Résumé
The best known vectors are insects in the order Homoptera, especially aphids. Based on thorough analysis of the morphology of the homopteran feeding apparatus, it is concluded that virus located past the OS in the intima-lined pharynx of the foregut can no longer be egested and is, therefore, noninoculable. Two subcategories of noncirculative viruses, nonpersistent and semipersistent, are usually differentiated on the basis of the time required for virus acquisition and inoculation, as well as the length of time for which the vector can retain virus in an infectious form. However, in some cases variable results have been obtained for semipersistent viruses and, in particular, for cauliflower mosaic virus (CaMV). There is no conclusive evidence of significant differences between the molecular mechanism of virus–vector interaction in nonpersistent and semipersistent transmission. Recent data has been on the molecular mechanisms of virus–vector interactions as the main criteria for defining two distinct viral strategies, the capsid strategy and the helper strategy. In the capsid strategy, the virus interacts directly with the vector via its coat protein, whereas in the helper strategy the virus–vector interaction is mediated by an additional virus-encoded nonstructural protein, generally designated as helper. The helper strategy is frequently found among noncirculative viruses in the genera Potyvirus, Caulimovirus, Waikavirus, Sequivirus, and presumably Closterovirus and even the nematode-transmitted Tobravirus. The molecular mechanisms of virus–vector interaction have been elucidated by researching the mode of action of the helper. This chapter illustrates that the helpers in different virus groups may not all function similarly by comparing the aphid transmission of caulimoviruses with that of potyviruses.