4.1.3: Plant defences against pathogens Flashcards
What are passive defenses?
- Defences present before infection, their role is to prevent entry and spread of the pathogen.
- Include physical barriers and chemicals.
What are some physical defences?
- Cellulose.
- Lignin thickening of cell walls.
- Waxy cuticles.
- Bark
- Stomatal closure
- Callose
- Tylose formation
Physical defence:
Cellulose
- Acts as physical barrier.
- plant cell walls contain a variety of chemical defences that can be activated when a pathogen is detected.
Physical defence:
Lignin thickening of cell walls
-Lignin (a phenolic compound) is waterproof and almost completely indigestible.
Physical defence:
Waxy cuticles
- 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.
Physical defence:
Bark
-Contains a variety of chemical defences that work against pathogenic organisms.
Physical defence:
Stomatal closure
- 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.
Physical defence:
Callose
- 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.
Physical defence:
Tylose formation
- 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.
Why are many chemicals not usually produced until the plant detects an infection?
Chemical production requires a lot of energy.
What are passive chemical plant defences?
- Plant tissues contain anti-pathogenic properties.
- Includes terpenoids, phenols, alkaloids and hydrolytic enzymes.
How are active plant defences activated?
- 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.
Name some active defences.
- Cell walls thickened and strengthened with additional cellulose.
- Deposition of callose between cell wall and cell membrane.
- Oxidative bursts.
- Increase in production of chemicals.
Active defence:
Deposition of callose between cell wall and cell membrane near invading pathogen.
- Callose deposits are polysaccharide chemicals that impede cellular penetration at the site of infection.
- It strengthens the cell wall and blocks plasmodesmata.
Active defence:
Oxidative bursts
-Produce highly reactive oxygen molecules capable of damaging the cells of invading organisms.
Chemical:
Terpenoids
- Essential oils.
- Antibacterial and antifungal properties.
- Create scent e.g. menthols produced in mint plants.
Chemical:
Phenols
- Antibiotic and antifungal properties
- Tannins found in bark inhibit attack by insects.
How do tannins in bark inhibit attack by insects?
- Bind to salivary proteins and digestive enzymes to deactivate them
- Insects that ingest high amounts of tannins do not grow and eventually die.
Chemical: Defensive proteins (defensins)
- 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.
Chemical:
Hydrolytic enzymes
-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.
Chemical:
Alkaloids
- 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.
What is necrosis?
- 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.
What is a canker?
- A sunken necrotic lesion in the woody tissue such as the main stem or branch.
- Causes death of the cambium tissue in the bark.
