Plant Coordination
Plants coordinate responses without nerves or muscles. Rapid electrical signalling in the Venus flytrap, cell elongation driven by auxin’s proton pump mechanism, and gibberellin’s orchestration of seed germination through DELLA protein degradation and gene activation.
Venus Flytrap — Rapid Plant Response
The Venus flytrap (Dionaea muscipula) is a carnivorous plant that obtains nitrogen from trapped insects. It demonstrates that plants can generate electrical signals and produce rapid responses — without nerves or muscles. The trap closure is driven by turgor pressure changes, not muscle contraction.
Trap closure mechanism
The modified leaf has two lobes joined at a midrib. Each lobe contains 3–4 stiff sensory (trigger) hairs on its inner surface. The edges of the lobes bear interlocking teeth (cilia) that seal the trap when closed.
A single touch does not close the trap. Two stimulations within ~20 seconds are required. This prevents accidental closure from rain drops or debris. The plant “counts” stimuli because the first touch generates a sub-threshold receptor potential; the second generates a full action potential.
- An insect contacts a trigger hair on a lobe, causing mechanical deformation
- Deformation opens mechanically gated ion channels in the trigger hair cell membrane
- Ion flux generates a receptor potential that, if a second stimulation occurs within ~20 s, generates a full action potential
- Action potentials are transmitted rapidly through the leaf via plasmodesmata (connecting adjacent cells) — no nervous system is needed
- Action potentials trigger rapid changes in turgor pressure in cells in the midrib region: water leaves certain cells rapidly by osmosis, changing cell shape
- The lobes snap shut in <100 milliseconds — driven by elastic energy stored in the leaf structure releasing rapidly as turgor changes
- Interlocking teeth seal the trap; glands on the inner surface secrete digestive enzymes
- Digested nutrients (particularly nitrogen compounds) are absorbed by the plant
Venus flytrap closure is driven by changes in turgor pressure (water movement), not by muscle contraction. There are no muscle proteins (actin/myosin). The action potentials are transmitted through plasmodesmata, not along nerve axons. The response time is fast because elastic energy is pre-stored in the leaf structure and released rapidly when turgor changes.
9700 exam scope: the core requirement is that plants can use electrical signals (action potentials) and rapid turgor pressure changes to produce fast responses — without nerves or muscles. Detailed molecular ion-channel mechanisms (gating, threshold depolarisation voltage) are useful for understanding but should not replace the core sequence: mechanical stimulation → action potentials → turgor pressure change → trap closure. Exam marks are awarded for this sequence, not for ion-channel molecular detail.
A Venus flytrap fails to close when a single trigger hair is touched once. Which of the following best explains this observation?
- (A) The plant is dormant and does not generate electrical signals
- (B) The digestive enzymes have not been secreted yet
- (C) A single stimulation generates a sub-threshold potential; a second stimulation within ~20 s is required to generate a full action potential
- (D) The turgor pressure in the lobe cells is already at maximum
Auxin (IAA) and Cell Elongation
Auxin (IAA — indole-3-acetic acid) is a plant hormone synthesised primarily in the apical meristems of shoots. It promotes cell elongation and is responsible for directional growth responses (tropisms). The primary mechanism is the acid growth hypothesis.
Proton pump and acid growth hypothesis
- Auxin (IAA) binds to a specific receptor protein on the cell surface membrane of target cells in the elongation zone
- Receptor activation stimulates H⁺-ATPase (proton pumps) in the plasma membrane
- The proton pump uses ATP to actively transport H⁺ ions from the cytoplasm into the cell wall (apoplast)
- The pH of the cell wall falls (becomes more acidic, approximately pH 4.5–5.0)
- The lower pH activates expansins — proteins that loosen the bonds between cellulose microfibrils and hemicellulose chains in the cell wall
- The cell wall becomes more plastic (stretchable) and less rigid
- The reduced wall tension allows water to enter by osmosis (down the water potential gradient), increasing turgor pressure
- The increased turgor pressure stretches the now-flexible wall → irreversible cell elongation
Auxin → H⁺-ATPase → H⁺ pumped out → wall acidifies → expansins activated → wall loosens → water enters (osmosis) → turgor increases → cell elongates
The critical step that students miss: auxin does not directly loosen the wall — it stimulates the proton pump, which acidifies the wall, which then activates expansins. H⁺ pumping is the intermediate step.
Phototropism and auxin distribution
When a shoot tip receives light from one side, auxin is redistributed laterally towards the shaded side. This results in a higher auxin concentration on the dark side of the shoot.
Higher auxin concentration on the shaded side → greater elongation on that side → shoot bends towards light (positive phototropism).
