Résumé
Most plant viruses are transmitted by living vectors that transport viruses to a new host plant. One discriminates between circulative transmission, where viruses must pass through the vector interior and arc usually inoculated with the saliva on a healthy plant, and non-circulative transmission, where viruses do not need to pass through the vector interior but are directly inoculated from the mouth parts into a new host. Especially transmission of non-circulative viruses has been regarded as a simple process where a vector more or less accidentally transports the virus. However, it becomes more and more evident that this scenario is unlikely, because transmission constitutes a dramatic bottleneck of the virus life cycle, where only very few viral genomes pass to a new host, and where a given virus must do everything to ensure successful transmission. We will show and discuss in this chapter that viruses -also in non-circulative transmission- deliberately manipulate their hosts and vectors in often very unexpected ways to optimize their transmission.