Plant Defences Against Pathogens

Unlike animals, plants cannot move away from pathogens, nor do they have an adaptive immune system or specialised mobile immune cells. Yet they live surrounded by bacteria, fungi, viruses and insect vectors, and most plants are not destroyed by disease. OCR specification 4.1.1 (d) requires you to describe the physical and chemical defences of plants against pathogens. This lesson covers the passive structural barriers, the inducible chemical defences, and the molecular signalling that recognises an invader.

Key Definitions:

• Callose — a polysaccharide (β-1,3-glucan) deposited between the cell wall and plasma membrane at sites of infection.
• Tylose — a balloon-like outgrowth of parenchyma cells into xylem vessels, blocking them and sealing off infection.
• Phytoalexin — an antimicrobial compound produced by a plant in response to infection.
• Hypersensitive response (HR) — rapid, localised cell death at the site of infection, isolating the pathogen.

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Physical (Passive) Defences

Physical defences are structures that are already in place before infection occurs. They form the first line of defence.

1. Bark

The outer layer of woody stems is a dead, corky, lignified tissue. It physically prevents entry of pathogens and many insects. Cork cells contain suberin — a waxy substance that also resists water loss and microbial attack.

2. Waxy Cuticle

A hydrophobic layer of cutin covers the epidermis of leaves and young stems. Because pathogens need moisture to germinate and grow, the cuticle provides a dry, chemically inert barrier. Stomata are the main route for many pathogens precisely because they are gaps in this otherwise perfect cuticle.

3. Cell Walls

The cellulose cell wall is a mechanical barrier that pathogens must digest or burst through. Lignification of the wall in older tissues makes this much harder. Many pathogens secrete cellulases and pectinases to dissolve the wall — so the strength of the wall and the presence of lignin are crucial defences.

4. Callose Deposition

On detecting an invader, plants rapidly synthesise callose (a β-1,3-glucan) and deposit it between the cell wall and plasma membrane at the point of attack. Callose plugs plasmodesmata (cytoplasmic connections between cells), preventing the pathogen from spreading cell-to-cell. Callose is especially important in defence against fungal hyphae and viruses.

5. Tylose Formation

In response to infection of the xylem, parenchyma cells next to the vessels swell and push into the vessel lumen, forming tyloses. These balloon-like outgrowths physically block the vessel, preventing the pathogen (and any toxins) spreading in the transpiration stream. However, this also cuts off water flow — a trade-off.

6. Wound Sealing

Damage to plant tissue is immediately sealed with a mixture of suberin, lignin and phenolic compounds. This prevents pathogens from entering through the wound site.

flowchart TD
    A[Pathogen lands on plant] --> B{Physical barrier?}
    B -->|Yes| C[Cuticle, bark, cell wall block entry]
    B -->|Breached| D[Cell recognises PAMPs]
    D --> E[Callose deposited at plasmodesmata]
    D --> F[Tyloses form in xylem]
    D --> G[Chemical defences produced]
    G --> H[Terpenoids]
    G --> I[Phenols]
    G --> J[Alkaloids]
    G --> K[Defensive proteins]
    G --> L[Chitinases]

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Chemical (Active) Defences

Chemical defences are produced in response to infection. Many plants also store antimicrobial compounds in vacuoles or specialised cells, ready to be released on wounding.

1. Terpenoids

Terpenoids are a diverse family of lipid-soluble compounds that give plants their characteristic scents (e.g., menthol in mint, camphor in camphor tree, limonene in citrus). Many terpenoids have antibacterial and antifungal activity. Conifers release terpenoid resins that trap insects and kill fungi.

2. Phenols

Phenolic compounds such as tannins inhibit fungal and bacterial enzymes. Tannins bind to proteins, denaturing them and inactivating digestive enzymes — this is why unripe fruits taste bitter and astringent. Phenols also reduce the palatability of plant tissue to herbivorous insects.