Cytoplasmic male sterility-regulated novel microRNAs from maize

Effects of differentially expressed microRNAs induced by rootstocks and silicon on improving chilling tolerance of cucumber seedlings (Cucumis sativus L.)

BACKGROUND

Rootstocks can improve the chilling tolerance of grafted cucumbers, but their effectiveness varies. Rootstocks with strong de-blooming capacity may result in lower chilling tolerance of grafted cucumbers compared to those with weak de-blooming capacity, while also reducing the silicon absorption. However, it remains unclear whether this reduction in chilling tolerance is due to differences in rootstock genotypes or the reduction in silicon absorption.

RESULTS

The chilling tolerance of cucumber seedlings was improved by using rootstocks and silicon nutrition. Rootstocks had a more significant effect than silicon nutrition, and the weak de-blooming rootstock 'Yunnan figleaf gourd' was superior to the strong de-blooming rootstock 'Huangchenggen No. 2'. Compared to self-rooted cucumber, twelve miRNAs were regulated by two rootstocks, including seven identical miRNAs (novel-mir23, novel-mir26, novel-mir30, novel-mir37, novel-mir46, miR395a and miR398a-3p) and five different miRNAs (novel-mir32, novel-mir38, novel-mir65, novel-mir78 and miR397a). Notably, four of these miRNAs (novel-mir38, novel-mir65, novel-mir78 and miR397a) were only identified in 'Yunnan figleaf gourd'-grafted cucumbers. Furthermore, six miRNAs (miR168a-5p, miR390a-5p, novel-mir26, novel-mir55, novel-mir67 and novel-mir70) were found to be responsive to exogenous silicon. Target gene prediction for 20 miRNAs resulted in 520 genes. Functional analysis of these target genes showed that 'Yunnan figleaf gourd' improves the chilling tolerance of cucumber by regulating laccase synthesis and sulfate metabolism, while 'Huangchenggen No. 2' and exogenous silicon reduced chilling stress damage to cucumber by regulating ROS scavenging and protein protection, respectively.

CONCLUSION

Among the identified miRNAs, novel-mir46 and miR398a-3p were found in cucumbers in response to chilling stress and two types of rootstocks. However, no identical miRNAs were identified in response to chilling stress and silicon. In addition, the differential expression of novel-mir38, novel-mir65, novel-mir78 and miR397a may be one of the important reasons for the differences in chilling tolerance of grafted cucumbers caused by two types of rootstocks.

High-temperature stress in crops: male sterility, yield loss and potential remedy approaches

Global food security is one of the utmost essential challenges in the 21st century in providing enough food for the growing population while coping with the already stressed environment. High temperature (HT) is one of the main factors affecting plant growth, development and reproduction and causes male sterility in plants. In male reproductive tissues, metabolic changes induced by HT involve carbohydrates, lipids, hormones, epigenetics and reactive oxygen species, leading to male sterility and ultimately reducing yield. Understanding the mechanism and genes involved in these pathways during the HT stress response will provide a new path to improve crops by using molecular breeding and biotechnological approaches. Moreover, this review provides insight into male sterility and integrates this with suggested strategies to enhance crop tolerance under HT stress conditions at the reproductive stage.

Toxic mechanism on phenanthrene-triggered cell apoptosis, genotoxicity, immunotoxicity and activity changes of immunity protein in Eisenia fetida: Combined analysis at cellular and molecular levels

Phenanthrene (PHE) is a harmful organic contaminant and exists extensively in the soil environment. The accumulation of PHE would potentially threaten soil invertebrates, including earthworms, and the toxicity is also high. Currently, the possible mechanisms underlying apoptotic pathways induced by PHE and its immunotoxicity and genotoxicity in earthworms remain unclear. Thus, Eisenia fetida coelomocytes and immunity protein lysozyme (LYZ) were chosen as targeted receptors to reveal the apoptotic pathways, genotoxicity, and immunotoxicity triggered by PHE and its binding mechanism with LYZ, using cellular, biochemical, and molecular methods. Results indicated that PHE exposure can cause cell membrane damage, increase cell membrane permeability, and ultimately trigger mitochondria-mediated apoptosis. Increased 8-hydroxy-2-deoxyguanosine (8-OHdG) levels indicated PHE had triggered DNA oxidative damage in cells after PHE exposure. Occurrence of detrimental effects on the immune system in E. fetida coelomocytes due to decreased phagocytic efficacy and destroyed the lysosomal membrane. The LYZ activity in coelomocytes after PHE exposure was consistent with the molecular results, in which the LYZ activity was inhibited. After PHE binding, the protein structure (secondary structure and protein skeleton) and protein environment (the micro-environment of aromatic amino acids) of LYZ were destroyed, forming a larger particle size of the PHE-LYZ complex, and causing a significant sensitization effect on LYZ fluorescence. Molecular simulation indicated the key residues Glu 35, Asp 52, and Trp 62 for protein function located in the binding pocket, suggesting PHE preferentially binds to the active center of LYZ. Additionally, the primary driving forces for the binding interaction between PHE and LYZ molecule are hydrophobicity forces and hydrogen bonds. Taken together, PHE exposure can induce apoptosis by mitochondria-mediated pathway, destroy the normal immune system, and trigger DNA oxidative damage in earthworms. Besides, this study provides a comprehensive evaluation of phenanthrene toxicity to earthworms on molecular and cellular level.

