4.1.3 Plant Defences Against Pathogens

Question 1

What do passive mechanisms for plant defence do

Answer 1

• Always present
• Physical or chemical

Question 2

What do active mechanisms in plant defence do

Answer 2

• Activated when pathogen invades
• Hypersensitivity deprives pathogens of resources
• Formation of physical barriers by callose
• Cell signalling

Question 3

Passive defence mechanism examples

Answer 3

Physical
- Waxy cuticle (wounds defeat this)
- Cellulose cell wall
- Closed stomata
- Bark
- Casparian Strip

Chemical
- Catechol
- Sticky resin on bark
- Yeast on leaf surface encourage growth of competing microbes
- Enzyme inhibitors like Tannins
- Receptor molecules detecting pathogens triggering other defence mechanisms

Question 4

Active defence mechanism examples

Answer 4

• Hyper sensitivity (tissue death around infection site)

Physical
- Release of callose and lignin (deposited between cell membrane and wall)
- Plasmodesmata narrow reducing size of channels connecting neighboring plant cells
- Callose grows in xylem vessels to block them
- Phloem is blocked by callose preventing phloem sap from being transported

Question 5

How can cell signalling work in plant defence?

Answer 5

• Cellulose broken down in injury by cellulase enzyme from pathogens
• Molecules produced by this act as signals to cell surface receptors
• When these receptors are stimulated, phytoalexins are released (disrupt reproduction and metabolism of pathogen)
• Salicylic acid migrates to uninfected areas, activating defence mechanisms protecting plant
• Ethylene lets plants communicate