MicroRNAS and their regulatory roles in plants

MiRNA160 is associated with local defense and systemic acquired resistance against Phytophthora infestans infection in potato

To combat pathogen infection, plants employ local defenses in infected sites and elicit systemic acquired resistance (SAR) in distant tissues. MicroRNAs have been shown to play a significant role in local defense, but their association with SAR is unknown. In addition, no such studies of the interaction between potato and Phytophthora infestans have been reported. We investigated the role of miR160 in local and SAR responses to P. infestans infection in potato. Expression analysis revealed induced levels of miR160 in both local and systemic leaves of infected wild-type plants. miR160 overexpression and knockdown plants exhibited increased susceptibility to infection, suggesting that miR160 levels equivalent to those of wild-type plants may be necessary for mounting local defense responses. Additionally, miR160 knockdown lines failed to elicit SAR, and grafting assays indicated that miR160 is required in both local and systemic leaves to trigger SAR. Consistently, SAR-associated signals and genes were dysregulated in miR160 knockdown lines. Furthermore, analysis of the expression of defense and auxin pathway genes and direct regulation of StGH3.6, a mediator of salicylic acid-auxin cross-talk, by the miR160 target StARF10 revealed the involvement of miR160 in antagonistic cross-talk between salicylic acid-mediated defense and auxin-mediated growth pathways. Overall, our study demonstrates that miR160 plays a crucial role in local defense and SAR responses during the interaction between potato and P. infestans.

Tamarix microRNA Profiling Reveals New Insight into Salt Tolerance

The halophyte tamarisk (Tamarix) is extremely salt tolerant, making it an ideal material for salt tolerance-related studies. Although many salt-responsive genes of Tamarix were identified in previous studies, there are no reports on the role of post-transcriptional regulation in its salt tolerance. We constructed six small RNA libraries of Tamarix chinensis roots with NaCl treatments. High-throughput sequencing of the six libraries was performed and microRNA expression profiles were constructed. We investigated salt-responsive microRNAs to uncover the microRNA-mediated genes regulation. From these analyses, 251 conserved and 18 novel microRNA were identified from all small RNAs. From 191 differentially expressed microRNAs, 74 co-expressed microRNAs were identified as salt-responsive candidate microRNAs. The most enriched GO (gene ontology) terms for the 157 genes targeted by differentially expressed microRNAs suggested that transcriptions factors were highly active. Two hub microRNAs (miR414, miR5658), which connected by several target genes into an organic microRNA regulatory network, appeared to be the key regulators of post-transcriptional salt-stress responses. As the first survey on the tamarisk small RNAome, this study improves the understanding of tamarisk salt-tolerance mechanisms and will contribute to the molecular-assisted resistance breeding.

Identification of miRNAs and their targets in maize in response to Sugarcane mosaic virus infection

MicroRNAs (miRNAs) are endogenous non-coding small RNAs that play essential regulatory roles in plant development and environmental stress responses. Maize (Zea mays L.) is a global economically important food and forage crop. To date, a number of maize miRNAs have been identified as being involved in plant development and stress responses. However, the miRNA-mediated gene regulatory networks responsive to virus infections in maize remain largely unknown. In this study, the profiles of small RNAs in buffer- and Sugarcane mosaic virus (SCMV)-inoculated maize plants were obtained by high-throughput sequencing, respectively. A total of 154 known miRNAs and 213 novel miRNAs were profiled and most of the miRNAs identified were differentially expressed after SCMV infection. In addition, 70 targets of 13 known miRNAs and 3 targets of a novel miRNA were identified by degradome analysis. The results of Northern blotting and quantitative real-time PCR showed that the expression levels of the selected miRNAs and their targets were mostly influenced by SCMV infection at 12 days post inoculation, including up-regulation of miR168 and miR528, and down-regulation of miR159, miR397 and miR827. These results provide new insights into the regulatory networks of miRNAs and their targets in maize plants responsive to SCMV infection.

Control of Cell Fate Reprogramming Towards De Novo Shoot Organogenesis

Many plants have a high regenerative capacity, which can be used to induce de novo organogenesis and produce various valuable plant species and products. In the classic two-step protocol for de novo shoot organogenesis, small pieces of plant parts or tissues known as explants are initially cultured on an auxin-rich medium to produce a cell mass called callus. Upon transfer to a cytokinin-rich medium, a subpopulation of cells within the callus acquire shoot cell fate and subsequently develop into a fertile shoot. Cell fate reprogramming during de novo organogenesis is thus recognized as the decisive step to acquire new cell types progressively, in response to a change in the levels of plant hormones auxin and cytokinin. Currently, the molecular mechanisms underlying the onset and completion of cell fate reprogramming remains partly understood. In this review, we sought to summarize the most recent progress made in the study of cell fate reprogramming during de novo shoot organogenesis, and highlight the critical roles of epigenetic and transcription factors in the developmental timing of cell fate reprogramming.

Metazoan MicroRNAs

MicroRNAs (miRNAs) are ∼22 nt RNAs that direct posttranscriptional repression of mRNA targets in diverse eukaryotic lineages. In humans and other mammals, these small RNAs help sculpt the expression of most mRNAs. This article reviews advances in our understanding of the defining features of metazoan miRNAs and their biogenesis, genomics, and evolution. It then reviews how metazoan miRNAs are regulated, how they recognize and cause repression of their targets, and the biological functions of this repression, with a compilation of knockout phenotypes that shows that important biological functions have been identified for most of the broadly conserved miRNAs of mammals.

The Polycistronic miR166k-166h Positively Regulates Rice Immunity via Post-transcriptional Control of EIN2

MicroRNAs (miRNAs) are small RNAs acting as regulators of gene expression at the post-transcriptional level. In plants, most miRNAs are generated from independent transcriptional units, and only a few polycistronic miRNAs have been described. miR166 is a conserved miRNA in plants targeting the HD-ZIP III transcription factor genes. Here, we show that a polycistronic miRNA comprising two miR166 family members, miR166k and miR166h, functions as a positive regulator of rice immunity. Rice plants with activated MIR166k-166h expression showed enhanced resistance to infection by the fungal pathogens Magnaporthe oryzae and Fusarium fujikuroi, the causal agents of the rice blast and bakanae disease, respectively. Disease resistance in rice plants with activated MIR166k-166h expression was associated with a stronger expression of defense responses during pathogen infection. Stronger induction of MIR166k-166h expression occurred in resistant but not susceptible rice cultivars. Notably, the ethylene-insensitive 2 (EIN2) gene was identified as a novel target gene for miR166k. The regulatory role of the miR166h-166k polycistron on the newly identified target gene results from the activity of the miR166k-5p specie generated from the miR166k-166h precursor. Collectively, our findings support a role for miR166k-5p in rice immunity by controlling EIN2 expression. Because rice blast is one of the most destructive diseases of cultivated rice worldwide, unraveling miR166k-166h-mediated mechanisms underlying blast resistance could ultimately help in designing appropriate strategies for rice protection.

