Synergistic and independent actions of multiple terminal nucleotidyl transferases in the 3' tailing of small RNAs in Arabidopsis

HC-Pro inhibits HEN1 methyltransferase activity, leading to autophagic degradation of AGO1

Helper-component proteinase (HC-Pro), encoded by potyviruses, function as viral suppressors of RNA silencing (VSRs). Despite their conserved role, HC-Pros share approximately 40% similarity, implying potential differences in VSR efficiency, particularly in their ability to inhibit HEN1 methyltransferase activity. This study investigated the inhibitory potential of HC-Pros from different potyviruses in transgenic plants. P1/HC-Pro from turnip mosaic virus (P1/HC-ProTu) exhibited the most potent inhibition of HEN1, followed by P1/HC-Pro from zucchini yellow mosaic virus (P1/HC-ProZy), while P1/HC-Pro from tobacco etch virus (P1/HC-ProTe) showed the weakest inhibitory effect. These differential effectual effects corresponded to variations in unmethylated microRNAs (unMet-miRNAs) accumulation across the transgenic lines. Fluorescence resonance energy transfer (FRET) analysis indicated that HC-ProTu recruits HEN1 and ATG8a to HC-Pro bodies (H-bodies) and indirectly associates with AGO1, potentially influencing the assembly of the RNA-induced silencing complex (RISC) and leading to the accumulation of free-form miRNA duplexes. The ability of HC-ProTu to sequester HEN1 and AGO1 in H-bodies may, therefore, modulate miRNA loading. This observation aligns with the finding that P1/HC-ProTu plants harbored approximately 50% unMet-miRNAs and exhibited the lowest AGO1 levels, suggesting a positive correlation between HEN1 inhibition and autophagic degradation of AGO1. Interestingly, unMet-miRNAs are absent in the AGO1 of P1/HC-ProTu plants but reappeared in P1/HC-ProTu/hen1-8/heso1-1 plants, accompanied by signs of AGO1 recovery. These findings highlight the functional diversity of HC-Pro VSRs and provide new insights into their differential effects on miRNA methylation, RISC assembly, and the regulation of RNA silencing pathways.

Scalable Approach to Evaluate Plant microRNA Trimming and Tailing from Small RNA-Seq Data

MicroRNAs are small regulatory RNAs of 20-24 nt, which guide the RNA-induced silencing complex to silence several genes post-transcriptionally. Plant miRNA biogenesis involves many steps, including their 2'-O-methylation at their 3' end, which protects them from degradation. In addition, there are two other types of modifications involved in miRNA turnover: trimming and tailing. This method describes a bioinformatics analysis procedure to evaluate miRNA trimming and tailing from sRNA-Seq data. The pipeline includes the steps required to download the raw data from public repositories and important considerations to analyze a large number of samples. It starts from the raw sequencing reads and involves 3' adaptor removal, quality filtering, alignment to a masked reference genome to discard unwanted reads, and the assessment of the degree of trimming and tailing for each miRNA. Finally, an index for these modifications is calculated, and the results are evaluated graphically and statistically. In conclusion, this scalable pipeline allows researchers to begin with raw data from various sRNA-seq studies and progress to meaningful results and visual representations.

PBOX-sRNA-seq uncovers novel features of miRNA modification and identifies selected 5'-tRNA fragments bearing 2'-O-modification

The concomitant cloning of RNA degradation products is a major concern in standard small RNA-sequencing practices. This not only complicates the characterization of bona fide sRNAs but also hampers cross-batch experimental replicability and sometimes even results in library construction failure. Given that all types of plant canonical small RNAs possess the 3' end 2'-O-methylation modification, a new small RNA sequencing (sRNA-seq) method, designated as PBOX-sRNA-seq, has been developed specifically to capture this modification. PBOX-sRNA-seq, as its name implies, relies on the sequential treatment of RNA samples with phenylboronic acid-polyacrylamide gel electrophoresis (PBA-PAGE) and sodium periodate (NaIO4) oxidation, before sRNA library construction and sequencing. PBOX-sRNA-seq outperformed separate treatments (i.e. PBA-PAGE only or NaIO4 only) in terms of the depletion of unmethylated RNA species and capture 2'-O-modified sRNAs with extra-high purity. Using PBOX-sRNA-seq, we discovered that nascent miRNA-5p/-3p duplexes may undergo mono-cytidylation/uridylation before 2'-O-methylation. We also identified two highly conserved types of 5'-tRNA fragments (tRF) bearing HEN1-independent 2'-O modification (mainly the 13-nt tRF-5aAla and the 26-nt tRF-5bGly). We believe that PBOX-sRNA-seq is powerful for both qualitative and quantitative analyses of sRNAs in plants and piRNAs in animals.

Biomolecular condensates in plant RNA silencing: insights into formation, function, and stress responses

Biomolecular condensates are dynamic structures formed through diverse mechanisms, including liquid-liquid phase separation. These condensates have emerged as crucial regulators of cellular processes in eukaryotic cells, enabling the compartmentalization of specific biological reactions while allowing for dynamic exchange of molecules with the surrounding environment. RNA silencing, a conserved gene regulatory mechanism mediated by small RNAs (sRNAs), plays pivotal roles in various biological processes. Multiple types of biomolecular condensate, including dicing bodies, processing bodies, small interfering RNA bodies, and Cajal bodies, have been identified as key players in RNA silencing pathways. These biomolecular condensates provide spatial compartmentation for the biogenesis, loading, action, and turnover of small RNAs. Moreover, they actively respond to stresses, such as viral infections, and modulate RNA silencing activities during stress responses. This review summarizes recent advances in understanding of dicing bodies and other biomolecular condensates involved in RNA silencing. We explore their formation, roles in RNA silencing, and contributions to antiviral resistance responses. This comprehensive overview provides insights into the functional significance of biomolecular condensates in RNA silencing and expands our understanding of their roles in gene expression and stress responses in plants.

Identification and Expression Analysis of the Nucleotidyl Transferase Protein (NTP) Family in Soybean (Glycine max) under Various Abiotic Stresses

Nucleotidyl transferases (NTPs) are common transferases in eukaryotes and play a crucial role in nucleotide modifications at the 3' end of RNA. In plants, NTPs can regulate RNA stability by influencing 3' end modifications, which in turn affect plant growth, development, stress responses, and disease resistance. Although the functions of NTP family members have been extensively studied in Arabidopsis, rice, and maize, there is limited knowledge about NTP genes in soybeans. In this study, we identified 16 members of the NTP family in soybeans, including two subfamilies (G1 and G2) with distinct secondary structures, conserved motifs, and domain distributions at the protein level. Evolutionary analysis of genes in the NTP family across multiple species and gene collinearity analysis revealed a relatively conserved evolutionary pattern. Analysis of the tertiary structure of the proteins showed that NTPs have three conserved aspartic acids that bind together to form a possible active site. Tissue-specific expression analysis indicated that some NTP genes exhibit tissue-specific expression, likely due to their specific functions. Stress expression analysis showed significant differences in the expression levels of NTP genes under high salt, drought, and cold stress. Additionally, RNA-seq analysis of soybean plants subjected to salt and drought stress further confirmed the association of soybean NTP genes with abiotic stress responses. Subcellular localization experiments revealed that GmNTP2 and GmNTP14, which likely have similar functions to HESO1 and URT1, are located in the nucleus. These research findings provide a foundation for further investigations into the functions of NTP family genes in soybeans.

