Inquiry Question 2: How does a plant or animal respond to infection?
Investigate the response of a named Australian plant to a named pathogen through the application of physical and chemical defences
A focused answer to the HSC Biology Module 7 dot point on plant defences. Covers the waxy cuticle, bark, stomatal closure, callose deposition, phytoalexins and the hypersensitive response, with a named Australian example (jarrah and Phytophthora cinnamomi).
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What this dot point is asking
NESA wants you to describe how plants defend themselves against infection, distinguish between physical and chemical defences, and provide a named Australian plant and a named pathogen. Plant immunity is often examined in 3 to 6 mark short responses, and the command words range from "outline" up to "assess" - so match your depth to the verb and always anchor your answer to a named example.
The answer
Plants lack mobile immune cells and circulating antibodies. Instead they rely on a combination of pre-existing structural defences and induced biochemical responses. Two ideas unlock most of the marks: the split between physical (a barrier/structure) and chemical (an antimicrobial molecule) defences, and the split between pre-formed (constitutive) defences that are always present and induced defences that switch on only after the plant detects a pathogen.
Physical defences (passive and structural)
- Cuticle and bark
- The outer surfaces of leaves and stems are covered by a waxy cuticle (cutin and waxes) that resists water loss and pathogen entry. Woody stems have lignified bark, a tough physical barrier that few pathogens can penetrate.
- Cell walls
- Each plant cell is enclosed in a rigid cellulose cell wall. Pathogens must produce wall-degrading enzymes (cellulases, pectinases) to enter.
- Trichomes and thorns
- Hair-like trichomes and physical spines deter macroparasites and reduce pathogen contact.
- Stomatal closure
- Stomata are the main entry point for airborne pathogens. Guard cells detect pathogen-associated molecular patterns (PAMPs) such as flagellin and close the stomatal pore.
The diagram below collects the main physical barriers - the surfaces a pathogen meets before it can ever reach living tissue.
Induced physical defences
Callose deposition. When a pathogen attempts to enter, the plant deposits callose (beta-1,3-glucan) into the cell wall at the site of attack, forming a localised plug.
Tylose formation. In xylem vessels, neighbouring cells extrude into the vessel lumen, forming tyloses that block fungal spread through the vascular system.
Chemical defences
- Phytoalexins
- Small antimicrobial molecules (often terpenes, alkaloids or phenolics) synthesised in response to infection. Examples include camalexin in Arabidopsis and the terpene-based oils in Eucalyptus species.
- Reactive oxygen species (ROS)
- Plants produce hydrogen peroxide and superoxide at the infection site, damaging pathogen membranes and triggering further defence signalling.
- Defensive enzymes
- Plants produce chitinases (degrade fungal cell walls), glucanases and protease inhibitors that disable pathogen enzymes.
- Pre-formed antimicrobials
- Many plants store compounds in vacuoles or specialised cells that are released on wounding. Eucalyptus essential oils (cineole, pinene) and tea tree oil (terpinen-4-ol) are antimicrobial constituents of native Australian plants.
The hypersensitive response
The most dramatic plant defence. On detecting pathogen effector proteins, infected cells trigger programmed cell death, killing themselves and the pathogen at the infection site. The result is a small lesion of dead tissue that isolates the pathogen.
A linked response, systemic acquired resistance (SAR), primes the rest of the plant against future infection. Salicylic acid acts as the systemic signal.
The diagram below shows the induced chemical line of defence at a single infected cell: antimicrobial molecules are deployed, and if the threat is serious the cell sacrifices itself to seal the pathogen off.
Worked example: jarrah and Phytophthora cinnamomi
The jarrah tree (Eucalyptus marginata), a keystone species in Western Australia, is severely affected by jarrah dieback, caused by Phytophthora cinnamomi.
Pathogen. P. cinnamomi is an oomycete (water mould). It produces motile zoospores in moist soil that swim toward root exudates and infect fine roots.
Plant defences.
- Lignified bark and a waxy cuticle on stems and leaves prevent surface infection.
- Infected root cells deposit callose and lignin to seal off the infection.
- Eucalyptus species accumulate phytoalexins (terpenes and phenolic compounds) and produce reactive oxygen species at infection sites.
- Hypersensitive cell death isolates infected root tips.
Despite these defences, P. cinnamomi often overwhelms the plant in wet soils, and jarrah dieback has become one of Australia's most damaging plant diseases. Management focuses on hygiene and quarantine of soil and vehicles in affected areas.
More named examples in context
Eucalyptus essential oils against post-fire fungi. Mature Eucalyptus camaldulensis (river red gum) trees along the Murray-Darling produce essential oils dominated by 1,8-cineole (eucalyptol) and alpha-pinene, stored in pellucid oil glands visible in fresh leaves. These terpenes are constitutive chemical defences with broad antimicrobial activity against fungi such as Armillaria luteobubalina, a wood-decay basidiomycete common after fire. When fungal hyphae penetrate the bark, they encounter terpenes that disrupt fungal cell membranes. This is an example of constitutive (always-present) chemical defence rather than an induced response.
