Characterization and Cloning of Grape Circular RNAs Identified the Cold Resistance-Related Vv-circATS1

The circular RNA circANK suppresses rice resistance to bacterial blight by inhibiting microRNA398b-mediated defense

Circular RNAs (circRNAs) are prevalent in eukaryotic cells and have been linked to disease progressions. Their unique circular structure and stability make them potential biomarkers and therapeutic targets. Compared with animal models, plant circRNA research is still in its infancy. The lack of effective tools to specifically knock down circRNAs without affecting host gene expression has slowed the progress of plant circRNA research. Here, we have developed a CRISPR-Cas13d tool that can specifically knock down circRNAs in plant systems, successfully achieving the targeted knockdown of circRNAs in rice (Oryza sativa). We further focused on Os-circANK (a circRNA derived from Ankyrin repeat-containing protein), a circRNA differentially expressed in rice upon pathogen infection. Physiological and biochemical experiments revealed that Os-circANK functions as a sponge for miR398b, suppressing the cleavage of Cu/Zn-superoxidase dismutase (CSD)1/CSD2/copper chaperone for superoxide dismutase/superoxidase dismutaseX through competing endogenous RNA, leading to reduced reactive oxygen species levels following Xanthomonas oryzae pv. oryzae (Xoo) infection and a negative regulation of rice resistance to bacterial blight. Our findings indicate Os-circANK inhibits rice resistance to bacterial blight via the microRNA398b(miR398b)/CSD/SOD pathway.

CircZmMED16 delays plant flowering by negatively regulating starch content through its binding to ZmAPS1

Circular RNAs (circRNAs), a type of head-to-tail closed RNA molecules, have been implicated in various aspects of plant development and stress responses through transcriptome sequencing; however, the precise functional roles of circRNAs in plants remain poorly understood. In this study, we identified a highly expressed circular RNA, circZmMED16, derived from exon 8 of the mediator complex subunit 16 (ZmMED16) across different maize (Zea mays L.) inbred lines using circRNA-seq analysis. This circRNA is predominantly expressed in maize tassels and functions in the cytoplasm. Overexpression of circZmMED16 resulted in increased expression of ZmMED16/AtMED16 and delayed flowering in both maize and Arabidopsis thaliana, compared with that in wild-type plants. In contrast, overexpression of the parent gene ZmMED16 did not alter the flowering time of transgenic plants in Arabidopsis, suggesting that circZmMED16 plays a specific role in regulating flowering, distinct from that of linear ZmMED16. To further understand the mechanisms underlying the regulation of flowering time by circZmMED16, we performed RNA pull-down, dual-luciferase, RNA interference (RNAi), and ribonuclease protection assays (RPA). These results indicate that circZmMED16 interacts with small subunit 1 of ADP-glucose pyrophosphorylase (APS1) mRNA in both maize and Arabidopsis. The knockdown of circZmMED16 increased the expression of ZmAPS1, whereas the overexpression of circZmMED16 led to the downregulation of ZmAPS1 RNA and protein. By affecting ZmAPS1 expression, circZmMED16 reduced ADP-glucose pyrophosphorylase (AGPase) activity and led to delayed flowering. These results revealed a novel regulatory mechanism for circRNAs in flowering time and shed light on their functional and regulatory roles in plants.

A novel peptide encoded by a rice circular RNA confers broad-spectrum disease resistance in rice plants

Circular RNAs (circRNAs) are a significant class of endogenous RNAs that exert crucial biological functions in human and animal systems, but little is currently understood regarding their roles in plants. Here, we identified a circRNA originating from the back-splicing of exon 4 and exon 5 of a rice gene, OsWRKY9, and named it circ-WRKY9. It is upregulated in rice stripe mosaic virus (RSMV)-infected rice plants. Notably, circ-WRKY9 contains two open reading frames with an internal ribosome entry site. We found that circ-WRKY9 encoded a peptide of 88 amino acids (aa) and named it WRKY9-88aa. Overexpression of WRKY9-88aa suppresses RSMV infection in rice plants, with increased reactive oxygen species production. Furthermore, WRKY9-88aa enhances resistance to blast disease and bacterial leaf blight, suggesting its potential to provide broad-spectrum disease resistance. Our findings provide the first evidence of a peptide encoded by a circRNA in planta and highlight its potential application to control a wide spectrum of plant diseases.

circGDSL-induced OPR3 expression regulates jasmonate signaling and copper tolerance in rice (Oryza sativa)

