Unique and conserved microRNAs in wheat chromosome 5D revealed by next-generation sequencing

MicroRNA miR1118 contributes to wheat (Triticum aestivum L.) salinity tolerance by regulating the Plasma Membrane Intrinsic Proteins1;5 (PIP1;5) gene

microRNAs (miRNAs) are important regulators of various adaptive stress responses in crops; however, many details about associations among miRNAs, their target genes and physiochemical responses of crops under salinity stress remain poorly understood. We designed this study in a systems biology context and used a collection of computational, experimental and statistical procedures to uncover some regulatory functions of miRNAs in the response of the important crop, wheat, to salinity stress. Accordingly, under salinity conditions, wheat roots' Expressed Sequence Tag (EST) libraries were computationally mined to identify the most reliable differentially expressed miRNA and its related target gene(s). Then, molecular and physiochemical evaluations were carried out in a separate salinity experiment using two contrasting wheat genotypes. Finally, the association between changes in measured characteristics and wheat salinity tolerance was determined. From the results, miR1118 was assigned as a reliable salinity-responsive miRNA in wheat roots. The expression profiles of miR1118 and its predicted target gene, Plasma Membrane Intrinsic Proteins1,5 (PIP1;5), significantly differed between wheat genotypes. Moreover, results revealed that expression profiles of miR1118 and PIP1;5 significantly correlate to Relative Water Content (RWC), root hydraulic conductance (Lp), photosynthetic activities, plasma membrane damages, osmolyte accumulation and ion homeostasis of wheat. Our results suggest a plausible regulatory node through miR1118 adjusting the wheat water status, maintaining ion homeostasis and mitigating membrane damages, mainly through the PIP1;5 gene, under salinity conditions. To our knowledge, this is the first report on the role of miR1118 and PIP1;5 in wheat salinity response.

The CaCA superfamily genes in Saccharum: comparative analysis and their functional implications in response to biotic and abiotic stress

BACKGROUND

In plants, Calcium (Ca2+) acts as a universal messenger in various signal transduction pathways, including responses to biotic and abiotic stresses and regulation of cellular and developmental processes. The Ca2+/cation antiporter (CaCA) superfamily proteins play vital roles in the transport of Ca2+ and/or other cations. However, the characteristics of these superfamily members in Saccharum and their evolutionary and functional implications have remained unclear.

RESULTS

A total of 34 CaCA genes in Saccharum spontaneum, 5 CaCA genes in Saccharum spp. R570, and 14 CaCA genes in Sorghum bicolor were identified and characterized. These genes consisted of the H+/cation exchanger (CAX), cation/Ca2+ exchanger (CCX), EF-hand / CAX (EFCAX), and Mg2+/H+ exchanger (MHX) families, among which the CCX and EFCAX could be classified into three groups while the CAX could be divided into two groups. The exon/intron structures and motif compositions suggested that the members in the same group were highly conserved. Synteny analysis of CaCAs established their orthologous and paralogous relationships among the superfamily in S. spontaneum, R570, and S. bicolor. The results of protein-protein interactions indicated that these CaCA proteins had direct or indirect interactions. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis demonstrated that most members of Saccharum CaCA genes exhibited a similar expression pattern in response to hormonal (abscisic acid, ABA) treatment but played various roles in response to biotic (Sporisorium scitamineum) and abiotic (cold) stresses. Furthermore, ScCAX4, a gene encoding a cytoplasm, plasma membrane and nucleus positioning protein, was isolated from sugarcane. This gene was constitutively expressed in different sugarcane tissues and its expression was only induced at 3 and 6 h time points after ABA treatment, however was inhibited and indued in the whole process under cold and S. scitamineum stresses, respectively.

CONCLUSIONS

This study systematically conducted comparative analyses of CaCA superfamily genes among S. spontaneum, R570, and S. bicolor, delineating their sequence and structure characteristics, classification, evolutionary history, and putative functions. These results not only provided rich gene resources for exploring the molecular mechanism of the CaCA superfamily genes but also offered guidance and reference for research on other gene families in Saccharum.

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.

Novel approaches on identification of conserved miRNAs for broad-spectrum Potyvirus control measures

Potyviridae comprises more than 200 ssRNA viruses, many of which have a broad host range and geographical distributions. Potyvirids (members of Potyviridae) infect several economically important plants such as saffron, cardamom, cucumber, pepper, potato, tomato, yam, etc. Cumulatively, potyvirids cause a substantial economic loss. The major bottleneck in developing an efficient antiviral strategy is that viruses quickly evade host immunity owing to their higher mutation and recombination rates. Due to this reason, the emergence of newer and improved broad-spectrum approaches to combat viral infections is essential. The use of microRNA's (miRNA) to circumvent viral infection against animal viruses has been successfully employed. Fewer studies reported the development of efficient miRNA-based antivirus resistant strategies against plant viruses and none focused on multiple virus resistance. We focused on potyviruses since studies are limited and identification of conserved miRNAs among various host plants would be an initiative to design broad-spectrum antivirus strategies. In this study, we predicted evolutionarily conserved miRNAs by BLAST searching of reported miRNAs from 15 plants against the GSS and EST sequences of banana. A total of nine miRNAs were predicted and screened in nine diverse potyvirids' hosts (Banana, Tomato, Green gram, Jasmine, Chilli, Coriander, Onion, Rose and Colocasia) belonging to eight different orders (Zingiberales, Solanales, Fabales, Lamiales, Apiales, Asperagales, Rosales and Alismatales). Results suggested that miR168 and miR162 are conserved among all the selected plants. This comprehensive study laid the foundations to design broad-spectrum antivirus resistance using miRNAs. To conclude miR168 and miR162 are conserved among many plants and play a crucial role in evading virus infection which could be used as a potential candidate for developing antiviral strategies against potyvirid infections.

