Technical Abstract: Systemic acquired resistance (SAR) immunizes distal tissues of plants against secondary infections. We report that an acyl carrier protein, ACP4, is essential for perception but not generation of the SAR signal in Arabidopsis. A mutation in acp4 reduces the fatty acid flux, resulting in impaired cuticle biogenesis and compromised SAR. Genetic mutations or physical treatment of systemic tissues that damage the cuticle impair SAR, suggesting that an intact cuticle is required for decoding the mobile SAR signal generated upon primary infection. The acp4 mutation also impairs the plastidal acyltransferase catalyzed reaction, and thereby suppresses constitutive defense signaling in ssi2 plants.