Plant viruses and the vectors, especially hemipterans like aphids, that they use for plant-to-plant transmission, cause tremendous damage in agriculture. Conventional control strategies rely on insecticides to eliminate the aphids, but appearance of resistances, ecological issues and new legislation demand alternative control strategies that are more respectful of the environment and compatible with sustainable agricultural practices. Evidence emerges that viruses manipulate plant traits, vector behavior and vector performance to optimize their transmission. Thus, research targeting plant viruses, vectors and hosts, and exploring the tritrophic interactions between them may show ways to more sustainable pest control strategies. This is the objective of our project. We want to identify the molecular mechanisms driving these interactions on all three levels, with an emphasis on their role in transmission. For this, we will study two viruses sharing the same hosts and aphid vectors, and representing the two most common transmission modes used by plant viruses, non-circulative (NC) and circulative (C) transmission. NC viruses are transmitted by binding to and being released from a specific site on the external mouthparts of vectors, whereas C viruses must traverse the intestine of their aphid vectors, cycle through the hemocoel and invade the salivary glands, before they can be inoculated with saliva when aphids feed on a new host. In a comparative study, we want to elaborate the similarities and differences between NC and C transmission and to determine whether they can be host-specific. For this, we use NC Cauliflower mosaic virus (CaMV) and C Turnip mosaic virus (TuYV) as viruses, the model plant Arabidopsis thaliana and the biofuel plant Camelina sativa as hosts for the two viruses, and the economically important pest, green peach aphid (Myzus persicae L.), as a vector transmitting both viruses. Our first objective is to confirm, by aphid performance analysis (individual growth rate, fecundity, starvation survival, arrestment, behavior), previous studies showing that these viruses do modify aphid behavior and traits in a way favorable for transmission, and analyze these traits in detail. Then we will identify and characterize virus-induced sRNAs and viral proteins involved in aphid modifications. This will be achieved by analyzing aphid performance on plants expressing the candidates stably or transiently. It is then our second objective to create, by RNA profiling, a comprehensive catalog of aphid and plant genes whose transcription is altered by viral infection and/or aphid infestation. Our third objective is to validate and characterize, by functional analysis, the host and aphid genes and pathways identified by RNA profiling and that are primarily involved in modifying plant and aphid traits in a way conducive for transmission. Taken together, this project will show which host and vector traits are modified by the viral infection for a better transmission, which viral factors provoke these changes, and which host and vector genes and pathways are targeted by the two viruses.