In silico determination of transposon-derived miRNAs and targets in Aegilops species

Transposable elements (TEs) are found almost in all living organism, shaping organisms' genomes. miRNAs are noncoding RNA types which are especially important in gene expression regulations. Many previously determined plant miRNAs are identical/homologous to transposons (TE-MIR). The aim of this study was computational characterization of novel TE-related miRNAs and their targets in Aegilops genome by using stringent criteria. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were performed by BLAST2GO. Seventeen novel TE-related miRNAs in Aegilops genome were identified for the first time. GO analyses indicated that 40 targets played different roles in biological processes, cellular components and molecular functions. Moreover, these genes were involved in 10 metabolic pathways such as purine metabolism, nitrogen metabolism, oxidative phosphorylation, etc. as a result of KEGG analyses. Identification of miRNAs and their targets are significant to understand miRNA-TEs relationships and even how TEs affect plant growth and development. Obtaining results of this study are expected to provide possible new insight into Aegilops and its related species, wheat, with respect to miRNAs evolution and domestication.Communicated by Ramaswamy H. Sarma.

Transcriptome-wide identification and characterization of microRNAs responsive to phosphate starvation in Populus tomentosa

miRNAs (microRNAs) are ~ 21-nt non-coding small RNAs (sRNAs) that play crucial regulatory roles in plant biotic and abiotic stress responses. Phosphorus (Pi) deficiency constrains plant growth and reduces yields worldwide. To identify tree miRNAs and evaluate their functions in the response to low Pi, we identified 261 known and 31 candidate novel miRNA families from three sRNA libraries constructed from Populus tomentosa subjected to sufficient or Pi deficiency condition or to restoration of a sufficient Pi level after Pi deficiency. Pi deficiency resulted in significant changes in the abundance of TPM (transcript per million) of 65 known and 3 novel miRNAs. Interestingly, four miRNAs responsive to low N-miR167, miR394, miR171, and miR857-were found to be involved in the response to low Pi. Thirty-five known and one novel miRNAs responded dynamically to Pi fluctuations, suggesting their involvement in the response to Pi deficiency. miRNA clusters comprising 36 miRNAs were identified in 10 chromosomes. Intriguingly, nine pairs of sense and antisense miRNAs transcribed from the same loci were detected in P. tomentosa, which is the first such report in woody plants. Moreover, target genes of the known miRNAs and novel miRNA candidates with significantly changed abundance were predicted, and their functions were annotated. Degradome sequencing supported the identified targets of miRNAs in P. tomentosa. These findings will enhance our understanding of universal and specific molecular regulatory mechanisms of trees under nutrition stress and may facilitate improvement of the Pi utilization efficiency of woody plants.

Discovery and profiling of small RNAs from Puccinia triticina by deep sequencing and identification of their potential targets in wheat

Cross-kingdom RNAi is a well-documented phenomenon where sRNAs generated by host and pathogens may govern resistance or susceptible phenotypes during host-pathogen interaction. With the first example of the direct involvement of fungal generated sRNAs in virulence of plant pathogenic fungi Botrytis cinerea and recently from Puccinia striiformis f. sp. tritici, we attempted to identify sRNAs in Puccinia triticina (P. triticina). Four sRNA libraries were prepared and sequenced using Illumina sequencing technology and a total of ~ 1-1.28 million potential sRNAs and two microRNA-like small RNA (mil-RNAs) candidates were identified. Computational prediction of targets using a common set of sRNAs and P. triticina mil-RNAs (pt-mil-RNAs) within P. triticina and wheat revealed the majority of the targets as repetitive elements in P. triticina whereas in wheat, the target genes were identified to be involved in many biological processes including defense-related pathways. We found 9 receptor-like kinases (RLKs) and 14 target genes of each related to reactive oxygen species (ROS) pathway and transcription factors respectively, including significant numbers of target genes from various other categories. Expression analysis of twenty selected sRNAs, targeting host genes pertaining to ROS related, disease resistance, metabolic processes, transporter, apoptotic inhibitor, and transcription factors along with two pt-mil-RNAs by qRT-PCR showed distinct patterns of expression of the sRNAs in urediniospore-specific libraries. In this study, for the first time, we report identification of novel sRNAs identified in P. triticina including two pt-mil-RNAs that may play an important role in biotrophic growth and pathogenicity.

Chromosome-based survey sequencing reveals the genome organization of wild wheat progenitor Triticum dicoccoides

Wild emmer wheat (Triticum turgidum ssp. dicoccoides) is the progenitor of wheat. We performed chromosome-based survey sequencing of the 14 chromosomes, examining repetitive sequences, protein-coding genes, miRNA/target pairs and tRNA genes, as well as syntenic relationships with related grasses. We found considerable differences in the content and distribution of repetitive sequences between the A and B subgenomes. The gene contents of individual chromosomes varied widely, not necessarily correlating with chromosome size. We catalogued candidate agronomically important loci, along with new alleles and flanking sequences that can be used to design exome sequencing. Syntenic relationships and virtual gene orders revealed several small-scale evolutionary rearrangements, in addition to providing evidence for the 4AL-5AL-7BS translocation in wild emmer wheat. Chromosome-based sequence assemblies contained five novel miRNA families, among 59 families putatively encoded in the entire genome which provide insight into the domestication of wheat and an overview of the genome content and organization.

