Phytoplasmas are insect-transmitted Prokariotes associated with several plant diseases. They are responsible for significant economic losses in several crops and are mainly spread through infected plant material and by insect vectors. The latter, during their feeding activity, acquire the pathogen from infected plants and then transmit it, after a latency period, to new healthy plants. This process has been considered as having a passive nature for a long time but a growing body of evidences suggests that Phytoplasmas are able to actively modulate this process. Indeed, they interact with plant metabolism and negatively affect defense response against insects. This thesis presents work done to identify the nature of this interaction and especially the component explained by the effector SAP11, a protein molecule secreted into plant by Phytoplasmas during pathogenic process and involved in plant-insect interaction. Previous studies have shown that SAP11 is able to manipulate plant metabolism, causing various developmental changes and contributing to show typical symptoms associated with the presence of Phytoplasmas. Additionally, SAP11 reduces plant defense response to insect vectors, allowing them to lay more eggs and to produce more nymphs. In this paper, the role of this effector in the Phytoplasma pathogenic process is analyzed, allowing to elucidate several fundamental points of this complex plant-pathogen-insect interaction.