In this class we will cover

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In this class we will cover

• The genetic and molecular basis of plant disease
  resistance
• Balint-Kurti
• The biochemical basis of plant disease resistance
• Opperman
• The deployment of disease resistance in the field
• Cowger
• Breeding for disease resistance
• Marshall

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Class Structure

• Four sections
• Each with a guest speaker
• Each with a quiz
• Each quiz is 25 of the grade

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Required reading

• Elicitors, Effectors, and R Genes The New
  Paradigm and a Lifetime Supply of Questions
• A. F. Bent and D. Mackey
• Annual Review of PhytopathologyVol. 45 399-436

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A few words on Disease Resistance

• A specialized topic?
• Analogous to other important but specialized
  topics?
• Metabolism
• PhotosynthesisTaxonomy
• I think not!

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Consider the following

• Why do organisms have sex?
• Wasteful, time-consuming, messy etc.
• To generate and maintain diversity
• But why do we need to do that?

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Consider the following

• "For sex to be advantageous requires constant
  selection for changing conditions. One factor
  that might cause this is the constant arms race
  between parasites and their hosts. Parasites
  generally evolve quickly, due to their short
  lifespans. As they evolve, they attack their
  hosts in a variety of ways. Two consecutive
  generations might be faced with very different
  selective pressures. If this change is rapid
  enough, it might explain the persistence of sex.
• wikipedia

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Red Queen Hypothesis

• It takes all the running you can do, to keep in
  the same place.

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• Sex is about disease. It is used to combat the
  threat from parasites. Organisms need sex to
  keep their genes one step ahead of their
  parasites. Men are not redundant after all
• Matt Ridley

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Not only that.

• Disease resistance is also important for mate
  choice.
• The Major Histocompatibility Complex (MHC) is a
  large genomic region found in most vertebrates.
  It plays an important role in the immune system
  and is extremely variable.

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• Generally the more diverse the MHC genes of the
  parents, the more robust the offspring
• Somehow mice can recognize and preferentially
  mate with individuals with different MHC
• Works for fish too

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And in humans

• Females sniffing the T-shirts recently worn by
  males favored the scent of those whose immune
  response genes were different from their own
• Wedekind, C. et al. (1995). "MHC-dependent
  preferences in humans." Proceedings of the Royal
  Society of London 260 245-49.

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Disease Resistance and Evolution

• Constant selection for disease resistance has
  likely been a profound factor in the evolution of
  most metazoans.
• May underlie the evolution of sex and mate choice
• Disease resistance is really important!

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Also

• Disease results in losses of about 20 per year
  on a global basis
• reduced plant production
• reduced plant quality
• reduced food safety
• reduced natural resources
• reduced genetic resources
• increased cost to producers, consumers, society
• Resistance gene analogues make up 1 of
  Arabidopsis gene
• The Arabidopsis Genome Initiative (2000) Analysis
  of the genome sequence of the flowering plant
  Arabidopsis thaliana. Nature, 408, 796-815.
• Thats just the known genes involved in detection
• If you include genes involved in response plus
  all the unknown genes, the percentage is likely
  much higher

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Disease Resistance genes

• May be involved in speciation
• May become susceptibility genes
• May affect yield

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• Economic Impact of Host Plant Resistance
• Return on research investment
• (Marasas et al. The Economic Impact in Developing
  Countries of Leaf Rust Resistance Breeding in
  CIMMYT-RelatedSpring Bread Wheat 271 return on
  investment)
• Other evidence of economic benefit
• Overall genetic improvement of crops ranges from
  400-600 million/year (Frisvold et al, Ag Econ
  2003 V28 P109).
• Peanut cultivars with resistance to rosette virus
  benefit farmers in Uganda by 2.3-4.1
  million/year (Moyo et al, Am J Ag Econ 2007 V89
  P448)

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Wheat progenitor field, central Anatolian
Plateau, Turkey, 1992
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General Concepts

• Most plants are resistant to most diseases.
  Resistance is the rule and susceptibility is the
  exception.
• Disease resistance is a relative trait in plants
  rather than an absolute quality.
• However, most plant species are attacked by
  certain pathogens that can incite severe disease.
• Disease Resistance
• Reduced expense to producer
• Shifts burden of control away from producer
• No environmental side effects
• Compatible with other disease control techniques

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• Plant Terminology (for now)
• Immunity Plant cannot be infected by a given
  pathogen absolute freedom from disease.-
  Qualtitative trait
• Resistance - Any inherited characteristic of a
  host plant which lessens the effects of a
  pathogen. Pathogen development is hindered to
  some degree. May be race-specific or
  non-specific. Quantitative/Qualtitative
• Susceptibility The inability of a plant to
  resist the effects of a pathogen.
  Quantitative/Qualtitative
• Tolerance The ability of a plant to suffer
  little damage, despite high levels of disease.
  The maintenance of yield in the face of high
  disease. Two lines with the same apparent
  susceptibility may have quite different
  tolerances. Quantitative
• Escape - Occurs when inherently susceptible
  plants do not become infected because of
  development, absence of inoculum, or conditions
  unfavorable to infection.
• Klenducity - Disease escape due to morphological
  features of the plant, or the way in which the
  plant grows.

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• Pathogen Terminology (for now)
• Pathogenic The ability of a pathogen to cause
  disease (host range). a qualitative trait
• Virulence The ability of a particulat race of a
  pathogen to cause disease on a particular variety
  of the host (see gene-for-gene relationship).- a
  qualitative trait
• Aggressiveness The relative disease-producing
  ability of a pathogen on a susceptible host.- a
  quantitative trait
• Important to understand the different use of
  these terms based on the type of pathogen in
  question.