TaMIR1139: a wheat miRNA responsive to Pi-starvation, acts a critical mediator in modulating plant tolerance to Pi deprivation

Wheat miRNA member TaMIR1139 targets genes functional in various families and plays crucial roles in regulating plant Pi starvation tolerance. Through regulating target genes at posttranscriptional or translational level, plant miRNAs are involved in mediating diverse biological processes associated with growth, development, and responses to adverse stresses. In this study, we characterized the expression pattern and function of TaMIR1139, a miRNA member of wheat (T. aestivum) under Pi deprivation. TaMIR1139 precursor is also present in N. tabucum, suggesting the conserved nature of miR1139 across monocots and eudicots. TaMIR1139 targets seven genes within different families. The transcripts abundance of TaMIR1139 was induced upon Pi deprivation and the upregulated expression under Pi starvation was downregulated by the Pi recovery treatment, In contrast, the genes targeted by TaMIR1139 exhibited reduced transcripts upon Pi starvation and their downregulated expression was recovered by Pi-recovery condition, suggesting the regulation of them under TaMIR1139 through a cleavage mechanism. TaMIR1139 overexpression conferred the Pi-deprived plants improved phenotype, biomass, photosynthesis, and Pi acquisition. Transcriptome analysis identified numerous genes involving biological process, cellular components, and molecular function were differentially expressed in the TaMIR1139 overexpression lines, which suggests the TaMIR1139-mediated plant Pi starvation tolerance to be associated with the role of miRNA in extensively modulating the transcript profiling. A phosphate transporter (PT) gene NtPT showed significantly upregulated expression in TaMIR1139 overexpression lines; overexpression of it conferred plants improved Pi acquisition upon Pi starvation, suggesting its contribution to the TaMIR1139-mediated plant low-Pi stress resistance. Our investigation indicates that TaMIR1139 is critical in plant Pi starvation tolerance through transcriptionally regulating the target genes and modulating the Pi stress-defensiveness processes.

Identification and molecular characterization of a trans-acting small interfering RNA producing locus regulating leaf rust responsive gene expression in wheat (Triticum aestivum L.)

A novel leaf rust responsive ta-siRNA-producing locus was identified in wheat showing similarity to 28S rRNA and generated four differentially expressing ta-siRNAs by phasing which targeted stress responsive genes. Trans-acting-small interfering RNAs (Ta-siRNAs) are plant specific molecules generally involved in development and are also stress responsive. Ta-siRNAs identified in wheat till date are all responsive to abiotic stress only. Wheat cultivation is severely affected by rusts and leaf rust particularly affects grain filling. This study reports a novel ta-siRNA producing locus (TAS) in wheat which is a segment of 28S ribosomal RNA but shows differential expression during leaf rust infestation. Four small RNA libraries prepared from wheat Near Isogenic Lines were treated with leaf rust pathogen and compared with untreated controls. A TAS with the ability to generate four ta-siRNAs by phasing events was identified along with the microRNA TamiR16 as the phase initiator. The targets of the ta-siRNAs included α-gliadin, leucine rich repeat, trans-membrane proteins, glutathione-S-transferase, and fatty acid desaturase among others, which are either stress responsive genes or are essential for normal growth and development of plants. Expression of the TAS, its generated ta-siRNAs, and their target genes were profiled at five different time points after pathogen inoculation of susceptible and resistant wheat isolines and compared with mock-inoculated controls. Comparative analysis of expression unveiled differential and reciprocal relationship as well as discrete patterns between susceptible and resistant isolines. The expression profiles of the target genes of the identified ta-siRNAs advocate more towards effector triggered susceptibility favouring pathogenesis. The study helps in discerning the functions of wheat genes regulated by ta-siRNAs in response to leaf rust.

Isolation of Early-Responsive MicroRNA From Diuraphis noxia (Hemiptera: Aphididae)-Resistant Wheat

The Russian wheat aphid (Diuraphis noxia Kurdjumov) is an economically important pest of small grains in many countries. The past decades have seen the deployment of resistance-carrying wheat (Triticum aestivum L.) cultivars to control D. noxia. However, the emergence of resistance-breaking biotypes is negating this strategy. The role that noncoding RNA (ncRNA) molecules play in the wheat-D. noxia interaction has not been studied to date. This study aimed to isolate differentially regulated microRNA from a resistant and susceptible near-isogenic wheat line after aphid infestation. Twenty-seven identified miRNA were mostly related to stress-linked miRNA, and their predicted targets were linked with known D. noxia-feeding regulated proteins. These included transcription factors, signaling proteins, carbohydrate metabolism, and disease resistance pathways. Gene expression of three putative miRNAs and a predicted nucleotide-binding leucine-rich repeat gene with an identified miRNA target site in the NB-ARC domain displayed differential regulation between the resistant and susceptible plants. This study marks the initial investigation into understanding the role of ncRNA in a D. noxia-resistant wheat line after infestation and reports a correlation between a miRNA and its putative target for this interaction.

Identification and characterization of durum wheat microRNAs in leaf and root tissues

MicroRNAs are a class of post-transcriptional regulators of plant developmental and physiological processes and responses to environmental stresses. Here, we present the study regarding the annotation and characterization of MIR genes conducted in durum wheat. We characterized the miRNAome of leaf and root tissues at tillering stage under two environmental conditions: irrigated with 100% (control) and 55% of evapotranspiration (early water stress). In total, 90 microRNAs were identified, of which 32 were classified as putative novel and species-specific miRNAs. In addition, seven microRNA homeologous groups were identified in each of the two genomes of the tetraploid durum wheat. Differential expression analysis highlighted a total of 45 microRNAs significantly differentially regulated in the pairwise comparisons leaf versus root. The miRNA families, miR530, miR395, miR393, miR5168, miR396 and miR166, miR171, miR319, and miR167, were the most expressed in leaves in comparison to roots. Putative microRNA targets were predicted for both five and three prime sequences derived from the stem-loop of the MIR gene. Gene ontology analysis showed significant overrepresented gene categories in microRNA targets belonging to transcription factors, phenylpropanoids, oxydases, and lipid binding-protein. This work represents one of the first genome wide characterization of MIR genes in durum wheat, identifying leaf and root tissue-specific microRNAs. This genomic identification of microRNAs together with the analysis of their expression profiles is a well-accepted starting point leading to a better comprehension of the role of MIR genes in the genus Triticum.

