Abstract : Dickeya dadantii is an enterobacterium causing soft rot diseases in a wide range of plant species. The maceration of plant tissues is essentially caused by the secretion of a set of plant cell wall degrading enzymes. This thesis focused on the identification of molecules involved in signalling processes during biological interactions. Various secondary metabolites and phytoconstituants are known to play a role in the interactions and communication between plants and microorganisms. In D. dadantii, we have characterized two genomic loci, lfa and lfg, coding for proteins that are expressed only during plant infection or in the presence of plant extracts. The expression of the genes lfaA and lfgN is modulated by repressors that act differently depending on the presence or absence of the inducer. These regulators, LfaR and LfgR, also control the expression of genes involved in the transport of the inducer in the bacterial cytoplasm. After achievement of a screening test to analyse the fractions, a search for the inducer was performed in extracts from chicory leaves and potato tubers. Several chromatographic fractionations allowed the recovery of different active fractions, including a highly active fraction from potato. This fraction contained two compounds which were further purified. The main compound was identified as a nucleoside, guanosine, but the pure molecule has no inducing capacity. The low amount of the second molecule was insufficient for the final identification of the inducer. The results of this work show the involvement and mechanism of action of small hydrophilic molecules acting as signals in the interactions between plants and D. dadantii.