Systemic acquired resistance (SAR) is a "whole-plant" resistance response that occurs following an earlier localized exposure to a pathogen. SAR is analogous to the innate immune system found in animals, and although there are many shared aspects between the two systems, it is thought to be a result of convergent evolution. Plants use pattern-recognition receptors to recognize conserved microbial signatures. This recognition triggers an immune response. Plants also carry immune receptors that recognize highly variable pathogen effectors, these include the NBS-LRR class of proteins. SAR is associated with the induction of a wide range of genes and the activation of SAR requires the accumulation of endogenous salicylic acid (SA). The pathogen-induced SA signal activates a molecular signal transduction pathway that is identified by a gene called NIM1, NPR1 or SAI1 .One reaction of plants to pathogen infection is the induction of a long-lasting, broad-spectrum, systemic resistance to subsequent infections. This induced disease-resistance response has been known for many years under different names such as physiological acquired immunity or induced resistance; for the purpose of this update we will refer to it as SAR. Our perspective is that SAR is one component of plants' integrated disease-resistance repertoire. SAR appearsto be distinct from preexisting resistance mechanisms such as physical bamers or protein cross-linking and also from other inducible resistance mechanisms such as phytoalexin biosynthesis, the hypersensitive response, and ethylene-induced physiological changes. Furthermore, SAR is not related to responses induced by wounding or osmotic stress.