Uncovering leaf rust responsive miRNAs in wheat (Triticum aestivum L.) using high-throughput sequencing and prediction of their targets through degradome analysis

Deep sequencing identified 497 conserved and 559 novel miRNAs in wheat, while degradome analysis revealed 701 targets genes. QRT-PCR demonstrated differential expression of miRNAs during stages of leaf rust progression. Bread wheat (Triticum aestivum L.) is an important cereal food crop feeding 30 % of the world population. Major threat to wheat production is the rust epidemics. This study was targeted towards identification and functional characterizations of micro(mi)RNAs and their target genes in wheat in response to leaf rust ingression. High-throughput sequencing was used for transcriptome-wide identification of miRNAs and their expression profiling in retort to leaf rust using mock and pathogen-inoculated resistant and susceptible near-isogenic wheat plants. A total of 1056 mature miRNAs were identified, of which 497 miRNAs were conserved and 559 miRNAs were novel. The pathogen-inoculated resistant plants manifested more miRNAs compared with the pathogen infected susceptible plants. The miRNA counts increased in susceptible isoline due to leaf rust, conversely, the counts decreased in the resistant isoline in response to pathogenesis illustrating precise spatial tuning of miRNAs during compatible and incompatible interaction. Stem-loop quantitative real-time PCR was used to profile 10 highly differentially expressed miRNAs obtained from high-throughput sequencing data. The spatio-temporal profiling validated the differential expression of miRNAs between the isolines as well as in retort to pathogen infection. Degradome analysis provided 701 predicted target genes associated with defense response, signal transduction, development, metabolism, and transcriptional regulation. The obtained results indicate that wheat isolines employ diverse arrays of miRNAs that modulate their target genes during compatible and incompatible interaction. Our findings contribute to increase knowledge on roles of microRNA in wheat-leaf rust interactions and could help in rust resistance breeding programs.

A large-scale chromosome-specific SNP discovery guideline

Single-nucleotide polymorphisms (SNPs) are the most prevalent type of variation in genomes that are increasingly being used as molecular markers in diversity analyses, mapping and cloning of genes, and germplasm characterization. However, only a few studies reported large-scale SNP discovery in Aegilops tauschii, restricting their potential use as markers for the low-polymorphic D genome. Here, we report 68,592 SNPs found on the gene-related sequences of the 5D chromosome of Ae. tauschii genotype MvGB589 using genomic and transcriptomic sequences from seven Ae. tauschii accessions, including AL8/78, the only genotype for which a draft genome sequence is available at present. We also suggest a workflow to compare SNP positions in homologous regions on the 5D chromosome of Triticum aestivum, bread wheat, to mark single nucleotide variations between these closely related species. Overall, the identified SNPs define a density of 4.49 SNPs per kilobyte, among the highest reported for the genic regions of Ae. tauschii so far. To our knowledge, this study also presents the first chromosome-specific SNP catalog in Ae. tauschii that should facilitate the association of these SNPs with morphological traits on chromosome 5D to be ultimately targeted for wheat improvement.

Water-deficit stress-responsive microRNAs and their targets in four durum wheat genotypes

MicroRNAs (miRNAs) guide regulation at the post-transcriptional level by inducing messenger RNA (mRNA) degradation or translational inhibition of their target protein-coding genes. Durum wheat miRNAs may contribute to the genotypic water-deficit stress response in different durum varieties. Further investigation of the interactive miRNA-target regulatory modules and experimental validation of their response to water stress will contribute to our understanding of the small RNA-mediated molecular networks underlying stress adaptation in durum wheat. In this study, a comprehensive genome-wide in silico analysis using the updated Triticum transcriptome assembly identified 2055 putative targets for 113 conserved durum miRNAs and 131 targets for four novel durum miRNAs that putatively contribute to genotypic stress tolerance. Predicted mRNA targets encode various transcription factors, binding proteins and functional enzymes, which play vital roles in multiple biological pathways such as hormone signalling and metabolic processes. Quantitative PCR profiling further characterised 43 targets and 5 miRNAs with stress-responsive and/or genotype-dependent differential expression in two stress-tolerant and two stress-sensitive durum genotypes subjected to pre-anthesis water-deficit stress. Furthermore, a 5' RLM-RACE approach validated nine mRNA targets cleaved by water-deficit stress-responsive miRNAs, which, to our knowledge, has not been previously reported in durum wheat. The present study provided experimental evidence of durum miRNAs and target genes in response to water-deficit stress in contrasting durum varieties, providing new insights into the regulatory roles of the miRNA-guided RNAi mechanism underlying stress adaptation in durum wheat.

The miRNAome of durum wheat: isolation and characterisation of conserved and novel microRNAs and their target genes

Background

The allotetraploid durum wheat [Triticum turgidum subsp. durum (Desf.) Husn.] is a highly economically important species especially in the Mediterranean basin. However, its genomics, transcriptomics and in particular microRNAome are still largely unknown.

Results

In the present work, two small RNA libraries from durum wheat Ciccio and Svevo cultivars were generated from different tissues at the late milk (Z77) developmental stage. A total of 167 conserved and 98 potential novel miRNAs were identified in the two libraries and interestingly, three novel miRNAs were found to be derived from ribosomal RNA. Putative target genes were predicted for conserved and novel miRNAs, the majority of which interact with nucleic acids, according to GO terms relative to molecular function. Quantitative qPCR analysis showed that several miRNAs identified were differentially expressed in the mature (Z77) developmental stage compared to young (Z14) tissues. Moreover, target gene expression analysis suggested that in roots, the putative genes encoding for the SQUAMOSA SPL2 and TGA1 proteins are regulated by ttu-miR156n, while MYB3 transcription factor by ttu-miR319f. Additionally, the Photosystem II P680 chlorophyll A apoprotein gene showed an expression level negatively correlated to that of ttu-novel-48 in leaves.

Conclusion

Our results suggest that, in durum wheat, these genes may play important roles in root/leaf development and are subjected to miRNA regulation. The prediction of novel miRNAs putatively derived from ribosomal RNA opens new perspectives on the study of plant miRNAs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2838-4) contains supplementary material, which is available to authorized users.

