Parallel analysis of RNA ends enhances global investigation of microRNAs and target RNAs of Brachypodium distachyon

Widespread changes to the translational landscape in a maize microRNA biogenesis mutant

MicroRNAs are short, non-coding RNAs that repress gene expression in both plants and animals and have diverse functions related to growth, development, and stress responses. The ribonuclease, DICER-LIKE1 (DCL1) is required for two steps in plant miRNA biogenesis: cleavage of the primary miRNAs (pri-miRNAs) to release a hairpin structure, called the precursor miRNA (pre-miRNA) and cleavage of the pre-miRNA to generate the miRNA/miRNA* duplex. The mature miRNA guides the RNA-induced silencing complex to target RNAs with complementary sequences, resulting in translational repression and/or RNA cleavage of target mRNAs. However, the relative contribution of translational repression versus mRNA degradation by miRNAs remains unknown at the genome-level in crops, especially in maize. The maize fuzzy tassel (fzt) mutant contains a hypomorphic mutation in DCL1 resulting in broad developmental defects. While most miRNAs are reduced in fzt, the levels of miRNA-targeted mRNAs are not dramatically increased, suggesting that translational regulation by miRNAs may be common. To gain insight into the repression mechanism of plant miRNAs, we combined ribosome profiling and RNA-sequencing to globally survey miRNA activities in maize. Our data indicate that translational repression contributes significantly to regulation of most miRNA targets and that approximately one-third of miRNA targets are regulated primarily at the translational level. Surprisingly, ribosomes appear altered in fzt mutants suggesting that DCL1 may also have a role in ribosome biogenesis. Thus, DICER-LIKE1 shapes the translational landscape in plants through both miRNA-dependent and miRNA-independent mechanisms.

Overview of Computational and Experimental Methods to Identify Tissue-Specific MicroRNA Targets

As ubiquitous posttranscriptional regulators of gene expression, microRNAs (miRNAs) play key roles in cell physiology and function across taxa. In the last two decades, we have gained a good understanding about miRNA biogenesis pathways, modes of action, and consequences of miRNA-mediated gene regulation. More recently, research has focused on exploring causes for miRNA dysregulation, miRNA-mediated crosstalk between genes and signaling pathways, and the role of miRNAs in disease.This chapter discusses methods for the identification of miRNA-target interactions and causes for tissue-specific miRNA-target regulation. Computational approaches for predicting miRNA target sites and assessing tissue-specific target regulation are discussed. Moreover, there is an emphasis on features that affect miRNA target recognition and how high-throughput sequencing protocols can help in assessing miRNA-mediated gene regulation on a genome-wide scale. In addition, this chapter introduces some experimental approaches for the validation of miRNA targets as well as web-based resources sharing predicted and validated miRNA-target interactions.

The identification of conserved sequence features of co-translationally decayed mRNAs and upstream open reading frames in angiosperm transcriptomes

RNA turnover is essential in maintaining messenger RNA (mRNA) homeostasis during various developmental stages and stress responses. Co-translational mRNA decay (CTRD), a process in which mRNAs are degraded while still associated with translating ribosomes, has recently been discovered to function in yeast and three angiosperm transcriptomes. However, it is still unclear how prevalent CTRD across the plant lineage. Moreover, the sequence features of co-translationally decayed mRNAs have not been well-studied. Here, utilizing a collection of publicly available degradome sequencing datasets for another seven angiosperm transcriptomes, we have confirmed that CTRD is functioning in at least 10 angiosperms and likely throughout the plant lineage. Additionally, we have identified sequence features shared by the co-translationally decayed mRNAs in these species, implying a possible conserved triggering mechanism for this pathway. Given that degradome sequencing datasets can also be used to identify actively translating upstream open reading frames (uORFs), which are quite understudied in plants, we have identified numerous actively translating uORFs in the same 10 angiosperms. These findings reveal that actively translating uORFs are prevalent in plant transcriptomes, some of which are conserved across this lineage. We have also observed conserved sequence features in the regions flanking these uORFs' stop codons that might contribute to ribosome stalling at these sequences. Finally, we discovered that there were very few overlaps between the mRNAs harboring actively translating uORFs and those sorted into the co-translational decay pathway in the majority of the studied angiosperms, suggesting that these two processes might be nearly mutually exclusive in those species. In total, our findings provide the identification of CTRD and actively translating uORFs across a broad collection of plants and provide novel insights into the important sequence features associated with these collections of mRNAs and regulatory elements, respectively.

Parallel analysis of RNA ends reveals global microRNA-mediated target RNA cleavage in maize

MicroRNAs (miRNAs) are endogenous 20-24-nucleotide non-coding RNAs that play important regulatory roles in many biological processes in eukaryotes. miRNAs modulate the expression of target genes at the post-transcriptional level by transcript cleavage or translational inhibition. The identification of miRNA target genes has been extensively investigated in Arabidopsis and rice, but an in-depth global analysis of miRNA-mediated target regulation is still lacking in maize. Here, we report a transcriptome-wide identification of miRNA targets by analyzing parallel analysis of RNA ends (PARE) datasets derived from nine different tissues at five developmental stages of the maize (Zea mays L.) B73 cultivar. In total, 246 targets corresponding to 60 miRNAs from 25 families were identified, including transcription factors and other genes. In addition, PARE analysis revealed that miRNAs guide specific target transcript cleavage in a tissue-preferential manner. Primary transcripts of MIR159c and MIR169e were found to be cleaved by mature miR159 and miR169, respectively, indicating a negative-feedback regulatory mechanism in miRNA biogenesis. Moreover, several miRNA-target gene pairs involved in seed germination were identified and experimentally validated. Our PARE analyses generated a wide and detailed miRNA-target interaction atlas, which provides a valuable resource for investigating the roles of miRNAs and their targets in maize.

TRUEE; a bioinformatic pipeline to define the functional microRNA targetome of Arabidopsis

Central to plant microRNA (miRNA) biology is the identification of functional miRNA-target interactions (MTIs). However, the complementarity basis of bioinformatic target prediction results in mostly false positives, and the degree of complementarity does not equate with regulation. Here, we develop a bioinformatic workflow named TRUEE (Targets Ranked Using Experimental Evidence) that ranks MTIs on the extent to which they are subjected to miRNA-mediated cleavage. It sorts predicted targets into high (HE) and low evidence (LE) groupings based on the frequency and strength of miRNA-guided cleavage degradome signals across multiple degradome experiments. From this, each target is assigned a numerical value, termed a Category Score, ranking the extent to which it is subjected to miRNA-mediated cleavage. As a proof-of-concept, the 428 Arabidopsis miRNAs annotated in miRBase were processed through the TRUEE pipeline to determine the miRNA 'targetome'. The majority of high-ranking Category Score targets corresponded to highly conserved MTIs, validating the workflow. Very few Arabidopsis-specific, Brassicaceae-specific, or Conserved-passenger miRNAs had HE targets with high Category Scores. In total, only several hundred MTIs were found to have Category Scores characteristic of currently known physiologically significance MTIs. Although non-exhaustive, clearly the number of functional MTIs is much narrower than many studies claim. Therefore, using TRUEE to numerically rank targets  directly on experimental evidence has given insights into the scope of the functional miRNA targetome of Arabidopsis.

A unique miR775-GALT9 module regulates leaf senescence in Arabidopsis during post-submergence recovery by modulating ethylene and the abscisic acid pathway

The submergence-induced hypoxic condition negatively affects the plant growth and development, and causes early onset of senescence. Hypoxia alters the expression of a number of microRNAs (miRNAs). However, the molecular function of submergence stress-induced miRNAs in physiological or developmental changes and recovery remains poorly understood. Here, we show that miR775 is an Arabidopsis thaliana-specific young and unique miRNA that possibly evolved non-canonically. miR775 post-transcriptionally regulates GALACTOSYLTRANSFERASE 9 (GALT9) and their expression is inversely affected at 24 h of complete submergence stress. The overexpression of miR775 (miR775-Oe) confers enhanced recovery from submergence stress and reduced accumulation of RBOHD and ROS, in contrast to wild-type and MIM775 Arabidopsis shoot. A similar recovery phenotype in the galt9 mutant indicates the role of the miR775-GALT9 module in post-submergence recovery. We predicted that Golgi-localized GALT9 is potentially involved in protein glycosylation. The altered expression of senescence-associated genes (SAG12, SAG29 and ORE1), ethylene signalling (EIN2 and EIN3) and abscisic acid (ABA) biosynthesis (NCED3) pathway genes occurs in miR775-Oe, galt9 and MIM775 plants. Thus, our results indicate the role for the miR775-GALT9 module in post-submergence recovery through a crosstalk between the ethylene signalling and ABA biosynthesis pathways.

