First and third authors: Station de Pathologie Comparée, Unité Mixte de Recherche 5087 INRA/CNRS/Université Montpellier II, 30380 Saint-Christol-lez-Alès France; and second author: Centre d'étude; sur le Polymorphisme des Micro-organismes, UMR 9926, IRD, 911 avenue d'Agropolis, 34032 Montpellier Cedex 01 France
Plant viruses are most frequently transmitted from one host plant to another by vectors. In noncirculative vector transmission, the virus does not process through a cycle within the vector body. Instead, upon acquisition by the vector, viruses are retained in the mouth parts or the anterior gut; from there, they will be subsequently released in a new host plant. Two molecular strategies have been described for the virus—vector interaction. In the capsid strategy, the virus coat interacts directly with binding sites in the vector mouth parts, whereas an additional nonstructural protein, designated helper component (HC), is required in the helper strategy. The HC and virus particles can be acquired sequentially, and this property introduces the possibility that an HC acquired first by the vector assists the transmission of virus particles located in the same cell, in other cells, or even in other host plants probed by the vector. Such a phenomenon is here called HC-transcomplementation. Surprisingly, the existing definition of HC does not explicitly include the concept of HC-transcomplementation, and it is generally omitted in the literature in any consideration of the virus biology other than the molecular interaction with the vector. Here we propose an extended definition of HC and emphasize the concept of HC-transcomplementation that distinguishes the helper strategy from any other type of vector transmission and may have consequences at the level of the virus population genetics and evolution.