12.4 Plant Defences Against Pathogens

Question 1

How do plants recognise an attack

Answer 1

1. cells have receptors that respond to chemicals released by the pathogen;
2. these receptors trigger the release of signalling molecules.
3. signalling molecules switch on genes in the nucleus which trigger cellular responses, including:
   - production of defensive chemicals
   - sending signals to unaffected cells
   - strengthening cell walls

Question 2

How does callose act as a physical barrier in plants

Answer 2

It’s a polysaccharide; it is thought it acts as a barrier because prevents the pathogen from entering the cell around the site of infection.
This is because minutes after the attack, callose is deposited in between space linking cell wall and cell membrane; in plasmodesmata blocking off infected cells from healthy ones; and it also blocks sieve plates in the phloem to block off healthy areas of the plant from infection.

Question 3

What are examples of plant defensive chemicals?

Answer 3

insect repellents (citronella),
insecticides (neurotoxins/caffeine),
antibacterial compounds including antibiotics (phenols, antiseptics),
antifungal compounds (toxic to fungi/insects; produced by cotton, caffeine, CHITINASES are enzymes that break down chitin in fungal cell walls),
anti-oomycetes (glucanases that break down glucans),
general toxins (make chemicals that break down to form cyanide compounds when the cell is attacked; cyanide is toxic to most living things)