Transcript No Slide Title

Inheritance,
Expression, and
Deployment of Host
Plant Resistance
Margaret Smith
Department of Plant Breeding
and Genetics
Topics to Cover
• The challenge
• Strategies to deal
with it
• An interesting case
The challenge
• Evolution
– Natural genetic variation
– Survival of the fittest
– Change in allele frequency over time
• What defines “the fittest”?
– With natural selection
– With artificial selection
The Good News: Selection
Reduces Genetic Diversity
• Increases frequency
of desired alleles
The Bad News: Selection
Reduces Genetic Diversity
• Increases frequency
of desired alleles
Selection - Reduces
Genetic Diversity
• Increases frequency
of desired alleles
• This has some
risks…
Iowa
Hazard of Crop Uniformity
• Large, homogeneous
target for pests
• Potato late blight
Hazard of Crop Uniformity
• Large, homogeneous
target for pests
• Potato late blight
Potato research: Fighting the blight
Irish famine fungus makes comeback through
stronger strain
October 23, 1998
ITHACA, New York (CNN) -- More than 150 years
after the Irish potato famine, aggressive relatives of the
disastrous fungus are making themselves at home all
over the world. But scientists are fighting the blight
with efforts to develop new disease-resistant plants.
Hazard of Crop Uniformity
• Large, homogeneous
target for pests
• Potato late blight
• Wheat stem rust
Stem Rust Ug99
Current Distribution
2008
2007
2003
1999 1990s
Resistance rating
Resistance Evolution Powdery Mildew in Barley
9
8
7
6
5
4
3
2
1
0
Sultan
Wing
Triumph
Kym
Pipkin
67 970 973 976 979 982 985 988 991 994
9
1
1
1
1
1
1
1
1
1
1
Year
Crute et al., 1997, The gene-for-gene relationship in
plant-parasite interactions. CAB International
Resistance evolution
• To Bacillus thuringiensis
– Diamondback moth
– Indian mealmoth
– Cabbage looper
• To Bt crops?
Resistance evolution
• Rate of evolution affected by:
– Pest biology and behavior
– Factors affecting pest population density
(predators, parasites, host distribution)
– Frequency of resistance alleles
– Nature of resistance alleles (recessive,
additive, dominant)
– Intensity of selection
Resistance Management
• Developing more durable
types of host plant
resistance
• Deploying host plant
resistance genes in ways
that minimize the chance of
pest evolution to overcome
them
Strategies: Partial Resistance
•
•
•
•
•
•
“Vertical” vs. “horizontal” resistance
Single gene vs. polygenic resistance
Qualitative vs. quantitative resistance
Durable resistance
Hypersensitive response
Antibiosis vs. antixenosis and tolerance
Strategies: Partial Resistance
Gray leaf spot of corn
Strategies: Partial Resistance
Northern leaf
blight of corn
Strategies: Physical Mixtures
• Mixing resistant and susceptible plants in a
given field
– Zhu et al.,
2000, Nature
– Rice blast
in China
Susceptible
varieties
Monoculture
Mixture
Resistant
varieties
Strategies: Physical Mixtures
• Mixing resistant and susceptible plants in a
given field
• Mixing varieties that each carry different
resistance alleles
– Multilines
– Lannou and Pope, 2009, Variety
Mixtures in Theory and Practice, SCRI
Wheat inoculated with yellow rust
Pure line
Multiline
Strategies: Physical Mixtures
• Mixing resistant and susceptible plants in a
given field
• Mixing varieties that each carry different
resistance alleles
• Regional deployment of resistance genes
– Theoretical use for wheat rust
Wheat rust pathway in the US
Strategies: Genetic Mixtures
• Pyramiding resistance genes
– Almost impossible with classical
breeding methods
– Molecular markers allow pyramiding
– Tested with genetically engineered Bt
broccoli (Zhao et al., 2003, Nature
Biotechnology)
Strategies: Genetic Mixtures
Strategies: High Dose + Refuge
• Developed by entomologists for genetically
engineered Bt varieties
• Combines two elements:
– Very strong resistance gene
– Refuge - mandatory planting of a variety
that does not carry the resistance gene
Bt - High Dose, Refuge Strategy
An Interesting Case
• Biological systems are interactive and fluid
• Changing any one component will cause
other alterations
• A “simple” single gene trait may not be
simple…
– Genes do not act independently of the
rest of the genetic background
– Genes do not necessarily have only one
function in the organism
The Modern Cucumber
• 100 years ago - fruits for
sale were notched
• Dutch found a non-bitter
type in the 1930s
• US breeders traded disease
resistance for non-bitter in
the 1950s
The Modern Cucumber
• Bitterness = cucurbitacin
• Attractant to cucumber
beetle
• Repellant to two-spotted
spider mite
Summary
• Host plant resistance is an interaction
between host and pest
• Both players in the interaction have
potential to evolve
• Care must be taken when selection pressure
on pest populations is intense
• Appropriate strategies will depend on pest
biology, host production systems, and
many other factors