The Oncogenic Relevance of miR-17-92 Cluster and Its Paralogous miR-106b-25 and miR-106a-363 Clusters in Brain Tumors

The fundamental function of ribonucleic acids is to transfer genetic information from DNA to protein during translation process, however, this is not the only way connecting active RNA sequences with essential biological processes. Up until now, many RNA subclasses of different size, structure, and biological function were identified. Among them, there are non-coding single-stranded microRNAs (miRNAs). This subclass comprises RNAs of 19–25 nucleotides in length that modulate the activity of well-defined coding RNAs and play a crucial role in many physiological and pathological processes. miRNA genes are located both in exons, introns, and also within non-translated regions. Several miRNAs that are transcribed from the adjacent miRNA genes are called cluster. One of the largest ones is miR-17-92 cluster known as OncomiR-1 due to its strong link to oncogenesis. Six miRNAs from the OncomiR-1 have been shown to play important roles in various physiological cellular processes but also through inhibition of cell death in many cancer-relevant processes. Due to the origin and similarity of the sequence, miR-17-92 cluster and paralogs, miR-106b-25 and miR-106a-363 clusters were defined. Here we discuss the oncogenic function of those miRNA subgroups found in many types of cancers, including brain tumors.

Identification and characterization of microRNAs in tree peony during chilling induced dormancy release by high-throughput sequencing

Tree peony, one of the most valuable horticultural and medicinal plants in the world, has to go through winter to break dormancy. Growing studies from molecular aspects on dormancy release process have been reported, but inadequate study has been done on miRNA-guided regulation in tree peony. In this study, high-throughput sequencing was employed to identify and characterize miRNAs in three libraries (6 d, 18 d and 24 d chilling treatments). There were 7,122, 10,076 and 9,097 unique miRNA sequences belonging to 52, 87 and 68 miRNA families, respectively. A total of 32 conserved miRNAs and 17 putative novel miRNAs were identified during dormancy release. There were 771 unigenes as potential targets of 62 miRNA families. Total 112 known miRNAs were differentially expressed, of which 55 miRNAs were shared among three libraries and 28 miRNAs were only found in 18 d chilling duration library. The expression patterns of 15 conserved miRNAs were validated and classified into four types by RT-qPCR. Combining with our microarray data under same treatments, five miRNAs (miR156k, miR159a, miR167a, miR169a and miR172a) were inversely correlated to those of their target genes. Our results would provide new molecular basis about dormancy release in tree peony.

Selection of reference genes for miRNA qRT-PCR under abiotic stress in grapevine

Grapevine is among the fruit crops with high economic value, and because of the economic losses caused by abiotic stresses, the stress resistance of Vitis vinifera has become an increasingly important research area. Among the mechanisms responding to environmental stresses, the role of miRNA has received much attention recently. qRT-PCR is a powerful method for miRNA quantitation, but the accuracy of the method strongly depends on the appropriate reference genes. To determine the most suitable reference genes for grapevine miRNA qRT-PCR, 15 genes were chosen as candidate reference genes. After eliminating 6 candidate reference genes with unsatisfactory amplification efficiency, the expression stability of the remaining candidate reference genes under salinity, cold and drought was analysed using four algorithms, geNorm, NormFinder, deltaCt and Bestkeeper. The results indicated that U6 snRNA was the most suitable reference gene under salinity and cold stresses; whereas miR168 was the best for drought stress. The best reference gene sets for salinity, cold and drought stresses were miR160e + miR164a, miR160e + miR168 and ACT + UBQ + GAPDH, respectively. The selected reference genes or gene sets were verified using miR319 or miR408 as the target gene.

Coordinated regulation of Arabidopsis microRNA biogenesis and red light signaling through Dicer-like 1 and phytochrome-interacting factor 4

Light and microRNAs (miRNAs) are key external and internal signals for plant development, respectively. However, the relationship between the light signaling and miRNA biogenesis pathways remains unknown. Here we found that miRNA processer proteins DCL1 and HYL1 interact with a basic helix-loop-helix (bHLH) transcription factor, phytochrome-interacting factor 4 (PIF4), which mediates the destabilization of DCL1 during dark-to-red-light transition. PIF4 acts as a transcription factor for some miRNA genes and is necessary for the proper accumulation of miRNAs. DCL1, HYL1, and mature miRNAs play roles in the regulation of plant hypocotyl growth. These results uncovered a previously unknown crosstalk between miRNA biogenesis and red light signaling through the PIF4-dependent regulation of miRNA transcription and processing to affect red-light-directed plant photomorphogenesis.

External light and internal miRNAs are important for plant development. This study revealed that the miRNA-processing enzyme DCL1 interacts with the red-light-regulated transcription factor PIF4, which modulates the stability of DCL1 during dark-to-red-light or red-light-to-dark transitions and acts as a transcription factor for some miRNA genes. This study revealed that DCL1 and mature miRNAs play roles in the red light signaling pathway to regulate plant photomorphogenesis. These results shed light on the crosstalk between miRNA and red light signaling pathways.

Plant Response to Metal-Containing Engineered Nanomaterials: An Omics-Based Perspective

The increasing use of engineered nanomaterials (ENMs) raises questions regarding their environmental impact. Improving the level of understanding of the genetic and molecular basis of the response to ENM exposure in biota is necessary to accurately assess the true risk to sensitive receptors. The aim of this Review is to compare the plant response to several metal-based ENMs widely used, such as quantum dots, metal oxides, and silver nanoparticles (NPs), integrating available "omics" data (transcriptomics, miRNAs, and proteomics). Although there is evidence that ENMs can release their metal components into the environment, the mechanistic basis of both ENM toxicity and tolerance is often distinct from that of metal ions and bulk materials. We show that the mechanisms of plant defense against ENM stress include the modification of root architecture, involvement of specific phytohormone signaling pathways, and activation of antioxidant mechanisms. A critical meta-analysis allowed us to identify relevant genes, miRNAs, and proteins involved in the response to ENMs and will further allow a mechanistic understanding of plant-ENM interactions.

