Question
List the five major plant hormones and describe their primary effects. How do auxin and cytokinin interact in controlling plant growth?
(NEET and CBSE 11 — tested as matching, assertion-reason, or MCQs)
Solution — Step by Step
Indole-3-acetic acid (IAA) is the most common natural auxin. It promotes cell elongation, apical dominance, and phototropism. Produced mainly at the shoot tip.
Key applications: 2,4-D (a synthetic auxin) is used as a weedkiller. Auxin also promotes root initiation in cuttings — that is why gardeners dip stems in rooting hormone.
Gibberellins (GA3): Promote stem elongation, seed germination (break dormancy), and bolting in rosette plants. Think of gibberellin as the “growth spurt” hormone.
Cytokinins: Promote cell division (cytokinesis — hence the name). Delay leaf senescence. Produced in root tips and transported upward.
Abscisic acid (ABA): The stress hormone. Promotes stomatal closure during drought, inhibits growth, induces seed dormancy. It opposes gibberellin in many contexts.
Ethylene: The ripening hormone. A gaseous hormone that promotes fruit ripening, leaf abscission, and senescence. One rotten apple spoils the bunch — that is ethylene at work.
The auxin:cytokinin ratio determines the type of growth in tissue culture:
- High auxin, low cytokinin → root formation
- Low auxin, high cytokinin → shoot formation
- Equal ratio → callus (undifferentiated mass)
This is the basis of plant tissue culture and is a favourite NEET question.
graph TD
A[Plant Hormones] --> B[Growth Promoters]
A --> C[Growth Inhibitors]
B --> D["Auxin - Cell elongation"]
B --> E["Gibberellin - Stem elongation"]
B --> F["Cytokinin - Cell division"]
C --> G["ABA - Stress response"]
C --> H["Ethylene - Ripening"]
D ---|"Ratio controls"| F
G ---|"Antagonistic to"| EWhy This Works
Plant hormones work not in isolation but as an interactive network. The effect of any hormone depends on its concentration and the ratio with other hormones present. Auxin at low concentration promotes root growth, but at high concentration it inhibits root growth — this is why we say “the dose makes the effect.”
ABA and gibberellins are classic antagonists — GA promotes seed germination while ABA maintains dormancy. Similarly, auxin maintains apical dominance by suppressing lateral bud growth, and cytokinin promotes lateral bud growth. Remove the shoot tip (source of auxin), and the lateral buds sprout — that is pruning science.
Alternative Method
Use this mnemonic for quick recall: “AAGCE” — Auxin (elongation), ABA (stress), Gibberellin (germination), Cytokinin (cell division), Ethylene (ripening). For the NEET matching-type question, link each hormone to its one-word function first, then add details.
Common Mistake
Students often classify ethylene as purely a “growth inhibitor.” Ethylene is more nuanced — it promotes fruit ripening and abscission (which are active developmental processes) while inhibiting stem elongation. Calling it simply an inhibitor loses marks in NEET assertion-reason questions.
Also, ABA is called the “stress hormone,” not the “death hormone.” It helps the plant survive stress by closing stomata and conserving water — it is protective, not destructive.