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 s 2007

Resistance Evolution - Powdery Mildew in Barley 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 Resistant varieties Monoculture Mixture

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 lineMultiline

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