Biochemical immunity

The chemical defences include:

1. Receptors that recognise the pathogen and switch on defences. Healthy plants already contain recognition compounds; after infection their concentration increases. Their job is to detect the presence of a pathogen and stimulate the plant's immune system.
2. Antimicrobial chemicals. Some plants secrete compounds that fight pathogens. These may already be present before infection or form only because of infection. Examples:
   • Phenols and glycosides – toxic chemicals that kill microbes such as bacteria or inhibit their growth.
   • Non-protein amino acids – amino acids which do not take part in the plant's own proteins but are poisonous to pathogens, for example canavanine (and cephalosporin, as named in the book).
3. Antimicrobial proteins. Some plants make proteins that were absent before infection. These proteins react with the toxins released by the pathogen and convert them into compounds that are harmless to the plant. In other cases the plant makes detoxifying enzymes that react with the pathogen's toxins and cancel their poisonous effect.
4. Inducible post-infection defences. After one infection, some plants strengthen and "prime" their defences so they are better protected against any new infection.