Electron microscopy studies on luteovirid transmission by aphids

Transmission electron microscopy (TEM) observations have been extensively applied to follow the route of luteovirids in their vectors. Luteovirids are icosahedral plant viruses which are phloem-limited and strictly transmitted in a circulative manner by aphids. Virus particles, acquired by aphids while feeding on an infected plant, circulate in the aphid's body without replication and are internalized during this process in two different cell types (intestinal and accessory salivary gland cells). The endocytosis mechanism at the gut level seems to rely on a clathrin-mediated entry process and virions are observed in the aphid's gut cells in various vesicular structures. After exocytosis from intestinal cells, virions are released in the aphid's body cavity where they are thought to bind to symbionin, an endosymbiotic protein. Transcytosis of the accessory salivary gland cells occurs similarly as at the gut level but in the reverse direction. Using engineered virus mutants, viral proteins required for transmission and involved in virus retention in the hemocoel have been identified. Virus mutants poorly or non aphid-transmitted have also been localized in the aphid's body by TEM. These observations reveal the crucial role of the minor capsid protein in gut internalization. While not strictly required, this protein seems to play an important role in the efficiency of this process by interacting with putative virus receptors localized on the gut apical membrane. More recently, some aphid proteins have also been shown to exhibit in vitro virus binding capacity and could potentially be components of the endocytotic apparatus.