Efficient silencing of endogenous microRNAs using artificial microRNAs in Arabidopsis thaliana

Recent allopolyploidy alters Spartina microRNA expression in response to xenobiotic-induced stress

Environmental contamination by xenobiotics represents a major threat for natural ecosystems and public health. In response, xenobiotic detoxification is a fundamental trait of organisms for developmental plasticity and stress tolerance, but the underlying molecular mechanisms remain poorly understood in plants. To decipher this process, we explored the consequences of allopolyploidy on xenobiotic tolerance in the genus Spartina Schreb. Specifically, we focused on microRNAs (miRNAs) owing to their central function in the regulation of gene expression patterns, including responses to stress. Small RNA-Seq was conducted on the parents S. alterniflora and S. maritima, their F1 hybrid S. x townsendii and the allopolyploid S. anglica under phenanthrene-induced stress (phe), a model Polycyclic Aromatic Hydrocarbon (PAH) compound. Differentially expressed miRNAs in response to phe were specifically identified within species. In complement, the respective impacts of hybridization and genome doubling were detected, through changes in miRNA expression patterns between S. x townsendii, S. anglica and the parents. The results support the impact of allopolyploidy in miRNA-guided regulation of plant response to phe. In total, we identified 17 phe-responsive miRNAs in Spartina among up-regulated MIR156 and down-regulated MIR159. We also describe novel phe-responsive miRNAs as putative Spartina-specific gene expression regulators in response to stress. Functional validation using Arabidopsis (L.) Heynh. T-DNA lines inserted in homologous MIR genes was performed, and the divergence of phe-responsive miRNA regulatory networks between Arabidopsis and Spartina was discussed.

Tomato SlMYB15 transcription factor targeted by sly-miR156e-3p positively regulates ABA-mediated cold tolerance

Cold is a common abiotic stress that seriously affects plant growth and development. MYB transcription factors are regulatory molecules that play important roles in various biological processes. We have previously demonstrated that SlMYB15 positively regulates cold tolerance in tomato. However, the underlying mechanism of SlMYB15-induced cold tolerance remains largely unexplored. Here, cold-induced SlMYB15 was found to be targeted by Solanum lycopersicum (sly)-miR156e-3p, which was decreased by cold stimulus in tomato. Tomato plants overexpressing sly-MIR156e-3p displayed significant enhancement in susceptibility to cold stress, while silencing of sly-miR156e-3p by an artificial microRNA interference strategy caused tomato plants to be more tolerant to cold. Moreover, both overexpression of SlMYB15 and silencing of sly-miR156e-3p increased the accumulation of ABA. SlMYB15 directly binds to the promoter regions of ABA biosynthesis and signalling genes, SlNCED1 and SlABF4, resulting in enhanced cold tolerance. Further experiments showed that SlMYB15 and sly-miR156e-3p also coordinated the cold tolerance of tomato via the reactive oxygen species (ROS) signalling pathway, as reflected by the increased expression of SlRBOH1, enhanced H2O2 and O2•-accumulation, and amplified activity of antioxidant enzymes in SlMYB15-overexpressing and sly-miR156e-3p-silenced plants. Taken together, our results demonstrate that SlMYB15 targeted by sly-miR156e-3p confers higher survivability to cold stress via ABA and ROS signals. This study provides valuable information for breeding improved crop cultivars better equipped with cold tolerance.

miR825-5p targets the TIR-NBS-LRR gene MIST1 and down-regulates basal immunity against Pseudomonas syringae in Arabidopsis

Plants encode numerous intracellular receptors known as nucleotide-binding leucine-rich repeat receptors (NLRs) that recognize pathogen-derived effectors or their activity to activate defenses. miRNAs regulate NLR genes in many species, often triggering the production of phased siRNAs (phasiRNAs). Most such examples involve genes encoding NLRs carrying coiled-coil domains, although a few include genes encoding NLRs carrying a Toll/interleukin-1 domain (TNL). Here, we characterize the role of miR825-5p in Arabidopsis, using a combination of bioinformatics, transgenic plants with altered miRNA levels and/or reporters, small RNAs, and virulence assays. We demonstrate that miR825-5p down-regulates the TNL MIST1 by targeting for endonucleolytic cleavage the sequence coding for TIR2, a highly conserved amino acid motif, linked to a catalytic residue essential for immune function. miR825-5p acts as a negative regulator of basal resistance against Pseudomonas syringae. miR825-5p triggers the production from MIST1 of a large number of phasiRNAs that can mediate cleavage of both MIST1 and additional TNL gene transcripts, potentially acting as a regulatory hub. miR825-5p is expressed in unchallenged leaves and transcriptionally down-regulated in response to pathogen-associated molecular patterns (PAMPs). Our results show that miR825-5p, which is required for full expression of PAMP-triggered immunity, establishes a link between PAMP perception and expression of uncharacterized TNL genes.

A tobacco ringspot virus-based vector system for gene and microRNA function studies in cucurbits

Cucurbits are economically important crops worldwide. The genomic data of many cucurbits are now available. However, functional analyses of cucurbit genes and noncoding RNAs have been impeded because genetic transformation is difficult for many cucurbitaceous plants. Here, we developed a set of tobacco ringspot virus (TRSV)-based vectors for gene and microRNA (miRNA) function studies in cucurbits. A TRSV-based expression vector could simultaneously express GREEN FLUORESCENT PROTEIN (GFP) and heterologous viral suppressors of RNA silencing in TRSV-infected plants, while a TRSV-based gene silencing vector could knock down endogenous genes exemplified by PHYTOENE DESATURASE (PDS) in Cucumis melo, Citrullus lanatus, Cucumis sativus, and Nicotiana benthamiana plants. We also developed a TRSV-based miRNA silencing vector to dissect the functions of endogenous miRNAs. Four representative miRNAs, namely, miR159, miR166, miR172, and miR319, from different cucurbits were inserted into the TRSV vector using a short tandem target mimic strategy and induced characteristic phenotypes in TRSV-miRNA-infected plants. This TRSV-based vector system will facilitate functional genomic studies in cucurbits.

