Many aphid species are restricted to one or few host plants, while some aphids, many of which are of agricultural importance, can infest a wide range of plant species. An important observation is that aphids spend a considerable time on nonhost species, where they probe the leaf tissue and secrete saliva, but for unknown reasons are unable to ingest phloem sap. This suggest that aphids, like plant pathogens, interact with nonhost plants at the molecular level, but potentially are not successful in suppressing plant defenses and/or releasing nutrients. To date, however, the plant cellular changes and the involvement of immune response, such as ETI and PTI, in aphid-host and -nonhost interactions remain elusive. The aim of the proposed project is to gain insight into the level of cellular host reprogramming that takes place during aphid-host interactions, the cellular processes involved in aphid nonhost resistance, and the role of aphid effectors in determining host range. We will compare interactions of two economically important aphid species, Myzus persicae (green peach aphid) and Rhopalosiphum padi (bird cherry oat aphid), with host and nonhost plants. We will investigate local changes in plant cellular processes during aphid-host and -nonhost interactions using microscopy and biochemistry approaches. We will apply a comparative transcriptomics approach and functional assays to identify aphid effectors as potential determinants of host range. Herein we will specifically looks for aphids-species specific effectors and those that are expressed in specific host interactions. To gain insight into molecular mechanisms of effector activities we will identify host targets and investigate the contribution of effector-target interactions to host range. The expected outcomes of the project will, in the long term, contribute to the development of novel strategies to control infestations by aphids and potentially other pests and pathogens, thereby improving food security.