Identification of microRNAs involved in lipid biosynthesis and seed size in developing sea buckthorn seeds using high-throughput sequencing

Sea buckthorn is a plant of medicinal and nutritional importance owing in part to the high levels of essential fatty acids, linoleic (up to 42%) and α-linolenic (up to 39%) acids in the seed oil. Sea buckthorn can produce seeds either via the sexual pathway or by apomixis. The seed development and maturation programs are critically dependent on miRNAs. To understand miRNA-mediated regulation of sea buckthorn seed development, eight small RNA libraries were constructed for deep sequencing from developing seeds of a low oil content line ‘SJ1’ and a high oil content line ‘XE3’. High-throughput sequencing identified 137 known miRNA from 27 families and 264 novel miRNAs. The potential targets of the identified miRNAs were predicted based on sequence homology. Nineteen (four known and 15 novel) and 22 (six known and 16 novel) miRNAs were found to be involved in lipid biosynthesis and seed size, respectively. An integrated analysis of mRNA and miRNA transcriptome and qRT-PCR identified some key miRNAs and their targets (miR164d-ARF2, miR168b-Δ9D, novelmiRNA-108-ACC, novelmiRNA-23-GPD1, novelmiRNA-58-DGAT1, and novelmiRNA-191-DGAT2) potentially involved in seed size and lipid biosynthesis of sea buckthorn seed. These results indicate the potential importance of miRNAs in regulating lipid biosynthesis and seed size in sea buckthorn.

Small but powerful: function of microRNAs in plant development

MicroRNAs (miRNAs) are a group of endogenous noncoding small RNAs frequently 21 nucleotides long. miRNAs act as negative regulators of their target genes through sequence-specific mRNA cleavage, translational repression, or chromatin modifications. Alterations of the expression of a miRNA or its targets often result in a variety of morphological and physiological abnormalities, suggesting the strong impact of miRNAs on plant development. Here, we review the recent advances on the functional studies of plant miRNAs. We will summarize the regulatory networks of miRNAs in a series of developmental processes, including meristem development, establishment of lateral organ polarity and boundaries, vegetative and reproductive organ growth, etc. We will also conclude the conserved and species-specific roles of plant miRNAs in evolution and discuss the strategies for further elucidating the functional mechanisms of miRNAs during plant development.

Non-coding RNAs in hepatocellular carcinoma: molecular functions and pathological implications

Hepatocellular carcinoma (HCC) is a leading lethal malignancy worldwide. However, the molecular mechanisms underlying liver carcinogenesis remain poorly understood. Over the past two decades, overwhelming evidence has demonstrated the regulatory roles of different classes of non-coding RNAs (ncRNAs) in liver carcinogenesis related to a number of aetiologies, including HBV, HCV and NAFLD. Among the ncRNAs, microRNAs, which belong to a distinct class of small ncRNAs, have been proven to play a crucial role in the post-transcriptional regulation of gene expression. Deregulation of microRNAs has been broadly implicated in the inactivation of tumour-suppressor genes and activation of oncogenes in HCC. Modern high-throughput sequencing analyses have unprecedentedly identified a very large number of non-coding transcripts. Divergent groups of long ncRNAs have been implicated in liver carcinogenesis through interactions with DNA, RNA or proteins. Overall, ncRNAs represent a burgeoning field of cancer research, and we are only beginning to understand the importance and complicity of the ncRNAs in liver carcinogenesis. In this Review, we summarize the common deregulation of small and long ncRNAs in human HCC. We also comprehensively review the pathological roles of ncRNAs in liver carcinogenesis, epithelial-to-mesenchymal transition and HCC metastasis and discuss the potential applications of ncRNAs as diagnostic tools and therapeutic targets in human HCC.

Analysis of physiological and miRNA responses to Pi deficiency in alfalfa (Medicago sativa L.)

The induction of miR399 and miR398 and the inhibition of miR156, miR159, miR160, miR171, miR2111, and miR2643 were observed under Pi deficiency in alfalfa. The miRNA-mediated genes involved in basic metabolic process, root and shoot development, stress response and Pi uptake. Inorganic phosphate (Pi) deficiency is known to be a limiting factor in plant development and growth. However, the underlying miRNAs associated with the Pi deficiency-responsive mechanism in alfalfa are unclear. To elucidate the molecular mechanism at the miRNA level, we constructed four small RNA (sRNA) libraries from the roots and shoots of alfalfa grown under normal or Pi-deficient conditions. In the present study, alfalfa plants showed reductions in biomass, photosynthesis, and Pi content and increases in their root-to-shoot ratio and citric, malic, and succinic acid contents under Pi limitation. Sequencing results identified 47 and 44 differentially expressed miRNAs in the roots and shoots, respectively. Furthermore, 909 potential target genes were predicted, and some targets were validated by RLM-RACE assays. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed prominent enrichment in signal transducer activity, binding and basic metabolic pathways for carbohydrates, fatty acids and amino acids; cellular response to hormone stimulus and response to auxin pathways were also enriched. qPCR results verified that the differentially expressed miRNA profile was consistent with sequencing results, and putative target genes exhibited opposite expression patterns. In this study, the miRNAs associated with the response to Pi limitation in alfalfa were identified. In addition, there was an enrichment of miRNA-targeted genes involved in biological regulatory processes such as basic metabolic pathways, root and shoot development, stress response, Pi transportation and citric acid secretion.

Non-coding RNAs and plant male sterility: current knowledge and future prospects

Latest outcomes assign functional role to non-coding (nc) RNA molecules in regulatory networks that confer male sterility to plants. Male sterility in plants offers great opportunity for improving crop performance through application of hybrid technology. In this respect, cytoplasmic male sterility (CMS) and sterility induced by photoperiod (PGMS)/temperature (TGMS) have greatly facilitated development of high-yielding hybrids in crops. Participation of non-coding (nc) RNA molecules in plant reproductive development is increasingly becoming evident. Recent breakthroughs in rice definitively associate ncRNAs with PGMS and TGMS. In case of CMS, the exact mechanism through which the mitochondrial ORFs exert influence on the development of male gametophyte remains obscure in several crops. High-throughput sequencing has enabled genome-wide discovery and validation of these regulatory molecules and their target genes, describing their potential roles performed in relation to CMS. Discovery of ncRNA localized in plant mtDNA with its possible implication in CMS induction is intriguing in this respect. Still, conclusive evidences linking ncRNA with CMS phenotypes are currently unavailable, demanding complementing genetic approaches like transgenics to substantiate the preliminary findings. Here, we review the recent literature on the contribution of ncRNAs in conferring male sterility to plants, with an emphasis on microRNAs. Also, we present a perspective on improved understanding about ncRNA-mediated regulatory pathways that control male sterility in plants. A refined understanding of plant male sterility would strengthen crop hybrid industry to deliver hybrids with improved performance.

