Plant defences against pathogens

• Plants have evolved defences against pathogens which can either take the form of physical or chemical defences
• Physical barriers of pathogen entry include cellulose cell walls, a lignin layer which thickens the cell walls and waxy cuticles. Old vascular tissue is also blocked to prevent the spread of pathogens inside the plant
• In a case where a pathogen is detected, various mechanisms are activated to prevent the spread of infection through the plant
• To protect against pathogens plants can include closing of the stomata to prevent entry to leaves, additional thickening of cell walls with cellulose, callose deposits between cell wall and cell membrane near the site of infection to strengthen the cell wall.
• Other mechanisms include necrosis which is when cells near the site of infection are killed with the help of intracellular enzymes which are activated by injury, that is in a case where cell damage occurs
• Another variation of necrosis is known as canker and this is the necrosis of woody tissue in the main stem or branch
• Chemical defences include menthols produced by mint which are an example of terpenoids- these are essential oils with antibacterial properties
• Other examples of chemical defences include phenols such as tannin which interfere with digestion thus inhibiting insects from attacking the plant, alkaloids such as caffeine and morphine which have a bitter flavour- preventing herbivores from feeding on the plant.
• Defensins are cysteine-rich proteins involved in inhibition of transport channels whereas hydrolytic enzymes such as chitinases are released with the purpose of breaking down the cell wall of an invading organism.