Editing miR482b and miR482c Simultaneously by CRISPR/Cas9 Enhanced Tomato Resistance to Phytophthora infestans

Late blight, caused by Phytophthora infestans, is severely damaging to the global tomato industry. Micro-RNAs (miRNAs) have been widely demonstrated to play vital roles in plant resistance by repressing their target genes. Recently, the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) method has been continuously improved and extensively applied to edit plant genomes. However, editing multiplex miRNAs by CRISPR/Cas9 in tomato has not been studied yet. We knocked out miR482b and miR482c simultaneously in tomato through the multiplex CRISPR/Cas9 system. Two transgenic plants with silenced miR482b and miR482c simultaneously and one transgenic line with silenced miR482b alone were obtained. Compared with wild-type plants, the disease symptoms of three transgenic plants upon infection were reduced, accompanied by increased expression of their common target nucleotide binding site-leucine-rich repeat genes and decreased levels of reactive oxygen species. Furthermore, silencing miR482b and miR482c simultaneously was more resistant than silencing miR482b alone in tomato. More importantly, we found that knocking out miR482b and miR482c can elicit expression perturbation of other miRNAs, suggesting cross-regulation between miRNAs. Our study demonstrated that editing miR482b and miR482c simultaneously with CRISPR/Cas9 is an efficient strategy for generating pathogen-resistant tomatoes, and cross-regulation between miRNAs may reveal the novel mechanism in tomato-P. infestans interactions.

MicroRNA Techniques: Valuable Tools for Agronomic Trait Analyses and Breeding in Rice

MicroRNAs (miRNAs) are a class of small noncoding RNAs that regulate gene expression at the post-transcriptional level. Extensive studies have revealed that miRNAs have critical functions in plant growth, development, and stress responses and may provide valuable genetic resources for plant breeding research. We herein reviewed the development, mechanisms, and characteristics of miRNA techniques while highlighting widely used approaches, namely, the short tandem target mimic (STTM) approach. We described STTM-based advances in plant science, especially in the model crop rice, and introduced the CRISPR-based transgene-free crop breeding. Finally, we discussed the challenges and unique opportunities related to combining STTM and CRISPR technology for crop improvement and agriculture.

Interactions and links among the noncoding RNAs in plants under stresses

The complex interplay among sRNAs, lncRNAs and circRNAs has been implicated in plants under biotic and abiotic stresses. Here, we review current advances in our understanding of ncRNA interactions and links, which have considerable potential for improving the agronomic traits and the environmental adaptability of plants. Plants can respond to biotic or abiotic stresses. To cope with various conditions, numerous intricate molecular regulatory mechanisms have evolved in plants. Noncoding RNAs (ncRNAs) can be divided into small noncoding RNAs (sRNAs), long noncoding RNAs (lncRNAs) and circular RNAs (circRNAs). Emerging evidence has demonstrated that interplay among the ncRNAs acts as a novel layer in the regulatory mechanisms, which has attracted substantial interest. Links between sRNAs can affect plant immune responses and development in synergistic or antagonistic manners. Additionally, multiple interactions between lncRNAs and sRNAs are involved in crop breeding, disease resistance and high tolerance to environmental stresses. Here, we summarize current knowledge of the interactions and links among the ncRNAs in plant responses to stresses and the methods for identifying ncRNA interactions. Furthermore, challenges and prospects for further progress in elucidating ncRNA interactions and links are highlighted.

Research Tools for the Functional Genomics of Plant miRNAs During Zygotic and Somatic Embryogenesis

During early plant embryogenesis, some of the most fundamental decisions on fate and identity are taken making it a fascinating process to study. It is no surprise that higher plant embryogenesis was intensively analysed during the last century, while somatic embryogenesis is probably the most studied regeneration model. Encoded by the MIRNA, short, single-stranded, non-coding miRNAs, are commonly present in all Eukaryotic genomes and are involved in the regulation of the gene expression during the essential developmental processes such as plant morphogenesis, hormone signaling, and developmental phase transition. During the last few years dedicated to miRNAs, analytical methods and tools have been developed, which have afforded new opportunities in functional analyses of plant miRNAs, including (i) databases for in silico analysis; (ii) miRNAs detection and expression approaches; (iii) reporter and sensor lines for a spatio-temporal analysis of the miRNA-target interactions; (iv) in situ hybridisation protocols; (v) artificial miRNAs; (vi) MIM and STTM lines to inhibit miRNA activity, and (vii) the target genes resistant to miRNA. Here, we attempted to summarise the toolbox for functional analysis of miRNAs during plant embryogenesis. In addition to characterising the described tools/methods, examples of the applications have been presented.

An artificial miRNA as a new tool to silence and explore gene functions in apple

Artificial miRNA (amiRNA) is a powerful technology to silence genes of interest. It has a high efficiency and specificity that can be used to explore gene function through targeted gene regulation or to create new traits. To develop this gene regulation tool in apple, we designed two amiRNA constructs based on an apple endogenous miRNA backbone previously characterized (Md-miR156h), and we checked their efficiency on an easily scorable marker gene: the phytoene desaturase gene (MdPDS in apple). Two pairs of miRNA:miRNA* regions were designed (named h and w). The monocistronic Md-miR156h with these MdPDS targets was placed under the control of the CaMV 35S promoter to generate the two plasmids: pAmiRNA156h-PDSh and pAmiRNA156h-PDSw. Two Agrobacterium-mediated transformation experiments were performed on the cultivar 'Gala'. A total of 11 independent transgenic clones were obtained in the first experiment and 5 in the second. Most transgenic lines had a typical albino and dwarf phenotype. However, six clones had a wild type green phenotype. Molecular analyses indicated clear relationships between the degree of albino phenotype, the level of MdPDS gene expression and the amount of mature amiRNAs. This study demonstrated for the first time in apple the functionality of an artificial miRNA based on an endogenous miRNA backbone. It provides important opportunities for apple genetic functional studies as well as apple genetic improvement projects.