Can-miRn37a mediated suppression of ethylene response factors enhances the resistance of chilli against anthracnose pathogen Colletotrichum truncatum L

Pepper anthracnose, caused by Colletotrichum species complex is the most destructive disease of chilli (Capsicum annuum L.). miRNAs are key modulators of transcriptional and post- transcriptional expression of genes during defense responses. In the present study, we performed a comparative miRNA profiling of susceptible (Arka Lohit-AL) and resistant (Punjab Lal-PL) chilli cultivars to identify 35 differentially expressed miRNAs that could be classified as positive, negative or basal regulators of defense against C. truncatum, the most potent anthracnose pathogen. Interestingly, a novel microRNA can-miRn37a was significantly induced in PL but largely repressed in AL genotype post pathogen attack. Subsequent over-expression of can-miRn37a in AL showed enhanced resistance to anthracnose, as evidenced by decreased fungal growth and induced expression of defense-related genes. Consequently, the expression of its three target genes encoding the ethylene response factors (ERFs) was down-regulated in PL as well as in the over-expression lines of AL genotypes. The ability of these targets to be regulated by can-miRn37a was further confirmed by transient co-expression in Nicotiana benthamiana. Additionally, the virus-induced silencing of the three targets in the susceptible AL cultivar revealed their role in fungal colonization and induction of C. truncatum pathogenicity in chilli. Taken together, our study suggests that can-miRn37a provides a potential miRNA mediated approach of engineering anthracnose resistance in chilli by repressing ERFs and preventing fungal colonization.

Expression dynamics of miRNAs and their targets in seed germination conditions reveals miRNA-ta-siRNA crosstalk as regulator of seed germination

Seed germination paves the way for the dormant embryo to establish itself as a new plant marking the first critical step in postembryonic plant growth and development. Germination starts with the uptake of water (imbibition), followed by induction of transcription, translation, energy metabolism, and cell division processes. Although small RNAs have been implicated in many developmental processes, their role during seed germination stages and conditions remained elusive. Here we show that seed germination conditions, like imbibition and temperature, dynamically regulate the expression of many developmentally important miRNAs and their targets. We have identified 58 miRNAs belonging to 30 different families at different seed germination conditions. Amongst these, 15 miRNAs and their targets were significantly differentially expressed in Arabidopsis seeds in dry and 12 h, 24 h and 48 h of imbibition. Interestingly, differential expression of miR390, which targets trans-acting siRNA locus (TAS3) derived transcripts, resulted in alteration of tasiR-ARF mediated regulation of expression of target AUXIN RESPONSE FACTORs (ARF2/3/4). Our results suggest that the dynamic expression of several miRNAs, their targets, and a crosstalk between miRNA and ta-siRNA pathways contribute to the regulation of seed germination in Arabidopsis thaliana.

Discovery of microRNA-target modules of African rice (Oryza glaberrima) under salinity stress

Oryza glaberrima is the second edible rice in the genus Oryza. It is grown in the African countries. miRNAs are regulatory molecules that are involved in every domains of gene expression including salinity stress response. Although several miRNAs have been reported from various species of Oryza, yet none of them are from this species. Salt treated (200 mM NaCl for 48 h) and control smallRNA libraries of RAM-100, a salt tolerant genotype, each with 2 replications generated 150 conserve and 348 novel miRNAs. We also used smallRNAseq data of NCBI of O. glaberrima to discover additional 246 known miRNAs. Totally, 29 known and 32 novel miRNAs were differentially regulated under salinity stress. Gene ontology and KEGG analysis indicated several targets were involved in vital biological pathways of salinity stress tolerance. Expression of selected miRNAs as indicated by Illumina data were found to be coherent with real time-PCR analysis. However, target gene expression was inversely correlated with their corresponding miRNAs. Finally based upon present results as well as existing knowledge of literature, we proposed the miRNA-target modules that were induced by salinity stress. Therefore, the present findings provide valuable information about miRNA-target networks in salinity adaption of O. glaberrima.

Advances in Transcriptomics of Plants

The current global population of 7.3 billion is estimated to reach 9.7 billion in the year 2050. Rapid population growth is driving up global food demand. Additionally, global climate change, environmental degradation, drought, emerging diseases, and salty soils are the current threats to global food security. In order to mitigate the adverse effects of these diverse agricultural productivity constraints and enhance crop yield and stress-tolerance in plants, we need to go beyond traditional and molecular plant breeding. The powerful new tools for genome editing, Transcription Activator-Like Effector Nucleases (TALENs) and Clustered Regulatory Interspaced Short Palindromic Repeats (CRISPR)/Cas systems (CRISPR-Cas9), have been hailed as a quantum leap forward in the development of stress-resistant plants. Plant breeding techniques, however, have several drawbacks. Hence, identification of transcriptional regulatory elements and deciphering mechanisms underlying transcriptional regulation are crucial to avoiding unintended consequences in modified crop plants, which could ultimately have negative impacts on human health. RNA splicing as an essential regulated post-transcriptional process, alternative polyadenylation as an RNA-processing mechanism, along with non-coding RNAs (microRNAs, small interfering RNAs and long non-coding RNAs) have been identified as major players in gene regulation. In this chapter, we highlight new findings on the essential roles of alternative splicing and alternative polyadenylation in plant development and response to biotic and abiotic stresses. We also discuss biogenesis and the functions of microRNAs (miRNAs) and small interfering RNAs (siRNAs) in plants and recent advances in our knowledge of the roles of miRNAs and siRNAs in plant stress response. Graphical Abstract.

Deep Sequencing Uncovers Rice Long siRNAs and Its Involvement in Immunity Against Rhizoctonia solani

Small RNA (sRNA) is a class of noncoding RNA that can silence the expression of target genes. In rice, the majority of characterized sRNAs are within the range of 21 to 24 nucleotides (nt) long, whose biogenesis and function are associated with a specific sets of components, such as Dicer-like (OsDCLs) and Argonaute proteins (OsAGOs). Rice sRNAs longer than 24 nt are occasionally reported, with biogenesis and functional mechanism uninvestigated, especially in a context of defense responses against pathogen infection. By using deep sequencing, we identified a group of rice long small interfering RNAs (lsiRNAs) that are within the range of 25 to 40 nt in length. Our results show that some rice lsiRNAs are differentially expressed upon infection of Rhizoctonia solani, the causal agent of the rice sheath blight disease. Bioinformatic analysis and experimental validation indicate that some rice lsiRNAs can target defense-related genes. We further demonstrate that rice lsiRNAs are neither derived from RNA degradation nor originated as secondary small interfering RNAs (siRNAs). Moreover, lsiRNAs require OsDCL4 for biogenesis and OsAGO18 for function. Therefore, our study indicates that rice lsiRNAs are a unique class of endogenous sRNAs produced in rice, which may participate in response against pathogens.

