid: Only climacteric fruits (e.g., banana, apple) show a respiratory burst and ethylene surge during ripening.

Practice MCQs (5 questions)

Q1. Which of the following is a naturally occurring auxin?
A) 2,4-D
B) IAA
C) NAA
D) IBN

Answer: B
Explanation: Indole-3-acetic acid (IAA) is the primary natural auxin.
Why others fail: 2,4-D, NAA, and IBN are synthetic auxins used in labs and agriculture.

Q2. The Richmond–Lang effect refers to:
A) Promotion of flowering by gibberellins
B) Delay of leaf senescence by cytokinins
C) Induction of parthenocarpy by auxins
D) Stomatal closure by ABA

Answer: B
Explanation: Cytokinins delay senescence, known as the Richmond–Lang effect.
Why others fail: Gibberellins affect flowering, auxins induce parthenocarpy, ABA causes stomatal closure — but only cytokinins delay senescence.

Q3. Which hormone is responsible for inducing ?-amylase production during seed germination?
A) Auxin
B) Gibberellin
C) Cytokinin
D) Ethylene

Answer: B
Explanation: Gibberellins activate ?-amylase in aleurone layer of barley seeds to mobilize stored starch.
Why others fail: Students may confuse with auxin or cytokinin roles, but GA specifically triggers enzyme synthesis in germinating cereals.

Q4. Which of the following is NOT a function of ethylene?
A) Promotes fruit ripening
B) Induces thickening of stems
C) Breaks seed dormancy
D) Promotes abscission

Answer: C
Explanation: Gibberellins, not ethylene, break seed dormancy.
Why others fail: Ethylene does promote ripening, abscission, and stem thickening (epinasty), making C the correct exception.

Q5. In tissue culture, formation of shoots is favored when:
A) Auxin concentration is high
B) Cytokinin concentration is high
C) ABA is added
D) Ethylene is suppressed

Answer: B
Explanation: High cytokinin to auxin ratio promotes shoot differentiation.
Why others fail: High auxin favors roots, not shoots; ABA and ethylene are not primary regulators of organogenesis in this context.

Last?Minute Revision (15–20 one?liners)

• IAA = Indole-3-acetic acid — the natural auxin.
• Avena curvature test measures auxin activity — developed by Went.
• Apical dominance is due to auxin — removal allows lateral growth.
• Bakanae disease in rice caused by Gibberella fujikuroi — source of gibberellins.
• GA? overcomes genetic dwarfism in pea plants.
• Zeatin is a cytokinin first isolated from corn (Zea mays).
• High auxin : cytokinin-roots; high cytokinin : auxin-shoots.
• ABA = stress hormone — closes stomata during drought.
• ABA maintains seed dormancy; GA breaks it.
• Ethylene is a gas — only gaseous PGR.
• Ethylene-climacteric rise in respiration during fruit ripening.
• 2,4-D is a synthetic auxin used as a selective herbicide.
• Parthenocarpy induced by auxins — e.g., seedless tomatoes.
• Cytokinins delay senescence — Richmond–Lang effect.
• Coconut water is rich in cytokinins.
• ABA does NOT cause abscission — ethylene and auxin gradient do.
• Five classes of PGRs: Auxins, Gibberellins, Cytokinins, ABA, Ethylene — no sixth class in NCERT.
• Gibberellins induce ?-amylase in barley grains — key for malt production.
• Ethylene promotes epinasty — epinastic curvature due to differential growth.
• Mnemonic: "A Girl Can Eat Apples"-Auxin, Gibberellin, Cytokinin, Ethylene, ABA.