Viroid–insect–plant interactions

Viroids are small, non-protein-encoding, single-stranded, circular RNA molecules that can cause several diseases in economically important crops, including ornamentals. Their swift and easy spread via mechanical transmission (contaminated pruning gear, vegetative propagation) has been intensively studied for decades. However, the role of insect-mediated transmission remains unclear. Initial studies delivered doubtful results, and more recent research is largely characterized by contradictory and/or incomparable outcomes. While several studies point to a vectoring role of aphids and bumblebees, others believe that this is not the case. Parallels with the transmission of plant viruses can certainly be drawn, but one must not forget the unorthodox molecular features of viroids, which can lead to potentially complex interactions with viruses (e.g. through transencapsidation and complementation), insect vectors and plants. Molecular pathways of contact, entry and dispersal within the insect vector and the host also need to be investigated. Additionally, recent European surveys reported the latent presence of many viroids in different Solanaceae ornamentals. This observation raises new questions about their epidemiology and causes anxiety among growers.
This chapter examines plant-infecting viroids that, similar to plant viruses, can depend upon insect vectors to inoculate plant hosts. For more information about the association of plant-infecting viruses with their hosts we refer to reviews from Gray & Banerjee (1999) Microbiol Mol Biol Rev 63(1):128, Power (2000) Curr Opin Plant Biol 3(4):336–340, and Fereres & Moreno (2009) Virus Res 141(2):158–168. Here we introduce the prominent aspects of viroid epidemiology in Europe, as well as the transmission pathways of viroids, with an emphasis on insect vectors. Finally, we suggest possible avenues deserving exploration in future viroid research.