Plant Defences

• What are 3 things which act as physical barriers to pathogens in plants?

  1. Waxy cuticle of leaves

  2. Bark on trees

  3. Cellulose cell wall
• What is different about plants' and animals' responses to infection?

  Animals attempt to heal damaged tissue, but plants just seal it off and sacrifice it
• What are 3 cellular responses to infection in plants?

  1. Producing defensive chemicals

  2. Physically strengthening cell walls

  3. Sending alarm signals to unaffected cells to trigger their defences
• What 2 things can receptors in plant cells respond to upon infection?

  1. Molecules from the pathogen

  2. Chemicals produced when the plant cell wall is attacked by pathogenic enzymes
• What is stimulated when receptors in a plant cell detect an infection?

  The release of signalling molecules, which appear to switch on genes in the nucleus, in turn causing the 3 cellular responses to infection
• What two polysaccharides are made to strengthen plant cell walls upon infection?

  Callose and lignin
• What is the structure of callose?

  Polysaccharide made up of beta glucose with 1-3 and 1-6 carbon linkages
• What does callose do when a plant cell is infected?

  1. Block sieve plates in the phloem, sealing off the infected part

  2. Is deposited in the plasmodesmata between healthy cells and their neighbours

  3. Within minutes of attack, synthesised and deposited between cell membrane and cell wall of cells neighbouring infected one. More callose and lignin then added, strengthening the wall even further
• What are some examples of plant chemical defences against pathogens?

  1. Insect repellants

  2. Anti-fungals

  3. Insecticides

  4. Antibacterial compounds (including antibiotics)

  5. Anti-oomycetes

  6. General toxins
• What is an example of a general toxin made by plant cells as a chemical defence against pathogens?

  Chemicals which can be broken down to make cyanide compounds
• What are some examples of insect repellents made by plants?

  Citronella from lemon grass or pine resin
• What are some examples of insecticides made by plants?

  Caffeine or pyrethrins, which are made by chrysanthemums and is an insect neurotoxin
• What are some example of some antibacterial compounds made by plant cells?

  1. Phenols

  2. Gossypol

  3. Defensins, plant proteins which disrupt bacterial and fungal cell membranes
• What is gossypol?


  A chemical made by cotton which has antibacterial and antifungal properties
• What are phenols used for in plants?

  As an antiseptic and antifungal agent
• What are some examples of antifungal chemicals made by plants?

  1. Phenols

  2. Gossypol

  3. Caffeine

  4. Saponins- chemicals found in many plant cell membranes which interfere with fungal cell membranes

  5. Chitinases- enzymes to break down chitin in fungal cell walls
• What is caffeine toxic to?

  Insects and fungi