Plants possess inducible systemic defense responses when locally infected by pathogens. Bacterial infection results in the increased accumulation of the mobile metabolite azelaic acid, a nine-carbon dicarboxylic acid, in the vascular sap of Arabidopsis that confers local and systemic resistance against the pathogen Pseudomonas syringae. Azelaic acid primes plants to accumulate salicylic acid (SA), a known defense signal, upon infection. Mutation of the AZELAIC ACID INDUCED 1 (AZI1) gene, which is induced by azelaic acid, results in the specific loss of systemic immunity triggered by pathogen or azelaic acid and of the priming of SA induction in plants. Furthermore, the predicted secreted protein AZI1 is also important for generating vascular sap that confers disease resistance. Thus, azelaic acid and AZI1 are components of plant systemic immunity involved in priming defenses.