Isochorismate synthase is required to synthesize salicylic acid for plant defence

Salicylic acid (SA) mediates plant defences against pathogens, accumulating in both infected and distal leaves in response to pathogen attack(1-5). Pa thogenesis-related gene expression and the synthesis of defensive compounds associated with both local and systemic acquired resistance (LAR and SAR) in plants require SA. In Arabidopsis, exogenous application of SA suffices to establish SAR, resulting in enhanced resistance to a variety of pathogen s. However, despite its importance in plant defence against pathogens, SA b iosynthesis is not well defined. Previous work has suggested that plants sy nthesize SA from phenylalanine(6-10); however, SA could still be produced w hen this pathway was inhibited(6,8), and the specirc activity of radiolabel led SA in feeding experiments was often lower than expected(7,8). Some bact eria such as Pseudomonas aeruginosa synthesize SA using isochorismate synth ase (ICS) and pyruvate lyase(11). Here we show, by cloning and characterizi ng an Arabidopsis defence-related gene (SID2) defined by mutation, that SA is synthesized from chorismate by means of ICS, and that SA made by this pa thway is required for LAR and SAR responses.