The 9700 syllabus requires understanding that auxin causes differential elongation resulting in phototropism. The mechanism of auxin redistribution is understood, but very detailed molecular mechanisms of auxin transport are not required.
Key point: it is the differential distribution of auxin (more on one side) rather than the absolute concentration that drives bending growth.
Describe how auxin stimulates the elongation of cells in a shoot tip. [5]
- Auxin (IAA) binds to a receptor on the cell surface membrane of shoot cells [1]
- Binding stimulates H⁺-ATPase (proton pumps) in the plasma membrane to actively pump H⁺ ions from the cytoplasm into the cell wall [1]
- The cell wall pH decreases (acidification); the lower pH activates expansins [1]
- Expansins loosen the bonds between cellulose microfibrils and hemicellulose, making the wall more plastic/flexible [1]
- Water enters the cell by osmosis; increased turgor stretches the flexible wall, causing irreversible cell elongation [1]
Gibberellin and Barley Seed Germination
Gibberellins (GAs) are plant hormones with roles in seed germination, stem elongation, and fruit development. The 9700 syllabus focuses on the role of GA in barley (Hordeum vulgare) seed germination as a detailed case study.
Barley germination — aleurone layer and amylase
- Imbibition: the dry barley grain absorbs water; the embryo becomes metabolically active
- The embryo synthesises and secretes gibberellin (GA)
- GA diffuses from the embryo to the aleurone layer — a single cell layer surrounding the starchy endosperm
- GA binds to receptors in the aleurone cells and triggers gene expression: the genes encoding hydrolytic enzymes (primarily α-amylase) are transcribed and translated
- α-Amylase is secreted into the endosperm; it hydrolyses starch to maltose and glucose
- Soluble sugars diffuse to the embryo, providing substrate for cellular respiration and building materials for growth
- The embryo grows: the radicle (root) and plumule (shoot) emerge from the grain
The aleurone layer acts as a signal amplifier. The embryo produces GA, which travels a short distance to the aleurone layer and switches on enzyme production. The aleurone cells then flood the endosperm with amylase. This means the embryo does not need to synthesise all the enzymes itself — a small GA signal triggers a much larger enzyme response.
α-Amylase hydrolyses α-1,4 glycosidic bonds in amylose and amylopectin → producing maltose (disaccharide) and glucose (monosaccharide). These are used by the embryo for:
- Cellular respiration (ATP for growth)
- Building block synthesis (cell walls, membranes)
Describe the role of gibberellin in the germination of a barley grain. [4]
- The embryo synthesises gibberellin after water is absorbed (imbibition) [1]
- Gibberellin diffuses from the embryo to the aleurone layer (single cell layer surrounding the endosperm) [1]
- Gibberellin stimulates aleurone cells to produce (transcribe and translate genes for) α-amylase [1]
- α-Amylase hydrolyses starch in the endosperm to maltose/glucose, providing soluble substrates for the embryo to respire and grow [1]
DELLA Proteins and Gene Regulation by Gibberellin
GA stimulates gene expression by removing DELLA proteins — repressor proteins that block the transcription of growth genes. This is the molecular mechanism linking the GA signal to changes in gene expression.
- In the absence of GA, DELLA proteins bind to and inhibit transcription factors that would activate growth/germination genes. Gene expression is suppressed.
- GA is present (e.g. after embryo synthesis in germinating barley)
- GA binds to the GID1 receptor inside the cell
- The GA-GID1 complex interacts with DELLA proteins, targeting them for degradation (via the ubiquitin-proteasome pathway)
- With DELLA repressors removed, transcription factors are freed to bind to the promoters of target genes
- Target genes (e.g. the gene for α-amylase in aleurone cells) are transcribed and translated
- The gene products (e.g. α-amylase) carry out the cellular response (starch hydrolysis)
GA does not directly activate genes. Instead, it removes an inhibitor (DELLA protein) — a mechanism called derepression. The transcription factor was always capable of activating the gene; it was being held back by DELLA. This “removing the brake” strategy is efficient because gene activation is immediate once the repressor is removed.
Synoptic link to Topic 16.3: DELLA protein degradation is the 9700 example of how a non-DNA signal (a hormone) regulates gene expression in eukaryotes through protein intermediates, complementing the lac operon example of prokaryotic gene regulation.
