Plant Hormones and Responses

This lesson covers plant hormones (growth regulators) and how plants respond to environmental stimuli as required by the Edexcel A-Level Biology specification (9BI0), Topic 9 -- Control Systems. You need to understand the roles of auxins, gibberellins, and other plant hormones, the mechanisms of tropisms, and how plant responses are coordinated.

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Plant Responses to the Environment

Unlike animals, plants cannot move to escape unfavourable conditions. Instead, they respond to environmental stimuli by changing their growth patterns. These growth responses are called tropisms.

Tropism	Stimulus	Direction of Growth
Phototropism	Light	Shoots grow towards light (positive phototropism); roots grow away from light (negative phototropism)
Gravitropism (geotropism)	Gravity	Roots grow towards gravity (positive gravitropism); shoots grow away from gravity (negative gravitropism)
Thigmotropism	Touch	Tendrils grow towards the surface they touch (positive thigmotropism)
Hydrotropism	Water	Roots grow towards water (positive hydrotropism)
Chemotropism	Chemicals	Pollen tubes grow towards the ovule (positive chemotropism)

Exam Tip: 'Positive tropism' means growth towards the stimulus; 'negative tropism' means growth away from it. Always specify whether the response is positive or negative and which organ (root or shoot) you are describing.

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Auxins

Auxins (primarily indole-3-acetic acid, IAA) are the most important plant hormones for growth responses. IAA is produced in the tips of shoots and roots (meristematic regions) and is transported downwards in the plant.

The Role of Auxin in Cell Elongation

Auxin promotes cell elongation in shoots at low-to-moderate concentrations:

1. Auxin binds to a receptor on the cell surface (or inside the cell).
2. This activates proton pumps (H+-ATPases) in the plasma membrane.
3. H+ ions are pumped out of the cell into the cell wall (acid growth hypothesis).
4. The lowered pH activates expansins -- proteins that loosen the bonds between cellulose microfibrils in the cell wall.
5. The cell wall becomes more flexible (plastic).
6. Water enters the cell by osmosis (due to the low water potential of the cell contents).
7. The cell elongates as turgor pressure pushes against the loosened cell wall.

Auxin Concentration and Growth

Organ	Effect of LOW auxin	Effect of MODERATE auxin	Effect of HIGH auxin
Shoot	Little growth	Maximum growth (promotion)	Inhibition of growth
Root	Maximum growth (promotion)	Inhibition of growth	Strong inhibition

This difference in sensitivity is crucial for understanding tropisms: the same concentration of auxin can promote growth in shoots while inhibiting growth in roots.

Exam Tip: The different sensitivity of roots and shoots to auxin concentration is a very commonly tested concept. Roots are much more sensitive than shoots -- they respond to lower concentrations and are inhibited by concentrations that promote shoot growth.

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Phototropism: The Mechanism

The explanation of phototropism involves the lateral redistribution of auxin:

In Shoots (Positive Phototropism)

1. Light hits the shoot tip from one side.
2. Auxin is redistributed to the shaded side of the shoot tip.
3. The shaded side has a higher concentration of auxin than the lit side.
4. In the shoot, higher auxin promotes more cell elongation on the shaded side.
5. Unequal elongation causes the shoot to bend towards the light.

In Roots (Negative Phototropism)

1. Light hits the root tip from one side.
2. Auxin is redistributed to the shaded side.
3. Higher auxin concentration on the shaded side inhibits cell elongation in the root (roots are more sensitive).
4. The lit side elongates more.
5. The root bends away from the light.

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Gravitropism: The Mechanism

Gravitropism involves statoliths -- starch-containing organelles (amyloplasts) that sediment under gravity.

In Roots (Positive Gravitropism)

1. When a root is placed horizontally, statoliths settle to the lower side of root cap cells.
2. This triggers the redistribution of auxin to the lower side of the root.
3. The high auxin concentration on the lower side inhibits cell elongation (roots are sensitive).
4. The upper side elongates more.
5. The root bends downwards (towards gravity).

In Shoots (Negative Gravitropism)

1. When a shoot is placed horizontally, auxin accumulates on the lower side.
2. Higher auxin on the lower side promotes more cell elongation in the shoot.
3. The lower side elongates more than the upper side.
4. The shoot bends upwards (away from gravity).

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Gibberellins

Gibberellins (GAs, particularly GA₃) are a large family of plant hormones involved in several growth processes:

Role	Mechanism
Stem elongation	Gibberellins promote cell elongation and cell division in the internodes (stem sections between nodes)
Seed germination	Gibberellins stimulate the production of amylase in the aleurone layer of cereal seeds; amylase hydrolyses starch → maltose → glucose, providing energy for the embryo to grow
Flowering	Gibberellins can promote bolting (rapid stem elongation before flowering) in some species
Fruit development	Gibberellins promote fruit growth; can be used to produce seedless grapes
Breaking dormancy	Gibberellins can overcome dormancy in seeds and buds

Gibberellins and Seed Germination (Detailed)

1. The seed absorbs water (imbibition).
2. The embryo produces gibberellins (GA).
3. GA diffuses to the aleurone layer (a protein-rich layer surrounding the endosperm).
4. GA stimulates the aleurone cells to produce alpha-amylase (and other hydrolytic enzymes).
5. Alpha-amylase hydrolyses starch in the endosperm to maltose.
6. Maltose is further hydrolysed to glucose by maltase.
7. Glucose provides the energy (via respiration) and carbon skeletons for the growing embryo.