MicroRNAs Involved in Regulatory Cytoplasmic Male Sterility by Analysis RNA-seq and Small RNA-seq in Soybean

Cytoplasmic male sterility (CMS) is an important plant characteristic for exploiting heterosis to enhance crop traits during breeding. However, the CMS regulatory network remains unclear in plants, even though researchers have attempted to isolate genes associated with CMS. In this study, we performed high-throughput sequencing and degradome analyses to identify microRNAs (miRNAs) and their targets in a soybean CMS line (JLCMS9A) and its maintainer line (JLCMS9B). Additionally, the differentially expressed genes during reproductive development were identified using RNA-seq data. A total of 280 miRNAs matched soybean miRNA sequences in miRBase, including mature miRNAs and pre-miRNAs. Of the 280 miRNAs, 30, 23, and 21 belonged to the miR166, miR156, and miR171 families, respectively. Moreover, 410 novel low-abundant miRNAs were identified in the JLCMS9A and JLCMS9B flower buds. Furthermore, 303 and 462 target genes unique to JLCMS9A and JLCMS9B, respectively, as well as 782 common targets were predicted based on the degradome analysis. Target genes differentially expressed between the CMS line and the maintainer line were revealed by an RNA-seq analysis. Moreover, all target genes were annotated with diverse functions related to biological processes, cellular components, and molecular functions, including transcriptional regulation, the nucleus, meristem maintenance, meristem initiation, cell differentiation, auxin-activated signaling, plant ovule development, and anther development. Finally, a network was built based on the interactions. Analyses of the miRNA, degradome, and transcriptome datasets generated in this study provided a comprehensive overview of the reproductive development of a CMS soybean line. The data presented herein represent useful information for soybean hybrid breeding. Furthermore, the study results indicate that miRNAs might contribute to the soybean CMS regulatory network by modulating the expression of CMS-related genes. These findings lay the foundation for future studies on the molecular mechanisms underlying soybean CMS.

Identification of microRNAs and their gene targets in cytoplasmic male sterile and fertile maintainer lines of pigeonpea

Comparative analysis of genome-wide miRNAs and their gene targets between cytoplasmic male sterile (CMS) and fertile lines of pigeonpea suggests a possible role of miRNA-regulated pathways in reproductive development. Exploitation of hybrid vigor using CMS technology has delivered nearly 50% yield gain in pigeonpea. Among various sterility-inducing cytoplasms (A₁-A₉) reported so far in pigeonpea, A₂ and A₄ are the two major sources that facilitate hybrid seed production. Recent evidence suggests involvement of micro RNA in vast array of biological processes including plant reproductive development. In pigeonpea, information about the miRNAs is insufficient. In view of this, we sequenced six small RNA libraries of CMS line UPAS 120A and isogenic fertile line UPAS 120B using Illumina technology. Results revealed 316 miRNAs including 248 known and 68 novel types. A total of 637 gene targets were predicted for known miRNAs, while 324 genes were associated with novel miRNAs. Degradome analysis revealed 77 gene targets of predicted miRNAs, which included a variety of transcription factors playing key roles in plant reproduction such as F-box family proteins, apetala 2, auxin response factors, ethylene-responsive factors, homeodomain-leucine zipper proteins etc. Differential expression of both known and novel miRNAs implied roles for both conserved as well as species-specific players. We also obtained several miRNA families such as miR156, miR159, miR167 that are known to influence crucial aspects of plant fertility. Gene ontology and pathway level analyses of the target genes showed their possible implications for crucial events during male reproductive development such as tapetal degeneration, pollen wall formation, retrograde signaling etc. To the best of our knowledge, present study is first to combine deep sequencing of small RNA and degradome for elucidating the role of miRNAs in flower and male reproductive development in pigeonpea.

