12.4 Plant Defences Against Pathogens

Question 1

How to plants recognise an attack

Answer 1

• plants respond rapidly to pathogen attacks
• receptors in the cells respond to molecules from the pathogens, or to chemicals produced when the plant cell wall is attacked
• this stimulates the release of signalling molecules that appear to switch on genes in the nucleus
• this is turn triggers cellular responses, which include producing defensive chemicals, sending alarm signals to unaffected cells to trigger their defences, and physically strengthening the cell walls

Question 2

Describe the role of callose as a physical plant defence against pathogens

Answer 2

• they produce high levels of the polysaccharide, callose
• within minutes of an initial attack, callose is synthesised and deposited between the cells walls and the cell membrane in cells next to the infected cells
• callose acts as a barrier preventing the pathogens entering the plant cells
• lignin is added, making mechanical barrier
  to invasion even thicker and stronger
• callose blocks sieve plates in the phloem, sealing off the infected part and preventing the spread of pathogens
• callose is deposited in the plasmodesmata between infected cells and their neighbours, sealing the off from the healthy cells
• plant leaves have a waxy cuticle , which provides a physical barrier against pathogen entry

Question 3

Physical plant defences against pathogens

Answer 3

• plant leaves and stems have a waxy cuticle, which provides a physical barrier against pathogen entry
  It may also water collecting on the leaf, which could reduce the risk of infection by pathogens that are transferred between plants and water
• plants cells are surrounded by cellulose cell walls. These form a physical barrier against pathogens that make it past the waxy cuticle
• plants also produce the polysaccharide callose.
  Callose gets deposited between plant cell walls and plant membranes during times of stress, e.g. pathogen invasion.
  Callose deposition makes it harder for pathogens to enter cells.
  Callose deposition at the plasmodesmata at a may limit the spread of viruses between cells

Question 4

Chemical plant defences against pathogens

Answer 4

Plants also produce anti microbial chemical (e.g. antibiotics) which kill pathogens or inhibit their growth
E.g.
- some plants produce chemicals called saponins, which destroy the cell membranes of fungi and other pathogens
- plants also produce chemical called phytoalexins, which inhibit the growth of fungi and other pathogens

Plants also make general toxins, e.g. cyanide is toxic to insects — this reduces the risk of infection by plant viruses carried by insect vectors