Cauliflower mosaic virus (CaMV) is transmitted by aphids. For acquisition by the vector, a transmissible complex must form, composed of the virus particle, the viral coat-associated protein P3 and the helper protein P2. However, the components of the transmissible complex are largely separated in infected plant cells: most P3 virions are confined in electron-dense inclusion bodies, whereas P2 is sequestered in electron-lucent inclusion bodies (elIBs). This spatial separation controls virus acquisition by favouring the binding of virus-free P2 to the vector first, rendering the vector competent for later uptake of P3 virions. Consequently, sequential acquisition of virus from different cells or tissues is possible, with important implications for the biology of CaMV transmission. CaMV strains Campbell and CM1841 contain a single amino acid mutation (G94R) in the helper protein P2, rendering them non-transmissible from plant to plant. However, the mutant P2-94 protein supports aphid transmission when expressed heterologously and supplied to P3–CaMV complexes in vitro. The non-transmissibility of P2-94 was re-examined in vivo and it is shown here that the non-transmissibility of this P2 mutant is not due to low accumulation levels in infected plants, as suggested previously, but more specifically to the failure to form elIBs within infected plant cells. This demonstrates that elIBs are complex viral structures specialized for aphid transmission and suggests that viral inclusion bodies other than viral factories, most often considered as ‘garbage cans’, can in fact exhibit specific functions.
In vitro Biological Activity Associated with the Aphid Transmission Factor of Cauliflower Mosaic Virus