OCR AS Biology

Revision Notes

4.1.3 Plant Defences Against Pathogens

Plant Defences Against Pathogens

  • Just like animals, plants have defence mechanisms to protect themselves against infection and disease
  • The different mechanisms are classified into two categories: passive and active
  • Passive defence mechanisms are always present
    • Some of these mechanisms are physical barriers that prevent pathogens from entering
    • Some are chemicals that reduce or prevent the growth of pathogens
  • Active defence mechanisms in plants are activated when pathogens invade
    • Hypersensitivity deprives pathogens of resources
    • The formation of physical barriers by callose plays a major role in limiting the spread of pathogens
  • Cell signalling plays an important role in coordinating the active defence mechanisms

Passive defence mechanisms

  • Physical barriers make it harder for pathogens to gain entry into plants
  • Examples of physical barriers:
    • Waxy cuticle
      • The only way that viruses and bacteria can penetrate the waxy cuticle of a leaf is if there is a wound on the leaf surface or stem. Wounds are commonly caused by grazing herbivores
    • Cellulose cell wall
    • Closed stomata
    • Bark
    • 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 defences prevent pathogens from growing on the surface of the plant by creating acidic conditions
  • Examples of chemical defences:
    • Toxic compounds
      • E.g. Catechol
    • Sticky resin found in the bark
      • This traps the pathogens so they can’t spread
    • Compounds that encourage the growth of competing microorganisms
      • Microorganisms such as yeast found on the leaf surface are completely harmless to plants. They are strong competitors against harmful pathogens
    • Enzyme inhibitors
      • E.g. Tannins
    • Receptor molecules
      • They detect the presence of pathogens and trigger other defence mechanisms

Active defence mechanisms

  • Unlike animal cells, plant cells have cell walls. This means that cells can not freely move around the entire plant as the immune cells do in some animals
  • The active defence mechanisms of a plant are activated once a pathogen has invaded
  • Hypersensitivity is the rapid death of tissue surrounding the infection site
    • Although quite an extreme response, it is very effective as it deprives the pathogens of host tissue, nutrients and energy
  • Plants also create physical barriers to reduce the spread of a pathogen
    • Reinforced cell walls are formed when fungi and bacteria invade
      • The invasion of pathogens stimulates the release of compounds callose and lignin
      • These molecules are deposited between the cell surface membrane and the cell wall
      • Callose is a polysaccharide that forms a matrix shape. Antimicrobial compounds that kill pathogens (hydrogen peroxide and phenols) can be deposited in this shape
    • Narrowing of the plasmodesmata
      • Callose helps to reduce the size of the channels that connect neighbouring plant cells
    • Ingrowths into the xylem vessels (tyloses)
      • The cytoplasm of nearby cells grows into the xylem to create a wall made of callose
    • Blockage of the phloem
      • The sieve pores are filled with callose which prevents phloem sap from being transported

The importance of cell signalling in plant defence

  • Pathogens possess cellulase enzymes that digest the cellulose in plant cell walls
  • The molecules produced from this breakdown of cellulose act as signals to cell surface receptors
  • By stimulating these receptors they cause the release of defence chemicals called phytoalexins
  • Phytoalexins have several modes of action
    • Disrupt pathogen metabolism
    • Delay pathogen reproduction
    • Disrupt bacterial cell surface membranes
    • Stimulate the release of chitinases (enzymes that break down the cell walls of fungi)
  • Salicylic acid is another important signalling molecule involved in plant defence
    • It migrates through the plant to uninfected areas. Once there is activates defence mechanisms that protect against pathogens for a period of time
    • This long-term protection is called systemic acquired resistance
  • Ethylene is a signalling compound that allows plants to communicate
    • Plants under attack from pathogens secrete ethylene onto their leaves. The ethylene vaporises, stimulating other leaves on the same plant as well as other plants
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