Successful systemic infection of a plant by Tobacco mosaic virus (TMV) requires three processes that repeat over time: initial establishment and accumulation in invaded cells, intercellular movement, and systemic transport. Accumulation and intercellular movement of TMV necessarily involves intracellular transport by complexes containing virus and host proteins and virus RNA during a dynamic process that can be visualized. Multiple membranes appear to assist TMV accumulation, while membranes, microfilaments and microtubules appear to assist TMV movement. Here we review cell biological studies that describe TMV-membrane, -cytoskeleton, and -other host protein interactions which influence virus accumulation and movement in leaves and callus tissue. The importance of understanding the developmental phase of the infection in relationship to the observed virus-membrane or -host protein interaction is emphasized. Utilizing the latest observations of TMV-membrane and -host protein interactions within our evolving understanding of the infection ontogeny, a model for TMV accumulation and intracellular spread in a cell biological context is provided.

The movement protein VP37 of broad bean wilt virus 2 (BBWV 2) forms tubules in the plasmodesmata (PD) for the transport of virions between cells. This paper reports a mutual association between the BBWV 2 VP37-tubule complex and PD at the cytological level as determined by transmission electron microscopy. The generation of VP37-tubules within different PD leads to a different occurrence frequency as well as different morphology lines of virus-like particles. In addition, the frequency of VP37-tubules was different between PD found at different cellular interfaces, as well as between single-lined PD and branched PD. VP37-tubule generation also induced structural alterations of PD as well as modifications to the cell wall (CW) in the vicinity of the PD. A structural comparison using three-dimensional (3D) electron tomography (ET), determined that desmotubule structures found in the center of normal PD were absent in PD containing VP37-tubules. Using gold labeling, modification of the CW by callose deposition and cellulose reduction was observable on PD containing VP37-tubule. These cytological observations provide evidence of a mutual association of MP-derived tubules and PD in a natural host, improving our fundamental understanding of interactions between viral MP and PD that result in intercellular movement of virus particles.

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