Promiscuous splicing-derived hairpins are dominant substrates of tailing-mediated defense of miRNA biogenesis in mammals

Canonical microRNA (miRNA) hairpins are processed by the RNase III enzymes Drosha and Dicer into ∼22 nt RNAs loaded into an Argonaute (Ago) effector. In addition, splicing generates numerous intronic hairpins that bypass Drosha (mirtrons) to yield mature miRNAs. Here, we identify hundreds of previously unannotated, splicing-derived hairpins in intermediate-length (∼50-100 nt) but not small (20-30 nt) RNA data. Since we originally defined mirtrons from small RNA duplexes, we term this larger set as structured splicing-derived RNAs (ssdRNAs). These associate with Dicer and/or Ago complexes, but generally accumulate modestly and are poorly conserved. We propose they contaminate the canonical miRNA pathway, which consequently requires defense against the siege of splicing-derived substrates. Accordingly, ssdRNAs/mirtrons comprise dominant hairpin substrates for 3' tailing by multiple terminal nucleotidyltransferases, notably TUT4/7 and TENT2. Overall, the rampant proliferation of young mammalian mirtrons/ssdRNAs, coupled with an inhibitory molecular defense, comprises a Red Queen's race of intragenomic conflict.

Tweaking the Small Non-Coding RNAs to Improve Desirable Traits in Plant

Plant transcriptome contains an enormous amount of non-coding RNAs (ncRNAs) that do not code for proteins but take part in regulating gene expression. Since their discovery in the early 1990s, much research has been conducted to elucidate their function in the gene regulatory network and their involvement in plants' response to biotic/abiotic stresses. Typically, 20-30 nucleotide-long small ncRNAs are a potential target for plant molecular breeders because of their agricultural importance. This review summarizes the current understanding of three major classes of small ncRNAs: short-interfering RNAs (siRNAs), microRNA (miRNA), and transacting siRNAs (tasiRNAs). Furthermore, their biogenesis, mode of action, and how they have been utilized to improve crop productivity and disease resistance are discussed here.

MicroRNAs play regulatory roles in genomic balance

Classic genetics studies found that genomic imbalance caused by changing the dosage of part of the genome (aneuploidy) has more detrimental effects than altering the dosage of the whole genome (ploidy). Previous analysis revealed global modulation of gene expression triggered by aneuploidy across various species, including maize (Zea mays), Arabidopsis, yeast, mammals, etc. Plant microRNAs (miRNAs) are a class of 20- to 24-nt endogenous small noncoding RNAs that carry out post-transcriptional gene expression regulation. That miRNAs and their putative targets are preferentially retained as duplicates after whole-genome duplication, as are many transcription factors and signaling components, indicates miRNAs are likely to be dosage-sensitive and potentially involved in genomic balance networks. This review addresses the following questions regarding the role of miRNAs in genomic imbalance. (1) How do aneuploidy and polyploidy impact the expression of miRNAs? (2) Do miRNAs play a regulatory role in modulating the expression of their targets under genomic imbalance?

microRNA production in Arabidopsis

In plants, microRNAs (miRNAs) associate with ARGONAUTE (AGO) proteins and act as sequence-specific repressors of target gene expression, at the post-transcriptional level through target transcript cleavage and/or translational inhibition. MiRNAs are mainly transcribed by DNA-dependent RNA polymerase II (POL II) and processed by DICER LIKE1 (DCL1) complex into 21∼22 nucleotide (nt) long. Although the main molecular framework of miRNA biogenesis and modes of action have been established, there are still new requirements continually emerging in the recent years. The studies on the involvement factors in miRNA biogenesis indicate that miRNA biogenesis is not accomplished separately step by step, but is closely linked and dynamically regulated with each other. In this article, we will summarize the current knowledge on miRNA biogenesis, including MIR gene transcription, primary miRNA (pri-miRNA) processing, miRNA AGO1 loading and nuclear export; and miRNA metabolism including methylation, uridylation and turnover. We will describe how miRNAs are produced and how the different steps are regulated. We hope to raise awareness that the linkage between different steps and the subcellular regulation are becoming important for the understanding of plant miRNA biogenesis and modes of action.

Recent Insights into Plant miRNA Biogenesis: Multiple Layers of miRNA Level Regulation

MicroRNAs are small RNAs, 20-22 nt long, the main role of which is to downregulate gene expression at the level of mRNAs. MiRNAs are fundamental regulators of plant growth and development in response to internal signals as well as in response to abiotic and biotic factors. Therefore, the deficiency or excess of individual miRNAs is detrimental to particular aspects of a plant's life. In consequence, the miRNA levels must be appropriately adjusted. To obtain proper expression of each miRNA, their biogenesis is controlled at multiple regulatory layers. Here, we addressed processes discovered to influence miRNA steady-state levels, such as MIR transcription, co-transcriptional pri-miRNA processing (including splicing, polyadenylation, microprocessor assembly and activity) and miRNA-encoded peptides synthesis. MiRNA stability, RISC formation and miRNA export out of the nucleus and out of the plant cell also define the levels of miRNAs in various plant tissues. Moreover, we show the evolutionary conservation of miRNA biogenesis core proteins across the plant kingdom.

Characterization of anatomical features, developmental roadmaps, and key genes of bamboo leaf epidermis

The exact developmental roadmaps of bamboo leaf epidermis and the regulating genes are largely unknown. In this study, we comprehensively investigated the morphological features of the leaf epidermis of bamboo, Pseudosasa japonica. We also established the developmental roadmaps of the abaxial epidermis along the linearly growing leaf. A variant of P. japonica, P. japonica var. tsutsumiana, with smaller stomata and higher stomata density, was identified. Further analysis revealed that the higher stomata density of the variant was due to the abnormal increase in stomata columns within the single stomata band. This abnormal development of stomata bands was observed as early as the guard mother cell stage in the leaf division zone (DZ). Interestingly, the developmental pattern of the single stomata was similar in P. japonica and the variant. Molecular data showed that PjDLT (Dwarf and Low Tillering) was significantly downregulated in leaves DZ of the variant. Overexpression of PjDLT in Arabidopsis and rice results in smaller plants with lower stomata density, whereas downregulation or mutation of OsDLT results in increased stomata density. Our results highlight the morphological features and developmental schedule of the leaf epidermis of bamboo and provide evidence that DLT plays an important role in regulating stomata in bamboo and rice.

Molecular mechanism underlying the di-uridylation activity of Arabidopsis TUTase URT1

In Arabidopsis, HESO1 and URT1 act cooperatively on unmethylated miRNA and mRNA uridylation to induce their degradation. Their collaboration significantly impacts RNA metabolism in plants. However, the molecular mechanism determining the functional difference and complementarity of these two enzymes remains unclear. We previously solved the three-dimensional structure of URT1 in the absence and presence of UTP. In this study, we further determined the structure of URT1 in complex with a 5′-AAAU-3′ RNA stretch that mimics the post-catalytic state of the mRNA poly(A) tail after the addition of the first uridine. Structural analysis and enzymatic assays revealed that L527 and Y592 endow URT1 with a preference to interact with purine over pyrimidine at the -1 RNA binding position, thus controlling the optimal number of uridine added to the 3′ extremity of poly(A) as two. In addition, we observed that a large-scale conformational rearrangement in URT1 occurs upon binding with RNA from an ‘open’ to a ‘closed’ state. Molecular dynamic simulation supports an open-closed conformational selection mechanism employed by URT1 to interact with RNA substrates and maintain distributive enzymatic activity. Based on the above results, a model regarding the catalytic cycle of URT1 is proposed to explain its di-uridylation activity.