Wheat miRNA ancestors: evident by transcriptome analysis of A, B, and D genome donors

MicroRNAs are critical players of post-transcriptional gene regulation with profound effects on the fundamental processes of cellular life. Their identification and characterization, together with their targets, hold great significance in exploring and exploiting their roles on a functional context, providing valuable clues into the regulation of important biological processes, such as stress tolerance or environmental adaptation. Wheat is a hardy crop, extensively harvested in temperate regions, and is a major component of the human diet. With the advent of the next generation sequencing technologies considerably decreasing sequencing costs per base-pair, genomic, and transcriptomic data from several wheat species, including the progenitors and wild relatives have become available. In this study, we performed in silico identification and comparative analysis of microRNA repertoires of bread wheat (Triticum aestivum L.) and its diploid progenitors and relatives, Aegilops sharonensis, Aegilops speltoides, Aegilops tauschii, Triticum monococcum, and Triticum urartu through the utilization of publicly available transcriptomic data. Over 200 miRNA families were identified, majority of which have not previously been reported. Ancestral relationships expanded our understanding of wheat miRNA evolution, while T. monococcum miRNAs delivered important clues on the effects of domestication on miRNA expression. Comparative analyses on wild Ae. sharonensis accessions highlighted candidate miRNAs that can be linked to stress tolerance. The miRNA repertoires of bread wheat and its diploid progenitors and relatives provide important insight into the diversification and distribution of miRNA genes, which should contribute to the elucidation of miRNA evolution of Poaceae family. A thorough understanding of the convergent and divergent expression profiles of miRNAs in different genetic backgrounds can provide unique opportunities to modulation of gene regulation for better crop performance.

Synteny and comparative analysis of miRNA retention, conservation, and structure across Brassicaceae reveals lineage- and sub-genome-specific changes

The recent availability of genome sequences together with syntenic block information for Brassicaceae offers an opportunity to study microRNA (miRNA) evolution across this family. We employed a synteny-based comparative genomics strategy to unambiguously identify miRNA homologs from the genome sequence of members of Brassicaceae. Such an analysis of miRNA across Brassicaceae allowed us to classify miRNAs as conserved, lineage-, karyotype- and sub-genome-specific. The differential loss of miRNA from sub-genomes in polyploid genomes of Brassica rapa and Brassica oleracea shows that miRNA also follows the rules of gene fractionation as observed in the case of protein-coding genes. The study of mature and miR* region of precursors revealed instances of in-dels and SNPs which reflect the evolutionary history of the genomes. High level of conservation in miR* regions in some cases points to their functional relevance which needs to be further investigated. We further show that sequence and length variability in precursor sequences can affect the free energy and foldback structure of miRNA which may ultimately affect their biogenesis and expression in the biological system.

Identification and Comparative Analysis of microRNA in Wheat (Triticum aestivum L.) Callus Derived from Mature and Immature Embryos during In vitro Culture

Feasible and efficient tissue culture plays an important role in plant genetic engineering. Wheat (Triticum aestivum L.) immature embryos (IMEs) are preferred for tissue culture to mature embryos (MEs) because IMEs easily generate embryogenic callus, producing large number of plants. The molecular mechanisms of regulation and the biological pathways involved in embryogenic callus formation in wheat remain unclear. Here, microRNAs (miRNAs) potentially involved in embryogenic callus formation and somatic embryogenesis were identified through deep sequencing of small RNAs (sRNAs) and analyzed with bioinformatics tools. Six sRNA libraries derived from calli of IMEs and MEs after 3, 6, or 15 d of culture (DC) were constructed and sequenced. A total of 85 known miRNAs were identified, of which 30, 33, and 18 were differentially expressed (P < 0.05) between the IME and ME libraries at 3, 6, and 15 DC, respectively. Additionally, 171 novel and 41 candidate miRNAs were also identified, of the novel miRNA, 69, 67, and 37 were differentially expressed (P < 0.05) between the two types of libraries at 3, 6, and 15 DC, respectively. The expression patterns of eight known and eight novel miRNAs were validated using quantitative real-time polymerase chain reaction. Gene ontology annotation of differentially expressed miRNA targets provided information regarding the underlying molecular functions, biological processes, and cellular components involved in embryogenic callus development. Functional miRNAs, such as miR156, miR164, miR1432, miR398, and miR397, differentially expressed in IMEs and MEs might be related to embryogenic callus formation and somatic embryogenesis. This study suggests that miRNA plays an important role in embryogenic callus formation and somatic embryogenesis in wheat, and our data provide a useful resource for further research.

Dissecting miRNAs in Wheat D Genome Progenitor, Aegilops tauschii

As the post-transcriptional regulators of gene expression, microRNAs or miRNAs comprise an integral part of understanding how genomes function. Although miRNAs have been a major focus of recent efforts, miRNA research is still in its infancy in most plant species. Aegilops tauschii, the D genome progenitor of bread wheat, is a wild diploid grass exhibiting remarkable population diversity. Due to the direct ancestry and the diverse gene pool, A. tauschii is a promising source for bread wheat improvement. In this study, a total of 87 Aegilops miRNA families, including 51 previously unknown, were computationally identified both at the subgenomic level, using flow-sorted A. tauschii 5D chromosome, and at the whole genome level. Predictions at the genomic and subgenomic levels suggested A. tauschii 5D chromosome as rich in pre-miRNAs that are highly associated with Class II DNA transposons. In order to gain insights into miRNA evolution, putative 5D chromosome miRNAs were compared to its modern ortholog, Triticum aestivum 5D chromosome, revealing that 48 of the 58 A. tauschii 5D miRNAs were conserved in orthologous T. aestivum 5D chromosome. The expression profiles of selected miRNAs (miR167, miR5205, miR5175, miR5523) provided the first experimental evidence for miR5175, miR5205 and miR5523, and revealed differential expressional changes in response to drought in different genetic backgrounds for miR167 and miR5175. Interestingly, while miR5523 coding regions were present and expressed as pre-miR5523 in both T. aestivum and A. tauschii, the expression of mature miR5523 was observed only in A. tauschii under normal conditions, pointing out to an interference at the downstream processing of pre-miR5523 in T. aestivum. Overall, this study expands our knowledge on the miRNA catalog of A. tauschii, locating a subset specifically to the 5D chromosome, with ample functional and comparative insight which should contribute to and complement efforts to develop drought tolerant wheat varieties.