Molecular regulation of aluminum resistance and sulfur nutrition during root growth

Aluminum toxicity and sulfate deprivation both regulate microRNA395 expression, repressing its low-affinity sulfate transporter ( SULTR2;1 ) target. Sulfate deprivation also induces the high-affinity sulfate transporter gene ( SULTR12 ), allowing enhanced sulfate uptake. Few studies about the relationships between sulfate, a plant nutrient, and aluminum, a toxic ion, are available; hence, the molecular and physiological processes underpinning this interaction are poorly understood. The Al-sulfate interaction occurs in acidic soils, whereby relatively high concentrations of trivalent toxic aluminum (Al³⁺) may hamper root growth, limiting uptake of nutrients, including sulfur (S). On the other side, Al³⁺ may be detoxified by complexation with sulfate in the acid soil solution as well as in the root-cell vacuoles. In this review, we focus on recent insights into the mechanisms governing plant responses to Al toxicity and its relationship with sulfur nutrition, emphasizing the role of phytohormones, microRNAs, and ion transporters in higher plants. It is known that Al³⁺ disturbs gene expression and enzymes involved in biosynthesis of S-containing cysteine in root cells. On the other hand, Al³⁺ may induce ethylene biosynthesis, enhance reactive oxygen species production, alter phytohormone transport, trigger root growth inhibition and promote sulfate uptake under S deficiency. MicroRNA395, regulated by both Al toxicity and sulfate deprivation, represses its low-affinity Sulfate Transporter 2;1 (SULTR2;1) target. In addition, sulfate deprivation induces High Affinity Sulfate Transporters (HAST; SULTR1;2), improving sulfate uptake from low-sulfate soil solutions. Identification of new microRNAs and cloning of their target genes are necessary for a better understanding of the role of molecular regulation of plant resistance to Al stress and sulfate deprivation.

RNA-binding protein immunoprecipitation as a tool to investigate plant miRNA processing interference by regulatory proteins of diverse origin

BACKGROUND

Due to the nature of viral RNA genomes, RNA viruses depend on many RNA-binding proteins (RBP) of viral and host origin for replication, dissemination and evasion of host RNA degradation pathways. Some viruses interfere with the microRNA (miRNA) pathway to generate better fitness. The development of an adjusted, reliable and sensitive ribonucleoprotein immunoprecipitation (RIP) assay is needed to study the interaction between RBP of different origin (including viral origin) and miRNA precursors. The method could be further applied to transiently expressed heterologous proteins in different plant species.

RESULTS

Here we describe a modified RIP assay applied to nuclear epitope-tagged proteins of heterologous origin and transiently expressed in Nicotiana benthamiana. The assay includes a combination of optimized steps as well as the careful selection of control samples and rigorous data analysis. It has proven efficient to detect and quantify miRNA processing intermediates associated with regulatory proteins.

CONCLUSIONS

The RIP method described here provides a reliable tool to study the interaction of RBPs, such as transiently expressed regulatory proteins with lowly represented host RNA, as is the case of miRNA precursors. This modified method was efficiently adjusted to recover nuclear proteins and reduce unspecific background. The purification scheme optimized here for GFP-tagged proteins can be applied to a wide array of RBPs. The subsequent application of next-generation sequencing technologies will permit to sequence and characterize all RNA species bound in vivo by a given RBP.

Regulation of low temperature stress in plants by microRNAs

Low temperature is one of the most common environmental stresses that seriously affect the growth and development of plants. However, plants have the plasticity in their defence mechanisms enabling them to tolerate and, sometimes, even survive adverse environmental conditions. MicroRNAs (miRNAs) are small non-coding RNAs, approximately 18-24 nucleotides in length, and are being increasingly recognized as regulators of gene expression at the post-transcriptional level and have the ability to influence a broad range of biological processes. There is growing evidence in the literature that reprogramming of gene expression mediated through miRNAs is a major defence mechanism in plants enabling them to respond to stresses. To date, numerous studies have established the importance of miRNA-based regulation of gene expression under low temperature stress. Individual miRNAs can modulate the expression of multiple mRNA targets, and, therefore, the manipulation of a single miRNA has the potential to affect multiple biological processes. Numerous functional studies have attempted to identify the miRNA-target interactions and have elaborated the role of several miRNAs in cold-stress regulation. This review summarizes the current understanding of miRNA-mediated modulation of the expression of key genes as well as genetic and regulatory pathways, involved in low temperature stress responses in plants.

An APETALA2 Homolog, RcAP2, Regulates the Number of Rose Petals Derived From Stamens and Response to Temperature Fluctuations

Rosa chinensis, which is a famous traditional flower in China, is a major ornamental plant worldwide. Long-term cultivation and breeding have resulted in considerable changes in the number of rose petals, while most wild Rosaceae plants have only one whorl consisting of five petals. The petals of double flowers reportedly originate from stamens, but the underlying molecular mechanism has not been fully characterized. In this study, we observed that the number of petals of R. chinensis 'Old Blush' flowers increased and decreased in response to low- and high-temperature treatments, respectively, similar to previous reports. We characterized these variations in further detail and found that the number of stamens exhibited the opposite trend. We cloned an APETALA2 homolog, RcAP2. A detailed analysis of gene structure and promoter cis-acting elements as well as RcAP2 temporospatial expression patterns and responses to temperature changes suggested that RcAP2 expression may be related to the number of petals from stamen origin. The overexpression of RcAP2 in Arabidopsis thaliana transgenic plants may induce the transformation of stamens to petals, thereby increasing the number of petals. Moreover, silencing RcAP2 in 'Old Blush' plants decreased the number of petals. Our results may be useful for clarifying the temperature-responsive mechanism involved in petaloid stamen production, which may be relevant for the breeding of new rose varieties with enhanced flower traits.

Plant Small RNAs Responsive to Fungal Pathogen Infection

Accumulating evidence indicates that small noncoding RNAs (sRNAs) can be transferred across species for interkingdom communication. In addition to the artificial transgene-derived small interfering RNAs (siRNAs), endogenous microRNAs (miRNAs) can also influence interacting organisms to execute a regulatory function. For instance, we have recently found that, in response to infection with Verticillium dahliae (V. dahliae), cotton plants increase accumulation of miR166 and miR159, which can be exported to the fungal hyphae for specific silencing of virulence genes. These findings suggest a great potential for applying interkingdom mobile miRNAs for crop protection against fungal pathogens. The methods described here provide an approach to identify plant miRNAs and their potential targets in invading fungal pathogens, which will help in revealing the underlying mechanisms of these crosstalk phenomena.

Plant Gene Regulation Using Multiplex CRISPR-dCas9 Artificial Transcription Factors

Besides genome editing, the CRISPR-Cas9-based platform provides a new way of engineering artificial transcription factors (ATFs). Multiplex of guide RNA (gRNA) expression cassettes holds a great promise for many useful applications of CRISPR-Cas9. In this chapter, we provide a detailed protocol for building advanced multiplexed CRISPR-dCas9-Activator/repressor T-DNA vectors for carrying out transcriptional activation or repression experiments in plants. We specifically describe the assembly of multiplex T-DNA vectors that can express multiple gRNAs to activate a silenced gene, or to repress two independent miRNA genes simultaneously in Arabidopsis. We then describe a "higher-order" vector assembly method for increased multiplexing capacity. This higher-order assembly method in principle allows swift stacking of gRNAs cassettes that are only limited by the loading capacity of a cloning or expression vector.