Induced callose deposition in cereals. Wheat (Triticum aestivum) infected by Puccinia graminis stem rust triggers a localised induced response: within hours of fungal penetration, plant cells around the infection site deposit callose (beta-1,3-glucan) at the penetration peg, physically blocking further hyphal entry. If the plant carries a resistance gene matching the fungal effector, it also mounts the hypersensitive response - rapid death of the infected and surrounding cells - depriving the biotrophic fungus of living host tissue. This illustrates how induced physical (callose) and the hypersensitive response work together; for a native Australian named pair in the exam, keep to jarrah and P. cinnamomi.
Exam-style practice questions
Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2024 HSC4 marksDescribe a plant disease and its effect on agricultural production. (Name the plant disease.)Show worked answer →
Full marks (4) require describing a named plant disease AND its effects on agriculture. Sample answer: Stone fruit scab, a fungal disease affecting stone fruit (plums, peaches, nectarines). It begins as small dark spots on the fruit that become scabby and may cause the fruit to crack, shrivel and fall off. Effect on agriculture: it downgrades fruit quality and decreases yield, leading to economic losses. Marker feedback: you must name a real plant disease and state a clear, directional effect on agricultural production (e.g. reduced yield/quality), not a vague "it has an impact." Source: NESA 2024 HSC Biology examination and marking guidelines.
Practice questions
Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.
foundation2 marksDistinguish between a physical defence and a chemical defence in plants, giving one example of each.Show worked solution →
1 mark - physical defence. A physical (structural) defence is a barrier or structure that physically blocks or impedes a pathogen, for example the waxy cuticle, bark, cell wall or callose deposits.
1 mark - chemical defence. A chemical defence is an antimicrobial substance the plant produces, for example phytoalexins, reactive oxygen species or defensive enzymes such as chitinases.
The mark hinges on the contrast: physical = a structure/barrier; chemical = a molecule that attacks the pathogen. A valid example must accompany each definition.
foundation3 marksOutline the role of each of the following plant defences: (a) the waxy cuticle, (b) stomatal closure, (c) callose deposition.Show worked solution →
- 1 mark - waxy cuticle
- The waxy cuticle (cutin and waxes) coats the leaf and stem surface and acts as a pre-formed physical barrier that resists pathogen entry and water loss.
- 1 mark - stomatal closure
- Guard cells detect pathogen-associated molecular patterns (PAMPs) and close the stomatal pore, shutting the main entry route for airborne pathogens.
- 1 mark - callose deposition
- When a pathogen tries to penetrate, the plant deposits callose (beta-1,3-glucan) at the attack site, forming a localised plug that physically blocks further entry.
Each defence must be tied to its specific job. Note the contrast between the cuticle (pre-formed) and callose/stomatal closure (induced).
foundation2 marksExplain why naming a specific Australian plant and a specific pathogen is essential when answering this dot point.Show worked solution →
1 mark - the syllabus requirement. The dot point asks for the response of a named Australian plant to a named pathogen, so a generic answer about "a plant" or "a fungus" does not meet the question and earns no marks for the example.
1 mark - a valid named pair. A correct named pair is jarrah (Eucalyptus marginata) infected by Phytophthora cinnamomi (causing jarrah dieback). (Accept other valid Australian pairs, e.g. wheat and Puccinia graminis is not Australian-native, so prefer eucalypt/banksia/wattle examples.)
Markers award the application marks only when both the plant and pathogen are correctly named and Australian.
core4 marksDescribe the hypersensitive response and explain how it limits the spread of a pathogen through a plant.Show worked solution →
- 1 mark - trigger
- On detecting pathogen effector proteins (often via a matching plant resistance gene), the infected cell initiates the hypersensitive response.
- 1 mark - mechanism
- The infected and immediately surrounding cells undergo programmed cell death (apoptosis), deliberately killing themselves.
- 1 mark - the outcome
- This produces a small lesion of dead tissue that seals off the infection site, isolating the pathogen.
- 1 mark - why it works
- Many pathogens (especially biotrophic fungi) need living host tissue to survive; by sacrificing local cells the plant deprives the pathogen of nutrients and physically contains it, preventing spread to healthy tissue.
Full marks need the trigger, the cell-death mechanism, the isolating lesion AND the reason cell death stops the pathogen (depriving it of living tissue).
core4 marksA study measures callose deposition in barley leaves at 6, 12 and 24 hours after powdery mildew inoculation, recording 5, 38 and 62 callose papillae per mm² respectively. Describe the trend and explain how it contributes to resistance.Show worked solution →
- 1 mark - describe the trend
- Callose deposition increases over time, rising from 5 to 38 to 62 papillae per mm² across the 6 to 24 hour period (a roughly 12-fold increase).
