Bernie Carroll
School of Chemistry and Molecular Biosciences, University of Queensland

Silencing of introduced transgenes constitutes a major bottleneck in the production of transgenic crops. Commonly, these transgenes contain no introns, a feature shared with transposons, which are also prime targets for gene silencing. Given that introns are very common in endogenous genes, but are often lacking in transgenes and transposons, we hypothesized that introns may suppress gene silencing. To investigate this, we conducted a genome-wide analysis of small RNA densities in exons from intronless versus intron-containing genes in Arabidopsis thaliana (1). We found that small RNA libraries are strongly enriched for exon sequences derived from intronless genes. Small RNA densities in exons of intronless genes were comparable to exons of transposable elements. To test these findings in vivo, we used a transgenic reporter system to determine whether introns are able to suppress gene silencing in Arabidopsis (1). Introducing an intron into a transgene reduced silencing by more than four-fold. Compared to intronless transcripts, the spliced transcripts were less effective substrates for RNA-dependent RNA polymerase 6 (RDR6)-mediated gene silencing. This intron suppression of transgene silencing requires efficient intron splicing and is dependent on ABH1, the Arabidopsis orthologue of human mRNA Cap Binding Protein 80 (1).Remarkably, when gene silencing is triggered in plants it can spread cell to cell and/or systemically throughout the organism (2). We are also interested in the genetic determinants involved in systemic spreading of transgene silencing and the molecular nature of mobile silencing signals (3). Our progress in this area will also be described.
References:
1. Christie M., Croft L.J. and Carroll B.J. (2011) Intron splicing suppresses RNA silencing in Arabidopsis. Plant Journal doi: 10.1111/j.1365-313X.2011.04676.x.
2. Gursanscky N., Searle I.R. and Carroll B.J. (2011) Mobile microRNAs hit the target. Traffic doi: 10.1111/j.1600-0854.2011.01253.x.
3. Brosnan, C.A., Mitter, N., Christie, M., Smith, N.A., Waterhouse, P.M. and Carroll, B.J. (2007) Nuclear gene silencing directs reception of long-distance mRNA silencing in Arabidopsis. PNAS 104, 14741-14746.