12.4 Plant defences against pathogens

Question 1

What are the common barriers plants have to defend themselves against pathogens?

Answer 1

• Waxy cuticle of leaves
• Bark on treas
• Cellulose cell wall of plant cells

Question 2

What do plants do with diseased tissue?

Answer 2

Seal it off and sacrifice it.

Damaged parts will be replaced due to continuous growth at the meristem.

Question 3

How do plants recognise an attack?

Answer 3

1. Receptors in plant cells respond to molecules from pathogens or chemicals produced when the plant cell wall is attacked
2. This stimulates release of signalling molecules that switch on genes in the nucleus
3. This triggers a cellular response

Question 4

What 3 things can happen during a cellular response in plants when a pathogen is recognised?

Answer 4

1. Producing defensive chemicals
2. Alarm signalling to unaffected cells to trigger defences
3. Physically strengthening the cell wall

Question 5

What do plants release as part of a physical defense to a pathogen?

Answer 5

Callose - contains β-1,3 linkages and β-1,6 linkages between glucose monomers

Question 6

Describe the 4 stage process of the physical defence in plants, in relation to callose?

Answer 6

1. Within minutes of an attack, callose is synthesised + deposited between cell walls + cell membranes next to infected cells.
   - These callose papillae act as barriers, preventing pathogens entering the plant cells around the site of infection
2. Large amounts of callose continue to be deposited in cell walls after the initial infection.
   - Lignin is added to make the mechanical barrier stronger and thicker
3. Callose blocks the sieve plates in phloem, sealing off the infected part and preventing the pathogen spreading
4. Callose is deposited in plasmodesmata between infected cells and cells next to them.
   - This seals them off from healthy cells and prevents the pathogen spreading

Question 7

What is the function of powerful chemicals in plants?

Answer 7

Repel insect vectors

Kill invading pathogens

Question 8

What are the 6 examples of plant defensive chemicals?

Answer 8

1. Insect repellents
2. Instecticides
3. Antibacterial compounds, including antibiotics
4. Antifungal compounds
5. Anti-oomycetes
6. General toxins

Question 9

What is the plant defensive chemical ‘Insect repellent’?

Answer 9

E.g. pine resin + citronella from lemon grass

Question 10

What is the plant defensive chemical ‘Insecticide’?

Answer 10

E.g. pyrethrins made by chrysantheums, act as insect neurotoxins
Caffeine - toxic to insects and fungi

Question 11

What antibacterial compounds act as plant defensive chemicals? There are 4. Explain what each one is/does.

Answer 11

Example:
Phenols - antiseptics made by many plants
Antibacterial gossypol produced by cotton
Defensins - plant proteins that disrupt bacterial and fungal cell membranes
Lysosomes - organelles containing enzymes that break down bacterial cell wall

Question 12

What is the plant defensive chemical ‘Antifungal compounds’?

Answer 12

Example:
Phenols - antifungals made in many plants
Antifungal gossypol produced by cotton
Caffeine - toxic to insects and fungi
Saponins - Chemicals in many plant cell membranes that interfere with fungal cell membranes
Chitinases - Enzymes that break down chitin in fungal cell walls

Question 13

What is the plant defensive chemical ‘Anti-oomycete’?

Answer 13

Example:
Glucanases - Enzymes made by some plants that break down glucans
Polymers found in cell walls of oomycete

Question 14

What is the plant defensive chemical ‘General Toxins’?

Answer 14

when plant cell is attacked, some plants make chemicals that can break down into cyanide compounds.
Cyanide is toxic to most living things.