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Concepts Terminology
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Expression or Description of Resistance

• Resistance can be functionally described using 3
  criteria
• - Magnitude or Effect of the resistance (high or
  low).
• - Whether or not the resistance is differential
  (effective against one race/strain/biotype/pathoty
  pe or against many races).
• - Its inheritance or genetic basis (single genes
  or multiple genes).

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Theoretical types of resistance
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Qualitative Disease Resistance

• aka Vertical Qualitative Inoculum-reducing
  Major-effect Hypersensitive Monogenic R
  gene. Complete Race-specific Single gene
  Vertical Gene-for-gene

100 severity
Amount of disease
0 severity
Race 4
Race 3
Race 2
Race 1

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Quantitative Disease Resistance

• aka Horizontal Rate-reducing Minor-effect
  General Polygenic Additive Incomplete, Partial
  Race-nonspecific Multi-genic

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• Biotrophic pathogens
• Derive nutrition from living host cells, Usually
  establish a long-term interaction with the plant.
• Necrotrophic pathogens
• Kill host cells. Derive nutrition from dead cells

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The Molecular Basis of Qualitative/Major gene
resistancea chronological summary
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1940s
H. H. Flor
MOLECULAR PLANT PATHOLOGY 8 349364
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Pathogen
Host
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Pathogen
avr1avr2
avr1AVR2
AVR1avr2
AVR1AVR2
 
Compatible (Susceptible)
Incompatible (Resistant)
Incompatible (Resistant)
Incompatible (Resistant)
R1-R2-
 Compatible (Susceptible)
Compatible (Susceptible)
Incompatible (Resistant)
 Incompatible (Resistant)
 R1-r2r2
Host
Compatible (Susceptible)
 Incompatible (Resistant)
Compatible (Susceptible)
 Incompatible (Resistant)
r1r1R2-
Compatible (Susceptible)
Compatible (Susceptible)
Compatible (Susceptible)
Compatible (Susceptible)
r1r1r2r2
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What does this mean?

• Implies, interaction (direct of otherwise) of
  dominant Resistance and Avirulence gene products
  leads to resistance.
• The loss of an AVR gene in the pathogen can
  render the corresponding R gene essentially
  useless (or can it?).
• It doesnt matter how many interactions there are
  leading to compatibility. A single R/Avr match
  will lead to resistance.

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What did classical genetics tell us?

• Gene-for-gene interactions were identified in
  many different interactions with many different
  types of pathogens
• R-genes were often clustered in complex loci.
• Avr genes were not clustered
• What does this suggest to you?

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Predictions

• Pathogen is trying to lose Avr genes.
• Why does a pathogen have AVR genes in the first
  place?
• Host is trying to develop new types of R genes

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Slight Tangent
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Hypersensitvie Response

• A rapid, localized host cell death that occurs
  at the site of pathogen penetration, associated
  with qualitative resistance.
• Usually is associated with an incompatible
  gene-for-gene interaction

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Hypersensitive Response
A barley epidermal cell (cv. Manchuria) that
underwent hypersensitive response (HR) at 18 h
after inoculation with the inappropriate wheat
powdery mildew fungus. The HR cell displays
autofluorescence due to the accumulation of
phenolic substances, which is indicative for cell
death. Neighboring cells show nitroblue
tetrazolium staining as a dark granulate
indicative for superoxide (O(2)(-)). Trujillo
et al MPMI
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So an R-gene does two things

• Detection
• Triggers response
• What happens if these two things are uncoupled?

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Its not pretty!
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Generation of the Rp1-D21 recombinant
7
9
8
X
3
2
5
4
7
1
9
6
8
2/9
Rp1-D21
1
A dominant lesion mimic mutant
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The Rp1-D21 genotype varies in strength depending
on genetic background
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End Tangent
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What was known about R-genes?

• Often associated with hypersensitive response
  (HR)
• In some cases, usually where complex loci are
  involved, they were quite unstable.

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Three elegant studies

• Confirms our understanding of Gene-for-Gene
• Helps us understand R-gene structure and
  variability
• Helps us understand R-gene function

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Elicitors from the Cladosporium fulvum/ tomato
system

• Pierre de Wit
• Several Avr/R gene interactions were
  characterized
• Avr2/Cf2
• Avr4/Cf4
• Avr5/Cf5
• Avr9/Cf9

http//www.php.wur.nl/UK/Research/Cladosporium/?wb
c_purposeBasicWBCMODEPresentationUnpublished/a
vr
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• Fungus grows strictly in the apoplastic space-
  doesnt invade host cells
• DE WIT, P., 1977 A light and scanning-electron
  microscopic study of infection of tomato plants
  by virulent and avirulent races of Cladosporium
  fulvum. European Journal of Plant Pathology 83
  109-122.
• Can you isolate Avr elicitors from apoplastic
  fluid ?

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Cf2
Cf4
Isolate intercellular fluid and inject into Cf2,
Cf4, Cf5 Cf9 leaves
Cf5
Cf9
http//www.php.wur.nl/UK/Research/Cladosporium/
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SCHOLTENS-TOMA, I. M. J., G. J. M. DE WIT and P.
DE WIT, 1989 Characterization of elicitor
activities of apoplastic fluids isolated from
tomato lines infected with new races of
Cladosporium fulvum. European Journal of Plant
Pathology 95 161-168.
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Conclusions

• Identified specific elicitors associated with
  R-gene-mediated defense response.
• Specific peptide elicitors were identified from
  intercellular fluid
• Dont necessarily need pathogen itself to be
  present to invoke R-gene mediated resistance