Analysis of phosphorus-deficient responsive miRNAs and cis-elements from soybean (Glycine max L.)

MicroRNAs as potent regulators in nitrogen and phosphorus signaling transduction and their applications

Nitrogen (N) and phosphorus (Pi) are essential macronutrients that affect plant growth and development by influencing the molecular, metabolic, biochemical, and physiological responses at the local and whole levels in plants. N and Pi stresses suppress the physiological activities of plants, resulting in agricultural productivity losses and severely threatening food security. Accordingly, plants have elaborated diverse strategies to cope with N and Pi stresses through maintaining N and Pi homeostasis. MicroRNAs (miRNAs) as potent regulators fine-tune N and Pi signaling transduction that are distinct and indivisible from each other. Specific signals, such as noncoding RNAs (ncRNAs), interact with miRNAs and add to the complexity of regulation. Elucidation of the mechanisms by which miRNAs regulate N and Pi signaling transduction aids in the breeding of plants with strong tolerance to N and Pi stresses and high N and Pi use efficiency by fine-tuning MIR genes or miRNAs. However, to date, there has been no detailed and systematic introduction and comparison of the functions of miRNAs in N and Pi signaling transduction from the perspective of miRNAs and their applications. Here, we summarized and discussed current advances in the involvement of miRNAs in N and Pi signaling transduction and highlighted that fine-tuning the MIR genes or miRNAs involved in maintaining N and Pi homeostasis might provide valuable sights for sustainable agriculture.

Changes in epigenetic features in legumes under abiotic stresses

Legume crops are rich in nutritional value for human and livestock consumption. With global climate change, developing stress-resilient crops is crucial for ensuring global food security. Because of their nitrogen-fixing ability, legumes are also important for sustainable agriculture. Various abiotic stresses, such as salt, drought, and elevated temperatures, are known to adversely affect legume production. The responses of plants to abiotic stresses involve complicated cellular processes including stress hormone signaling, metabolic adjustments, and transcriptional regulations. Epigenetic mechanisms play a key role in regulating gene expressions at both transcriptional and posttranscriptional levels. Increasing evidence suggests the importance of epigenetic regulations of abiotic stress responses in legumes, and recent investigations have extended the scope to the epigenomic level using next-generation sequencing technologies. In this review, the current knowledge on the involvement of epigenetic features, including DNA methylation, histone modification, and noncoding RNAs, in abiotic stress responses in legumes is summarized and discussed. Since most of the available information focuses on a single aspect of these epigenetic features, integrative analyses involving omics data in multiple layers are needed for a better understanding of the dynamic chromatin statuses and their roles in transcriptional regulation. The inheritability of epigenetic modifications should also be assessed in future studies for their applications in improving stress tolerance in legumes through the stable epigenetic optimization of gene expressions.

The roles of miR156 in abiotic and biotic stresses in plants

MicroRNAs (miRNAs), known as a kind of non-coding RNA, can negatively regulate its target genes. To date, the roles of various miRNAs in plant development and resistance to abiotic and biotic stresses have been widely explored. The present review summarized and discussed the functions of miR156 or miR156-SPL module in abiotic and biotic stresses, such as drought, salt, heat, cold stress, UV-B radiation, heavy mental hazards, nutritional starvation, as well as plant viruses, plant diseases, etc. Based on this, the regulation of miR156-involved stress tolerance was better understood, thus, it would be much easier for plant biologists to carry out suitable strategies to help plants suffer from unfavorable living environments.

Advances in the Study of the Transcriptional Regulation Mechanism of Plant miRNAs

MicroRNAs (miRNA) are a class of endogenous, non-coding, small RNAs with about 22 nucleotides (nt), that are widespread in plants and are involved in various biological processes, such as development, flowering phase transition, hormone signal transduction, and stress response. The transcriptional regulation of miRNAs is an important process of miRNA gene regulation, and it is essential for miRNA biosynthesis and function. Like mRNAs, miRNAs are transcribed by RNA polymerase II, and these transcription processes are regulated by various transcription factors and other proteins. Consequently, the upstream genes regulating miRNA transcription, their specific expression, and the regulating mechanism were reviewed to provide more information for further research on the miRNA regulatory mechanism and help to further understand the regulatory networks of plant miRNAs.

Identification of microRNAs involved in the Phosphate starvation response in Oil Palm (Elaeis guineensis Jacq.)

BACKGROUND

Plant microRNA, often known as miRNA, is a novel form of gene expression regulator that is known to play a significant role in phosphate starvation. The identification of microRNAs involved in the response to phosphate starvation in oil palms is beneficial for breeding programs.

METHOD

The main nursery stage seedlings of two oil palm progenies were treated with three different fertiliser namely: complete fertiliser with urea, P2O5, K2O, and MgO based on the standard procedure as a control (C); fertiliser with urea, K2O, MgO without P2O5 (P0); and no fertiliser (F0) for 24 weeks. A total of six oil palm roots were subjected to RNA isolation, followed by miRNA sequencing using the Illumina HiSeq 4000 platform, and all reads were computationally analysed.

RESULTS

In total, 119 potential miRNAs related to 5,891 genes were identified. The P-specific miRNAs were assumed based on the miRNAs that identified without P fertilizer treatment, resulted of twenty miRNA sequences in the treatment comparison of (C vs P0) vs (C vs F0). Those 20 miRNA sequences were grouped into 9 families, namely EgmiR319; EgmiR399; EgmiR396; EgmiR172; EgmiR156; EgmiR157; miR5648; miR5645; and EgmiRNA_unidentified. Two miRNAs were selected for RT-qPCR validation, namely EgMir399 and EgMir172. Their expression pattern was similar with the RNA sequencing results and shown opposite expression pattern with their target genes, UBC E2 24 and APETALA2, respectively.

CONCLUSIONS

The nine micro RNA families was identified in oil palm root tissue at phosphate starvation.

Improving phosphorus acquisition efficiency through modification of root growth responses to phosphate starvation in legumes

Phosphorus (P) is one of the essential macronutrients for plant growth and development, and it is an integral part of the major organic components, including nucleic acids, proteins and phospholipids. Although total P is abundant in most soils, a large amount of P is not easily absorbed by plants. Inorganic phosphate (Pi) is the plant-available P, which is generally immobile and of low availability in soils. Hence, Pi starvation is a major constraint limiting plant growth and productivity. Enhancing plant P efficiency can be achieved by improving P acquisition efficiency (PAE) through modification of morpho-physiological and biochemical alteration in root traits that enable greater acquisition of external Pi from soils. Major advances have been made to dissect the mechanisms underlying plant adaptation to P deficiency, especially for legumes, which are considered important dietary sources for humans and livestock. This review aims to describe how legume root growth responds to Pi starvation, such as changes in the growth of primary root, lateral roots, root hairs and cluster roots. In particular, it summarizes the various strategies of legumes to confront P deficiency by regulating root traits that contribute towards improving PAE. Within these complex responses, a large number of Pi starvation-induced (PSI) genes and regulators involved in the developmental and biochemical alteration of root traits are highlighted. The involvement of key functional genes and regulators in remodeling root traits provides new opportunities for developing legume varieties with maximum PAE needed for regenerative agriculture.

Overexpression of soybean microRNA156b enhanced tolerance to phosphorus deficiency and seed yield in Arabidopsis

microRNAs (miRNAs) are endogenous small RNAs that are key regulatory factors participating in various biological activities such as the signaling of phosphorus deficiency in the plant. Previous studies have shown that miR156 expression was modulated by phosphorus starvation in Arabidopsis and soybean. However, it is not clear whether the over-expression of soybean miR156b (GmmiR156b) can improve a plant's tolerance to phosphorus deficiency and affect yield component traits. In this study, we generated Arabidopsis transgenic lines overexpressing GmmiR156b and investigated the plant's response to phosphorus deficiency. Compared with the wild type, the transgenic Arabidopsis seedlings had longer primary roots and higher phosphorus contents in roots under phosphorus-deficit conditions, but lower fresh weight root/shoot ratios under either phosphorus-deficient or sufficient conditions. Moreover, the GmmiR156b overexpression transgenic lines had higher phosphorus content in shoots of adult plants and grew better than the wide type under phosphorus-deficient conditions, and exhibited increased seed yields as well as strong pleiotropic developmental morphology such as dwarfness, prolonged growth period, bushy shoot/branching, and shorter silique length, suggesting that the transgenic lines were more tolerant to phosphorus deficiency. In addition, the expression level of four SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) genes (i.e., AtSPL4/5/6/15) were markedly suppressed in transgenic plants, indicating that they were the main targets negatively regulated by GmmiR156b (especially AtSPL15) and that the enhanced tolerance to phosphorus deficiency and seed yield is conferred mainly by the miR156-mediated downregulation of AtSPL15.

