Gram-negative bacteria continuously release membrane vesicles, pinching off portions of their outer membrane. These outer membrane vesicles (OMVs) are an important component of the bacterial lifestyle and are involved in multiple processes including cell-cell communication, biofilm formation, stress tolerance, horizontal gene transfer and virulence. OMVs are also known modulators of the mammalian immune response. Despite the well-documented role of OMVs in mammalian-bacterial communication, their interaction with plants is not well studied. To examine whether OMVs of plant pathogens modulate the plant immune response we purified OMVs from three different plant pathogens and used them to treat Arabidopsis thaliana. OMVs rapidly induced ROS burst, defense gene induction and medium alkalinization in A. thaliana leaf discs, seedlings and cell cultures, respectively. Western blot analysis revealed that EF-Tu is secreted in OMVs and that it serves as an elicitor of plant immunity in this form. Our results further indicate that other immune elicitors both proteinaceous and non-proteinaceous, are present in OMVs and are responsible for the activation of the plant immune response. Taken together, our results reveal a new facet of plant-bacterial interactions and demonstrate that plants can detect and respond to OMV-associated elicitors by activation of their immune response.
