reportRNA silencing in plants : Flash report !

Earlier this year plant scientists met in Santa Fe, New Mexico at the Keystone Symposium "RNA Silencing Mechanisms in Plants". Sessions included small RNA biogenesis and signalling, development and stress responses, small RNAdirected DNA methylation, and interaction with pathogens. This report highlights some of the prominent and recurring themes at the meeting and emerging arenas of future research. Introduction Many of the seminal discoveries in RNA silencing were made in plant systems and, due in large part to excellent genetics, plant science continues to contribute greatly to our understanding of small RNA biology. At the Keystone Symposium "RNA Silencing Mechanisms in Plants" in Santa Fe, New Mexico, over 150 plant scientists met to share their latest research. Many of the topics discussed will undoubtedly increase understanding of RNA silencing in diverse systems. The great migration Movement of small RNAs over both long and short distances was a major topic at the meeting. The systemic aspect of RNA silencing is well documented [1,2], yet the nature of the systemic signal and the biological significance of this movement are unknown. Marja Timmermans (Cold Spring Harbor Laboratory, USA) presented evidence that diffusion of tasi-ARFs from their point of synthesis at the adaxial leaf surface generates a gradient of ARF3 repression and restricts ARF3 activity to the abaxial domain[3]. The tasi-ARF gradient combined with abaxial ARF3 expression establishes a discreet domain of ARF3 accumulation, helping define leaf polarity. To investigate whether cell-to-cell mobility is a general feature of small RNAs, Olivier Voinnet (Institut de Biologie Moleculaire des Plantes, France) bombarded GFP siRNAs into Arabidopsis leaves that constitutively expressed GFP and detected patches of silencing spreading from the bombarded cells [4]. To rule out movement of silencing intermediates generated from the target transcript, fluorescently-labelled siRNAs were also bombarded and shown to spread into neighbouring cells. Interestingly, when bombarded near a vein, siRNAs entered the vasculature system and moved long distances. To investigate the potential for long-range movement of siRNAs, the Voinnet group isolated a T-DNA mutant at the siRNAgenerating inverted repeat IR71. This mutant lacks siRNAs from a portion of the inverted repeat, yet when a wild-type scion was grafted onto mutant rootstock, siRNAs were detected in roots from the entire inverted repeat, demonstrating that endogenous siRNAs or an siRNA precursor transcript from this locus move from shoot to root [5]. David Baulcombe (University of Cambridge, UK) also described analysis of Arabidopsis grafts to demonstrate that small RNAs are capable of long distance travel. Using deep sequencing, long-distance movement of siRNAs from thousands of genomic loci was identified [6]. Grafting of wild-type scions onto dcl2 dcl3 dcl4 triple mutant rootstocks eliminated the possibility that longer transcripts might transit before being cleaved into siRNAs in the distal tissue. These analyses indicate that siRNAs from some loci are mobile, while other siRNAs are not, and that movement of siRNAs might be responsible for many of the siRNAs that accumulate in root tissue. The Voinnet and Baulcombe groups demonstrate that mobile siRNAs are functionally indistinguishable from locally synthesized siRNAs in that they target DNA methylation in the cells that receive the signal [5,6]. There is evidence that the plant vasculature system is a highway of small RNA molecules. Julia Kehr (Campus Montegancedo Universidad Politecnica, Spain) addressed the biological relevance of this transport route by deep sequencing the small RNA population in Brassica napus phloem sap under different nutrient conditions. Several * Correspondence: rm536@cam.ac. uk 1 Department of Plant Sciences, University of Cambridge, Cambridge, CB2 3EA,