Plants, in general, appear to be able to detect the presence of incompatible Pseudomonas syringae strains by a hypothetical cell-cell recognition process to initiate inducible defense mechanisms that contribute to disease resistance. A 25-kb hrp/hrm gene cluster isolated from P. syringae pv. syringae 61(pHIR11) enables Escherichia coil to elicit a hypersensitive response (HR), a plant response generally considered to be a manifestation of recognition and resistance. To identify the nature of the HR-eliciting signal produced by E. coli cells carrying pHIR11, bacterial surface features were surveyed by immunological and biochemical procedures. No immunoreactive epitopes or outer membrane proteins were detected that were associated with expression of the P. syringae pv. syringae 61 hrplhrm cluster in E. coli MC4100. Phenotypic expression of the P. syringae pv. syringae 61 hrp/hrm cluster in E. coli MC4100, however, was found to be dependent upon ompC and ompF, which control outer membrane permeability to hydrophilic solutes. The results suggest that deployment of the HR-eliciting signal occurs via outer membrane porins and imply that a low-molecular-weight, hydrophilic factor mediates signal exchange between the bacterium and the' responding plant cell.