Uridylation and the SKI complex orchestrate the Calvin cycle of photosynthesis through RNA surveillance of TKL1 in Arabidopsis

RNA uridylation, catalyzed by terminal uridylyl transferases (TUTases), represents a conserved and widespread posttranscriptional RNA modification in eukaryotes that affects RNA metabolism. In plants, several TUTases, including HEN1 SUPPRESSOR 1 (HESO1) and UTP: RNA URIDYLYLTRANSFERASE (URT1), have been characterized through genetic and biochemical approaches. However, little is known about their physiological significance during plant development. Here, we show that HESO1 and URT1 act cooperatively with the cytoplasmic 3'-5' exoribonucleolytic machinery component SUPERKILLER 2 (SKI2) to regulate photosynthesis through RNA surveillance of the Calvin cycle gene TRANSKETOLASE 1 (TKL1) in Arabidopsis. Simultaneous dysfunction of HESO1, URT1, and SKI2 resulted in leaf etiolation and reduced photosynthetic efficiency. In addition, we detected massive illegitimate short interfering RNAs (siRNAs) from the TKL1 locus in heso1 urt1 ski2, accompanied by reduced TKL1/2 expression and attenuated TKL activities. Consequently, the metabolic analysis revealed that the abundance of many Calvin cycle intermediates is dramatically disturbed in heso1 urt1 ski2. Importantly, all these molecular and physiological defects were largely rescued by the loss-of-function mutation in RNA-DEPENDENT RNA POLYMERASE 6 (RDR6), demonstrating illegitimate siRNA-mediated TKL silencing. Taken together, our results suggest that HESO1- and URT1-mediated RNA uridylation connects to the cytoplasmic RNA degradation pathway for RNA surveillance, which is crucial for TKL expression and photosynthesis in Arabidopsis.

TENT2, TUT4, and TUT7 selectively regulate miRNA sequence and abundance

TENTs generate miRNA isoforms by 3' tailing. However, little is known about how tailing regulates miRNA function. Here, we generate isogenic HEK293T cell lines in which TENT2, TUT4 and TUT7 are knocked out individually or in combination. Together with rescue experiments, we characterize TENT-specific effects by deep sequencing, Northern blot and in vitro assays. We find that 3' tailing is not random but highly specific. In addition to its known adenylation, TENT2 contributes to guanylation and uridylation on mature miRNAs. TUT4 uridylates most miRNAs whereas TUT7 is dispensable. Removing adenylation has a marginal impact on miRNA levels. By contrast, abolishing uridylation leads to dysregulation of a set of miRNAs. Besides let-7, miR-181b and miR-222 are negatively regulated by TUT4/7 via distinct mechanisms while the miR-888 cluster is upregulated specifically by TUT7. Our results uncover the selective actions of TENTs in generating 3' isomiRs and pave the way to investigate their functions.

Current progress of miRNA-derivative nucleotide drugs: Modifications, delivery systems, applications

INTRODUCTION

miRNA-derivative clinical nucleotide drugs (mdCNDs) effectively treat several diseases, with numerous undergoing clinical trials. In early-stage trials in disease therapeutics such as malignant pleural mesothelioma and hepatic virus C infection, mdCND's therapeutic potency is undeniably good for effectiveness and safety.

AREAS COVERED

15 mdCNDs undergoing clinical trials are introduced in this review. MiRNA modifications methods have been summarized including phosphorothioate, cholesterol, locked nucleic acid, 2'-O-methyl, N,N-diethyl-4-(4-nitronaphthalen1-ylazo)-phenylamine modifications and many more. Moreover, delivery systems, including self-assembled, inorganic ions nanoparticles, exosomes, and lipid-based nanosystems for mdCNDs targeted delivery, are presented. Among that, EnGeneIC, N-Acetylgalactosamine, liposomal nanoparticles, and cholesterol-conjugated for mdCNDs delivery are currently undergoing clinical trials. The pH, light, temperature, redox-responsive, enzyme, and specific-substance modes to trigger the release of miRNAs to target sites on-demand and the prospects of mdCNDs are discussed in this review.

EXPERT OPINION

mdNCDs are one type of promising clinical drugs, however, it is still in the infancy. During the development process, it is imperative to advance in modifying miRNAs, especially at the 5'-end, to enhance targetability and stability against nucleases, develop a stimuli-responsive mode to control the release of mdCNDs to tissue cell-type-specific sites.

Mechanisms of MicroRNA Biogenesis and Stability Control in Plants

MicroRNAs (miRNAs), a class of endogenous, non-coding RNAs, which is 20-24 nucleotide long, regulate the expression of its target genes post-transcriptionally and play critical roles in plant normal growth, development, and biotic and abiotic stresses. In cells, miRNA biogenesis and stability control are important in regulating intracellular miRNA abundance. In addition, research on these two aspects has achieved fruitful results. In this review, we focus on the recent research progress in our understanding of miRNA biogenesis and their stability control in plants.

Transcriptome and Small RNA Profiling of Potato Virus Y Infected Potato Cultivars, Including Systemically Infected Russet Burbank

Potatoes are the world’s most produced non-grain crops and an important food source for billions of people. Potatoes are susceptible to numerous pathogens that reduce yield, including Potato virus Y (PVY). Genetic resistance to PVY is a sustainable way to limit yield and quality losses due to PVY infection. Potato cultivars vary in their susceptibility to PVY and include susceptible varieties such as Russet Burbank, and resistant varieties such as Payette Russet. Although the loci and genes associated with PVY-resistance have been identified, the genes and mechanisms involved in limiting PVY during the development of systemic infections have yet to be fully elucidated. To increase our understanding of PVY infection, potato antiviral responses, and resistance, we utilized RNA sequencing to characterize the transcriptomes of two potato cultivars. Since transcriptional responses associated with the extreme resistance response occur soon after PVY contact, we analyzed the transcriptome and small RNA profile of both the PVY-resistant Payette Russet cultivar and PVY-susceptible Russet Burbank cultivar 24 h post-inoculation. While hundreds of genes, including terpene synthase and protein kinase encoding genes, exhibited increased expression, the majority, including numerous genes involved in plant pathogen interactions, were downregulated. To gain greater understanding of the transcriptional changes that occur during the development of systemic PVY-infection, we analyzed Russet Burbank leaf samples one week and four weeks post-inoculation and identified similarities and differences, including higher expression of genes involved in chloroplast function, photosynthesis, and secondary metabolite production, and lower expression of defense response genes at those time points. Small RNA sequencing identified different populations of 21- and 24-nucleotide RNAs and revealed that the miRNA profiles in PVY-infected Russet Burbank plants were similar to those observed in other PVY-tolerant cultivars and that during systemic infection ~32% of the NLR-type disease resistance genes were targeted by 21-nt small RNAs. Analysis of alternative splicing in PVY-infected potato plants identified splice variants of several hundred genes, including isoforms that were more dominant in PVY-infected plants. The description of the PVYN-Wi-associated transcriptome and small RNA profiles of two potato cultivars described herein adds to the body of knowledge regarding differential outcomes of infection for specific PVY strain and host cultivar pairs, which will help further understanding of the mechanisms governing genetic resistance and/or virus-limiting responses in potato plants.

An atypical class of non-coding small RNAs is produced in rice leaves upon bacterial infection

Non-coding small RNAs (sRNA) act as mediators of gene silencing and regulate plant growth, development and stress responses. Early insights into plant sRNAs established a role in antiviral defense and they are now extensively studied across plant-microbe interactions. Here, sRNA sequencing discovered a class of sRNA in rice (Oryza sativa) specifically associated with foliar diseases caused by Xanthomonas oryzae bacteria. Xanthomonas-induced small RNAs (xisRNAs) loci were distinctively upregulated in response to diverse virulent strains at an early stage of infection producing a single duplex of 20-22 nt sRNAs. xisRNAs production was dependent on the Type III secretion system, a major bacterial virulence factor for host colonization. xisRNA loci overlap with annotated transcripts sequences, with about half of them encoding protein kinase domain proteins. A number of the corresponding rice cis-genes have documented functions in immune signaling and xisRNA loci predominantly coincide with the coding sequence of a conserved kinase motif. xisRNAs exhibit features of small interfering RNAs and their biosynthesis depend on canonical components OsDCL1 and OsHEN1. xisRNA induction possibly mediates post-transcriptional gene silencing but they do not broadly suppress cis-genes expression on the basis of mRNA-seq data. Overall, our results identify a group of unusual sRNAs with a potential role in plant-microbe interactions.

Up-to-date on the evidence linking miRNA-related epitranscriptomic modifications and disease settings. Can these modifications affect cross-kingdom regulation?