Solanum lycopersicum microRNA1916 targets multiple target genes and negatively regulates the immune response in tomato

MicroRNA1916 (miR1916) is one of the nonconserved miRNAs that respond to various stresses in plants, but little has been known at present about its mechanisms in biotic stresses. In this study, the expression of Solanum lycopersicum (sly)-miR1916 in tomato was found to be down-regulated after infection with Phytophthora infestans or Botrytis cinerea. Tomato plants that overexpressed sly-miR1916 displayed significant enhancement in susceptibility to P. infestans and B. cinerea infection, as well as increased tendency to produce reactive oxygen species. Silencing of sly-miR1916 by short tandem target mimic and artificial microRNA strategies caused the tomato plants to become more tolerant to adverse conditions. In addition, lower sly-miR1916 expression could up-regulate the expression of strictosidine synthase (STR-2), UDP-glycosyltransferases (UGTs), late blight resistance protein homolog R1B-16, disease resistance protein RPP13-like, and MYB transcription factor (MYB12), which ultimately resulted in the accumulation of α-tomatine and anthocyanins via STR-2, UGT, and MYB12. Furthermore, ectopic expression of sly-miR1916/STR-2 significantly changed the tolerance of tobacco to B. cinerea. Taken together, the results demonstrated that sly-miR1916 might regulate the expression of STR-2, UGT, and MYB12 in tomato plant, conferring sensitivity to biotic stress via modulating α-tomatine and anthocyanins.

Phyto-miRNAs-based regulation of metabolites biosynthesis in medicinal plants

Medicinal plants, are known to produce a wide range of plant secondary metabolites (PSMs) applied as insecticides, drugs, dyes and toxins in agriculture, medicine, industry and bio-warfare plus bio-terrorism, respectively. However, production of PSMs is usually in small quantities, so we need to find novel ways to increase both quantity and quality of them. Fortunately, biotechnology suggests several options through which secondary metabolism in plants can be engineered in innovative ways to: 1) over-produce the useful metabolites, 2) down-produce the toxic metabolites, 3) produce the new metabolites. Among the ways, RNA interference (RNAi) technology which involves gene-specific regulation by small non-coding RNAs (sncRNAs) have been recently emerged as a promising tool for plant biotechnologist, not only to decipher the function of plant genes, but also for development of the plants with improved and novel traits through manipulation of both desirable and undesirable genes. Among sncRNAs, miRNAs have been recorded various regulatory roles in plants such as development, signal transduction, response to environmental stresses, metabolism. Certainly, the use of miRNAs in metabolic engineering requires identification of miRNAs involved in metabolites biosynthesis, understanding of the biosynthetic pathways, as well as the identification of key points of the pathways in which the miRNAs have their own effect. Thus, we firstly consider these three issues on metabolic engineering of medicinal plants. Our review shows, application of miRNAs can open a novel perspective to metabolic engineering of medicinal plants.

Target-mimicry based diminution of miRNA167 reinforced flowering-time phenotypes in tobacco via spatial-transcriptional biases of flowering-associated miRNAs

Evolutionarily conserved microRNAs such as miR156, miR159, miR167 and miR172 tightly regulate the extensive array of gene expression during flowering in plants, through instant and long-term alterations in the expression of their target genes. Here we employed a novel target-mimicry approach for the diminution of auxin signalling regulator miRNA167 by developing mimic-transgenic lines in tobacco, to investigate the transcriptional biases of flowering-associated miRNAs in apical and floral meristematic tissues and their phenotypic implications. Recorded morpho-alterations such as uneven flowering-time phenotypes, anomalous floral organ formation, and large variations in the seed forming characteristics permitted us to determine the consequence of the extent of miR167 expression diminution accompanying the transcriptional biases of interrelated miRNAs. We demonstrate that percent diminution of miR167 gene expression is proportionally associated with both early and late flowering-time phenotypes in mimic lines. Also, the associated miRNAs, miR156, miR159, and miR172 showed >90% transcriptional diminution in at least 'early-flowering' miR167 mimic lines. On contrary, low percentages of their respective diminution were recorded in 'late-flowering' lines. Evidently, the misexpression of miR156, miR159, and miR172 led to the over-expression of their respective target genes SPL9, AtMYB33-like and AP2 genes in mimic lines which resulted in assorted phenotypes. We describe the scope of spatial regulation of these microRNAs in floral bud tissues of mimic lines which showed negative- or very low (<25%) misexpression levels in early/late-flowering lines highlighting their roles in the acquisition of flowering mechanism. To our knowledge, this study represents the first characterization of transcriptional biases of flowering associated miRNAs in miR167-mimic lines and certainly augments our understanding of the importance of microRNA-mediated regulation of flowering in plants.

Chinese Wheat Mosaic Virus-Induced Gene Silencing in Monocots and Dicots at Low Temperature

Virus-induced gene silencing (VIGS) is an important tool for functional genomics studies in plants. With this method, it is possible to target most endogenous genes and downregulate the messenger RNA (mRNA) in a sequence-specific manner. Chinese wheat mosaic virus (CWMV) has a bipartite, single-strand positive RNA genome, and can infect both wheat and Nicotiana benthamiana, and the optimal temperature for systemic infection in plants is 17°C. To assess the potential of the virus as a vector for gene silencing at low temperature, a fragment of the N. benthamiana or wheat phytoene desaturase (PDS) gene was expressed from a modified CWMV RNA2 clone and the resulting photo bleaching in infected plants was used as a reporter for silencing. Downregulation of PDS mRNA was also measured by quantitative reverse-transcriptase polymerase chain reaction (RT-qPCR). In experiments using fragments of PDS ranging from 500 to 1500 nucleotides, insert length influenced the stability and the efficiency of VIGS. The CWMV induced silencing system was also used to suppress miR165/166 and miR3134a through expression of miRNA target mimics. The relative expression levels of mature miR165/166 and miR3134a decreased whereas the transcript levels of their target genes increased. Interestingly, we also found the CWMV-induced silencing system was more efficient compare with the vector based on Barley stripe mosaic virus (BSMV) or Foxtail mosaic virus (FoMV) in wheat or the vector based on TRV in N. benthamiana at 17°C. In summary, the CWMV vector is effective in silencing endogenous genes and miRNAs at 17°C, thereby providing a powerful tool for gene function analysis in both N. benthamiana and wheat at low temperature.