Lipid Metabolism: Critical Roles in Male Fertility and Other Aspects of Reproductive Development in Plants

Fatty acids and their derivatives are essential building blocks for anther cuticle and pollen wall formation. Disruption of lipid metabolism during anther and pollen development often leads to genic male sterility (GMS). To date, many lipid metabolism-related GMS genes that are involved in the formation of anther cuticle, pollen wall, and subcellular organelle membranes in anther wall layers have been identified and characterized. In this review, we summarize recent progress on characterizing lipid metabolism-related genes and their roles in male fertility and other aspects of reproductive development in plants. On the basis of cloned GMS genes controlling biosynthesis and transport of anther cutin, wax, sporopollenin, and tryphine in Arabidopsis, rice, and maize as well as other plant species, updated lipid metabolic networks underlying anther cuticle development and pollen wall formation were proposed. Through bioinformatics analysis of anther RNA-sequencing datasets from three maize inbred lines (Oh43, W23, and B73), a total of 125 novel lipid metabolism-related genes putatively involved in male fertility in maize were deduced. More, we discuss the pathways regulating lipid metabolism-related GMS genes at the transcriptional and post-transcriptional levels. Finally, we highlight recent findings on lipid metabolism-related genes and their roles in other aspects of plant reproductive development. A comprehensive understanding of lipid metabolism, genes involved, and their roles in plant reproductive development will facilitate the application of lipid metabolism-related genes in gene editing, haploid and callus induction, molecular breeding and hybrid seed production in crops.

Comparative Analysis of miRNA Expression Profiles Between Heat-Tolerant and Heat-Sensitive Genotypes of Flowering Chinese Cabbage Under Heat Stress Using High-Throughput Sequencing

Heat stress disturbs cellular homeostasis, thus usually impairs yield of flowering Chinese cabbage (Brassica campestris L. ssp. chinensis var. utilis Tsen et Lee). MicroRNAs (miRNAs) play a significant role in plant responses to different stresses by modulating gene expression at the post-transcriptional level. However, the roles that miRNAs and their target genes may play in heat tolerance of flowering Chinese cabbage remain poorly characterized. The current study sequenced six small RNA libraries generated from leaf tissues of flowering Chinese cabbage collected at 0, 6, and 12 h after 38 °C heat treatment, and identified 49 putative novel miRNAs and 43 known miRNAs that differentially expressed between heat-tolerant and heat-sensitive flowering Chinese cabbage. Among them, 14 novel and nine known miRNAs differentially expressed only in the heat-tolerant genotype under heat-stress, therefore, their target genes including disease resistance protein TAO1-like, RPS6, reticuline oxidase-like protein, etc. might play important roles in enhancing heat-tolerance. Gene Ontology (GO) analysis revealed that targets of these differentially expressed miRNAs may play key roles in responses to temperature stimulus, cell part, cellular process, cell, membrane, biological regulation, binding, and catalytic activities. Furthermore, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis identified their important functions in signal transduction, environmental adaptation, global and overview maps, as well as in stress adaptation and in MAPK signaling pathways such as cell death. These findings provide insight into the functions of the miRNAs in heat stress tolerance of flowering Chinese cabbage.

CircRNA Expression Pattern and ceRNA and miRNA-mRNA Networks Involved in Anther Development in the CMS Line of Brassica campestris

Male-sterile plants provide an important breeding tool for the heterosis of hybrid crops, such as Brassicaceae. In the last decade, circular RNAs (circRNAs), as a novel class of covalently closed and single-stranded endogenous non-coding RNAs (ncRNAs), have received much attention because of their functions as “microRNA (miRNA) sponges” and “competing endogenous RNAs” (ceRNAs). However, the information about circRNAs in the regulation of male-sterility and anther development is limited. In this study, we established the Polima cytoplasm male sterility (CMS) line “Bcpol97-05A”, and the fertile line, “Bcajh97-01B”, in Brassicacampestris L. ssp. chinensis Makino, syn. B. rapa ssp. chinensis, and performed RNA expression profiling comparisons between the flower buds of the sterile line and fertile line by whole-transcriptome sequencing. A total of 31 differentially expressed (DE) circRNAs, 47 DE miRNAs, and 4779 DE mRNAs were identified. By using Cytoscape, the miRNA-mediated regulatory network and ceRNA network were constructed, and the circRNA A02:23507399|23531438 was hypothesized to be an important circRNA regulating anther development at the post-transcriptional level. The gene ontology (GO) analysis demonstrated that miRNAs and circRNAs could regulate the orderly secretion and deposition of cellulose, sporopollenin, pectin, and tryphine; the timely degradation of lipids; and the programmed cell death (PCD) of tapetum cells, which play key roles in anther development. Our study revealed a new circRNA–miRNA–mRNA network, which is involved in the anther development of B. campestris, which enriched the understanding of CMS in flowering plants, and laid a foundation for further study on the functions of circRNAs and miRNAs during anther development.

Identification of Differentially Expressed miRNAs between a Wheat K-type Cytoplasmic Male Sterility Line and Its Near-Isogenic Restorer Line

K-type cytoplasmic male sterility (KCMS) lines were ideal material for three-line hybrid wheat system due to the major role in hybrid wheat production. In this study, the morphology of developing microspore and mature pollen was compared between a KCMS line and its near-isogenic restorer line (KCMS-NIL). The most striking difference is that the microspore was unable to develop into tricellular pollen in the KCMS line. MicroRNA plays vital roles in flowering and gametophyte development. Small RNA sequencing identified a total of 274 known and 401 novel miRNAs differentially expressed between two lines or two developmental stages. Most of miRNAs with high abundance were differentially expressed at the uninucleate stage, and their expression level recovered or remained at the binucleate stage. Further degradome sequencing identified target genes which were mainly enriched in transcription regulation, phytohormone signaling and RNA degradation pathways. Combining with the transcriptome data, a correlation was found between the abnormal anther development, such as postmeiotic mitosis cessation, deformative pollen wall and the chromosome condensation of the vegetative cell, and the alterations in the related miRNA and their targets expression profiles. According to the correlation and pathway analysis, we propose a hypothetic miRNA-mediated network for the control of KCMS restoration.

