4.1.3 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 substances can not freely move around the entire plant as the immune cells do in some animals, making cell signalling vital for plant defence
• 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
  • Disrupting pathogen metabolism
  • Delaying pathogen reproduction
  • Disrupting bacterial cell surface membranes
  • Stimulating the release of chitinases (enzymes that break down the chitin cell walls in fungi)

• Salicylic acid is another important signalling molecule involved in plant defence
  • It migrates through the plant to uninfected areas. Once there it activates defence mechanisms that protect the plant 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 to react (as well as other plants)