Cold-responsive miRNAs and their target genes in the wild eggplant species Solanum aculeatissimum

Background

Low temperature is an important abiotic stress in plant growth and development, especially for thermophilic plants. Eggplants are thermophilic vegetables, although the molecular mechanism of their response to cold stress remains to be elucidated. MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs that play an essential role during plant development and stress responses. Although the role of many plant miRNAs in facilitating chilling tolerance has been verified, little is known about the mechanisms of eggplant chilling tolerance.

Results

Here, we used high-throughput sequencing to extract the miRNA and target genes expression profiles of Solanum aculeatissimum (S. aculeatissimum) under low temperature stress at different time periods(0 h, 2 h, 6 h, 12 h, 24 h). Differentially regulated miRNAs and their target genes were analyzed by comparing the small RNA (sRNA) and miRBase 20.0 databases using BLAST or BOWTIE, respectively. Fifty-six down-regulated miRNAs and 28 up-regulated miRNAs corresponding to 220 up-regulated mRNAs and 94 down-regulated mRNAs, respectively, were identified in S. aculeatissimum. Nine significant differentially expressed miRNAs and twelve mRNAs were identified by quantitative Real-time PCR and association analysis, and analyzed for their GO function enrichment and KEGG pathway association.

Conclusions

In summary, numerous conserved and novel miRNAs involved in the chilling response were identified using high-throughput sequencing, which provides a theoretical basis for the further study of low temperature stress-related miRNAs and the regulation of cold-tolerance mechanisms of eggplant at the miRNA level.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4341-y) contains supplementary material, which is available to authorized users.

Virus-Based MicroRNA Silencing and Overexpressing in Common Wheat (Triticum aestivum L.)

MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs that arise from large RNA precursors with a stem-loop structure and play important roles in plant development and responses to environmental stress. Although a hundred and nineteen wheat miRNAs have been identified and registered in the miRBase (Release 21.0, June, 2014; http://www.mirbase.org), the functional characterization of these miRNAs in wheat growth and development is lagging due to lack of effective techniques to investigate endogenous miRNA functions in wheat. Here we report barley stripe mosaic virus(BSMV)-based miRNA overexpression and silence systems that can be applied to study miRNA functions in wheat. By utilizing the BSMV system, we successfully knocked down endogenous miR156 and miR166 levels and over-expressed endogenous miR156 and artificial miRNA against phytoene desaturase gene PDS (amiR-PDS) in wheat. amiR-PDS expression caused a great reduction in endogenous mRNA abundance of PDS gene in wheat plant, leading to leaf obviously photobleaching. miR156 silencing led to a great increase in the mRNA level of its target gene SPL2, resulting in a leaf-curl phenotype in wheat seedlings. In contrast, overexpression of miR156 led to a significant reduction in the mRNA level of SPL2 in wheat seedlings, resulting in serious delay of the vegetative phase transitions as well as booting and flowering in wheat. These confirmed that miR156 regulates wheat development and booting time through SPL genes. In summary, the BSMV-based miRNA overexpression and silence systems have extraordinary potential not only for functional study of protein-encoding genes but also for miRNA genes in wheat.

Genomic profiling of exogenous abscisic acid-responsive microRNAs in tomato (Solanum lycopersicum)

BACKGROUND

Plant microRNAs (miRNAs) are involved in various biological pathways and stress responses as negative regulators at the posttranscriptional level. Abscisic acid (ABA) is a key signaling molecule that mediates plant stress response by activating many stress-related genes. Although some miRNAs in plants are previously identified to respond to ABA, a comprehensive profile of ABA-responsive miRNAs has not yet been elucidated.

RESULTS

Here, we identified miRNAs responding to exogenous application of ABA, and their predicted target genes in the model plant organism tomato (Solanum lycopersicum). Deep sequencing of small RNAs from ABA-treated and untreated tomatoes revealed that miRNAs can be up- or down-regulated upon treatment with ABA. A total of 1067 miRNAs were detected (including 365 known and 702 candidate novel miRNAs), of those, 416 miRNAs which had an abundance over two TPM (transcripts per million) were selected for differential expression analysis. We identified 269 (180 known and 89 novel) miRNAs that respond to exogenous ABA treatment with a change in expression level of |log2FC|≥0.25. 136 of these miRNAs (90 known and 46 novel) were expressed at significantly different levels |log2FC|≥1 between treatments. Furthermore, stem-loop RT-PCR was applied to validate the RNA-seq data. Target prediction and analysis of the corresponding ABA-responsive transcriptome data uncovered that differentially expressed miRNAs are involved in condition stress and pathogen resistance, growth and development. Among them, approximately 90 miRNAs were predicted to target transcription factors and pathogen resistance genes. Some miRNAs had functional overlap in biotic and abiotic stress. Most of these miRNAs were down-regulated following exposure to exogenous ABA, while their related target genes were inversely up-regulated, which is consistent with their negative regulatory role in gene expression.

CONCLUSIONS

Exogenous ABA application influences the composition and expression level of tomato miRNAs. ABA mainly down-regulates miRNAs that their target genes involve in abiotic stress adaption and disease resistance. ABA might increase expression of stress-related genes via miRNA-mediated posttranscriptional regulation, and our results indicate that ABA treatment has the potential to improve both abiotic stress tolerance and pathogen resistance. This study presents a comprehensive profile of ABA-regulated miRNAs in the tomato, and provides a robust database for further investigation of ABA regulatory mechanisms.