Exploration of miRNA-mediated fertility regulation network of cytoplasmic male sterility during flower bud development in soybean

Cytoplasmic male sterility (CMS) plays an important role in the production of soybean hybrid seeds. MicroRNAs (miRNAs) are a class of non-coding endogenous ~ 21 nt small RNAs that play crucial roles in flower and pollen development by targeting genes in plants. To dissect the function of miRNAs in soybean CMS, a total of 558 known miRNAs, 10 novel miRNAs, and 466 target genes were identified in flower buds of the soybean CMS line NJCMS1A and its restorer line NJCMS1C through small RNA sequencing and degradome analysis. In addition, miRNA-mediated editing events were also observed, and the two most frequently observed editing types (A → G and C → U) were validated by cloning and sequencing. And as the base editing occurred, some targets were filtered, such as gma-miR2118b-P6GT with Glyma.08G122000.2. Further integrated analysis of transcriptome and small RNA found some miRNAs and their targets' expression patterns showing a negative correlation, such as gma-miR156b/GmSPL9a and gma-miR4413b/GmPPR. Furthermore, opposite expression pattern was observed between gma-miR156b and GmSPL9 during early stage of flower bud development. Taken together, the regulatory network of gma-miR156b/GmSPL9 and gma-miR4413b/GmPPR with flower bud development in soybean CMS was developed. Most importantly, previous reports showed that these targets might be related to pollen development and male sterility, suggesting that both conserved and species-specific miRNAs might act as regulators of flower bud development in soybean CMS. These findings may provide a better understanding of the miRNA-mediated regulatory networks of CMS mechanisms in soybean.

Uncovering key small RNAs associated with gametocidal action in wheat

Gametocidal (Gc) chromosomes can kill gametes that lack them by causing chromosomal breakage to ensure their preferential transmission, and they have been exploited in genetic breeding. The present study investigated the possible roles of small RNAs (sRNAs) in Gc action. By sequencing two small RNA libraries from the anthers of Triticum aestivum cv. Chinese Spring (CS) and the Chinese Spring-Gc 3C chromosome monosomic addition line (CS-3C), we identified 239 conserved and 72 putative novel miRNAs, including 135 differentially expressed miRNAs. These miRNAs were predicted to target multiple genes with various molecular functions relevant to the features of Gc action, including sterility and genome instability. The transgenic overexpression of miRNA, which was up-regulated in CS-3C, reduced rice fertility. The CS-3C line exhibited a genome-wide reduction in 24 nt siRNAs compared with that of the CS line, particularly in transposable element (TE) and repetitive DNA sequences. Corresponding to this reduction, the bisulfite sequencing analysis of four retro-TE sequences showed a decrease in CHH methylation, typical of RNA-directed DNA methylation (RdDM). These results demonstrate that both miRNA-directed regulation of gene expression and siRNA-directed DNA methylation of target TE loci could play a role in Gc action.

Oxidative Stress and Aberrant Programmed Cell Death Are Associated With Pollen Abortion in Isonuclear Alloplasmic Male-Sterile Wheat

Cytoplasmic male sterility is crucial for the utilization of hybrid heterosis and it possibly occurs in parallel with tapetal programmed cell death (PCD) and oxidative metabolism responses. However, little is known about the mechanisms that underlie pollen abortion in wheat. Therefore, we obtained two isonuclear alloplasmic male sterile lines (IAMSLs) with Aegilops kotschyi and Ae. juvenalis cytoplasm. Compared with the maintainer line, cytochemical analyses of the anthers demonstrated that the IAMSLs exhibited anomalous tapetal PCD and organelles, with premature PCD in K87B1-706A and delayed PCD in Ju87B1-706A. We also found that the dynamic trends in reactive oxygen species (ROS) were consistent in these two IAMSLs during anther development and they were potentially associated with the initiation of tapetal PCD. In addition, the activities of ROS-scavenging enzymes increased rapidly, whereas non-enzymatic antioxidants were downregulated together with excess ROS production in IAMSLs. Real-time PCR analysis showed that the expression levels of superoxide dismutase, catalase, and ascorbate peroxidase genes, which encode important antioxidant enzymes, were significantly upregulated during early pollen development. Thus, we inferred that excessive ROS and the abnormal transcript levels of antioxidant enzyme genes disrupted the balance of the antioxidant system and the presence of excess ROS may have been related to aberrant tapetal PCD progression, thereby affecting the development of microspores and ultimately causing male sterility. These relationships between the mechanism of PCD and ROS metabolism provide new insights into the mechanisms responsible for abortive pollen in wheat.

