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Plants are constantly exposed to pathogens including bacteria, viruses, fungi, and insects. Unlike animals, plants do not have a mobile immune system with specialised immune cells. Instead, they rely on a combination of physical barriers, chemical defences, and inducible responses. This lesson covers plant defence mechanisms as required by the Edexcel A-Level Biology (9BI0) specification.
Plants are sessile (non-motile) organisms that cannot flee from pathogens. They are exposed to a wide range of threats:
| Threat | Examples |
|---|---|
| Bacterial pathogens | Agrobacterium tumefaciens (crown gall), Pseudomonas syringae (leaf spot) |
| Viral pathogens | Tobacco mosaic virus (TMV), cauliflower mosaic virus |
| Fungal pathogens | Phytophthora infestans (potato blight), Botrytis cinerea (grey mould) |
| Herbivorous insects | Aphids, caterpillars, beetles |
Exam Tip: Plants do not have an adaptive immune system like animals. They cannot produce antibodies or have immune memory. All plant defences are innate (non-specific or pattern-recognition based).
Plants have several physical barriers that prevent pathogen entry:
| Defence | Description | How it protects |
|---|---|---|
| Cellulose cell wall | Rigid wall surrounding every plant cell | Acts as a physical barrier to pathogen entry; most pathogens cannot digest cellulose |
| Waxy cuticle | Layer of cutin/wax on the epidermis of leaves and stems | Waterproof barrier that prevents entry of waterborne pathogens and reduces moisture on the surface (which pathogens need to grow) |
| Bark | Outer protective layer of woody plants (composed of dead cork cells) | Tough, impermeable barrier; contains suberin (waterproof substance) |
| Stomatal closure | Guard cells can close stomata in response to pathogen detection | Prevents entry of pathogens through stomatal pores |
| Callose deposition | Polysaccharide (β-1,3-glucan) deposited at plasmodesmata and cell walls | Blocks plasmodesmata to prevent pathogen spread between cells; strengthens cell walls at the site of infection |
| Lignification | Deposition of lignin in cell walls | Strengthens walls; lignin is resistant to enzymatic degradation by fungi |
| Thorns, spines, trichomes | Physical projections from the plant surface | Deter herbivores and insects; trichomes may be glandular (producing toxic substances) |
| Leaf abscission | Dropping of infected leaves | Removes the infected tissue from the plant |
Plants produce a wide range of secondary metabolites (chemicals not directly involved in growth) that have antimicrobial or anti-herbivore properties.
| Chemical | Source | Function |
|---|---|---|
| Tannins | Leaves, bark, unripe fruits | Bind to proteins in herbivore digestive systems, reducing palatability and digestibility; antimicrobial |
| Alkaloids | Various tissues | Toxic to herbivores and pathogens. Examples: caffeine, nicotine, morphine, quinine |
| Terpenoids | Leaves, flowers, resin | Toxic or repellent to herbivores and pathogens. Example: menthol, citronella |
| Cyanogenic glycosides | Stored in vacuoles; released on tissue damage | Release hydrogen cyanide (HCN) when cells are broken, which is toxic to herbivores |
| Saponins | Various tissues | Disrupt cell membranes of fungi and other pathogens |
| Chemical | Trigger | Function |
|---|---|---|
| Phytoalexins | Pathogen detection (elicitors) | Small antimicrobial molecules produced rapidly at the site of infection; inhibit pathogen growth |
| Chitinases and glucanases | Pathogen attack | Enzymes that break down fungal cell walls (chitin and glucan), directly killing fungi |
| Protease inhibitors | Herbivore damage | Inhibit digestive enzymes in the herbivore gut, reducing the nutritional value of the plant tissue |
| Reactive oxygen species (ROS) | Pathogen detection | Toxic to pathogens; contribute to the hypersensitive response |
Exam Tip: Be able to distinguish between constitutive defences (always present) and inducible defences (produced in response to attack). Examiners may ask you to classify specific examples.
The hypersensitive response is a rapid, localised cell death response that prevents the spread of infection.
The visible result is often a small brown or black spot on the leaf — the necrotic lesion.
Following a localised hypersensitive response, the plant can develop systemic acquired resistance — a heightened state of defence throughout the entire plant.
Exam Tip: Systemic acquired resistance is sometimes compared to the animal immune system's "memory" — but it is not the same. SAR provides a temporary, non-specific enhancement of defences, not a permanent, antigen-specific memory.
