plant defences

Question 1

Plant defences against pathogens

Answer 1

Plants have passive defences that are always present

Prevent entry and spread of a pathogen

Incl physical barriers and chemicals

Active defences

Induced when a plant detects a pathogen

Question 2

Passive defences (physical)

Answer 2

Cellulose cell wall & bark – physical barrier

Waxy cuticles – prevent water collecting on the cell surfaces (moisture allows pathogen growth)

Lignin thickening of cell wall – waterproof and almost completely indigestible

Stomatal closure – when pathogenic organisms are detected, the guard cells will close the stomata in that part of the plant

Question 3

Chemical defences in plants

Answer 3

Terpenoids – antibacterial and antifungal properties (essential oils)

Phenols – antibacterial and antifungal properties e.g. tannins in bark prevent insect attack

Alkaloids – nitrogen- containing compounds e.g. caffeine, nicotine, cocaine, morphine. Bitter taste–> prevent herbivory; inhibit enzyme action or protein synthesis

Defensive proteins – small, cysteine-rich proteins that are antimicrobial, acting on plasma membranes

Hydrolytic enzymes – found in spaces between cells e.g. chitinases(chitin found in fungal walls, glucanases (hydrolyse glycosidic bonds in glucans) and lysosomes (degrade bacterial wall)

Question 4

Active plant defences : recognising an attack

Answer 4

Receptors respond to molecules from pathogens/ from chemicals released when the cell wall Is attacked

Cells release signalling molecules

Switch on genes in the nucleus

Triggers cellular responses : producing defence chemicals/ sending alarm signals to other cells/strengthening cell wall

Cell walls become thickened & strengthened with additional cellulose

Deposition of callose between the plant cell wall and cell membrane near the invading pathogen (plasmodesmata)

Increase in production of defence chemicals

Question 5

Callose

Answer 5

Large polysaccharide

deposited in the sieve tube blocking them (forms a barrier) at the end of growing season

Prevent a pathogen spreading around the plant

Question 6

Tyloses

Answer 6

Ballon-like swelling

Fills the xylem vessel

Prevent the spread of pathogens

Contains a high concentration of chemicals e.g. terpenes –> toxic to pathogens

Question 7

Necrosis

Answer 7

Deliberate cell suicide

Few of the cells are sacrificed to save the rest of the plant

Intracellular enzymes released

Question 8

Canker

Answer 8

Sunken necrotic lesion in the woody tissue of plant

Caused by pathogens

Death of cambium bark tissue