Exploration of miRNAs and target genes of cytoplasmic male sterility line in cotton during flower bud development

Cytoplasmic male sterility (CMS) lines provide crucial material to harness heterosis for crop plants, which serves as an important strategy for hybrid seed production. However, the molecular mechanism remains obscure. Although microRNAs (miRNAs) play important roles in vegetative growth and reproductive growth, there are few reports on miRNAs regulating the development of male sterility in Upland cotton. In present study, 12 small RNA libraries were constructed and sequenced for two development stages of flower buds from a CMS line and its maintainer line. Based on the results, 256 novel miRNAs were allocated to 141 new miRNA families, and 77 known miRNAs belonging to 54 conserved miRNA families were identified as well. Comparative analysis revealed that 61 novel and 10 conserved miRNAs were differentially expressed. Further transcriptome analysis identified 232 target genes for these miRNAs, which participated in cellular developmental process, cell death, pollen germination, and sexual reproduction. In addition, expression patterns of typical miRNA and the negatively regulated target genes, such as PPR, ARF, AP2, and AFB, were verified by qRT-PCR in cotton flower buds. These targets were previously reported to be related to reproduction development and male sterility, suggesting that miRNAs might act as regulators of CMS occurrence. Some miRNAs displayed specific expression profiles in special developmental stages of CMS line and its fertile hybrid (F₁). Present study offers new information on miRNAs and their related target genes in exploiting CMS mechanism, and revealing the miRNA regulatory networks in Upland cotton.

Comprehensive analysis of small RNAs expressed in developing male strobili of Cryptomeria japonica

Deep sequencing of small RNAs (sRNAs) in developing male strobili of second-generation offspring originating from a nuclear genic male sterile tree of Cryptomeria japonica were performed to characterize sRNA populations in the male strobili at early pollen developmental stages. Comparing to sequences of microRNA (miRNA) families of plant species and sRNAs expressed in the reproductive organs of representative vascular plants, 37 conserved miRNA families were detected, of which eight were ubiquitously expressed in the reproductive organs of land plant species. In contrast, miR1083 was common in male reproductive organs of gymnosperm species but absent in angiosperm species. In addition to conserved miRNAs, 199 novel miRNAs candidates were predicted. The expression patterns of the obtained sRNAs were further investigated to detect the differentially expressed (DE) sRNAs between genic male sterile and fertile individuals. A total of 969 DE sRNAs were obtained and only three known miRNA families were included among them. These results suggest that both conserved and species-specific sRNAs contribute to the development of male strobili in C. japonica.

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

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

The Role of Non-Coding RNAs in Cytoplasmic Male Sterility in Flowering Plants

The interactions between mitochondria and nucleus substantially influence plant development, stress response and morphological features. The prominent example of a mitochondrial-nuclear interaction is cytoplasmic male sterility (CMS), when plants produce aborted anthers or inviable pollen. The genes responsible for CMS are located in mitochondrial genome, but their expression is controlled by nuclear genes, called fertility restorers. Recent explosion of high-throughput sequencing methods enabled to study transcriptomic alterations in the level of non-coding RNAs under CMS biogenesis. We summarize current knowledge of the role of nucleus encoded regulatory non-coding RNAs (long non-coding RNA, microRNA as well as small interfering RNA) in CMS. We also focus on the emerging data of non-coding RNAs encoded by mitochondrial genome and their possible involvement in mitochondrial-nuclear interactions and CMS development.

Uncovering Male Fertility Transition Responsive miRNA in a Wheat Photo-Thermosensitive Genic Male Sterile Line by Deep Sequencing and Degradome Analysis

MicroRNAs (miRNAs) are endogenous small RNAs which play important negative regulatory roles at both the transcriptional and post-transcriptional levels in plants. Wheat is the most commonly cultivated plant species worldwide. In this study, RNA-seq analysis was used to examine the expression profiles of miRNA in the spikelets of photo-thermosenisitive genic male sterile (PTGMS) wheat line BS366 during male fertility transition. Through mapping on their corresponding precursors, 917-7,762 novel miRNAs were found in six libraries. Six novel miRNAs were selected for examination of their secondary structures and confirmation by stem-loop RT-PCR. In a differential expression analysis, 20, 22, and 58 known miRNAs exhibited significant differential expression between developmental stages 1 (secondary sporogenous cells had formed), 2 (all cells layers were present and mitosis had ceased), and 3 (meiotic division stage), respectively, of fertile and sterile plants. Some of these differential expressed miRNAs, such as tae-miR156, tae-miR164, tae-miR171, and tae-miR172, were shown to be associated with their targets. These targets were previously reported to be related to pollen development and/or male sterility, indicating that these miRNAs and their targets may be involved in the regulation of male fertility transition in the PTGMS wheat line BS366. Furthermore, target genes of miRNA cleavage sites were validated by degradome sequencing. In this study, a possible signal model for the miRNA-mediated signaling pathway during the process of male fertility transition in the PTGMS wheat line BS366 was developed. This study provides a new perspective for understanding the roles of miRNAs in male fertility in PTGMS lines of wheat.

