Plants cannot flee from attackers, such as plant-eating animals or pathogens. Nevertheless, the world around us is green. What defense mechanisms did plants evolve to combat these attacks?

The tough layers of the epidermis and other structural defenses, such as hairs and thorns, deter plant-eating animals—also called herbivores.

When pathogens, such as bacteria, overcome structural defenses, plants resort to another line of defense. Membrane-bound receptors recognize molecular patterns that signal the presence of a pathogen. 

For instance, plants recognize the protein flagellin, which is specific to bacteria. The recognition of flagellin triggers a signaling cascade in plant cells, leading to diverse immune responses, such as the closure of stomata, the production of antimicrobial chemicals, and the strengthening of the cell wall.

Some pathogens evolved the ability to overcome these general plant defense mechanisms by injecting effector molecules that perturb the immune response.

As a countermeasure, some plant species evolved resistance proteins that detect the effector molecules in a highly specific manner. Resistance proteins induce a signaling cascade that often results in the expression of pathogenesis-related genes.

The detection of pathogen effectors or tissue-damage signals may also induce rapid programmed cell death, also called the hypersensitive response, near the area of infection. 

The hypersensitive response rapidly limits the spread of the pathogen by initiating structural changes of the cell wall and inducing systemic acquired resistance. Systemic acquired resistance is a long-term defense against pathogens in parts of the plant that are distant from the site of infection.

Herbivore attack often induces chemical plant defenses that disturb the digestive system or negatively affect the skin of the herbivore.

As an indirect defense strategy, plants may release volatiles that attract a parasite, which ultimately kills the herbivore.