Aegilops tauschii genome assembly Aet v5.0 features greater sequence contiguity and improved annotation

Aegilops tauschii is the donor of the D subgenome of hexaploid wheat and an important genetic resource. The reference-quality genome sequence Aet v4.0 for Ae. tauschii acc. AL8/78 was therefore an important milestone for wheat biology and breeding. Further advances in sequencing acc. AL8/78 and release of the Aet v5.0 sequence assembly are reported here. Two new optical maps were constructed and used in the revision of pseudomolecules. Gaps were closed with Pacific Biosciences long-read contigs, decreasing the gap number by 38,899. Transposable elements and protein-coding genes were reannotated. The number of annotated high-confidence genes was reduced from 39,635 in Aet v4.0 to 32,885 in Aet v5.0. A total of 2245 biologically important genes, including those affecting plant phenology, grain quality, and tolerance of abiotic stresses in wheat, was manually annotated and disease-resistance genes were annotated by a dedicated pipeline. Disease-resistance genes encoding nucleotide-binding site domains, receptor-like protein kinases, and receptor-like proteins were preferentially located in distal chromosome regions, whereas those encoding transmembrane coiled-coil proteins were dispersed more evenly along the chromosomes. Discovery, annotation, and expression analyses of microRNA (miRNA) precursors, mature miRNAs, and phasiRNAs are reported, including miRNA target genes. Other small RNAs, such as hc-siRNAs and tRFs, were characterized. These advances enhance the utility of the Ae. tauschii genome sequence for wheat genetics, biotechnology, and breeding.

Posttranscriptional regulation of cellulose synthase genes by small RNAs derived from cellulose synthase antisense transcripts

Transcriptional regulatory mechanisms governing plant cell wall biosynthesis are incomplete. Expression programs that activate wall biosynthesis are well understood, but mechanisms that control the attenuation of gene expression networks remain elusive. Previous work has shown that small RNAs (sRNAs) derived from the HvCESA6 (Hordeum vulgare, Hv) antisense transcripts are naturally produced and are capable of regulating aspects of wall biosynthesis. Here, we further test the hypothesis that CESA-derived sRNAs generated from CESA antisense transcripts are involved in the regulation of cellulose and broader cell wall biosynthesis. Antisense transcripts were detected for some but not all members of the CESA gene family in both barley and Brachypodium distachyon. Phylogenetic analysis indicates that antisense transcripts are detected for most primary cell wall CESA genes, suggesting a possible role in the transition from primary to secondary cell wall biosynthesis. Focusing on one antisense transcript, HvCESA1 shows dynamic expression throughout development, is correlated with corresponding sRNAs over the same period and is anticorrelated with HvCESA1 mRNA expression. To assess the broader impacts of CESA-derived sRNAs on the regulation of cell wall biosynthesis, transcript profiling was performed on barley tissues overexpressing CESA-derived sRNAs. Together, the data support the hypothesis that CESA antisense transcripts function through an RNA-induced silencing mechanism, to degrade cis transcripts, and may also trigger trans-acting silencing on related genes to alter the expression of cell wall gene networks.

Overlapping and stress-specific transcriptomic and hormonal responses to flooding and drought in soybean

Flooding and drought are serious constraints that reduce crop productivity worldwide. Previous studies identified genes conferring tolerance to both water extremes in various plants. However, overlapping responses to flooding and drought at the genome-scale remain obscure. Here, we defined overlapping and stress-specific transcriptomic and hormonal responses to submergence, drought and recovery from these stresses in soybean (Glycine max). We performed comparative RNA-sequencing and hormone profiling, identifying genes, hormones and biological processes that are differentially regulated in an overlapping or stress-specific manner. Overlapping responses included positive regulation of trehalose and sucrose metabolism and negative regulation of cellulose, tubulin, photosystem II and I, and chlorophyll biosynthesis, facilitating the economization of energy reserves under both submergence and drought. Additional energy-consuming pathways were restricted in a stress-specific manner. Downregulation of distinct pathways for energy saving under each stress suggests energy-consuming processes that are relatively unnecessary for each stress adaptation are turned down. Our newly developed transcriptomic-response analysis revealed that abscisic acid and ethylene responses were activated in common under both stresses, whereas stimulated auxin response was submergence-specific. The energy-saving strategy is the key overlapping mechanism that underpins adaptation to both submergence and drought in soybean. Abscisic acid and ethylene are candidate hormones that coordinate transcriptomic energy-saving processes under both stresses. Auxin may be a signaling component that distinguishes submergence-specific regulation of the stress response.

Pan-Genome miRNomics in Brachypodium

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

Genome-Wide Changes of Regulatory Non-Coding RNAs Reveal Pollen Development Initiated at Ecodormancy in Peach

Bud dormancy is under the regulation of complex mechanisms including genetic and epigenetic factors. To study the function of regulatory non-coding RNAs in winter dormancy release, we analyzed the small RNA and long non-coding RNA (lncRNA) expression from peach (Prunus persica) floral buds in endodormancy, ecodormancy and bud break stages. Small RNAs underwent a major shift in expression primarily between dormancy and flowering with specific pairs of microRNAs and their mRNA target genes undergoing coordinated differential expression. From endodormancy to ecodormancy, ppe-miR6285 was significantly upregulated while its target gene, an ASPARAGINE-RICH PROTEIN involved in the regulation of abscisic acid signaling, was downregulated. At ecodormancy, ppe-miR2275, a homolog of meiosis-specific miR2275 across angiosperms, was significantly upregulated, supporting microsporogenesis in anthers at a late stage of dormancy. The expression of 785 lncRNAs, unlike the overall expression pattern in the small RNAs, demonstrated distinctive expression signatures across all dormancy and flowering stages. We predicted that a subset of lncRNAs were targets of microRNAs and found 18 lncRNA/microRNA target pairs with both differentially expressed across time points. The genome-wide differential expression and network analysis of non-coding RNAs and mRNAs from the same tissues provide new candidate loci for dormancy regulation and suggest complex noncoding RNA interactions control transcriptional regulation across these key developmental time points.

Small Non-Coding RNAs at the Crossroads of Regulatory Pathways Controlling Somatic Embryogenesis in Seed Plants

Small non-coding RNAs (sncRNAs) are molecules with important regulatory functions during development and environmental responses across all groups of terrestrial plants. In seed plants, the development of a mature embryo from the zygote follows a synchronized cell division sequence, and growth and differentiation events regulated by highly regulated gene expression. However, given the distinct features of the initial stages of embryogenesis in gymnosperms and angiosperms, it is relevant to investigate to what extent such differences emerge from differential regulation mediated by sncRNAs. Within these, the microRNAs (miRNAs) are the best characterized class, and while many miRNAs are conserved and significantly represented across angiosperms and other seed plants during embryogenesis, some miRNA families are specific to some plant lineages. Being a model to study zygotic embryogenesis and a relevant biotechnological tool, we systematized the current knowledge on the presence and characterization of miRNAs in somatic embryogenesis (SE) of seed plants, pinpointing the miRNAs that have been reported to be associated with SE in angiosperm and gymnosperm species. We start by conducting an overview of sncRNA expression profiles in the embryonic tissues of seed plants. We then highlight the miRNAs described as being involved in the different stages of the SE process, from its induction to the full maturation of the somatic embryos, adding references to zygotic embryogenesis when relevant, as a contribution towards a better understanding of miRNA-mediated regulation of SE.

