RNA silencing is a nucleotide sequence-specific process that includes RNA degradation (PTGS), DNA methylation (RdDM), heterochromatin formation and protein translation inhibition in eukaryotic genomes.

In plants, effective silencing of a transgene includes a silencing phenomenon termed silencing transitivity requiring siRNA biogenesis and spreading of methylation along the target DNA sequence, as well as a non-cell autonomous process allowing RNA silencing to spread from cell-to-cell and throughout the whole plant. However, exogenous inverted repeat sequence (exo-IR) transgenes expressed in plants are not always able to efficiently silence related endogenous coding genes. How do plants control or prevent unwanted silencing of active protein-coding genes?

To address this issue, supporting by grants from the National Science Foundation of China, Prof. GUO Hui-Shan and her colleagues in the State Key Laboratory of Plant Genomics, Institute of Microbiology, Chinese Academy of Sciences, used a previously reported chemical-inducible RNAi system for detail analysis. They found that non-cell autonomous silencing involves the production of secondary siRNAs, and DNA methylation/spreading occurred only in the exo-IR (exo-Pdsi) silencer. Neither secondary siRNAs nor DNA methylation/spreading were found in the endogenous-PDS silencing target, which simply occurred PTGS. Endo-PDS silencing was enhanced in RdDM related mutant plants, such as drd1, pol V (nrpe1 and nrpd2). Their results reveal that DRD1-Pol V-dependent RdDM required to reinforce self-silencing of exo-IR may play a role to prevent inappropriate silencing of the endogenous targettranscript in wild-type plants, which may represent a surveillance system that was evolved to guarantee the activity of endogenous genes when subjecting to exogenous silencer. Their findings explain a general phenomenon of low and diverse efficiencies in inducing silencing of endogenous coding genes even when using inverted repeat transgenes as a silencer.

The related paper has been published in The Plant Journal（http://www.ncbi.nlm.nih.gov/pubmed/21771120). Reviewers of The Plant Journal commented: “This work provides novel valuable information to understand the complex interactions between cell-autonomous and non-cell-autonomous effects of transcriptional and posttranscriptional gene silencing (TGS and PTGS) pathways.” “This is an interesting paper that uses a clever transgene system to study connections between PTGS and TGS, transitivity, and cell-to-cell movement and sensing of silencing signals. All of these are important topics of broad interest. This paper is the first to combine all of these features together in a single silencing system.”