Integrated mRNA and microRNA expression analysis of root response to phosphate deficiency in Medicago sativa

The deficiency of available phosphate significantly limits plant growth and development. This study sought to investigate how alfalfa (Medicago sativa), a high-yielding and high-quality forage widely cultivated worldwide, responds to phosphate deficiency stress by integrating transcriptional and post-transcriptional data. In this study, 6,041 differentially expressed genes (DEGs) were identified in alfalfa roots under phosphate deficiency conditions. Furthermore, psRNATarget, RNAhybrid, and TargetFinder were used to predict the target genes of 137 differentially expressed miRNAs (DEMs) in the root. In total, 3,912 DEGs were predicted as target genes. Pearson correlation analysis revealed 423 pairs of miRNA-mRNA regulatory relationships. MiRNA negatively regulates mRNA involved in regulatory pathways of phosphate deficiency responses in alfalfa. miR156e targeted squamosa promoter-binding-like protein 13A (SPL13), miR160c targeted auxin response factor 18 (ARF18), and miR2587a controlled glycolysis and citrate cycle via Phosphoenolpyruvate carboxykinase (ATP) (PCKA). Novel-miR27 regulated SPX domain-containing protein that controls phosphate transport in alfalfa root, novel-miR3-targeted sulfoquinovosyl transferase SQD2 controlled sulfolipid synthesis and glutathione S-transferase (GST; mediated by miR169j/k and novel-miR159) regulated glutathione metabolism. miR399l regulated auxin-responsive protein SAUR72 involved in IAA signal transduction, while abscisic acid receptor PYL4 (regulated by novel-miR205 and novel-miR83) participated in ABA signal transduction. Combined miRNA-mRNA enrichment analysis showed that most miRNAs regulate the phosphate starvation response of alfalfa by modulating target genes involved in carbohydrate metabolism, sulfolipid metabolism, glutathione metabolism, and hormone signal transduction. Therefore, this study provides new insights into the post-transcriptional regulation mechanism of phosphate deficiency responses and new perspectives on phosphate assimilation pathways in alfalfa and other legumes.

Differential microRNA expression, microRNA arm switching, and microRNA:long noncoding RNA interaction in response to salinity stress in soybean

BACKGROUND

Soybean is a major legume crop with high nutritional and environmental values suitable for sustainable agriculture. Noncoding RNAs (ncRNAs), including microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), are important regulators of gene functions in eukaryotes. However, the interactions between these two types of ncRNAs in the context of plant physiology, especially in response to salinity stress, are poorly understood.

RESULTS

Here, we challenged a cultivated soybean accession (C08) and a wild one (W05) with salt treatment and obtained their small RNA transcriptomes at six time points from both root and leaf tissues. In addition to thoroughly analyzing the differentially expressed miRNAs, we also documented the first case of miRNA arm-switching (miR166m), the swapping of dominant miRNA arm expression, in soybean in different tissues. Two arms of miR166m target different genes related to salinity stress (chloroplastic beta-amylase 1 targeted by miR166m-5p and calcium-dependent protein kinase 1 targeted by miR166m-3p), suggesting arm-switching of miR166m play roles in soybean in response to salinity stress. Furthermore, two pairs of miRNA:lncRNA interacting partners (miR166i-5p and lncRNA Gmax_MSTRG.35921.1; and miR394a-3p and lncRNA Gmax_MSTRG.18616.1) were also discovered in reaction to salinity stress.

CONCLUSIONS

This study demonstrates how ncRNA involves in salinity stress responses in soybean by miRNA arm switching and miRNA:lncRNA interactions. The behaviors of ncRNAs revealed in this study will shed new light on molecular regulatory mechanisms of stress responses in plants, and hence provide potential new strategies for crop improvement.

Noncoding-RNA-Mediated Regulation in Response to Macronutrient Stress in Plants

Macronutrient elements including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) are required in relatively large and steady amounts for plant growth and development. Deficient or excessive supply of macronutrients from external environments may trigger a series of plant responses at phenotypic and molecular levels during the entire life cycle. Among the intertwined molecular networks underlying plant responses to macronutrient stress, noncoding RNAs (ncRNAs), mainly microRNAs (miRNAs) and long ncRNAs (lncRNAs), may serve as pivotal regulators for the coordination between nutrient supply and plant demand, while the responsive ncRNA-target module and the interactive mechanism vary among elements and species. Towards a comprehensive identification and functional characterization of nutrient-responsive ncRNAs and their downstream molecules, high-throughput sequencing has produced massive omics data for comparative expression profiling as a first step. In this review, we highlight the recent findings of ncRNA-mediated regulation in response to macronutrient stress, with special emphasis on the large-scale sequencing efforts for screening out candidate nutrient-responsive ncRNAs in plants, and discuss potential improvements in theoretical study to provide better guidance for crop breeding practices.

Non-Coding RNAs in Legumes: Their Emerging Roles in Regulating Biotic/Abiotic Stress Responses and Plant Growth and Development

Noncoding RNAs, including microRNAs (miRNAs), small interference RNAs (siRNAs), circular RNA (circRNA), and long noncoding RNAs (lncRNAs), control gene expression at the transcription, post-transcription, and translation levels. Apart from protein-coding genes, accumulating evidence supports ncRNAs playing a critical role in shaping plant growth and development and biotic and abiotic stress responses in various species, including legume crops. Noncoding RNAs (ncRNAs) interact with DNA, RNA, and proteins, modulating their target genes. However, the regulatory mechanisms controlling these cellular processes are not well understood. Here, we discuss the features of various ncRNAs, including their emerging role in contributing to biotic/abiotic stress response and plant growth and development, in addition to the molecular mechanisms involved, focusing on legume crops. Unravelling the underlying molecular mechanisms and functional implications of ncRNAs will enhance our understanding of the coordinated regulation of plant defences against various biotic and abiotic stresses and for key growth and development processes to better design various legume crops for global food security.

Phosphate or nitrate imbalance induces stronger molecular responses than combined nutrient deprivation in roots and leaves of chickpea plants

The negative effects of phosphate (Pi) and/or nitrate (NO₃^- ) fertilizers on the environment have raised an urgent need to develop crop varieties with higher Pi and/or nitrogen use efficiencies for cultivation in low-fertility soils. Achieving this goal depends upon research that focuses on the identification of genes involved in plant responses to Pi and/or NO₃^- starvation. Although plant responses to individual deficiency in either Pi (-Pi/+NO₃^- ) or NO₃^- (+Pi/-NO₃^- ) have been separately studied, our understanding of plant responses to combined Pi and NO₃^- deficiency (-Pi/-NO₃^- ) is still very limited. Using RNA-sequencing approach, transcriptome changes in the roots and leaves of chickpea cultivated under -Pi/+NO₃^- , +Pi/-NO₃^- or -Pi/-NO₃^- conditions were investigated in a comparative manner. -Pi/-NO₃^- treatment displayed lesser effect on expression changes of genes related to Pi or NO₃^- transport, signalling networks, lipid remodelling, nitrogen and Pi scavenging/remobilization/recycling, carbon metabolism and hormone metabolism than -Pi/+NO₃^- or +Pi/-NO₃^- treatments. Therefore, the plant response to -Pi/-NO₃^- is not simply an additive result of plant responses to -Pi/+NO₃^- and +Pi/-NO₃^- treatments. Our results indicate that nutrient imbalance is a stronger stimulus for molecular reprogramming than an overall deficiency.

Genome-wide identification of low phosphorus responsive microRNAs in two soybean genotypes by high-throughput sequencing

MicroRNAs (miRNAs) have been reported to be correlated with various stress responses in soybean, but only a few miRNAs have been demonstrated to respond to low phosphorus (LP) stress. To unravel the response mechanisms of miRNAs to low-P stress, the roots of two representative soybean genotypes with different P efficiency, Nannong94-156 (a LP-tolerant genotype) and Bogao (a LP-sensitive genotype), were used for the construction of RNA sequencing (RNA-seq) libraries under low/normal-P treatment by high-throughput sequencing. In total, 603 existing miRNAs and 1699 novel miRNAs belonging to 248 and 1582 families in all samples were identified, respectively. Among these miRNAs, 777 miRNAs were differentially expressed (DE) across different P levels and genotypes. Furthermore, putative targets of DE miRNAs were predicted, and these miRNAs mainly targeted ERF (ethylene responsive factor), auxin response factors (ARF), zinc finger protein, MYB, and NAC domain transcription factors. Gene ontology (GO) analysis showed that targets of DE miRNAs were significantly enriched in binding, metabolic processes, biological regulation, response to stress, and phosphorus metabolic processes. In addition, the expression profiles of chosen P-responsive miRNAs and target genes were validated by quantitative real-time PCR (qRT-PCR). Our study focused on genome-wide miRNA identification in two representative soybean genotypes under low-P stress. Overall, the DE miRNAs across different P levels and genotypes and their putative target genes will provide useful information for further study of miRNAs mediating low-P response and facilitate improvements in soybean breeding.

