Biology · Year 12 · Module 7 · Lesson 8
How Plants Respond to Pathogens
Build HSC band 5–6 extended-response technique on plant defence mechanisms — evaluating the hypersensitive response, SAR, and the Banksia–Phytophthora case study under real conservation pressure.
1. Extended response — evaluate plant defence mechanisms in the Banksia–Phytophthora system (Band 5–6)
8 marks Band 5–6
Stimulus. Phytophthora cinnamomi — an oomycete pathogen introduced from Southeast Asia — has been described by the IUCN as one of the world's 100 worst invasive species. In south-western Australia it threatens an estimated 40% of native plant species, including multiple Banksia species across the Swan Coastal Plain and Stirling Range. Field surveys of infected sites consistently show that a small proportion of Banksia individuals (typically 5–15%) survive at high pathogen loads for years. Laboratory analysis of these survivors reveals significantly elevated root phytoalexin concentrations, more rapid R-protein–triggered hypersensitive responses, and measurable systemic acquired resistance (SAR) activity compared to susceptible individuals from the same population. A 2022 conservation program in the Stirling Range has begun selecting seeds from these putatively resistant survivors for ex situ seed banking and future replanting trials. Conservation ecologists debate whether survivor resistance reflects genuine constitutive or induced defence differences, or simply a chance delay in infection, and what this means for breeding programs.
Q1. Evaluate the plant defence mechanisms available to Banksia in resisting Phytophthora cinnamomi infection. In your response you must:
- Explain the role of both physical and chemical defences in the initial resistance of Banksia to Phytophthora entry and spread.
- Describe the sequence and function of the hypersensitive response as the primary cellular defence once infection has occurred, and explain why its effectiveness is limited in most Australian Banksia species.
- Explain how systemic acquired resistance could benefit a Banksia individual that survives an initial infection, and identify one key limitation of SAR that reduces its conservation value.
- Reach a justified, evidence-based evaluation of whether the survivor phenotype described in the stimulus is more likely to reflect genuine defence superiority or chance delay in infection, and what this implies for the seed-banking program's effectiveness.
2. Evaluate this claim — science communication and the hypersensitive response (Band 5–6)
7 marks Band 5–6
"Plants have an immune system that works just like ours. When a pathogen attacks, the plant gets a kind of fever — cells heat up and get inflamed at the infection site. The hypersensitive response is basically the plant's equivalent of an allergic reaction: cells overreact, which unfortunately kills them. That's why you see the dark spots on infected leaves — the immune system has gone haywire and damaged the plant's own tissue. Once this has happened, the plant is weakened and the pathogen can spread freely, so the spots are a bad sign."
— Paraphrased from a popular-science blog post, 2024.
Q2. Evaluate this claim. For each of the four underlined biological ideas in the passage, identify whether it is correct, partially correct, or wrong, explain the correct biology, and — for any claims you identify as wrong — describe how the claim could be tested experimentally.
The four ideas to evaluate: (i) "plant immune system works just like ours" · (ii) "gets a kind of fever — cells heat up and get inflamed" · (iii) "hypersensitive response is basically an allergic reaction — cells overreact" · (iv) "dark spots mean the immune system has gone haywire and the pathogen can spread freely".
