Free Plant Defences Against Pathogens revision notes for OCR A Level Biology – covering specification point 4.1.1(c).
lant 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.
