Pathogen attack

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Pathogen attack

• Attraction to plant
• Adhesion to plant
• Differentiation of infection structures
• Attack mechanisms

Different pathogen groups use different methods
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Attraction to plant
Pratylenchus attracted to root hairs
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Zoospores swim to nutrients
Drop sugar
Microscope slide
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Germtube Septoria tritici growing through stomata
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Adhesion to Plant - nonspecific

• Magnaporthe grisea conidia - tip mucilage
  (glue)

Glues the spore to the hydrophobic rice surface
through rain or dew, which are critical for
penetration.

• Mistletoe viscin

Micrographs by Rick Howard Lab, DuPont
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Adhesion to Plant - specific
Ghigo lab, Pasture Institute
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Adhesion to Plant - specific
OToole lab, Dartmouth
Ghigo lab, Pasture Institute
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• Polysaccharides
• Cellulose (NOT Erwinia chrysanthemi)
• Agrobacterium tumefaciens
• Salmonella enterica
• Pathogenic E. coli
• Poly-b-1.6-n-acetyl-d-glucosamine
• Colanic acid (NOT S. enterica)
• Adhesins
• Fimbriae
• Curli
• Type 3 Secretion System
• Flagella
• Pathogenicity complex
• Enteric bacteria

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Adhesion to Plant - specific
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N-acyl homoserine lactone (AHL)-mediated quorum
sensing

• Cells in aggregates have a greater ability to
  survive desiccation stress
• Increased tolerance to desiccation is related
  directly to the size of the cell aggregate
• Is leaf fitness cell-density-dependent?

AHL signal
regulator
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Differentiation of infection structures (fungi)

• Physical (thigmotropism) or chemical stimulus
  from plant leads to differentiation
• Thigmotropism
• Powdery Mildew gt junctions of epidermal cells
• Puccinia spp. gt leaf surface topography
• Uromyces phaseoli gt height of lip of a stoma, 0.5
  µm

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Transmembrane protein secretion machinery
Marlovits, et al., 2004
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Attack Mechanisms

• a. Destruction of Structural Integrity
• b. Altered Host Metabolism
• c. Altered Genetic Controls
• d. Interference with Transport (translocation and
  transpiration)

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Destruction of Structural Integrity

• Enzymatic degradation of host
• Enzyme name often ends in ase
• Endopolygalacturonase gt attacks polymers of
  galacturonic acid. Endo indicates attack of
  internal part of galacturonic chain.
• Exopoly gt ends of the polymer
• Lyase, splits polymer by removing water

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Plant Cell Walllipid and sugar polymers with
imbedded proteins
CW layer
Biochemical composition
Pathogen tool
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Other enzymes

• Proteinases
• Amylases - starch
• Lipases, phospholipses - oils, fats, membranes

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Cellulose
b 1-4 D glucan chains held together by hydrogen
bonds

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Mixed linked b glucans

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Why make cellulose AND cellulase?
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Pectin structure and where enzymes attack
galacturonic acid
rhamnose
arabinose
galacturonic acid
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Erwinia soft rots

• 2 cellulases
• 8 endopectate lyases
• 1 Exopectate lyases
• 1 Pectin acetylesterase
• 2 Pectin methylesterases
• 3 Pectin lyases
• 1 Polygalacturonase
• 1 Rhamnogalacturonate lyase
• 3 Endopolygalacturonases

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Altered Host Metabolism

• Phytotoxins low molecular weight compounds
  produced by pathogen and toxic to host
• Host specific
• Non host specific
• Detoxification
• Growth Regulators
• avr and Type III Secretion

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Phytotoxins, Host Specific

• Victorin
• Cochliobolus victoria, Victoria Blight of Oats
• inhibition of glycine decarboxylase, involved in
  photorespiration
• T-toxin
• Cochliobolus heterostrophus, race T southern corn
  leaf blight
• uncouples electrolyte gradient in mitochondria
• HC toxin
• Cochliobolus carbonum, Carbonum leaf spot of corn
• inhibition of histone deacetylase
• Ptr toxin
• Pyrenophora tritici-repentis, tan spot of wheat

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Tan Spot of Wheat Pyrenophora tritici-repentis
Courtesy of Dr.William Bockus, Kansas State Univ.
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Toxin alone causes chlorosis on susceptible wheat
Lines
Katepwa S Chl
6B365 R
fungus
Glenlea R
Salamouni R
S Chl
R
Prt toxin
R
R
S Control
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Pyrenophora tritici-repentis isolates lacking the
Prt gene are less virulent on susceptible wheat
Susceptible wheat
R wheat
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Phytotoxins, Non Host Specific

• Tabtoxin
• Pseudomonas syringae pv. tabaci, wildfire disease
  of tobacco
• Glutamine synthetase gt toxic conc ammonia
• Phaseolotoxin
• Pseudomonas syringae pv. phaseolicola, halo
  blight of bean
• Ornithine carbamoyltransferase gt alters amino
  acid pools and inhibits pyrimidine biosynthesis
• Pyricularin
• Pyricularia (Magnaporthe) grisea, rice blast
  disease
• Tentoxin
• Alternaria alternata, seedling disease
• Disrupts chloroplast development and inhibits
  polyphenol oxidase
• Cerato-ulmin
• Ophiostoma ulmi, Dutch elm disease

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Coronatine Pseudomonas syringae pv. tomato
Stomata
Guard Cells
CLOSED
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Growth Regulators

• Can be formed or induced by pathogen
• Auxins (cell elongation)
• Pathogen derived inhibitors of auxin
• Cytokinins (cell division)
• Mistletoes on conifers
• Gibberellins (cell elongation)
• Gibberella fujikuroi, foolish seedling disease of
  rice
• Elsinoë brasiliensis, superelongation disease of
  cassava
• Ethylene (many functions)
• Implicated in premature defoliation of some
  plants diseases

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Giant cell formation by Melodigyne
Female nematode
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Galls - growth regulationaltered genetic control
Agrobacterium tumefaciens
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Interference with Transport

• Plugging vascular tissues
• Wilting
• Pathogen
• Mycelium and spores
• Extracellular polysaccharide (EPS)
• May mask elicitors of resistant response
• Host
• Tyloses (Overgrowth of the protoplast of a
  parenchyma cell into an adjacent xylelm vessel or
  tracheid), gums, gels
• Girdling by cankers
• Root damage
• Nematodes
• fungi