Signaling in plants by intercellular RNA and protein movement

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Abstract
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 inter- cellular signaling. As an example, plant cells are sepa- rated from each other by a substantial extracellular ma- trix, the cell wall. Although the cell wall is not imper- vious, it is likely to hinder cell-to-cell communication. Moreover, because plant cells cannot migrate during de- velopment, location, instead of lineage, is the primary determinant of cell fate in plants, making communica- tion 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 organ- isms, in that whole tissues are symplastically connected. The channels that connect plant cells are called plasmo- desmata (PD), and their investigation offers tantalizing clues as to how plant cells may use them to communi- cate with each other and to coordinate development. Al- though once seen as constricted channels through which only molecules <1 kD in size could pass (Terry and Ro- bards 1987; Burnell 1988), PD have recently been shown to be far more dynamic and allow passage of both pro- teins and nucleic acids. Other findings, such as the ob- servation 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. Be- cause of the importance of PD, we first summarize what we know about them, and then go on to discuss inter- cellular movement of proteins and RNA in general.