A conserved sequence signature is essential for robust plant miRNA biogenesis

Principles of miRNA/miRNA* function in plant MIRNA processing

MicroRNAs (miRNAs) are essential regulators of gene expression, defined by their unique biogenesis, which requires the precise excision of the small RNA from an imperfect fold-back precursor. Unlike their animal counterparts, plant miRNA precursors exhibit variations in sizes and shapes. Plant MIRNAs can undergo processing in a base-to-loop or loop-to-base direction, with DICER-LIKE1 (DCL1) releasing the miRNA after two cuts (two-step MIRNAs) or more (sequential MIRNAs). In this study, we demonstrate the critical role of the miRNA/miRNA* duplex region in the processing of miRNA precursors. We observed that endogenous MIRNAs frequently experience suboptimal processing in vivo due to mismatches in the miRNA/miRNA* duplex, a key region that fine-tunes miRNA levels. Enhancing the interaction energy of the miRNA/miRNA* duplex in two-step MIRNAs results in a substantial increase in miRNA levels. Conversely, sequential MIRNAs display distinct and specific requirements for the miRNA/miRNA* duplexes along their foldback structure. Our work establishes a connection between the miRNA/miRNA* structure and precursor processing mechanisms. Furthermore, we reveal a link between the biological function of miRNAs and the processing mechanism of their precursors with the evolution of plant miRNA/miRNA* duplex structures.

A high-efficiency gene silencing in plants using two-hit asymmetrical artificial MicroRNAs

MicroRNAs (miRNAs) are small non-coding RNA molecules that play a crucial role in gene regulation. They are produced through an enzyme-guided process called dicing and have an asymmetrical structure with two nucleotide overhangs at the 3' ends. Artificial microRNAs (amiRNAs or amiRs) are designed to mimic the structure of miRNAs and can be used to silence specific genes of interest. Traditionally, amiRNAs are designed based on an endogenous miRNA precursor with certain mismatches at specific positions to increase their efficiency. In this study, the authors modified the highly expressed miR168a in Arabidopsis thaliana by replacing the single miR168 stem-loop/duplex with tandem asymmetrical amiRNA duplexes that follow the statistical rules of miRNA secondary structures. These tandem amiRNA duplexes, called "two-hit" amiRNAs, were shown to have a higher efficiency in silencing GFP and endogenous PDS reporter genes compared to traditional "one-hit" amiRNAs. The authors also demonstrated the effectiveness of "two-hit" amiRNAs in silencing genes involved in miRNA, tasiRNA, and hormone signalling pathways, individually or in families. Importantly, "two-hit" amiRNAs were also able to over-express endogenous miRNAs for their functions. The authors compare "two-hit" amiRNA technology with CRISPR/Cas9 and provide a web-based amiRNA designer for easy design and wide application in plants and even animals.

Revitalizing miRNAs mediated agronomical advantageous traits improvement in rice

One of the key enigmas in conventional and modern crop improvement programmes is how to introduce beneficial traits without any penalty impairment. Rice (Oryza sativa L.), among the essential staple food crops grown and utilized worldwide, needs to improve genotypes in multifaceted ways. With the global view to feed ten billion under the climatic perturbation, only a potent functional master regulator can withstand with hope for the next green revolution and food security. miRNAs are such, miniature, fine tuners for crop improvement and provide a value addition in emerging technologies, namely large-scale genotyping, phenotyping, genome editing, marker-assisted selection, and genomic selection, to make rice production feasible. There has been surplus research output generated since the last decade on miRNAs in rice, however, recent functional knowledge is limited to reaping the benefits for conventional and modern improvements in rice to avoid ambiguity and redundancy in the generated data. Here, we present the latest functional understanding of miRNAs in rice. In addition, their biogenesis, intra- and inter-kingdom signaling and communication, implication of amiRNAs, and consequences upon integration with CRISPR-Cas9. Further, highlights refer to the application of miRNAs for rice agronomical trait improvements, broadly classified into three functional domains. The majority of functionally established miRNAs are responsible for growth and development, followed by biotic and abiotic stresses. Tabular cataloguing reveals and highlights two multifaceted modules that were extensively studied. These belong to miRNA families 156 and 396, orchestrate multifarious aspects of advantageous agronomical traits. Moreover, updated and exhaustive functional aspects of different supplemental miRNA modules that would strengthen rice improvement are also being discussed.

