Pseudomonas syringae pv. syringae is recognized by the non-host plant Oryza sativa which reacts with defense responses leading to induced resistance towards the rice blast fungus Pyricularia oryzae. We have previously shown that syringolin, a novel and unusual peptide secreted by P. syringae pv. syringae under appropriate conditions, is one of the molecular determinants by which rice plants can perceive the non-host pathogen. Here we show that syringolin is a member of a family of related compounds. We have isolated syringolin variants B to F and determined their structure using mass and NMR spectroscopic methods. These variants, which differ from each other by various modifications, are all able to induce defense responses upon application onto rice leaves.