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IN BRIEF

A CLASSY RNA Silencing Signaling Mutant in Arabidopsis

neckardt{at}aspb.org

RNA silencing in plants is associated with movement of a silencing signal from cell to cell and through the phloem. It is likely that the mobile silencing signals are RNA molecules: either short interfering RNAs generated by Dicer-liker (DCL) RNaseIII enzymes or longer double stranded RNAs. Smith et al. (pages 1507–1521) describe a genetic screen designed to recover RNA silencing mutations in Arabidopsis that block or enhance the spread of silencing between cells. Their approach involved a silencing transgene expressed specifically in phloem companion cells such that a silencing phenotype observed in cells located away from vascular cells would require the spread of a silencing signal. Use of phytoene desaturase as the reporter of silencing allowed for a simple visual screen: movement of the silencing signal out of the phloem would lead to silencing in the surrounding mesophyll cells manifested as photobleaching. Use of this screen led to the identification of a novel SNF2-related gene, named CLASSY1, involved in the spread of silencing. CLASSY1 is in the same plant-specific family of SNF2 proteins as DRD1, which is implicated in DNA methylation and transcriptional gene silencing. It was also found that RDR2, NRPD1a, and NRPD2a, but not AGO4 or DCL3, were required for spreading of the silencing signal to surrounding tissues. The authors propose that AGO4 and DCL3 are involved in self-silencing of the target gene, whereas NRPD1a (a subunit of Pol IVa), RDR2, DCL4, and CLSY1 influence the spread of silencing via a signal amplification process in cells that receive the signal.

View larger version (163K): [in this window] [in a new window] [as a PowerPoint slide]   An RNA silencing phenotype. Loss of the phenotype in mutant plants indicated defects in the spread of a silencing signal out of the phloem.

www.plantcell.org/cgi/doi/10.1105/tpc.107.190510

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An SNF2 Protein Associated with Nuclear RNA Silencing and the Spread of a Silencing Signal between Cells in Arabidopsis

Lisa M. Smith, Olga Pontes, Iain Searle, Nataliya Yelina, Faridoon K. Yousafzai, Alan J. Herr, Craig S. Pikaard, and David C. Baulcombe
Plant Cell 2007 19: 1507-1521. [Abstract] [Full Text]  

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