The field of epitranscriptomics is rapidly developing. Several modifications (e.g. methylations) have been identified for different RNA types. Current evidence shows that chemical RNA modifications can influence the whole molecule's secondary structure, translatability, functionality, stability, and degradation, and some are dynamically and reversibly modulated. miRNAs, in particular, are not only post-transcriptional modulators of gene expression but are themselves submitted to regulatory mechanisms. Understanding how these modifications are regulated and the resulting pathological consequences when dysregulation occurs is essential for the development of new therapeutic targets. In humans and other mammals, dietary components have been shown to affect miRNA expression and may also induce chemical modifications in miRNAs. The identification of chemical modifications in miRNAs (endogenous and exogenous) that can impact host gene expression opens up an alternative way to select new specific therapeutic targets.Hence, the aim of this review is to briefly address how RNA epitranscriptomic modifications can affect miRNA biogenesis and to summarize the existing evidence showing the connection between the (de)regulation of these processes and disease settings. In addition, we hypothesize on the potential effect certain chemical modifications could have on the potential cross-kingdom journey of dietary plant miRNAs.

MicroRNAs: Tiny, powerful players of metal stress responses in plants

Metal contamination of the environment is a widespread problem threatening sustainable and safe crop production. Physio-biochemical and molecular mechanisms of plant responses to metal exposure have been studied to establish the best possible agronomical or biotechnological methods to tackle metal contamination. Metal stress tolerance is regulated by several molecular effectors among which microRNAs are one of the key master regulators of plant growth and stress responses in plants. MicroRNAs are known to coordinate multitude of plant responses to metal stress through antioxidant functions, root growth, hormonal signalling, transcription factors and metal transporters. The present review discusses integrative functions of microRNAs in the regulation of metal stress in plants, which will be useful for engineering stress tolerance traits for improved plant growth and productivity in metal stressed situations.

The TUTase URT1 connects decapping activators and prevents the accumulation of excessively deadenylated mRNAs to avoid siRNA biogenesis

Uridylation is a widespread modification destabilizing eukaryotic mRNAs. Yet, molecular mechanisms underlying TUTase-mediated mRNA degradation remain mostly unresolved. Here, we report that the Arabidopsis TUTase URT1 participates in a molecular network connecting several translational repressors/decapping activators. URT1 directly interacts with DECAPPING 5 (DCP5), the Arabidopsis ortholog of human LSM14 and yeast Scd6, and this interaction connects URT1 to additional decay factors like DDX6/Dhh1-like RNA helicases. Nanopore direct RNA sequencing reveals a global role of URT1 in shaping poly(A) tail length, notably by preventing the accumulation of excessively deadenylated mRNAs. Based on in vitro and in planta data, we propose a model that explains how URT1 could reduce the accumulation of oligo(A)-tailed mRNAs both by favoring their degradation and because 3' terminal uridines intrinsically hinder deadenylation. Importantly, preventing the accumulation of excessively deadenylated mRNAs avoids the biogenesis of illegitimate siRNAs that silence endogenous mRNAs and perturb Arabidopsis growth and development.

Extensive Analysis of miRNA Trimming and Tailing Indicates that AGO1 Has a Complex Role in miRNA Turnover

MicroRNAs are small regulatory RNAs involved in several processes in plants ranging from development and stress responses to defense against pathogens. In order to accomplish their molecular functions, miRNAs are methylated and loaded into one ARGONAUTE (AGO) protein, commonly known as AGO1, to stabilize and protect the molecule and to assemble a functional RNA-induced silencing complex (RISC). A specific machinery controls miRNA turnover to ensure the silencing release of targeted-genes in given circumstances. The trimming and tailing of miRNAs are fundamental modifications related to their turnover and, hence, to their action. In order to gain a better understanding of these modifications, we analyzed Arabidopsis thaliana small RNA sequencing data from a diversity of mutants, related to miRNA biogenesis, action, and turnover, and from different cellular fractions and immunoprecipitations. Besides confirming the effects of known players in these pathways, we found increased trimming and tailing in miRNA biogenesis mutants. More importantly, our analysis allowed us to reveal the importance of ARGONAUTE 1 (AGO1) loading, slicing activity, and cellular localization in trimming and tailing of miRNAs.

Genome-wide analysis of CCHC-type zinc finger (ZCCHC) proteins in yeast, Arabidopsis, and humans

The diverse eukaryotic proteins that contain zinc fingers participate in many aspects of nucleic acid metabolism, from DNA transcription to RNA degradation, post-transcriptional gene silencing, and small RNA biogenesis. These proteins can be classified into at least 30 types based on structure. In this review, we focus on the CCHC-type zinc fingers (ZCCHC), which contain an 18-residue domain with the CX2CX4HX4C sequence, where C is cysteine, H is histidine, and X is any amino acid. This motif, also named the "zinc knuckle", is characteristic of the retroviral Group Antigen protein and occurs alone or with other motifs. Many proteins containing zinc knuckles have been identified in eukaryotes, but only a few have been studied. Here, we review the available information on ZCCHC-containing factors from three evolutionarily distant eukaryotes-Saccharomyces cerevisiae, Arabidopsis thaliana, and Homo sapiens-representing fungi, plants, and metazoans, respectively. We performed systematic searches for proteins containing the CX2CX4HX4C sequence in organism-specific and generalist databases. Next, we analyzed the structural and functional information for all such proteins stored in UniProtKB. Excluding retrotransposon-encoded proteins and proteins harboring uncertain ZCCHC motifs, we found seven ZCCHC-containing proteins in yeast, 69 in Arabidopsis, and 34 in humans. ZCCHC-containing proteins mainly localize to the nucleus, but some are nuclear and cytoplasmic, or exclusively cytoplasmic, and one localizes to the chloroplast. Most of these factors participate in RNA metabolism, including transcriptional elongation, polyadenylation, translation, pre-messenger RNA splicing, RNA export, RNA degradation, microRNA and ribosomal RNA biogenesis, and post-transcriptional gene silencing. Several human ZCCHC-containing factors are derived from neofunctionalized retrotransposons and act as proto-oncogenes in diverse neoplastic processes. The conservation of ZCCHCs in orthologs of these three phylogenetically distant eukaryotes suggests that these domains have biologically relevant functions that are not well known at present.

Hyponastic Leaves 1 protects pri-miRNAs from nuclear exosome attack

Biogenesis of plant microRNAs (miRNAs) takes place in nuclear dicing bodies (D-bodies), where the ribonulease III-type enzyme Dicer-like 1 (DCL1) processes primary transcripts of miRNAs (pri-miRNAs) into miRNA/miRNA* (*, passenger strand) duplexes from either base-to-loop or loop-to-base directions. Hyponastic Leaves 1 (HYL1), a double-stranded RNA-binding protein, is crucial for efficient and accurate processing. However, whether HYL1 has additional function remains unknown. Here, we report that HYL1 plays a noncanonical role in protecting pri-miRNAs from nuclear exosome attack in addition to ensuring processing. Loss of functions in SOP1 or HEN2, two cofactors of the nucleoplasmic exosome, significantly suppressed the morphological phenotypes of hyl1-2 Remarkably, mature miRNAs generated from loop-to-base processing were partially but preferentially restored in the hyl1 sop1 and hyl1 hen2 double mutants. Accordingly, loop-to-base-processed pri-miRNAs accumulated to higher levels in double mutants. In addition, dysfunction of HEN2, but not of SOP1, in hyl1-2 resulted in overaccumulation of many base-to-loop-processed pri-miRNAs, with most of their respective miRNAs unaffected. In summary, our findings reveal an antagonistic action of exosome in pri-miRNA biogenesis and uncover dual roles of HYL1 in stabilizing and processing of pri-miRNAs.