A Comprehensive Prescription for Plant miRNA Identification

microRNAs (miRNAs) are tiny ribo-regulatory molecules involved in various essential pathways for persistence of cellular life, such as development, environmental adaptation, and stress response. In recent years, miRNAs have become a major focus in molecular biology because of their functional and diagnostic importance. This interest in miRNA research has resulted in the development of many specific software and pipelines for the identification of miRNAs and their specific targets, which is the key for the elucidation of miRNA-modulated gene expression. While the well-recognized importance of miRNAs in clinical research pushed the emergence of many useful computational identification approaches in animals, available software and pipelines are fewer for plants. Additionally, existing approaches suffers from mis-identification and annotation of plant miRNAs since the miRNA mining process for plants is highly prone to false-positives, particularly in cereals which have a highly repetitive genome. Our group developed a homology-based in silico miRNA identification approach for plants, which utilizes two Perl scripts "SUmirFind" and "SUmirFold" and since then, this method helped identify many miRNAs particularly from crop species such as Triticum or Aegliops. Herein, we describe a comprehensive updated guideline by the implementation of two new scripts, "SUmirPredictor" and "SUmirLocator," and refinements to our previous method in order to identify genuine miRNAs with increased sensitivity in consideration of miRNA identification problems in plants. Recent updates enable our method to provide more reliable and precise results in an automated fashion in addition to solutions for elimination of most false-positive predictions, miRNA naming and miRNA mis-annotation. It also provides a comprehensive view to genome/transcriptome-wide location of miRNA precursors as well as their association with transposable elements. The "SUmirPredictor" and "SUmirLocator" scripts are freely available together with a reference high-confidence plant miRNA list.

Nitrate Starvation Induced Changes in Root System Architecture, Carbon:Nitrogen Metabolism, and miRNA Expression in Nitrogen-Responsive Wheat Genotypes

Improvement of nutrient use efficiency in cereal crops is highly essential not only to reduce the cost of cultivation but also to save the environmental pollution, reduce energy consumption for production of these chemical fertilizers, improve soil health, and ultimately help in mitigating climate change. In the present investigation, we have studied the morphological (with special emphasis on root system architecture) and biochemical responses (in terms of assay of the key enzymes involved in N assimilation) of two N-responsive wheat genotypes, at the seedling stage, under nitrate-optimum and nitrate-starved conditions grown in hydroponics. Expression profile of a few known wheat micro RNAs (miRNAs) was also studied in the root tissue. Total root size, primary root length, and first- and second-order lateral root numbers responded significantly under nitrate-starved condition. Morphological parameters in terms of root and shoot length and fresh and dry weight of roots and shoots have also been observed to be significant between N-optimum and N-starved condition for each genotypes. Nitrate reductase (NR), glutamine synthatase (GS), and glutamate dehydrogenase (GDH) activity significantly decreased under N-starved condition. Glutamine oxoglutarate amino transferase (GOGAT) and pyruvate kinase (PK) activity was found to be genotype dependent. Most of the selected miRNAs were expressed in root tissues, and some of them showed their differential N-responsive expression. Our studies indicate that one of the N-responsive genotype (NP-890) did not get affected significantly under nitrogen starvation at seedling stage.

Root precursors of microRNAs in wild emmer and modern wheats show major differences in response to drought stress

MicroRNAs, small regulatory molecules with significant impacts on the transcriptional network of all living organisms, have been the focus of several studies conducted mostly on modern wheat cultivars. In this study, we investigated miRNA repertoires of modern durum wheat and its wild relatives, with differing degrees of drought tolerance, to identify miRNA candidates and their targets involved in drought stress response. Root transcriptomes of Triticum turgidum ssp. durum variety Kızıltan and two Triticum turgidum ssp. dicoccoides genotypes TR39477 and TTD-22 under control and drought conditions were assembled from individual RNA-Seq reads and used for in silico identification of miRNAs. A total of 66 miRNAs were identified from all species, across all conditions, of which 46 and 38 of the miRNAs identified from modern durum wheat and wild genotypes, respectively, had not been previously reported. Genotype- and/or stress-specific miRNAs provide insights into our understanding of the complex drought response. Particularly, miR1435, miR5024, and miR7714, identified only from drought-stress roots of drought-tolerant genotype TR39477, can be candidates for future studies to explore and exploit the drought response to develop tolerant varieties.

Plant miRNAs: biogenesis, organization and origins

MicroRNAs, or miRNAs, are posttranscriptional regulators of gene expression. A wealth of observations and findings suggest highly complex, multicomponent, and intermingled pathways governing miRNA biogenesis and miRNA-mediated gene silencing. Plant miRNA genes are usually found as individual entities scattered around the intergenic and-to a much lesser extent-intragenic space, while miRNA gene clusters, formed by tandem or segmental duplications, also exist in plant genomes. Genome duplications are proposed to contribute to miRNA family expansions, as well. Evolutionarily young miRNAs retaining extensive homology to their loci of origin deliver important clues into miRNA origins and evolution. Additionally, imprecisely processed miRNAs evidence noncanonical routes of biogenesis, which may affect miRNA expression levels or targeting capabilities. Majority of the knowledge regarding miRNAs comes from model plant species. As ongoing research progressively expands into nonmodel systems, our understanding of miRNAs and miRNA-related pathways changes which opens up new perspectives and frontiers in miRNA research.

