Plant Defences Against Pathogens (OCR A-Level Biology A): Revision Notes
Plant Defences Against Pathogens
Plants face constant threats from bacterial, fungal, and viral pathogens. Despite appearing vulnerable, they have evolved sophisticated defence systems that operate continuously or activate upon infection. Plant defences are classified into two categories: passive and active mechanisms.
Unlike animals with adaptive immune systems, plants have developed unique defence strategies that work at both structural and cellular levels. These mechanisms operate 24/7, providing continuous protection even without specialized immune cells.
Passive defence mechanisms
Passive defences are structural and chemical barriers that exist permanently, providing constant protection against pathogen entry and establishment. These mechanisms do not require pathogen detection or cell activation.
Passive defences are the plant's first line of defence and are always active – they don't wait for pathogen invasion to begin working. This continuous protection is energy-efficient because it requires only maintenance rather than active synthesis.
Physical barriers
Plants possess several structural features that physically prevent pathogen invasion:
Waxy cuticle – The leaf epidermis is covered by a hydrophobic waxy layer that viruses and bacteria cannot penetrate. Entry can only occur through wounds created by herbivores such as caterpillars or through natural openings.
Bark – This thick, protective tissue on stems and trunks provides an even more substantial barrier than cuticle, making penetration extremely difficult for most pathogens.
Cellulose cell walls – The rigid polysaccharide structure surrounding every plant cell acts as a first line of defence, resisting enzymatic breakdown by many pathogens.
Casparian strip – Located in the root endodermis, this waterproof band of suberin prevents pathogens (particularly fungi) from moving beyond the cortex into the vascular tissue. Many fungi can grow through root tissues but cannot penetrate this impermeable barrier.
Stomatal closure – Guard cells can close stomatal pores in response to pathogen detection, blocking a potential entry route into the leaf interior.
How Physical Barriers Work Together:
Consider a fungal spore landing on a leaf surface. To establish infection, it must:
- Penetrate the waxy cuticle (first barrier)
- Break through the cellulose cell wall (second barrier)
- If attempting root entry, bypass the Casparian strip (third barrier)
This multi-layered defence means pathogens must overcome several obstacles before reaching living cells, greatly reducing infection success rates.
Chemical defences
Plants deploy various chemical strategies to prevent pathogen growth:
Competitive microorganisms – Leaves and roots secrete nutrients that support harmless microorganisms such as yeasts. These beneficial microbes colonize plant surfaces and outcompete pathogenic species for space and resources.
pH modification – By secreting acidic compounds onto leaf and root surfaces, plants create an environment too hostile for many pathogens to establish.
Toxic compounds – Plants synthesize substances directly harmful to pathogens. For example, catechol disrupts pathogen cells and inhibits their growth.
Enzyme inhibitors – Compounds such as tannins block the action of cellulases and other enzymes that pathogens use to digest cell walls and gain entry to cells.
Receptor molecules – Cell surface membranes contain specialized proteins that recognize pathogen-associated molecular patterns, triggering immediate defensive responses.
Sticky resins – Bark produces viscous resins that trap pathogens and prevent their spread through the plant.
Chemical defences often work synergistically with physical barriers. For instance, toxic compounds may be concentrated in the waxy cuticle, making it not just a physical obstacle but also a chemical deterrent.
Active defence mechanisms
Unlike mammals with mobile immune cells, plants cannot send defensive cells to infection sites because their cells are fixed by rigid walls. Instead, plants employ active defences – responses triggered specifically when pathogens invade.
Why Plants Need Active Defences:
Plant cells are immobile due to rigid cell walls, so they cannot mount immune responses like animals do with white blood cells traveling to infection sites. Active defences compensate for this limitation by triggering localized responses at the infection site and systemic responses throughout the plant.
Hypersensitivity response
Hypersensitivity is the rapid, programmed death of cells immediately surrounding an infection site. Although this appears drastic, it is highly effective because most pathogens require living host tissue to obtain nutrients and energy. By killing infected and nearby cells, the plant starves the pathogen and contains the infection.