Comparative Analysis of Transcriptome and sRNAs Expression Patterns in the Brachypodium distachyon- Magnaporthe oryzae Pathosystems

The hemibiotrophic fungus Magnaporthe oryzae (Mo) is the causative agent of rice blast and can infect aerial and root tissues of a variety of Poaceae, including the model Brachypodium distachyon (Bd). To gain insight in gene regulation processes occurring at early disease stages, we comparatively analyzed fungal and plant mRNA and sRNA expression in leaves and roots. A total of 310 Mo genes were detected consistently and differentially expressed in both leaves and roots. Contrary to Mo, only minor overlaps were observed in plant differentially expressed genes (DEGs), with 233 Bd-DEGs in infected leaves at 2 days post inoculation (DPI), compared to 4978 at 4 DPI, and 138 in infected roots. sRNA sequencing revealed a broad spectrum of Mo-sRNAs that accumulated in infected tissues, including candidates predicted to target Bd mRNAs. Conversely, we identified a subset of potential Bd-sRNAs directed against fungal cell wall components, virulence genes and transcription factors. We also show a requirement of operable RNAi genes from the DICER-like (DCL) and ARGONAUTE (AGO) families for fungal virulence. Overall, our work elucidates the extensive reprogramming of transcriptomes and sRNAs in both plant host (Bd) and fungal pathogen (Mo), further corroborating the critical role played by sRNA species in the establishment of the interaction and its outcome.

Differential Expression of Maize and Teosinte microRNAs under Submergence, Drought, and Alternated Stress

Submergence and drought stresses are the main constraints to crop production worldwide. MicroRNAs (miRNAs) are known to play a major role in plant response to various stresses. In this study, we analyzed the expression of maize and teosinte miRNAs by high-throughput sequencing of small RNA libraries in maize and its ancestor teosinte (Zea mays ssp. parviglumis), under submergence, drought, and alternated stress. We found that the expression patterns of 67 miRNA sequences representing 23 miRNA families in maize and other plants were regulated by submergence or drought. miR159a, miR166b, miR167c, and miR169c were downregulated by submergence in both plants but more severely in maize. miR156k and miR164e were upregulated by drought in teosinte but downregulated in maize. Small RNA profiling of teosinte subject to alternate treatments with drought and submergence revealed that submergence as the first stress attenuated the response to drought, while drought being the first stress did not alter the response to submergence. The miRNAs identified herein, and their potential targets, indicate that control of development, growth, and response to oxidative stress could be crucial for adaptation and that there exists evolutionary divergence between these two subspecies in miRNA response to abiotic stresses.

Global Analysis of the Human RNA Degradome Reveals Widespread Decapped and Endonucleolytic Cleaved Transcripts

RNA decay is an important regulatory mechanism for gene expression at the posttranscriptional level. Although the main pathways and major enzymes that facilitate this process are well defined, global analysis of RNA turnover remains under-investigated. Recent advances in the application of next-generation sequencing technology enable its use in order to examine various RNA decay patterns at the genome-wide scale. In this study, we investigated human RNA decay patterns using parallel analysis of RNA end-sequencing (PARE-seq) data from XRN1-knockdown HeLa cell lines, followed by a comparison of steady state and degraded mRNA levels from RNA-seq and PARE-seq data, respectively. The results revealed 1103 and 1347 transcripts classified as stable and unstable candidates, respectively. Of the unstable candidates, we found that a subset of the replication-dependent histone transcripts was polyadenylated and rapidly degraded. Additionally, we identified 380 endonucleolytically cleaved candidates by analyzing the most abundant PARE sequence on a transcript. Of these, 41.4% of genes were classified as unstable genes, which implied that their endonucleolytic cleavage might affect their mRNA stability. Furthermore, we identified 1877 decapped candidates, including HSP90B1 and SWI5, having the most abundant PARE sequences at the 5′-end positions of the transcripts. These results provide a useful resource for further analysis of RNA decay patterns in human cells.

The small RNA microRNA-212 regulates sirtuin 2 expression in a cellular model of oxygen-glucose deprivation

MicroRNA-212 has been found to play an important role in several types of diseases, but the functional and potential mechanisms of microRNA-212 in ischemic brain injury are still unclear. The aims of this study were to investigate the potential role of microRNA-212 in ischemic brain injury and to reveal potential molecular mechanisms. The rat oxygen-glucose deprivation and simulated reperfusion model was established to study the role of microRNA-212 in ischemic brain injury. The expression of microRNA-212 in oxygen-glucose deprivation and simulated reperfusion model and its effect on cell proliferation were measured by quantitative reverse transcription PCR and Cell Counting Kit-8 assay, respectively. The relationships between microRNA-212 and sirtuin 2 were confirmed by luciferase-reporter assay. We observed that microRNA-212 was downregulated after oxygen-glucose deprivation and simulated reperfusion treatment. Besides, the cells viabilities were increased/decreased in oxygen-glucose deprivation and simulated reperfusion model after transfection with microRNA-212 agomir (agonist of microRNA-212 action) and microRNA-212 antagomir (inhibitor of microRNA-212 action). In addition, luciferase and western blot experiments showed that microRNA-212 directly regulated sirtuin 2 changes. Furthermore, promotion of neuronal survival by microRNA-212 was blocked by overexpression of sirtuin 2, whereas the neuronal death induced by microRNA-212 inhibition was rescued by sirtuin 2 inhibition. Taken together, our study revealed that the role of miR-212 in the modulation of ischemic brain injury might be achieved by regulating sirtuin 2, which provides potential biomarkers and candidates for the treatment of cerebral ischemia.

Genome-wide analysis of brassinosteroid responsive small RNAs in Arabidopsis thaliana

BACKGROUND

Brassinosteroids (BRs) are a class of phytohormones with important roles in regulating physiological and developmental processes. Small RNAs, including small interfering RNAs and microRNAs (miRNAs), are non-protein coding RNAs that regulate gene expression at the transcriptional and post-transcriptional levels. However, the roles of small RNAs in BR response have not been studied well.

OBJECTIVE

In this study, we aimed to identify BR-responsive small RNA clusters and miRNAs in Arabidopsis. In addition, the effect of BR-responsive small RNAs on their transcripts and target genes were examined.

METHODS

Small RNA libraries were constructed from control and epibrassinolide-treated seedlings expressing wild-type BRI1-Flag protein under its native promoter in the bri1-5 mutant. After sequencing the small RNA libraries, differentially expressed small RNA clusters were identified by examining the expression levels of small RNAs in 100-nt bins of the Arabidopsis genome. To identify the BR-responsive miRNAs, the expression levels of all the annotated mature miRNAs, registered in miRBase, were analyzed. Previously published RNA-seq data were utilized to monitor the BR-responsive expression patterns of differentially expressed small RNA clusters and miRNA target genes.

RESULTS

In results, 38 BR-responsive small RNA clusters, including 30 down-regulated and eight up-regulated clusters, were identified. These differentially expressed small RNA clusters were from miRNA loci, transposons, protein-coding genes, pseudogenes and others. Of these, a transgene, BRI1, accumulates small RNAs, which are not found in the wild type. Small RNAs in this transgene are up-regulated by BRs while BRI1 mRNA is down-regulated by BRs. By analyzing the expression patterns of mature miRNAs, we have identified BR-repressed miR398a-5p and BR-induced miR156g. Although miR398a-5p is down-regulated by BRs, its predicted targets were not responsive to BRs. However, SPL3, a target of BR-inducible miR156g, is down-regulated by BRs.

CONCLUSION

BR-responsive small RNAs and miRNAs identified in this study will provide an insight into the role of small RNAs in BR responses in plants. Especially, we suggest that miR156g/SPL3 module might play a role in BR-mediated growth and development in Arabidopsis.