Transcriptional Regulation of zma-MIR528a by Action of Nitrate and Auxin in Maize

In recent years, miR528, a monocot-specific miRNA, has been assigned multifaceted roles during development and stress response in several plant species. However, the transcription regulation and the molecular mechanisms controlling MIR528 expression in maize are still poorly explored. Here we analyzed the zma-MIR528a promoter region and found conserved transcription factor binding sites related to diverse signaling pathways, including the nitrate (TGA1/4) and auxin (AuxRE) response networks. Accumulation of both pre-miR528a and mature miR528 was up-regulated by exogenous nitrate and auxin treatments during imbibition, germination, and maize seedling establishment. Functional promoter analyses demonstrated that TGA1/4 and AuxRE sites are required for transcriptional induction by both stimuli. Overall, our findings of the nitrogen- and auxin-induced zma-MIR528a expression through cis-regulatory elements in its promoter contribute to the knowledge of miR528 regulome.

Plant responses to drought stress: microRNAs in action

Drought is common in most regions of the world, and it has a significant impact on plant growth and development. Plants, on the other hand, have evolved their own defense systems to deal with the extreme weather. The reprogramming of gene expression by microRNAs (miRNAs) is one of these defense mechanisms. miRNAs are short noncoding RNAs that have emerged as key post-transcriptional gene regulators in a variety of species. Drought stress modulates the expression of certain miRNAs that are functionally conserved across plant species. These characteristics imply that miRNA-based genetic changes might improve drought resistance in plants. This study highlights current knowledge of plant miRNA biogenesis, regulatory mechanisms and their role in drought stress responses. miRNAs functions and their adaptations by plants during drought stress has also been explained that can be exploited to promote drought-resistance among economically important crops.

Genome-Wide Identification and Expression Analysis of Wall-Associated Kinase (WAK) Gene Family in Cannabis sativa L

Wall-associated kinases (WAKs) are receptors that bind pectin or small pectic fragments in the cell wall and play roles in cell elongation and pathogen response. In the Cannabis sativa (Cs) genome, 53 CsWAK/CsWAKL (WAK-like) protein family members were identified and characterized; their amino acid lengths and molecular weights varied from 582 to 983, and from 65.6 to 108.8 kDa, respectively. They were classified into four main groups by a phylogenetic tree. Out of the 53 identified CsWAK/CsWAKL genes, 23 CsWAK/CsWAKL genes were unevenly distributed among six chromosomes. Two pairs of genes on chromosomes 4 and 7 have undergone duplication. The number of introns and exons among CsWAK/CsWAKL genes ranged from 1 to 6 and from 2 to 7, respectively. The promoter regions of 23 CsWAKs/CsWAKLs possessed diverse cis-regulatory elements that are involved in light, development, environmental stress, and hormone responsiveness. The expression profiles indicated that our candidate genes (CsWAK1, CsWAK4, CsWAK7, CsWAKL1, and CsWAKL7) are expressed in leaf tissue. These genes exhibit different expression patterns than their homologs in other plant species. These initial findings are useful resources for further research work on the potential roles of CsWAK/CsWAKL in cellular signalling during development, environmental stress conditions, and hormone treatments.

Cultivar-specific miRNA-mediated RNA silencing in grapes

In-depth comparative degradome analysis of two domesticated grape cultivars with diverse secondary metabolite accumulation reveals differential miRNA-mediated targeting. Small (s)RNAs such as micro(mi)RNAs and secondary small interfering (si) often work as negative switches of gene expression. In plants, it is well known that miRNAs target and cleave mRNAs that have high sequence complementarity. However, it is not known if there are variations in miRNA-mediated targeting between subspecies and cultivars that have been subjected to vast genetic modifications through breeding and other selections. Here, we have used PAREsnip2 tool for analysis of degradome datasets derived from two contrasting domesticated grape cultivars having varied fruit color, habit and leaf shape. We identified several interesting variations in sRNA targeting using degradome and 5'RACE analysis between two contrasting grape cultivars that was further correlated using RNA-seq analysis. Several of the differences we identified are associated with secondary metabolic pathways. We propose possible means by which sRNAs might contribute to diversity in secondary metabolites and other development pathways between two domesticated cultivars of grapes.