Stable Inactivation of MicroRNAs in Medicago truncatula Roots

MicroRNAs are key regulators in the development processes or stress responses in plants. In the last decade, several conserved or non-conserved microRNAs have been identified in Medicago truncatula. Different strategies leading to the inactivation of microRNAs in plants have been described. Here, we propose a protocol for an effective inactivation of microRNAs using a STTM strategy in M. truncatula transgenic roots.

Small RNAs, emerging regulators critical for the development of horticultural traits

Small RNAs (sRNAs) have been recently recognized as key genetic and epigenetic regulators in various organisms, ranging from the modification of DNA and histone methylations to the modulation of the abundance of coding or non-coding RNAs. In plants, major regulatory sRNAs are classified as respective microRNA (miRNA) and small interfering RNA (siRNA) species, with the former primarily engaging in posttranscriptional regulation while the latter in transcriptional one. Many of these characterized sRNAs are involved in regulation of diverse biological programs, processes, and pathways in response to developmental cues, environmental signals/stresses, pathogen infection, and pest attacks. Recently, sRNAs-mediated regulations have also been extensively investigated in horticultural plants, with many novel mechanisms unveiled, which display far more mechanistic complexity and unique regulatory features compared to those studied in model species. Here, we review the recent progress of sRNA research in horticultural plants, with emphasis on mechanistic aspects as well as their relevance to trait regulation. Given that major and pioneered sRNA research has been carried out in the model and other plants, we also discuss ongoing sRNA research on these plants. Because miRNAs and phased siRNAs (phasiRNAs) are the most studied sRNA regulators, this review focuses on their biogenesis, conservation, function, and targeted genes and traits as well as the mechanistic relation between them, aiming at providing readers comprehensive information instrumental for future sRNA research in horticulture crops.

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.

RNase III Nucleases and the Evolution of Antiviral Systems

Every living entity requires the capacity to defend against viruses in some form. From bacteria to plants to arthropods, cells retain the capacity to capture genetic material, process it in a variety of ways, and subsequently use it to generate pathogen-specific small RNAs. These small RNAs can then be used to provide specificity to an otherwise non-specific nuclease, generating a potent antiviral system. While small RNA-based defenses in chordates are less utilized, the protein-based antiviral invention in this phylum appears to have derived from components of the same ancestral small RNA machinery. Based on recent evidence, it would seem that RNase III nucleases have been reiteratively repurposed over billions of years to provide cells with the capacity to recognize and destroy unwanted genetic material. Here we describe an overview of what is known on this subject and provide a model for how these defenses may have evolved.

Comparative Analysis of Fruit Ripening-Related miRNAs and Their Targets in Blueberry Using Small RNA and Degradome Sequencing

MicroRNAs (miRNAs) play vital roles in the regulation of fruit development and ripening. Blueberry is an important small berry fruit crop with economical and nutritional value. However, nothing is known about the miRNAs and their targets involved in blueberry fruit ripening. In this study, using high-throughput sequencing of small RNAs, 84 known miRNAs belonging to 28 families and 16 novel miRNAs were identified in white fruit (WF) and blue fruit (BF) libraries, which represent fruit ripening onset and in progress, respectively. Among them, 41 miRNAs were shown to be differentially expressed during fruit maturation, and 16 miRNAs representing 16 families were further chosen to validate the sRNA sequencing data by stem-loop qRT-PCR. Meanwhile, 178 targets were identified for 41 known and 7 novel miRNAs in WF and BF libraries using degradome sequencing, and targets of miR160 were validated using RLM-RACE (RNA Ligase-Mediated (RLM)-Rapid Amplification of cDNA Ends) approach. Moreover, the expression patterns of 6 miRNAs and their targets were examined during fruit development and ripening. Finally, integrative analysis of miRNAs and their targets revealed a complex miRNA-mRNA regulatory network involving a wide variety of biological processes. The findings will facilitate future investigations of the miRNA-mediated mechanisms that regulate fruit development and ripening in blueberry.

High-Throughput Sequencing of Small RNA Transcriptomes in Maize Kernel Identifies miRNAs Involved in Embryo and Endosperm Development

Maize kernel development is a complex biological process that involves the temporal and spatial expression of many genes and fine gene regulation at a transcriptional and post-transcriptional level, and microRNAs (miRNAs) play vital roles during this process. To gain insight into miRNA-mediated regulation of maize kernel development, a deep-sequencing technique was used to investigate the dynamic expression of miRNAs in the embryo and endosperm at three developmental stages in B73. By miRNA transcriptomic analysis, we characterized 132 known miRNAs and six novel miRNAs in developing maize kernel, among which, 15 and 14 miRNAs were commonly differentially expressed between the embryo and endosperm at 9 days after pollination (DAP), 15 DAP and 20 DAP respectively. Conserved miRNA families such as miR159, miR160, miR166, miR390, miR319, miR528 and miR529 were highly expressed in developing embryos; miR164, miR171, miR393 and miR2118 were highly expressed in developing endosperm. Genes targeted by those highly expressed miRNAs were found to be largely related to a regulation category, including the transcription, macromolecule biosynthetic and metabolic process in the embryo as well as the vitamin biosynthetic and metabolic process in the endosperm. Quantitative reverse transcription-PCR (qRT-PCR) analysis showed that these miRNAs displayed a negative correlation with the levels of their corresponding target genes. Importantly, our findings revealed that members of the miR169 family were highly and dynamically expressed in the developing kernel, which will help to exploit new players functioning in maize kernel development.

MicroRNA and Putative Target Discoveries in Chrysanthemum Polyploidy Breeding

MicroRNAs (miRNAs), around 22 nucleotides (nt) in length, are a class of endogenous and noncoding RNA molecule that play an essential role in plant development, either by suppressing the transcription of target genes at a transcriptional level or inhibiting translation at a posttranscriptional level. To understand the roles of miRNAs and their target genes in chrysanthemum polyploidy breeding, three sRNA libraries of normal and abnormal embryos after hybridization were performed by RNA-Seq. As a result, a total of 170 miRNAs were identified and there are 41 special miRNAs in cross of paternal chromosome doubling, such as miR169b, miR440, and miR528-5p. miR164c and miR159a were highly expressed in a normal embryo at 18 days after pollination, suggesting the regulatory role at the late stage of embryonic development. miR172c was only detected in the normal embryo at 18 days after pollination, which means that miR172c mainly mediates gene expression in postembryonic development and these genes may promote embryo maturation. Other miRNAs, including miR414, miR2661, and miR5021, may regulate the genes participated in pathways of auxin response and energy metabolism; then they regulate the complex embryonic development together.

