Systemic transport of RNA in plants.

vasculature was once thought to function as a mere conduit for nutrients and hormones. However, recent evidence indicates that this transport system can participate in the dissemination of various signal molecules throughout the plant. Interestingly, these signals include not only traditional signaling factors, such as proteins and growth-regulating small molecules, but also RNA. Once RNA was thought to be an information carrier, functioning as a passive template for protein synthesis. It is now also thought to act as an active signal molecule, regulating gene expression and development in plants. Various types of RNA molecules travel long distances from their site of synthesis to different parts of the plant. This systemic movement occurs through plant vasculature by as yet unknown mechanisms. Here, three major types of RNA systemic transport are summarized (Fig. 1): • Long distance movement of plant virus genomic RNA. • Systemic transport of RNA elicitors of post-transcriptional gene silencing (PTGS). • Long distance transport of specific en-dogenous RNA molecules. Following initial infection, usually by mechanical inoculation, plant RNA viruses spread from cell to cell through plasmodes-mata until they reach the vascular system; the virus is then transported systemically through the vasculature (reviewed in Ref. 1; Fig. 1). Virus-encoded nonstructural movement proteins (reviewed in Refs 2,3) mediate the cell-to-cell spread of infection. The best studied movement protein is the 30 kDa protein of the tobacco mosaic virus (TMV). A TMV movement protein binds viral RNA, forming an extended TMV movement protein–RNA transport complex 4 , which is targeted to plas-modesmata, potentially, through its interaction with cytoskeletal elements of the host cell 5,6. The movement protein then acts to increase the size exclusion limit of plasmodesmata 7,8 , allowing intercellular movement of TMV movement protein–RNA complexes. In addition to the TMV movement protein, these protein activities have also been demonstrated for movement proteins of many other plant viruses (reviewed in Refs 2,3). Although movement proteins alone are sufficient to transport viral RNA genomes cell to cell to the vicinity of the host plant vascular system, virus entry into the vasculature and its subsequent systemic spread require an additional function generally encoded by viral coat proteins (reviewed in Refs 2,3). The exact nature of coat protein activity in viral systemic movement is unknown. Potentially, a coat protein, in concert with a movement protein, might act to dilate plasmodesmata at the boundary between vascular and non-vascular tissues. Indeed, although the TMV movement …