Plant Immunity

Question 1

What are the five major classes of plant hormones and their primary functions?

Answer 1

1. Auxins: Cell elongation, apical dominance.
2. Cytokinins: Cell division, delay senescence.
3. Gibberellins (GA): Stem elongation, seed germination.
4. Abscisic Acid (ABA): Stress responses, stomatal closure.
5. Ethylene: Fruit ripening, leaf abscission.

Question 2

How is auxin transported in plants, and what proteins are involved?

Answer 2

Auxin is transported polar (unidirectional) via:
- PIN proteins: Export auxin out of cells.
- AUX1/LAX: Import auxin into cells.

Question 3

What are the roles of gibberellins (GA) in plants?

Answer 3

GA promotes:
1. Stem elongation by stimulating cell division and elongation.
2. Seed germination by mobilizing stored nutrients.

Question 4

How does abscisic acid (ABA) regulate stomatal closure during drought stress?

Answer 4

ABA triggers:
1. Ca2+ influx into guard cells.
2. K+ efflux, reducing turgor pressure.
3. Stomatal closure to reduce water loss.

Question 5

What is the ethylene triple response in seedlings?

Answer 5

1. Reduced elongation.
2. Radial swelling.
3. Apical hook formation.

Question 6

How do plant hormones interact in growth and defence?

Answer 6

JA (Jasmonic Acid): Antagonizes SA (Salicylic Acid) pathways (JA for herbivory, SA for pathogens).
ABA antagonizes GA to maintain seed dormancy.

Question 7

What is the role of jasmonic acid (JA) in defence?

Answer 7

JA regulates responses to herbivores by inducing protease inhibitors and secondary metabolites like alkaloids.

Question 8

How does salicylic acid (SA) defend against pathogens?

Answer 8

SA triggers Systemic Acquired Resistance (SAR) and hypersensitive response (HR) against biotrophic pathogens.

Question 9

How do ROS participate in plant defence?

Answer 9

ROS act as signalling molecules to:
1. Strengthen cell walls (lignification).
2. Induce programmed cell death (PCD).
3. Activate defence pathways.

Question 10

What is the hypersensitive response, and how does it work?

Answer 10

HR involves localized cell death at the infection site to prevent pathogen spread.

Question 11

What is PTI, and how is it initiated?

Answer 11

PTI detects Pathogen-Associated Molecular Patterns (PAMPs) using plant PRRs (Pattern Recognition Receptors) and activates basal defences.

Question 12

How does ETI provide a stronger immune response than PTI?

Answer 12

ETI involves plant R-genes recognizing pathogen effectors, triggering robust and specific immune responses.

Question 13

Name three classes of secondary metabolites involved in plant defence.

Answer 13

1. Alkaloids: Toxic to herbivores (e.g., nicotine).
2. Terpenoids: Antimicrobial.
3. Phenolics: Strengthen cell walls (e.g., lignin).

Question 14

How do plants use VOCs in defence?

Answer 14

VOCs attract predators of herbivores or warn neighbouring plants to activate defences.

Question 15

What is systemin, and how does it signal herbivore damage?

Answer 15

Systemin is a peptide hormone that activates jasmonic acid (JA) signalling, inducing defence against herbivory.

Question 16

How does callose deposition contribute to plant defence?

Answer 16

Callose strengthens cell walls at infection sites, limiting pathogen entry and spread.

Question 17

How do GA and ABA regulate seed germination?

Answer 17

GA: Promotes germination by mobilizing nutrients.
ABA: Maintains dormancy by inhibiting germination.

Question 18

How was jasmonic acid’s role in herbivory defence proven experimentally?

Answer 18

Wounded tobacco plants produced JA, and mutants deficient in JA signalling were more susceptible to herbivory.

Question 19

Why is there a trade-off between growth and defence in plants?

Answer 19

Energy allocated to defence reduces resources available for growth, leading to trade-offs between survival and reproduction.

Question 20

What experiment demonstrated SA’s role in pathogen defence?

Answer 20

SA-deficient mutants failed to mount effective defences against biotrophic pathogens, proving SA is essential for SAR.