Viral infection has long been a powerful experimental system for fundamental discoveries in biology, ranging from molecular mechanisms of gene expression at the cellular level to cell-to-cell communication at the organismal level. In early molecular studies on plant viral infection, the central focus was on the identification of viral factors responsible for replication, cell-to-cell and long-distance movement, elicitation of plant defense responses, and induction of symptoms. Findings from these studies laid the foundation for subsequent investigations of the host factors involved in these processes. The research discoveries themselves and the cutting-edge multidisciplinary research tools that were developed have firmly established the cell biology of plant-virus interactions as a vibrant field of current biological research. They have significantly advanced and will continue to have impact on investigations of basic biological principles far beyond virology. This Focus Issue showcases these exciting developments with five invited reviews and nine research papers covering replication, movement, resistance, and inter-viral interaction mechanisms. It is our special privilege and honor to introduce these articles. --Biao Ding and John Carr, Senior Editors