Integrated analyses of miRNAome and transcriptome reveal zinc deficiency responses in rice seedlings

BACKGROUND

Zinc (Zn) deficiency is one of the most widespread soil constraints affecting rice productivity, but the molecular mechanisms underlying the regulation of Zn deficiency response is still limited. Here, we aim to understand the molecular mechanisms of Zn deficiency response by integrating the analyses of the global miRNA and mRNA expression profiles under Zn deficiency and resupply in rice seedlings by integrating Illumina's high-throughput small RNA sequencing and transcriptome sequencing.

RESULTS

The transcriptome sequencing identified 360 genes that were differentially expressed in the shoots and roots of Zn-deficient rice seedlings, and 97 of them were recovered after Zn resupply. A total of 68 miRNAs were identified to be differentially expressed under Zn deficiency and/or Zn resupply. The integrated analyses of miRNAome and transcriptome data showed that 12 differentially expressed genes are the potential target genes of 10 Zn-responsive miRNAs such as miR171g-5p, miR397b-5p, miR398a-5p and miR528-5p. Some miRNA genes and differentially expressed genes were selected for validation by quantitative RT-PCR, and their expressions were similar to that of the sequencing results.

CONCLUSION

These results provide insights into miRNA-mediated regulatory pathways in Zn deficiency response, and provide candidate genes for genetic improvement of Zn deficiency tolerance in rice.

Gold nanoparticles-based biosensor can detect drought stress in tomato by ultrasensitive and specific determination of miRNAs

Drought stress can significantly affect the yield and quality of tomato production. However, the development of a sensitive and specific method for the determination of drought stress is somehow challenging since plant common morpho-physiological and biochemical characteristic are not generally specific to biotic and abiotic stresses. As a solution, the concentration of miRNAs in plant tissues can be a selective and specific indicator of plant stress. In this study, an optical biosensor based on gold nanoparticles is introduced to determine miRNA-1886 in tomato plant roots. Results showed that irrigation levels from 100% to 60% of field capacity increased the concentration of miRNA-1886 in a range from ca. 100 to 6800 fM (fM) causing a linear change in the biosensor response (R² = 0.97). Results also revealed that in contrast with plant conventional morpho-physiological and biochemical characteristic, miRNA-1886 concentration was not significantly affected (P < 0.01) by other stresses, i.e., salinity and temperature during the growth period. The biosensor introduced in this study is a reliable method to study stress-related functions of miRNAs in plants and their application in specific plant stress determination.

Transcriptome profiles of soybean leaves and roots in response to zinc deficiency

Zinc (Zn) deficiency is a widespread agricultural problem in arable soils of the whole world. However, the molecular mechanisms underlying Zn-deficiency response are largely unknown. Here, we analyzed the transcriptomic profilings of soybean leaves and roots in response to Zn deficiency through Illumina's high-throughput RNA sequencing in order to understand the molecular basis of Zn-deficiency response in the plants. A total of 614 and 1011 gene loci were found to be differentially expressed in leaves and roots, respectively, and 88 loci were commonly found in both leaves and roots. Twelve differentially expressed genes (DEGs) were randomly selected for validation by quantitative reverse transcription polymerase chain reaction, and their fold changes were similar to those of RNA-seq. Gene ontology enrichment analysis showed that ion transport, nicotianamine (NA) biosynthetic process and queuosine biosynthetic process were enriched in the upregulated genes, while oxidation-reduction process and defense response were enriched in the downregulated genes. Among the DEGs, 20 DEGs are potentially involved in Zn homeostasis, including seven ZRT, IRT-related protein (ZIP) transporter genes, three NA synthase genes, and seven metallothionein genes; 40 DEGs are possibly involved in diverse hormonal signals such as auxin, cytokinin, ethylene and gibberellin; nine DEGs are putatively involved in calcium signaling; 85 DEGs are putative transcription factor genes. Nine DEGs were found to contain zinc-deficiency-response element in their promoter regions. These results could provide comprehensive insights into the soybean response to Zn deficiency and will be helpful for further elucidation of the molecular mechanisms of Zn-deficiency response and Zn-deficiency tolerance in plants.

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.

Stress-responsive miRNAome of Glycine max (L.) Merrill: molecular insights and way forward

MAIN CONCLUSION

Analysis of stress-associated miRNAs of Glycine max (L.) Merrill reveals wider ramifications of small RNA-mediated (conserved and legume-specific miRNAs) gene regulatory foot prints in molecular adaptive responses. MicroRNAs (miRNAs) are indispensable components of gene regulatory mechanism of plants. Soybean is a crop of immense commercial potential grown worldwide for its edible oil and soy meal. Intensive research efforts, using the next generation sequencing and bioinformatics techniques, have led to the identification and characterization of numerous small RNAs, especially microRNAs (miRNAs), in soybean. Furthermore, studies have unequivocally demonstrated the significance of miRNAs during the developmental processes and various stresses in soybean. In this review, we summarize the current state of understanding of miRNA-based abiotic and biotic stress responses in soybean. In addition, the molecular insights gained from the stress-related soybean miRNAs have been compared to the miRNAs of other crops, especially legumes, and the core commonalities have been highlighted, though differences among them were not ignored. Nature of response of soybean-derived conserved miRNAs during various stresses was also analyzed to gain deeper insights regarding sRNAome-based defense responses. This review further provides way forward in legume small RNA transcriptomics based on the adaptive responses of soybean and other legume-derived miRNAs.

Identification of microRNAs and their targets responding to low-potassium stress in two barley genotypes differing in low-K tolerance

MicroRNAs (miRNAs) have diverse and crucial roles in plant growth and development, including in the response to abiotic stresses. Although plant responses to K deficiency are well documented at the physiological and transcriptional levels, the miRNA-mediated post-transcriptional pathways are still not clearly elucidated. In this study, high-throughput sequencing and degradome analysis were performed using two barley genotypes differing in low-K tolerance (XZ149, tolerant and ZD9, sensitive), to determine the genotypic difference in miRNAs profiling. A total of 270 miRNAs were detected in the roots of XZ149 and ZD9 at 2 d and 10 d after low-K treatment, of which 195 were commonly found in both genotypes. Their targets were further investigated by bioinformatics prediction and degradome sequencing approach. The results showed that ata-miR1432-5p might act as a regulator participating in Ca²⁺ signaling pathways in response to low-K stress. The difference in the miR444/MADS-box model as well as pathways mediated by miR319/TCP4 and miR396/GRF could be attributed to high tolerance to low-K stress in XZ149. In addition, other conserved and novel miRNAs families associated with low-K tolerance were also detected. The current results provide molecular evidence for understanding the possible involvement of miRNAs in the regulation of low-K tolerance.

Long Non-Coding RNAs as Endogenous Target Mimics and Exploration of Their Role in Low Nutrient Stress Tolerance in Plants

Long non-coding RNA (lncRNA) research in plants has recently gained momentum taking cues from studies in animals systems. The availability of next-generation sequencing has enabled genome-wide identification of lncRNA in several plant species. Some lncRNAs are inhibitors of microRNA expression and have a function known as target mimicry with the sequestered transcript known as an endogenous target mimic (eTM). The lncRNAs identified to date show diverse mechanisms of gene regulation, most of which remain poorly understood. In this review, we discuss the role of identified putative lncRNAs that may act as eTMs for nutrient-responsive microRNAs (miRNAs) in plants. If functionally validated, these putative lncRNAs would enhance current understanding of the role of lncRNAs in nutrient homeostasis in plants.

