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Asymmetric bulges within hairpin RNA transgenes influence small RNA size, secondary siRNA production and viral defence II

ABSTRACT: Small RNAs (sRNAs) are essential for normal plant development and range in size classes of 21-24 nucleotides. The 22nt small interfering RNAs (siRNAs) and miRNAs are processed by Dicer-like 2 (DCL2) and DCL1 respectively and can initiate secondary siRNA production from the target transcript amplifying the silencing signal in plants. 22nt siRNAs are under-represented due to competition with DCL4, while only a small number of 22nt miRNAs exist due to the rare occurrence of an asymmetric bulge in the precursor miRNA stem. Here we report a strategy to produce abundant 22nt siRNAs and other desired siRNA size classes using long hairpin RNA (hpRNA) transgenes. By introducing asymmetric bulges periodically into the antisense strand of hpRNA, we successfully shifted the dominant siRNA size class from 21nt of the traditional hpRNA to 22, 23 and 24nt of the asymmetric hpRNAs. We showed that the asymmetric hpRNA constructs effectively silenced a β-glucuronidase (GUS) reporter transgene and the endogenous ethylene insensitive-2 (EIN2) and chalcone synthase (CHS) genes. Furthermore, plants containing the asymmetric hpRNA transgenes targeting both GUS and EIN2 showed increased amount of 21nt siRNAs downstream of the hpRNA target site compared to plants with the traditional hpRNA transgenes. This indicates that these asymmetric hpRNAs are more effective at inducing secondary siRNA production to amplify silencing signals. Consistent with the production of secondary siRNAs, the 22nt asymmetric hpRNA constructs increased gene silencing phenotypes and enhanced virus resistance in plants compared to the traditional hpRNA constructs.

ORGANISM(S): Arabidopsis thaliana

PROVIDER: GSE243296 | GEO | 2024/06/27

REPOSITORIES: GEO

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Project description:To assay siRNA movement in the pollen grain, we took advantage of the known behavior of 22nt siRNAs to target transcripts for cleavage and initiate secondary siRNA biogenesis. Previous research has demonstrated that when targeted by a 22nt microRNA, a transcript containing a truncated non-fluorescent ~400 bp version of GFP (trGFP) will produce GFP secondary siRNAs that can target a second copy of a functional full-length GFP mRNA in trans. Therefore, we created a ~400 bp non-fluorescent version of trGFP with a 5’ 22nt microRNA target site driven by the vegetative cell-specific KRP6 promoter and transformed this construct into a line homozygous for the sperm-specific functional full-length GFP driven by the MGH3 promoter. If cleaved by a 22nt microRNA, the vegetative cell-driven trGFP will be cleaved into secondary siRNAs, and if the RNA components of this system are mobile and transferred into the sperm cells, the sperm-specific fluorescence of GFP will be reduced. We used two versions of the microRNA target site, which included a mock target site that is not targeted by small RNAs, the target site of microRNA173 (which is a 22nt known to induce the production of secondary siRNAs). We demonstrate that when the 22nt microRNA173 target site is used in the vegetative cell transcript, a corresponding decrease in sperm cell fluorescence is observed. In the mock small RNA target site this reduction in fluorescence was not observed. To ensure the transitive silencing sperm cell transcripts is occurring via secondary small RNAs, we transferred this trGFP experiment into a dcl2/dcl4 mutant background that is defective in TE 21-22nt siRNA silencing. In dcl2/dcl4 double mutants with the trGFP system, we find that the GFP fluorescence of the mock target site transgene does not change, while the repression found in wild-type pollen with the 22nt microRNA173 target site is alleviated. The data for microRNA173 is particularly important, as DCL2 and DCL4 are not necessary for microRNA173 production, and therefore this demonstrates that specifically secondary siRNAs acting downstream of microRNA action are required to silence the sperm cell GFP. We further analyzed the production of sRNAs derived from GFP in pollen grains of transgenic lines expressing the 22nt miR173 or 22nt mock target sites inserted on the 5’ region of the trGFP transgene in both wt and dcl2/dcl4 backgrounds.

Project description:Endogenous 24nt short interfering RNAs (siRNAs) derived mostly from intergenic and repetitive genomic regions constitute a major class of endogenous small RNAs in Arabidopsis thaliana. Accumulation of A. thaliana 24nt siRNAs requires the Dicer family member DCL3, and clear homologs of DCL3 exist in both flowering and non-flowering plants. However, the absence of a conspicuous 24nt peak in the total RNA populations of several non-flowering plants has raised the question of whether this class of siRNAs might, in contrast to the ancient 21nt microRNAs (miRNAs) and 21-22nt trans-acting siRNAs (tasiRNAs), be an angiosperm-specific innovation. Analysis of non-miRNA, non-tasiRNA hotspots of small RNA production within the genome of the moss Physcomitrella patens revealed multiple loci which consistently produced a mixture of 21-24nt siRNAs with a peak at 23nts. These Pp23SR loci were significantly enriched in transposon content, depleted in overlap with annotated genes, and typified by dense concentrations of the 5-methyl cytosine (5mC) DNA modification. Deep sequencing of small RNAs from two independent Ppdcl3 mutants showed that the P. patens DCL3 homolog is required for the accumulation of 22-24nt siRNAs, but not 21nt siRNAs, at Pp23SR loci. The 21nt component of Pp23SR-derived siRNAs was also unaffected by a mutation in the RNA-dependent RNA polymerase mutant Pprdr6. Transcriptome-wide, Ppdcl3 mutants specifically failed to accumulate 23 and 24nt small RNAs from repetitive regions. We conclude that intergenic/repeat-derived siRNAs are indeed a broadly conserved, distinct class of small regulatory RNAs within land plants. Our results also suggest that Pp DCL3 produces siRNAs of heterogenous size, unlike its A. thaliana homolog which generates exclusively 24nt siRNAs. Small RNAs from Wild-type (two technical replicates), rdr6-19 (two technical replicates), dcl3-5 (one sample), and dcl3-10 (one sample) were sequenced using a Solexa GA. Three different linkers were used for different samples and mixed prior to sequencing. linker 1: CTGTAGGCACCATCAAT (GPL7174 AND GSM313212, GSM313213) linker 2: CACTCGGGCACCAAGGA (GPL7175 AND GSM313214, GSM313215) linker 3: TTTAACCGCGAATTCCAG (GPL7176 AND GSM313216, GSM313217) Raw data: Two FASTQ files represent the raw data underlying the processed sample data in GSE12468. 080616_s_7_seq_GAK-2.fastq corresponds to GSM313212, GSM313214, and GSM313216. 080616_s_8_seq_GAK-3.fastq corresponds to GSM313213, GSM313215, and GSM313217. Experimentally, samples GSM313212, GSM313214, and GSM313216 (i.e., all three genotypes) were sequenced in the same run (hence the same FASTQ file). The sequencing run was a mixture of three different libraries with different linker sequences (as indicated above). These 'barcoded' samples were computationally extracted based on the linker sequences. This is also true for samples GSM313213, GSM313215, and GSM313217. Raw data were filtered to identify small RNAs associated with each linker. Subsequently, small RNAs were filtered to remove rRNA fragments. The remainder were mapped to the Physcomitrella patens draft genome version 1.2 (using oligomap) and filtered to retain only those with one or more perfect match. See Cho et al. for details.

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Asymmetric bulges within hairpin RNA transgenes influence small RNA size, secondary siRNA production and viral defence II

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