This study investigates the regulatory function of circular RNA (circRNA) as a competing endogenous RNA (ceRNA) in rice (Oryza sativa L.) under toxic levels of copper (Cu) stress. Physiological parameters and differences in Cu accumulation were analyzed through a hydroponic experiment. RNA sequencing (RNA-seq) identified 1051 circRNAs, of which 26 were differentially expressed (FDR <0.05, |log2FC| > 1) under Cu stress. A Cu-responsive ceRNA network mediated by circRNAs was constructed, comprising 16 circRNAs, 34 miRNAs, and 126 mRNAs. Topological analysis identified the circGDSL/miR1850.1/OPR3 triplet as a key regulatory hub, which was experimentally validated by RT-qPCR. Overexpression of circGDSL conferred significant resistance to Cu stress, characterized by enhanced antioxidant enzyme activity, reduced reactive oxygen species (ROS) levels, and alleviated Cu-induced growth suppression. Functional studies indicated that circGDSL upregulates the expression of the key jasmonic acid (JA) synthesis gene OPR3 by sponging miR1850.1, thereby activating the JA signaling pathway. The increased endogenous JA concentration represses the expression of genes (IRT1, Nramp5, and HMA2) that promote Cu uptake and translocation, resulting in decreased Cu concentration in rice. Conversely, overexpression of miR1850.1 reduces endogenous JA concentration and increases sensitivity to Cu, a phenotype that can be rescued by exogenous methyl jasmonate (MeJA). In conclusion, we identified a Cu-responsive circRNA in rice and confirmed its role in activating JA synthesis pathway as miRNA sponge, thereby enhancing rice tolerance to Cu stress.

Identification and Network Construction of mRNAs, miRNAs, lncRNAs, and circRNAs in Sweetpotato (Ipomoea batatas L.) Adventitious Roots Under Salt Stress via Whole-Transcriptome RNA Sequencing

Sweetpotato is the seventh largest crop worldwide, and soil salinization is a major environmental stress limiting its yield. Recent studies have shown that noncoding RNAs (ncRNAs) play important regulatory roles in plant responses to abiotic stress. However, ncRNAs in sweetpotato remain largely unexplored. This study analyzed the characteristics of salt-responsive ncRNAs in sweetpotato adventitious roots under salt stress via whole-transcriptome RNA sequencing. The results revealed that 3175 messenger RNAs (mRNAs), 458 microRNAs (miRNAs), 544 long-chain ncRNAs (lncRNAs), and 23 circular RNAs (circRNAs) were differentially expressed. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed that most differentially expressed mRNAs (DEmRNAs) and miRNAs (DEmiRNAs) were enriched primarily in phenylpropanoid biosynthesis, starch and sucrose metabolism, the Mitogen-Activated Protein Kinase (MAPK) signaling pathway, plant hormone signal transduction, the mRNA surveillance pathway, and ATP-binding cassette (ABC) transporters. Gene Ontology (GO) enrichment analysis revealed that the majority of DEmRNAs, their target DEmiRNAs, and differentially expressed lncRNAs (DElncRNAs) were associated with the cell wall, oxidation-reduction, the plasma membrane, protein phosphorylation, metabolic processes, transcription factor activity, and the regulation of transcription. Additionally, based on the competitive endogenous RNA (ceRNA) hypothesis, we predicted interactions among different RNAs and constructed a salt-responsive ceRNA network comprising 22 DEmiRNAs, 42 DEmRNAs, 27 DElncRNAs, and 10 differentially expressed circRNAs (DEcircRNAs). Some miRNAs, such as miR408, miR169, miR160, miR5139, miR5368, and miR6179, were central to the network, suggesting their crucial roles in the sweetpotato salt response. Our findings provide a foundation for further research into the potential functions of ncRNAs and offer new targets for salt stress resistance improvement through the manipulation of ncRNAs.

The antisense CircRNA VvcircABH controls salt tolerance and the brassinosteroid signaling response by suppressing cognate mRNA splicing in grape

Soil salinization is a major factor limiting the sustainable development of the grape industry. Circular RNAs (circRNAs) are more stable than linear mRNAs and are involved in stress responses. However, the biological functions and molecular mechanisms underlying antisense circRNAs in plants remain unclear. We identified the antisense circRNA VvcircABH through high-throughput sequencing. Using genetic transformation methods and molecular biological techniques, we analyzed the effects of VvcircABH on the response to salt stress and the mechanisms underlying its effects. VvcircABH was located in the nucleus and upregulated by salt stress, while the expression level of its cognate gene VvABH (alpha/beta-hydrolase) was downregulated. VvcircABH overexpression or VvABH silencing greatly enhanced grape salt tolerance. VvcircABH could bind to the overlapping region and inhibits VvABH pre-mRNA splicing, thereby decreasing the expression level of VvABH. Additionally, VvcircABH repressed the additive effect of VvABH on the interaction between VvBRI1 (brassinosteroid-insensitive 1) and VvBKI1 (BRI1 kinase inhibitor 1), thus influencing the plant's response to BR, which plays important roles in plant salt tolerance. We conclude that VvcircABH and VvABH play distinct roles in the salt tolerance and brassinosteroid signaling response, and VvcircABH could govern the expression of VvABH by inhibiting its splicing.

Maize transcriptome profiling reveals low temperatures affect photosynthesis during the emergence stage

Introduction

Earlier sowing is a promising strategy of ensuring sufficiently high maize yields in the face of negative environmental factors caused by climate change. However, it leads to the low temperature exposure of maize plants during emergence, warranting a better understanding of their response and acclimation to suboptimal temperatures.

