inheritance, expression, and deployment of host plant resistance margaret smith department of plant...
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Inheritance, Inheritance, Expression, and Expression, and
Deployment of Host Deployment of Host Plant ResistancePlant Resistance
Margaret Smith
Department of Plant Breeding and Genetics
Topics to CoverTopics to Cover
• The challenge
• Strategies to deal with it
• An interesting case
The challengeThe 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 The Good News: Selection Reduces Genetic DiversityReduces Genetic Diversity
• Increases frequency of desired alleles
The Bad News: Selection The Bad News: Selection Reduces Genetic DiversityReduces Genetic Diversity
• Increases frequency of desired alleles
Selection - Reduces Selection - Reduces Genetic DiversityGenetic Diversity
• Increases frequency of desired alleles
• This has some risks…
Iowa
Hazard of Crop UniformityHazard of Crop Uniformity
• Large, homogeneous target for pests
• Potato late blight
Hazard of Crop UniformityHazard 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 UniformityHazard of Crop Uniformity
• Large, homogeneous target for pests
• Potato late blight
• Wheat stem rust
Stem Rust Ug99
Current DistributionCurrent Distribution
1999
2008
2003
1990s
2007
Resistance Evolution - Resistance Evolution - Powdery Mildew in BarleyPowdery Mildew in Barley
Crute et al., 1997, The gene-for-gene relationship in plant-parasite interactions. CAB International
Resistance evolutionResistance evolution
• To Bacillus thuringiensis
– Diamondback moth
– Indian mealmoth
– Cabbage looper
• To Bt crops?
Resistance evolutionResistance 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 ManagementResistance 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 ResistanceStrategies: 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 ResistanceStrategies: Partial Resistance
Gray leaf spot of corn
Strategies: Partial ResistanceStrategies: Partial Resistance
Northern leaf blight of corn
Strategies: Physical MixturesStrategies: Physical Mixtures• Mixing resistant and susceptible plants in a
given field
– Zhu et al., 2000, Nature
– Rice blast in China
Susceptible varieties
Resistant varieties
Monoculture Mixture
Strategies: Physical MixturesStrategies: 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 MixturesStrategies: 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 MixturesStrategies: 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 MixturesStrategies: Genetic Mixtures
Strategies: High Dose + RefugeStrategies: 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 StrategyBt - High Dose, Refuge Strategy
An Interesting CaseAn 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 CucumberThe 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 CucumberThe Modern Cucumber
• Bitterness = cucurbitacin
• Attractant to cucumber beetle
• Repellant to two-spotted spider mite
SummarySummary• 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
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