MicroRNAs in crop improvement: fine-tuners for complex traits

One of the most common challenges for both conventional and modern crop improvement is that the appearance of one desirable trait in a new crop variety is always balanced by the impairment of one or more other beneficial characteristics. The best way to overcome this problem is the flexible utilization of regulatory genes, especially genes that provide more efficient and precise regulation in a targeted manner. MicroRNAs (miRNAs), a type of short non-coding RNA, are promising candidates in this area due to their role as master modulators of gene expression at the post-transcriptional level, targeting messenger RNAs for cleavage or directing translational inhibition in eukaryotes. We herein highlight the current understanding of the biological role of miRNAs in orchestrating distinct agriculturally important traits by summarizing recent functional analyses of 65 miRNAs in 9 major crops worldwide. The integration of current miRNA knowledge with conventional and modern crop improvement strategies is also discussed.

Identification and Functional Analysis of microRNAs Involved in the Anther Development in Cotton Genic Male Sterile Line Yu98-8A

Hybrid vigor contributes in a large way to the yield and quality of cotton (Gossypium hirsutum) fiber. Although microRNAs play essential regulatory roles in flower induction and development, it is still unclear if microRNAs are involved in male sterility, as the regulatory molecular mechanisms of male sterility in cotton need to be better defined. In this study, two independent small RNA libraries were constructed and sequenced from the young buds collected from the sporogenous cell formation to the meiosis stage of the male sterile line Yu98-8A and the near-isogenic line. Sequencing revealed 1588 and 1536 known microRNAs and 347 and 351 novel miRNAs from male sterile and male fertile libraries, respectively. MicroRNA expression profiles revealed that 49 conserved and 51 novel miRNAs were differentially expressed. Bioinformatic and degradome analysis indicated the regulatory complexity of microRNAs during flower induction and development. Further RT-qPCR and physiological analysis indicated that, among the different Kyoto Encyclopedia Gene and Genomes pathways, indole-3-acetic acid and gibberellic acid signaling transduction pathways may play pivotal regulatory functions in male sterility.

High-throughput sequencing and degradome analysis reveal altered expression of miRNAs and their targets in a male-sterile cybrid pummelo (Citrus grandis)

Background

G1 + HBP is a male sterile cybrid line with nuclear genome from Hirado Buntan pummelo (C. grandis Osbeck) (HBP) and mitochondrial genome from “Guoqing No.1” (G1, Satsuma mandarin), which provides a good opportunity to study male sterility and nuclear-cytoplasmic cross talk in citrus. High-throughput sRNA and degradome sequencing were applied to identify miRNAs and their targets in G1 + HBP and its fertile type HBP during reproductive development.

Results

A total of 184 known miRNAs, 22 novel miRNAs and 86 target genes were identified. Some of the targets are transcription factors involved in floral development, such as auxin response factors (ARFs), SQUAMOSA promoter binding protein box (SBP-box), MYB, basic region-leucine zipper (bZIP), APETALA2 (AP2) and transport inhibitor response 1 (TIR1). Eight target genes were confirmed to be sliced by corresponding miRNAs using 5’ RACE technology. Based on the sequencing abundance, 42 differentially expressed miRNAs between sterile line G1 + HBP and fertile line HBP were identified. Differential expression of miRNAs and their target genes between two lines was validated by quantitative RT-PCR, and reciprocal expression patterns between some miRNAs and their targets were demonstrated. The regulatory mechanism of miR167a was investigated by yeast one-hybrid and dual-luciferase assays that one dehydrate responsive element binding (DREB) transcription factor binds to miR167a promoter and transcriptionally repress miR167 expression.

Conclusion

Our study reveals the altered expression of miRNAs and their target genes in a male sterile line of pummelo and highlights that miRNA regulatory network may be involved in floral bud development and cytoplasmic male sterility in citrus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2882-0) contains supplementary material, which is available to authorized users.