Materials and Methods

To achieve this goal, whole transcriptome sequencing was performed on two maize inbred lines - tolerant/susceptible to low temperatures, at the 5-day-old seedling stage. Sampling was performed after 6h and 24h of treatment (10/8°C). The data was filtered, mapped, and the identified mRNAs, lncRNAs, and circRNAs were quantified. Expression patterns of the RNAs, as well as the interactions between them, were analyzed to reveal the ones important for low-temperature response.

Results and Discussion

Genes involved in different steps of photosynthesis were downregulated in both genotypes: psa, psb, lhc, and cab genes important for photosystem I and II functioning, as well as rca, prk, rbcx1 genes necessary for the Calvin cycle. The difference in low-temperature tolerance between genotypes appeared to arise from their ability to mitigate damage caused by photoinhibition: ctpa2, grx, elip, UF3GT genes showed higher expression in the tolerant genotype. Certain identified lncRNAs also targeted these genes, creating an interaction network induced by the treatment (XLOC_016169-rca; XLOC_002167-XLOC_006091-elip2). These findings shed light on the potential mechanisms of low-temperature acclimation during emergence and lay the groundwork for subsequent analyses across diverse maize genotypes and developmental stages. As such, it offers valuable guidance for future research directions in the molecular breeding of low-temperature tolerant maize.

Genome‑wide identification of circular RNAs and MAPKs reveals the regulatory networks in response to green peach aphid infestation in peach (Prunus persica)

The green peach aphid (GPA), Myzus persicae (Sulzer), is a serious agricultural pest with a worldwide distribution and a vector of over 100 plant viruses. Various pathways, such as the mitogen-activated protein kinase (MAPK) cascades, play pivotal roles in signaling plant defense against pest attack, and circular RNAs (circRNAs) regulate the expression of mRNAs in response to pest attack. However, the mechanism underlying peach (Prunus persica) response to GPA attack remains unclear. The present study initially identified and characterized 316 circRNAs and 18 PpMAPKs from healthy and GPA-infested peach leaves by whole-transcriptome sequencing and predicted the differentially expressed circRNAs (DECs) after GPA infestation. PCR and Sanger sequencing confirmed the presence of six DECs in peach samples. Besides, RNA sequencing analysis detected 13 DECs, including 5 upregulated and 8 downregulated ones, in peach in response to the GPA attack. Gene ontology (GO) enrichment analysis indicated that specific DECs play crucial roles in the MAPK signaling pathway, and qRT-PCR revealed that GPA infestation altered the expression patterns of PpMAPKs. Finally, five circRNAs, three microRNA (miRNAs), and two MAPK target genes were identified to interact as a network and perform critical roles in modulating the MAPK pathway in the peach during GPA infestation.

PlantCircRNA: a comprehensive database for plant circular RNAs

Circular RNAs (circRNAs) represent recently discovered novel regulatory non-coding RNAs. While they are present in many eukaryotes, there has been limited research on plant circRNAs. We developed PlantCircRNA (https://plant.deepbiology.cn/PlantCircRNA/) to fill this gap. The two most important features of PlantCircRNA are (i) it incorporates circRNAs from 94 plant species based on 39 245 RNA-sequencing samples and (ii) it imports the original AtCircDB and CropCircDB databases. We manually curated all circRNAs from published articles, and imported them into the database. Furthermore, we added detailed information of tissue as well as abiotic stresses to the database. To help users understand these circRNAs, the database includes a detection score to measure their consistency and a naming system following the guidelines recently proposed for eukaryotes. Finally, we developed a comprehensive platform for users to visualize, analyze, and download data regarding specific circRNAs. This resource will serve as a home for plant circRNAs and provide the community with unprecedented insights into these mysterious molecule.

Viroids and Retrozymes: Plant Circular RNAs Capable of Autonomous Replication

Among the long non-coding RNAs that are currently recognized as important regulatory molecules influencing a plethora of processes in eukaryotic cells, circular RNAs (circRNAs) represent a distinct class of RNAs that are predominantly produced by back-splicing of pre-mRNA. The most studied regulatory mechanisms involving circRNAs are acting as miRNA sponges, forming R-loops with genomic DNA, and encoding functional proteins. In addition to circRNAs generated by back-splicing, two types of circRNAs capable of autonomous RNA-RNA replication and systemic transport have been described in plants: viroids, which are infectious RNAs that cause a number of plant diseases, and retrozymes, which are transcripts of retrotransposon genomic loci that are capable of circularization due to ribozymes. Based on a number of common features, viroids and retrozymes are considered to be evolutionarily related. Here, we provide an overview of the biogenesis mechanisms and regulatory functions of non-replicating circRNAs produced by back-splicing and further discuss in detail the currently available data on viroids and retrozymes, focusing on their structural features, replication mechanisms, interaction with cellular components, and transport in plants. In addition, biotechnological approaches involving replication-capable plant circRNAs are discussed, as well as their potential applications in research and agriculture.

