Abstract
BackgroundRNA-DEPENDENT RNA POLYMERASE6 (RDR6) and SUPPRESSOR of GENE SILENCING 3 (SGS3) are required for DNA methylation and post-transcriptional gene silencing (PTGS) mediated by 21-nt siRNAs produced by sense transgenes (S-PTGS). In contrast, RDR2, but not RDR6, is required for DNA methylation and TGS mediated by 24-nt siRNAs, and for cell-to-cell spreading of IR-PTGS mediated by 21-nt siRNAs produced by inverted repeat transgenes under the control of a phloem-specific promoter.Principal FindingsIn this study, we examined the role of RDR2 and RDR6 in S-PTGS. Unlike RDR6, RDR2 is not required for DNA methylation of transgenes subjected to S-PTGS. RDR6 is essential for the production of siRNAs by transgenes subjected to S-PTGS, but RDR2 also contributes to the production of transgene siRNAs when RDR6 is present because rdr2 mutations reduce transgene siRNA accumulation. However, the siRNAs produced via RDR2 likely are counteractive in wildtype plants because impairement of RDR2 increases S-PTGS efficiency at a transgenic locus that triggers limited silencing, and accelerates S-PTGS at a transgenic locus that triggers efficient silencing.Conclusions/SignificanceThese results suggest that RDR2 and RDR6 compete for RNA substrates produced by transgenes subjected to S-PTGS. RDR2 partially antagonizes RDR6 because RDR2 action likely results in the production of counteractive siRNA. As a result, S-PTGS efficiency is increased in rdr2 mutants.
Highlights
Most Eukaryotic genomes produce small RNAs, 20 to 30 nucleotides in length, which regulate endogenous genes at either the transcriptional or posttranscriptional level [1,2,3]
Conclusions/Significance: These results suggest that RDR2 and RNA-DEPENDENT RNA POLYMERASE6 (RDR6) compete for RNA substrates produced by transgenes subjected to S-post-transcriptional gene silencing (PTGS)
Based on early genetic screens, RDR6 has been associated to 21-nt short-interfering RNAs (siRNAs)-related PTGS mediated by amplicons (A-PTGS) or sense transgenes (S-PTGS) [14,28], and RDR2 to 24-nt siRNA-related DNA methylation and transcriptional gene silencing (TGS) [9,18]
Summary
Most Eukaryotic genomes produce small RNAs, 20 to 30 nucleotides (nt) in length, which regulate endogenous genes at either the transcriptional or posttranscriptional level [1,2,3]. Exogenous siRNAs can be produced in response to invasive DNA or RNA (transgenes, viruses, bacteria, etc). This de novo production of siRNAs relies on the existing cellular small RNA machineries [5]. In contrast to endogenous small RNAs that usually are processed from their precursor RNAs by one or the other cellular machinery, exogenous precursor RNAs can be processed into various forms of siRNAs by the different cellular machineries. 24-nt siRNAs produced by DCL3 trigger transcriptional gene silencing (TGS) when they are homologous to promoter regions, either by guiding DNA methylation or histone modification [7]. RDR2, but not RDR6, is required for DNA methylation and TGS mediated by 24-nt siRNAs, and for cellto-cell spreading of IR-PTGS mediated by 21-nt siRNAs produced by inverted repeat transgenes under the control of a phloem-specific promoter
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