12.4

• Receptors in plant cells respond to molecules from pathogens or chemicals produced when the cell wall is attacked, which stimulates the release of signalling molecules
• Signalling molecules switch on genes in the nucleus which triggers a cellular response e.g defensive chemicals, alarm signals
• Callose is a polysaccharide containing b-1,3 & b,1-6 linkage between glucose monomers
• Callose papillae are synthesized and deposited between cell walls/membranes of cells next to infected cells
• Lignin is added to callose to make the mechanical barrier thicker & stronger
• Callose blocks sieve plates in the phloem which seals off the infected part of the plant
• Callose is deposited in the plasmodesmata to seal off healthy cells
• Chemical defences include: Insect repellents, insecticides, antibacterial compounds, antifungal compounds, anti-oomycetes, and general toxins
• Chitinases are anti-fungal compounds which break down the chitin in fungal cell walls
• Plant chemicals can be broken down to form cyanide compounds which are toxic