AScirRNA: A novel computational approach to discover abiotic stress-responsive circular RNAs in plant genome

In the realm of plant biology, understanding the intricate regulatory mechanisms governing stress responses stands as a pivotal pursuit. Circular RNAs (circRNAs), emerging as critical players in gene regulation, have garnered attention in recent days for their potential roles in abiotic stress adaptation. A comprehensive grasp of circRNAs' functions in stress response offers avenues for breeders to manipulating plants to develop abiotic stress resistant crop cultivars to thrive in challenging climates. This study pioneers a machine learning-based model for predicting abiotic stress-responsive circRNAs. The K-tuple nucleotide composition (KNC) and Pseudo KNC (PKNC) features were utilized to numerically represent circRNAs. Three different feature selection strategies were employed to select relevant and non-redundant features. Eight shallow and four deep learning algorithms were evaluated to build the final predictive model. Following five-fold cross-validation process, XGBoost learning algorithm demonstrated superior performance with LightGBM-chosen 260 KNC features (Accuracy: 74.55 %, auROC: 81.23 %, auPRC: 76.52 %) and 160 PKNC features (Accuracy: 74.32 %, auROC: 81.04 %, auPRC: 76.43 %), over other combinations of learning algorithms and feature selection techniques. Further, the robustness of the developed models were evaluated using an independent test dataset, where the overall accuracy, auROC and auPRC were found to be 73.13 %, 72.34 % and 72.68 % for KNC feature set and 73.52 %, 79.53 % and 73.09 % for PKNC feature set, respectively. This computational approach was also integrated into an online prediction tool, AScirRNA (https://iasri-sg.icar.gov.in/ascirna/) for easy prediction by the users. Both the proposed model and the developed tool are poised to augment ongoing efforts in identifying stress-responsive circRNAs in plants.

Assessment of heat tolerance and identification of miRNAs during high-temperature response in grapevine

With global warming, heat stress has been recognized as a significant factor limiting grapevine development and fruit quality. MicroRNAs (miRNAs) are a class of small non-coding RNAs known to play crucial regulatory roles in stress resistance. Hence, there is an immediate requirement to cultivate and identify grapevine varieties that are resistant to heat and explore miRNA-mediated heat stress defense mechanisms. In this study, we assessed the thermal resistance of 38 grape germplasm resources and identified a series of miRNAs involved in heat stress resistance. The CK (25°C) and HS (45°C) groups of "Shenyue" cuttings of grapes were used as experimental materials for next-generation sequencing and construct libraries of small RNAs. A total of 177 known and 20 novel miRNAs were detected in the libraries. Differential expression analysis identified 65 differentially expressed miRNAs (DEMs) using the DE-Seq procedure. Furthermore, RT-qPCR validation confirmed complementary expression profiles of eight DEMs and their target genes between the HS and CK groups. Heterologous transformation further identified the function of Vvi-miR3633a downregulated under heat stress in Arabidopsis. In the heterologous expression lines, the survival rate was reduced by high temperature treatment indicating the ability of Vvi-miR3633a to regulate heat resistance. Assessing the heat resistance of grape species and the expression patterns of miRNA in response to high temperatures may reveal the molecular processes of heat resistance regulation mediated by miRNA in grapes under heat stress.

Expression and drought functional analysis of one circRNA PecircCDPK from moso bamboo (Phyllostachys edulis)

Drought stress can affect the growth of bamboo. Circle RNAs (CircRNAs) have been found to play a role in drought stress in plants, but their function in moso bamboo is not well understood. In previous studies, we observed that under drought stress, the expression of some circRNAs were altered and predicted to be involved in calcium-dependent protein kinase phosphorylation, as indicated by KEGG enrichment analysis. In this study, we cloned a circRNA called PecircCDPK in moso bamboo that is responsive to drought stress. To further investigate its function, we constructed an overexpression vector using flanking intron sequences supplemented by reverse complementary sequences. When this vector was transferred to Arabidopsis plants, we observed that the roots of the transgenic lines were more developed, the water loss rate decreased, the stomata became smaller, and the activity of antioxidant enzymes increased under drought stress. These findings suggest that overexpression of PecircCDPK can enhance the drought resistance of Arabidopsis thaliana, providing valuable insights for the breeding of moso bamboo with improved resistance to drought.

Advances in CircRNAs in the Past Decade: Review of CircRNAs Biogenesis, Regulatory Mechanisms, and Functions in Plants

Circular RNA (circRNA) is a type of non-coding RNA with multiple biological functions. Whole circRNA genomes in plants have been identified, and circRNAs have been demonstrated to be widely present and highly expressed in various plant tissues and organs. CircRNAs are highly stable and conserved in plants, and exhibit tissue specificity and developmental stage specificity. CircRNAs often interact with other biomolecules, such as miRNAs and proteins, thereby regulating gene expression, interfering with gene function, and affecting plant growth and development or response to environmental stress. CircRNAs are less studied in plants than in animals, and their regulatory mechanisms of biogenesis and molecular functions are not fully understood. A variety of circRNAs in plants are involved in regulating growth and development and responding to environmental stress. This review focuses on the biogenesis and regulatory mechanisms of circRNAs, as well as their biological functions during growth, development, and stress responses in plants, including a discussion of plant circRNA research prospects. Understanding the generation and regulatory mechanisms of circRNAs is a challenging but important topic in the field of circRNAs in plants, as it can provide insights into plant life activities and their response mechanisms to biotic or abiotic stresses as well as new strategies for plant molecular breeding and pest control.