Evolution of PHAS loci in the young spike of Allohexaploid wheat

BACKGROUND

PhasiRNAs (phased secondary siRNAs) play important regulatory roles in the development processes and biotic or abiotic stresses in plants. Some of phasiRNAs involve in the reproductive development in grasses, which include two categories, 21-nt (nucleotide) and 24-nt phasiRNAs. They are triggered by miR2118 and miR2275 respectively, in premeiotic and meiotic anthers of rice, maize and other grass species. Wheat (Triticum aestivum) with three closely related subgenomes (subA, subB and subD), is a model of allopolyploid in plants. Knowledge about the role of phasiRNAs in the inflorescence development of wheat is absent until now, and the evolution of PHAS loci in polyploid plants is also unavailable.

RESULTS

Using 261 small RNA expression datasets from various tissues, a batch of PHAS (phasiRNA precursors) loci were identified in the young spike of wheat, most of which were regulated by miR2118 and miR2275 in their target site regions. Dissection of PHAS and their trigger miRNAs among the diploid (AA and DD), tetraploid (AABB) and hexaploid (AABBDD) genomes of Triticum indicated that distribution of PHAS loci were dominant randomly in local chromosomes, while miR2118 was dominant only in the subB genome. The diversity of PHAS loci in the three subgenomes of wheat and their progenitor genomes (AA, DD and AABB) suggested that they originated or diverged at least before the occurrence of the tetraploid AABB genome. The positive correlation between the PHAS loci or the trigger miRNAs and the ploidy of genome indicated the expansion of genome was the major drive force for the increase of PHAS loci and their trigger miRNAs in Triticum. In addition, the expression profiles of the PHAS transcripts suggested they responded to abiotic stresses such as cold stress in wheat.

CONCLUSIONS

Altogether, non-coding phasiRNAs are conserved transcriptional regulators that display quick plasticity in Triticum genome. They may be involved in reproductive development and abiotic stress in wheat. It could be referred to molecular research on male reproductive development in Triticum.

Integrated Genome-Scale Analysis and Northern Blot Detection of Retrotransposon siRNAs Across Plant Species

Cells have sophisticated RNA-directed mechanisms to regulate genes, destroy viruses, or silence transposable elements (TEs). In terrestrial plants, a specialized non-coding RNA machinery involving RNA polymerase IV (Pol IV) and small interfering RNAs (siRNAs) targets DNA methylation and silencing to TEs. Here, we present a bioinformatics protocol for annotating and quantifying siRNAs that derive from long terminal repeat (LTR) retrotransposons. The approach was validated using small RNA northern blot analyses, comparing the species Arabidopsis thaliana and Brachypodium distachyon. To assist hybridization probe design, we configured a genome browser to show small RNA-seq mappings in distinct colors and shades according to their nucleotide lengths and abundances, respectively. Samples from wild-type and pol IV mutant plants, cross-species negative controls, and a conserved microRNA control validated the detected siRNA signals, confirming their origin from specific TEs and their Pol IV-dependent biogenesis. Moreover, an optimized labeling method yielded probes that could detect low-abundance siRNAs from B. distachyon TEs. The integration of de novo TE annotation, small RNA-seq profiling, and northern blotting, as outlined here, will facilitate the comparative genomic analysis of RNA silencing in crop plants and non-model species.

RNA degradomes reveal substrates and importance for dark and nitrogen stress responses of Arabidopsis XRN4

XRN4, the plant cytoplasmic homolog of yeast and metazoan XRN1, catalyzes exoribonucleolytic degradation of uncapped mRNAs from the 5' end. Most studies of cytoplasmic XRN substrates have focused on polyadenylated transcripts, although many substrates are likely first deadenylated. Here, we report the global investigation of XRN4 substrates in both polyadenylated and nonpolyadenylated RNA to better understand the impact of the enzyme in Arabidopsis. RNA degradome analysis demonstrated that xrn4 mutants overaccumulate many more decapped deadenylated intermediates than those that are polyadenylated. Among these XRN4 substrates that have 5' ends precisely at cap sites, those associated with photosynthesis, nitrogen responses and auxin responses were enriched. Moreover, xrn4 was found to be defective in the dark stress response and lateral root growth during N resupply, demonstrating that XRN4 is required during both processes. XRN4 also contributes to nonsense-mediated decay (NMD) and xrn4 accumulates 3' fragments of select NMD targets, despite the lack of the metazoan endoribonuclease SMG6 in plants. Beyond demonstrating that XRN4 is a major player in multiple decay pathways, this study identified intriguing molecular impacts of the enzyme, including those that led to new insights about mRNA decay and discovery of functional contributions at the whole-plant level.

Small RNA profiling in Pinus pinaster reveals the transcriptome of developing seeds and highlights differences between zygotic and somatic embryos

Regulation of seed development by small non-coding RNAs (sRNAs) is an important mechanism controlling a crucial phase of the life cycle of seed plants. In this work, sRNAs from seed tissues (zygotic embryos and megagametophytes) and from somatic embryos of Pinus pinaster were analysed to identify putative regulators of seed/embryo development in conifers. In total, sixteen sRNA libraries covering several developmental stages were sequenced. We show that embryos and megagametophytes express a large population of 21-nt sRNAs and that substantial amounts of 24-nt sRNAs were also detected, especially in somatic embryos. A total of 215 conserved miRNAs, one third of which are conifer-specific, and 212 high-confidence novel miRNAs were annotated. MIR159, MIR171 and MIR394 families were found in embryos, but were greatly reduced in megagametophytes. Other families, like MIR397 and MIR408, predominated in somatic embryos and megagametophytes, suggesting their expression in somatic embryos is associated with in vitro conditions. Analysis of the predicted miRNA targets suggests that miRNA functions are relevant in several processes including transporter activity at the cotyledon-forming stage, and sulfur metabolism across several developmental stages. An important resource for studying conifer embryogenesis is made available here, which may also provide insightful clues for improving clonal propagation via somatic embryogenesis.

Submergence and Waterlogging Stress in Plants: A Review Highlighting Research Opportunities and Understudied Aspects

Soil flooding creates composite and complex stress in plants known as either submergence or waterlogging stress depending on the depth of the water table. In nature, these stresses are important factors dictating the species composition of the ecosystem. On agricultural land, they cause economic damage associated with long-term social consequences. The understanding of the plant molecular responses to these two stresses has benefited from research studying individual components of the stress, in particular low-oxygen stress. To a lesser extent, other associated stresses and plant responses have been incorporated into the molecular framework, such as ion and ROS signaling, pathogen susceptibility, and organ-specific expression and development. In this review, we aim to highlight known or suspected components of submergence/waterlogging stress that have not yet been thoroughly studied at the molecular level in this context, such as miRNA and retrotransposon expression, the influence of light/dark cycles, protein isoforms, root architecture, sugar sensing and signaling, post-stress molecular events, heavy-metal and salinity stress, and mRNA dynamics (splicing, sequestering, and ribosome loading). Finally, we explore biotechnological strategies that have applied this molecular knowledge to develop cultivars resistant to flooding or to offer alternative uses of flooding-prone soils, like bioethanol and biomass production.

Cis-directed cleavage and nonstoichiometric abundances of 21-nucleotide reproductive phased small interfering RNAs in grasses

Post-transcriptional gene silencing in plants results from independent activities of diverse small RNA types. In anthers of grasses, hundreds of loci yield noncoding RNAs that are processed into 21- and 24-nucleotide (nt) phased small interfering RNAs (phasiRNAs); these are triggered by miR2118 and miR2275. We characterized these 'reproductive phasiRNAs' from rice (Oryza sativa) panicles and anthers across seven developmental stages. Our computational analysis identified characteristics of the 21-nt reproductive phasiRNAs that impact their biogenesis, stability, and potential functions. We demonstrate that 21-nt reproductive phasiRNAs can function in cis to target their own precursors. We observed evidence of this cis regulatory activity in both rice and maize (Zea mays). We validated this activity with evidence of cleavage and a resulting shift in the pattern of phasiRNA production. We characterize biases in phasiRNA biogenesis, demonstrating that the Pol II-derived 'top' strand phasiRNAs are consistently higher in abundance than the bottom strand. The first phasiRNA from each precursor overlaps the miR2118 target site, and this impacts phasiRNA accumulation or stability, evident in the weak accumulation of this phasiRNA position. Additional influences on this first phasiRNA duplex include the sequence composition and length, and we show that these factors impact Argonaute loading.

