Systemic Acquired Resistance.

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 appears to 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. In this update we will provide a brief history of SAR research and review recent results important to our understanding of this response. We will also present a working model of the critica1 steps that lead to the induction and maintenance of the resistant state and point out steps in the response that need further experimentation to extend our understanding of SAR.