Extensive profiling in Arabidopsis reveals abundant polysome-associated 24-nt small RNAs including AGO5-dependent pseudogene-derived siRNAs.
ABSTRACT: In a reductionist perspective, plant silencing small (s)RNAs are often classified as mediating nuclear transcriptional gene silencing (TGS) or cytosolic posttranscriptional gene silencing (PTGS). Among the PTGS diagnostics is the association of AGOs and their sRNA cargos with the translation apparatus. In Arabidopsis, this is observed for AGO1 loaded with micro(mi)RNAs and, accordingly, translational-repression (TR) is one layer of plant miRNA action. Using AGO1:miRNA-mediated TR as a paradigm, we explored, with two unrelated polysome-isolation methods, which, among the ten Arabidopsis AGOs and numerous sRNA classes, interact with translation. We found that representatives of all three AGO-clades associate with polysomes, including the TGS-effector AGO4 and stereotypical 24-nt sRNAs that normally mediate TGS of transposons/repeats. Strikingly, approximately half of these annotated 24-nt siRNAs displayed unique matches in coding regions/introns of genes, and in pseudogenes, but not in transposons/repeats commonly found in their vicinity. Protein-coding gene-derived 24-nt sRNAs correlate with gene-body methylation. Those derived from pseudogenes belong to two main clusters defined by their parental-gene expression patterns, and are vastly enriched in AGO5, itself found on polysomes. Based on their tight expression pattern in developing and mature siliques, their biogenesis, and genomic/epigenomic features of their loci-of-origin, we discuss potential roles for these hitherto unknown polysome-enriched, pseudogene-derived siRNAs.
Project description:To systematically investigate viral sRNA production and sRNA-target interaction, we sequenced sRNAs from Tobacco Rattle Virus (TRV)-infected Nicotiana benthamiana at an early (1 week post infection) and late time point (3 weeks post infection). The N. benthamiana 16c plants were infected with TGS-inducing viruses (TRV-35S and TRV-35-2M) and PTGS-inducing viruses (TRV-GFP and TRV-GFP-2M), respectively. TRV-35S is a recombinant TRV containing a 120 nt segment of the 35S promoter. Its derivative, TRV-35S-2M, carrying single nucleotide substitutions (SNS) at every 10 nt within the 120 nt 35S target segment. Same strategy was used to create recombinant TRV-GFP and TRV-GFP-2M targeting GFP coding sequence. According to SNS content, sRNAs from TRV-35S-2M/TRV-GFP-2M infected plants can be separated to yield primary (containing SNSs) and secondary sRNAs (lacking SNSs). Wild Type TRV was used along as viral infection control. Libraries were indexed during PCR amplification (16 cycles) according to the Illumina protocol. See individual sample information for specific index primers used.
Project description:The identification of viroid-derived small RNAs (vd-sRNAs) of 21 to 24 nucleotides (nt) in plants infected by viroids (infectious non-protein-coding RNAs of just 250 to 400 nt) supports their targeting by Dicer-like enzymes, the first host RNA-silencing barrier. However, whether viroids, like RNA viruses, are also targeted by the RNA-induced silencing complex (RISC) remains controversial. At the RISC core is one Argonaute (AGO) protein that, guided by endogenous or viral sRNAs, targets complementary RNAs. To examine whether AGO proteins also load vd-sRNAs, leaves of Nicotiana benthamiana infected by potato spindle tuber viroid (PSTVd) were agroinfiltrated with plasmids expressing epitope-tagged versions of AGO1, AGO2, AGO3, AGO4, AGO5, AGO6, AGO7, AGO9, and AGO10 from Arabidopsis thaliana. Immunoprecipitation analyses of the agroinfiltrated halos revealed that all AGOs except AGO6, AGO7, and AGO10 associated with vd-sRNAs: AGO1, AGO2, and AGO3 preferentially with those of 21 and 22 nt, while AGO4, AGO5, and AGO9 additionally bound those of 24 nt. Deep-sequencing analyses showed that sorting of vd-sRNAs into AGO1, AGO2, AGO4, and AGO5 depended essentially on their 5'-terminal nucleotides, with the profiles of the corresponding