Exam Tip: The gibberellin-aleurone-amylase pathway in seed germination is a commonly examined sequence. Make sure you include: gibberellin produced by the embryo → stimulates aleurone layer → amylase produced → starch hydrolysed to maltose → glucose for respiration.

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Other Plant Hormones

Hormone	Key Roles
Cytokinins	Promote cell division; delay senescence (ageing); work with auxins to control differentiation
Abscisic acid (ABA)	Promotes stomatal closure during drought stress; promotes seed dormancy; inhibits growth
Ethylene (ethene)	Promotes fruit ripening; promotes leaf abscission (leaf fall); stimulated by auxin at high concentrations

Interactions Between Plant Hormones

Plant hormones do not act in isolation -- they interact:

Interaction	Example
Auxin:cytokinin ratio	Controls differentiation: high auxin → root formation; high cytokinin → shoot formation
Gibberellin + auxin	Together promote stem elongation more than either alone (synergy)
Auxin promotes ethylene	High auxin concentrations stimulate ethylene production → fruit ripening and leaf abscission
ABA antagonises gibberellin	ABA promotes dormancy; gibberellins break dormancy

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Commercial Uses of Plant Hormones

Application	Hormone Used	Effect
Rooting powder	Synthetic auxin (e.g. IBA, NAA)	Promotes root growth on cuttings
Seedless fruit	Auxin or gibberellin	Promotes fruit development without fertilisation (parthenocarpy)
Selective herbicides	Synthetic auxins (e.g. 2,4-D)	Kill dicot weeds by overstimulating growth; monocot crops are unaffected
Fruit ripening	Ethylene	Applied to unripe fruit during transport to induce ripening
Delayed ripening	Ethylene inhibitors	Prevents premature ripening during storage
Malting in brewing	Gibberellin	Speeds up germination and amylase production in barley grains

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Investigating Tropisms: Required Practical Techniques

The Edexcel specification includes practical work on plant responses. Key techniques:

Technique	Description
Coleoptile experiments	Observing the growth of oat or wheat coleoptiles in response to unilateral light
Agar blocks	Auxin can be collected in agar blocks and placed asymmetrically on decapitated coleoptiles to demonstrate bending
Clinostat	A rotating device that eliminates the effect of gravity; used as a control
Light-proof covers	Covering the tip or base of the coleoptile to identify where light is detected

Exam Tip: In practical questions, always describe a suitable control (e.g. a coleoptile in uniform light or on a clinostat) and explain how you would measure the response (e.g. angle of bending, length of growth over a set time period).

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Summary

• Plants respond to stimuli via tropisms -- directional growth responses mediated by hormones.
• Auxin (IAA) is the primary hormone controlling phototropism and gravitropism. It is redistributed to the shaded side (phototropism) or lower side (gravitropism) and has different effects on roots and shoots due to their different sensitivities.
• The acid growth hypothesis explains how auxin promotes cell elongation.
• Gibberellins promote stem elongation, seed germination (amylase production), and flowering.
• Other hormones (cytokinins, ABA, ethylene) interact with auxins and gibberellins to coordinate plant growth and development.
• Plant hormones have many commercial applications in agriculture and horticulture.

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A-Level Deep Dive: Plant Hormones and Responses

Spec mapping

This lesson sits in Edexcel 9BI0 Topic 8 — Grey Matter (Coordination, Response and Gene Technology) and is the canonical worked example of chemical signalling without a circulatory system. The content statements paraphrase to: describe the role of plant growth substances (auxins, gibberellins, cytokinins, abscisic acid, ethylene); explain phototropism and gravitropism (geotropism) in shoots and roots in terms of differential cell elongation driven by lateral redistribution of auxin (indole-3-acetic acid, IAA); describe gibberellins in seed germination via induction of alpha-amylase in the aleurone layer of cereal grains; describe commercial applications (rooting compounds, selective herbicides, fruit-ripening control, malting) — refer to the official Pearson Edexcel 9BI0 specification document for exact wording. Examined on Paper 2 — Energy, Exercise and Coordination. Synoptic: contrasts with animal hormones in Lesson 1 (different chemistry — auxin is a tryptophan-derived indole acting at picomolar concentrations — but the same long-distance signalling logic, with vascular tissues replacing the circulatory system); the homeostasis principles of Lesson 6 apply (water balance via stomatal closure; negative-feedback architecture is identical); transpiration in Topic 7 is regulated by abscisic-acid-driven stomatal closure; seed germination by gibberellin-induced amylase links to Topic 5 carbohydrate metabolism and the barley malting industry; and auxin's TIR1 F-box receptor and SCF ubiquitin-ligase complex — beyond the spec but credit-worthy as AO3 — situates plant signalling within shared molecular vocabulary with animals.

