4.1.3: Plant defences against pathogens

Question 1

What are passive defenses?

Answer 1

• Defences present before infection, their role is to prevent entry and spread of the pathogen.
• Include physical barriers and chemicals.

Question 2

What are some physical defences?

Answer 2

• Cellulose.
• Lignin thickening of cell walls.
• Waxy cuticles.
• Bark
• Stomatal closure
• Callose
• Tylose formation

Question 3

Physical defence:

Cellulose

Answer 3

• Acts as physical barrier.

- plant cell walls contain a variety of chemical defences that can be activated when a pathogen is detected.

Question 4

Physical defence:

Lignin thickening of cell walls

Answer 4

-Lignin (a phenolic compound) is waterproof and almost completely indigestible.

Question 5

Physical defence:

Waxy cuticles

Answer 5

• Prevents water from collecting on cell surfaces.

- Since pathogens collect in water and need water to survive, the absence of water is a passive defence.

Question 6

Physical defence:

Bark

Answer 6

-Contains a variety of chemical defences that work against pathogenic organisms.

Question 7

Physical defence:

Stomatal closure

Answer 7

• Stomata are possible points of entry for a pathogen,
• Stomatal closure is controlled by guard cells.
• When pathogenic organisms are detected, the guard cells will close the stomata in that part of the plant.

Question 8

Physical defence:

Callose

Answer 8

• Large polysaccharide.
• Deposited in sieve tubes at the end of a growing season.
• It is deposited around the sieve plates and blocks the flow in the sieve tube.
• This can prevent a pathogen spreading around the plant.

Question 9

Physical defence:

Tylose formation

Answer 9

• A balloon-like swelling or projection that fills xylem vessel.
• When a tylose is fully formed, it plugs the vessel and the vessel can no longer carry water.
• Prevents spread of pathogens through the heartwood.
• Tylose contains a high conc. of chemicals such as terpenes that are toxic to pathogens.

Question 10

Why are many chemicals not usually produced until the plant detects an infection?

Answer 10

Chemical production requires a lot of energy.

Question 11

What are passive chemical plant defences?

Answer 11

• Plant tissues contain anti-pathogenic properties.

- Includes terpenoids, phenols, alkaloids and hydrolytic enzymes.

Question 12

How are active plant defences activated?

Answer 12

• Pathogens attack.
• Specific chemicals in their cell walls (glycoproteins and glycolipids) detected by plant cells.
• Plant responds by fortifying the defences already present.
• Includes increasing physical defences and producing defensive chemicals.

Question 13

Name some active defences.

Answer 13

• Cell walls thickened and strengthened with additional cellulose.
• Deposition of callose between cell wall and cell membrane.
• Oxidative bursts.
• Increase in production of chemicals.

Question 14

Active defence:

Deposition of callose between cell wall and cell membrane near invading pathogen.

Answer 14

• Callose deposits are polysaccharide chemicals that impede cellular penetration at the site of infection.
• It strengthens the cell wall and blocks plasmodesmata.

Question 15

Active defence:

Oxidative bursts

Answer 15

-Produce highly reactive oxygen molecules capable of damaging the cells of invading organisms.

Question 16

Chemical:

Terpenoids

Answer 16

• Essential oils.
• Antibacterial and antifungal properties.
• Create scent e.g. menthols produced in mint plants.

Question 17

Chemical:

Phenols

Answer 17

• Antibiotic and antifungal properties

- Tannins found in bark inhibit attack by insects.

Question 18

How do tannins in bark inhibit attack by insects?

Answer 18

• Bind to salivary proteins and digestive enzymes to deactivate them
• Insects that ingest high amounts of tannins do not grow and eventually die.

Question 19

Chemical:
Defensive proteins (defensins)

Answer 19

• Small cystine-rich proteins that have anti-microbial activity.
• Act on molecules in plasma membrane of pathogens, possibly inhibiting the action of ion transport channels.

Question 20

Chemical:

Hydrolytic enzymes

Answer 20

-Found in spaces between cells.
Chitinase: break down chitin in fungal cell walls.
Glucanase: hydrolyse glycosidic bonds in glucans
Lysozymes: capable of degrading bacterial dell walls.

Question 21

Chemical:

Alkaloids

Answer 21

• Nitrogen containing compounds
• e.g. caffeine, nicotine, cocaine, morphine
• Bitter taste inhibits herbivores feeding.
• Act on variety of metabolic reactions via. inhibiting or activating enzyme action.
• Some inhibit protein synthesis.

Question 22

What is necrosis?

Answer 22

• Deliberate cell suicide
• A few cells sacrificed to save the rest of the plant.
• By killing cells surrounding the infection, the plant can limit the pathogen’s access to water and nutrients.
• Therefore, preventing it from spreading further around the plant.
• Brought about by intracellular enzymes, activated by injury.

Question 23

What is a canker?

Answer 23

• A sunken necrotic lesion in the woody tissue such as the main stem or branch.
• Causes death of the cambium tissue in the bark.