AGO-loaded vd-sRNAs adopting specific hot spot distributions along the viroid genome. Furthermore, agroexpression of AGO1, AGO2, AGO4, and AGO5 on PSTVd-infected tissue attenuated the level of the genomic RNAs, suggesting that they, or their precursors, are RISC targeted. In contrast to RNA viruses, PSTVd infection of N. benthamiana did not affect miR168-mediated regulation of the endogenous AGO1, which loaded vd-sRNAs with specificity similar to that of its A. thaliana counterpart. Importance: To contain invaders, particularly RNA viruses, plants have evolved an RNA-silencing mechanism relying on the generation by Dicer-like (DCL) enzymes of virus-derived small RNAs of 21 to 24 nucleotides (nt) that load and guide Argonaute (AGO) proteins to target and repress viral RNA. Viroids, despite their minimal genomes (non-protein-coding RNAs of only 250 to 400 nt), infect and incite disease in plants. The accumulation in these plants of 21- to 24-nt viroid-derived small RNAs (vd-sRNAs) supports the notion that DCLs also target viroids but does not clarify whether vd-sRNAs activate one or more AGOs. Here, we show that in leaves of Nicotiana benthamiana infected by potato spindle tuber viroid, the endogenous AGO1 and distinct AGOs from Arabidopsis thaliana that were overexpressed were associated with vd-sRNAs displaying the same properties (5'-terminal nucleotide and size) previously established for endogenous and viral small RNAs. Overexpression of AGO1, AGO2, AGO4, and AGO5 attenuated viroid accumulation, supporting their role in antiviroid defense.
Project description:ARGONAUTE (AGO) proteins are the major effectors of RNA silencing. In Arabidopsis, loading of 21-22-nt long small RNAs into AGOs results in post-transcriptional gene silencing (PTGS) by mRNA cleavage and/or translational repression. On the other hand, loading of 24-nt sRNAs results in transcriptional gene silencing (TGS) by RNA directed DNA methylation (RdDM). The Arabidopsis genome encodes 10 AGOs, which are known, for most, to belong to either pathway. Here we characterized the cell specific role of Arabidopsis AGO3. It is specifically expressed in tissue close to vascular termination and strongly accumulates in chalazal seed integuments. AGO3 encodes a functional AGO able to bind sRNAs with a preference for 24nt in length with a 5’ nucleotide bias for Adenosine. Down-regulation of AGO3 affects gene expression in siliques and its expression is strongly induced in the plant vasculature upon proteasome inhibition. Our results suggest that AGO3 might act to regulate gene expression by a novel RNA silencing pathway involving 24nt sRNA-directed PTGS. Overall design: Transcriptome of Arabidopsis Col-0 Wt and ago3-3 mutant siliques were generated by stranded paired-end deep sequencing, in duplicate, using Illumina HiSeq 2000
Project description:ARGONAUTE (AGO) proteins are the major effectors of RNA silencing. In Arabidopsis, loading of 21-22-nt long small RNAs into AGOs results in post-transcriptional gene silencing (PTGS) by mRNA cleavage and/or translational repression. On the other hand, loading of 24-nt sRNAs results in transcriptional gene silencing (TGS) by RNA directed DNA methylation (RdDM). The Arabidopsis genome encodes 10 AGOs, which are known, for most, to belong to either pathway. Here we characterized the cell specific role of Arabidopsis AGO3. It is specifically expressed in tissue close to vascular termination and strongly accumulates in chalazal seed integuments. AGO3 encodes a functional AGO able to bind sRNAs with a preference for 24nt in length with a 5’ nucleotide bias for Adenosine. Down-regulation of AGO3 affects gene expression in siliques and its expression is strongly induced in the plant vasculature upon proteasome inhibition. Our results suggest that AGO3 might act to regulate gene expression by a novel RNA silencing pathway involving 24nt sRNA-directed PTGS. Overall design: small RNAs co-Immunoprecipitating with FLAG AGO3 protein from siliques and, after a SyrginolinA treatment, from plantlets were sequenced using Illumina platform.
