Part H - Plant Breeding and Pathogen Diversity

Question 1

Single gene resistance generally offers complete resistance to a plant pathogen, sometimes referred to as “immunity”. However, there are concerns about deploying this type of resistance. Explain why? What strategies would you use to reduce these concerns?

Answer 1

It can lead to the development of resistant pathogen populations.
1. Single gene resistance is often based on a single mechanism of action.
2. Single gene resistance is often very effective at first, which can lead to the selection of pathogen populations that are highly susceptible to that type of resistance.

Strategies:
Use multiple genes for resistance instead of a one fit all solution.
Use a variety of resistance strategies such as:
Gene stacking
Gene rotation
Spatial isolation
Cultural practices
Biological control
Chemical control

Question 2

Describe how genetic diversity originates in fungi and how it is maintained. Indicate what the practical impacts are on plant production of the occurrence of genetic diversity within fungal plant pathogens.

Answer 2

Genetic variation in fungi originates from nay different way including:
Mutations in DNA caused spontaneously or by environmental factors such as chemicals used in pesticides
Recombination of the genetic material between two individuals which happens during sexual reproduction AND asexual reproduction in some fungi.

This genetical variation is maintained through:
Sexual reproduction
Asexual reproduction
Horizontal gene transfer

The practical impacts of genetic variation include:
Reduced effectiveness of fungicides
Increased difficulty of disease control
Increased risk of new diseases

Question 3

Briefly describe the conditions that favour post-harvest diseases of root crops

Answer 3

Moisture provides a favorable environment for the growth of pathogens.
Temperature
Injury provides entry points for pathogens.
Pests: Some pests, such as insects and nematodes, can damage root crops and make them more susceptible to post-harvest diseases.
Storage conditions: Improper storage conditions, such as storing root crops at too high a temperature or humidity, can increase the risk of post-harvest diseases.

Question 4

Describe the differences between qualitative and quantitative forms of genetic resistance and explain why plant breeders consider quantitative forms of resistance more durable and less likely to be overcome by the pathogen.

Answer 4

Qualitative resistance is determined by a single gene and results in complete resistance to the pathogen. This type of resistance is often overcome by the pathogen through mutation.

Quantitative resistance is determined by multiple genes and results in a reduction in the severity of disease symptoms. This type of resistance is more durable because it is more difficult for the pathogen to overcome.

Plant breeders prefer quantitative resistance pathways because the pathogen would need to mutate multiple genes in order to overcome quantitative resistance. Additionally, quantitative resistance is often more effective against a wider range of pathogens.

Question 5

Describe what is meant by Flor’s gene-for-gene hypothesis?

Answer 5

Resistance and virulence are determined by pairs of genes that interact with each other.
R genes are typically dominant genes, meaning that a plant only needs to inherit one copy of the gene to be resistant.
The genes in the pathogen that determine virulence are called avirulence (Avr) genes. Avr genes are typically recessive genes, meaning that a pathogen must inherit two copies of the gene to be virulent.
When a plant with an R gene encounters a pathogen with an Avr gene, the R gene product will interact with the Avr gene product. This interaction will trigger a signal transduction pathway in the plant that will lead to the plant’s defence response.
If the plant does not have an R gene that matches the Avr gene of the pathogen, the pathogen will be able to infect the plant and cause disease.