Efficient and specific inhibition of plant microRNA function by anti-microRNA oligonucleotides (AMOs) in vitro and in vivo

Anti-microRNA oligonucleotides (AMOs) are efficient and sequence-specific inhibitors of plant miRNA function both in vitro and in vivo. MicroRNAs (miRNAs) are small non-coding RNAs that play critical roles in developmental and physiological processes in plants and animals. Although miRNA knockdown by chemically modified antisense oligonucleotides prevails in animal and therapeutic studies, no such application has ever been reported in plants. Here, we show that sucrose-mediated delivery of 2'-O-methyl (2'-O-Me) anti-miRNA oligonucleotides (AMOs) is an efficient and sequence-specific way of inhibiting plant miRNA activity both in vitro and in vivo. Administration of AMOs to rice protoplasts and intact leaves resulted in efficient inhibition of miRNAs with concurrent de-repression of their target genes. AMOs caused simultaneous inhibition of miRNAs from the same family but exerted negligible effects on miRNAs from different families. In rice seedlings, a single-dose AMO treatment conferred long-lasting miRNA inhibition for at least 7 days. Although simultaneous dysregulation of multiple miRNAs by an AMO-and-miRNA-mimic mixture resulted in severe root defects, the phenotypic effects of individual AMOs and miRNA mimics were negligible, suggesting that those miRNAs function together in regulatory networks to ensure homeostasis. Our results validate the utility of AMOs as an efficient tool for plant miRNA loss-of-function studies in vivo, and this approach may prove to be a highly promising general method for unraveling miRNA-mediated gene-regulatory networks.

Essential RNA-Based Technologies and Their Applications in Plant Functional Genomics

Genome sequencing has not only extended our understanding of the blueprints of many plant species but has also revealed the secrets of coding and non-coding genes. We present here a brief introduction to and personal account of key RNA-based technologies, as well as their development and applications for functional genomics of plant coding and non-coding genes, with a focus on short tandem target mimics (STTMs), artificial microRNAs (amiRNAs), and CRISPR/Cas9. In addition, their use in multiplex technologies for the functional dissection of gene networks is discussed.

Artificial miRNA-mediated down-regulation of two monolignoid biosynthetic genes (C3H and F5H) cause reduction in lignin content in jute

Artificial microRNAs (amiRNA) provide a new feature in the gene silencing era. Concomitantly, reducing the amount of lignin in fiber-yielding plants such as jute holds significant commercial and environmental potential, since this amount is inversely proportional to the quality of the fiber. The present study aimed at reducing the lignin content in jute, by introducing amiRNA based vectors for down-regulation of two monolignoid biosynthetic genes of jute, coumarate 3-hydroxylase (C3H) and ferulate 5-hydroxylase (F5H). The transgenic lines of F5H-amiRNA and C3H-amiRNA showed a reduced level of gene expression, which resulted in about 25% reduction in acid insoluble lignin content for whole stem and 12-15% reduction in fiber lignin as compared to the non-transgenic plants. The results indicate successful F5H-amiRNA and C3H-amiRNA transgenesis for lignin reduction in jute. This is likely to have far-reaching commercial implications and economic acceleration for jute producing countries.

Small RNAs in Plant Responses to Abiotic Stresses: Regulatory Roles and Study Methods

To survive under abiotic stresses in the environment, plants trigger a reprogramming of gene expression, by transcriptional regulation or translational regulation, to turn on protective mechanisms. The current focus of research on how plants cope with abiotic stresses has transitioned from transcriptomic analyses to small RNA investigations. In this review, we have summarized and evaluated the current methodologies used in the identification and validation of small RNAs and their targets, in the context of plant responses to abiotic stresses.

Inhibiting plant microRNA activity: molecular SPONGEs, target MIMICs and STTMs all display variable efficacies against target microRNAs

Elucidation of microRNA (miRNA) function through a loss-of-function approach has proven difficult due to extensive genetic redundancy among most plant and animal miRNA families. Consequently, miRNA decoy technologies such as target MIMICs (MIMs) and short tandem target MIMICs (STTMs) in plants or molecular SPONGEs (SPs) in animals have been developed to generate loss-of-function phenotypes by perturbing endogenous miRNA activity. To test whether SPs can inhibit plant miRNA activity, synthetic SP transgenes containing multiple miRNA binding sites targeting different Arabidopsis miRNA families were generated. Additionally, their silencing efficacies were compared to the corresponding MIM and STTM transgenes via scoring the frequency and severity of phenotypic abnormalities elicited by each transgene. While SPs with wild-type miRNA binding sites have no apparent impact, SPs containing miRNA binding sites with two central mismatches (cmSPs) can generate strong loss-of-function phenotypes. However, their efficacy varied dramatically, from inducing strong loss-of-function phenotypes to failing to produce any phenotypic impact. Variability was also observed when MIMs and STTMs were compared to cmSPs. While cmSP165/166 and STTM165/166 showed a stronger efficacy than MIM165/166, MIM159 was stronger than cmSP159 and STTM159. Although increasing the number of miRNA binding sites or strengthening the free energy of the miRNA binding site interaction can improve decoy efficacy, clearly additional unknown overriding factors are at play. In conclusion, we demonstrate that no one approach guarantees the strongest miRNA inhibition, but rather distinct miRNA families respond differently to the various approaches, suggesting that multiple approaches may need to be taken to generate the desired loss-of-function outcome.

Barley Stripe Mosaic Virus (BSMV) Induced MicroRNA Silencing in Common Wheat (Triticum aestivum L.)

MicroRNAs (miRNAs) play important roles in growth, development, and response to environmental changes in plants. Based on the whole-genome shotgun sequencing strategy, more and more wheat miRNAs have been annotated. Now, there is a need for an effective technology to analyse endogenous miRNAs function in wheat. We report here that the modified barley stripe mosaic virus (BSMV)-induced miRNAs silencing system can be utilized to silence miRNAs in wheat. BSMV-based miRNA silencing system is performed through BSMV-based expression of miRNA target mimics to suppress miR159a and miR3134a. The relative expression levels of mature miR159a and miR3134a decrease with increasing transcript levels of their target genes in wheat plants. In summary, the developed approach is effective in silencing endogenous miRNAs, thereby providing a powerful tool for biological function analyses of miRNA molecules in common wheat.