AGO-bound mature miRNAs are oligouridylated by TUTs and subsequently degraded by DIS3L2

MicroRNAs (miRNAs) associated with Argonaute proteins (AGOs) regulate gene expression in mammals. miRNA 3' ends are subject to frequent sequence modifications, which have been proposed to affect miRNA stability. However, the underlying mechanism is not well understood. Here, by genetic and biochemical studies as well as deep sequencing analyses, we find that AGO mutations disrupting miRNA 3' binding are sufficient to trigger extensive miRNA 3' modifications in HEK293T cells and in cancer patients. Comparing these modifications in TUT4, TUT7 and DIS3L2 knockout cells, we find that TUT7 is more robust than TUT4 in oligouridylating mature miRNAs, which in turn leads to their degradation by the DIS3L2 exonuclease. Our findings indicate a decay machinery removing AGO-associated miRNAs with an exposed 3' end. A set of endogenous miRNAs including miR-7, miR-222 and miR-769 are targeted by this machinery presumably due to target-directed miRNA degradation.

AtXRN4 Affects the Turnover of Chosen miRNA*s in Arabidopsis

Small RNA (sRNA) turnover is a key but poorly understood mechanism that determines the homeostasis of sRNAs. Animal XRN genes contribute the degradation of sRNAs, AtXRN2 and AtXRN3 also contribute the pri-miRNA processing and miRNA loop degradation in plants. However, the possible functions of the plant XRN genes in sRNA degradation are far from known. Here, we find that AtXRN4 contributes the turnover of plant sRNAs in Arabidopsis thaliana mainly by sRNA-seq, qRT-PCR and Northern blot. The mutation of AtXRN4 alters the sRNA profile and the accumulation of 21 nt sRNAs was increased. Some miRNA*s levels are significantly increased in xrn4 mutant plants. However, the accumulation of the primary miRNAs (pri-miRNAs) and miRNA precursors (pre-miRNAs) were generally unchanged in xrn4 mutant plants which indicates that AtXRN4 contributes the degradation of some miRNA*s. Moreover, AtXRN4 interacts with Arabidopsis Argonaute 2 (AtAGO2). This interaction takes place in Processing bodies (P-bodies). Taken together, our observations identified the interaction between XRN4 with AtAGO2 and suggested that plant XRN4 also contributes the turnover of sRNAs.

Crystal structure of Arabidopsis terminal uridylyl transferase URT1

3' uridylation is an essential modification associated with coding and noncoding RNA degradation in eukaryotes. In Arabidopsis, HESO1 was first identified as the major nucleotidyl transferase that uridylates most unmethylated miRNAs, and URT1 was later reported to play a redundant but important role in miRNA uridylation when HESO1 is absent. Two enzymes work sequentially and collaboratively to tail different forms of the same miRNAs in vivo. For mRNA, however, URT1 becomes the main enzyme to uridylate the majority of mRNA and repairs their deadenylated ends to restore the binding site for Poly(A) Binding Protein (PABP). HESO1, on the other hand, targets mostly the mRNAs with very short oligo(A) tails and fails in fulfilling the same task. To understand the structural basis these two functional homologues possess for their different substrate preferences and catalytic behaviors, we first determined the crystal structures of URT1 in the absence and presence of UTP. Our structures, together with functional assay and sequence analysis, indicated that URT1 has a conserved UTP-recognition mechanism analogue to the terminal uridylyl transferases from other species whereas HESO1 may evolve separately to recognize UTP in a different way. Moreover, URT1 N552 may be an important residue in interacting with 3' nucleotide of RNA substrate. The URT1 structure we determined represents the first structure of uridylyl transferase from plants, shedding light on the mechanisms of URT1/HESO1-dependent RNA metabolism.

Prevalent cytidylation and uridylation of precursor miRNAs in Arabidopsis

A key step in microRNA biogenesis is the processing of a primary precursor RNA by the microprocessor into a precursor miRNA (pre-miRNA) intermediate. In plants, little is known about the processes that act on pre-miRNAs to influence miRNA biogenesis. Here, we performed 3' rapid amplification of complementary DNA ends sequencing to profile pre-miRNA 3' ends in Arabidopsis. 3' end heterogeneity was prevalent, and the three microprocessor components promoted 3' end precision. Extensive cytidylation and uridylation of precise and imprecise pre-miRNA 3' ends were uncovered. The nucleotidyl transferase HESO1 uridylated pre-miRNAs in vitro and was responsible for most pre-miRNA uridylation in vivo. HESO1, NTP6 and NTP7 contribute to pre-miRNA cytidylation. Tailing of pre-miRNAs tended to restore trimmed pre-miRNAs to their intact length to promote further processing. In addition, HESO1-mediated uridylation led to the degradation of certain imprecisely processed pre-miRNAs. Thus, we uncovered widespread cytidylation and uridylation of pre-miRNAs and demonstrated diverse functions of pre-miRNA tailing in plants.

Extensive profiling in Arabidopsis reveals abundant polysome-associated 24-nt small RNAs including AGO5-dependent pseudogene-derived siRNAs

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.

Analysis of Methylation Status of Plant MicroRNAs

Small noncoding RNAs of 20-30 nucleotides in length are key mediators of gene silencing. 2'-O-Methylation on the 3' terminal nucleotide of several types of small RNAs, including microRNAs (miRNAs) and small interfering RNAs (siRNAs) in plants, PIWI-interacting RNAs (piRNAs) in animals, and some siRNAs in Drosophila and Caenorhabditis elegans, provides a key protective mechanism against 3' tailing- and trimming-mediated destabilization. The methylation reaction is catalyzed by the small RNA methyltransferase HUA ENHANCER 1 (HEN1). In this chapter, we describe a detailed protocol for analyzing 3' end methylation status of plant miRNAs, which can be applicable to other types of small RNAs as well.

MicroRNAs and Their Regulatory Roles in Plant-Environment Interactions

MicroRNAs (miRNAs) are 20-24 nucleotide noncoding RNAs abundant in plants and animals. The biogenesis of plant miRNAs involves transcription of miRNA genes, processing of primary miRNA transcripts by DICER-LIKE proteins into mature miRNAs, and loading of mature miRNAs into ARGONAUTE proteins to form miRNA-induced silencing complex (miRISC). By targeting complementary sequences, miRISC negatively regulates gene expression, thereby coordinating plant development and plant-environment interactions. In this review, we present and discuss recent updates on the mechanisms and regulation of miRNA biogenesis, miRISC assembly and actions as well as the regulatory roles of miRNAs in plant developmental plasticity, abiotic/biotic responses, and symbiotic/parasitic interactions. Finally, we suggest future directions for plant miRNA research.

Plant microRNAs: Biogenesis, Homeostasis, and Degradation

MicroRNAs (miRNAs), a class of endogenous, tiny, non-coding RNAs, are master regulators of gene expression among most eukaryotes. Intracellular miRNA abundance is regulated under multiple levels of control including transcription, processing, RNA modification, RNA-induced silencing complex (RISC) assembly, miRNA-target interaction, and turnover. In this review, we summarize our current understanding of the molecular components and mechanisms that influence miRNA biogenesis, homeostasis, and degradation in plants. We also make comparisons with findings from other organisms where necessary.

RNA uridylation and decay in plants

RNA uridylation consists of the untemplated addition of uridines at the 3' extremity of an RNA molecule. RNA uridylation is catalysed by terminal uridylyltransferases (TUTases), which form a subgroup of the terminal nucleotidyltransferase family, to which poly(A) polymerases also belong. The key role of RNA uridylation is to regulate RNA degradation in a variety of eukaryotes, including fission yeast, plants and animals. In plants, RNA uridylation has been mostly studied in two model species, the green algae Chlamydomonas reinhardtii and the flowering plant Arabidopsis thaliana Plant TUTases target a variety of RNA substrates, differing in size and function. These RNA substrates include microRNAs (miRNAs), small interfering silencing RNAs (siRNAs), ribosomal RNAs (rRNAs), messenger RNAs (mRNAs) and mRNA fragments generated during post-transcriptional gene silencing. Viral RNAs can also get uridylated during plant infection. We describe here the evolutionary history of plant TUTases and we summarize the diverse molecular functions of uridylation during RNA degradation processes in plants. We also outline key points of future research.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.

