Plant Defense

• Passive defence mechanisms


  Always present in plants to protect against infection and disease
• Passive defence mechanisms


  • Physical barriers that prevent pathogens from entering
  • Chemicals that reduce or prevent the growth of pathogens
• Active defence mechanisms


  Activated in plants when pathogens invade
• Active defence mechanisms


  • Hypersensitivity deprives pathogens of resources
  • Formation of physical barriers by callose to limit pathogen spread
  • Cell signalling coordinates the active defence mechanisms
• Physical barriers


  • Waxy cuticle
  • Cellulose cell wall
  • Closed stomata
  • Bark
  • Casparian strip
• Viruses and bacteria can only penetrate the waxy cuticle if there is a wound on the leaf surface or stem, commonly caused by grazing herbivores
• Chemical defences


  • Create acidic conditions to prevent pathogen growth
  • Toxic compounds like catechol
  • Sticky resin that traps pathogens
  • Compounds that encourage growth of competing microorganisms
  • Enzyme inhibitors like tannins
  • Receptor molecules that detect pathogens and trigger other defences
• Microorganisms like yeast on the leaf surface are harmless to plants and compete against harmful pathogens
• Unlike animal cells, plant cells have cell walls so substances cannot freely move around the entire plant, making cell signalling vital for plant defence
• Active defence mechanisms activated when pathogens invade

  1. Hypersensitivity - rapid death of tissue surrounding infection site

  2. Formation of reinforced cell walls with callose and lignin

  3. Narrowing of plasmodesmata channels between cells

  4. Ingrowths into xylem vessels (tyloses)

  5. Blockage of phloem sieve pores with callose
• Phytoalexins


  Defence chemicals released in response to pathogen detection that disrupt pathogen metabolism, reproduction, and cell membranes
• Salicylic acid


  Signalling molecule that migrates through plant to activate long-term systemic acquired resistance against pathogens
• Ethylene


  Signalling compound that allows plants to communicate about pathogen attack, stimulating defensive reactions in other leaves and plants