Long distance signaling during systemic acquired resistance

Systemic acquired resistance (SAR) is an inducible defense response triggered in response to a localized infection that confers whole-plant immunity against secondary infections. SAR is thought to involve generation of mobile signal(s) at the site of infection that, upon translocation to the distal tissues, activate broad-spectrum disease resistance. Several plant metabolites that fit the criteria defined for mobile signals have been identified. The SAR conferred by these structurally unrelated chemicals, including azelaic acid (AA), methylated derivative of salicylic acid (SA), dehydroabietinal, pipecolic acid and glycerol-3-phosphate (G3P), is dependent on SA. Many of these chemical SAR inducers require the lipid transfer proteins, DIR1 and AZI1, for their function in SAR. Thus, signaling induced by these diverse chemicals is likely to converge into a common pathway. For example recent findings show that AA functions upstream of G3P and confers SAR by inducing G3P biosynthesis. This review summarizes recent findings related to these mobile chemical inducers of SAR.