Plant Defences against Pathogens

Physical Defences

- Waxy cuticle

- Bark on trees

- Cellulose cell wall

Plants seal off diseased tissue and sacrifice it

Plant Defences

1. When a plant is attacked, the receptors in the cell surface membrane detect the presence of the pathogen.

2. The receptors stimulate the release of signalling molecules

3. Signalling molecules trigger cellular responses, such as:

   • Producing defensive chemicals

   • Sending alarm signals to unaffected cells

   • Strengthening their cell walls with callose and lignin

What polysaccharide do plants produce when attacked by pathogens?

Callose

What does callose do with lignin?

Callose is deposited into the cell walls and reinforced with lignin to provide a thicker barrier for the pathogen to penetrate through

What does callose do in the phloem?

It blocks the sieve tube end plates in the phloem, sealing off the infected area and preventing the spread of pathogens

What does callose do in the plasmodesmata?

It blocks the plasmodesmata between infected cells and their neighbours

Chemical Defences

Plants produce chemicals that can repel the insect vectors of disease, or kill invading pathogens

Tannins

Create a bitter taste

Insect repellents

- Pine resin

- Citronella from lemon grass

Insecticides

Caffeine acting as a neurotoxin

Antibacterial compounds

Disrupts the cell wall/membrane of the bacteria

Antifungal Compounds

Chitinases - enzymes released to break down the chitin in the cell wall

Anti-oomycetes

Breaks down the cell walls on fungi like eukaryotic organisms

General toxins

- Some plants make chemicals that can be broken down to form cyanide compounds when the plant cell is attacked.

- Cyanide is toxic to most living things.

Silica in Grass

- Acts like glass

- Herbivores (caterpillars) that eat silica don't grow as well