miR160 and miR166/165 Contribute to the LEC2-Mediated Auxin Response Involved in the Somatic Embryogenesis Induction in Arabidopsis

MicroRNAs are non-coding small RNA molecules that are involved in the post-transcriptional regulation of the genes that control various developmental processes in plants, including zygotic embryogenesis (ZE). miRNAs are also believed to regulate somatic embryogenesis (SE), a counterpart of the ZE that is induced in vitro in plant somatic cells. However, the roles of specific miRNAs in the regulation of the genes involved in SE, in particular those encoding transcription factors (TFs) with an essential function during SE including LEAFY COTYLEDON2 (LEC2), remain mostly unknown. The aim of the study was to reveal the function of miR165/166 and miR160 in the LEC2-controlled pathway of SE that is induced in in vitro cultured Arabidopsis explants.In ZE, miR165/166 controls the PHABULOSA/PHAVOLUTA (PHB/PHV) genes, which are the positive regulators of LEC2, while miR160 targets the AUXIN RESPONSE FACTORS (ARF10, ARF16, ARF17) that control the auxin signaling pathway, which plays key role in LEC2-mediated SE. We found that a deregulated expression/function of miR165/166 and miR160 resulted in a significant accumulation of auxin in the cultured explants and the spontaneous formation of somatic embryos. Our results show that miR165/166 might contribute to SE induction via targeting PHB, a positive regulator of LEC2 that controls embryogenic induction via activation of auxin biosynthesis pathway (Wójcikowska et al., 2013). Similar to miR165/166, miR160 was indicated to control SE induction through auxin-related pathways and the negative impact of miR160 on ARF10/ARF16/ARF17 was shown in an embryogenic culture. Altogether, the results suggest that the miR165/166- and miR160-node contribute to the LEC2-mediated auxin-related pathway of embryogenic transition that is induced in the somatic cells of Arabidopsis. A model summarizing the suggested regulatory interactions between the miR165/166-PHB and miR160-ARF10/ARF16/ARF17 nodes that control SE induction in Arabidopsis was proposed.

Regulation of tRNA biogenesis in plants and its link to plant growth and response to pathogens

The field of tRNA biology, encompassing the functional and structural complexity of tRNAs, has fascinated scientists over the years and is continuously growing. Besides their fundamental role in protein translation, new evidence indicates that tRNA-derived molecules also regulate gene expression and protein synthesis in all domains of life. This review highlights some of the recent findings linking tRNA transcription and modification with plant cell growth and response to pathogens. In fact, mutations in proteins directly involved in tRNA synthesis and modification most often lead to pleiotropic effects on plant growth and immunity. As plants need to optimize and balance their energy and nutrient resources towards growth and defense, regulatory pathways that play a central role in integrating tRNA transcription and protein translation with cell growth control and organ development, such as the auxin-TOR signaling pathway, also influence the plant immune response against pathogens. As a consequence, distinct pathogens employ an array of effector molecules including tRNA fragments to target such regulatory pathways to exploit the plant's translational capacity, gain access to nutrients and evade defenses. An example includes the RNA polymerase III repressor MAF1, a conserved component of the TOR signaling pathway that controls ribosome biogenesis and tRNA synthesis required for plant growth and which is targeted by a pathogen effector molecule to promote disease. This article is part of a Special Issue entitled: SI: Regulation of tRNA synthesis and modification in physiological conditions and disease edited by Dr. Boguta Magdalena.

Comparative Analysis of Cotton Small RNAs and Their Target Genes in Response to Salt Stress

Small RNAs play an important role in regulating plant responses to abiotic stress. Depending on the method of salt application, whether sudden or gradual, plants may experience either salt shock or salt stress, respectively. In this study, small RNA expression in response to salt shock and long-term salt stress in parallel experiments was described. Cotton small RNA libraries were constructed and sequenced under normal conditions, as well as sudden and gradual salt application. A total of 225 cotton microRNAs (miRNAs) were identified and of these 24 were novel miRNAs. There were 88 and 75 miRNAs with differential expression under the salt shock and long-term salt stress, respectively. Thirty one transcripts were found to be targets of 20 miRNA families. Eight targets showed a negative correlation in expression with their corresponding miRNAs. We also identified two TAS3s with two near-identical 21-nt trans-acting small interfering RNA (tasiRNA)-Auxin Response Factors (ARFs) that coaligned with the phases D7(+) and D8(+) in three Gossypium species. The miR390/tasiRNA-ARFs/ARF4 pathway was identified and showed altered expression under salt stress. The identification of these small RNAs as well as elucidating their functional significance broadens our understanding of post-transcriptional gene regulation in response to salt stress.

The phenotypic and molecular assessment of the non-conserved Arabidopsis MICRORNA163/S-ADENOSYL-METHYLTRANSFERASE regulatory module during biotic stress

In plants, microRNAs (miRNAs) have evolved in parallel to the protein-coding genes that they target for expression regulation, and miRNA-directed gene expression regulation is central to almost every cellular process. MicroRNA, miR163, is unique to the Arabidopsis genus and is processed into a 24-nucleotide (nt) mature small regulatory RNA (sRNA) from a single precursor transcript transcribed from a single locus, the MIR163 gene. The MIR163 locus is a result of a recent inverted duplication event of one of the five closely related S-ADENOSYL-METHYLTRANSFERASE genes that the mature miR163 sRNA targets for expression regulation. Currently, however, little is known about the role of the miR163/S-ADENOSYL-METHYLTRANSFERASE regulatory module in response to biotic stress. Here, we document the expression domains of MIR163 and the S-ADENOSYL-METHYLTRANSFERASE target genes following fusion of their putative promoter sequences to the β-glucuronidase (GUS) reporter gene and subsequent in planta expression. Further, we report on our phenotypic and molecular assessment of Arabidopsis thaliana plants with altered miR163 accumulation, namely the mir163-1 and mir163-2 insertion knockout mutants and the miR163 overexpression line, the MIR163-OE plant. Finally, we reveal miR163 accumulation and S-ADENOSYL-METHYLTRANSFERASE target gene expression post treatment with the defence elicitors, salicylic acid and jasmonic acid, and following Fusarium oxysporum infection, wounding, and herbivory attack. Together, the work presented here provides a comprehensive new biological insight into the role played by the Arabidopsis genus-specific miR163/S-ADENOSYL-METHYLTRANSFERASE regulatory module in normal A. thaliana development and during the exposure of A. thaliana plants to biotic stress.