Respective Contributions of URT1 and HESO1 to the Uridylation of 5' Fragments Produced From RISC-Cleaved mRNAs

In plants, post-transcriptional gene silencing (PTGS) represses gene expression by translation inhibition and cleavage of target mRNAs. The slicing activity is provided by argonaute 1 (AGO1), and the cleavage site is determined by sequence complementarity between the target mRNA and the microRNA (miRNA) or short interfering RNA (siRNA) loaded onto AGO1, to form the core of the RNA induced silencing complex (RISC). Following cleavage, the resulting 5' fragment is modified at its 3' end by the untemplated addition of uridines. Uridylation is proposed to facilitate RISC recycling and the degradation of the RISC 5'-cleavage fragment. Here, we detail a 3' RACE-seq method to analyze the 3' ends of 5' fragments produced from RISC-cleaved transcripts. The protocol is based on the ligation of a primer at the 3' end of RNA, followed by cDNA synthesis and the subsequent targeted amplification by PCR to generate amplicon libraries suitable for Illumina sequencing. A detailed data processing pipeline is provided to analyze nibbling and tailing at high resolution. Using this method, we compared the tailing and nibbling patterns of RISC-cleaved MYB33 and SPL13 transcripts between wild-type plants and mutant plants depleted for the terminal uridylyltransferases (TUTases) HESO1 and URT1. Our data reveal the respective contributions of HESO and URT1 in the uridylation of RISC-cleaved MYB33 and SPL13 transcripts, with HESO1 being the major TUTase involved in uridylating these fragments. Because of its depth, the 3' RACE-seq method shows at high resolution that these RISC-generated 5' RNA fragments are nibbled by a few nucleotides close to the cleavage site in the absence of uridylation. 3' RACE-seq is a suitable approach for a reliable comparison of uridylation and nibbling patterns between mutants, a prerequisite to the identification of all factors involved in the clearance of RISC-generated 5' mRNA fragments.

Target RNAs Strike Back on MicroRNAs

MicroRNAs are extensively studied regulatory non-coding small RNAs that silence animal genes throughout most biological processes, typically doing so by binding to partially complementary sequences within target RNAs. A plethora of studies has described detailed mechanisms for microRNA biogenesis and function, as well as their temporal and spatial regulation during development. By inducing translational repression and/or degradation of their target RNAs, microRNAs can contribute to achieve highly specific cell- or tissue-specific gene expression, while their aberrant expression can lead to disease. Yet an unresolved aspect of microRNA biology is how such small RNA molecules are themselves cleared from the cell, especially under circumstances where fast microRNA turnover or specific degradation of individual microRNAs is required. In recent years, it was unexpectedly found that binding of specific target RNAs to microRNAs with extensive complementarity can reverse the outcome, triggering degradation of the bound microRNAs. This emerging pathway, named TDMD for Target RNA-Directed MicroRNA Degradation, leads to microRNA 3'-end tailing by the addition of A/U non-templated nucleotides, trimming or shortening from the 3' end, and highly specific microRNA loss, providing a new layer of microRNA regulation. Originally described in flies and known to be triggered by viral RNAs, novel endogenous instances of TDMD have been uncovered and are now starting to be understood. Here, we review our current knowledge of this pathway and its potential role in the control and diversification of microRNA expression patterns.

Mechanisms of microRNA-mediated gene regulation in unicellular model alga Chlamydomonas reinhardtii

MicroRNAs are a class of endogenous non-coding RNAs that play a vital role in post-transcriptional gene regulation in eukaryotic cells. In plants and animals, miRNAs are implicated in diverse roles ranging from immunity against viral infections, developmental pathways, molecular pathology of cancer and regulation of protein expression. However, the role of miRNAs in the unicellular model green alga Chlamydomonas reinhardtii remains unclear. The mode of action of miRNA-induced gene silencing in C. reinhardtii is very similar to that of higher eukaryotes, in terms of the activation of the RNA-induced silencing complex and mRNA targeting. Certain studies indicate that destabilization of mRNAs and mRNA turnover could be the major possible functions of miRNAs in eukaryotic algae. Here, we summarize recent findings that have advanced our understanding of miRNA regulatory mechanisms in C. reinhardtii.

Structural and biochemical insights into small RNA 3' end trimming by Arabidopsis SDN1

A family of DEDDh 3′→5′ exonucleases known as Small RNA Degrading Nucleases (SDNs) initiates the turnover of ARGONAUTE1 (AGO1)-bound microRNAs in Arabidopsis by trimming their 3′ ends. Here, we report the crystal structure of Arabidopsis SDN1 (residues 2-300) in complex with a 9 nucleotide single-stranded RNA substrate, revealing that the DEDDh domain forms rigid interactions with the N-terminal domain and binds 4 nucleotides from the 3′ end of the RNA via its catalytic pocket. Structural and biochemical results suggest that the SDN1 C-terminal domain adopts an RNA Recognition Motif (RRM) fold and is critical for substrate binding and enzymatic processivity of SDN1. In addition, SDN1 interacts with the AGO1 PAZ domain in an RNA-independent manner in vitro, enabling it to act on AGO1-bound microRNAs. These extensive structural and biochemical studies may shed light on a common 3′ end trimming mechanism for 3′→5′ exonucleases in the metabolism of small non-coding RNAs.

Small RNA degrading nucleases (SDNs) can degrade short RNAs. Here the authors report the crystal structure of Arabidopsis SDN1 in complex with a single-stranded RNA, and provide new insight into 3′ end trimming mechanism of 3′ to 5′ riboexonucleases in the metabolism of various species of small RNAs.

Biogenesis of a 22-nt microRNA in Phaseoleae species by precursor-programmed uridylation

Phased, secondary siRNAs (phasiRNAs) represent a class of small RNAs in plants generated via distinct biogenesis pathways, predominantly dependent on the activity of 22-nt miRNAs. Most 22-nt miRNAs are processed by DCL1 from miRNA precursors containing an asymmetric bulge, yielding a 22/21-nt miRNA/miRNA* duplex. Here we show that miR1510, a soybean miRNA capable of triggering phasiRNA production from numerous nucleotide-binding leucine-rich repeat (NB-LRRs), previously described as 21 nt in its mature form, primarily accumulates as a 22-nt isoform via monouridylation. We demonstrate that, in Arabidopsis, this uridylation is performed by HESO1. Biochemical experiments showed that the 3' terminus of miR1510 is only partially 2'-O-methylated because of the terminal mispairing in the miR1510/miR1510* duplex that inhibits HEN1 activity in soybean. miR1510 emerged in the Phaseoleae ∼41-42 million years ago with a conserved precursor structure yielding a 22-nt monouridylated form, yet a variant in mung bean is processed directly in a 22-nt mature form. This analysis of miR1510 yields two observations: (i) plants can utilize postprocessing modification to generate abundant 22-nt miRNA isoforms to more efficiently regulate target mRNA abundances; and (ii) comparative analysis demonstrates an example of selective optimization of precursor processing of a young plant miRNA.

Biogenesis of a 22-nt microRNA in Phaseoleae species by precursor-programmed uridylation

Phased, secondary siRNAs (phasiRNAs) represent a class of small RNAs in plants generated via distinct biogenesis pathways, predominantly dependent on the activity of 22-nt miRNAs. Most 22-nt miRNAs are processed by DCL1 from miRNA precursors containing an asymmetric bulge, yielding a 22/21-nt miRNA/miRNA* duplex. Here we show that miR1510, a soybean miRNA capable of triggering phasiRNA production from numerous nucleotide-binding leucine-rich repeat (NB-LRRs), previously described as 21 nt in its mature form, primarily accumulates as a 22-nt isoform via monouridylation. We demonstrate that, in Arabidopsis, this uridylation is performed by HESO1. Biochemical experiments showed that the 3' terminus of miR1510 is only partially 2'-O-methylated because of the terminal mispairing in the miR1510/miR1510* duplex that inhibits HEN1 activity in soybean. miR1510 emerged in the Phaseoleae ∼41-42 million years ago with a conserved precursor structure yielding a 22-nt monouridylated form, yet a variant in mung bean is processed directly in a 22-nt mature form. This analysis of miR1510 yields two observations: (i) plants can utilize postprocessing modification to generate abundant 22-nt miRNA isoforms to more efficiently regulate target mRNA abundances; and (ii) comparative analysis demonstrates an example of selective optimization of precursor processing of a young plant miRNA.

