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Qualitative Disease Resistance

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Qualitative Disease Resistance aka: Vertical; Qualitative; Inoculum-reducing; Major-effect; Hypersensitive; Monogenic; R gene.Complete Race-specific Single gene ... – PowerPoint PPT presentation

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Title: Qualitative Disease Resistance


1
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

8
The Molecular Basis of Qualitative/Major gene
resistancea chronological summary
9
1940s
H. H. Flor
MOLECULAR PLANT PATHOLOGY 8 349364
10
Pathogen
Host
11
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
12
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.
  • It doesnt matter how many interactions there are
    leading to compatibility. A single R/Avr match
    will lead to resistance.

13
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?

14
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

15
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.

16
Three elegant studies
  • Confirms our understanding of Gene-for-Gene
  • Helps us understand R-gene structure and
    variability
  • Helps us understand R-gene function

17
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
18
  • Fungus grows strictly in the apoplastic space-
    doesnt invade host cells
  • Can you isolate Avr elicitors from apoplastic
    fluid

19
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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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

22
Unstable nature of R-genes
  • Seems to be associated with meiosis
  • R-genes dont spontaneously cease functioning in
    an existing plant.

23
tester - no R gene
Homozygous for R gene, heterzygous for flanking
markers
A
a
a
A
Select susceptible progeny
r
X
r
OR
R
R
25/15,646
b
b
B
B
24/25 show non-parental combinations
Sudapak et al, 1993 Genetics, Vol 133, 119-125
24
Sudapak et al, 1993 Genetics, Vol 133, 119-125
25
Conclusion
  • Complex nature of R-gene loci leads to their
    unstable nature.
  • NB many , but not all, R-genes occur in complex
    loci.

26
Cell Autonomous Nature of R-genes
27
Developing maize embryos were exposed to X-rays
rp1
Rp1
rp1
oy
Oy
oy
28
  • HR in green sectors
  • Pustules in yellow sectors
  • HR doesnt cross into yellow sectors (much-1F).
  • Rp1 is not diffusible
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