Small RNA Profiles of the Rice PTGMS Line Wuxiang S Reveal miRNAs Involved in Fertility Transition

MicroRNAs (miRNAs) play key roles in the regulation of plant growth and developmental processes. In this study, RNA-seq was used to examine the expression profiles of miRNAs in a novel, photo-thermosensitive genic male sterile (PTGMS) rice line, Wuxiang S (WXS), during fertility transition. A total of 497 known miRNAs and 273 novel miRNAs were identified. In a differential expression analysis, 26 miRNAs exhibited significant differential expression between WXS (Sterile, S) and WXS (Fertile, F). Some of these miRNAs were validated by quantitative real-time PCR. Among these miRNAs, 11 showed decreased expression levels, and 15 showed increased expression levels in WXS (S) compared to WXS (F). Some of these miRNAs, such as osa-miR156a-j, osa-miR164d, and osa-miR528, were shown to be negatively correlated with their targets. These targets have previously been reported to be related to pollen development and male sterility, suggesting that these miRNAs may be involved in the regulation of pollen development in the rice PTGMS line WXS. Furthermore, miRNA-mediated editing events were also observed. In this study, a possible model for the control of signaling pathways during the process of fertility transition in the rice PTGMS line WXS by miRNAs was developed. These findings contribute to our understanding of the roles of miRNAs during anther development in PTGMS lines in rice.

Comparative Small RNA Analysis of Pollen Development in Autotetraploid and Diploid Rice

MicroRNAs (miRNAs) play key roles in plant reproduction. However, knowledge on microRNAome analysis in autotetraploid rice is rather limited. Here, high-throughput sequencing technology was employed to analyze miRNAomes during pollen development in diploid and polyploid rice. A total of 172 differentially expressed miRNAs (DEM) were detected in autotetraploid rice compared to its diploid counterpart, and 57 miRNAs were specifically expressed in autotetraploid rice. Of the 172 DEM, 115 and 61 miRNAs exhibited up- and down-regulation, respectively. Gene Ontology analysis on the targets of up-regulated DEM showed that they were enriched in transport and membrane in pre-meiotic interphase, reproduction in meiosis, and nucleotide binding in single microspore stage. osa-miR5788 and osa-miR1432-5p_R+1 were up-regulated in meiosis and their targets revealed interaction with the meiosis-related genes, suggesting that they may involve in the genes regulation associated with the chromosome behavior. Abundant 24 nt siRNAs associated with transposable elements were found in autotetraploid rice during pollen development; however, they significantly declined in diploid rice, suggesting that 24 nt siRNAs may play a role in pollen development. These findings provide a foundation for understanding the effect of polyploidy on small RNA expression patterns during pollen development that cause pollen sterility in autotetraploid rice.

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.

miRNAs and lncRNAs in reproductive development

Non-coding RNAs (ncRNAs) regulate gene expression at the transcriptional and post-transcriptional levels. Many ncRNAs have been identified in the past decade, including small ncRNAs, such as microRNAs (miRNAs), and long ncRNAs (lncRNAs). These novel molecules have important roles in a wide range of biological processes such as the regulation of reproduction and sex determination. Due to their ability to regulate specific genes or entire gene families, these molecules have the potential for uses in the development of breeding strategies as well as in the genetic modification of agronomic traits. In this review, we summarize recent progress on the understanding of plant miRNAs and lncRNAs in male and female development. We also discuss future challenges of using these molecules in agricultural applications, including transgenic plants in hybrid breeding, for novel genetic trait selection, for rapid character screening, and genetic modification for crop improvement.

Comparative expression profiling of miRNAs between the cytoplasmic male sterile line MeixiangA and its maintainer line MeixiangB during rice anther development

miRNAs are involved in the pollen development during the CMS occurrence in rice. miRNAs are 20-24 nt endogenously expressed small RNAs that play key roles in the regulation of many growth and developmental processes in plants. The knowledge on cytoplasmic male sterility (CMS) regulation by miRNAs in rice is rather limited. In this study, Illumina sequencing was employed to examine the expression profiles of rice anther miRNAs from the CMS line MeixiangA (MxA) and its maintainer line MeixiangB (MxB). A total of 518 known miRNAs and 144 novel miRNAs were identified during rice anther development. Based on the number of sequencing reads, a total of 24 miRNAs were discovered to be differentially expressed between MxA and MxB, and the results were partially validated by qRT-PCR. Among these, 16 miRNAs were decreased and 8 miRNAs were increased in MxA compared with MxB. Target prediction showed that they target genes encoding EF-hand family proteins, F-box domain-containing proteins, MYB transcription factors, PPR-containing proteins and transposons. The expression patterns for targets of osa-miR528, osa-miR5793, osa-miR1432, osa-miR159, osa-miR812d, osa-miR2118c, osa-miR172d and osa-miR5498 were selectively examined, and the results showed that there was a negative correlation on the expression patterns between miRNAs and their targets. These targets have previously been reported to be related with pollen development and male sterility, suggesting that miRNAs might act as regulators of CMS occurrence in rice anthers. Furthermore, miRNA editing events were observed. The U → C and U → A editing phenomenon was validated by molecular cloning and sequencing. These findings contribute to our understanding of the roles of miRNAs during anther development and CMS occurrence in rice.

Comparative transcript profiling of the fertile and sterile flower buds of pol CMS in B. napus

BACKGROUND

The Polima (pol) system of cytoplasmic male sterility (CMS) and its fertility restoration gene Rfp have been used in hybrid breeding in Brassica napus, which has greatly improved the yield of rapeseed. However, the mechanism of the male sterility transition in pol CMS remains to be determined.