Plants also defend against animal herbivores:
| Defence | Mechanism |
|---|---|
| Spines and thorns | Physical deterrents (e.g. cacti, roses) |
| Stinging hairs | Trichomes that inject irritant chemicals (e.g. nettles release histamine and serotonin) |
| Mimicry | Some plants mimic the appearance of insect eggs on their leaves to deter egg-laying (e.g. Passiflora) |
| Volatile organic compounds (VOCs) | Released when damaged; attract predators of the herbivore (indirect defence). Example: maize releases VOCs that attract parasitic wasps when attacked by caterpillars. |
| Toughened leaves | High fibre content and silica deposits make leaves difficult to eat |
| Feature | Detail |
|---|---|
| Pathogen | TMV (RNA virus) |
| Host | Tobacco and other solanaceous plants |
| Symptoms | Mosaic pattern of light and dark green on leaves; stunted growth; reduced photosynthesis |
| Transmission | Contact (sap-to-sap); contaminated tools; no vector needed |
| Mechanism of damage | Virus replicates in mesophyll cells, disrupting chloroplast function and reducing chlorophyll production |
| Plant response | Hypersensitive response, callose deposition, and RNA silencing (an additional antiviral mechanism) |
| Term | Definition |
|---|---|
| Constitutive defence | A defence mechanism that is always present, regardless of infection |
| Inducible defence | A defence mechanism that is activated in response to pathogen attack |
| Phytoalexin | A small antimicrobial molecule produced by plants in response to pathogen infection |
| Hypersensitive response | Rapid, localised cell death at the site of pathogen infection to prevent spread |
| Systemic acquired resistance | A plant-wide enhanced state of defence triggered by a local infection |
| Callose | A polysaccharide deposited at plasmodesmata and cell walls to prevent pathogen spread |
| PAMP | Pathogen-associated molecular pattern; a molecule from a pathogen recognised by plant receptors |
The Edexcel 9BI0 specification places plant defences within Topic 6: Immunity, Infection and Forensics as a deliberate counter-point to the animal innate-and-adaptive system covered in lessons 6–9. Plants demonstrate that sophisticated, multi-layered defence is possible without circulating cells, antibodies or lymphoid organs, sharpening what is distinctive about the animal immune response. Synoptic links run backwards to lesson 1 (plants face fungal, oomycete, bacterial and viral attack, plus arthropod herbivory absent from the animal-pathogen list) and lesson 4 (plant pathogens spread by wind-blown spores, water-splash, soil contact, insect vectors and contaminated tools). Forward links run to Topic 5, where carbon and nitrogen diverted from photosynthate into defence chemistry imposes a growth-cost trade-off; to Topic 7, where the waxy cuticle and stomata are simultaneously the apparatus of transpiration and the principal entry point for foliar bacterial pathogens such as Pseudomonas syringae; and to Topic 8, where Mendelian inheritance of single-locus R-genes (encoding intracellular NLR receptors) underpins resistance-breeding programmes. Relevant statements concern physical and chemical plant defences, the hypersensitive response and systemic acquired resistance, and the comparison between plant and animal defence (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).
Question (8 marks):
A leaf cell of Arabidopsis thaliana is invaded by Pseudomonas syringae, a Gram-negative bacterial pathogen that enters the leaf through open stomata and injects effector proteins into the host cytoplasm via a type III secretion system.
(a) Outline the cellular and molecular events of the hypersensitive response (HR) triggered when an intracellular NLR (R-protein) receptor recognises a pathogen effector. (5)
(b) Explain how the localised hypersensitive response leads to systemic acquired resistance (SAR) in distal, uninfected tissue. (3)
Solution with mark scheme:
(a) Stage 1 — recognition. Cytosolic NLR (nucleotide-binding leucine-rich repeat) receptors — encoded by plant R-genes — bind a pathogen-derived effector protein delivered by the P. syringae type III secretion system. Direct or indirect recognition of the effector triggers a conformational change in the NLR.
M1 (AO1.1) — NLR receptor recognises pathogen effector. Common error: writing "the cell detects bacteria" without naming receptor or ligand class.
Stage 2 — oxidative burst. NLR activation triggers a rapid reactive oxygen species (ROS) burst at the plasma membrane (via the NADPH-oxidase RBOHD homologue), generating superoxide and hydrogen peroxide that are directly toxic to the pathogen and act as second messengers.
M1 (AO1.2) — ROS burst; named role both as antimicrobial agent and as signalling molecule.
Stage 3 — calcium influx and callose deposition. Calcium channels open, raising cytosolic Ca2+; this activates callose synthase, which deposits callose (β-1,3-glucan) at plasmodesmata, sealing the symplastic continuity between the infected cell and its neighbours and preventing cell-to-cell viral or bacterial movement.