Degradation of unmethylated miRNA/miRNA*s by a DEDDy-type 3' to 5' exoribonuclease Atrimmer 2 in Arabidopsis

The 3' end methylation catalyzed by HUA Enhancer 1 (HEN1) is a crucial step of small RNA stabilization in plants, yet how unmethylated small RNAs undergo degradation remains largely unknown. Using a reverse genetic approach, we here show that Atrimmer 2 (ATRM2), a DEDDy-type 3' to 5' exoribonuclease, acts in the degradation of unmethylated miRNAs and miRNA*s in Arabidopsis Loss-of-function mutations in ATRM2 partially suppress the morphological defects caused by HEN1 malfunction, with restored levels of a subset of miRNAs and receded expression of corresponding miRNA targets. Dysfunction of ATRM2 has negligible effect on miRNA trimming, and further increase the fertility of hen1 heso1 urt1, a mutant with an almost complete abolishment of miRNA uridylation, indicating that ATRM2 may neither be involved in 3' to 5' trimming nor be the enzyme that specifically degrades uridylated miRNAs. Notably, the fold changes of miRNAs and their corresponding miRNA*s were significantly correlated in hen1 atrm2 versus hen1 Unexpectedly, we observed a marked increase of 3' to 5' trimming of several miRNA*s but not miRNAs in ATRM2 compromised backgrounds. These data suggest an action of ATRM2 on miRNA/miRNA* duplexes, and the existence of an unknown exoribonuclease for specific trimming of miRNA*. This asymmetric effect on miRNA/miRNA* is likely related to Argonaute (AGO) proteins, which can distinguish miRNAs from miRNA*s. Finally, we show that ATRM2 colocalizes and physically interacts with Argonaute 1 (AGO1). Taken together, our results suggest that ATRM2 may be involved in the surveillance of unmethylated miRNA/miRNA* duplexes during the initiation step of RNA-induced silencing complex assembly.

A Suppressor Screen for AGO1 Degradation by the Viral F-Box P0 Protein Uncovers a Role for AGO DUF1785 in sRNA Duplex Unwinding

In Arabidopsis thaliana, ARGONAUTE1 (AGO1) plays a central role in microRNA (miRNA) and small interfering RNA (siRNA)-mediated silencing and is a key component in antiviral responses. The polerovirus F-box P0 protein triggers AGO1 degradation as a viral counterdefense. Here, we identified a motif in AGO1 that is required for its interaction with the S phase kinase-associated protein1-cullin 1-F-box protein (SCF) P0 (SCF^P0) complex and subsequent degradation. The AGO1 P0 degron is conserved and confers P0-mediated degradation to other AGO proteins. Interestingly, the degron motif is localized in the DUF1785 domain of AGO1, in which a single point mutation (ago1-57, obtained by forward genetic screening) compromises recognition by SCF^P0 Recapitulating formation of the RNA-induced silencing complex in a cell-free system revealed that this mutation impairs RNA unwinding, leading to stalled forms of AGO1 still bound to double-stranded RNAs. In vivo, the DUF1785 is required for unwinding perfectly matched siRNA duplexes, but is mostly dispensable for unwinding imperfectly matched miRNA duplexes. Consequently, its mutation nearly abolishes phased siRNA production and sense transgene posttranscriptional gene silencing. Overall, our work sheds new light on the mode of AGO1 recognition by P0 and the in vivo function of DUF1785 in RNA silencing.

The 'how' and 'where' of plant microRNAs

Contents 1002 I. 1002 II. 1007 III. 1010 IV. 1013 1013 References 1013 SUMMARY: MicroRNAs (miRNAs) are small non-coding RNAs, of typically 20-24 nt, that regulate gene expression post-transcriptionally through sequence complementarity. Since the identification of the first miRNA, lin-4, in the nematode Caenorhabditis elegans in 1993, thousands of miRNAs have been discovered in animals and plants, and their regulatory roles in numerous biological processes have been uncovered. In plants, research efforts have established the major molecular framework of miRNA biogenesis and modes of action, and are beginning to elucidate the mechanisms of miRNA degradation. Studies have implicated restricted and surprising subcellular locations in which miRNA biogenesis or activity takes place. In this article, we summarize the current knowledge on how plant miRNAs are made and degraded, and how they repress target gene expression. We discuss not only the players involved in these processes, but also the subcellular sites in which these processes are known or implicated to take place. We hope to raise awareness that the cell biology of miRNAs holds the key to a full understanding of these enigmatic molecules.

RNA uridylation: a key posttranscriptional modification shaping the coding and noncoding transcriptome

RNA uridylation is a potent and widespread posttranscriptional regulator of gene expression. RNA uridylation has been detected in a range of eukaryotes including trypanosomes, animals, plants, and fungi, but with the noticeable exception of budding yeast. Virtually all classes of eukaryotic RNAs can be uridylated and uridylation can also tag viral RNAs. The untemplated addition of a few uridines at the 3' end of a transcript can have a decisive impact on RNA's fate. In rare instances, uridylation is an intrinsic step in the maturation of noncoding RNAs like for the U6 spliceosomal RNA or mitochondrial guide RNAs in trypanosomes. Uridylation can also switch specific miRNA precursors from a degradative to a processing mode. This switch depends on the number of uridines added which is regulated by the cellular context. Yet, the typical consequence of uridylation on mature noncoding RNAs or their precursors is to accelerate decay. Importantly, mRNAs are also tagged by uridylation. In fact, the advent of novel high throughput sequencing protocols has recently revealed the pervasiveness of mRNA uridylation, from plants to humans. As for noncoding RNAs, the main function to date for mRNA uridylation is to promote degradation. Yet, additional roles begin to be ascribed to U-tailing such as the control of mRNA deadenylation, translation control and possibly storage. All these new findings illustrate that we are just beginning to appreciate the diversity of roles played by RNA uridylation and its full temporal and spatial implication in regulating gene expression. WIREs RNA 2018, 9:e1440. doi: 10.1002/wrna.1440 This article is categorized under: RNA Processing > 3' End Processing RNA Processing > RNA Editing and Modification RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms.

Conservation and divergence of small RNA pathways and microRNAs in land plants

Background

As key regulators of gene expression in eukaryotes, small RNAs have been characterized in many seed plants, and pathways for their biogenesis, degradation, and action have been defined in model angiosperms. However, both small RNAs themselves and small RNA pathways are not well characterized in other land plants such as lycophytes and ferns, preventing a comprehensive evolutionary perspective on small RNAs in land plants.

Results

Using 25 representatives from major lineages of lycophytes and ferns, most of which lack sequenced genomes, we characterized small RNAs and small RNA pathways in these plants. We identified homologs of DICER-LIKE (DCL), ARGONAUTE (AGO), and other genes involved in small RNA pathways, predicted over 2600 conserved microRNA (miRNA) candidates, and performed phylogenetic analyses on small RNA pathways as well as miRNAs. Pathways underlying miRNA biogenesis, degradation, and activity were established in the common ancestor of land plants, but the 24-nucleotide siRNA pathway that guides DNA methylation is incomplete in sister species of seed plants, especially lycophytes. We show that the functional diversification of key gene families such as DCL and AGO as observed in angiosperms occurred early in land plants followed by parallel expansion of the AGO family in ferns and angiosperms. We uncovered a conserved AGO subfamily absent in angiosperms.

