Plant defence against pathogens

Question 1

What are some physical barriers in plants?

Answer 1

• Waxy cuticle
• Closing stomata
• Bark which has sticky resin
• Cellulose cell wall
• Casparian strip in endoderms of root tissue

Question 2

What are some chemical barriers in plants?

Answer 2

• Insect repellents e.g. pine resin from lemon grass
• Insecticides => production of chemicals that act as insect neurotoxins
• Antibacterial compounds including antibiotics e.g sitcky resin in bark, lysosomes contain hydrolytic enzymes
• Antifungal compounds e.g. caffeine is toxic to fungi and insects, chitinase enzyme breaks down the chitin in fungal cell walls
• Secretion of enzyme inhibitors e.g. cellulase inhibitors
• Secretion of toxins => e.g. in mint, garlic, cinamon, tea tree oil and some plants make chemicals that can be broken down to form cyanide when attacked. Cyanide is toxic to most living things

Question 3

What are passive defence mechanisms in plants?

Answer 3

• Mechanims which are present all the time
• This includes physical and chemical barriers

Question 4

What is callose?

Answer 4

• A branched polysaccharide
• Has β 1,3 and β 1,6 glycosidic bonds
• Its monomer is β glucose
• They act as barriers preventing pathogens from entering plant cells around hte site of infection

Question 5

How does callose provide defence against pathogens?

Answer 5

• Entry of pathogen stimulates the synthesis of callose
• It is deposited between the cell walls and plasma membranes in cells next to the infected cells
• Ligin is added making the barrier thicker and stronger
• It also blocks seive plate pores of the phloem, sealing off the infected part and preventing the spread of pathogens
• Callose is depositied in the plasmodesmata between infected cells and their neighbours, sealing them off from the healthy cells and preventing the spread of the pathogen

Question 6

How do phytoalexins provide defence against pathogens?

Answer 6

• It is produces as a response to breakdown products of cellulose by bacterial and fungal cellulases
• It disrupts the cell membrane of bacteria
• Stimulates the production of enzymes that break down fungal cell walls
• Delays production of pathogens

Question 7

How do tyloses provide defence against pathogens?

Answer 7

In-growth into xylem vessel to prevent moment of pathogens through plant from soil