MicroRNA-based biotechnology for plant improvement

MicroRNAs (miRNAs) are an extensive class of newly discovered endogenous small RNAs, which negatively regulate gene expression at the post-transcription levels. As the application of next-generation deep sequencing and advanced bioinformatics, the miRNA-related study has been expended to non-model plant species and the number of identified miRNAs has dramatically increased in the past years. miRNAs play a critical role in almost all biological and metabolic processes, and provide a unique strategy for plant improvement. Here, we first briefly review the discovery, history, and biogenesis of miRNAs, then focus more on the application of miRNAs on plant breeding and the future directions. Increased plant biomass through controlling plant development and phase change has been one achievement for miRNA-based biotechnology; plant tolerance to abiotic and biotic stress was also significantly enhanced by regulating the expression of an individual miRNA. Both endogenous and artificial miRNAs may serve as important tools for plant improvement.

MicroRNAs in fruit trees: discovery, diversity and future research directions

Since the first description of microRNAs (miRNAs) 20 years ago, the number of miRNAs identified in different eukaryotic organisms has exploded, largely due to the recent advances in DNA sequencing technologies. Functional studies, mostly from model species, have revealed that miRNAs are major post-transcriptional regulators of gene expression in eukaryotes. In plants, they are implicated in fundamental biological processes, from plant development and morphogenesis, to regulation of plant pathogen and abiotic stress responses. Although a substantial number of miRNAs have been identified in fruit trees to date, their functions remain largely uncharacterised. The present review aims to summarise the progress made in miRNA research in fruit trees, focusing specifically on the economically important species Prunus persica, Malus domestica, Citrus spp, and Vitis vinifera. We also discuss future miRNA research prospects in these plants and highlight potential applications of miRNAs in the on-going improvement of fruit trees.

Artificial microRNA mediated gene silencing in plants: progress and perspectives

Homology based gene silencing has emerged as a convenient approach for repressing expression of genes in order to study their functions. For this purpose, several antisense or small interfering RNA based gene silencing techniques have been frequently employed in plant research. Artificial microRNAs (amiRNAs) mediated gene silencing represents one of such techniques which can utilize as a potential tool in functional genomics. Similar to microRNAs, amiRNAs are single-stranded, approximately 21 nt long, and designed by replacing the mature miRNA sequences of duplex within pre-miRNAs. These amiRNAs are processed via small RNA biogenesis and silencing machinery and deregulate target expression. Holding to various refinements, amiRNA technology offers several advantages over other gene silencing methods. This is a powerful and robust tool, and could be applied to unravel new insight of metabolic pathways and gene functions across the various disciplines as well as in translating observations for improving favourable traits in plants. This review highlights general background of small RNAs, improvements made in RNAi based gene silencing, implications of amiRNA in gene silencing, and describes future themes for improving value of this technology in plant science.

Goat activin receptor type IIB knockdown by artificial microRNAs in vitro

Activin receptor type IIB (ACVR2B) has been known to negatively regulate the muscle growth through mediating the action of transforming growth factor beta superfamily ligands. Recently, the artificial microRNAs (amiRNAs) which are processed by endogenous miRNA processing machinery have been proposed as promising approach for efficient gene knockdown. We evaluated amiRNAs targeting goat ACVR2B in HEK293T and goat myoblasts cells. The amiRNAs were designed based on the miR-155 backbone and cloned in 5'- and 3'-UTR of GFP reporter gene under the CMV promoter. Although both 5'- and 3'-UTR-amiRNAs vectors showed efficient synthesis of GFP transcripts, amiRNAs in 5'-UTR drastically affected GFP protein synthesis in transfected goat myoblast cells. Among the four amiRNAs targeting ACVR2B derived from either 5'- or 3'-UTR, ami318 showed highest silencing efficiency against exogenously co-expressed ACVR2B in both 293T and goat myoblast cells whereas ami204 showed highest silencing efficiency against endogenous ACVR2B in goat myoblasts cells. The 3'-UTR-derived amiRNA exerted higher knockdown efficiency against endogenous ACVR2B at transcript level whereas 5'-UTR-derived amiRNAs exerted higher knockdown efficiency at protein level. The expression of ACVR2B showed positive correlation with the expression of MYOD (r = 0.744; p = 0.009) and MYOG (r = 0.959; p = 0.000) in the amiRNA-transfected myoblasts. Although both 5'- and 3'-UTR-amiRNA vectors led to substantial induction of interferon response, the magnitude of the response was found to be higher with the 3'-UTR-amiRNA vectors.

In Nicotiana species, an artificial microRNA corresponding to the virulence modulating region of Potato spindle tuber viroid directs RNA silencing of a soluble inorganic pyrophosphatase gene and the development of abnormal phenotypes

Potato spindle tuber viroid (PSTVd) is a small non-protein-coding RNA pathogen that can induce disease symptoms in a variety of plant species. How PSTVd induces disease symptoms is a long standing question. It has been suggested that PSTVd-derived small RNAs (sRNAs) could direct RNA silencing of a targeted host gene(s) resulting in symptom development. To test this, we expressed PSTVd sequences as artificial microRNAs (amiRNAs) in Nicotiana tabacum and Nicotiana benthamiana. One amiRNA, amiR46 that corresponds to sequences within the PSTVd virulence modulating region (VMR), induced abnormal phenotypes in both Nicotiana species that closely resemble those displayed by PSTVd infected plants. In N. tabacum amiR46 plants, phenotype severity correlated with amiR46 accumulation and expression down-regulation of the bioinformatically-identified target gene, a Nicotiana soluble inorganic pyrophosphatase (siPPase). Taken together, our phenotypic and molecular analyses suggest that disease symptom development in Nicotiana species following PSTVd infection results from sRNA-directed RNA silencing of the host gene, siPPase.