Identification of miRNAs in Response to Sweet Potato Weevil (Cylas formicarius) Infection by sRNA Sequencing

The sweet potato weevil (Cylas formicarius) is an important pest in the growing and storage of sweet potatoes. It is a common pest in the sweet potato production areas of southern China, causing serious harm to the development of the sweet potato industry. For the existing cultivars in China and abroad, there is no sweet potato variety with complete resistance to the sweet potato weevil. Thus, understanding the regulation mechanisms of sweet potato weevil resistance is the prerequisite for cultivating sweet potato varieties that are resistant to the sweet potato weevil. However, very little progress has been made in this field. In this study, we inoculated adult sweet potato weevils into sweet potato tubers. The infected sweet potato tubers were collected at 0, 24, 48, and 72 h. Then, a miRNA library was constructed for Eshu 6 and Guang 87 sweet potato tubers infected for different lengths of time. A total of 407 known miRNAs and 298 novel miRNAs were identified. A total of 174 differentially expressed miRNAs were screened out from the known miRNAs, and 247 differentially expressed miRNAs were screened out from the new miRNAs. Moreover, the targets of the differentially expressed miRNAs were predicted and their network was further investigated through GO analysis and KEGG analysis using our previous transcriptome data. More importantly, we screened 15 miRNAs and their target genes for qRT-PCR verification to confirm the reliability of the high-throughput sequencing data, which indicated that these miRNAs were detected and most of the expression results were consistent with the sequencing results. These results provide theoretical and data-based resources for the identification of miRNAs in response to sweet potato weevil infection and an analysis of the molecular regulatory mechanisms involved in insect resistance.

Endogenous MicroRNA Accurate Diagnostics to Guide Photothermal Therapy

Developing an intelligent theranostic nanoplatform with satisfied diagnostic accuracy and therapeutic efficiency holds great promise for personalized nanomedicine. Herein, we constructed a smart nanodevice for the accurate diagnosis of endogenous cancer microRNA (miRNA) biomarkers and efficient photothermal therapy (PTT). The nanodevice was composed of polydopamine (PDA)-functionalized CuS nanosheets (CuS@PDA NSs) and three elaborate DNA hairpin probes (TDHPs). The CuS@PDA NSs acted as efficient delivery vehicles and photothermal agents. They provided a large surface area available for an efficient and facile loading of TDHPs and a high-fluorescence (FL) quenching performance to achieve an ultralow background signal. The intracellular miRNA triggered TDHPs to assemble into three-arm branched junction structures for a strong fluorescence recovery as output signals to discriminate cancer cells from normal cells with an excellent sensitivity. The CuS@PAD NSs showed a good photothermal conversion efficiency in the near-infrared II (NIR II) region to mediate a good photothermal performance to kill cancer cells. A remarkable antitumor therapeutic effect was achieved in vivo. This work integrated highly sensitive detection to endogenous cancer biomarkers and valid therapeutic potency to tumor-bearing mice, indicating its promising biomedical applications.

