The mystery remains: How do potyviruses move within and between cells?

The genus Potyvirus is considered as the largest among plant single-stranded (positive-sense) RNA viruses, causing considerable economic damage to vegetable and fruit crops worldwide. Through the coordinated action of four viral proteins and a few identified host factors, potyviruses exploit the endomembrane system of infected cells for their replication and for their intra- and intercellular movement to and through plasmodesmata (PDs). Although a significant amount of data concerning potyvirus movement has been published, no synthetic review compiling and integrating all information relevant to our current understanding of potyvirus transport is available. In this review, we highlight the complexity of potyvirus movement pathways and present three potential nonexclusive mechanisms based on (1) the use of the host endomembrane system to produce membranous replication vesicles that are targeted to PDs and move from cell to cell, (2) the movement of extracellular viral vesicles in the apoplasm, and (3) the transport of virion particles or ribonucleoprotein complexes through PDs. We also present and discuss experimental data supporting these different models as well as the aspects that still remain mostly speculative.