RNA Silencing and the Mobile Silencing Signal

INTRODUCTION

RNA silencing is a sequence-specific RNA degradation mechanism that occurs in a broad range of eukaryotic organisms including fungi (quelling), animals (RNA interference [RNAi]), and plants (post-transcriptional gene silencing). In all these organisms, the process is triggered by double-stranded RNA (dsRNA) and requires a conserved set of gene products (for recent reviews of RNA silencing in plants, see Matzke et al., 2001; Vance and Vaucheret, 2001; Voinnet, 2001; Waterhouse et al., 2001; Baulcombe, 2002; in fungi or animals, see Cogoni and Macino, 2000; Bernstein et al., 2001a; Carthew, 2001; Zamore, 2001). The mechanism for RNA silencing involves an initial processing of the inducing dsRNA into small interfering RNAs (siRNAs) of 21 to 25 nucleotides, corresponding to both sense and antisense strands of the target gene (Hamilton and Baulcombe, 1999). These siRNAs become associated with a protein complex referred to as the RNA-induced silencing complex (RISC), where they serve as guides to select the target RNAs and effect their degradation (Hammond et al., 2000; Zamore et al., 2000). In plants, RNA silencing is typically correlated with methylation within the transcribed regions of the transgene that correspond to target RNA (reviewed in Wassenegger, 2000; Bender, 2001). Methylation of genomic DNA occurs even when the silencing is induced by an RNA virus that replicates exclusively in the cytoplasm (Jones et al., 1998), suggesting communication between the cytoplasm and the nucleus. A good deal of evidence suggests that RNA silencing plays a natural role in defense against foreign nucleic acids, including virus resistance in plants (Covey et al., 1997; Ratcliff et al., 1997, 1999; Mourrain et al., 2000; reviewed in Voinnet, 2001) and in control of transposons in a number of other organisms (Ketting et al., 1999; Tabara et al., 1999; Grishok et al., 2000; Djikeng et al., 2001; Elbashir et al., 2001b; Takeda et al., 2001).

 Consistent with the antiviral nature of RNA silencing in plants, many plant viruses have evolved proteins that suppress RNA silencing (reviewed in Li and Ding, 2001). One of the most intriguing aspects of RNA silencing is that it is non-cell-autonomous: in both plants and Caenorhabditis elegans it can be induced locally and then spread to distant sites throughout the organism (Palauqui et al., 1997; Voinnet and Baulcombe, 1997; Fire et al., 1998; Voinnet et al., 1998; Winston et al., 2002). The systemic spread of silencing reflects the existence of an as yet unidentified mobile silencing signal as an integral component of the RNA silencing pathway. In C. elegans , a protein with multiple transmembrane domains has been reported to be required specifically for systemic silencing (Winston et al., 2002). Given the putative membrane localization of this protein, it could act as a receptor or transmembrane channel for the mobile signal. Whereas no mutations specific to systemic silencing have yet been reported in plant systems, certain plant viral proteins interfere with RNA silencing at this step (Voinnet et al., 2000; Guo and Ding, 2002). The mechanisms involved in systemic RNA silencing in plant systems are being actively investigated using grafting and transient expression approaches (Figure 1) in conjunction with a variety of plant viral suppressors of silencing that act at different steps in the silencing pathway. This review focuses on advances in understanding the nature of systemic silencing in plants and the signal(s) that induces silencing at distant sites.