Plant defences against pathogens 12.4

Question 1

What do plants do to infected tissue instead of healing it, why would they do this

Answer 1

Plants seal it off and sacrifice it, because they are continually growing at the meristems, they can then replace the damaged parts

Question 2

How do plants recognise and respond to an attack from a pathogen

Answer 2

Plants are not passive - they 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 in turn tiggers cellular responses, which include producing defensive chemicals, sending alarm signals to unaffected cells to trigger their defences and physically strengthening the cell walls.

Question 3

What Physical defences arise when a pathogen is recognised in a plant

Answer 3

• They produce high levels of a polysaccharide called callose
• deposited between plasmodesmata to slow spread of pathogens

Question 4

What are the suspected roles of callose in physical plant defences

Answer 4

• within minutes of an initial attack, callose is synthesised and deposited between the cell walls and the cell membrane in cells next to the infected cells. These callose papillae act as barriers, preventing the pathogens entering the plant cells around the site of infection
• large amounts of callose continue to be deposited in cell walls after the initial infection, Lignin is added, making the 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 them off from the healthy cells and helping to prevent the pathogen spreading.

Question 5

What are some examples of chemical defences in plants

Answer 5

• insect repellents (for example, pine resin and citronella from lemon grass)
• insecticides ( for example, pyrethrins-these are made by chrysanthemums and act as insect neurotoxins; and caffeine toxic to insects and fungi)
• Antibacterial compounds including antibiotics (for examples phenols-antiseptics made in many different plants; defensins-plant proteins that disrupt bacterial and fungal cell membranes)
• Antifungal compounds ( for example phenols-antifungals made in many different plants)
• anti-oomycetes (for example glucanases-enzymes made by some plants that break down glucans; polymers found in the cell walls of oomycetes
• general toxins, some plants make chemicals that can be broken down to form cyanide compounds when the plan cell is attacked.