RNA silencing in plants: Flash report!

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.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 siRNA-generating 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 de