The use of artificial microRNA technology to control gene expression in Arabidopsis thaliana

In plants, double-stranded RNA (dsRNA) is an effective trigger of RNA silencing, and several classes of endogenous small RNA (sRNA), processed from dsRNA substrates by DICER-like (DCL) endonucleases, are essential in controlling gene expression. One such sRNA class, the microRNAs (miRNAs) control the expression of closely related genes to regulate all aspects of plant development, including the determination of leaf shape, leaf polarity, flowering time, and floral identity. A single miRNA sRNA silencing signal is processed from a long precursor transcript of nonprotein-coding RNA, termed the primary miRNA (pri-miRNA). A region of the pri-miRNA is partially self-complementary allowing the transcript to fold back onto itself to form a stem-loop structure of imperfectly dsRNA. Artificial miRNA (amiRNA) technology uses endogenous pri-miRNAs, in which the miRNA and miRNA* (passenger strand of the miRNA duplex) sequences have been replaced with corresponding amiRNA/amiRNA* sequences that direct highly efficient RNA silencing of the targeted gene. Here, we describe the rules for amiRNA design, as well as outline the PCR and bacterial cloning procedures involved in the construction of an amiRNA plant expression vector to control target gene expression in Arabidopsis thaliana.

A 22-nt artificial microRNA mediates widespread RNA silencing in Arabidopsis

It is known that 22-nucleotide (nt) microRNAs (miRNAs) derived from asymmetric duplexes trigger phased small-interfering RNA (phasiRNA) production from complementary targets. Here we investigate the efficacy of 22-nt artificial miRNA (amiRNA)-mediated RNA silencing relative to conventional hairpin RNA (hpRNA) and 21-nt amiRNA-mediated RNA silencing. CHALCONE SYNTHASE (CHS) was selected as a target in Arabidopsis thaliana due to the obvious and non-lethal loss of anthocyanin accumulation upon widespread RNA silencing. Over-expression of CHS in the pap1-D background facilitated visual detection of both local and systemic RNA silencing. RNA silencing was initiated in leaf tissues from hpRNA and amiRNA plant expression vectors under the control of an Arabidopsis RuBisCo small subunit 1A promoter (SSU). In this system, hpRNA expression triggered CHS silencing in most leaf tissues but not in roots or seed coats. Similarly, 21-nt amiRNA expression from symmetric miRNA/miRNA* duplexes triggered CHS silencing in all leaf tissues but not in roots or seed coats. However, 22-nt amiRNA expression from an asymmetric duplex triggered CHS silencing in all tissues, including roots and seed coats, in the majority of plant lines. This widespread CHS silencing required RNA-DEPENDENT RNA POLYMERASE6-mediated accumulation of phasiRNAs from the endogenous CHS transcript. These results demonstrate the efficacy of asymmetric 22-nt amiRNA-directed RNA silencing and associated phasiRNA production and activity, in mediating widespread RNA silencing of an endogenous target gene. Asymmetric 22-nt amiRNA-directed RNA silencing requires little modification of existing amiRNA technology and is expected to be effective in suppressing other genes and/or members of gene families.

SQUAMOSA promoter binding protein-like7 regulated microRNA408 is required for vegetative development in Arabidopsis

MicroRNAs (miRNAs) are endogenous small RNAs repressing target gene expression post-transcriptionally and are critically involved in various development processes and responses to environmental stresses. MiR408 is highly conserved in land plants and targets several transcripts encoding copper proteins. Although it has been well documented that expression level of miR408 is strongly influenced by a variety of environmental conditions including copper availability, the biological function of this miRNA is still unknown. Here we show that constitutive expression of miR408 results in enhanced growth of seedling and adult plant while knocking down miR408 level by T-DNA insertions or the artificial miRNA technique causes impaired growth. Further, we found that constitutively activated miR408 is able to complement the growth defects of the T-DNA lines. Regarding the molecular mechanism governing miR408 expression, we found that the transcription factors SQUAMOSA PROMOTER BINDING PROTEIN-LIKE7 (SPL7) binds to the GTAC motifs in the MIR408 promoter in response to copper deficiency. Interestingly, constitutive activation of miR408 in the spl7 background could partially rescue the severe growth defects of the mutant. Together these results demonstrate that miR408 is a powerful modulator of vegetative growth. Our finding thus reveals a novel control mechanism for vegetative development based on calculated miR408 expression in response to environmental cues.

DRB2, DRB3 and DRB5 function in a non-canonical microRNA pathway in Arabidopsis thaliana

DOUBLE-STRANDED RNA BINDING (DRB) proteins have been functionally characterized in viruses, prokaryotes and eukaryotes and are involved in all aspects of RNA biology. Arabidopsis thaliana (Arabidopsis) encodes five closely related DRB proteins, DRB1 to DRB5. DRB1 and DRB4 are required by DICER-LIKE (DCL) proteins DCL1 and DCL4 to accurately and efficiently process structurally distinct double-stranded RNA (dsRNA) precursor substrates in the microRNA (miRNA) and trans-acting small-interfering RNA (tasiRNA) biogenesis pathways respectively. We recently reported that DRB2 is also involved in the biogenesis of specific miRNA subsets. ( 1) Furthermore, the severity of the developmental phenotype displayed by the drb235 triple mutant plant, compared with those expressed by either drb2, drb3 and drb5 single mutants, or double mutant combinations thereof, indicates that DRB3 and DRB5 function in the same non-canonical miRNA pathway as DRB2. Through the use of our artificial miRNA (amiRNA) plant expression vector, pBlueGreen ( 2) (,) ( 3) we demonstrate here that unlike DRB2, DRB3 and DRB5 are not involved in the dsRNA processing stages of the miRNA biogenesis pathway, but are required to mediate RNA silencing of target genes of DRB2-associated miRNAs.

