When a plant leaf is attacked by certain bacterial pathogens, leaves located far away from the initial strike are primed to activate a defense response. Exactly how the warning signal is relayed through plant sap and what that warning signal is has kept plant scientists guessing. Now, research by Jean T. Greenberg of the University of Chicago and colleagues is pointing to a C9 dicarboxylic acid called azelaic acid as the defense messenger (Science 2009, 324, 89). The researchers found that a plant bacterial infection leads to the production of azelaic acid, which then travels through the plant vasculature and primes cells to produce infection-fighting salicylic acid when the next attack comes. The team carried out its work using the model plant Arabidopsis and the agricultural pathogen Pseudomonas syringae, which gains entry into plants by enabling the formation of ice on a leaf's surface above freezing temperature, causing plant tissue to burst open and allowing the bacteria inside. The new knowledge about azelaic acid could be useful for developing plant protection strategies and provides additional insight about systemic plant immunity, the researchers note.