Cis-directed cleavage and nonstoichiometric abundances of 21-nucleotide reproductive phased small interfering RNAs in grasses

Post-transcriptional gene silencing in plants results from independent activities of diverse small RNA types. In anthers of grasses, hundreds of loci yield noncoding RNAs that are processed into 21- and 24-nucleotide (nt) phased small interfering RNAs (phasiRNAs); these are triggered by miR2118 and miR2275. We characterized these 'reproductive phasiRNAs' from rice (Oryza sativa) panicles and anthers across seven developmental stages. Our computational analysis identified characteristics of the 21-nt reproductive phasiRNAs that impact their biogenesis, stability, and potential functions. We demonstrate that 21-nt reproductive phasiRNAs can function in cis to target their own precursors. We observed evidence of this cis regulatory activity in both rice and maize (Zea mays). We validated this activity with evidence of cleavage and a resulting shift in the pattern of phasiRNA production. We characterize biases in phasiRNA biogenesis, demonstrating that the Pol II-derived 'top' strand phasiRNAs are consistently higher in abundance than the bottom strand. The first phasiRNA from each precursor overlaps the miR2118 target site, and this impacts phasiRNA accumulation or stability, evident in the weak accumulation of this phasiRNA position. Additional influences on this first phasiRNA duplex include the sequence composition and length, and we show that these factors impact Argonaute loading.

Analysis of Brachypodium miRNA targets: evidence for diverse control during stress and conservation in bioenergy crops

BACKGROUND

Since the proposal of Brachypodium distachyon as a model for the grasses, over 500 Bdi-miRNAs have been annotated in miRBase making Brachypodium second in number only to rice. Other monocots, such as switchgrass, are completely absent from the miRBase database. While a significant number of miRNAs have been identified which are highly conserved across plants, little research has been done with respect to the conservation of miRNA targets. Plant responses to abiotic stresses are regulated by diverse pathways many of which involve miRNAs; however, it can be difficult to identify miRNA guided gene regulation when the miRNA is not the primary regulator of the target mRNA.

RESULTS

To investigate miRNA target conservation and stress response involvement, a set of PARE (Parallel Analysis of RNA Ends) libraries totaling over two billion reads was constructed and sequenced from Brachypodium, switchgrass, and sorghum representing the first report of RNA degradome data from the latter two species. Analysis of this data provided not only PARE evidence for miRNA guided cleavage of over 7000 predicted target mRNAs in Brachypodium, but also evidence for miRNA guided cleavage of over 1000 homologous transcripts in sorghum and switchgrass. A pipeline was constructed to compare RNA-seq and PARE data made from Brachypodium plants exposed to various abiotic stress conditions. This resulted in the identification of 44 miRNA targets which exhibit stress regulated cleavage. Time course experiments were performed to reveal the relationship between miR393ab, miR169a, miR394ab, and their respective targets throughout the first 36 h of the cold stress response in Brachypodium.

CONCLUSIONS

Knowledge gained from this study provides considerable insight into the RNA degradomes and the breadth of miRNA target conservation among these three species. Additionally, associations of a number of miRNAs and target mRNAs with the stress responses have been revealed which could aid in the development of stress tolerant transgenic crops.

Regulation and Evolution of NLR Genes: A Close Interconnection for Plant Immunity

NLR (NOD-like receptor) genes belong to one of the largest gene families in plants. Their role in plants' resistance to pathogens has been clearly described for many members of this gene family, and dysregulation or overexpression of some of these genes has been shown to induce an autoimmunity state that strongly affects plant growth and yield. For this reason, these genes have to be tightly regulated in their expression and activity, and several regulatory mechanisms are described here that tune their gene expression and protein levels. This gene family is subjected to rapid evolution, and to maintain diversity at NLRs, a plethora of genetic mechanisms have been identified as sources of variation. Interestingly, regulation of gene expression and evolution of this gene family are two strictly interconnected aspects. Indeed, some examples have been reported in which mechanisms of gene expression regulation have roles in promotion of the evolution of this gene family. Moreover, co-evolution of the NLR gene family and other gene families devoted to their control has been recently demonstrated, as in the case of miRNAs.

Grass phasiRNAs and male fertility

Recent studies have indicated that a special type of small noncoding RNAs, phased small-interfering RNAs (phasiRNAs) play crucial roles in many cellular processes of plant development. PhasiRNAs are generated from long RNA precursors at intervals of 21 or 24 nt in plants, and they are produced from both protein-coding gene and long noncoding RNA genes. Different from those in eudicots, grass phasiRNAs include a special class of small RNAs that are specifically expressed in reproductive organs. These grass phasiRNAs are associated with gametogenesis, especially with anther development and male fertility. In this review, we summarized current knowledge on these small noncoding RNAs in male germ cells and their possible biological functions and mechanisms in grass species.

Spearmint R2R3-MYB transcription factor MsMYB negatively regulates monoterpene production and suppresses the expression of geranyl diphosphate synthase large subunit (MsGPPS.LSU)

Many aromatic plants, such as spearmint, produce valuable essential oils in specialized structures called peltate glandular trichomes (PGTs). Understanding the regulatory mechanisms behind the production of these important secondary metabolites will help design new approaches to engineer them. Here, we identified a PGT-specific R2R3-MYB gene, MsMYB, from comparative RNA-Seq data of spearmint and functionally characterized it. Analysis of MsMYB-RNAi transgenic lines showed increased levels of monoterpenes, and MsMYB-overexpressing lines exhibited decreased levels of monoterpenes. These results suggest that MsMYB is a novel negative regulator of monoterpene biosynthesis. Ectopic expression of MsMYB, in sweet basil and tobacco, perturbed sesquiterpene- and diterpene-derived metabolite production. In addition, we found that MsMYB binds to cis-elements of MsGPPS.LSU and suppresses its expression. Phylogenetic analysis placed MsMYB in subgroup 7 of R2R3-MYBs whose members govern phenylpropanoid pathway and are regulated by miR858. Analysis of transgenic lines showed that MsMYB is more specific to terpene biosynthesis as it did not affect metabolites derived from phenylpropanoid pathway. Further, our results indicate that MsMYB is probably not regulated by miR858, like other members of subgroup 7.

Regulation of rice root development by a retrotransposon acting as a microRNA sponge

It is well documented that transposable elements (TEs) can regulate the expression of neighbouring genes. However, their ability to act in trans and influence ectopic loci has been reported rarely. We searched in rice transcriptomes for tissue-specific expression of TEs and found them to be regulated developmentally. They often shared sequence homology with co-expressed genes and contained potential microRNA-binding sites, which suggested possible contributions to gene regulation. In fact, we have identified a retrotransposon that is highly transcribed in roots and whose spliced transcript constitutes a target mimic for miR171. miR171 destabilizes mRNAs encoding the root-specific family of SCARECROW-Like transcription factors. We demonstrate that retrotransposon-derived transcripts act as decoys for miR171, triggering its degradation and thus results in the root-specific accumulation of SCARECROW-Like mRNAs. Such transposon-mediated post-transcriptional control of miR171 levels is conserved in diverse rice species.

Regulation of rice root development by a retrotransposon acting as a microRNA sponge

It is well documented that transposable elements (TEs) can regulate the expression of neighbouring genes. However, their ability to act in trans and influence ectopic loci has been reported rarely. We searched in rice transcriptomes for tissue-specific expression of TEs and found them to be regulated developmentally. They often shared sequence homology with co-expressed genes and contained potential microRNA-binding sites, which suggested possible contributions to gene regulation. In fact, we have identified a retrotransposon that is highly transcribed in roots and whose spliced transcript constitutes a target mimic for miR171. miR171 destabilizes mRNAs encoding the root-specific family of SCARECROW-Like transcription factors. We demonstrate that retrotransposon-derived transcripts act as decoys for miR171, triggering its degradation and thus results in the root-specific accumulation of SCARECROW-Like mRNAs. Such transposon-mediated post-transcriptional control of miR171 levels is conserved in diverse rice species.

