Genetic of resistance in pathogen-plant interactions

Question 1

Describe four processes that create genetic variation in bacteria.

Answer 1

Mutation
Conjugation
Transformation
Transduction

Question 2

Describe four processes that can create genetic variation in fungi.

Answer 2

Mutation and recombination
specialized processes such as parasexual (mitotic) recombination, hybridization and gains and losses of entire chromosomes (accessory chromosomes)

Question 3

Which are the three most important mutations occurring in pathogenic bacteria and fungi in the context of agricultural ecosystems?

Answer 3

Mutations from avirulence to cirulence
Mutations from susceptobility to resistance towards fungicides
from non-toxin production to toxin production

Question 4

Why is genetic resistance the preffered method to control plant diseases?

Answer 4

Many advantages compared to other methods:
disease control is in seed, it is easily moved and utilized, no sprays or special management practices requiring complex equipment are needed, disease resistance is ecologically sound and economically profitable, farmaers will pay a premium price for resistant seed, especially important for developing world where farmers produce their own seed, more sustainable control of disease

Question 5

What three properties are usually associated with major gene resistance?

Answer 5

race-specific
large effects
simple inheritance

Question 6

What three properties are usually associated with quantitative resistance?

Answer 6

non-specific
small effects
quantitative inheritance

Question 7

How do quantitative resistance and major gene resistance differ in their epidemiological effects?

Answer 7

large effect resistance will prevent an epidemic and often confers immunity to disease by activating the hypersensitive response

small effects resistance will reduce the rate of epidemic development, plants often have smaller lesions producing fewer pathogen spores adn exhibit a longer latent period
small effect resistance may not work in a conducive environment

Question 8

Define disease tolerance. How does disease tolerance differ from disease resistance? Why isn’t tolerance used in plant breeding programs?

Answer 8

Tolerance refers to the ability of some plants ro endure severe disease symptoms without severe losses in yield or quality
It doesn’t affect epidemic progress and is difficult to detect, that’s why it isn’t commonly used in agricultural crops

Question 9

Define disease escape. How does disease escape affect plant breeding programs that aim to increase genetic resistance?

Answer 9

Disease escape occurs when a genetically susceptible plant escapes infection because one or more components of the disease triangle are not present. Escape can easily be confused with resistance and can complicate selection of resistant plants in plant breeding programs

Question 10

Describe in a general way the experiments that H.H. Flor conducted with flax and Melampsora lini. What were the key findings that led him to develop his gene-for-gene theory of host-parasite interactions?

Answer 10

H. H. Flor noted that a single gene determines pathogenicity in Melampsora lini and corresponds to a single resistance gene in flax. Gene-for-gene concept is advanced and verified when avirulence and resistance genes are cloned and characterized in the 1880s and 90s. Flor essentially evaluated inheritance of different interactions between host and pathogen simultaneously. Through backgrossing he was able to put different resistance genes in flax, resulting in a set of differentials in which each member possessed a single, unique resistance gene. Resistance was found to be dominant in almost every cross.
He then concluded that virulence is recessive and conditioned by as many genes in the parasite as there are genes for resistance in the host.

Question 11

Prepare a diagram that uses the receptor-elicitor model to explain the gene-for-gene (GFG) quadratic check for resistance genes and avirulence genes.

Answer 11

Siehe Seite 51

Question 12

When GFG interactions follow the receptor-elicitor model, why is the avirulence allele dominant to the virulence allele?

Answer 12

The avirulence allele is often dominant because the host plant’s resistance gene recognizes the pathogen’s elicitor molecule. The virulence allele doesn’t
cause the production of the elicitor, which causes defenses to not be activated and successful infection.

Question 13

When GFG interactions follow the receptor-elicitor model, why is the resistance allele dominant to the susceptibility allele?

Answer 13

dominant allele causes the production of receptor

Question 14

Fill in the figure showing the interaction between two resistance genes and two avirulence genes.

Answer 14

S. 129

Question 15

In the following GFG interactions, would you expect to observe an incompatible or a compatible interaction between plant and pathogen?
S. 129, nr. 61

Answer 15

Siehe Dokument

(?)

Question 16

What is the “Guard Hypothesis” and how does it differ from the receptor-elicitor model of GFG interactions? Give an example of a well-characterized pathogen-plant interaction that appears to follow the guard model.

Answer 16

The guard hypothesis states that there does not
have to be a direct interaction between the avirulence and resistance gene product

The guard hypothesis proposes the existence of guard proteins that monitor the integrity of other important proteins and signal to activate defenses. While the receptor-elicitor model emphasizes specific recognition and binding of pathogen elicitors, the guard hypothesis focuses on detection of disruption to important cellular components. The interaction between the fungal pathogen C. fulvum and tomato is a good example of this interaction. The pathogen secretes Avr2, which targets Rcr3 (a tomato protease). Rcr3 is a guard protein that monitors the integrity of another protein, Rcr3pim. When Rcr3 is attacked by Avr2, it causes it to cleave Rcr3pim, resulting in a defense response.

Question 17

How many classes of major resistance genes have been discovered? What functional elements do most proteins encoded by major resistance genes have in common? What are functions of these conserved protein domains?

Answer 17

Eight classes depending on structure and function, most have LRR domains which are known to be involved in protein-protein interactions

Question 18

What is a leucine-rich repeat and what is its function in disease resistance?

Answer 18

key component of most R-gene proteins. The leucine-Rich repeat (LRR) domain can be located inside or outside of the cell. LRR domains are modular with conserved structural elements composed mainly of beta-strands and alpha-helices connected by loops.

Question 19

Prepare a diagram of the Xa21 resistance gene, identifying the five functional domains of this protein and their predicted function.

Answer 19

S. 54 Skript oder Dokument Frage 65

Question 20

Though Hm1 was the first cloned resistance gene, it ended up having unique properties. How does Hm1 make plants resistant and how does this differ from other R-genes?

Answer 20

Protein product detoxifies the host-selective HC-toxin of Chochliobolus carbonum.
Hm1 is an exception to the R-genes because it does not appear to involve a signal transduction pathway and the lack of the ability in the pathogen to produce the toxin renders it unable to invade the host.
Hm1 codes for a HC-toxin reductase enabling plants that contain it to detoxify the HC-toxin
It does not correspond to the classic gene-for-gene pattern, but it encodes an enzyme that detoxifies the toxin

Question 21

What is the connection between GFG resistance in plants and the innate immune system of insects and animals? Explain these connections.

Answer 21

LRR-based recognition systems are conserved across plants, insects and animals

(?) Dokument

Question 22

What is the evolutionary significance of the finding that major resistance genes exist as clusters in the plant genome? How does this finding affect plant breeding strategies?

Answer 22

they suggest an important role for gene duplication as a source of new R-genes. the clusters also suggest that recombination can play an important role by making possible new specificities, especially through recombination in the LRR region.

Question 23

What is the significance of the high variability seen in the LRR region of R-genes? How is this variation generated?

Answer 23

It is generated by recombination, deletion and duplications
…

Question 24

What is the significance of the conserved locations of R-genes on genetic maps of plants in the same family?

Answer 24

the relative positions of major R-genes are conserved across species boundaries within the Solanaceae and the cereals, indicating their importance on plant evolution