Biotic stress

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Biotic stress (Plant Defense)
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Plant Disease

• Can and has caused large-scale starvation
• Serious economic impact - quality and quantity
• Environmental impacts
• Damage to ecosystems
• Limit the kinds of plants grown in select
  environments

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Fire blight
(Erwinia amylovora)
Infects many rosaceous hosts, serious disease of
apple and pear which were imported by European
settlers. Now spread to Europe, Middle East,
New Zealand etc. First plant disease shown to
be caused by a bacterium.
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Citrus Canker
(Xanthomonas axonopodis pv. citri)
Spread from Southeast Asia and now in over 30
countries including US. Thought to be eradicated
from Florida 2X. Likely source of latest outbreak
is from ornamental citrus and has been greatly
exacerbated by numerous hurricanes. Currently
spending 12 million a year to control in
Florida.
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Tobacco Mosaic Virus
(TMV)
The first virus discovered, the first virus
crystalized and the first virus shown to contain
infectious nucleic acid. Also, the first virus
used for electron microscopy, solution
electrophoresis and analytical ultracentrifugation
.
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Ergot on Rye
(Claviceps purprea)
Little effect on yield but Alkaloids accumlate in
sclerotia Toxic cause hallucinations- St.
Anthonys Fire Burning crawling skin
Cause miscarriages and low fertility Leads
to gangrenous infections
Epidemics in the 1800s cause up to 40
mortality After cause discovered, potato
was adopted as peasants staple
Several useful pharmaceuticals developed
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Potato Late Blight
(Phytophthora infestans)
Epidemic in the 1840s lead to mass starvation,
especially in Ireland where 1.5 million starved
and 1.5 million emigrated- esp. to
USA Establishing cause started Plant Path
contributed to Pasteurs germ theory
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How Do Plants Defend Themselves?
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Definitions
Pathogen Race - subgroup or biotype within a
species or variety, distinguished from other
races by virulence, symptom expression, or host
range, but not by morphology Host Differential
(syn. differential cultivar) - a plant host that
on the basis of disease symptoms serves to
distinguish between various strains or races of a
given plant pathogen
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Host Differentials
Pathogen Races
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Extensive Genetic Analyses

• Crosses among races and analyses of ability to
  cause disease on host differentials
• Crosses among host differentials and analyses of
  disease response to races

Gene-for-Gene hypothesis - genes for resistance
and virulence exist in the host and pathogen,
respectively Resistance is genetically
dominant Aviurlence is genetically dominant
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Gene-for-Gene
gene-for-gene hypothesis - genes for resistance
and virulence exist in the host and pathogen,
respectively
Host
Pathogen
Typically genes for Avirulence in the pathogen
and Resistance in the host are genetically
dominant
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Different R gene products recognize different Avr
gene products
Host
R1 R2 R1 r2 r1 R2 r1 r2
R
R
R
S
A1 A2
A1 a2
S
S
R
R
Pathogen
a1 A2
R
S
S
R
a1 a2
S
S
S
S
Resistance is epistatic to susceptibility
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Host Differentials
R1 r2 r3
r1 R2 r3
r1 r2 R3
R
A1 A2 A3
R
R
R
S
a1 A2 A3
R
R
S
A1 a2 A3
R
R
R
S
A1 A2 a3
Pathogen Races
S
R
S
a1 a2 A3
R
S
S
A1 a2 a3
S
R
S
a1 A2 a3
S
S
S
a1 a2 a3
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Dominant Avirulence Genes?

• Why would a pathogen maintain them?
• Wouldnt this take a lot of resistance genes
  (i.e. recognition for all potential pathogens)?

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Why Are Genes Dominant?

• This is a Recognition System

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Why Are Genes Dominant?

• This is a Recognition System

A
Recognition!
Dominance implies function
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Molecular Genetics

• Identified Avirulence genes by making DNA
  libraries and putting genes into virulent
  pathogens
• Identified Plant Resistance genes mostly by
  Mapped-Based Cloning

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What are these gene products?
Avirulence Defeated virulence
products (virulence products are used by the
pathogen to incite disease)
Resistance Recognize the presence of
avirulence products (recognition leads to
signaling and the induction of the resistance
response )
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(No Transcript)
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Guard Hypothesis
Outside of Cell
Cell Wall
Type III secretion system
Plasma Membrane
Cytoplasm
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Guard Hypothesis
Outside of Cell
Cell Wall
Type III secretion system
Plasma Membrane
R
Cytoplasm
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Guard Hypothesis
Outside of Cell
Cell Wall
Type III secretion system
Plasma Membrane
Signals Resistance
R
Cytoplasm
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Resistance Signaling
Linked predominantly to ion fluxes and kinase
activation
Signals Changes in gene expression
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Resistance Signaling

• Hypersensitive Response Programmed Cell Death
  of the host cells
• Biosynthesis of Lytic enzymes So called
  Pathogenesis Related (PR) proteins
• Production of Reactive Oxygen
• Lignification and callose synthesis
• Activation of Systemic Acquired Resistance
• Secondary metabolites e.g. Phytoalexins