4.1.3 plant defences against pathogens

Question 1

define callose

Answer 1

large polysaccharide deposit that blocks old phloem sieve tubes

Question 2

what are passive defences

Answer 2

defences present before infection to prevent entry/spread of pathogen

Question 3

physical passive defences of plants to stop entry/spead of pathogen

Answer 3

• cellulose cell wall = physical barrier & contain chemical defences activated when pathogen detected
• lignin thickening of cell walls = lignin is waterproof & mostly indigestible
• waxy cuticles = prevent water collecting on cell surfaces (pathogens collect in water/need it for survival)
• bark = contains variety of chemical defences
• stomatal closure = possible entry points for pathogens & stomatal aperture controlled by guard cells (close if pathogen detected)
• callose = deposited in sieve tubes at end of growing season to block flow which prevents pathogen spreading around plant
• tylose formation = balloon-like swelling which fills xylem vessel to stop pathogen spreading & contains high concentration of chemicals toxic to pathogens

Question 4

chemical passive defences of plants to stop entry/spead of pathogen

Answer 4

• plant tissue contains many chemicals with anti-pathogenic properties (eg. terpenoids, phenols)
• some chemicals (eg. terpenes in tyloses, tannins in bark) present before infection

Question 5

outline the active defences of plants when pathogen detected

Answer 5

• cells walls thicken & strengthen with additional cellulose
• deposition of callose between plant cell wall & cell membrane near invading pathogen (strengthens wall & blocks plasmodesmata)
• oxidative bursts = produce highly reactive oxygen molecules capable of damaging cells of invading pathogens
• increase in production of chemicals
• necrosis (cell suicide) = few cells sacrificed surrounding infection, to limit pathogens access to water/nutrients which stops it spreading around plant
• canker = sunken necrotic lesion in woody tissue (eg. main stem, branch) & causes death of cambium tissue in bark

Question 6

which chemicals are produced when pathogen detected (active defence) & what do they do

Answer 6

1. terpernoids = essential oils with antibacterial/antifungal properties & may create scent
2. phenols =
   - antibiotic/antifungal properties
   - tannins in bark inhibit insect attacks
   - bind to salivary proteins/digestive enzymes which deactivates the enzymes
   - insects which ingest high amounts of tannins don’t grow & eventually die = helps prevent transmission of pathogens
3. alkaloids =
   - nitrogen-containing compounds (eg. caffeine, nicotine)
   - give bitter taste to inhibit herbivores feeding
   - act on many metabolic reactions via inhibiting/activating enzyme action
   - some inhibit protein synthesis
4. defensive proteins (defensins) =
   - small cytosine-rich proteins that have broad anti-microbial activity
   - act upon molecules in plasma membrane of pathogens
5. hydrolytic enzymes =
   - found in spaces between cells
   - include chitinases, glucanases & lysosomes

Question 7

how is necrosis brought about

Answer 7

• intracellular enzymes activated by injury
• enzymes destroy damaged cells & produce brown spots on leaves/dieback