Transcriptomic analysis of submergence-tolerant and sensitive Brachypodium distachyon ecotypes reveals oxidative stress as a major tolerance factor

When excessive amounts of water accumulate around roots and aerial parts of plants, submergence stress occurs. To find the integrated mechanisms of tolerance, we used ecotypes of the monocot model plant Brachypodium distachyon to screen for genetic material with contrasting submergence tolerance. For this purpose, we used a set of previously studied drought sensitive/tolerant ecotypes and the knowledge that drought tolerance is positively associated with submergence stress. We decided to contrast aerial tissue transcriptomes of the ecotype Bd21 14-day-old plants as sensitive and ecotype Bd2-3 as tolerant after 2 days of stress under a long-day photoperiod. Gene ontology and the grouping of transcripts indicated that tolerant Bd2-3 differentially down-regulated NITRATE REDUCTASE and ALTERNATIVE OXIDASE under stress and constitutively up-regulated HAEMOGLOBIN, when compared with the sensitive ecotype, Bd21. These results suggested the removal of nitric oxide, a gaseous phytohormone and concomitant reactive oxygen species as a relevant tolerance determinant. Other mechanisms more active in tolerant Bd2-3 were the pathogen response, glyoxylate and tricarboxylic acid cycle integration, and acetate metabolism. This data set could be employed to design further studies on the basic science of plant tolerance to submergence stress and its biotechnological application in the development of submergence-tolerant crops.

An efficient Potato virus X -based microRNA silencing in Nicotiana benthamiana

Plant microRNAs (miRNAs) play pivotal roles in many biological processes. Although many miRNAs have been identified in various plant species, the functions of these miRNAs remain largely unknown due to the shortage of effective genetic tools to block their functional activity. Recently, miRNA target mimic (TM) technologies have been applied to perturb the activity of specific endogenous miRNA or miRNA families. We previously reported that Tobacco rattle virus (TRV)-based TM expression can successfully mediate virus-based miRNA silencing/suppression (VbMS) in plants. In this study, we show the Potato virus X (PVX)-based TM expression causes strong miRNA silencing in Nicotiana benthamiana. The PVX-based expression of short tandem target mimic (STTMs) against miR165/166 and 159 caused the corresponding phenotype in all infected plants. Thus, a PVX-based VbMS is a powerful method to study miRNA function and may be useful for high-throughput investigation of miRNA function in N. benthamiana.

Functional and evolutionary analyses of the miR156 and miR529 families in land plants

BACKGROUND

MicroRNAs (miRNAs) are important regulatory elements of gene expression. Similarly to coding genes, miRNA genes follow a birth and death pattern of evolution likely reflecting functional relevance and divergence. For instance, miRNA529 is evolutionarily related to miRNA156 (a highly conserved miRNA in land plants), but it is lost in Arabidopsis thaliana. Interestingly, both miRNAs target sequences overlap in some members of the SQUAMOSA promoter-binding protein like (SPL) family, raising important questions regarding the diversification of the miR156/miR529-associated regulatory network in land plants.

RESULTS

In this study, through phylogenic reconstruction of miR156/529 target sequences from several taxonomic groups, we have found that specific eudicot SPLs, despite miRNA529 loss, retained the corresponding target site. Detailed molecular evolutionary analyses of miR156/miR529-target sequence showed that loss of miR529 in core eudicots, such as Arabidopsis, is correlated with a more relaxed selection of the miRNA529 specific target element, while miRNA156-specific target sequence is under stronger selection, indicating that these two target sites might be under distinct evolutionary constraints. Importantly, over-expression in Arabidopsis of MIR529 precursor from a monocot, but not from a basal eudicot, demonstrates specific miR529 regulation of AtSPL9 and AtSPL15 genes, which contain conserved responsive elements for both miR156 and miR529.

CONCLUSIONS

Our results suggest loss of functionality of MIR529 genes in the evolutionary history of eudicots and show that the miR529-responsive element present in some eudicot SPLs is still functional. Our data support the notion that particular miRNA156 family members might have compensated for the loss of miR529 regulation in eudicot species, which concomitantly may have favored diversification of eudicot SPLs.

Identification of miRNAs and Their Targets in the Liverwort Marchantia polymorpha by Integrating RNA-Seq and Degradome Analyses

Bryophytes (liverworts, hornworts and mosses) comprise the three earliest diverging lineages of land plants (embryophytes). Marchantia polymorpha, a complex thalloid Marchantiopsida liverwort that has been developed into a model genetic system, occupies a key phylogenetic position. Therefore, M. polymorpha is useful in studies aiming to elucidate the evolution of gene regulation mechanisms in plants. In this study, we used computational, transcriptomic, small RNA and degradome analyses to characterize microRNA (miRNA)-mediated pathways of gene regulation in M. polymorpha. The data have been integrated into the open access ContigViews-miRNA platform for further reference. In addition to core components of the miRNA pathway, 129 unique miRNA sequences, 11 of which could be classified into seven miRNA families that are conserved in embryophytes (miR166a, miR390, miR529c, miR171-3p, miR408a, miR160 and miR319a), were identified. A combination of computational and degradome analyses allowed us to identify and experimentally validate 249 targets. In some cases, the target genes are orthologous to those of other embryophytes, but in other cases, the conserved miRNAs target either paralogs or members of different gene families. In addition, the newly discovered Mpo-miR11707.1 and Mpo-miR11707.2 are generated from a common precursor and target MpARGONAUTE1 (LW1759). Two other newly discovered miRNAs, Mpo-miR11687.1 and Mpo-miR11681.1, target the MADS-box transcription factors MpMADS1 and MpMADS2, respectively. Interestingly, one of the pentatricopeptide repeat (PPR) gene family members, MpPPR_66 (LW9825), the protein products of which are generally involved in various steps of RNA metabolism, has a long stem-loop transcript that can generate Mpo-miR11692.1 to autoregulate MpPPR_66 (LW9825) mRNA. This study provides a foundation for further investigations of the RNA-mediated silencing mechanism in M. polymorpha as well as of the evolution of this gene silencing pathway in embryophytes.

Identification and Characterization of Erysiphe necator-Responsive MicroRNAs in Chinese Wild Vitis pseudoreticulata by High-Throughput Sequencing

Grapevine powdery mildew is one of the most damaging fungal diseases. Therefore, a precise understanding of the grapevine disease resistance system becomes a subject of significant importance. Plant microRNAs(miRNAs) have been implicated to play regulatory roles in plant biotic stress responses. In this study, high-throughput sequencing and miRDeep-P were employed to identify miRNAs in Chinese wild Vitis pseudoreticulata leaves following inoculation with Erysiphe necator. Altogether, 126 previously identified microRNAs and 124 novel candidates of miRNA genes were detected. Among them, 43 conserved miRNAs belong to 20 families and 23 non-conserved but previously-known miRNAs belong to 15 families. Following E. necator inoculation, 119 miRNAs were down-regulated and 131 were up-regulated. Furthermore, the expression changes occurring in 32 miRNAs were significant. The expression patterns of some miRNAs were validated by semi-quantitative RT-PCR and qRT-PCR. A total of 485 target genes were predicted and categorized by Gene Ontology (GO). In addition, 14 vvi-miRNAs were screened with 36 targets which may be involved in powdery mildew resistance in grape. Highly accumulated vvi-NewmiR2118 was detected from accession "Baihe-35-1," whose targets were mostly NBS-LRR resistance genes. It was down-regulated rapidly and strongly in "Baihe-35-1" leaves after inoculated with E. necator, indicating its involvement in grape powdery mildew resistance. Finally, the study verified interaction between vvi-NewmiR2118 and RPP13 by histochemical staining and GUS fluorescence quantitative assay.

