Question
Describe the roles of auxins, gibberellins, and cytokinins in plant growth and development. How do these hormones interact with each other?
(NEET 2022, similar pattern)
Solution — Step by Step
Discovery: First discovered by F.W. Went from oat coleoptile tips. The most common natural auxin is Indole-3-Acetic Acid (IAA).
Major functions:
- Apical dominance: Auxin produced by the apical bud suppresses growth of lateral buds. Removing the apex (pruning) releases lateral buds from inhibition — this is why pruned hedges become bushy.
- Cell elongation: Promotes elongation of cells in stems (at low concentrations). The acid growth hypothesis explains this — auxin activates proton pumps, acidifying the cell wall, loosening it, and allowing expansion.
- Phototropism and gravitropism: Unequal distribution of auxin causes differential growth, bending the plant toward light or in response to gravity.
- Root initiation: Used commercially to induce rooting in stem cuttings (synthetic auxins like IBA, NAA).
- Parthenocarpy: Induces fruit development without fertilization.
- Herbicides: Synthetic auxins like 2,4-D are used as selective herbicides (kill dicot weeds in cereal crops).
Discovery: Discovered from the fungus Gibberella fujikuroi, which causes “foolish seedling disease” (bakane disease) in rice — infected plants grew abnormally tall. The most common gibberellin is GA3 (Gibberellic acid).
Major functions:
- Stem elongation: Promotes internodal elongation in dwarf plants (can make genetically dwarf pea plants grow tall).
- Bolting: Triggers rapid stem elongation and flowering in rosette plants (like cabbage) under long-day conditions.
- Seed germination: Stimulates the production of -amylase in the aleurone layer of cereal seeds, which digests starch in the endosperm to provide glucose for the embryo.
- Fruit growth: Promotes elongation of grape stalks, leading to larger, better-spaced fruits.
- Breaks dormancy: Can substitute for cold treatment (vernalisation) or light requirement in some seeds.
Discovery: First identified as kinetin (from autoclaved herring sperm DNA, by Skoog and Miller). The most common natural cytokinin is zeatin (from maize endosperm).
Major functions:
- Cell division (cytokinesis): Promotes cell division, especially in the presence of auxin.
- Delays senescence: Keeps leaves green and fresh longer by preventing protein and chlorophyll degradation — the Richmond-Lang effect.
- Promotes lateral bud growth: Opposes auxin’s apical dominance, encouraging branching.
- Shoot differentiation in tissue culture: In the auxin:cytokinin ratio, high cytokinin promotes shoot formation, while high auxin promotes root formation.
Plant hormones rarely act alone — they interact:
- Auxin + Cytokinin ratio determines organ differentiation in tissue culture: high auxin:cytokinin → root formation, high cytokinin:auxin → shoot formation, equal → callus (undifferentiated mass).
- Auxin vs Cytokinin in apical dominance: auxin promotes apical dominance, cytokinin opposes it.
- Gibberellin + Auxin together promote fruit development more effectively than either alone.
Why This Works
Plant hormones are chemical messengers produced in minute quantities that regulate growth at distant sites. Unlike animal hormones, plant hormones are not produced in specific glands — they are synthesised in various tissues and transported through the vascular system or by cell-to-cell diffusion.
The ratio of hormones, rather than the absolute concentration of any single hormone, often determines the developmental outcome. This is why tissue culture relies on precise auxin:cytokinin ratios to control whether cells form roots, shoots, or remain undifferentiated.
NEET frequently tests: “Which hormone is responsible for bolting?” Answer: Gibberellin. “Which hormone delays leaf senescence?” Answer: Cytokinin. “Which synthetic auxin is used as a herbicide?” Answer: 2,4-D. These are direct recall questions — memorise them.
Common Mistake
Students often confuse the effects of auxin on stems vs roots. Auxin promotes elongation in stems (at low to moderate concentrations) but inhibits root elongation at the same concentration. Roots are much more sensitive to auxin — they require very low concentrations for growth. This differential sensitivity explains gravitropism: auxin accumulates on the lower side, inhibiting root growth (root bends down) but promoting stem growth (stem bends up).
Another error: writing that gibberellin “causes” the foolish seedling disease. Gibberellin is produced by the fungus Gibberella, not by the plant. The excess gibberellin from the fungal infection causes abnormal elongation. The plant’s own gibberellin levels are tightly regulated.