Q1 — Sample Band 6 response (8 marks), annotated
Banksia mounts a two-layer defence against Phytophthora cinnamomi — physical and chemical/cellular — though its effectiveness is limited by the pathogen's evolutionary novelty in Australia. [framing]
Physical and initial chemical defences: At the root surface, cell wall composition provides a first physical barrier to Phytophthora zoospore penetration. Importantly, Phytophthora degrades the cell wall enzymatically, so this barrier slows but does not prevent entry. Resistant Banksia individuals reinforce their walls with callose and phenolic compounds at infection sites, and some species grow proteoid cluster roots away from infected soil zones to partially compensate for root loss. Constitutive tannins and phenolics in root vacuoles are released when cells are damaged, adding chemical toxicity to the initial physical defence layer. [2 marks — physical + initial chemical with mechanisms]
Hypersensitive response: Once hyphae penetrate root cortex cells, R-protein receptors in resistant individuals detect Phytophthora PAMPs and trigger the HR cascade: (1) R-protein recognition → (2) ROS burst (hydrogen peroxide/superoxide — directly toxic + alarm signal) → (3) programmed cell death of infected and surrounding cells → (4) visible necrotic zone → (5) callose/lignin reinforcement of surrounding healthy cells. This is effective because Phytophthora is hemibiotrophic — it requires living host cells in early infection stages, so a dead zone deprives it of the resource it needs. However, in most Australian Banksia species the HR is delayed or absent: their R-protein systems did not co-evolve with this introduced pathogen, so recognition of its PAMPs is inefficient. Even where HR is mounted, Phytophthora spreads through multiple simultaneous root entry points via soil water, quickly overwhelming any localised necrotic containment. [2 marks — HR sequence with mechanism + explanation of limitation]
Systemic acquired resistance (SAR): Surviving individuals that mount a successful HR generate salicylic acid signals that travel through the phloem to uninfected tissues, activating PR gene expression (chitinases, glucanases, protease inhibitors) throughout the plant and priming phytoalexin production. This means a Banksia that survives one infection is faster and stronger in its defence response when new infections are established. A key limitation for conservation: SAR is not heritable — it cannot be passed to seeds. Each new seedling must mount its own initial exposure response. A seed banking program using survivor parents therefore cannot directly pass SAR activation to the next generation; what it can pass on are the genetic loci encoding more efficient R-proteins or higher-capacity phytoalexin synthase pathways — which are heritable. [2 marks — SAR benefit + limitation for conservation]
Evaluation of survivor phenotype: The stimulus provides three lines of evidence that favour genuine defence superiority over chance delay: (1) significantly elevated phytoalexin concentrations in survivors suggest a higher constitutive or induced phytoalexin capacity, not just a chance temporal gap in infection; (2) more rapid R-protein-triggered HR implies a heritable allele difference in pattern recognition; (3) measurable SAR activity in survivors indicates their defence machinery is being actively engaged, not bypassed. Taken together, this suggests the survivor phenotype is more likely to reflect genetic differences in physical cell wall composition, R-protein efficiency, or phytoalexin pathway capacity — all potentially heritable — than mere chance delay. The seed banking program's value therefore lies not in the plants' SAR (which is not heritable) but in capturing alleles for superior R-protein recognition and phytoalexin production. However, the program should be evaluated against the possibility that some survivors are chance-delayed: ideally, survivor seeds should be tested by inoculation challenge before inclusion in the banking program to confirm resistance, not just survival. [2 marks — evidence-based judgement on survivor phenotype + conservation implication]
Marking criteria.
- 1 mark — Explains the role of physical defences (cell wall reinforcement, callose/phenolics) in slowing Phytophthora entry at root surfaces, with mechanism.
- 1 mark — Explains the role of chemical defences (constitutive phenolics/tannins and induced phytoalexins) as a complementary second-layer defence, noting constitutive vs induced distinction.
- 1 mark — Describes the HR sequence correctly (R-protein → ROS → programmed cell death → necrotic zone → callose/lignin reinforcement) and its rationale (depriving hemibiotrophic pathogen of living cells).
- 1 mark — Explains why the HR is limited in most Australian Banksia: R-proteins did not co-evolve with the introduced pathogen; multi-site simultaneous infection overwhelms localised containment.
- 1 mark — Explains the SAR benefit: salicylic acid → PR gene expression throughout plant → faster, primed response on future infection events.
- 1 mark — Identifies a key limitation of SAR for conservation: SAR is not heritable; seeds cannot inherit activated SAR — only heritable alleles (R-protein efficiency, phytoalexin capacity) can be banked.
- 1 mark — Reaches an evidence-based evaluation of survivor phenotype: three stimulus data points (elevated phytoalexins, faster HR, SAR activity) are consistent with genuine heritable defence differences, not chance delay alone.
- 1 mark — States a clear conservation implication (program value is in capturing heritable defence alleles; recommends inoculation challenge to confirm resistance before seed banking) supported by the lesson framework.
