Module 4: Plant Defences Against Pathogens
These free OCR A Level Biology Plant Defences Against Pathogens revision notes have been written for specification point 4.1.1(c).
Plant Defences Against Pathogens
Plants have two types of defence against pathogens:
- Chemical defences are antibiotics, antifungals, hydrolytic enzymes, antimicrobial proteins or other metabolic products that can inhibit or kill microorganisms.
- Physical defences are structures that keep out microorganisms or contain their spread within the organism.
Different types of chemical and physical defences are categorised as:
- Active Defences: Produced in response to infection, helping to contain or destroy pathogens.
- Passive Defences: Always present, designed to prevent entry of pathogens.
Active Defences
The table below outlines some of the active chemical defences plants have:
| Examples | Description |
|---|---|
| Alkaloids, Phenols | Inhibit key metabolic processes (e.g. enzymes for digestion or protein synthesis). |
| Defensins, Oxidative bursts | Disrupt pathogen plasma membranes or ion transport. |
| Hydrolytic enzymes | Break down microbial cell walls (e.g. chitinases for fungi, lysozymes for bacteria). |
| Terpenoids, Phenols | Act as general antimicrobials (antibacterial and antifungal). |
The table below outlines some of the active physical defences plants have:
| Examples | Description |
|---|---|
| Cell wall thickening, Callose deposition | Strengthen cell walls and block plasmodesmata to prevent pathogen spread between cells. |
| Necrosis | Localised cell death to isolate infection. |
The table below outlines some of the passive chemical defences plants have:
| Examples | Description |
|---|---|
| Phenols, Tannins | Stored in bark and tissues, they have broad antimicrobial and antifungal properties. |
| Terpenoids, Alkaloids | Bitter or aromatic compounds that deter herbivores and inhibit pathogens. |
| Defensins | Protein-based inhibitors present in tissues act against microbial growth. |
| Hydrolytic enzymes | Pre-formed enzymes (e.g. chitinases, lysozymes) that degrade invading pathogen cell walls. |
The table below outlines some of the passive physical defences plants have:
| Examples | Description |
|---|---|
| Bark, Cellulose cell walls, Lignin | Physical barriers that are tough, waterproof, and contain antimicrobial compounds. |
| Callose, Tylose | Internal blockages that restrict pathogen movement in phloem or xylem. |
| Waxy cuticle, Stomatal closure | Surface defences that prevent pathogen entry. |