High-Throughput Sequencing Reveals H2O2 Stress-Associated MicroRNAs and a Potential Regulatory Network in Brachypodium distachyon Seedlings

Oxidative stress in plants can be triggered by many environmental stress factors, such as drought and salinity. Brachypodium distachyon is a model organism for the study of biofuel plants and crops, such as wheat. Although recent studies have found many oxidative stress response-related proteins, the mechanism of microRNA (miRNA)-mediated oxidative stress response is still unclear. Using next generation high-throughput sequencing technology, the small RNAs were sequenced from the model plant B. distachyon 21 (Bd21) under H₂O₂ stress and normal growth conditions. In total, 144 known B. distachyon miRNAs and 221 potential new miRNAs were identified. Further analysis of potential new miRNAs suggested that 36 could be clustered into known miRNA families, while the remaining 185 were identified as B. distachyon-specific new miRNAs. Differential analysis of miRNAs from the normal and H₂O₂ stress libraries identified 31 known and 30 new H₂O₂ stress responsive miRNAs. The expression patterns of seven representative miRNAs were verified by reverse transcription quantitative polymerase chain reaction (RT-qPCR) analysis, which produced results consistent with those of the deep sequencing method. Moreover, we also performed RT-qPCR analysis to verify the expression levels of 13 target genes and the cleavage site of 5 target genes by known or novel miRNAs were validated experimentally by 5' RACE. Additionally, a miRNA-mediated gene regulatory network for H₂O₂ stress response was constructed. Our study identifies a set of H₂O₂-responsive miRNAs and their target genes and reveals the mechanism of oxidative stress response and defense at the post-transcriptional regulatory level.

The Influence of Genotype and Environment on Small RNA Profiles in Grapevine Berry

Understanding the molecular mechanisms involved in the interaction between the genetic composition and the environment is crucial for modern viticulture. We approached this issue by focusing on the small RNA transcriptome in grapevine berries of the two varieties Cabernet Sauvignon and Sangiovese, growing in adjacent vineyards in three different environments. Four different developmental stages were studied and a total of 48 libraries of small RNAs were produced and sequenced. Using a proximity-based pipeline, we determined the general landscape of small RNAs accumulation in grapevine berries. We also investigated the presence of known and novel miRNAs and analyzed their accumulation profile. The results showed that the distribution of small RNA-producing loci is variable between the two cultivars, and that the level of variation depends on the vineyard. Differently, the profile of miRNA accumulation mainly depends on the developmental stage. The vineyard in Riccione maximizes the differences between the varieties, promoting the production of more than 1000 specific small RNA loci and modulating their expression depending on the cultivar and the maturation stage. In total, 89 known vvi-miRNAs and 33 novel vvi-miRNA candidates were identified in our samples, many of them showing the accumulation profile modulated by at least one of the factors studied. The in silico prediction of miRNA targets suggests their involvement in berry development and in secondary metabolites accumulation such as anthocyanins and polyphenols.

Extensive Families of miRNAs and PHAS Loci in Norway Spruce Demonstrate the Origins of Complex phasiRNA Networks in Seed Plants

In eudicot plants, the miR482/miR2118 superfamily regulates and instigates the production of phased secondary small interfering RNAs (siRNAs) from NB-LRR (nucleotide binding leucine-rich repeat) genes that encode disease resistance proteins. In grasses, this miRNA family triggers siRNA production specifically in reproductive tissues from long noncoding RNAs. To understand this functional divergence, we examined the small RNA population in the ancient gymnosperm Norway spruce (Picea abies). As many as 41 miRNA families in spruce were found to trigger phasiRNA (phased, secondary siRNAs) production from diverse PHAS loci, with a remarkable 19 miRNA families capable of targeting over 750 NB-LRR genes to generate phasiRNAs. miR482/miR2118, encoded in spruce by at least 24 precursor loci, targets not only NB-LRR genes to trigger phasiRNA production (as in eudicots) but also noncoding PHAS loci, generating phasiRNAs preferentially in male or female cones, reminiscent of its role in the grasses. These data suggest a dual function of miR482/miR2118 present in gymnosperms that was selectively yet divergently retained in flowering plants. A few MIR482/MIR2118 precursors possess an extremely long stem-loop structure, one arm of which shows significant sequence similarity to spruce NB-LRR genes, suggestive of an evolutionary origin from NB-LRR genes through gene duplication. We also characterized an expanded miR390-TAS3 (TRANS-ACTING SIRNA GENE 3)-ARF (AUXIN RESPONSIVE FACTOR) pathway, comprising 18 TAS3 genes of diverse features. Finally, we annotated spruce miRNAs and their targets. Taken together, these data expand our understanding of phasiRNA network in plants and the evolution of plant miRNAs, particularly miR482/miR2118 and its functional diversification.

miRVine: a microRNA expression atlas of grapevine based on small RNA sequencing

Background

miRNAs are the most abundant class of small non-coding RNAs, and they are involved in post-transcriptional regulations, playing a crucial role in the refinement of genetic programming during plant development. Here we present a comprehensive picture of miRNA regulation in Vitis vinifera L. plant during its complete life cycle. Furthering our knowledge about the post-transcriptional regulation of plant development is fundamental to understand the biology of such an important crop.

Results

We analyzed 70 small RNA libraries, prepared from berries, inflorescences, tendrils, buds, carpels, stamens and other samples at different developmental stages. One-hundred and ten known and 175 novel miRNAs have been identified and a wide grapevine expression atlas has been described. The distribution of miRNA abundance reveals that 22 novel miRNAs are specific to stamen, and two of them are, interestingly, involved in ethylene biosynthesis, while only few miRNAs are highly specific to other organs. Thirty-eight miRNAs are present in all our samples, suggesting a role in key regulatory circuit. On the basis of miRNAs abundance and distribution across samples and on the estimated correlation, we suggest that miRNA expression define organ identity. We performed target prediction analysis and focused on miRNA expression analysis in berries and inflorescence during their development, providing an initial functional description of the identified miRNAs.

Conclusions

Our findings represent a very extensive miRNA expression atlas in grapevine, allowing the definition of how the spatio-temporal distribution of miRNAs defines organ identity. We describe miRNAs abundance in specific tissues not previously described in grapevine and contribute to future targeted functional analyses. Finally, we present a deep characterization of miRNA involvement in berry and inflorescence development, suggesting a role for miRNA-driven hormonal regulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1610-5) contains supplementary material, which is available to authorized users.

MicroRNA-directed cleavage of targets: mechanism and experimental approaches

MicroRNAs (miRNAs) are a large family of post-transcriptional regulators, which are 21-24 nt in length and play a role in a wide variety of biological processes in eukaryotes. The past few years have seen rapid progress in our understanding of miRNA biogenesis and the mechanism of action, which commonly entails a combination of target degradation and translational repression. The target degradation mediated by Argonaute-catalyzed endonucleolytic cleavage exerts a significant repressive effect on target mRNA expression, particularly during rapid developmental transitions. This review outlines the current understanding of the mechanistic aspects of this important process and discusses several different experimental approaches to identify miRNA cleavage targets. [BMB Reports 2014; 47(8): 417-423]

Evolutionary Comparison of Two Combinatorial Regulators of SBP-Box Genes, MiR156 and MiR529, in Plants

A complete picture of the evolution of miRNA combinatorial regulation requires the synthesis of information on all miRNAs and their targets. MiR156 and miR529 are two combinatorial regulators of squamosa promoter binding protein-like (SBP-box) genes. Previous studies have clarified the evolutionary dynamics of their targets; however, there have been no reports on the evolutionary patterns of two miRNA regulators themselves to date. In this study, we investigated the evolutionary differences between these two miRNA families in extant land plants. Our work found that miR529 precursor, especially of its mature miRNA sequence, has a higher evolutionary rate. Such accelerating evolution of miR529 has significantly effects on its structural stability, and sequence conservation against existence of itself. By contrast, miR156 evolves more rapidly in loop region of the stable secondary structure, which may contribute to its functional diversity. Moreover, miR156 and miR529 genes have distinct rates of loss after identical duplication events. MiR529 genes have a higher average loss rate and asymmetric loss rate in duplicated gene pairs, indicating preferred miR529 gene losses become another predominant mode of inactivation, that are implicated in the contraction of this family. On the contrary, duplicated miR156 genes have a low loss rate, and could serve as another new source for functional diversity. Taken together, these results provide better insight into understanding the evolutionary divergence of miR156 and miR529 family in miRNA combinational regulation network.

