12.4

Question 1

How are plants different to animals in the way they treat disease?

Answer 1

• So plants have physical barriers, which prevent pathogens from getting in such as a waxy cuticle on plant leaves, the bark on trees and the cellulose cell walls of individual plant cells act as barriers which prevent pathogens getting in, reducing risk of infection by pathogens that use water as a vector
• Plants do not heal diseased tissue like animals, they seal it off and sacrifice it

Question 2

How do plants respond when a pathogens attack?

Answer 2

• receptors in the cells respond to the molecules from pathogens or to chemicals produced when a plant cell wall is attacked
• stimulates the release of signalling molecules which switch on genes in the nucleus

Question 3

What are physical defences

Answer 3

• when plants are attacked by pathogens they set up extra mechanical defences
• They produce high levels of the polysaccharide callose which contain beta 1,3 linkages and beta 1,6 linkages between the glucose monomers
• Within mins of the initial attack, callose is synthesised and deposited between the cell walls and cell membranes next to infected cells
• The callose papillae act as BarrIers, preventing pathogens entering 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 the sieve plates in the pholem, 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 the healthy cells and helping to prevent the pathogen spreading

Question 4

What are chemicals defences of the plant?

Answer 4

• Many plants produce chemicals that either repel the insect vectors of disease or kill invading pathogens
• Insect repellents- eg pine resin
• Insecticides eg pyrethrins act as insect neurotoxins and cafffeine is toxic to insects and fungi
• Antibacterial compounds including antibiotics eg phenols- antiseptics are made by many different plants
• Antifungal compounds
• Antiseptics
• General toxins- some plants make chemicals that can be broken down to form cynaide compounds when the plant cell is attacked. Cynaide is toxic to most living things.