Coronatine is a chlorosis-inducing toxin produced by the plant pathogen Pseudomonas syringae pv atropurpurea. This bacterium is the causal agent of chocolate spot disease, in which brown lesions with chlorotic margins develop on the leaves of Lolium multiflorum Lam. Among the many physiological changes to plants caused by coronatine is the stimulation of ethylene production from bean leaves. The ethyl-substituted side chain of coronatine is an analog of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC). We have examined the question of whether part or all of the released ethylene comes from the breakdown of coronatine itself. The rate of ethylene release from leaves of Nicotiana tabacum was proportional to the concentration of coronatine applied to the leaf surface. The lowest effective concentration of coronatine, applied to leaves at 15 pmol cm-2 of leaf cm-2 over a period of 4 h. The maximum rate of ethylene production occurred 28 to 32 h after application of coronatine. The specific activity of ethylene produced by discs cut from coronatine-treated Nicotiana tabacum leaves floating on a solution containing 10 mM [U-`4C]methionine was consistent with its exclusive origin from methionine. ACC accumulated in the coronatine-treated tissue. ACC synthase activity increased in Phaseolus aureus hypocotyls during a 6-h treatment with coronatine. Thus, coronatine induces the synthesis of ethylene from methionine.