Comparative genome-wide analysis of circular RNAs in Brassica napus L.: target-site versus non-target-site resistance to herbicide stress

Circular RNAs (circRNAs), a class of non-coding RNA molecules, are recognized for their unique functions; however, their responses to herbicide stress in Brassica napus remain unclear. In this study, the role of circRNAs in response to herbicide treatment was investigated in two rapeseed cultivars: MH33, which confers non-target-site resistance (NTSR), and EM28, which exhibits target-site resistance (TSR). The genome-wide circRNA profiles of herbicide-stressed and non-stressed seedlings were analyzed. The findings indicate that NTSR seedlings exhibited a greater abundance of circRNAs, shorter lengths of circRNAs and their parent genes, and more diverse functions of parent genes compared with TSR seedlings. Compared to normal-growth plants, the herbicide-stressed group exhibited similar trends in the number of circRNAs, functions of parent genes, and differentially expressed circRNAs as observed in NTSR seedlings. In addition, a greater number of circRNAs that function as competing microRNA (miRNA) sponges were identified in the herbicide stress and NTSR groups compared to the normal-growth and TSR groups, respectively. The differentially expressed circRNAs were validated by qPCR. The differntially expressed circRNA-miRNA networks were predicted, and the mRNAs targeted by these miRNAs were annotated. Our results suggest that circRNAs play a crucial role in responding to herbicide stress, exhibiting distinct responses between NTSR and TSR in rapeseed. These findings offer valuable insights into the mechanisms underlying herbicide resistance in rapeseed.

CircRNA: An emerging star in plant research: A review

CircRNAs are a class of covalently closed non-coding RNA formed by linking the 5' terminus and the 3' terminus after reverse splicing. CircRNAs are widely found in eukaryotes, and they are highly conserved, with spatio-temporal expression specificity and stability. CircRNAs can act as miRNA sponges to regulate the expression of downstream target genes, regulating the transcription of parental genes and some can even be translated into peptides or proteins. Research on circRNAs in plants is still in its infancy compared to that in animals. With the deepening of research, the results of a variety of plant circRNAs suggest that they play an important role in growth and development, and tolerance towards abiotic stresses such as salt, drought, low temperature, high temperature and other adverse environments. In this review paper, we elaborated the molecular characteristics, mechanism of action, function and bioinformatics databases of plant circRNAs, combined with the progress of circRNA research in animals, discussed the potential mechanism of action of plant circRNAs, and proposed the unsolved problems and prospects for future application of plant circRNAs.

Circular RNA ath-circ032768, a competing endogenous RNA, response the drought stress by targeting miR472-RPS5 module

CircRNAs (circular RNAs) reduce the abundance of miRNAs through ceRNA (competing endogenous RNA), to regulate many physiological processes and stress responses in plants. However, the role of circRNA in drought stress is poorly understood. Through ring identification and sequencing verification of ath-circ032768, bioinformatics analysis predicted the interaction of ath-circ032768-miR472-RPS5, and further obtained transgenic plants overexpressing ath-circ032768 and silencing STTM-miR472. The change in drought stress was analysed using biochemical and molecular biological methods. Sequencing and biological analysis confirmed that ath-circ032768, miR472 and RPS5 were responsive to drought stress, and changes in gene expression were consistent with the prediction of ceRNA. The silencing vectors ath-circ032768 and STTM-miR472 were constructed using molecular biology techniques, and stable transgenic plants with drought tolerance obtained. Further physiological and biochemical studies showed that ath-circ032768 could bind to miR472, and that miR472 could bind to the RPS5 gene, resulting in decreased expression of RPS5. Hence, ath-circ032768 can competitively inhibit degradation of RPS5 by miR472 through ceRNA. This process is accompanied by increased expression of DREB2A, RD29A and RD29B genes. Through the ath-circ032768-miR472-RPS5 pathway, the RPS5 stress resistance protein interacts with DREB2A protein to enhance expression of downstream drought resistance genes, RD29A and RD29B, and participate in the regulation mechanism of plant drought resistance, thereby improving drought tolerance of plants.

Long- and short-read sequencing methods discover distinct circular RNA pools in Lotus japonicus

Circular RNAs (circRNAs) are covalently closed single-stranded RNAs, generated through a back-splicing process that links a downstream 5' site to an upstream 3' end. The only distinction in the sequence between circRNA and their linear cognate RNA is the back splice junction. Their low abundance and sequence similarity with their linear origin RNA have made the discovery and identification of circRNA challenging. We have identified almost 6000 novel circRNAs from Lotus japonicus leaf tissue using different enrichment, amplification, and sequencing methods as well as alternative bioinformatics pipelines. The different methodologies identified different pools of circRNA with little overlap. We validated circRNA identified by the different methods using reverse transcription polymerase chain reaction and characterized sequence variations using nanopore sequencing. We compared validated circRNA identified in L. japonicus to other plant species and showed conservation of high-confidence circRNA-expressing genes. This is the first identification of L. japonicus circRNA and provides a resource for further characterization of their function in gene regulation. CircRNAs identified in this study originated from genes involved in all biological functions of eukaryotic cells. The comparison of methodologies and technologies to sequence, identify, analyze, and validate circRNA from plant tissues will enable further research to characterize the function and biogenesis of circRNA in L. japonicus.