Early Transcriptomic Response to Phosphate Deprivation in Soybean Leaves as Revealed by RNA-Sequencing

Low phosphate (Pi) availability is an important limiting factor affecting soybean production. However, the underlying molecular mechanisms responsible for low Pi stress response and tolerance remain largely unknown, especially for the early signaling events under low Pi stress. Here, a genome-wide transcriptomic analysis in soybean leaves treated with a short-term Pi-deprivation (24 h) was performed through high-throughput RNA sequencing (RNA-seq) technology. A total of 533 loci were found to be differentially expressed in response to Pi deprivation, including 36 mis-annotated loci and 32 novel loci. Among the differentially expressed genes (DEGs), 303 were induced and 230 were repressed by Pi deprivation. To validate the reliability of the RNA-seq data, 18 DEGs were randomly selected and analyzed by quantitative RT-PCR (reverse transcription polymerase chain reaction), which exhibited similar fold changes with RNA-seq. Enrichment analyses showed that 29 GO (Gene Ontology) terms and 8 KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways were significantly enriched in the up-regulated DEGs and 25 GO terms and 16 KEGG pathways were significantly enriched in the down-regulated DEGs. Some DEGs potentially involved in Pi sensing and signaling were up-regulated by short-term Pi deprivation, including five SPX-containing genes. Some DEGs possibly associated with water and nutrient uptake, hormonal and calcium signaling, protein phosphorylation and dephosphorylation and cell wall modification were affected at the early stage of Pi deprivation. The cis-elements of PHO (phosphatase) element, PHO-like element and P responsive element were present more frequently in promoter regions of up-regulated DEGs compared to that of randomly-selected genes in the soybean genome. Our transcriptomic data showed an intricate network containing transporters, transcription factors, kinases and phosphatases, hormone and calcium signaling components is involved in plant responses to early Pi deprivation.

Down-regulation of genes coding for core RNAi components and disease resistance proteins via corresponding microRNAs might be correlated with successful Soybean mosaic virus infection in soybean

Plants protect themselves from virus infections by several different defence mechanisms. RNA interference (RNAi) is one prominent antiviral mechanism, which requires the participation of AGO (Argonaute) and Dicer/DCL (Dicer-like) proteins. Effector-triggered immunity (ETI) is an antiviral mechanism mediated by resistance (R) genes, most of which encode nucleotide-binding site-leucine-rich repeat (NBS-LRR) family proteins. MicroRNAs (miRNAs) play important regulatory roles in plants, including the regulation of host defences. Soybean mosaic virus (SMV) is the most common virus in soybean and, in this work, we identified dozens of SMV-responsive miRNAs by microarray analysis in an SMV-susceptible soybean line. Amongst the up-regulated miRNAs, miR168a, miR403a, miR162b and miR1515a predictively regulate the expression of AGO1, AGO2, DCL1 and DCL2, respectively, and miR1507a, miR1507c and miR482a putatively regulate the expression of several NBS-LRR family disease resistance genes. The regulation of target gene expression by these seven miRNAs was validated by both transient expression assays and RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE) experiments. Transcript levels for AGO1, DCL1, DCL2 and five NBS-LRR family genes were repressed at different time points after SMV infection, whereas the corresponding miRNA levels were up-regulated at these same time points. Furthermore, inhibition of miR1507a, miR1507c, miR482a, miR168a and miR1515a by short tandem target mimic (STTM) technology compromised SMV infection efficiency in soybean. Our results imply that SMV can counteract soybean defence responses by the down-regulation of several RNAi pathway genes and NBS-LRR family resistance genes via the induction of the accumulation of their corresponding miRNA levels.

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

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

Comparative transcriptome analysis of nodules of two Mesorhizobium-chickpea associations with differential symbiotic efficiency under phosphate deficiency

Phosphate (Pi) deficiency is known to be a major limitation for symbiotic nitrogen fixation (SNF), and hence legume crop productivity globally. However, very little information is available on the adaptive mechanisms, particularly in the important legume crop chickpea (Cicer arietinum L.), which enable nodules to respond to low-Pi availability. Thus, to elucidate these mechanisms in chickpea nodules at molecular level, we used an RNA sequencing approach to investigate transcriptomes of the nodules in Mesorhizobium mediterraneum SWRI9-(MmSWRI9)-chickpea and M. ciceri CP-31-(McCP-31)-chickpea associations under Pi-sufficient and Pi-deficient conditions, of which the McCP-31-chickpea association has a better SNF capacity than the MmSWRI9-chickpea association during Pi starvation. Our investigation revealed that more genes showed altered expression patterns in MmSWRI9-induced nodules than in McCP-31-induced nodules (540 vs. 225) under Pi deficiency, suggesting that the Pi-starvation-more-sensitive MmSWRI9-induced nodules required expression change in a larger number of genes to cope with low-Pi stress than the Pi-starvation-less-sensitive McCP-31-induced nodules. The functional classification of differentially expressed genes (DEGs) was examined to gain an understanding of how chickpea nodules respond to Pi starvation, caused by soil Pi deficiency. As a result, more DEGs involved in nodulation, detoxification, nutrient/ion transport, transcriptional factors, key metabolic pathways, Pi remobilization and signalling were found in Pi-starved MmSWRI9-induced nodules than in Pi-starved McCP-31-induced nodules. Our findings have enabled the identification of molecular processes that play important roles in the acclimation of nodules to Pi deficiency, ultimately leading to the development of Pi-efficient chickpea symbiotic associations suitable for Pi-deficient soils.

Genome-wide identification of soybean microRNA responsive to soybean cyst nematodes infection by deep sequencing

BACKGROUND

The soybean cyst nematode (SCN), Heterodera glycines, is one of the most devastating diseases limiting soybean production worldwide. It is known that small RNAs, including microRNAs (miRNAs) and small interfering RNAs (siRNAs), play important roles in regulating plant growth and development, defense against pathogens, and responses to environmental changes.

RESULTS

In order to understand the role of soybean miRNAs during SCN infection, we analyzed 24 small RNA libraries including three biological replicates from two soybean cultivars (SCN susceptible KS4607, and SCN HG Type 7 resistant KS4313N) that were grown under SCN-infested and -noninfested soil at two different time points (SCN feeding establishment and egg production). In total, 537 known and 70 putative novel miRNAs in soybean were identified from a total of 0.3 billion reads (average about 13.5 million reads for each sample) with the programs of Bowtie and miRDeep2 mapper. Differential expression analyses were carried out using edgeR to identify miRNAs involved in the soybean-SCN interaction. Comparative analysis of miRNA profiling indicated a total of 60 miRNAs belonging to 25 families that might be specifically related to cultivar responses to SCN. Quantitative RT-PCR validated similar miRNA interaction patterns as sequencing results.

CONCLUSION

These findings suggest that miRNAs are likely to play key roles in soybean response to SCN. The present work could provide a framework for miRNA functional identification and the development of novel approaches for improving soybean SCN resistance in future studies.

Analysis of EF-Hand Proteins in Soybean Genome Suggests Their Potential Roles in Environmental and Nutritional Stress Signaling

Calcium ion (Ca2+) is a universal second messenger that plays a critical role in plant responses to diverse physiological and environmental stimuli. The stimulus-specific signals are perceived and decoded by a series of Ca2+ binding proteins serving as Ca2+ sensors. The majority of Ca2+ sensors possess the EF-hand motif, a helix-loop-helix structure which forms a turn-loop structure. Although EF-hand proteins in model plant such as Arabidopsis have been well described, the identification, classification, and the physiological functions of EF-hand-containing proteins from soybean are not systemically reported. In this study, a total of at least 262 genes possibly encoding proteins containing one to six EF-hand motifs were identified in soybean genome. These genes include 6 calmodulins (CaMs), 144 calmodulin-like proteins (CMLs), 15 calcineurin B-like proteins, 50 calcium-dependent protein kinases (CDPKs), 13 CDPK-related protein kinases, 2 Ca2+- and CaM-dependent protein kinases, 17 respiratory burst oxidase homologs, and 15 unclassified EF-hand proteins. Most of these genes (87.8%) contain at least one kind of hormonal signaling- and/or stress response-related cis-elements in their -1500 bp promoter regions. Expression analyses by exploring the published microarray and Illumina transcriptome sequencing data revealed that the expression of these EF-hand genes were widely detected in different organs of soybean, and nearly half of the total EF-hand genes were responsive to various environmental or nutritional stresses. Quantitative RT-PCR was used to confirm their responsiveness to several stress treatments. To confirm the Ca2+-binding ability of these EF-hand proteins, four CMLs (CML1, CML13, CML39, and CML95) were randomly selected for SDS-PAGE mobility-shift assay in the presence and absence of Ca2+. Results showed that all of them have the ability to bind Ca2+. This study provided the first comprehensive analyses of genes encoding for EF-hand proteins in soybean. Information on the classification, phylogenetic relationships and expression profiles of soybean EF-hand genes in different tissues and under various environmental and nutritional stresses will be helpful for identifying candidates with potential roles in Ca2+ signal-mediated physiological processes including growth and development, plant-microbe interactions and responses to biotic and abiotic stresses.

