Immunity in plants

Question 1

What are the general defences? (physical and chemical)

Answer 1

PHYSICAL:
* Wavy cuticle: made of lignin and cutin

• Cell wall: physically prevents pathogens from entering the cell
• Bark: external layer of dead cells
• Leaf structure:
  1. Thorns/hairs/bristles repel insects
  2. Drooping leaves deter contact with insects that may carry disease
  3. Leaf shape prevents water pooling

CHEMICAL:
* Antimicrobial –> plants produce antibiotics which kill bacteria specifically

• Toxins target insects that feed on plants, reducing the amount of insects in contact with the plant which lowers its risk of infections from viruses they may carry

Question 2

What’s the immune response in plants?

Answer 2

• Plants detect pathogens by microbe associated molecular patterns (MAMPs) on their surface

1. Basal resistance
2. Hypersensitive response

Question 3

What’s basal resistance?

Answer 3

The plant immediately fortifies itself against infection by becoming impenetrable

1. Physical
   * Closed stomata
   * Callose deposition: a polysaccharide that plants deposit between the cells walls and plasma membranes when stressed –> limits spread of disease between cells
2. Chemical
   * Saponins destroy pathogen’s cell membrane

• Defensins target bacteria and fungi. They act upon molecular targets in the pathogen’s plasma membrane

Question 4

What’s hypersensitive response?

Answer 4

Limits pathogen access to host plant and saves the rest of the plant by undergoing APOPTOSIS, restricting pathogen to the infected site

Question 5

Distinguish between passive and active defence mechanisms of plants

Answer 5

Passive: physical/chemical barriers that are always present as the plant’s physiology –> not induced by contact with a pathogen

Active: defences induced after contact with a pathogen –> they react specifically against the infection or damage

Question 6

Provide 2 examples of pathogens that infect specific native Australian plants

Answer 6

1. Hardenbergia comptoniana: Hardenbergia mosaic virus
2. Bottle brush: myrtle rust

Question 7

Outline how the pathogen infects the plant host

Answer 7

Aphids (vector) spread the disease when eating sap through their virus containing saliva