M1 (AO1.2) — Ca2+ influx triggers callose deposition at plasmodesmata.
Stage 4 — programmed cell death. The infected cell undergoes programmed cell death (PCD), producing a small dry necrotic lesion that starves the pathogen of water and nutrients and physically isolates it from healthy tissue. Phytoalexins (e.g. camalexin in Arabidopsis) are synthesised and accumulate in the lesion margin.
M1 (AO1.2) — programmed cell death isolates pathogen; phytoalexins accumulate.
Stage 5 — outcome. The host has sacrificed a small number of cells to save the whole plant; further pathogen replication is blocked and the lesion is visible as a brown spot on the leaf.
A1 (AO3.1a) — explicit "trade-off" framing: localised cell death is a defensive strategy, not a pathology.
(b) M1 (AO2.1) — the infected cell synthesises salicylic acid (SA), which accumulates locally and is converted (in part) to a mobile signal — methyl-salicylate or related derivatives — that travels through the phloem to distal, uninfected tissue.
M1 (AO2.1) — in distal tissue, SA signalling activates expression of pathogenesis-related (PR) proteins (chitinases, β-1,3-glucanases, defensins) and primes the tissue for faster, stronger response to subsequent attack.
A1 (AO3.1a) — SAR is therefore the plant analogue of immunological memory: durable, systemic, broad-spectrum resistance triggered by a localised infection — though it lacks the antigen-specificity of true adaptive immunity.
Total: 8 marks.
Question (6 marks): Compare the two-tier plant immune system (PAMP-triggered immunity and effector-triggered immunity) with the animal innate immune response, and explain why plants have evolved a sophisticated defence system despite lacking circulating immune cells.
Mark scheme decomposition by AO:
| Marking point | AO | Credit-worthy content |
|---|---|---|
| 1 | AO1.1 | States that PAMP-triggered immunity (PTI) is mediated by surface pattern-recognition receptors (PRRs) that detect conserved pathogen-associated molecular patterns (e.g. bacterial flagellin, fungal chitin) — analogous to the toll-like receptors of animal innate immunity. |
| 2 | AO1.2 | States that effector-triggered immunity (ETI) is mediated by intracellular NLR (R-protein) receptors encoded by plant R-genes, which detect pathogen effectors that successful pathogens use to suppress PTI; ETI activation typically drives the hypersensitive response with programmed cell death. |
| 3 | AO2.1 | States that the animal innate response shares pattern recognition (TLRs) but adds circulating phagocytes, complement, inflammation and a downstream adaptive arm (B and T cells, antibodies, immunological memory) that plants do not possess. |
| 4 | AO2.1 | Explains the architectural reason: plants are sessile and have rigid cellulose cell walls that prevent the kind of cell migration animal phagocytes rely on, so plant defence is necessarily cell-autonomous and chemical rather than cellular and migratory. |
| 5 | AO3.1a | Discusses the trade-off: plants invest carbon and nitrogen from photosynthesis into constitutive defences (cuticle, lignin, alkaloids, terpenes), and induced defences (callose, phytoalexins) — this growth-defence trade-off is a major target of crop-breeding research. |
| 6 | AO3.2a | Concludes that the two-tier plant system (PTI + ETI) plus systemic acquired resistance is functionally analogous to but mechanistically distinct from animal innate-plus-adaptive immunity; both reflect convergent solutions to the same selection pressure. |
Total: 6 marks split AO1 = 2, AO2 = 2, AO3 = 2. Edexcel rewards candidates who connect plant architecture to defence strategy (sessility + cell wall → cell-autonomous chemistry rather than cellular migration) rather than merely listing defence mechanisms in isolation.
| AO | Typical share on this topic | Earned by |
|---|---|---|
| AO1 (knowledge) | 35–45% | Defining constitutive vs inducible defences; naming physical barriers (cuticle, cellulose wall, lignin, bark, callose); naming chemical defences (tannins, alkaloids, terpenes, saponins, phytoalexins); reciting the steps of the hypersensitive response and SAR; recalling that salicylic acid is the SAR signal molecule |
| AO2 (application) | 35–45% | Applying defence categories to specific scenarios; predicting which defence suits which pathogen class (chitinases for fungi, RNA silencing for viruses); explaining why stomatal closure follows PAMP detection; matching constitutive vs induced examples to specific chemicals |
| AO3 (analysis / evaluation) | 15–25% | Evaluating the growth-defence trade-off; comparing plant and animal immune architectures; critiquing single-gene R-gene breeding as fragile; reasoning about why sessile organisms cannot use migratory phagocytic defence |
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