MicroRNAs in the moss Physcomitrella patens

Having diverged from the lineage that lead to flowering plants shortly after plants have established on land, mosses, which share fundamental processes with flowering plants but underwent little morphological changes by comparison with the fossil records, can be considered as an evolutionary informative place. Hence, they are especially useful for the study of developmental evolution and adaption to life on land. The transition to land exposed early plants to harsh physical conditions that resulted in key physiological and developmental changes. MicroRNAs (miRNAs) are an important class of small RNAs (sRNAs) that act as master regulators of development and stress in flowering plants. In recent years several groups have been engaged in the cloning of sRNAs from the model moss Physcomitrella patens. These studies have revealed a wealth of miRNAs, including novel and conserved ones, creating a unique opportunity to broaden our understanding of miRNA functions in land plants and their contribution to the latter's evolution. Here we review the current knowledge of moss miRNAs and suggest approaches for their functional analysis in P. patens.

DRB2 is required for microRNA biogenesis in Arabidopsis thaliana

Background

The Arabidopsis thaliana (Arabidopsis) DOUBLE-STRANDED RNA BINDING (DRB) protein family consists of five members, DRB1 to DRB5. The biogenesis of two developmentally important small RNA (sRNA) species, the microRNAs (miRNAs) and trans-acting small interfering RNAs (tasiRNAs) by DICER-LIKE (DCL) endonucleases requires the assistance of DRB1 and DRB4 respectively. The importance of miRNA-directed target gene expression in plant development is exemplified by the phenotypic consequence of loss of DRB1 activity (drb1 plants).

Principal Findings

Here we report that the developmental phenotype of the drb235 triple mutant plant is the result of deregulated miRNA biogenesis in the shoot apical meristem (SAM) region. The expression of DRB2, DRB3 and DRB5 in wild-type seedlings is restricted to the SAM region. Small RNA sequencing of the corresponding tissue of drb235 plants revealed altered miRNA accumulation. Approximately half of the miRNAs detected remained at levels equivalent to those of wild-type plants. However, the accumulation of the remaining miRNAs was either elevated or reduced in the triple mutant. Examination of different single and multiple drb mutants revealed a clear association between the loss of DRB2 activity and altered accumulation for both the elevated and reduced miRNA classes. Furthermore, we show that the constitutive over-expression of DRB2 outside of its wild-type expression domain can compensate for the loss of DRB1 activity in drb1 plants.

Conclusions/Significance

Our results suggest that in the SAM region, DRB2 is both antagonistic and synergistic to the role of DRB1 in miRNA biogenesis, adding an additional layer of gene regulatory complexity in this developmentally important tissue.

Fruit improvement using intragenesis and artificial microRNA

Genetic engineering methods based on the use of transgenes have been successfully adopted to improve crops. A novel all-native DNA gene technology consists of the creation of intragenic constructs by isolating genetic elements from a crop, rearranging them in vitro, and inserting them back into the plant. The ever-increasing genomic information and the elucidation of the molecular mechanisms that control fruit development could be exploited to confer the desired fruit phenotypes using endogenous DNA. The spatial/temporal regulation of genes can be modified by using appropriate endogenous regulatory elements, such as fruit-specific promoters. In addition, intragenic silencing can be employed to downregulate fruit-related genes. Here, we describe the available tools for intragenic manipulation of early phases of fleshy fruit initiation.

Silencing the silencer: strategies to inhibit microRNA activity

Plants and animals microRNAs (miRNAs) have been proposed to be key regulators of many fundamental processes. However defining miRNAs function has been problematic due to the paucity of miRNA loss-of-function mutants. This is likely due to their small gene size and redundancy as most miRNA have highly related family members. Consequently, the analysis of miRNA function has been primarily based on predictive bioinformatic or transgenic gain-of-function approaches. However, a number of new methodologies have been developed able to result in loss-of-function phenotypes. This includes miRNA sponges in animals and target mimicry in plants, both of which sequesters the mature miRNAs, disrupting endogenous miRNA:mRNA target relationships. Furthermore, artificial miRNAs and RNA interference in plants have been shown to be potent silencers of MIRNA genes. We will discuss the strengths and weaknesses of these methodologies which are potentially of great biotechnological use in medicine and agriculture.

Alternate approaches to repress endogenous microRNA activity in Arabidopsis thaliana

MicroRNAs (miRNAs) are an endogenous class of regulatory small RNA (sRNA). In plants, miRNAs are processed from short non-protein-coding messenger RNAs (mRNAs) transcribed from small miRNA genes (MIR genes). Traditionally in the model plant Arabidopsis thaliana (Arabidopsis), the functional analysis of a gene product has relied on the identification of a corresponding T-DNA insertion knockout mutant from a large, randomly-mutagenized population. However, because of the small size of MIR genes and presence of multiple, highly conserved members in most plant miRNA families, it has been extremely laborious and time consuming to obtain a corresponding single, or multiple, null mutant plant line. Our recent study published in Molecular Plant ( 1) outlines an alternate method for the functional characterization of miRNA action in Arabidopsis, termed anti-miRNA technology. Using this approach we demonstrated that the expression of individual miRNAs, or entire miRNA families, can be readily and efficiently knocked-down. Our approach is in addition to two previously reported methodologies that also allow for the targeted suppression of either individual miRNAs, or all members of a MIR gene family; these include miRNA target mimicry and transcriptional gene silencing (TGS) of MIR gene promoters. All three methodologies rely on endogenous gene regulatory machinery and in this article we provide an overview of these technologies and discuss their strengths and weaknesses in inhibiting the activity of their targeted miRNA(s).

Artificial microRNAs (amiRNAs) engineering - On how microRNA-based silencing methods have affected current plant silencing research

In recent years, endogenous microRNAs have been described as important regulators of gene expression in eukaryotes. Artificial microRNAs (amiRNAs) represent a recently developed miRNA-based strategy to silence endogenous genes. amiRNAs can be created by exchanging the miRNA/miRNA(∗) sequence within a miRNA precursor with a sequence designed to match the target gene, this is possible as long as the secondary RNA structure of the precursor is kept intact. In this review, we summarize the basic methodologies to design amiRNAs and detail their applications in plants genetic functional studies as well as their potential for crops genetic improvement.