12.4 - Plant defences against pathogens

Question 1

Recognising a attack

Answer 1

• Plants are not passive so they respond rapidly to pathogen attacks
• Receptors in the cells respond to molecules from the pathogens, or to chemicals produced when the plant cell is attacked.
• This stimulates the release of signalling molecules that appear to switch on genes in the nucleus
• This in 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

Physical defences

Answer 2

Within minutes of an initial attack, callose is synthesised and deposited between the cell walls and the cell membrane in cells next to infected cells. These callose papillae act as barriers, preventing the pathogen entering the plant cells around the site of infection

• Large amounts of callose continue to 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 3

Chemical defences

Answer 3

• Insect repellent: for example pine resin and cintronella from lemon grass
• insecticides: for example, pyfenthrins - these are made by chrysanthemums and act as insect neurotoxins; and caffeine - toxic to insects and fungi
• antibacterial compounds including antibiotics - for example , phenols - antiseptics made in many different plants; antibacterial gossypol produced by cotton; defensins - plant proteins that disrupt bacterial and fungal cell membranes; lysosomes - organelles containing enzymes that break down bacterial cell walls
• antifungal compounds - for example phenols - antifungals made in many different plants; antifungal gossypol produced by cotton: caffeine - toxic to fungi and insects; sponins - chemicals in many plant cell membranes that interefere wit fungal cell membranes; chitinases - enzyme that break down the chitin in fngal cell walls
• 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 be broken down to form cyanide compounds when the plant cell is attacked. Cyanide is toxic to most living living things