miRNA-based heavy metal homeostasis and plant growth

Plants have been naturally gifted with mechanisms to adjust under very high or low nutrient concentrations. Heavy metal toxicity is considered as a major growth and yield-limiting factor for plants. This stress includes essential as well as non-essential metals. MicroRNAs (miRNAs) are known for mediating post-transcriptional regulation by cleaving transcripts or translational inhibition. It is commonly agreed that an extensive understanding of plant miRNAs will significantly help in the induction of tolerance against environmental stresses. With the introduction of the latest technology like next generation sequencing (NGS), a growing figure of miRNAs has been productively recognized in several plants for their diverse roles. These miRNAs are well-known modulators of plant responses to heavy metal (HM) stress. Data regarding metal-responsive miRNAs point out the vital role of plant miRNAs in supplementing metal detoxification by means of transcription factors (TF) or gene regulation. Acting as systemic signals, miRNAs also synchronize different physiological processes for plant responses to metal toxicities. In contrast to practicing techniques, using miRNA is a greatly helpful, pragmatic, and feasible approach. The earlier findings point towards miRNAs as a prospective target to engineer heavy metal tolerance in plants. Therefore, there is a need to augment our knowledge about the orchestrated functions of miRNAs during HM stress. We reviewed the deterministic significance of plant miRNAs in heavy metal tolerance and their role in mediating plant responses to HM toxicities. This review also summarized the topical developments by identification and validation of different metal stress-responsive miRNAs.

Involvement of Small RNAs in Phosphorus and Sulfur Sensing, Signaling and Stress: Current Update

Plants require several essential mineral nutrients for their growth and development. These nutrients are required to maintain physiological processes and structural integrity in plants. The root architecture has evolved to absorb nutrients from soil and transport them to other parts of the plant. Nutrient deficiency affects several physiological and biological processes in plants and leads to reduction in crop productivity and yield. To compensate this adversity, plants have developed adaptive mechanisms to enhance the acquisition, conservation, and mobilization of these nutrients under deficient or adverse conditions. In addition, plants have evolved an intricate nexus of complex signaling cascades, which help in nutrient sensing and uptake as well as to maintain nutrient homeostasis. In recent years, small non-coding RNAs such as micro RNAs (miRNAs) and endogenous small interfering RNAs have emerged as important component in regulating plant stress responses. A set of these small RNAs (sRNAs) have been implicated in regulating various processes involved in nutrient uptake, assimilation, and deficiency. In response to phosphorus (P) and sulphur (S) deficiencies, role of sRNAs, miR395 and miR399, have been identified to be instrumental; however, many more miRNAs might be involved in regulating the plant response to these nutrient stresses. These sRNAs modulate expression of target genes in response to P and S deficiencies and regulate their uptake and utilization for proper growth and development of the plant. This review summarizes the current understanding of uptake, sensing, and signaling of P and S and highlights the regulatory role of sRNAs in adaptive responses to these nutrient stresses in plants.

Small RNA profiles in soybean primary root tips under water deficit

BACKGROUND

Soybean (Glycine max) production is significantly hampered by frequent droughts in many regions of the world including the United States. Identifying microRNA (miRNA)-controlled posttranscriptional gene regulation under drought will enhance our understanding of molecular basis of drought tolerance in this important cash crop. Indeed, miRNA profiles in soybean exposed to drought were studied but not from the primary root tips, which is not only a main zone of water uptake but also critical for water stress sensing and signaling.

METHODS

Here we report miRNA profiles specifically from well-watered and water-stressed primary root tips (0 to 8 mm from the root apex) of soybean. Small RNA sequencing confirmed the expression of vastly diverse miRNA (303 individual miRNAs) population, and, importantly several conserved miRNAs were abundantly expressed in primary root tips.

RESULTS

Notably, 12 highly conserved miRNA families were differentially regulated in response to water-deficit; six were upregulated while six others were downregulated at least by one fold (log2) change. Differentially regulated soybean miRNAs are targeting genes include auxin response factors, Cu/Zn Superoxide dismutases, laccases and plantacyanin and several others.

CONCLUSIONS

These results highlighted the importance of miRNAs in primary root tips both under control and water-deficit conditions; under control conditions, miRNAs could be important for cell division, cell elongation and maintenance of the root apical meristem activity including quiescent centre whereas under water stress differentially regulated miRNAs could decrease auxin signaling and oxidative stress as well as other metabolic processes that save energy and water.

The Small-RNA Profiles of Almond (Prunus dulcis Mill.) Reproductive Tissues in Response to Cold Stress

Spring frost is an important environmental stress that threatens the production of Prunus trees. However, little information is available regarding molecular response of these plants to the frost stress. Using high throughput sequencing, this study was conducted to identify differentially expressed miRNAs, both the conserved and the non-conserved ones, in the reproductive tissues of almond tolerant H genotype under cold stress. Analysis of 50 to 58 million raw reads led to identification of 174 unique conserved and 59 novel microRNAs (miRNAs). Differential expression pattern analysis showed that 50 miRNA families were expressed differentially in one or both of almond reproductive tissues (anther and ovary). Out of these 50 miRNA families, 12 and 15 displayed up-regulation and down-regulation, respectively. The distribution of conserved miRNA families indicated that miR482f harbor the highest number of members. Confirmation of miRNAs expression patterns by quantitative real- time PCR (qPCR) was performed in cold tolerant (H genotype) alongside a sensitive variety (Sh12 genotype). Our analysis revealed differential expression for 9 miRNAs in anther and 3 miRNAs in ovary between these two varieties. Target prediction of miRNAs followed by differential expression analysis resulted in identification of 83 target genes, mostly transcription factors. This study comprehensively catalogued expressed miRNAs under different temperatures in two reproductive tissues (anther and ovary). Results of current study and the previous RNA-seq study, which was conducted in the same tissues by our group, provide a unique opportunity to understand the molecular basis of responses of almond to cold stress. The results can also enhance the possibility for gene manipulation to develop cold tolerant plants.

Identification of chilling-responsive microRNAs and their targets in vegetable soybean (Glycine max L.)

Chilling stress is a major factor limiting the yield and quality of vegetable soybean (Glycine max L.) on a global scale. In the present study, systematic identification and functional analysis of miRNAs under chilling stress were carried out to clarify the molecular mechanism of chilling resistance. Two independent small RNA libraries from leaves of soybean were constructed and sequenced with the high-throughput Illumina Solexa system. A total of 434 known miRNAs and 3 novel miRNAs were identified. Thirty-five miRNAs were verified by qRT-PCR analysis. Furthermore, their gene targets were identified via high-throughput degradome sequencing. A total of 898 transcripts were targeted by 54 miRNA families attributed to five categories. More importantly, we identified 51 miRNAs differentially expressed between chilling stress and control conditions. The targets of these miRNAs were enriched in oxidation-reduction, signal transduction, and metabolic process functional categories. Our qRT-PCR analysis confirmed a negative relationship among the miRNAs and their targets under chilling stress. Our work thus provides comprehensive molecular evidence supporting the involvement of miRNAs in chilling-stress responses in vegetable soybean.

Small RNA and degradome sequencing reveals important microRNA function in Astragalus chrysochlorus response to selenium stimuli

Selenium (Se), an essential element, plays important roles in human health as well as environmental sustainability. Se hyperaccumulating plants are thought as an alternative selenium resource, recently. Astragalus species are known as hyperaccumulator of Se by converting it to nonaminoacid compounds. However, Se-metabolism-related hyperaccumulation is not elucidated in plants yet. MicroRNAs (miRNAs) are key molecules in many biological and metabolic processes via targeting mRNAs, which may also play an important role in Se accumulation in plants. In this study, we identified 418 known miRNAs, belonging to 380 families, and 151 novel miRNAs induced by Se exposure in Astragalus chyrsochlorus callus. Among known miRNAs, the expression of 287 families was common in both libraries, besides 71 families were expressed only in Se-treated sample, whereas 60 conserved families were expressed in control tissue. miR1507a, miR1869 and miR2867-3p were mostly up-regulated, whereas miR1507-5p and miR8781b were significantly down-regulated by Se exposure. Computational analysis shows that the targets of miRNAs are involved in different types of biological mechanisms including 47 types of cellular component, 103 types of molecular function and 144 types of biological process. Degradome analysis shows that 1256 mRNAs were targeted by 499 miRNAs. We conclude that some known and novel miRNAs such as miR167a, miR319, miR1507a, miR4346, miR7767-3p, miR7800, miR9748 and miR-n93 target transcription factors, disease resistance proteins and some specific genes like cysteine synthase and might be related to plant hormone signal transduction, plant-pathogen interaction and sulphur metabolism pathways.

