Plant growth regulators
CIE A-Level BiologyΒ· 15 min read
1. Auxins: Core Functions and Mechanism of Actionβ β ββββ± 5 min
Indole-3-acetic acid (IAA)
The most abundant naturally occurring auxin, produced in apical meristems and young developing leaves, that regulates multiple growth processes.
Example:
Synthetic auxins like 2,4-D are used as selective weedkillers.
Auxins have concentration-dependent and tissue-specific effects. Low concentrations promote elongation in both shoots and roots, while high concentrations promote shoot elongation but strongly inhibit root growth.
Promote cell elongation in growing shoots
Maintain apical dominance by inhibiting lateral bud growth
Promote root development in cuttings
Inhibit premature leaf and fruit abscission
Outline how auxin causes cell elongation in shoot cells according to the acid growth hypothesis.
- 1
Auxin diffuses to target shoot cells and binds to cell surface membrane receptors.
- 2
This binding activates proton pumps that move HβΊ ions from the cytoplasm into the cell wall.
- 3
The lowered pH activates expansin enzymes that break hydrogen bonds between cellulose microfibrils.
- 4
The cell wall becomes more flexible and less resistant to expansion.
- 5
Water enters the cell by osmosis, increasing turgor pressure that pushes the flexible wall outwards, causing net elongation.
Exam tip:
CIE examiners always award marks for linking auxin concentration to differing effects in roots vs shoots. Always mention the concentration dependence.
2. Gibberellins: Roles in Growth and Germinationβ β β βββ± 6 min
Gibberellins
A family of over 100 acidic PGRs best known for promoting stem elongation and breaking seed dormancy.
Gibberellins are produced in young leaves, developing seeds, and root tissue. Their key roles go far beyond just stem elongation, and they interact with other PGRs to regulate development.
Explain why applying exogenous gibberellin to a dwarf pea plant causes it to grow to a normal tall height.
- 1
Most dwarf pea mutants have a recessive mutation that blocks the synthesis of active endogenous gibberellin.
- 2
Without gibberellin, cell elongation and cell division in stem internodes do not occur, leading to a short phenotype.
- 3
Externally applied gibberellin replaces the missing endogenous hormone.
- 4
Gibberellin promotes cell elongation and division in internodes, leading to stem growth and a normal tall height.
3. Abscisic Acid (ABA): Stress Response and Dormancyβ β β βββ± 6 min
Abscisic Acid (ABA)
An inhibitory PGR that mediates adaptive responses to abiotic stress and maintains seed and bud dormancy.
Describe how ABA triggers stomatal closure during drought stress.
- 1
When soil water content drops, ABA is synthesised in root cells and transported up to leaf tissue in the xylem.
- 2
ABA binds to receptors on the plasma membrane of guard cells.
- 3
Binding triggers opening of calcium channels, allowing CaΒ²βΊ to enter the guard cell, which acts as a second messenger.
- 4
Calcium triggers opening of KβΊ channels, leading to efflux of potassium ions out of the guard cell.
- 5
Loss of solutes raises the water potential of the guard cell, so water leaves by osmosis.
- 6
Guard cells become flaccid, closing the stomatal pore to reduce further water loss by transpiration.
4. Ethene and Commercial Applicationsβ β ββββ± 4 min
Ethene
A small gaseous PGR that promotes fruit ripening, leaf abscission, and senescence (ageing) of plant tissues.
Many plant growth regulators are widely used in commercial agriculture and horticulture to manipulate growth and development for higher yield and easier transport.
Explain how PGRs are used to deliver ripe bananas to a market 10 days transport time away from the growing region.
- 1
Bananas are picked when unripe and hard, so they resist damage during transport.
- 2
During shipping, bananas are stored in a low-ethylene, cool environment to prevent premature ripening.
- 3
After arrival at the destination, unripe bananas are exposed to controlled levels of exogenous ethene (from ethephon, a compound that releases ethene).
- 4
Ethene triggers ripening: starch is converted to sugar, cell walls soften, and skin colour changes, producing ripe bananas ready for sale after 2-3 days.
Auxins: Promote rooting in cuttings, kill broadleaf weeds in cereal crops
Gibberellins: Increase berry size in grapes, delay senescence in citrus
ABA: Used as an anti-transpirant for transplanting young plants
5. Common Pitfalls
Wrong move:
Claiming ABA causes leaf abscission
Why:
The name is misleading; ABA does not directly trigger abscission
Correct move:
Ethene and reduced auxin concentration are the direct causes of leaf abscission
Wrong move:
Saying PGRs are produced in specialised glands like animal hormones
Why:
Plants do not have endocrine glands
Correct move:
PGRs are synthesised by a range of tissues including meristems, young leaves and developing seeds
Wrong move:
Claiming auxin only promotes growth and never inhibits it
Why:
Auxin effects depend on concentration and target tissue
Correct move:
High auxin inhibits root growth and suppresses lateral bud growth during apical dominance
Wrong move:
Stating gibberellins only function in stem elongation
Why:
Gibberellins have multiple core roles in plant development
Correct move:
Gibberellins also break seed dormancy, promote germination and induce bolting in biennials
6. Quick Reference Cheatsheet
PGR Class | Main Physiological Roles | Key Exam Note |
|---|---|---|
Auxin (IAA) | Cell elongation, apical dominance, root development, inhibit abscission | Effect depends on concentration; high = inhibits root growth |
Gibberellins | Stem elongation, break seed dormancy, promote germination | Dwarf mutants are often gibberellin-deficient |
ABA | Stomatal closure (drought), induce seed dormancy | Not responsible for leaf abscission |
Ethene | Fruit ripening, leaf abscission, senescence | Gaseous hormone; used commercially to ripen fruit |
7. Frequently Asked
Are plant growth regulators the same as animal hormones?
No. Unlike animal hormones, plant growth regulators are not produced by specialised endocrine glands, and they often have multiple overlapping effects depending on concentration and target tissue.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Role of auxin in cell elongation
- 2023 Β· 1
Gibberellin and stem elongation
- 2021 Β· 4
Commercial uses of PGRs
What's Next
Plant growth regulators are central to how plants coordinate growth, development and responses to changing environmental conditions. This sub-topic connects to broader themes like plant transport, abiotic stress responses, and applied agricultural biology, which are common topics for extended response questions in CIE Paper 4. Mastery of PGR roles and mechanisms also supports understanding of tropic responses and plant reproduction. Next, you can explore related topics to build your full understanding of control and coordination in plants.