Systemic acquired resistance (SAR) is a form of induced resistance that is activated throughout a plant after being exposed to elicitors from virulent, avirulent, or nonpathogenic microbes, or artificial chemical stimuli such as chitosan or salicylic acid (SA). Induction of SAR is characterized by the accumulation of salicylic acid to stimulate defense mechanisms, often resulting in a localized hypersensitive response. SA is not a mobile signal agent, but is proposed to be the first chemical in the induction of pathogenesis-related (PR) genes, especially PR-1 . PR genes code for proteins such as chitinases and β-1,3-glucanases among others, and play a role in either preventing or slowing colonization of pathogens in the host.
Induction of SAR has increased disease resistance to a number of pathogens and pests, including Ascochyta fabae and Uromyces viciae-fabae on faba beans and Meloidogyne incognita and Ralstonia solanacearum on tomatoes. It has been suggested that SAR is most effective against biotrophic and hemibiotrophic pathogens and not against necrotrophic pathogens since it stimulates the SA pathway. Repeated inoculations can increase the level of resistance, but it can take several days for SAR to develop throughout the host plant.