Molecular organization and comparative analysis of chromosome 5B of the wild wheat ancestor Triticum dicoccoides

Wild emmer wheat, Triticum turgidum ssp. dicoccoides is the wild relative of Triticum turgidum, the progenitor of durum and bread wheat, and maintains a rich allelic diversity among its wild populations. The lack of adequate genetic and genomic resources, however, restricts its exploitation in wheat improvement. Here, we report next-generation sequencing of the flow-sorted chromosome 5B of T. dicoccoides to shed light into its genome structure, function and organization by exploring the repetitive elements, protein-encoding genes and putative microRNA and tRNA coding sequences. Comparative analyses with its counterparts in modern and wild wheats suggest clues into the B-genome evolution. Syntenic relationships of chromosome 5B with the model grasses can facilitate further efforts for fine-mapping of traits of interest. Mapping of 5B sequences onto the root transcriptomes of two additional T. dicoccoides genotypes, with contrasting drought tolerances, revealed several thousands of single nucleotide polymorphisms, of which 584 shared polymorphisms on 228 transcripts were specific to the drought-tolerant genotype. To our knowledge, this study presents the largest genomics resource currently available for T. dicoccoides, which, we believe, will encourage the exploitation of its genetic and genomic potential for wheat improvement to meet the increasing demand to feed the world.

Harnessing NGS and Big Data Optimally: Comparison of miRNA Prediction from Assembled versus Non-assembled Sequencing Data--The Case of the Grass Aegilops tauschii Complex Genome

MicroRNAs (miRNAs) are small, endogenous, non-coding RNA molecules that regulate gene expression at the post-transcriptional level. As high-throughput next generation sequencing (NGS) and Big Data rapidly accumulate for various species, efforts for in silico identification of miRNAs intensify. Surprisingly, the effect of the input genomics sequence on the robustness of miRNA prediction was not evaluated in detail to date. In the present study, we performed a homology-based miRNA and isomiRNA prediction of the 5D chromosome of bread wheat progenitor, Aegilops tauschii, using two distinct sequence data sets as input: (1) raw sequence reads obtained from 454-GS FLX Titanium sequencing platform and (2) an assembly constructed from these reads. We also compared this method with a number of available plant sequence datasets. We report here the identification of 62 and 22 miRNAs from raw reads and the assembly, respectively, of which 16 were predicted with high confidence from both datasets. While raw reads promoted sensitivity with the high number of miRNAs predicted, 55% (12 out of 22) of the assembly-based predictions were supported by previous observations, bringing specificity forward compared to the read-based predictions, of which only 37% were supported. Importantly, raw reads could identify several repeat-related miRNAs that could not be detected with the assembly. However, raw reads could not capture 6 miRNAs, for which the stem-loops could only be covered by the relatively longer sequences from the assembly. In summary, the comparison of miRNA datasets obtained by these two strategies revealed that utilization of raw reads, as well as assemblies for in silico prediction, have distinct advantages and disadvantages. Consideration of these important nuances can benefit future miRNA identification efforts in the current age of NGS and Big Data driven life sciences innovation.

An integrative approach to identify hexaploid wheat miRNAome associated with development and tolerance to abiotic stress

Background

Wheat is a major staple crop with broad adaptability to a wide range of environmental conditions. This adaptability involves several stress and developmentally responsive genes, in which microRNAs (miRNAs) have emerged as important regulatory factors. However, the currently used approaches to identify miRNAs in this polyploid complex system focus on conserved and highly expressed miRNAs avoiding regularly those that are often lineage-specific, condition-specific, or appeared recently in evolution. In addition, many environmental and biological factors affecting miRNA expression were not yet considered, resulting still in an incomplete repertoire of wheat miRNAs.

Results

We developed a conservation-independent technique based on an integrative approach that combines machine learning, bioinformatic tools, biological insights of known miRNA expression profiles and universal criteria of plant miRNAs to identify miRNAs with more confidence. The developed pipeline can potentially identify novel wheat miRNAs that share features common to several species or that are species specific or clade specific. It allowed the discovery of 199 miRNA candidates associated with different abiotic stresses and development stages. We also highlight from the raw data 267 miRNAs conserved with 43 miRBase families. The predicted miRNAs are highly associated with abiotic stress responses, tolerance and development. GO enrichment analysis showed that they may play biological and physiological roles associated with cold, salt and aluminum (Al) through auxin signaling pathways, regulation of gene expression, ubiquitination, transport, carbohydrates, gibberellins, lipid, glutathione and secondary metabolism, photosynthesis, as well as floral transition and flowering.

Conclusion

This approach provides a broad repertoire of hexaploid wheat miRNAs associated with abiotic stress responses, tolerance and development. These valuable resources of expressed wheat miRNAs will help in elucidating the regulatory mechanisms involved in freezing and Al responses and tolerance mechanisms as well as for development and flowering. In the long term, it may help in breeding stress tolerant plants.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1490-8) contains supplementary material, which is available to authorized users.

Brachypodium: a promising hub between model species and cereals

Brachypodium distachyon was proposed as a model species for genetics and molecular genomics in cereals less than 10 years ago. It is now established as a standard for research on C3 cereals on a variety of topics, due to its close phylogenetic relationship with Triticeae crops such as wheat and barley, and to its simple genome, its minimal growth requirement, and its short life cycle. In this review, we first highlight the tools and resources for Brachypodium that are currently being developed and made available by the international community. We subsequently describe how this species has been used for comparative genomic studies together with cereal crops, before illustrating major research fields in which Brachypodium has been successfully used as a model: cell wall synthesis, plant-pathogen interactions, root architecture, and seed development. Finally, we discuss the usefulness of research on Brachypodium in order to improve nitrogen use efficiency in cereals, with the aim of reducing the amount of applied fertilizer while increasing the grain yield. Several paths are considered, namely an improvement of either nitrogen remobilization from the vegetative organs, nitrate uptake from the soil, or nitrate assimilation by the plant. Altogether, these examples position the research on Brachypodium as at an intermediate stage between basic research, carried out mainly in Arabidopsis, and applied research carried out on wheat and barley, enabling a complementarity of the studies and reciprocal benefits.

