How Pathogens Attack

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How Pathogens Attack
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Reading Assignment Chapter 26
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Pathogen Attack Concepts

• Successful pathogens enter the host, overcome
  plant defenses and cause disease
• Pathogens use strategies such as mechanical
  pressure, enzymes, toxins and hormones to attack
  plants and overcome plant resistant mechanisms.

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Selection Pressure

• Competition for nutrients is a source of
  selection pressure that promotes parasitism on
  plants
• Microorganisms have evolved as saprophytes,
  facultative saprophytes, facultative parasites,
  endophytes, necrotrophs, and biotrophs.

Antibiosis
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Endophytes

• Endophyte - microorganism living inside another
  one. The association runs from antagonism to
  mutualism.
• Neotyphodium (Acremonium) species provide
  mutualistic benefits to their grass hosts
  produce alkaloids and phenolic toxins that deter
  insects, nematodes and some fungal pathogens.

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Hemibiotrophs
Hemibiotrophs are microorganisms that have
developed with plants where early in their life
cycle they have a dependence on living cells but
later can survive as facultative parasites or
facultative saprophytes.
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Sequence of Pathogenesis Events

• Dissemination/ contact
• Penetration
• Establishment of food relationship within host
• Invasion

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How Pathogens Attack
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Dissemination/ Contact

• Propagules of pathogen must come in contact with
  host (efficient method of dissemination or
  produce high number of propagules.
• Propagules must adhere to cell surface.

The fungus Penicillium produces millions of
conidia on a single orange.
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Penetration

• When spores of fungi germinate, the tip of the
  germ tube may swell and from a specialized
  structure known as an appressorium.
• An appressorium adheres to the cell surface and
  has much higher turgor pressure than found within
  normal hyphae.

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Penetration

• From the appressorium, a penetration peg (pp)
  much narrower than regular hyphae and
  penetrates the cell walls.
• Once the cell wall is breached, hyphae from the
  tip of the pp resume their normal size.
• Penetration of the cell wall by the pp is aided
  by mechanical and biochemical mechanisms.

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Establishment of food relationship within host

• Haustorium a specialized feeding cell that
  invaginates (does not rupture the cell membrane).
• Digestive enzymes pass out of the haustorium,
  across the cell membrane and into the cytoplasm
  of the cell
• Food passes across the cell membrane from the
  cell to the haustorium.

Intracelluar hyphae of a downy mildew fungus
with haustoria
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Invasion
Xylella bacteria colonizing xylem vessels.
Soft rot of potato
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Chemical Attack

• Pathogens may use chemicals to overcome plant
  defenses to successfully penetrate, establish a
  food relationship and invade the host.
• These chemicals include enzymes, hormones,
  toxins, and extracellular polysaccharides.

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Enyzmes- used to attack host cells

• Pathogens may mobilize numerous enzymes to attack
  walls of cells (primary and secondary) or the
  middle lamella.
• These enzymes include cutinases, pectinases,
  cellulases, hemicellulases, ligninases,
  proteases, lipases, and amylases.

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Hormones

• Some genes involved with defense responses in
  plants are downregulated by hormones and
  susceptibility to some pathogens.
• Hypersensitive response can also be inhibited by
  hormones.

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Toxins

• Phytotoxins are almost always secondary
  metabolites.
• Selective toxins have a specific activity, have
  low molecular weights and are agents for
  virulence or pathogenicity.

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Toxins

• Sclerotium rolfsii produces a nonspecific
  phytotoxin, oxallic acid, that is kills cells
  prior to invasion by the fungus.
• S. rolfsii is a necrotroph.