Comprehensive non-coding RNA analysis reveals specific lncRNA/circRNA-miRNA-mRNA regulatory networks in the cotton response to drought stress

Root and leaf are essential organs of plants in sensing and responding to drought stress. However, comparative knowledge of non-coding RNAs (ncRNAs) of root and leaf tissues in the regulation of drought response in cotton is limited. Here, we used deep sequencing data of leaf and root tissues of drought-resistant and drought-sensitive cotton varieties for identifying miRNAs, lncRNAs and circRNAs. A total of 1531 differentially expressed (DE) ncRNAs was identified, including 77 DE miRNAs, 1393 DE lncRNAs and 61 DE circRNAs. The tissue-specific and variety-specific competing endogenous RNA (ceRNA) networks of DE lncRNA-miRNA-mRNA response to drought were constructed. Furthermore, the novel drought-responsive lncRNA 1 (DRL1), specifically and differentially expressed in root, was verified to positively affect phenotypes of cotton seedlings under drought stress, competitively binding to miR477b with GhNAC1 and GhSCL3. In addition, we also constructed another ceRNA network consisting of 18 DE circRNAs, 26 DE miRNAs and 368 DE mRNAs. Fourteen circRNA were characterized, and a novel molecular regulatory system of circ125- miR7484b/miR7450b was proposed under drought stress. Our findings revealed the specificity of ncRNA expression in tissue- and variety-specific patterns involved in the response to drought stress, and uncovered novel regulatory pathways and potentially effective molecules in genetic improvement for crop drought resistance.

The Pivotal Role of Noncoding RNAs in Flowering Time Regulation

Noncoding RNAs constitute a substantial portion of the transcriptome and play pivotal roles in plant growth and development. Among these processes, flowering stands out as a crucial trait, ensuring reproductive success and seed set, and is meticulously controlled by genetic and environmental factors. With remarkable advancements in the identification and characterization of noncoding RNAs in plants, it has become evident that noncoding RNAs are intricately linked to the regulation of flowering time. In this article, we present an overview of the classification of plant noncoding RNAs and delve into their functions in the regulation of flowering time. Furthermore, we review their molecular mechanisms and their involvement in flowering pathways. Our comprehensive review enhances the understanding of how noncoding RNAs contribute to the regulation of flowering time and sheds light on their potential implications in crop breeding.

Is winter coming? Impact of the changing climate on plant responses to cold temperature

Climate change is causing alterations in annual temperature regimes worldwide. Important aspects of this include the reduction of winter chilling temperatures as well as the occurrence of unpredicted frosts, both significantly affecting plant growth and yields. Recent studies advanced the knowledge of the mechanisms underlying cold responses and tolerance in the model plant Arabidopsis thaliana. However, how these cold-responsive pathways will readjust to ongoing seasonal temperature variation caused by global warming remains an open question. In this review, we highlight the plant developmental programmes that depend on cold temperature. We focus on the molecular mechanisms that plants have evolved to adjust their development and stress responses upon exposure to cold. Covering both genetic and epigenetic aspects, we present the latest insights into how alternative splicing, noncoding RNAs and the formation of biomolecular condensates play key roles in the regulation of cold responses. We conclude by commenting on attractive targets to accelerate the breeding of increased cold tolerance, bringing up biotechnological tools that might assist in overcoming current limitations. Our aim is to guide the reflection on the current agricultural challenges imposed by a changing climate and to provide useful information for improving plant resilience to unpredictable cold regimes.

Vv-circSIZ1 mediated by pre-mRNA processing machinery contributes to salt tolerance

CircRNAs exist widely in plants, but the regulatory mechanisms for the biogenesis and function of plant circRNAs remain largely unknown. Using extensive mutagenesis of expression plasmids and genetic transformation methods, we analyzed the biogenesis and anti-salt functions of a new grape circRNA Vv-circSIZ1. We identified Vv-circSIZ1 that is mainly expressed in the cytoplasm of xylem. CircSIZ1 is species-specific, and genomic circSIZ1-forming region of seven tested species could be backspliced in Nicotiana benthamiana, but not in Arabidopsis. The retention length of Vv-circSIZ1 flanking introns was significantly positively correlated with its generation efficiency. The precise splicing of Vv-circSIZ1 does not depend on its mature exon sequence or internal intron sequences, but on the AG/GT splicing signal sites and branch site of the flanking introns. The spliceosome activity was inversely proportional to the expression level of Vv-circSIZ1. Furthermore, RNA-binding proteins can regulate the expression of Vv-circSIZ1. The overexpression of Vv-circSIZ1 improved salt tolerance of grape and N. benthamiana. Additionally, Vv-circSIZ1 could relieve the repressive effect of VvmiR3631 on its target VvVHAc1. Vv-circSIZ1 also promoted transcription of its parental gene. Overall, these results broaden our understanding of circRNAs in plants.