Identification of miRNAs and their targets by high-throughput sequencing and degradome analysis in cytoplasmic male-sterile line NJCMS1A and its maintainer NJCMS1B of soybean

BACKGROUND

Cytoplasmic male sterility (CMS) provides crucial breeding materials that facilitate hybrid seed production in various crops, and thus plays an important role in the study of hybrid vigor (heterosis), in plants. However, the CMS regulatory network in soybean remains unclear. MicroRNAs (miRNAs) play crucial roles in flower and pollen development by targeting genes that regulate their expression in plants. To identify the miRNAs and their targets that exist in the soybean CMS line NJCMS1A and its maintainer NJCMS1B, high-throughput sequencing and degradome analysis were conducted in this study.

RESULTS

Two small RNA libraries were constructed from the flower buds of the soybean CMS line NJCMS1A and its maintainer NJCMS1B. A total of 105 new miRNAs present on the other arm of known pre-miRNAs, 23 new miRNA members, 158 novel miRNAs and 160 high-confidence soybean miRNAs were identified using high-throughput sequencing. Among the identified miRNAs, 101 differentially expressed miRNAs with greater than two-fold changes between NJCMS1A and NJCMS1B were discovered. The different expression levels of selected miRNAs were confirmed by stem-loop quantitative real-time PCR. A degradome analysis showed that 856 targets were predicted to be targeted by 296 miRNAs, including a squamosa promoter-binding protein-like transcription factor family protein, a pentatricopeptide repeat-containing protein, and an auxin response factor, which were previously shown to be involved in floral organ or anther development in plants. Additionally, some targets, including a MADS-box transcription factor, NADP-dependent isocitrate dehydrogenase and NADH-ubiquinone oxidoreductase 24 kDa subunit, were identified, and they may have some relationship with the programmed cell death, reactive oxygen species accumulation and energy deficiencies, which might lead to soybean male sterility.

CONCLUSIONS

The present study is the first to use deep sequencing technology to identify miRNAs and their targets in the flower buds of the soybean CMS line NJCMS1A and its maintainer NJCMS1B. The results revealed that the miRNAs might participate in flower and pollen development, which could facilitate our understanding of the molecular mechanisms behind CMS in soybean.

Boron-deficiency-responsive microRNAs and their targets in Citrus sinensis leaves

BACKGROUND

MicroRNAs play important roles in the adaptive responses of plants to nutrient deficiencies. Most research, however, has focused on nitrogen (N), phosphorus (P), sulfur (S), copper (Cu) and iron (Fe) deficiencies, limited data are available on the differential expression of miRNAs and their target genes in response to deficiencies of other nutrient elements. In this study, we identified the known and novel miRNAs as well as the boron (B)-deficiency-responsive miRNAs from citrus leaves in order to obtain the potential miRNAs related to the tolerance of citrus to B-deficiency.

METHODS

Seedlings of 'Xuegan' [Citrus sinensis (L.) Osbeck] were supplied every other day with B-deficient (0 μM H3BO3) or -sufficient (10 μM H3BO3) nutrient solution for 15 weeks. Thereafter, we sequenced two small RNA libraries from B-deficient and -sufficient (control) citrus leaves, respectively, using Illumina sequencing.

RESULTS

Ninety one (83 known and 8 novel) up- and 81 (75 known and 6 novel) down-regulated miRNAs were isolated from B-deficient leaves. The great alteration of miRNA expression might contribute to the tolerance of citrus to B-deficiency. The adaptive responses of miRNAs to B-deficiency might related to several aspects: (a) attenuation of plant growth and development by repressing auxin signaling due to decreased TIR1 level and ARF-mediated gene expression by altering the expression of miR393, miR160 and miR3946; (b) maintaining leaf phenotype and enhancing the stress tolerance by up-regulating NACs targeted by miR159, miR782, miR3946 and miR7539; (c) activation of the stress responses and antioxidant system through down-regulating the expression of miR164, miR6260, miR5929, miR6214, miR3946 and miR3446; (d) decreasing the expression of major facilitator superfamily protein genes targeted by miR5037, thus lowering B export from plants. Also, B-deficiency-induced down-regulation of miR408 might play a role in plant tolerance to B-deficiency by regulating Cu homeostasis and enhancing superoxide dismutase activity.

CONCLUSIONS

Our study reveals some novel responses of citrus to B-deficiency, which increase our understanding of the adaptive mechanisms of citrus to B-deficiency at the miRNA (post-transcriptional) level.

miRNA778 and SUVH6 are involved in phosphate homeostasis in Arabidopsis

microRNAs (miRNAs) play an important role in plant adaptation to phosphate (Pi) starvation. Histone methylation can remodel chromatin structure and mediate gene expression. This study identified Arabidopsis miR778, a Pi-responsive miRNA, and its target gene Su(var) 3-9 homologs 6 (SUVH6) encoding a histone H3 lysine 9 (H3K9) methyltransferase. Overexpression of miR778 moderately enhanced primary and lateral root growth, free phosphate accumulation in shoots, and accumulation of anthocyanin under Pi deficient conditions. miR778 overexpression relieved the arrest of columella cell development under Pi starvation. Conversely, transgenic plants overexpressing a miR778-target mimic (35S::MIM778), that act as a sponge and sequesters miR778, showed opposite phenotypes of 35S::miR778 plants under Pi deficiency. Expression of several Pi deficiency-responsive genes such as miR399, Phosphate Transporter (PHT1;4), Low Phosphate-Resistant1 (LPR1) and Production of Anthocyanin Pigment 1 (PAP1) were elevated in the miR778 overexpressing plants, suggesting that both miR778 and SUVH6 are involved in phosphate homeostasis in plants. This study has provided a basis for further investigation on how SUVH6 regulates its downstream genes through chromatin remodeling and DNA methylation in plants stressed by Pi deficiency.

Role of microRNAs involved in plant response to nitrogen and phosphorous limiting conditions

Plant microRNAs (miRNAs) are a class of small non-coding RNAs which target and regulate the expression of genes involved in several growth, development, and metabolism processes. Recent researches have shown involvement of miRNAs in the regulation of uptake and utilization of nitrogen (N) and phosphorus (P) and more importantly for plant adaptation to N and P limitation conditions by modifications in plant growth, phenology, and architecture and production of secondary metabolites. Developing strategies that allow for the higher efficiency of using both N and P fertilizers in crop production is important for economic and environmental benefits. Improved crop varieties with better adaptation to N and P limiting conditions could be a key approach to achieve this effectively. Furthermore, understanding on the interactions between N and P uptake and use and their regulation is important for the maintenance of nutrient homeostasis in plants. This review describes the possible functions of different miRNAs and their cross-talk relevant to the plant adaptive responses to N and P limiting conditions. In addition, a comprehensive understanding of these processes at molecular level and importance of biological adaptation for improved N and P use efficiency is discussed.

miRNA regulation of nutrient homeostasis in plants

Small RNAs including micro RNAs (miRNA) play an indispensable role in cell signaling mechanisms. Generally, miRNAs that are 20-24 nucleotides long bind to specific complementary transcripts, attenuating gene expression at the post-transcriptional level or via translational inhibition. In plants, miRNAs have emerged as the principal regulator of various stress responses, including low nutrient availability. It has been reported that miRNAs are vital for maintaining nutrient homeostasis in plants by regulating the expression of transporters that are involved in nutrient uptake and mobilization. The present review highlights the role of various miRNAs in several macro- or micronutrient deficiencies in plants. Understanding the regulation of different transporters by miRNAs will aid in elucidating the underlying molecular signal transduction mechanisms during nutritional stress. Recent findings regarding nutrient related-miRNAs and their gene regulation machinery may delineate a novel platform for improving the nutritional status of cereal grains or crop biofortification programs in the future.

