11.11

• Plants like animals can be infected by pathogens
• This impacts on humans directly or indirectly


  e.g. Food or resources
• Pathogens


  • Viroid
  • Viruses
  • Bacteria
  • Fungi
  • Protozoa
• Viroid


  An RNA molecule without a protein coat
• Ectoparasites


  A parasite that lives on the skin
• Phytophthora infestans is an oomycete that causes the serious potato and tomato disease known as potato blight
• Late blight was a major culprit in the potato famines
• The organism can also infect some other members of the Solanaceae
• Pathogen favorability


  Favored by moist, cool environments
• Life Cycle and environment

  1. Sporulation optimal at 12-18 °C in saturated environments

  2. Zoospore production favored below 15 °C

  3. Lesion growth rates optimal between 20 to 24 °C
• Plants lack an adaptive immune system, but have a large array of innate systems
• Plant innate systems


  • Inhibit entry
  • Detect invading organisms
  • Prevent extensive damage
• Most plant pathogens are host specific or infect groups of plants
• Some pathogens have evolved to overcome these defences
• Types of plant defences

  • Active
  • Passive
• Passive Defence Mechanisms


  • Physical Defence
  • Chemical Defence
• Physical Defence


  • Impenetrable barriers
  • Leaf hairs with chemicals
  • Thorns, spines and prickles
  • Leaf orientation with thick cuticle
  • Stomatal size relative to pathogens
• Optimal pH


  Each plant has an optimal pH in which they grow which can eliminate certain pathogens
• Phytoanticipins


  A chemical produced by plants to inhibit fungal spore germination e.g. Brown Onion
• Plant defensin


  A protein produced by a plant to inhibit the development of fungi, bacteria and viruses
• Rapid Defences


  • Change in membrane permeability
  • Increase in cellular respiration
  • Cell wall reinforcement
  • Death of hypersensitive cells
  • Phytoalexins
• Phytoalexins


  An antifungal or antibacterial compound synthesised in the plant in response to infection
• About 350 different Phytoalexins exist
• Different organs accumulate different types of Phytoalexins
• Phytoalexins are synthesised in cells adjacent to infected cells
• Delayed Active Defences


  • Wounds can be repaired or contained
  • Fleshy tissues sealed in layers
  • Special cells form around wounds
  • Pathogenesis related proteins produced
• Pathogenesis related proteins can result in a change in plant metabolism resulting in novel protein formation
• These proteins may act as enzymes that break down cell walls or act as antifungal or antibacterial agents
• Can result in a systemic acquired immunity to a range of pathogens