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Worked example with full mark scheme

Question (8 marks): A botany student observes that a wheat seedling, illuminated from one side, bends toward the light source over 24 hours. A second seedling, kept in darkness with its tip removed, fails to bend toward a unilateral light source applied subsequently.

(a) Explain the mechanism of phototropism, naming the photoreceptor, the hormone, the redistribution mechanism, and the cellular events that produce shoot bending. (5)

(b) Explain why the decapitated seedling fails to respond. (3)

Solution with mark scheme:

(a) Step 1 — light detection. Blue-light photoreceptors (phototropins, PHOT1/PHOT2) in the shoot tip — flavoprotein kinases that autophosphorylate on illumination — initiate the signal driving auxin redistribution. M1 (AO1) — names phototropins.

Step 2 — auxin redistribution. IAA is synthesised at the tip from tryptophan; phototropin activation relocalises PIN auxin-efflux carriers to drive lateral redistribution to the shaded side. Auxin is not synthesised asymmetrically — the tip produces it symmetrically; PIN redirects flux. M1 (AO1) — names PIN-mediated redistribution rather than asymmetric synthesis.

Step 3 — acid growth. Higher shaded-side auxin activates H⁺-ATPases which acidify the wall (apoplast pH ~4.5); expansins loosen non-covalent bonds between cellulose microfibrils. M1 (AO1) — names the H⁺-ATPase / expansin chain.

Step 4 — turgor-driven elongation. With the wall loosened, turgor pressure drives water uptake and cell elongation; lit-side cells with lower auxin elongate less. A1 (AO2) — links acid growth to differential elongation.

Step 5 — bending. Differential elongation bends the shoot toward the light — positive phototropism. A1 (AO2) — identifies asymmetric elongation as the source of directional bending.

(b) Step 1. Decapitation removes both phototropin photoreceptors and the IAA biosynthesis site at the tip. M1 (AO1).

Step 2. Without a tip, no IAA is produced and no PIN-mediated redistribution can occur; downstream elongation machinery is intact but receives no asymmetric signal. M1 (AO1).

Step 3 — classical logic. This is the Boysen-Jensen / Went result: bending requires an intact tip or a tip-derived diffusible substance (Went's agar-block transfer produced bending without light). The decapitation negative control established auxin's role. A1 (AO2).

Total: 8 marks (5 + 3).

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Compare and contrast auxin (IAA) and abscisic acid (ABA) in plant signalling, referring to their chemical class, the variable each primarily controls, the cellular mechanism of action, and the role of each in adapting the plant to environmental challenge.

Mark scheme decomposition by AO:

Mark	AO	Awarded for
1	AO1	Naming auxin (IAA) as a tryptophan-derived indole synthesised in shoot apical meristems and young leaves, transported by PIN auxin-efflux carriers in polarised cell-to-cell flow; primarily controls directional growth (elongation, tropisms, apical dominance, root initiation).
2	AO1	Naming ABA as a carotenoid-derived sesquiterpene synthesised in roots under water stress, transported via xylem to leaves; primarily controls stress responses (stomatal closure, seed dormancy, growth inhibition) — the canonical "stress hormone".
3	AO2	Auxin mechanism: binds TIR1 F-box receptor, promoting ubiquitin-mediated degradation of Aux/IAA repressors and release of ARF transcription factors. The cell-elongation pathway via plasma-membrane H⁺-ATPase activation is faster (minutes); the transcriptional pathway is slower (hours).
4	AO2	ABA mechanism: binds PYR/PYL receptors; complex inhibits PP2C phosphatases, releasing SnRK2 kinases; SnRK2 activates SLAC1 anion channels and inhibits inward K⁺ channels → guard cells lose ions/water → flaccid → stomata close within minutes.
5	AO2	Variable controlled: auxin positions organs in space (toward light, gravity, water) and patterns the body. ABA shuts the plant down under stress (closes stomata, halts growth, locks seeds dormant). Auxin says "where to grow"; ABA says "whether to grow".
6	AO3	Local-vs-systemic / growth-vs-dormancy as architectural axes: auxin = local polarised cell-to-cell flow shaping morphology; ABA = long-distance root-to-shoot stress signal. Antagonistic — ABA suppresses auxin-driven growth under drought. The same axis spans plant hormones: ethylene local (gas), gibberellin intermediate, ABA long-distance via xylem.

Total: 6 marks (AO1 = 2, AO2 = 3, AO3 = 1). AO3 is reserved for the local-vs-systemic / growth-vs-dormancy architectural synthesis, not for restating mechanisms.

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Synoptic links

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