Manipulating microRNAs for improved biomass and biofuels from plant feedstocks

Petroleum-based fuels are nonrenewable and unsustainable. Renewable sources of energy, such as lignocellulosic biofuels and plant metabolite-based drop-in fuels, can offset fossil fuel use and reverse environmental degradation through carbon sequestration. Despite these benefits, the lignocellulosic biofuels industry still faces many challenges, including the availability of economically viable crop plants. Cell wall recalcitrance is a major economic barrier for lignocellulosic biofuels production from biomass crops. Sustainability and biomass yield are two additional, yet interrelated, foci for biomass crop improvement. Many scientists are searching for solutions to these problems within biomass crop genomes. MicroRNAs (miRNAs) are involved in almost all biological and metabolic process in plants including plant development, cell wall biosynthesis and plant stress responses. Because of the broad functions of their targets (e.g. auxin response factors), the alteration of plant miRNA expression often results in pleiotropic effects. A specific miRNA usually regulates a biologically relevant bioenergy trait. For example, relatively low miR156 overexpression leads to a transgenic feedstock with enhanced biomass and decreased recalcitrance. miRNAs have been overexpressed in dedicated bioenergy feedstocks such as poplar and switchgrass yielding promising results for lignin reduction, increased plant biomass, the timing of flowering and response to harsh environments. In this review, we present the status of miRNA-related research in several major biofuel crops and relevant model plants. We critically assess published research and suggest next steps for miRNA manipulation in feedstocks for increased biomass and sustainability for biofuels and bioproducts.

BdVIL4 regulates flowering time and branching through repressing miR156 in ambient temperature dependent way in Brachypodium distachyon

Responsing to environmental signals, Vernalization Insensitive 3 (VIN3) family proteins are involved in plant development control by repressing the target genes epigenecticly together with Polycomb Repressive Complex 2 (PRC2) complex. BdVIL4 is a VIN3 like gene in Brachypodium distachyon, preferentially expressed in young tissues spatially. The RNAi plants were constructed to study the function of BdVIL4 on the development process. The plants with BdVIL4 RNA interferenced (BdVIL4 RNAi plants) had no obvious difference from the wild at 23 °C, but flowered significantly later and had more branches than the control at l6 °C. In BdVIL4 RNAi plants the expression of miR156 were upregulated, and much more at low temperature (l6 °C). Coincidentally, similar to the BdVIL4 RNAi plants, the miR156 overexpressors also showed late flowering and more branches, and the late flowering phynotype just only performanced at lower temperature. The results suggested that BdVIL4 are involved in the regulation of branching and flowering responsing to the ambient temperature by repressing the expression of miR156.

Stress-induced endogenous siRNAs targeting regulatory intron sequences in Brachypodium

Exposure to abiotic stresses triggers global changes in the expression of thousands of eukaryotic genes at the transcriptional and post-transcriptional levels. Small RNA (smRNA) pathways and splicing both function as crucial mechanisms regulating stress-responsive gene expression. However, examples of smRNAs regulating gene expression remain largely limited to effects on mRNA stability, translation, and epigenetic regulation. Also, our understanding of the networks controlling plant gene expression in response to environmental changes, and examples of these regulatory pathways intersecting, remains limited. Here, to investigate the role of smRNAs in stress responses we examined smRNA transcriptomes of Brachypodium distachyon plants subjected to various abiotic stresses. We found that exposure to different abiotic stresses specifically induced a group of novel, endogenous small interfering RNAs (stress-induced, UTR-derived siRNAs, or sutr-siRNAs) that originate from the 3' UTRs of a subset of coding genes. Our bioinformatics analyses predicted that sutr-siRNAs have potential regulatory functions and that over 90% of sutr-siRNAs target intronic regions of many mRNAs in trans. Importantly, a subgroup of these sutr-siRNAs target the important intron regulatory regions, such as branch point sequences, that could affect splicing. Our study indicates that in Brachypodium, sutr-siRNAs may affect splicing by masking or changing accessibility of specific cis-elements through base-pairing interactions to mediate gene expression in response to stresses. We hypothesize that this mode of regulation of gene expression may also serve as a general mechanism for regulation of gene expression in plants and potentially in other eukaryotes.

The miR9863 family regulates distinct Mla alleles in barley to attenuate NLR receptor-triggered disease resistance and cell-death signaling

Barley (Hordeum vulgare L.) Mla alleles encode coiled-coil (CC), nucleotide binding, leucine-rich repeat (NB-LRR) receptors that trigger isolate-specific immune responses against the powdery mildew fungus, Blumeria graminis f. sp. hordei (Bgh). How Mla or NB-LRR genes in grass species are regulated at post-transcriptional level is not clear. The microRNA family, miR9863, comprises four members that differentially regulate distinct Mla alleles in barley. We show that miR9863 members guide the cleavage of Mla1 transcripts in barley, and block or reduce the accumulation of MLA1 protein in the heterologous Nicotiana benthamiana expression system. Regulation specificity is determined by variation in a unique single-nucleotide-polymorphism (SNP) in mature miR9863 family members and two SNPs in the Mla miR9863-binding site that separates these alleles into three groups. Further, we demonstrate that 22-nt miR9863s trigger the biogenesis of 21-nt phased siRNAs (phasiRNAs) and together these sRNAs form a feed-forward regulation network for repressing the expression of group I Mla alleles. Overexpression of miR9863 members specifically attenuates MLA1, but not MLA10-triggered disease resistance and cell-death signaling. We propose a key role of the miR9863 family in dampening immune response signaling triggered by a group of MLA immune receptors in barley.

Plants rely on cell-surface and intracellular immune receptors to sense pathogen invasion and to mediate defense responses. However, uncontrolled activation of immune responses is harmful to plant growth and development. Small RNAs have recently been shown to fine-tune the expression of intracellular immune receptors and contribute to the regulation of defense signaling in dicot plants, while similar processes have not been well documented in monocot grain crops, such as barley and wheat. Here, we show that, in barley, some members of the miR9863 family target a subset of Mla alleles that confer race-specific disease resistance to the powdery mildew fungus. These miRNAs act on Mla transcripts by cleavage and translational repression. Production of a type of trans-acting small RNAs, designated as phasiRNAs, enhances the effects of miRNA regulation on Mla targets. We propose that Mla-mediated immune signaling is fine-tuned by the miRNAs at later stage of MLA activation to avoid overloading of immune responses in barley cells.

An atlas of soybean small RNAs identifies phased siRNAs from hundreds of coding genes

Small RNAs are ubiquitous, versatile repressors and include (1) microRNAs (miRNAs), processed from mRNA forming stem-loops; and (2) small interfering RNAs (siRNAs), the latter derived in plants by a process typically requiring an RNA-dependent RNA polymerase. We constructed and analyzed an expression atlas of soybean (Glycine max) small RNAs, identifying over 500 loci generating 21-nucleotide phased siRNAs (phasiRNAs; from PHAS loci), of which 483 overlapped annotated protein-coding genes. Via the integration of miRNAs with parallel analysis of RNA end (PARE) data, 20 miRNA triggers of 127 PHAS loci were detected. The primary class of PHAS loci (208 or 41% of the total) corresponded to NB-LRR genes; some of these small RNAs preferentially accumulate in nodules. Among the PHAS loci, novel representatives of TAS3 and noncanonical phasing patterns were also observed. A noncoding PHAS locus, triggered by miR4392, accumulated preferentially in anthers; the phasiRNAs are predicted to target transposable elements, with their peak abundance during soybean reproductive development. Thus, phasiRNAs show tremendous diversity in dicots. We identified novel miRNAs and assessed the veracity of soybean miRNAs registered in miRBase, substantially improving the soybean miRNA annotation, facilitating an improvement of miRBase annotations and identifying at high stringency novel miRNAs and their targets.