Title: In this class we will cover
1In 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
2Class Structure
- Four sections
- Each with a guest speaker
- Each with a quiz
- Each quiz is 25 of the grade
3Required 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
4A few words on Disease Resistance
- A specialized topic?
- Analogous to other important but specialized
topics? - Metabolism
- PhotosynthesisTaxonomy
- I think not!
5Consider 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?
6Consider 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
7Red Queen Hypothesis
- It takes all the running you can do, to keep in
the same place.
8- 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
9Not 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.
10- 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
11And 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.
12Disease 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!
13Also
- 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
14Disease Resistance genes
- May be involved in speciation
- May become susceptibility genes
- May affect yield
15- 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)
16Wheat progenitor field, central Anatolian
Plateau, Turkey, 1992
17General 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
18- 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.
19- 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.
20Concepts Terminology
21Expression 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).
22Theoretical types of resistance
23Qualitative 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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25Quantitative Disease Resistance
- aka Horizontal Rate-reducing Minor-effect
General Polygenic Additive Incomplete, Partial
Race-nonspecific Multi-genic
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29- 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
30The Molecular Basis of Qualitative/Major gene
resistancea chronological summary
311940s
H. H. Flor
MOLECULAR PLANT PATHOLOGY 8 349364
32Pathogen
Host
33Pathogen
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
34What 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.
35What 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?
36Predictions
- 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
37Slight Tangent
38Hypersensitvie 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
39Hypersensitive 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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41So an R-gene does two things
- Detection
- Triggers response
- What happens if these two things are uncoupled?
42Its not pretty!
43Generation 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
44The Rp1-D21 genotype varies in strength depending
on genetic background
45End Tangent
46What was known about R-genes?
- Often associated with hypersensitive response
(HR) - In some cases, usually where complex loci are
involved, they were quite unstable.
47Three elegant studies
- Confirms our understanding of Gene-for-Gene
- Helps us understand R-gene structure and
variability - Helps us understand R-gene function
48Elicitors 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
49- 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 ?
50Cf2
Cf4
Isolate intercellular fluid and inject into Cf2,
Cf4, Cf5 Cf9 leaves
Cf5
Cf9
http//www.php.wur.nl/UK/Research/Cladosporium/
51SCHOLTENS-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.
52Conclusions
- 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