NPK macronutrients and microRNA homeostasis

Macronutrients are essential elements for plant growth and development. In natural, non-cultivated systems, the availability of macronutrients is not a limiting factor of growth, due to fast recycling mechanisms. However, their availability might be an issue in modern agricultural practices, since soil has been frequently over exploited. From a crop management perspective, the nitrogen (N), phosphorus (P), and potassium (K) are three important limiting factors and therefore frequently added as fertilizers. NPK are among the nutrients that have been reported to alter post-embryonic root developmental processes and consequently, impairs crop yield. To cope with nutrients scarcity, plants have evolved several mechanisms involved in metabolic, physiological, and developmental adaptations. In this scenario, microRNAs (miRNAs) have emerged as additional key regulators of nutrients uptake and assimilation. Some studies have demonstrated the intrinsic relation between miRNAs and their targets, and how they can modulate plants to deal with the NPK availability. In this review, we focus on miRNAs and their regulation of targets involved in NPK metabolism. In general, NPK starvation is related with miRNAs that are involved in root-architectural changes and uptake activity modulation. We further show that several miRNAs were discovered to be involved in plant-microbe symbiosis during N and P uptake, and in this way we present a global view of some studies that were conducted in the last years. The integration of current knowledge about miRNA-NPK signaling may help future studies to focus in good candidates genes for the development of important tools for plant nutritional breeding.

Orthologous plant microRNAs: microregulators with great potential for improving stress tolerance in plants

Small RNAs that are highly conserved across many plant species are involved in stress responses. Plants are exposed to many types of unfavorable conditions during their life cycle that result in some degree of stress. Recent studies on microRNAs (miRNAs) have highlighted their great potential as regulators of stress tolerance in plants. One of the possible ways in which plants counter environmental stresses is by altering their gene expression by the action of miRNAs. miRNAs regulate the expression of target genes by hybridizing to their nascent reverse complementary sequences marking them for cleavage in the nucleus or translational repression in the cytoplasm. Some miRNAs have been reported to be key regulators in biotic as well as abiotic stress responses across many species. The present review highlights some of the regulatory roles of orthologous plant miRNAs in response to various types of stress conditions.

Ectopic expression of miR156 represses nodulation and causes morphological and developmental changes in Lotus japonicus

The effects of microRNA156 overexpression on general plant architecture, branching, flowering time and nodulation were investigated in the model legume, Lotus japonicus. We cloned an miR156 homolog, LjmiR156a, from L. japonicus, and investigated its SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) genes and its biological function at enhancing vegetative biomass yield, extending flowering time, and its impact on nodulation. Thirteen potential targets for LjmiR156 were identified in vitro and their expression profiles were determined in aerial and underground parts of mature plants, including genes coding for eight SPLs, one WD-40, one RNA-directed DNA polymerase, two transport proteins, and one histidine-phosphotransfer protein. Two SPL and one WD-40 cleavage targets for LjmiR156-TC70253, AU089191, and TC57859-were identified. Transgenic plants with ectopic expression of LjmiR156a showed enhanced branching, dramatically delayed flowering, underdeveloped roots, and reduced nodulation. We also examined the transcript levels of key genes involved in nodule organogenesis and infection thread formation to determine the role of miR156 in regulating symbiosis. Overexpression of LjmiR156a led to repression of several nodulation genes during the early stages of root development such as three ENOD genes, SymPK, POLLUX, CYCLOPS, Cerberus, and Nsp1, and the stimulation of NFR1. Our results show that miR156 regulates vegetative biomass yield, flowering time and nodulation by silencing downstream target SPLs and other genes, suggesting that the miR156 regulatory network could be modified in forage legumes (such as alfalfa and trefoils) and in leafy vegetables (like lettuce and spinach) to positively impact economically valuable crop species.

MicroRNAs involving in cold, wounding and salt stresses in Triticum aestivum L

MicroRNAs (miRNAs) play critical roles in post-transcriptional regulation and act as important endogenous regulators to various stresses. Cold, wounding and high-salinity are three common environmental stress stimuli influencing crops growth and development. In this study, we identified 31 known miRNAs and 3 novel miRNAs in wheat. Moreover, 19 stress-regulated miRNAs using RT-qPCR data in which the effects of three stresses were surveyed from the known miRNAs. Among them, 16, 12 and 8 miRNAs were regulated under cold, wounding and high-salinity treatments, respectively. Of which 4 miRNAs were highly responsive to cold stress in wheat by northern blot, and 6 wounding-regulated and 3 high-salinity-regulated miRNAs were detected. Meanwhile, miR159, miR393 and miR398 were responsive to multiple stress stimuli. Besides, 2 novel miRNAs were regulated by cold stress. While, the analyses of targets suggested miR159, miR398 and miR6001 could responses to stress conditions in regulation pathways. Taken together, the results of this study suggest that wheat miRNAs may play important roles in response to abiotic stress.

Identification of boron-deficiency-responsive microRNAs in Citrus sinensis roots by Illumina sequencing

BACKGROUND

Boron (B)-deficiency is a widespread problem in many crops, including Citrus. MicroRNAs (miRNAs) play important roles in nutrient deficiencies. However, little is known on B-deficiency-responsive miRNAs in plants. In this study, we first identified miRNAs and their expression pattern in B-deficient Citrus sinensis roots by Illumina sequencing in order to identify miRNAs that might be involved in the tolerance of plants to B-deficiency.

RESULTS

We isolated 52 (40 known and 12 novel) up-regulated and 82 (72 known and 10 novel) down-regulated miRNAs from B-deficient roots, demonstrating remarkable metabolic flexibility of roots, which might contribute to the tolerance of plants to B-deficiency. A model for the possible roles of miRNAs in the tolerance of roots to B-deficiency was proposed. miRNAs might regulate the adaptations of roots to B-deficiency through following several aspects: (a) inactivating reactive oxygen species (ROS) signaling and scavenging through up-regulating miR474 and down-regulating miR782 and miR843; (b) increasing lateral root number by lowering miR5023 expression and maintaining a certain phenotype favorable for B-deficiency-tolerance by increasing miR394 expression; (c) enhancing cell transport by decreasing the transcripts of miR830, miR5266 and miR3465; (d) improving osmoprotection (miR474) and regulating other metabolic reactions (miR5023 and miR821). Other miRNAs such as miR472 and miR2118 in roots increased in response to B-deficiency, thus decreasing the expression of their target genes, which are involved in disease resistance, and hence, the disease resistance of roots.

CONCLUSIONS

Our work demonstrates the possible roles of miRNAs and related mechanisms in the response of plant roots to B-deficiency.

Genome-wide identification of microRNAs in Medicago truncatula by high-throughput sequencing

MicroRNAs (miRNAs) are small, endogenous RNAs that play important regulatory roles in development and stress response in plants by negatively regulating gene expression post-transcriptionally. Medicago truncatula has been used as a model plant to study functional genomics of legume plants. It has also been widely used to functionally study miRNAs. Identification of miRNAs at the whole-genome level is essential for functional characterization of miRNAs in plants. High-throughput sequencing is a powerful technology to identify miRNAs. In this chapter, the methods used for construction of a small RNA library and high-throughput sequencing involving total RNA isolation, small RNA purification, adapter ligation, reverse transcription, PCR amplification, and Solexa sequencing are described. Bioinformatics and analysis of differential expression of miRNAs including primary disposal, miRNA identification, target prediction, and expression analysis are also discussed. These methodologies associated with identification and functional characterization of miRNAs may provide useful tools for readers to study miRNAs in plants in general and Medicago truncatula in particular.

Analysis of promoters of microRNAs from a Glycine max degradome library

OBJECTIVE

MicroRNAs (miRNAs) are genome-encoded, small non-coding RNAs that play important functions in development, biotic and abiotic stress responses, and other processes. Our aim was to explore the regulation of miRNA expression.

METHODS

We used bioinformatics methods to predict the core promoters of 440 miRNAs identified from a soybean (Glycine max) degradome library and to analyze cis-acting elements for 369 miRNAs.

RESULTS

The prediction results showed that 83.86% of the 440 miRNAs contained promoters in their upstream sequences, and 8.64% (38 loci) in their downstream sequences. The distributions of two core promoter elements, TATA-boxes and transcription start sites (TSSs), were similar. The cis-acting elements were examined to provide clues to the function and regulation of spatiotemporal expression of the miRNAs. Analyses of miRNA cis-elements and targets indicated a potential auxin response factor (ARF)- and gibberellin response factor (GARF)-mediated negative feedback loop for miRNA expression.

CONCLUSIONS

The features of miRNAs from a Glycine max degradome library obtained here provide insights into the transcription regulation and functions of miRNAs in soybean.