Project description:RNAi mediated by small-interfering RNAs (siRNAs) operates via transcriptional (TGS) and posttranscriptional gene silencing (PTGS). In Arabidopsis thaliana, TGS relies on DICER-LIKE-3 (DCL3)-dependent 24-nt siRNAs loaded into AGO4-clade ARGONAUTE effector proteins. PTGS operates via DCL4-dependent 21-nt siRNAs loaded into AGO1-clade proteins. We set up and validated a medium-throughput, semi-automatized procedure enabling chemical screening, in a 96-well in vitro format, of Arabidopsis transgenic seedlings expressing an inverted-repeat construct from the phloem companion cells. The ensuing quantitative PTGS phenotype was exploited to identify molecules, which, upon topical application, either inhibit or enhance siRNA biogenesis/activities. The vast majority of identified modifiers were enhancers, among which Sortin1, Isoxazolone, and [5-(3,4-dichlorophenyl)furan-2-yl]-piperidine-1-ylmethanethione (DFPM) provided the most robust and consistent results, including upon their application onto soil-grown plants in which their effect was nonautonomous and long lasting. The three molecules increased the RNAi potency of the inverted-repeat construct, in large part by enhancing 21-nt siRNA accumulation and loading into AGO1, and concomitantly reducing AGO4 and DCL3 levels in planta. A similar, albeit not identical effect, was observed on 22-nt siRNAs produced from a naturally occurring inverted-repeat locus, demonstrating that the molecules also enhance endogenous PTGS. In standardized assays conducted in seedling extracts, the three enhancers selectively increased DCL4-mediated processing of in vitro-synthesized double-stranded RNAs, indicating the targeting of a hitherto unknown PTGS component probably independent of the DCL4-cofactor DOUBLE-STRANDED RNA-BINDING 4 (DRB4). This study establishes the proof-of-concept that RNAi efficacy can be modulated by chemicals in a whole organism. Their potential applications and the associated future research are discussed.
Project description:Arabidopsis encodes ten ARGONAUTE (AGO) effectors of RNA silencing, canonically loaded with either 21-22nt small RNAs (sRNA) to mediate post-transcriptional-gene-silencing (PTGS) or 24nt sRNAs to promote RNA-directed-DNA-methylation. Using full-locus constructs, we characterized the expression, biochemical properties, and possible modes of action of AGO3. Although AGO3 arose from a recent duplication at the AGO2 locus, their expression differs drastically, with AGO3 prevailing in aerial vascular terminations and specifically in chalazal seed integuments; accordingly, AGO3 down-regulation alters gene expression in siliques. Similar to AGO2, AGO3 binds sRNAs with a strong 5’-adenosine bias, but unlike most Arabidopsis AGOs - AGO2 included - it binds efficiently both 24nt and 21nt sRNAs. AGO3 immuno precipitation experiments in siliques revealed that these sRNAs mostly associate to genes and intergenic regions and not to canonical AGO targets, such as transposable elements. AGO3 localizes to the cytoplasm and co-fractionates with polysomes to possibly mediate PTGS via translation inhibition. Overall design: For mRNA sequencing, 2 biological samples per genotype were assessed. Samples were: ago3-3 homozygous, ago3-3 heterozygous, and Columbia 0 wilt-type samples. For sRNA sequencing, no replication was done. Three samples were analyzed: Total sRNA extracted from Arabidopsis siliques (TOTAL_smRNA), sRNA fraction which was collected from immunoprecipitated AGO3:FLAG protein (IP_AGO3), and sRNA from a control Immunoprecipitation experiment, conducted in wild-type siliques using a FLAG antibody.
