12.4 Plant defences against pathogens

• primary defences in plants:

  • waxy cuticle
  • bark of tree trunks
  • cellulose cell walls
• Attack - 1

  • some molecules from the pathogen are recognised directly by the plant cell
  • breakdown products of the cellulose cell wall are also directly recognised by the plant cell
  • receptors on the plasma membrane are the mediators of this response
• Attack - 2

  • signal travels to the nucleus and new gene expression is initiated
  • this leads to the synthesis of new defence proteins
  • the newly made defensive molecules attack the pathogen
• physical defences:

  • polysaccharides are made which enhance the strength of the cell wall
  • callose
  • lignin
  • defensive chemicals signal alarm to other cells before they are also attacked
• production of callose:

  • when a pathogen is detected the plant cells rapidly produce high levels of callose
  • callose is a polysaccharide, made of beta glucose monomers
• role of callose:

  • deposited between the cell walls and cell membranes next to the infected cells - this prevents further spread
  • lignin is added to to the callose to strengthen the barrier
  • callose also blocks sieve plates in the phloem
  • callose is also deposited in the plasmodesmata of connecting cells
• Chemical defences:

  anti-fungal compounds - e.g caffeine (toxic to fungi and insects) e.g chitinase (enzyme that breaks down fungal cell walls)

  Insecticides

  Insect repellents e.g citronella

  anti-oomycetes e.g glucanase (break down glucans which are found in cell walls)