Plant defences

• Passive defence mechanisms

  Always present
• Active defence mechanisms

  Activated when pathogens invade
• Passive defence mechanisms

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

  • Hypersensitivity deprives pathogens of resources
  • Formation of physical barriers by callose to limit the spread of pathogens to adjacent cells(callose depositied in the plasmodesmata which conntexts cytoplams between adjacent cells)
  • Cell signalling coordinates the active defence mechanisms
• Passive defence mechanisms(Physical barrier)

  • Waxy cuticle
  • resin within layers of bark which pathogen get stuck in
  • Cellulose cell wall
  • Closed stomata
  • Bark
  • Casparian strip
• Casparian strip

  Some fungi species can invade a plant all the way to the endodermis but they are unable to push past the Casparian strip
• Chemical defencesIn plants( needs le be fixed)

  Prevent pathogens from growing on the surface of the plant by creating acidic conditions -
• Chemical defences

  • Toxic compounds (e.g. Catechol)
  • Sticky resin found in the bark (traps pathogens)
  • Compounds that encourage the growth of competing microorganisms
  • Enzyme inhibitors (e.g. Tannins)
  • Receptor molecules (detect pathogens and trigger other defences)
• Hypersensitivity

  Rapid death of tissue surrounding the infection site, depriving pathogens of host tissue, nutrients and energy
• Formation of physical barriers

  1. Invasion of pathogens stimulates release of callose and lignin

  2. Callose forms a matrix shape that can deposit antimicrobial compounds

  3. Callose reduces size of channels between plant cells (plasmodesmata)

  4. Cytoplasm grows into xylem vessels (tyloses)

  5. Callose blocks phloem sieve pores
• Cell signalling

  Vital for plant defence as substances can't freely move around entire plant like immune cells in animals
• Cell signalling in plant defence

  1. Pathogens' cellulase enzymes break down cellulose, producing signalling molecules

  2. Signalling molecules stimulate cell surface receptors to release phytoalexins

  3. Phytoalexins disrupt pathogen metabolism, reproduction, cell membranes, and stimulate chitinases

  4. Salicylic acid migrates through plant to activate long-term systemic acquired resistance

  5. Ethylene secreted by attacked plants stimulates defence in other leaves and plants
• Casparian Strip

  A waterproof barrier found in the endodermis (a layer of cells surrounding the xylem) of plant roots
• Casparian Strip

  • Acts as a waterproof barrier within cell walls
  • Blocks the apoplastic pathway, forcing water and solutes to take the symplastic route
  • Ensures selective nutrient uptake while excluding harmful substances
  • Contains phenolic compounds with antifungal properties
• The Casparian Strip is essential for efficient water and mineral ion transport in plants
• Hypersensitivity

  A response to pathogen infection that quickly shuts down resources (such as nutrients and water) to the infected area, making it difficult for the pathogen to survive and spread
• Formation of physical barriers by callose

  The production of a carbohydrate called callose in response to an infection, which forms a physical barrier around the infected cells and helps to prevent the spread of the pathogen
• Callose prevention of pathogen spread

  Callose prevents the spread of pathogens by forming a physical barrier around the infected cells, either by plugging the plasmodesmata, strengthening the cell wall, or creating a physical barrier