Rhizobacteria can stimulate pathogen resistance in various plant species. Many plant-associated bacteria use N-acylhomoserine lactones (AHL) for intra- and intergeneric communication to recognize environmental conditions in a cell-density dependent manner ('quorum sensing'). In our experiments tomato leaves developed systemic resistance to the fungal pathogen Alternaria alternata following root inoculation with Serratia liquefaciens. The biocontrol activity of S. liquefaciens depended on AHL production. Root-applied AHL affected free salicylic acid contents in roots and defence gene expression in leaves. S. liquefaciens inoculated plants also exhibited elevated salicylic acid levels and enhanced ethylene emission in leaves. It is concluded that salicylic acid and ethylene signaling pathways contribute to AHL -induced systemic resistance because of enhanced expression of both salicylic acid and ethylene dependent genes. Consequently, AHL molecules can be considered new general elicitors of plant defence reactions against pathogens.
«