Circular RNAs are associated with the resistance to Newcastle disease virus infection in duck cells

Introduction

Newcastle disease virus (NDV) is prevalent worldwide with an extensive host range. Among birds infected with velogenic NDV strains, chickens experience high pathogenicity and mortality, whereas ducks mostly experience mild symptoms or are asymptomatic. Ducks have a unique, innate immune system hypothesized to induce antiviral responses. Circular RNAs (circRNAs) are among the most abundant and conserved eukaryotic transcripts. These participate in innate immunity and host antiviral response progression.

Methods

In this study, circRNA expression profile differences post-NDV infection in duck embryo fibroblast (DEF) cells were analyzed using circRNA transcriptome sequencing. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were used to reveal significant enrichment of differentially expressed (DE) circRNAs. The circRNA-miRNA-mRNA interaction networks were used to predict the related functions of circRNAs. Moreover, circ-FBXW7 was selected to determine its effect on NDV infection in DEFs.

Results

NDV infection altered circRNA expression profiles in DEF cells, and 57 significantly differentially expressed circRNAs were identified post-NDV infection. DEF responded to NDV by forming circRNAs to regulate apoptosis-, cell growth-, and protein degradation-related pathways via GO and KEGG enrichment analyses. circRNA-miRNA-mRNA interaction networks demonstrated that DEF cells combat NDV infection by regulating cellular pathways or apoptosis through circRNA-targeted mRNAs and miRNAs. circ-FBXW7 overexpression and knockdown inhibited and promoted viral replication, respectively. DEF cells mainly regulated cell cycle alterations or altered cellular sensing to combat NDV infection.

Conclusion

These results demonstrate that DEF cells exert antiviral responses by forming circRNAs, providing novel insights into waterfowl antiviral responses.

Biogenesis, Mode of Action and the Interactions of Plant Non-Coding RNAs

The central dogma of genetics, which outlines the flow of genetic information from DNA to RNA to protein, has long been the guiding principle in molecular biology. In fact, more than three-quarters of the RNAs produced by transcription of the plant genome are not translated into proteins, and these RNAs directly serve as non-coding RNAs in the regulation of plant life activities at the molecular level. The breakthroughs in high-throughput transcriptome sequencing technology and the establishment and improvement of non-coding RNA experiments have now led to the discovery and confirmation of the biogenesis, mechanisms, and synergistic effects of non-coding RNAs. These non-coding RNAs are now predicted to play important roles in the regulation of gene expression and responses to stress and evolution. In this review, we focus on the synthesis, and mechanisms of non-coding RNAs, and we discuss their impact on gene regulation in plants.

Functional Differences of Grapevine Circular RNA Vv-circPTCD1 in Arabidopsis and Grapevine Callus under Abiotic Stress

Circular RNAs (circRNAs) serve as covalently closed single-stranded RNAs and have been proposed to influence plant development and stress resistance. Grapevine is one of the most economically valuable fruit crops cultivated worldwide and is threatened by various abiotic stresses. Herein, we reported that a circRNA (Vv-circPTCD1) processed from the second exon of the pentatricopeptide repeat family gene PTCD1 was preferentially expressed in leaves and responded to salt and drought but not heat stress in grapevine. Additionally, the second exon sequence of PTCD1 was highly conserved, but the biogenesis of Vv-circPTCD1 is species-dependent in plants. It was further found that the overexpressed Vv-circPTCD1 can slightly decrease the abundance of the cognate host gene, and the neighboring genes are barely affected in the grapevine callus. Furthermore, we also successfully overexpressed the Vv-circPTCD1 and found that the Vv-circPTCD1 deteriorated the growth during heat, salt, and drought stresses in Arabidopsis. However, the biological effects on grapevine callus were not always consistent with those of Arabidopsis. Interestingly, we found that the transgenic plants of linear counterpart sequence also conferred the same phenotypes as those of circRNA during the three stress conditions, no matter what species it is. Those results imply that although the sequences are conserved, the biogenesis and functions of Vv-circPTCD1 are species-dependent. Our results indicate that the plant circRNA function investigation should be conducted in homologous species, which supports a valuable reference for further plant circRNA studies.

Multilayered regulation of secondary metabolism in medicinal plants

Medicinal plants represent a huge reservoir of secondary metabolites (SMs), substances with significant pharmaceutical and industrial potential. However, obtaining secondary metabolites remains a challenge due to their low-yield accumulation in medicinal plants; moreover, these secondary metabolites are produced through tightly coordinated pathways involving many spatiotemporally and environmentally regulated steps. The first regulatory layer involves a complex network of transcription factors; a second, more recently discovered layer of complexity in the regulation of SMs is epigenetic modification, such as DNA methylation, histone modification and small RNA-based mechanisms, which can jointly or separately influence secondary metabolites by regulating gene expression. Here, we summarize the findings in the fields of genetic and epigenetic regulation with a special emphasis on SMs in medicinal plants, providing a new perspective on the multiple layers of regulation of gene expression.

