A genetic screen for impaired systemic RNAi highlights the crucial role of DICER-LIKE 2

Post-transcriptional gene silencing (PTGS) of transgenes involves abundant 21 nt small interfering RNAs (siRNAs) and low abundance 22 nt siRNAs produced from double-stranded RNA (dsRNA) by DCL4 and DCL2, respectively. However, DCL2 facilitates the recruitment of RNA-DEPENDENT RNA POLYMERASE 6 (RDR6) to ARGONAUTE 1 (AGO1)-derived cleavage products, resulting in more efficient amplification of secondary and transitive dsRNA and siRNAs. Here, we describe a reporter system where RDR6-dependent PTGS is initiated by restricted expression of an inverted-repeat dsRNA specifically in the root tip, allowing a genetic screen to identify mutants impaired in RDR6-dependent systemic PTGS. Our screen identified dcl2 but not dcl4 mutants. Moreover, grafting experiments showed that DCL2, but not DCL4, is required in both the source rootstock and recipient shoot tissue for efficient RDR6-dependent systemic PTGS. Furthermore, dcl4 rootstocks produced more DCL2-dependent 22 nt siRNAs than wild type and showed enhanced systemic movement of PTGS to grafted shoots. Thus, along with its role in recruiting RDR6 for further amplification of PTGS, DCL2 is crucial for RDR6-dependent systemic PTGS.Christelle Taochy, Nial R. Gursanscky, Jiangling Cao, Stephen J. Fletcher, Uwe Dressel, Neena Mitter, Matthew R. Tucker, Anna M.G. Koltunow, John L. Bowman, Hervé Vaucheret and Bernard J. Carrol

Small RNA-based antimicrobial immunity

Protection against microbial infection in eukaryotes is provided by diverse cellular and molecular mechanisms. Here, we present a comparative view of the antiviral activity of virus-derived small interfering RNAs in fungi, plants, invertebrates and mammals, detailing the mechanisms for their production, amplification and activity. We also highlight the recent discovery of viral PIWI-interacting RNAs in animals and a new role for mobile host and pathogen small RNAs in plant defence against eukaryotic pathogens. In turn, viruses that infect plants, insects and mammals, as well as eukaryotic pathogens of plants, have evolved specific virulence proteins that suppress RNA interference (RNAi). Together, these advances suggest that an antimicrobial function of the RNAi pathway is conserved across eukaryotic kingdoms