Nowadays, efficient disease management encompasses diverse concepts, including the activation and reinforcement of the plant immune system. Therefore, biocontrol agents integrate priming of crop plants for stronger defense and systemic responses. To ensure maximum crop yield even upon pathogen pressure, it is fundamental to understand the mode of action of new biocontrol agents. Due to the ability to modulate plant-microbe interactions, bacterial quorum sensing molecules might be a good alternative for modern plant protection strategies. Quorum sensing (QS) refers to the communication system by which bacteria regulate (in a densitydependent manner) genes involved in diverse behaviors, such as biofilm formation, antibiotic resistance or virulence. In Gram-negative bacteria, the role of QS molecules is often played by N-acyl homoserine lactones (AHLs). In plants, AHLs can induce priming for a faster and more efficient immune response against pathogens. The objective of this thesis was to study the immune response and the physiological alteration upon AHL-priming in the model plant Arabidopsis and to translate those results into important crop plants, like barley, wheat, tomato and alfalfa. Based on previous data, I chose to work with the long-chain oxo-C14-HSL and the oxo-C14-HSL-producing rhizobacterium Sinorhizobium meliloti.We could demonstrate that oxo-C14-HSL primed Arabidopsis plant for a broadspectrum resistance based on a salicylic acid/oxylipin-dependent systemic signal. In addition, oxo-C14-HSL caused enhanced production of reactive oxygen species and transcriptional activation of defense-related genes in crop plants. The outcomes of the oxo-C14-HSL-induced priming were reinforcement of plant cell wall and stomata defense response, which helped to avoid pathogen entry and proliferation. Furthermore, oxo-C14-HSL could arrest the proliferation of the human pathogen Salmonella enterica serovar Thyphimurium in Arabidopsis plants even though this resistance effect might be limited when AHL-producing S. meliloti forms a symbiotic relationship with its native plant host Medicago sativa. On the other side, Salmonella inject effector proteins into the host cells in order to manipulate the immune system. We could show that the effector protein SpvC deactivates AtMPK6 and AtMPK3.Taking together, quorum-sensing molecules have a positive impact on plants and AHL-induced resistance could be a model for plant priming and open new strategies for crop plant protection.