Hypersensitivity in Action:
When a bacterium invades a leaf cell:
- Infected cell detects the pathogen
- Cell triggers its own death program within minutes
- Surrounding cells also die, creating a "dead zone"
- Pathogen, unable to extract nutrients from dead tissue, cannot spread
- Visible result: small brown spots on leaves where tissue died
This "scorched earth" strategy sacrifices a few cells to save the entire leaf or plant.
Cell wall reinforcement
When pathogens attempt to breach cell walls, plants respond by strengthening these barriers:
Callose deposition – Callose is a polysaccharide composed primarily of -1,3-glycosidic bonds with some -1,6-glycosidic bonds. It is deposited between the cell surface membrane and the cell wall, creating several defensive functions:
- Forms a matrix in which antimicrobial compounds (hydrogen peroxide, phenols) accumulate, killing pathogens attempting to enter
- Reduces the diameter of plasmodesmata (cytoplasmic channels connecting adjacent cells), limiting virus spread from cell to cell
- Blocks sieve pores in phloem tissue, preventing pathogen movement through the vascular system
Lignin reinforcement – Cells deposit lignin alongside callose to thicken and strengthen cell walls, making them significantly harder to penetrate.
Callose deposition is reversible – once the infection is controlled, plants can degrade the callose to restore normal cell-to-cell communication through plasmodesmata. This flexibility allows plants to balance defence with normal metabolic functions.
Tyloses
Tyloses are physical blockages within xylem vessels that prevent pathogen spread through the vascular system. The cytoplasm of living cells adjacent to xylem vessels grows into the vessel lumen, forming a wall often composed of callose. This blocks the movement of water and any pathogens traveling within the xylem.
Phytoalexins and cell signalling
Plant cells detect pathogen invasion through cell signalling. When bacterial or fungal pathogens secrete cellulases to digest their way into cells, the breakdown products of cellulose act as chemical signals. Receptors on cell surfaces detect these signals and trigger the production of phytoalexins.
Phytoalexins are antimicrobial defence chemicals that combat pathogens through multiple mechanisms:
- Disrupting bacterial cell surface membranes, causing cell lysis
- Stimulating secretion of chitinases, enzymes that degrade the chitin in fungal hyphal cell walls
- Interfering with metabolic pathways essential for pathogen survival
- Delaying or preventing pathogen reproduction
Different plant species produce different phytoalexins. For example, legumes produce pterocarpans, while solanaceous plants (tomatoes, potatoes) produce sesquiterpenoids. This diversity reflects millions of years of co-evolution with various pathogens.
Systemic acquired resistance
Following a localized infection, plants develop long-lasting, whole-plant resistance through systemic acquired resistance. This process involves signalling molecules that travel throughout the plant:
Salicylic acid – This hormone-like molecule moves through vascular tissue to uninfected regions, activating defensive mechanisms and increasing resistance to future pathogen attacks.
Ethylene – When released by infected tissues, this volatile gas diffuses through air spaces to stimulate defensive responses in other leaves on the same plant and even in neighboring plants.
Systemic acquired resistance provides "immunological memory" in plants – after one infection, the entire plant becomes more resistant to subsequent attacks, even from different pathogen species. This priming effect can last for weeks or even the entire growing season.
Comparison of passive and active defences
| Feature | Passive defences | Active defences |
|---|---|---|
| Timing | Always present | Activated upon infection |
| Energy requirement | Low (maintenance only) | High (synthesis of new compounds) |
| Specificity | Non-specific | Can be specific to pathogen type |
| Response time | Immediate | Minutes to hours |
| Examples | Waxy cuticle, bark, tannins | Callose, phytoalexins, hypersensitivity |
Key Points to Remember:
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Passive defences (physical and chemical barriers) operate continuously to prevent pathogen entry and include the waxy cuticle, bark, Casparian strip, toxic compounds, and enzyme inhibitors
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Active defences are triggered by infection and include hypersensitivity (programmed cell death), callose deposition, lignin reinforcement, and phytoalexin production
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Callose is a polysaccharide that blocks plasmodesmata and sieve pores, traps antimicrobial compounds, and strengthens cell walls
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Phytoalexins are antimicrobial chemicals that disrupt pathogen membranes, stimulate chitinase secretion, interfere with metabolism, and delay reproduction
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Systemic acquired resistance provides whole-plant protection through signalling molecules (salicylic acid and ethylene) that activate defences in uninfected tissues