Every plant cell can defend itself from attacking pathogenic microorganisms and invertebrates.
Some defenses, such as antimicrobial secondary metabolites, are constitutive, being located in specific cellular compartments and ready to be released on cell damage. Other defense responses, such as those induced by pathogen invasion, require detection of the pathogen by the plant. Defense activation is correlated with rapid activation of defense-related genes and often culminates in the HR, localized cell death, to impair pathogen spread. Avr proteins exhibit extensive sequence diversity, and their function in the pathogen is poorly understood. R proteins both detect pathogens and initiate signal transduction to activate defense mechanisms.