Unpaired nucleotides on the stem of microRNA precursor are important for precise cleavage by Dicer-Like1 in Arabidopsis

Dicer-Like1 (DCL1) is a core component of the plant microRNA (miRNA) biogenesis machinery. MiRNA is transcribed as a precursor RNA, termed primary miRNA (pri-miRNA), which is cleaved by DCL1 in two steps to generate miRNA/miRNA* duplex. Pri-miRNA is a single-stranded RNA that forms a hairpin structure with a number of unpaired bases, hereafter called mismatches, on its stem. In the present study, by using purified recombinant Arabidopsis DCL1, we presented evidence that mismatches on the stem of pri-miRNA are important for precise DCL1 cleavage. We showed that a mismatch at the loop-distal side of the end of miRNA/miRNA* duplex is important for efficient cleavage of pri-miRNA in vitro, as previously suggested in planta. On the other hand, mismatches distant from the miRNA/miRNA* duplex region are important for determining the cleavage position by DCL1. The purified DCL1 proteins cleaved mutant pri-miRNA variants without such mismatches at a position at which wild-type pri-miRNA variants are not usually cleaved, resulting in an increased accumulation of small RNA different from miRNA. Therefore, our results suggest that, in addition to the distance from the ssRNA-dsRNA junction, mismatches on the stem of pri-miRNA function as a determinant for precise processing of pri-miRNA by DCL1 in plants.

Degradome comparison between wild and cultivated rice identifies differential targeting by miRNAs

Background

Small non-coding (s)RNAs are involved in the negative regulation of gene expression, playing critical roles in genome integrity, development and metabolic pathways. Targeting of RNAs by ribonucleoprotein complexes of sRNAs bound to Argonaute (AGO) proteins results in cleaved RNAs having precise and predictable 5` ends. While tools to study sliced bits of RNAs to confirm the efficiency of sRNA-mediated regulation are available, they are sub-optimal. In this study, we provide an improvised version of a tool with better efficiency to accurately validate sRNA targets.

Results

Here, we improvised the CleaveLand tool to identify additional micro (mi)RNA targets that belong to the same family and also other targets within a specified free energy cut-off. These additional targets were otherwise excluded during the default run. We employed these tools to understand the sRNA targeting efficiency in wild and cultivated rice, sequenced degradome from two rice lines, O. nivara and O. sativa indica Pusa Basmati-1 and analyzed variations in sRNA targeting. Our results indicate the existence of multiple miRNA-mediated targeting differences between domesticated and wild species. For example, Os5NG4 was targeted only in wild rice that might be responsible for the poor secondary wall formation when compared to cultivated rice. We also identified differential mRNA targets of secondary sRNAs that were generated after miRNA-mediated cleavage of primary targets.

Conclusions

We identified many differentially targeted mRNAs between wild and domesticated rice lines. In addition to providing a step-wise guide to generate and analyze degradome datasets, we showed how domestication altered sRNA-mediated cascade silencing during the evolution of indica rice.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08288-5.

Plant RNA-mediated gene regulatory network

Not all transcribed RNAs are protein-coding RNAs. Many of them are non-protein-coding RNAs in diverse eukaryotes. However, some of them seem to be non-functional and are resulted from spurious transcription. A lot of non-protein-coding transcripts have a significant function in the translation process. Gene expressions depend on complex networks of diverse gene regulatory pathways. Several non-protein-coding RNAs regulate gene expression in a sequence-specific system either at the transcriptional level or post-transcriptional level. They include a significant part of the gene expression regulatory network. RNA-mediated gene regulation machinery is evolutionarily ancient. They well-evolved during the evolutionary time and are becoming much more complex than had been expected. In this review, we are trying to summarizing the current knowledge in the field of RNA-mediated gene silencing.

In Vitro Selection of Short DNA Aptamers that Can Inhibit or Alleviate Cocaine and MK-801 Inhibition of Muscle-Type Nicotinic Acetylcholine Receptors

Ligands of high specificity and selectivity have been selected for biological molecules of interest including nicotinic acetylcholine receptor (nAChR) using combinatorial libraries of nucleic acids. The nAChR belongs to a group of structurally related proteins that regulate signal transmission between ~ 10¹² cells of the mammalian nervous system. It is inhibited by both therapeutic agents and abused drugs, including cocaine. A mechanism-based approach to alleviating noncompetitive inhibition of the mucle-type nAChR, including Torpedo, resulted in the selection of very short DNA aptamers only seven nucleotides long. By transient kinetic measurements, these DNA aptamers, which displaced cocaine from its binding site on the muscle-type nAChR, were classified into two groups based on their effects on the nAChR: Class I aptamers inhibit agonist-induced current in the muscle-type nAChR and Class II molecules alleviate inhibition by MK-801 [(+)-dizocilpine] without affecting the receptor function. The most potent Class I DNA aptamer, which inhibits the muscle-type nAChR, has an apparent dissociation constant (K_Iapt) of 5 μM, while the most efficient Class II DNA aptamer, which alleviates MK-801-induced inhibition, has an apparent dissociation constant (K_Apt) of 1.8 μM. An innovative aspect of the work is the identification of very short DNA aptamers with these properties that makes them attractive for therapeutic and diagnostic applications.