RESULTS

To investigate the transcriptome during the male sterility transition in pol CMS, a near-isogenic line (NIL) of pol CMS was constructed. The phenotypic features and sterility stage were confirmed by anatomical analysis. Subsequently, we compared the genomic expression profiles of fertile and sterile young flower buds by RNA-Seq. A total of 105,481,136 sequences were successfully obtained. These reads were assembled into 112,770 unigenes, which composed the transcriptome of the bud. Among these unigenes, 72,408 (64.21%) were annotated using public protein databases and classified into functional clusters. In addition, we investigated the changes in expression of the fertile and sterile buds; the RNA-seq data showed 1,148 unigenes had significantly different expression and they were mainly distributed in metabolic and protein synthesis pathways. Additionally, some unigenes controlling anther development were dramatically down-regulated in sterile buds.

CONCLUSIONS

These results suggested that an energy deficiency caused by orf224/atp6 may inhibit a series of genes that regulate pollen development through nuclear-mitochondrial interaction. This results in the sterility of pol CMS by leading to the failure of sporogenous cell differentiation. This study may provide assistance for detailed molecular analysis and a better understanding of pol CMS in B. napus.

Combined small RNA and degradome sequencing reveals microRNA regulation during immature maize embryo dedifferentiation

Genetic transformation of maize is highly dependent on the development of embryonic calli from the dedifferentiated immature embryo. To better understand the regulatory mechanism of immature embryo dedifferentiation, we generated four small RNA and degradome libraries from samples representing the major stages of dedifferentiation. More than 186 million raw reads of small RNA and degradome sequence data were generated. We detected 102 known miRNAs belonging to 23 miRNA families. In total, we identified 51, 70 and 63 differentially expressed miRNAs (DEMs) in the stage I, II, III samples, respectively, compared to the control. However, only 6 miRNAs were continually up-regulated by more than fivefold throughout the process of dedifferentiation. A total of 87 genes were identified as the targets of 21 DEM families. This group of targets was enriched in members of four significant pathways including plant hormone signal transduction, antigen processing and presentation, ECM-receptor interaction, and alpha-linolenic acid metabolism. The hormone signal transduction pathway appeared to be particularly significant, involving 21 of the targets. While the targets of the most significant DEMs have been proved to play essential roles in cell dedifferentiation. Our results provide important information regarding the regulatory networks that control immature embryo dedifferentiation in maize.

Expression characterization of genes for CMS-C in maize

Cytoplasmic male sterility (CMS)-C is one of the most attractive sources of male sterility in the production of hybrid maize. However, the abortion mechanism of CMS-C is currently unknown. The major aim of this work was to characterize the expression of genes and proteins during pollen abortion. The materials assayed included CMS-C line C48-2, its maintainer line N48-2, and fertile F(1) (C48-2 × 18 white). A total of 20 unique genes and 25 proteins were identified by suppression subtractive hybridization and 2-D electrophoresis, respectively. Most of the genes and proteins identified are closely related to energy metabolism, stress responses, molecular chaperones, and cell death, which are generally considered to be essential to pollen development. Based on the function of these identified genes and proteins, reactive oxygen species in isolated mitochondria and DNA fragments were analyzed. The results from this study indicate that the oxidative stress which was associated with the specific expression patterns of some genes may be the physiological cause for the abortion of premature microspores in the maize CMS-C line.

Genomic organization, phylogenetic comparison and differential expression of the SBP-box family of transcription factors in tomato

SBP-box genes represent transcription factors ubiquitously found in the plant kingdom and recognized as important regulators of many different aspects of plant development. In this study, 15 SBP-box gene family members were identified in tomato and analyzed with respect to their genomic organization and other structural features. Phylogenetic reconstruction based on the DNA-binding SBP-domain, allowed the classification of the SlySBP proteins into eight groups representing clear orthologous relationships to family members of other flowering plants and the moss Physcomitrella. In order to have a better understanding of their possible function in the development of a fleshy-fruit species like tomato, the mRNA expression levels of all SlySBP genes were quantified in vegetative and reproductive organs of plants, at different stages of growth. As transcripts of ten SlySBP genes were found to carry putative miR156- and miR157-response elements, the expression levels of the corresponding microRNAs were determined as well, revealing different patterns of expression. In addition, eight putative miR156 and four miR157 encoding loci could be identified in the tomato genome, four of them forming a polycistronic cluster. Whereas miR156 and miR157 levels were highest in seedlings, leaves and anthers of young flowers, most miR156-targeted SlySBP genes were found to be expressed in young inflorescences and during fruit development and ripening, suggesting a particularly important role during tomato reproductive growth. The data presented provide a basis for future clarification of the various functions that SBP-box gene family members play in tomato growth and development.