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