Pan-Genome miRNomics in Brachypodium

Pan-genomes are efficient tools for the identification of conserved and varying genomic sequences within lineages of a species. Investigating genetic variations might lead to the discovery of genes present in a subset of lineages, which might contribute into beneficial agronomic traits such as stress resistance or yield. The content of varying genomic regions in the pan-genome could include protein-coding genes as well as microRNA(miRNAs), small non-coding RNAs playing key roles in the regulation of gene expression. In this study, we performed in silico miRNA identification from the genomic sequences of 54 lineages of Brachypodium distachyon, aiming to explore varying miRNA contents and their functional interactions. A total of 115 miRNA families were identified in 54 lineages, 56 of which were found to be present in all lineages. The miRNA families were classified based on their conservation among lineages and potential mRNA targets were identified. Obtaining information about regulatory mechanisms stemming from these miRNAs offers strong potential to provide a better insight into the complex traits that were potentially present in some lineages. Future work could lead us to introduce these traits to different lineages or other economically important plant species in order to promote their survival in different environmental conditions.

Regulation of pri-MIRNA processing: mechanistic insights into the miRNA homeostasis in plant

miRNAs in plant plays crucial role in controlling proper growth, development and fitness by modulating the expression of their target genes. Therefore to modulate the expression of any stress/development related gene specifically, it is better to modulate expression of the miRNA that can target that gene. To modulate the expression level of miRNA, it is prerequisite to uncover the underlying molecular mechanism of its biogenesis. The biogenesis pathway consists of two major steps, transcription of MIR gene to pri-MIRNA and processing of pri-MIRNA into mature miRNA via sequential cleavage steps. Both of these pathways are tightly controlled by several different factors involving structural and functional molecules. This review is mainly focused on different aspects of pri-MIRNA processing mechanism to emphasize on the fact that to modulate the level of a miRNA in the cell only over-expression or knock-down of that MIR gene is not always sufficient rather it is also crucial to take processing regulation into consideration. The data collected from the recent and relevant literatures depicts that processing regulation is controlled by several aspects like structure and size of the pri-MIRNA, presence of introns in MIR gene and their location, interaction of processing factors with the core components of processing machinery etc. These detailed information can be utilized to figure out the particular point which can be utilized to modulate the expression of the miRNA which would ultimately be beneficial for the scientist and researcher working in this field to generate protocol for engineering plant with improved yield and stress tolerance.

Expression and processing of polycistronic artificial microRNAs and trans-acting siRNAs from transiently introduced transgenes in Solanum lycopersicum and Nicotiana benthamiana

Targeted gene silencing using small regulatory RNAs is a widely used technique for genetic studies in plants. Artificial microRNAs are one common approach, as they have the advantage of producing just a single functional small RNA, which can be designed for high target specificity and low off-target effects. Simultaneous silencing of multiple targets with artificial microRNAs can be achieved by producing polycistronic microRNA precursors. Alternatively, specialized trans-acting short interfering RNA (tasiRNA) precursors can be designed to produce several specific tasiRNAs at once. Here we tested several artificial microRNA- and tasiRNA-based methods for multiplexed gene silencing in Solanum lycopersicum (tomato) and Nicotiana benthamiana. All analyses used transiently expressed transgenes delivered by infiltration of leaves with Agrobacterium tumefacians. Small RNA sequencing analyses revealed that many previously described approaches resulted in poor small RNA processing. The 5'-most microRNA precursor hairpins on polycistronic artificial microRNA precursors were generally processed more accurately than precursors at the 3'-end. Polycistronic artificial microRNAs where the hairpin precursors were separated by transfer RNAs had the best processing precision. Strikingly, artificial tasiRNA precursors failed to be processed in the expected phased manner in our system. These results highlight the need for further development of multiplexed artificial microRNA and tasiRNA strategies. The importance of small RNA sequencing, as opposed to single-target assays such as RNA blots or real-time polymerase chain reaction, is also discussed.

