Signaling in plants by intercellular RNA and protein movement

Plants and animals have had about 1.6 billion years—the time that has passed since they diverged from their last unicellular ancestor—to evolve different mechanisms for solving unique problems of development and intercellular signaling. As an example, plant cells are separated from each other by a substantial extracellular matrix, the cell wall. Although the cell wall is not impervious, it is likely to hinder cell-to-cell communication. Moreover, because plant cells cannot migrate during development, location, instead of lineage, is the primary determinant of cell fate in plants, making communication over both long and short distances essential for the coordination of plant growth. It appears that plants have significantly overcome the downside of having a cell wall by forming channels between cells to allow the transit of signaling and other important molecules. Plants may even be considered as supracellular organisms, in that whole tissues are symplastically connected. The channels that connect plant cells are called plasmodesmata (PD), and their investigation offers tantalizing clues as to how plant cells may use them to communicate with each other and to coordinate development. Although once seen as constricted channels through which only molecules <1 kD in size could pass (Terry and Robards 1987; Burnell 1988), PD have recently been shown to be far more dynamic and allow passage of both proteins and nucleic acids. Other findings, such as the observation that transcription factors can move between cells, and that RNA movement may be behind a range of long-distance signals in plants, raise further questions regarding a regulatory role of PD in development. Because of the importance of PD, we first summarize what we know about them, and then go on to discuss intercellular movement of proteins and RNA in general.