Identification and characterization of Prunus persica miRNAs in response to UVB radiation in greenhouse through high-throughput sequencing

Background

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression of target mRNAs involved in plant growth, development, and abiotic stress. As one of the most important model plants, peach (Prunus persica) has high agricultural significance and nutritional values. It is well adapted to be cultivated in greenhouse in which some auxiliary conditions like temperature, humidity, and UVB etc. are needed to ensure the fruit quality. However, little is known about the genomic information of P. persica under UVB supplement. Transcriptome and expression profiling data for this species are therefore important resources to better understand the biological mechanism of seed development, formation and plant adaptation to environmental change. Using a high-throughput miRNA sequencing, followed by qRT-PCR tests and physiological properties determination, we identified the responsive-miRNAs under low-dose UVB treatment and described the expression pattern and putative function of related miRNAs and target genes in chlorophyll and carbohydrate metabolism.

Results

A total of 164 known peach miRNAs belonging to 59 miRNA families and 109 putative novel miRNAs were identified. Some of these miRNAs were highly conserved in at least four other plant species. In total, 1794 and 1983 target genes for known and novel miRNAs were predicted, respectively. The differential expression profiles of miRNAs between the control and UVB-supplement group showed that UVB-responsive miRNAs were mainly involved in carbohydrate metabolism and signal transduction. UVB supplement stimulated peach to synthesize more chlorophyll and sugars, which was verified by qRT-PCR tests of related target genes and metabolites’ content measurement.

Conclusion

The high-throughput sequencing data provided the most comprehensive miRNAs resource available for peach study. Our results identified a series of differentially expressed miRNAs/target genes that were predicted to be low-dose UVB-responsive. The correlation between transcriptional profiles and metabolites contents in UVB supplement groups gave novel clues for the regulatory mechanism of miRNAs in Prunus. Low-dose UVB supplement could increase the chlorophyll and sugar (sorbitol) contents via miRNA-target genes and therefore improve the fruit quality in protected cultivation of peaches.

Electronic supplementary material

The online version of this article (doi: 10.1186/s12864-017-4347-5) contains supplementary material, which is available to authorized users.

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.

MicroRNA396a-5p and -3p induce tomato disease susceptibility by suppressing target genes and upregulating salicylic acid

Plants have evolved a variety of mechanisms to perceive and resist the assault of pathogens. The biotrophs, necrotrophs and hemibiotrophs are types of plant pathogens that activate diverse salicylic acid (SA) and jasmonic acid (JA) signaling pathways. In this study we showed that the expressions of miR396a-5p and -3p in Solanum lycopersicum (S. lycopersicum) were both down-regulated after infection by hemibiotroph Phytophthora infestans (P. infestans) and necrotroph Botrytis cinerea (B. cinerea) infection. Overexpression of miR396a-5p and -3p in transgenic tomato enhanced the susceptibility of S. lycopersicum to P. infestans and B. cinerea infection and the tendency to produce reactive oxygen species (ROS) under pathogen-related biotic stress. Additionally, miR396a regulated growth-regulating factor1 (GRF1), salicylic acid carboxyl methyltransferase (SAMT), glycosyl hydrolases (GH) and nucleotide-binding site-leucine-rich repeat (NBS-LRR) and down-regulated their levels. This ultimately led to inhibition of the expression of pathogenesis-related 1 (PR1), TGA transcription factors1 and 2 (TGA1 and TGA2) and JA-dependent proteinase inhibitors I and II (PI I and II), but enhanced the endogenous SA content and nonexpressor of pathogenesis-related genes 1 (NPR1) expression. Taken together, our results showed that negative regulation of target genes and their downstream genes expressions by miR396a-5p and -3p are critical for tomato abiotic stresses via affecting SA or JA signaling pathways.

Bioinformatics resources for deciphering the biogenesis and action pathways of plant small RNAs

The next-generation sequencing (NGS) technology has revolutionized our previous understanding of the plant genomes, relying on its innate advantages, such as high throughput and deep sequencing depth. In addition to the protein-coding gene loci, massive transcription signals have been detected within intergenic or intragenic regions. Most of these signals belong to non-coding ones, considering their weak protein-coding potential. Generally, these transcripts could be divided into long non-coding RNAs and small non-coding RNAs (sRNAs) based on their sequence length. In addition to the well-known microRNAs (miRNAs), many plant endogenous sRNAs were collectively referred to as small interfering RNAs. However, an increasing number of unclassified sRNA species are being discovered by NGS. The high heterogeneity of these novel sRNAs greatly hampered the mechanistic studies, especially on the clear description of their biogenesis and action pathways. Fortunately, public databases, bioinformatics softwares and NGS datasets are increasingly available for plant sRNA research. Here, by summarizing these valuable resources, we proposed a general workflow to decipher the RDR (RNA-dependent RNA polymerase)- and DCL (Dicer-like)-dependent biogenesis pathways, and the Argonaute-mediated action modes (such as target cleavages and chromatin modifications) for specific sRNA species in plants. Taken together, we hope that by summarizing a list of the public resources, this work will facilitate the plant biologists to perform classification and functional characterization of the interesting sRNA species.

An expression atlas of miRNAs in Arabidopsis thaliana

MicroRNAs (miRNAs) are small non-coding RNAs that regulate a variety of biological processes. MiRNA expression often exhibits spatial and temporal specificity. However, genome-wide miRNA expression patterns in different organs during development of Arabidopsis thaliana have not yet been systemically investigated. In this study, we sequenced small RNA libraries generated from 27 different organ/tissue types, which cover the entire life cycle of Arabidopsis. Analysis of the sequencing data revealed that most miRNAs are ubiquitously expressed, whereas a small set of miRNAs display highly specific expression patterns. In addition, different miRNA members within the same family have distinct spatial and temporal expression patterns. Moreover, we found that some miRNAs are produced from different arms of their hairpin precursors at different developmental stages. This work provides new insights into the regulation of miRNA biogenesis and a rich resource for future investigation of miRNA functions in Arabidopsis.

Genome-wide analysis of miRNAs in Carya cathayensis

BACKGROUND

MicroRNA (miRNA) plays an important role in plant development regulation. Hickory is an economically important plant in which the amount of flowering determines its production.

RESULTS

Here, 51 conserved miRNAs, which belong to 16 families and 195 novel miRNAs were identified in hickory genome. For each conserved miRNA family, we used sequences from hickory and other plants to construct a phylogenetic tree, which shows that each family has members in hickory. Some of the conserved miRNA families (i.e., miR167 and miR397) have more members in hickory than in other plants because of gene expansion. MiR166 exhibited tandem duplication with three copies being observed. Many members of these conserved miRNA families were detected in hickory flowers, and the expression patterns of target genes were opposite to those of the related miRNAs, indicating that miRNAs may have important functions in floral regulation of hickory.

CONCLUSIONS

Taken together, a comprehensive analysis was conducted to identify miRNAs produced in hickory flower organs, demonstrating functional conservation and diversity of miRNA families among hickory, Arabidopsis, grape, and poplar.