Study Guide

AHL: Plant hormone interactions

IB Biology Higher LevelΒ· Theme C: Interaction and Interdependence, Unit 3 AHLΒ· 20 min read

1. Synergistic vs Antagonistic Interactionsβ˜…β˜…β˜†β˜†β˜†HL only⏱ 5 min

Plant responses rarely rely on a single hormone. Instead, multiple hormones interact to produce coordinated outcomes. Interactions are classified by their combined effect relative to individual hormone effects.

πŸ“˜ Definition

Hormone interaction

The combined effect of multiple plant hormones on a biological process, differing from the effect of any single hormone acting alone.

Example:

Auxin and gibberellin interact synergistically to promote stem elongation.

πŸ“ Worked Example

A researcher measures stem elongation in dwarf pea plants: 4 cm after auxin only treatment, 5 cm after gibberellin only treatment, and 11 cm after treatment with both hormones. Classify this interaction.

  1. 1

    Step 1: Compare the combined effect to the sum of individual effects

  2. 2

    Sum of individual elongation = 4 cm + 5 cm = 9 cm. The combined effect (11 cm) is greater than the sum of individual effects.

  3. 3

    Step 2: Classify the interaction: Any combined effect larger than individual effects alone is classified as synergistic.

Exam tip:

When interpreting experimental data, always compare the combined response to individual responses to classify the interaction type.

2. Auxin-Cytokinin Interactionβ˜…β˜…β˜…β˜†β˜†HL only⏱ 6 min

The auxin-cytokinin interaction is one of the best-documented hormone interactions, regulating apical dominance and organogenesis in plant tissue culture.

πŸ“˜ Definition

Ratio-dependent organogenesis

The development of roots or shoots in tissue culture depends on the ratio of auxin to cytokinin, not just their absolute concentrations.

Example:

High auxin relative to cytokinin produces roots, while high cytokinin relative to auxin produces shoots.

πŸ“ Worked Example

Predict the outcome of growing plant explants on tissue culture medium with (a) high auxin:cytokinin ratio (b) low auxin:cytokinin ratio (c) equal auxin and cytokinin.

  1. 1

    Step 1: Recall the core rule for auxin-cytokinin interaction in organogenesis: high auxin promotes root development, high cytokinin promotes shoot development.

  2. 2

    (a) High auxin:cytokinin ratio: The ratio favors auxin, so root tissue will develop.

  3. 3

    (b) Low auxin:cytokinin ratio (high cytokinin): The ratio favors cytokinin, so shoot tissue will develop.

  4. 4

    (c) Equal ratio of auxin and cytokinin: Neither hormone dominates, so undifferentiated callus tissue will grow.

Exam tip:

This interaction is a common 3-5 mark short answer question, always remember the ratio rule.

3. Gibberellin-ABA Antagonism in Seed Germinationβ˜…β˜…β˜…β˜†β˜†HL only⏱ 7 min

Gibberellin (GA) and abscisic acid (ABA) act antagonistically to regulate seed dormancy and germination. This interaction ensures seeds only germinate when environmental conditions are favorable.

πŸ“ Worked Example

Explain how GA and ABA interact to control barley seed germination after water uptake.

  1. 1

    Step 1: Dormant dry seeds have high ABA concentrations, which maintain dormancy and inhibit embryo growth.

  2. 2

    Step 2: When water is absorbed, the barley embryo produces gibberellin that diffuses to the aleurone layer of the seed.

  3. 3

    Step 3: Gibberellin triggers the aleurone layer to produce amylase, which breaks down stored starch into glucose to fuel embryo growth.

  4. 4

    Step 4: ABA opposes gibberellin by inhibiting amylase production. When ABA levels drop after favorable conditions, GA action dominates and germination proceeds.

4. Auxin-Ethylene Interaction in Abscissionβ˜…β˜…β˜†β˜†β˜†HL only⏱ 4 min

Leaf and fruit abscission (natural shedding) is regulated by the interaction of auxin and ethylene. This interaction is senescence-dependent, so only old tissue is shed.

Young healthy leaves produce constant auxin that moves to the abscission zone at the base of the leaf stalk, inhibiting abscission and making the zone insensitive to ethylene. As leaves age, auxin production drops.

πŸ“ Worked Example

Explain why storing unripe apples with a ripe banana in a sealed bag speeds up apple ripening.

  1. 1

    Step 1: Ripe fruits produce large amounts of ethylene, a gaseous hormone that triggers fruit ripening.

  2. 2

    Step 2: Ethylene stimulates additional ethylene production in unripe fruits, creating a positive feedback loop that accelerates ripening.

  3. 3

    Step 3: A sealed bag traps ethylene gas, increasing its concentration around the unripe apples and speeding up the ripening process.

5. Common Pitfalls

Wrong move:

Claiming ABA directly causes leaf abscission

Why:

ABA has no direct role in abscission; abscission is controlled by auxin and ethylene

Correct move:

Describe abscission as: low auxin makes the abscission zone sensitive to ethylene, which triggers abscission

Wrong move:

Memorizing high auxin produces shoots in tissue culture

Why:

The ratio is reversed: high cytokinin relative to auxin promotes shoot growth

Correct move:

Use the mnemonic: 'High Auxin Roots, High Cytokinin Shoots' to remember the ratio rule

Wrong move:

Defining synergy as equal to the sum of individual effects

Why:

Synergy requires a combined effect greater than the effect of each hormone alone

Correct move:

State synergy as an interaction where the combined effect is larger than any individual effect

Wrong move:

Claiming gibberellin and ABA are always antagonistic

Why:

They only act antagonistically in the context of seed dormancy, not all processes

Correct move:

Only describe their antagonism for seed germination, the core syllabus example

6. Quick Reference Cheatsheet

Interaction Type

Hormones

Process

Outcome

Synergistic

Auxin + Gibberellin

Stem elongation

Increased growth

Ratio-dependent

Auxin:Cytokinin

Tissue culture

High auxin = roots; High CK = shoots

Balanced Synergy

Auxin + Cytokinin

Tissue culture

Undifferentiated callus growth

Antagonistic

Gibberellin:ABA

Seed germination

GA = break dormancy; ABA = maintain dormancy

Antagonistic

Auxin:Ethylene

Leaf abscission

Auxin = inhibit; Ethylene = promote

Positive Synergy

Ethylene

Fruit ripening

Accelerates ripening across adjacent fruits

7. Frequently Asked

Do I need to memorize all possible hormone interactions for the exam?

No, you only need to master the core interactions covered in this topic: auxin-cytokinin, gibberellin-ABA, and auxin-ethylene. Focus on how their interaction regulates the core processes, not rare secondary interactions.

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.

  • 2025 Β· 2

    Auxin-cytokinin interaction in tissue culture

  • 2023 Β· 1

    Gibberellin-ABA interaction in dormancy

  • 2022 Β· 2

    Hormone control of leaf abscission

Going deeper

What's Next

Understanding plant hormone interactions is foundational for exploring plant responses to abiotic stress, biotechnological applications in agriculture, and ecological interactions between plants and their environment. This topic builds on your existing knowledge of basic hormone function and cell signaling, and prepares you for more advanced study of plant adaptations and modern crop improvement. Mastery of core hormone interactions and interpretation of experimental data is key for scoring well on extended response questions in IB Biology HL.