Genome-wide identification of non-coding RNAs interacted with microRNAs in soybean

A wide range of RNA species interacting with microRNAs (miRNAs) form a complex gene regulation network and play vital roles in diverse biological processes. In this study, we performed a genome-wide identification of endogenous target mimics (eTMs) for miRNAs and phased-siRNA-producing loci (PHAS) in soybean with a focus on those involved in lipid metabolism. The results showed that a large number of eTMs and PHAS genes could be found in soybean. Additionally, we found that lipid metabolism related genes were potentially regulated by 28 miRNAs, and nine of them were potentially further regulated by a number of eTMs with expression evidence. Thirty-three miRNAs were found to trigger production of phasiRNAs from 49 PHAS genes, which were able to target lipid metabolism related genes. Degradome data supported miRNA- and/or phasiRNA-mediated cleavage of genes involved in lipid metabolism. Most eTMs for miRNAs involved in lipid metabolism and phasiRNAs targeting lipid metabolism related genes showed a tissue-specific expression pattern. Our bioinformatical evidences suggested that lipid metabolism in soybean is potentially regulated by a complex non-coding network, including miRNAs, eTMs, and phasiRNAs, and the results extended our knowledge on functions of non-coding RNAs.

Identification and comparative analysis of cadmium tolerance-associated miRNAs and their targets in two soybean genotypes

MicroRNAs (miRNAs) play crucial roles in regulating the expression of various stress responses genes in plants. To investigate soybean (Glycine max) miRNAs involved in the response to cadmium (Cd), microarrays containing 953 unique miRNA probes were employed to identify differences in the expression patterns of the miRNAs between different genotypes, Huaxia3 (HX3, Cd-tolerant) and Zhonghuang24 (ZH24, Cd-sensitive). Twenty six Cd-responsive miRNAs were identified in total. Among them, nine were detected in both cultivars, while five were expressed only in HX3 and 12 were only in ZH24. The expression of 16 miRNAs was tested by qRT-PCR and most of the identified miRNAs were found to have similar expression patterns with microarray. Three hundred and seventy six target genes were identified for 204 miRNAs from a mixture degradome library, which was constructed from the root of HX3 and ZH24 with or without Cd treatment. Fifty five genes were identified to be cleaved by 14 Cd-responsive miRNAs. Gene ontology (GO) annotations showed that these target transcripts are implicated in a broad range of biological processes. In addition, the expression patterns of ten target genes were validated by qRT-PCR. The characterization of the miRNAs and the associated target genes in response to Cd exposure provides a framework for understanding the molecular mechanism of heavy metal tolerance in plants.

Elucidation of miRNAs-mediated responses to low nitrogen stress by deep sequencing of two soybean genotypes

Nitrogen (N) is a major limiting factor in crop production, and plant adaptive responses to low N are involved in many post-transcriptional regulation. Recent studies indicate that miRNAs play important roles in adaptive responses. However, miRNAs in soybean adaptive responses to N limitation have been not reported. We constructed sixteen libraries to identify low N-responsive miRNAs on a genome-wide scale using samples from 2 different genotypes (low N sensitive and low N tolerant) subjected to various periods of low nitrogen stress. Using high-throughput sequencing technology (Illumina-Solexa), we identified 362 known miRNAs variants belonging to 158 families and 90 new miRNAs belonging to 55 families. Among these known miRNAs variants, almost 50% were not different from annotated miRNAs in miRBase. Analyses of their expression patterns showed 150 known miRNAs variants as well as 2 novel miRNAs with differential expressions. These differentially expressed miRNAs between the two soybean genotypes were compared and classified into three groups based on their expression patterns. Predicted targets of these miRNAs were involved in various metabolic and regulatory pathways such as protein degradation, carbohydrate metabolism, hormone signaling pathway, and cellular transport. These findings suggest that miRNAs play important roles in soybean response to low N and contribute to the understanding of the genetic basis of differences in adaptive responses to N limitation between the two soybean genotypes. Our study provides basis for expounding the complex gene regulatory network of these miRNAs.

Molecular dissection of mercury-responsive transcriptome and sense/antisense genes in Medicago truncatula

We described a newly developed approach, namely next-generation tag sequencing, to identify global gene transcripts and complexity regulated by heavy metals in Medicago truncatula. Two cDNA libraries were generated from M. truncatula seedlings: treated and non-treated with the toxic heavy metal mercury Hg(II). With the large number of read-mapped genes generated, we observed that most of the genes were differentially expressed between the two libraries. In addition, several classes of new transcripts including transcription factors, antisense transcripts, and stress responsive genes were detected. The forty genes most altered in expression levels were associated with tolerance to environmental stress and secondary metabolism. Validation of genes by quantitative RT-PCR confirmed the results from deep-sequencing. Most of genes coding for metal transporters, sulfate metabolism, and cell wall solidification were significantly altered by Hg exposure. We also examined altered expression ratios of sense and antisense (S-AS) transcripts between the two libraries. By analyzing strand-specific information of read sequences, S-AS transcripts were found to be enriched with metal treatment. The transcriptome sequences were analyzed further with Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) and showed diverse biological functions and metabolic pathways under the metal stress.

Genome-wide identification of soybean microRNAs and their targets reveals their organ-specificity and responses to phosphate starvation

BACKGROUND

Phosphorus (P) plays important roles in plant growth and development. MicroRNAs involved in P signaling have been identified in Arabidopsis and rice, but P-responsive microRNAs and their targets in soybean leaves and roots are poorly understood.

RESULTS

Using high-throughput sequencing-by-synthesis (SBS) technology, we sequenced four small RNA libraries from leaves and roots grown under phosphate (Pi)-sufficient (+Pi) and Pi-depleted (-Pi) conditions, respectively, and one RNA degradome library from Pi-depleted roots at the genome-wide level. Each library generated ~21.45-28.63 million short sequences, resulting in ~20.56-27.08 million clean reads. From those sequences, a total of 126 miRNAs, with 154 gene targets were computationally predicted. This included 92 new miRNA candidates with 20-23 nucleotides that were perfectly matched to the Glycine max genome 1.0, 70 of which belong to 21 miRNA families and the remaining 22 miRNA unassigned into any existing miRNA family in miRBase 18.0. Under both +Pi and -Pi conditions, 112 of 126 total miRNAs (89%) were expressed in both leaves and roots. Under +Pi conditions, 12 leaf- and 2 root-specific miRNAs were detected; while under -Pi conditions, 10 leaf- and 4 root-specific miRNAs were identified. Collectively, 25 miRNAs were induced and 11 miRNAs were repressed by Pi starvation in soybean. Then, stem-loop real-time PCR confirmed expression of four selected P-responsive miRNAs, and RLM-5' RACE confirmed that a PHO2 and GmPT5, a kelch-domain containing protein, and a Myb transcription factor, respectively are targets of miR399, miR2111, and miR159e-3p. Finally, P-responsive cis-elements in the promoter regions of soybean miRNA genes were analyzed at the genome-wide scale.

CONCLUSIONS

Leaf- and root-specific miRNAs, and P-responsive miRNAs in soybean were identified genome-wide. A total of 154 target genes of miRNAs were predicted via degradome sequencing and computational analyses. The targets of miR399, miR2111, and miR159e-3p were confirmed. Taken together, our study implies the important roles of miRNAs in P signaling and provides clues for deciphering the functions for microRNA/target modules in soybean.

Regulation of miR399f transcription by AtMYB2 affects phosphate starvation responses in Arabidopsis

Although a role for microRNA399 (miR399) in plant responses to phosphate (Pi) starvation has been indicated, the regulatory mechanism underlying miR399 gene expression is not clear. Here, we report that AtMYB2 functions as a direct transcriptional activator for miR399 in Arabidopsis (Arabidopsis thaliana) Pi starvation signaling. Compared with untransformed control plants, transgenic plants constitutively overexpressing AtMYB2 showed increased miR399f expression and tissue Pi contents under high Pi growth and exhibited elevated expression of a subset of Pi starvation-induced genes. Pi starvation-induced root architectural changes were more exaggerated in AtMYB2-overexpressing transgenic plants compared with the wild type. AtMYB2 directly binds to a MYB-binding site in the miR399f promoter in vitro, as well as in vivo, and stimulates miR399f promoter activity in Arabidopsis protoplasts. Transcription of AtMYB2 itself is induced in response to Pi deficiency, and the tissue expression patterns of miR399f and AtMYB2 are similar. Both genes are expressed mainly in vascular tissues of cotyledons and in roots. Our results suggest that AtMYB2 regulates plant responses to Pi starvation by regulating the expression of the miR399 gene.