Do plants have an immune system?
Plants may defend themselves in two ways. Constitutive characteristics are defined as defenses that are always present on the body of the organism. These include thorns, sharp leaves, bark and even leaf cuticle. Plants may also have induced defenses, which act as a sort of immune system. Especially against pathogens, plants may have four responses to combat damage.
- When a pathogen enters a cell, the plant may isolate that infected cell. Extensin and lignin, tough proteins, can be used to seal off gaps in the cell wall called plasmodesmada. The cell will die, but so will the harmful pathogen. All other cells will remain undamaged.
- Pathogen-specific responses occur when the organism is able to recognize the pathogen. Protein receptors are able to trigger local and systemic responses to the presence of infection, which allows the plant to fight off the pathogen.
- Hypersensitive responses activate phytoalexins. These are non-specific, which allow them to kill off a variety of bacteria and fungi, but plant cells are also killed in the process. This mechanism is called apoptosis. Infected and neighboring cells are killed, but the damage is isolated and other cells remain protected.
- Salicylic acid, a volatile substance, may be released by infected plants. This spreads to other organisms nearby and triggers an immune response in them. Although there is no pathogen in these plants, the hypersensitive response causes a systemic acquired resistance which allows them to be better prepared if they come into contact with that particular pathogen in the future.