- 1 mark - identify it as induced
- This rising response shows callose deposition is an induced defence - it builds up after the plant detects the pathogen, rather than being pre-formed.
- 1 mark - the mechanism of resistance
- Callose (beta-1,3-glucan) is laid down as papillae at the penetration sites, physically reinforcing the cell wall and blocking the fungal penetration peg.
- 1 mark - link to resistance
- By the time deposition peaks, most attempted penetrations are physically halted, so fewer hyphae establish inside the leaf - the plant resists infection.
A response that only describes the numbers without explaining how callose blocks penetration caps at 2 marks.
core5 marksUsing a named Australian plant and a named pathogen, describe two physical and two chemical defences the plant uses against that pathogen.Show worked solution →
- Named pair (1 mark)
- Jarrah (Eucalyptus marginata) defending against Phytophthora cinnamomi, the oomycete causing jarrah dieback. (Both the plant and pathogen must be named.)
- Two physical defences (2 marks)
- (1) Lignified bark and the waxy cuticle on stems and leaves form a pre-formed barrier to surface infection. (2) Infected root cells deposit callose and lignin to wall off and seal the infection site (an induced physical defence).
- Two chemical defences (2 marks)
- (1) Phytoalexins - eucalypts accumulate terpenes and phenolic compounds that are antimicrobial. (2) Reactive oxygen species (hydrogen peroxide, superoxide) produced at the infection site damage pathogen membranes and trigger further signalling. (Accept also Eucalyptus essential oils such as 1,8-cineole and pinene.)
Each defence must be correctly classified as physical or chemical. Mislabelling (e.g. calling phytoalexins a physical defence) loses that mark.
exam7 marksJarrah (Eucalyptus marginata) is severely affected by jarrah dieback caused by Phytophthora cinnamomi. Assess the effectiveness of the plant's physical and chemical defences in resisting this pathogen.Show worked solution →
"Assess" requires a judgement about how effective the defences are, supported by the strengths and limitations of each. A Band 6 response reaches a reasoned conclusion, not a list.
- The defences and their strengths (2-3 marks)
- Jarrah has a layered defence: lignified bark and waxy cuticle resist surface infection; callose and lignin are deposited to seal infected root cells; phytoalexins (terpenes, phenolics) and reactive oxygen species chemically attack the pathogen; and the hypersensitive response sacrifices infected root tips to isolate the oomycete. Against many soil pathogens this combination is effective and the tree survives.
- The limitations against this pathogen (2-3 marks)
- P. cinnamomi is an oomycete that produces motile zoospores in moist/waterlogged soil, swimming to and infecting fine roots below the protective bark and cuticle. It secretes cell-wall-degrading enzymes and can tolerate some plant chemical defences, and root infection in wet conditions is rapid and systemic, often outpacing the localised callose/hypersensitive responses. The defences evolved against other threats and are only partially effective here.
- Judgement (1-2 marks)
- A supported conclusion: jarrah's defences are effective against many pathogens but inadequate against P. cinnamomi, because the pathogen attacks the unprotected roots, thrives in wet soils and overwhelms the localised induced responses, which is why jarrah dieback is one of Australia's most damaging plant diseases and why management relies on hygiene and quarantine rather than the tree's own defences. An answer lacking an explicit, justified judgement caps below full marks.
exam6 marksCompare physical and chemical defences in plants, and explain why a plant typically requires both to resist infection effectively.Show worked solution →
Target a genuine comparison (shared features and differences) plus a clear reason both are needed.
The comparison (3-4 marks). Both are defences that reduce pathogen establishment, and both include pre-formed (constitutive) and induced examples. Physical defences are structures/barriers - the waxy cuticle, bark, cell walls (pre-formed) and callose, tyloses and lignin (induced) - that block or impede pathogen entry and spread. Chemical defences are antimicrobial molecules - pre-formed essential oils/antimicrobials and induced phytoalexins, reactive oxygen species and defensive enzymes (chitinases, glucanases) - that damage or disable the pathogen directly.
Why both are required (2-3 marks). Physical barriers slow or prevent entry but a determined pathogen can secrete wall-degrading enzymes to breach them; chemical defences then attack any pathogen that gets through, while the hypersensitive response isolates it. Relying on barriers alone fails once they are breached; relying on chemicals alone wastes resources and may not stop a fast invader at the surface. The layered combination - block at the surface, attack and isolate internally - gives far more reliable resistance than either alone.
Full marks need both the shared/contrasting features AND the layered-defence reasoning for why both are necessary.