Myocardial infarction cardiomyocytes-derived exosomal miR-328-3p promote apoptosis via Caspase signaling

Exosomal miRNAs are used as novel non-invasive biomarkers for detection strategies of human disease. Here, we aimed to investigate the potential clinical value of exosomal miRNAs for myocardial infarction (MI) diagnosis and treatment. Differentially expressed miRNAs were obtained from normal cardiomyocytes, MI cardiomyocytes and adjacent normal cardiomyocytes using miRNA microarray analysis. Exosomes were isolated by centrifugation and identified by transmission electron microscopy (TEM) and western blot. The expression of miR-328-3p in exosomes was then verified by qRT-PCR. Cell apoptosis was measured using flow cytometry and TUNEL analysis. The MI severity was confirmed by masson's trichrome staining and echocardiography. MiR-328-3p was significantly increased in the MI cardiomyocytes and adjacent normal cardiomyocytes. We further confirmed miR-328-3p increasing in the exosomes from MI cardiomyocytes, which can be taken into normal cardiomyocytes. Furthermore, exogenous exosomal miR-328-3p increased apoptosis of cardiomyocytes and promoted MI. Genes regulated by miR-328-3p are mainly enriched in Caspase signaling, which is an important apoptosis regulating signaling pathway. Additionally, Caspase-3 inhibitor, Z-DEVD-FMK, reversed apoptosis and MI promoting function of miR-328-3p. Exosomal miR-328-3p is a potential novel diagnostic biomarker and therapeutic target for MI, and Z-DEVD-FMK could reverse the apoptosis progression induced by miR-328-3p.

Recent advances in the regulation of plant miRNA biogenesis

MicroRNAs (miRNAs) are essential non-coding riboregulators of gene expression in plants and animals. In plants, miRNAs guide their effector protein named ARGONAUTE (AGO) to find target RNAs for gene silencing through target RNA cleavage or translational inhibition. miRNAs are derived from primary miRNA transcripts (pri-miRNAs), most of which are transcribed by the DNA-dependent RNA polymerase II. In plants, an RNase III enzyme DICER-LIKE1-containing complex processes pri-miRNAs in the nucleus into miRNAs. To ensure proper function of miRNAs, plants use multiple mechanisms to control miRNA accumulation. On one hand, pri-miRNA levels are controlled through transcription and stability. On the other hand, the activities of the DCL1 complex are regulated by many protein factors at transcriptional, post-transcriptional and post-translational levels. Notably, recent studies reveal that pri-miRNA structure/sequence features and modifications also play important roles in miRNA biogenesis. In this review, we summarize recent progresses on the mechanisms regulating miRNA biogenesis.

Can genetic engineering-based methods for gene function identification be eclipsed by genome editing in plants? A comparison of methodologies

Finding and explaining the functions of genes in plants have promising applications in crop improvement and bioprospecting and hence, it is important to compare various techniques available for gene function identification in plants. Today, the most popular technology among researchers to identify the functions of genes is the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9)-based genome editing method. But by no means can we say that CRISPR/Cas9 is the go-to method for all purposes. It comes with its own baggage. Researchers will agree and have lived through at least seven more technologies deployed to find the functions of genes, which come under three umbrellas: 1. genetic engineering, 2. transient expression, and 3. chemical/physical mutagenesis. Each of the methods evolved when the previous one ran into an insurmountable problem. In this review, we compare the eight technologies against one another on 14 parameters. This review lays bare the pros and cons, and similarities and dissimilarities of various methods. Every method comes with its advantages and disadvantages. For example, the CRISPR/Cas9-based genome editing is an excellent method for modifying gene sequences, creating allelic versions of genes, thereby aiding the understanding of gene function. But it comes with the baggage of unwanted or off-target mutations. Then, we have methods based on random or targeted knockout of the gene, knockdown, and overexpression of the gene. Targeted disruption of genes is required for complete knockout of gene function, which may not be accomplished by editing. We have also discussed the strategies to overcome the shortcomings of the targeted gene-knockout and the CRISPR/Cas9-based methods. This review serves as a comprehensive guide towards the understanding and comparison of various technologies available for gene function identification in plants and hence, it will find application for crop improvement and bioprospecting related research.