Explain how gibberellin activates the expression of the amylase gene in aleurone cells. [3]
- DELLA proteins normally bind to transcription factors and prevent them from activating the amylase gene [1]
- Gibberellin binds to GID1 receptor; the GA-GID1 complex causes degradation of DELLA proteins [1]
- Released transcription factors bind to the promoter of the amylase gene; the gene is transcribed and α-amylase is produced [1]
Le/le Alleles and Gibberellin-Mediated Stem Elongation
The Le gene in peas (Pisum sativum) illustrates how a single gene controls a phenotype through its effect on a hormone synthesis pathway. This is a key example of the gene → protein → phenotype relationship required by the 9700 syllabus.
| Allele | Enzyme | Gibberellin level | Phenotype |
|---|---|---|---|
| Le (dominant) | Functional enzyme in GA synthesis pathway | Normal GA produced | Tall plant — normal internode elongation |
| le (recessive) | Non-functional enzyme (loss-of-function mutation) | Very little GA produced | Dwarf plant — short internodes |
Le allele: Le gene → transcription → mRNA → translation → functional enzyme → GA synthesised from precursor → DELLA proteins degraded → growth genes expressed → cells elongate → tall phenotype
le allele: mutated Le gene → non-functional enzyme → GA not synthesised → DELLA proteins persist → growth genes not expressed → cells do not elongate → dwarf phenotype
This is the molecular explanation for Mendel’s famous tall vs dwarf pea character from the 1860s. He observed that tall (Le—) is dominant over dwarf (lele), and that tall:dwarf plants appear in a 3:1 ratio in F2 crosses. The molecular mechanism was only understood over a century later.
Applying exogenous (externally applied) GA to dwarf le/le plants restores tall growth — confirming that the dwarf phenotype is caused by GA deficiency, not a structural defect.
The Le/le example is shared between Topic 15C (plant coordination) and Topic 16C (gene control). In Topic 15 it illustrates how a hormone controls growth. In Topic 16 it illustrates the gene → protein → phenotype relationship with a specific named gene and enzyme. Make sure you know it from both perspectives.
A pea plant is homozygous for the recessive le allele. Explain why this plant has a dwarf phenotype. [4]
- The le allele codes for a non-functional enzyme in the gibberellin (GA) synthesis pathway [1]
- Little or no functional GA is produced in the dwarf plant [1]
- Without GA, DELLA repressor proteins are not degraded and remain bound to transcription factors [1]
- Growth genes (required for internode cell elongation) are not transcribed; cells in internodes do not elongate, producing the dwarf phenotype [1]
Practice Questions
Compare the mechanisms by which (i) the Venus flytrap closes its trap and (ii) a shoot cell elongates in response to auxin. In your answer, refer to the role of turgor pressure in both cases. [6]
- Mechanical stimulation of trigger hairs generates action potentials propagated via plasmodesmata [1]
- Action potentials trigger rapid water movement out of cells in the midrib, decreasing turgor in those cells → leaf lobes snap shut [1]
- Response time <100 ms; movement is driven by release of elastic energy as turgor changes [1]
- Auxin stimulates H⁺-ATPase to pump H⁺ into the cell wall; lower pH activates expansins that loosen wall [1]
- Water enters the cell by osmosis; increased turgor stretches the loosened wall [1]
- Cell elongation is irreversible; driven by increase in turgor rather than a decrease [1]
Common Mistakes
- ⚡Venus flytrap uses musclesNo muscle proteins exist in plants. Trap closure is driven entirely by turgor pressure changes. Action potentials in plants are transmitted through plasmodesmata, not axons.
- 📈Saying auxin directly loosens the cell wallAuxin does not directly loosen the wall. The chain is: auxin → H⁺-ATPase → H⁺ pumped out → wall acidification → expansins activated → wall loosened. Missing the H⁺ pump step loses the mechanism mark.
- 🌿Gibberellin directly activates amylase gene expressionGA does not directly bind DNA. GA degrades DELLA repressor proteins, which releases transcription factors that then activate gene expression. “GA removes the brake (DELLA)” — not “GA accelerates the car”.
- 🔰Confusing aleurone layer and endospermThe aleurone layer is the single cell layer surrounding the endosperm (starch store). GA acts on the aleurone layer to produce amylase. Amylase is then secreted INTO the endosperm to hydrolyse starch. The embryo does not produce amylase directly.
- 🍄Le/le: saying the le plant has no GA receptorThe le allele affects GA synthesis (an enzyme in the pathway), not the GA receptor. The le/le plant cannot make adequate GA, so the receptor is never properly activated — but the receptor itself is functional.
Plant coordination is a high-value synoptic topic. Connect it to: Topic 6 (transcription/translation — how GA leads to amylase gene expression), Topic 14 (ABA uses Ca²⁺ not cAMP; guard cell osmosis), Topic 15A/B (action potentials in Venus flytrap are analogous to nerve action potentials), Topic 16C (DELLA mechanism as gene control example; Le/le as gene→phenotype example). Highest-yield exam items: auxin proton pump sequence (5 steps), gibberellin→aleurone→amylase→starch hydrolysis, DELLA degradation mechanism, Le/le chain of causation.