Discovery of Novel Leaf Rust Responsive microRNAs in Wheat and Prediction of Their Target Genes

MicroRNAs are endogenous small noncoding RNAs which play critical roles in gene regulation. Few wheat (Triticum aestivum L.) miRNA sequences are available in miRBase repertoire and knowledge of their biological functions related to biotic stress is limited. We identified 52 miRNAs, belonging to 19 families, from next-generation transcriptome sequence data based on homology search. One wheat specific novel miRNA was identified but could not be ascribed or assigned to any known miRNA family. Differentially expressed 22 miRNAs were found between susceptible and resistant wheat near-isogenic lines inoculated with leaf rust pathogen Puccinia triticina and compared with mock inoculated controls. Most miRNAs were more upregulated in susceptible NIL compared to resistant NIL. We identified 1306 potential target genes for these 52 miRNAs with vital roles in response to stimuli, signaling, and diverse metabolic and cellular processes. Gene ontology analysis showed 66, 20, and 35 target genes to be categorized into biological process, molecular function, and cellular component, respectively. A miRNA-mediated regulatory network revealed relationships among the components of the targetome. The present study provides insight into potential miRNAs with probable roles in leaf rust pathogenesis and their target genes in wheat which establish a foundation for future studies.

History and current status of wheat miRNAs using next-generation sequencing and their roles in development and stress

As small molecules that aid in posttranscriptional silencing, microRNA (miRNA) discovery and characterization have vastly benefited from the recent development and widespread application of next-generation sequencing (NGS) technologies. Several miRNAs were identified through sequencing of constructed small RNA libraries, whereas others were predicted by in silico methods using the recently accumulating sequence data. NGS was a major breakthrough in efforts to sequence and dissect the genomes of plants, including bread wheat and its progenitors, which have large, repetitive and complex genomes. Availability of survey sequences of wheat whole genome and its individual chromosomes enabled researchers to predict and assess wheat miRNAs both in the subgenomic and whole genome levels. Moreover, small RNA construction and sequencing-based studies identified several putative development- and stress-related wheat miRNAs, revealing their differential expression patterns in specific developmental stages and/or in response to stress conditions. With the vast amount of wheat miRNAs identified in recent years, we are approaching to an overall knowledge on the wheat miRNA repertoire. In the following years, more comprehensive research in relation to miRNA conservation or divergence across wheat and its close relatives or progenitors should be performed. Results may serve valuable in understanding both the significant roles of species-specific miRNAs and also provide us information in relation to the dynamics between miRNAs and evolution in wheat. Furthermore, putative development- or stress-related miRNAs identified should be subjected to further functional analysis, which may be valuable in efforts to develop wheat with better resistance and/or yield.

Whole-genome discovery of miRNAs and their targets in wheat (Triticum aestivum L.)

BACKGROUND

MicroRNAs (miRNAs) are small, non-coding RNAs playing essential roles in plant growth, development, and stress responses. Sequencing of small RNAs is a starting point for understanding their number, diversity, expression and possible roles in plants.

RESULTS

In this study, we conducted a genome-wide survey of wheat miRNAs from 11 tissues, characterizing a total of 323 novel miRNAs belonging to 276 families in wheat. A miRNA conservation analysis identified 191 wheat-specific miRNAs, 2 monocot-specific miRNAs, and 30 wheat-specific variants from 9 highly conserved miRNA families. To understand possible roles of wheat miRNAs, we determined 524 potential targets for 124 miRNA families through degradome sequencing, and cleavage of a subset of them was validated via 5' RACE. Based on the genome-wide identification and characterization of miRNAs and their associated target genes, we further identified 64 miRNAs preferentially expressing in developing or germinating grains, which could play important roles in grain development.

CONCLUSION

We discovered 323 wheat novel miRNAs and 524 target genes for 124 miRNA families in a genome-wide level, and our data will serve as a foundation for future research into the functional roles of miRNAs in wheat.

A comprehensive genome-wide study on tissue-specific and abiotic stress-specific miRNAs in Triticum aestivum

Productivity of wheat crop is largely dependent on its growth and development that, in turn, is mainly regulated by environmental conditions, including abiotic stress factors. miRNAs are key regulators of gene expression networks involved in diverse aspects of development and stress responses in plants. Using high-throughput sequencing of eight small RNA libraries prepared from diverse abiotic stresses and tissues, we identified 47 known miRNAs belonging to 20 families, 49 true novel and 1030 candidate novel miRNAs. Digital gene expression analysis revealed that 257 miRNAs exhibited tissue-specific expression and 74 were associated with abiotic stresses. Putative target genes were predicted for miRNAs identified in this study and their grouping into functional categories indicated that the putative targets were involved in diverse biological processes. RLM-RACE of predicted targets of three known miRNAs (miR156, miR160 and miR164) confirmed their mRNA cleavage, thus indicating their regulation at post-transcriptional level by the corresponding miRNAs. Mapping of the sequenced data onto the wheat progenitors and closely related monocots revealed a large number of evolutionary conserved miRNAs. Additional expression profiling of some of these miRNAs in other abiotic stresses underline their involvement in multiple stresses. Our findings provide valuable resource for an improved understanding of the role of miRNAs in stress tolerance as well as plant development.

New wheat microRNA using whole-genome sequence

MicroRNAs are post-transcriptional regulators of gene expression, taking roles in a variety of fundamental biological processes. Hence, their identification, annotation and characterization are of great significance, especially in bread wheat, one of the main food sources for humans. The recent availability of 5× coverage Triticum aestivum L. whole-genome sequence provided us with the opportunity to perform a systematic prediction of a complete catalogue of wheat microRNAs. Using an in silico homology-based approach, stem-loop coding regions were derived from two assemblies, constructed from wheat 454 reads. To avoid the presence of pseudo-microRNAs in the final data set, transposable element related stem-loops were eliminated by repeat analysis. Overall, 52 putative wheat microRNAs were predicted, including seven, which have not been previously published. Moreover, with distinct analysis of the two different assemblies, both variety and representation of putative microRNA-coding stem-loops were found to be predominant in the intergenic regions. By searching available expressed sequences and small RNA library databases, expression evidence for 39 (out of 52) putative wheat microRNAs was provided. Expression of three of the predicted microRNAs (miR166, miR396 and miR528) was also comparatively quantified with real-time quantitative reverse transcription PCR. This is the first report on in silico prediction of a whole repertoire of bread wheat microRNAs, supported by the wet-lab validation.