Systemic Acquired Resistance

Systemic acquired resistance (SAR) is a form of induced resistance that is activated by pathogens that induce localized necrotic disease lesions or a hypersensitive response. A major characteristic of SAR is the broad spectrum nature of the protection it confers against a wide range of pathogens, although recent studies suggest that the resistance is most effective against biotrophic and hemibiotrophic pathogens and less effective against necrotrophs. SAR is dependent on salicylic acid signaling and is typically associated with systemic expression of pathogenesis-related protein genes and other putative defenses. Once induced, SAR-expressing plants are primed to respond to subsequent pathogen infection by induction of defenses that are localized at the site of attempted pathogen ingress. Finally, SAR typically does not provide full resistance to disease indicating that the practical application of this form of resistance will require the use of other disease management tools. On the basis of these types of observations, it is likely that SAR and other forms of induced resistance are based on the enhanced ability to express basal defenses.