Conclusions

Our phylogenetic analyses of miRNAs in bryophytes, lycophytes, ferns, and angiosperms refine the time-of-origin for conserved miRNA families as well as small RNA machinery in land plants.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1291-2) contains supplementary material, which is available to authorized users.

RISC-interacting clearing 3'- 5' exoribonucleases (RICEs) degrade uridylated cleavage fragments to maintain functional RISC in Arabidopsis thaliana

RNA-induced silencing complex (RISC) is composed of miRNAs and AGO proteins. AGOs use miRNAs as guides to slice target mRNAs to produce truncated 5' and 3' RNA fragments. The 5' cleaved RNA fragments are marked with uridylation for degradation. Here, we identified novel cofactors of Arabidopsis AGOs, named RICE1 and RICE2. RICE proteins specifically degraded single-strand (ss) RNAs in vitro; but neither miRNAs nor miRNA*s in vivo. RICE1 exhibited a DnaQ-like exonuclease fold and formed a homohexamer with the active sites located at the interfaces between RICE1 subunits. Notably, ectopic expression of catalytically-inactive RICE1 not only significantly reduced miRNA levels; but also increased 5' cleavage RISC fragments with extended uridine tails. We conclude that RICEs act to degrade uridylated 5’ products of AGO cleavage to maintain functional RISC. Our study also suggests a possible link between decay of cleaved target mRNAs and miRNA stability in RISC.

DOI:http://dx.doi.org/10.7554/eLife.24466.001

DNA contains all the information needed to build a body, yet molecules of RNA carry these instructions to the sites in the cell where they can be used. Cells must control how much RNA they produce in order to ensure that they develop properly and can respond well to their environment. RNA silencing refers to a collection of mechanisms that use smaller RNA molecules called microRNAs to incapacitate certain RNA molecules and selectively switch off the genes that encode them to stop more from being made.

One key player in RNA silencing is the multi-protein complex called RISC, which contains microRNA and a group of proteins called AGOs. Once the microRNA has identified its RNA target, the AGOs cut the RNA into two pieces, known as the 5’ cleavage fragment and 3’ cleavage fragment. The two resulting fragments need to be cleared away swiftly, so that the RISC can move on to the next target. While it was known how the 3’ cleavage fragment was removed, it was less clear how the 5’ cleavage fragment was dealt with.

Previous studies had shown that the 5’ cleavage fragment was marked with a chemical called uridine, which somehow signals to the RISC that this fragment needs to be destroyed. Now, using biochemical techniques, Zhang et al. have identified two new proteins in the model plant Arabidopsis that attach to the AGO proteins and degrade the 5’ cleavage fragments that are marked with uridine. The two proteins are named RICE1 and RICE2.

Zhang et al. then analyzed the three-dimensional shape of RICE1 and identified the ‘active’ region that is responsible for degrading the RNA fragments. When these active regions were blocked, the microRNA levels were low, but the uridine-marked 5’ cleavage fragments were high. Also, the RISC complex could not work properly, which lead to problems during the development of the plant. These results suggest that RICE proteins degrade 5’ cleavage fragments modified with uridine to activate RISC.

RICE proteins are conserved between plants and animals, and it is likely that their counterparts in humans will have a similar role to the plant proteins. The next challenge will be to explore how RICE proteins work in more details, which may lead to new ways to manipulate the levels of microRNAs to change the architecture of the plant and to improve their tolerance to various stress conditions.

DOI:http://dx.doi.org/10.7554/eLife.24466.002

ARGONAUTE10 promotes the degradation of miR165/6 through the SDN1 and SDN2 exonucleases in Arabidopsis

The degradation of small RNAs in plants and animals is associated with small RNA 3′ truncation and 3′ uridylation and thus relies on exonucleases and nucleotidyl transferases. ARGONAUTE (AGO) proteins associate with small RNAs in vivo and are essential for not only the activities but also the stability of small RNAs. AGO1 is the microRNA (miRNA) effector in Arabidopsis, and its closest homolog, AGO10, maintains stem cell homeostasis in meristems by sequestration of miR165/6, a conserved miRNA acting through AGO1. Here, we show that SMALL RNA DEGRADING NUCLEASES (SDNs) initiate miRNA degradation by acting on AGO1-bound miRNAs to cause their 3′ truncation, and the truncated species are uridylated and degraded. We report that AGO10 reduces miR165/6 accumulation by enhancing its degradation by SDN1 and SDN2 in vivo. In vitro, AGO10-bound miR165/6 is more susceptible to SDN1-mediated 3′ truncation than AGO1-bound miR165/6. Thus, AGO10 promotes the degradation of miR165/6, which is contrary to the stabilizing effect of AGO1. Our work identifies a class of exonucleases responsible for miRNA 3′ truncation in vivo and uncovers a mechanism of specificity determination in miRNA turnover. This work, together with previous studies on AGO10, suggests that spatially regulated miRNA degradation underlies stem cell maintenance in plants.

MicroRNAs (miRNAs) are 21–24 nucleotide regulatory RNAs that impact nearly all biological processes in plants and animals. The abundance of miRNAs is determined by their biogenesis and degradation. miRNA degradation is associated with trimming and tailing of their 3′ ends. The relationship between 3′ trimming and 3′ tailing as well as the enzyme(s) responsible for 3′ trimming was unknown in Arabidopsis. Mechanisms that protect miRNAs from degradation include 3′ terminal methylation and association with ARGONAUTE (AGO) proteins. In this study, we show that two members of the SMALL RNA DEGRADING NUCLEASE (SDN) family, SDN1 and SDN2, are partly responsible for the miRNA 3′ trimming activities in Arabidopsis. We further elucidate the relationship between 3′ trimming and 3′ tailing—miRNAs are first trimmed by SDNs, and the 3′ trimmed-and-unmethylated miRNAs are tailed by the nucleotidyl transferase HESO1. Furthermore, we demonstrate that SDN1 is able to act on AGO1-bound and methylated miRNAs. We also reveal a special function of AGO10, which has the highest affinity for miR165/6 in vivo. We show that AGO10-bound miR165/6 is more susceptible to SDN-mediated degradation than AGO1-bound miR165/6. This study provides a general molecular framework for miRNA degradation and an example of specificity determination in miRNA degradation.

Messages on small RNA duplexes in plants

Small RNA-mediated gene silencing encompasses diverse developmental events, stress responses, defense against pathogens, and maintenance of genome integrity. Extensive studies in model organisms have unveiled the molecular mechanisms underpinning the RNA silencing phenomena, and the accumulating knowledge have characterized the intricate pathways and the repertoire of proteins responsible for the actions of small RNAs characterized as microRNAs (miRNAs) or small interfering RNAs (siRNAs). Although the single-stranded, matured guide small RNAs direct the effector ribonucleoprotein complexes to induce gene silencing in sequence-specific manner, the double-stranded intermediate, the small RNA duplexes, which are processed as nascent products of the RNase III enzyme activities, act as key to determine the downstream molecular pathways and the fate of small RNAs. Based at the small RNA duplex-centered view, this review describes the recent advances in understanding the small RNA pathways in plants.

Analysis of the Uridylation of Both ARGONAUTE-Bound MiRNAs and 5' Cleavage Products of Their Target RNAs in Plants

Uridylation (3' untemplated uridine addition) provides a mechanism to trigger the degradation of miRNAs and the 5' cleavage products (5' CP) that are produced from miRNA-directed ARGONAUTE (AGO) cleavage of target RNAs. We have recently shown that HEN1 SUPPRESSOR 1 (HESO1), a terminal uridylyltransferase, and its homolog UTP:RNA uridylyltransferase 1 (URT1) catalyze the uridylation of miRNAs and 5' CPs within the AGO complex in higher plants. In this chapter, we describe detailed protocols for analyzing 3' end uridylation of both AGO-bound miRNAs and 5' CP.