Project description:Small RNAs (sRNAs) are loaded into ARGONAUTE (AGO) proteins to induce gene silencing. In plants, the 5'-terminal nucleotide is important for sRNA sorting into different AGOs. Here we show that microRNA (miRNA) duplex structure also contributes to miRNA sorting. Base pairing at the 15th nucleotide of a miRNA duplex is important for miRNA sorting in both Arabidopsis AGO1 and AGO2. AGO2 favours miRNA duplexes with no middle mismatches, whereas AGO1 tolerates, or prefers, duplexes with central mismatches. AGO structure modelling and mutational analyses reveal that the QF-V motif within the conserved PIWI domain contributes to recognition of base pairing at the 15th nucleotide of a duplex, while the DDDE catalytic core of AtAGO2 is important for recognition of the central nucleotides. Finally, we rescued the adaxialized phenotype of ago1-12, which is largely due to miR165 loss-of-function, by changing miR165 duplex structure which we predict redirects it to AGO2.
Project description:As important components of small RNA (smRNA) pathways, Argonaute (AGO) proteins mediate the interaction of incorporated smRNAs with their targets. Arabidopsis contains 10 AGO proteins with specialized or redundant functions. Among them, AGO1 mainly acts in microRNA (miRNA) and small-interfering RNA (siRNA) pathways for post-transcriptional gene silencing (PTGS), whereas AGO4 regulates transcriptional gene silencing (TGS) via endogenous 24-nucleotide (nt) smRNAs. To fully characterize smRNAs associated with AGO1 and AGO4, we developed a two-step protocol to purify AGO/smRNA complexes from flowers, leaves, roots and seedlings with enhanced purity, and sequenced the smRNAs by Illumina technology. Besides recovering most previously annotated smRNAs, we also identified some additional miRNAs, phased smRNA clusters and small-interfering RNAs derived from the overlapping region of natural antisense transcript pairs (NAT) (nat-siRNAs). We also identified a smRNA distribution feature on miRNA precursors which may help to identify authentic miRNAs. Organ-specific sequencing provided digital expression profiles of all obtained smRNAs, especially miRNAs. The presence and conservation of collateral miRNAs on known miRNA precursors were also investigated. Intriguingly, about 30% of AGO1-associated smRNAs were 24-nt long and unrelated to the 21-nt species. Further analysis showed that DNA-dependent RNA polymerase IV (Pol IV)-dependent smRNAs were mainly 24?nt and associated with AGO4, whereas the majority of the potential Pol V-dependent ones were 21-nt smRNAs and bound to AGO1, suggesting the potential involvement of AGO1 in Pol V-related pathways.
Project description:We grafted Transcriptional Gene Silencing (TGS)-inducing wild type Arabidopsis and a mutant that is compromised in 24 nucleotide (nt) small RNA (sRNA) production onto a wild type reporter line. We observed that 21-24 nt sRNAs were transmitted across a graft union yet only the 24 nt sRNAs directed RNA-dependent DNA methylation (RdDM) and TGS of a transgene promoter in meristematic cells. Analysis of sRNAs associated with mobile TGS by deep sequencing, biological replicates. 10 unique samples, 5 biological replicates.
Project description:The experimental induction of RNA silencing in plants often involves expression of transgenes encoding inverted repeat (IR) sequences to produce abundant dsRNAs that are processed into small RNAs (sRNAs). These sRNAs are key mediators of post-transcriptional gene silencing (PTGS) and determine its specificity. Despite its application in agriculture and broad utility in plant research, the mechanism of IR-PTGS is incompletely understood. We generated four sets of 60 Arabidopsis plants, each containing IR transgenes expressing different configurations of uidA and CHALCONE Synthase (At-CHS) gene fragments. Levels of PTGS were found to depend on the orientation and position of the fragment in the IR construct. Deep sequencing and mapping of sRNAs to corresponding transgene-derived and endogenous transcripts identified distinctive patterns of differential sRNA accumulation that revealed similarities among sRNAs associated with IR-PTGS and endogenous sRNAs linked to uncapped mRNA decay. Detailed analyses of poly-A cleavage products from At-CHS mRNA confirmed this hypothesis. We also found unexpected associations between sRNA accumulation and the presence of predicted open reading frames in the trigger sequence. In addition, strong IR-PTGS affected the prevalence of endogenous sRNAs, which has implications for the use of PTGS for experimental or applied purposes.