Rice-specific Argonaute 17 controls reproductive growth and yield-associated phenotypes

This manuscript describes the functions of an Argonaute protein named AGO17 in rice. AGO17 is required for the development of rice reproductive tissues. Argonaute (AGO) proteins are a well-conserved multigene family of regulators mediating gene silencing across eukaryotes. Monocot plants have additional members of AGO, the functions of which are poorly understood. Among the non-dicot AGO1 clade members in monocots, AGO17 expresses highly in reproductive tissues. Here we show that overexpression of Oryza sativa indica AGO17 in rice resulted in robust growth and increased yield, whereas its silencing resulted in reduced panicle length, less fertility, and poor growth. Small (s)RNA transcriptome analysis revealed misregulation of several miRNAs and other categories of sRNAs in silenced and overexpression lines, in agreement with its likely competition with other AGO1 clade members. Targets of differentially expressed miRNAs included previously unreported target RNAs coding for proteins involved in development, phase transition, and transport. Our results indicate a distinctive role for OsAGO17 in rice reproductive development that could be harnessed to improve yield.

Identification of key sequence features required for microRNA biogenesis in plants

MicroRNAs (miRNAs) are endogenous small RNAs of ∼21 nt that regulate multiple biological pathways in multicellular organisms. They derive from longer transcripts that harbor an imperfect stem-loop structure. In plants, the ribonuclease type III DICER-LIKE1 assisted by accessory proteins cleaves the precursor to release the mature miRNA. Numerous studies highlight the role of the precursor secondary structure during plant miRNA biogenesis; however, little is known about the relevance of the precursor sequence. Here, we analyzed the sequence composition of plant miRNA primary transcripts and found specifically located sequence biases. We show that changes in the identity of specific nucleotides can increase or abolish miRNA biogenesis. Most conspicuously, our analysis revealed that the identity of the nucleotides at unpaired positions of the precursor plays a crucial role during miRNA biogenesis in Arabidopsis.

The secondary structure of miRNA precursor sequences is known to affect processing by DICER-like proteins. Here Rojas et al. show that additional sequence features also play a regulatory role in plants with nucleotide identity at unpaired positions substantially impacting processing efficiency.

Artificial Small RNA-Based Silencing Tools for Antiviral Resistance in Plants

Artificial small RNAs (art-sRNAs), such as artificial microRNAs (amiRNAs) and synthetic trans-acting small interfering RNAs (syn-tasiRNAs), are highly specific 21-nucleotide small RNAs designed to recognize and silence complementary target RNAs. Art-sRNAs are extensively used in gene function studies or for improving crops, particularly to protect plants against viruses. Typically, antiviral art-sRNAs are computationally designed to target one or multiple sites in viral RNAs with high specificity, and art-sRNA constructs are generated and introduced into plants that are subsequently challenged with the target virus(es). Numerous studies have reported the successful application of art-sRNAs to induce resistance against a large number of RNA and DNA viruses in model and crop species. However, the application of art-sRNAs as an antiviral tool has limitations, such as the difficulty to predict the efficacy of a particular art-sRNA or the emergence of virus variants with mutated target sites escaping to art-sRNA-mediated degradation. Here, we review the different classes, features, and uses of art-sRNA-based tools to induce antiviral resistance in plants. We also provide strategies for the rational design of antiviral art-sRNAs and discuss the latest advances in developing art-sRNA-based methodologies for enhanced resistance to plant viruses.