Microbial Evasion of the Plant Immune Response

Question 1

What are effectors?

Answer 1

Virulence factors released by the pathogen, that either act within the apoplast or within the cytoplasm of the target cell.

Question 2

Give suggestions of methods by which fungi may release effectors into plant cells.

Answer 2

• through vesicle secretion

- receptors/translocons on the plant cell membrane may allow transport of effectors into the host cell

Question 3

Why can effectors be described as a plant gene in the pathogen genome, when looking at plant hosts?

Answer 3

Encodes a protein that only acts inside the plant cell to directly influence plant cell physiology.

Question 4

Why must effectors adapt quickly?

Answer 4

To evade the plant immune system and recognition by NLRs. The pathogen cannot know the genotype/phenotype of the plant it will infect next so must adapt quickly.

Question 5

How is rapid evolution of effectors achieved in P. infestans?

Answer 5

Effectors are within gene sparse regions of the genome - more mutations and more sequence variation.
Core genes required for pathogen lifestyle are within a separate gene dense region of the genome - protects them from the genome changes occurring in effector genes.

Question 6

Why and how do some pathogens release effectors that prevent chitinase activity?

Answer 6

Chitin released by chitinase can bind the chitin PRR to stimulate PRR-triggered immunity. Apoplastic effectors can bind chitin to prevent its degradation, e.g. AVR4, released by Clodosporium fulvum.

Question 7

Why must there be redundancy in effectors?

Answer 7

Effectors are not 100% efficient - cannot rely on one method of immune evasion.

Question 8

How is prevention of chitin recognition further achieved by C. fulvum?

Answer 8

ECP6 effector competes with the chitin PRR for chitin binding.

Question 9

Why and how do some pathogens release effectors that prevent peroxidase activity? Give an example of an effector.

Answer 9

Peroxidases are upregulated to give ROS release at the site of infection, and increased callose cross-linking. Inhibition of peroxidases reduces callose cross-linking, allowing the pathogen to penetrate the plant cell. Example is Pep1, released by U. maydis (infects corn), which inhibits Pox12.

Question 10

Give an example of a protease inhibitor released by C. fulvum during infection of tomato plants.

Answer 10

AVR2 - inhibits RCR3 and Pip1 proteases released by tomato plant cells. AVR2 is recognised by CF-2 to trigger the hypersensitive response.

Question 11

What is thought to be the role of RCR3?

Answer 11

To capture AVR2, so that it can be recognised by CF-2. It is thought that AVR2 causes a conformational change in RCR3 that allows recognition by CF-2.

Question 12

How has P.infestans further evolved to evade the protease activity in tomato plants?

Answer 12

Releases EPIC1 - a protease inhibitor that is not recognised by CF-2 as it does not cause a conformational change in RCR3.

Question 13

Why does each strain of P. infestans only secrete 100 out of the 500 effectors encoded by its genome?

Answer 13

• immune evasion technique.

- each spore has different effector expression patterns that can be changed to allow adaptation to new plant landscapes.

Question 14

What is the function of the AVR-blb2 effector in P.infestans?

Answer 14

Accumulates at the haustoria and binds the C14 protease to prevent its secretion from plant cells and to inhibit its activity.

Question 15

Give the effector that inhibits C14 protease in the apoplast.

Answer 15

EPIC

Question 16

What term describes effectors that accumulate at the haustoria?

Answer 16

Perihaustorial effectors.

Question 17

Describe how co-immunoprecipitation is used to identify effector targets.

Answer 17

• Protein of interest is separated by antibodies bound to agarose beads.
• This is spun down to separate the agarose beads.
• Agarose beads have a huge mass, and this pulls out the protein of interest, along with any protein interacting with protein of interest.
• The supernatant is removed and the complexes are run on a gel – identifies proteins that are interacting with the effector.
• Some interactors cannot be detected from the gel – may need mass spectrometry to identify the peptides.

Question 18

How is the C14 protease in tomato plants inhibited during P. infestans infection?

Answer 18

• inhibited by EPIC in the apoplast.

- inhibited by AVR-blb2 at the haustoria; preventing C14 secretion.

Question 19

Is there redundancy in pathogen effectors?

Answer 19

Yes;

• effectors from a given pathogen tend to converge on particular host pathways.
• many effectors affect different steps of the same pathway or converge on the same target.
• effectors from phylogenetically unrelated pathogens may converge on the same host target.

Question 20

How could effectors suppress host immunity?

Answer 20

• binding and inhibiting PRRs.
• binding and inhibiting downstream MAPKs.
• suppression of immune gene expression.
• targeting gene transcripts for degradation.

Question 21

Explain how AvrPtoB results in immune receptor deletion.

Answer 21

Acts a ubiquitin ligase to cause lysosomal degradation of FLS2,

Question 22

Explain how HopB1 results in co-receptor deletion.

Answer 22

Acts as a protease to cleave BAK1 when it is complexed with FLS2.

Question 23

Why has HopB1 evolved to only cleave BAK1 when it is complexed with FLS2?

Answer 23

BAK1 is only involved in the immune response when it is complexed with FLS2 - it would be a waste of resources to cleave single BAK1 as it would not contribute to immune evasion.

Question 24

Describe the molecular arms race surrounding sterol uptake in P. infestans infection.

Answer 24

• P. infestans are sterol auxotrophs and rely on host-derived sterols for survival.
• P. infestans use INF1 to bind sterols in the apoplast.
• Plant evolved PR1 to compete for sterol binding.
• Plant evolved receptor against INF1 called ELR1 - triggers HR.
• P. infestans evolved Avr3aKI to block ERL1 activity.
• Plant evolved R3a NLR that recognises Avr3aKI.
• P. infestans evolved Avr3aEM which is not recognised by the NLR.

Question 25

How can pathogens evolve their effectors?

Answer 25

Via mutations in aa seq, removal of effector, loss of effector from genome. Could evolve a new effector but this is often less effective.

Question 26

Why is it better for a pathogen to switch off effector expression rather than lose it from the genome completely?

Answer 26

Allows the effector to be reactivated when the pathogen infects a different plant environment.