Q2 — Sample Band 6 response (7 marks), annotated
(i) "Plant immune system works just like ours" — Wrong. Plant and animal immune systems are analogous but mechanistically entirely different. Animals use specialised mobile immune cells (B cells, T cells, macrophages, neutrophils) and produce antibodies targeting specific antigens. Plants have no mobile immune cells and produce no antibodies. Instead, every plant cell is capable of its own defence response through pattern recognition (R-proteins detecting PAMPs), and communication occurs via chemical signals (salicylic acid) through the phloem rather than immune-cell migration. The analogy is useful for conceptual orientation but the mechanisms are fundamentally different. Experimental test: infect a plant and then extract phloem sap to confirm salicylic acid signalling (not immune cell migration) is the defence signal; compare with a blood sample from an animal showing immune cell migration after infection. [1 — correct evaluation + experimental test]
(ii) "Gets a kind of fever — cells heat up and get inflamed" — Partially correct but misleading. Plants do mount a localised reactive oxygen species (ROS) burst at infection sites, and the ROS can cause localised chemical changes; however, plants do not thermoregulate and do not produce fever (elevated whole-body temperature). Plant cells do not "get inflamed" in the mammalian sense — inflammation involves vasodilation, increased blood flow, and immune cell recruitment, none of which apply to plants. The analogous plant response is a ROS burst and local ion flux, not thermal or vascular inflammation. Experimental test: measure temperature at the infection site on a plant leaf using an infrared thermometer over 24 hours — a true fever would show sustained elevated temperature, which plants do not produce. [1 — correct/partial verdict + experimental test]
(iii) "HR is basically an allergic reaction — cells overreact" — Wrong. An allergic reaction in animals is a maladaptive, excessive immune response to a harmless antigen (e.g. pollen triggering mast cell degranulation and histamine release). It is pathological — it causes harm to the organism without benefit. The HR is the opposite: it is a deliberately adaptive, controlled mechanism of programmed cell death that the plant uses strategically. Infected cells die to create a zone of dead tissue that the pathogen (if biotrophic/hemibiotrophic) cannot use to reproduce. The lesion is the plant's successful containment strategy, not a sign of overreaction or malfunction. Experimental test: compare plants that successfully mount an HR (showing necrotic lesions) with susceptible plants that lack HR capability — if HR is "overreaction", susceptible plants with no HR should survive better; instead, the opposite is typically observed in biotroph infections (resistant HR-capable plants contain the pathogen; susceptible plants show systemic disease). [2 marks — clear verdict + correct biology + experimental test]
(iv) "Dark spots = immune system gone haywire and pathogen can spread freely" — Wrong. The dark necrotic lesion is evidence of a successful HR — the infected cells died rapidly before the pathogen could spread to surrounding tissue. The pathogen's spread is being contained, not enabled. In a susceptible plant (no HR), the absence of necrotic spots is actually the worse outcome — the pathogen is spreading through living cells without being stopped. Experimental test: inoculate two plant lines (one HR-capable, one HR-deficient) with a biotrophic pathogen, then section the tissue and stain for hyphal presence; if lesion = spread freely, hyphae should be concentrated near lesions in the HR-capable line — in practice they are concentrated in the lesion-free areas of susceptible plants, not in the necrotic zone. [2 marks — clear verdict + correct biology + experimental test]
Overall evaluation: The passage contains three incorrect claims and one that is only partially correct. The core error is framing plant defence as a pathological overreaction and the HR as damaging — exactly the opposite of the correct interpretation. The blog conflates animal fever/inflammation with plant ROS responses, misidentifies strategic programmed cell death as an "allergic reaction", and interprets the necrotic lesion (a marker of defence success) as a sign of failure. [1 mark — overall evaluative judgement]
Marking criteria.
- 1 mark — Correctly evaluates claim (i): plant and animal immune systems are analogous but not mechanistically alike; no mobile immune cells or antibodies in plants; R-protein + phloem signalling vs immune cell migration.
- 1 mark — Correctly evaluates claim (ii): plants do not produce fever or inflammation in the mammalian sense; the analogous response is a ROS burst + ion flux, not vascular inflammation; provides a testable experimental contrast.
- 1 mark — Correctly identifies claim (iii) as wrong and distinguishes HR from an allergic reaction: allergic reaction is maladaptive/pathological; HR is deliberate, adaptive programmed cell death that contains a biotrophic/hemibiotrophic pathogen.
- 1 mark — Provides a valid experimental test for claim (iii): comparison of HR-capable vs HR-deficient plant lines inoculated with a biotrophic pathogen, with measurement of infection spread.
- 1 mark — Correctly identifies claim (iv) as wrong: the necrotic lesion is evidence of a successful HR and pathogen containment, not failure; absent lesions in susceptible plants correlates with worse, not better, outcomes.
- 1 mark — Provides a valid experimental test for claim (iv): inoculation + tissue sectioning with hyphal staining to map pathogen spread relative to necrotic zones.
- 1 mark — Provides an overall evaluative judgement that correctly characterises the pattern of errors in the passage (the blog consistently misframes defence as pathological failure rather than adaptive strategy).