Characterization and identification of grapevine heat stress-responsive microRNAs revealed the positive regulated function of vvi-miR167 in thermostability

High temperature stress is one of the primary abiotic stresses that restrict fruit tree production. Grapevine (Vitis vinifera) with high economic value throughout the world is a cultivated fruit crop, and its growth and development is often influenced by high temperature stress. Studying the heat stress-response mechanism of grapevine has great significance for understanding the acclimation to heat stress. In this study, we identified a series of heat stress responsive miRNAs and analyzed their function during the heat tolerance response. CK (control group, 25 °C) and heat treatment stress (TS, 45 °C) small RNA (sRNA) libraries were constructed and sequenced by high-throughput sequencing in 'Thompson seedless' grapevine. 873 known-miRNAs and 86 novel-miRNAs were identified, of which 88 known and three novel miRNAs were expressed differentially under heat stress. 322 genes were predicted to be targeted by the miRNAs. Eight selected miRNAs and its targets were confirmed by real time quantitative PCR (RT - qPCR), indicating that these "miRNA - target" were responsive to heat stress. In addition, most of the predicted target genes were negatively regulated by corresponding miRNAs. Gene function and pathway analyses indicated that these genes probably play crucial roles in heat stress tolerance. Vvi-miR167b transiently overexpression in grapevine leaves decreased target gene vvARF6, vvARF6-like and vvARF8 expression. The function of vvi-miR167 was verified by ectopic transformation in Arabidopsis thaliana, and the heat tolerance in transgenic lines was enhanced significantly, suggesting that the vvi-miR167 plays a positive regulatory role in grape thermostability. Comparison of miRNA expression patterns between heat treatment stress and CK can help elucidate the heat stress response and resistance mechanisms in grapes. In conclusion, these results gave us useful information to better understand the heat stress-response during domestication as well as for breeding new cultivars with heat stress resistance in fruit trees.

Identification of circular RNAs of Cannabis sativa L. potentially involved in the biosynthesis of cannabinoids

We identified circRNAs in the Cannabis sativa L. genome and examined their association with 28 cannabinoids in three tissues of C. sativa. Nine circRNAs are potentially involved in the biosynthesis of six cannabinoids. Cannabis sativa L. has been widely used in the production of medicine, textiles, and food for over 2500 years. The main bioactive compounds in C. sativa are cannabinoids, which have multiple important pharmacological actions. Circular RNAs (circRNAs) play essential roles in growth and development, stress resistance, and the biosynthesis of secondary metabolites. However, the circRNAs in C. sativa remain unknown. In this study, to explore the role of circRNAs in cannabinoid biosynthesis, we performed RNA-Seq and metabolomics analysis on the leaves, roots, and stems of C. sativa. We identified 741 overlapping circRNAs by three tools, of which 717, 16, and 8 circRNAs were derived from exonic, intronic, and intergenic, respectively. Functional enrichment analysis indicated that the parental genes (PGs) of circRNAs were enriched in many processes related to biological stress responses. We found that most of the circRNAs showed tissue-specific expression and 65 circRNAs were significantly correlated with their PGs (P < 0.05, |r|≥ 0.5). We also determined 28 cannabinoids by High-performance liquid chromatography-ESI-triple quadrupole-linear ion trap mass spectrometry. Ten circRNAs, including ciR0159, ciR0212, ciR0153, ciR0149, ciR0016, ciR0044, ciR0022, ciR0381, ciR0006, and ciR0025 were found to be associated with six cannabinoids by weighted gene co-expression network analysis. Twenty-nine of 53 candidate circRNAs, including 9 cannabinoids related were validated successfully using PCR amplification and Sanger sequencing. Taken together, all these results would help to enhance our acknowledge of the regulation of circRNAs, and lay the foundation for breeding new C. sativa cultivars with high cannabinoids through manipulating circRNAs.

Advances in understanding cold tolerance in grapevine

Grapevine (Vitis ssp.) is a deciduous perennial fruit crop, and the canes and buds of grapevine should withstand low temperatures annually during winter. However, the widely cultivated Vitis vinifera is cold-sensitive and cannot survive the severe winter in regions with extremely low temperatures, such as viticulture regions in northern China. By contrast, a few wild Vitis species like V. amurensis and V. riparia exhibit excellent freezing tolerance. However, the mechanisms underlying grapevine cold tolerance remain largely unknown. In recent years, much progress has been made in elucidating the mechanisms, owing to the advances in sequencing and molecular biotechnology. Assembly of grapevine genomes together with resequencing and transcriptome data enable researchers to conduct genomic and transcriptomic analyses in various grapevine genotypes and populations to explore genetic variations involved in cold tolerance. In addition, a number of pivotal genes have been identified and functionally characterized. In this review, we summarize recent major advances in physiological and molecular analyses of cold tolerance in grapevine and put forward questions in this field. We also discuss the strategies for improving the tolerance of grapevine to cold stress. Understanding grapevine cold tolerance will facilitate the development of grapevines for adaption to global climate change.

Evolutionary Landscape of Tea Circular RNAs and Its Contribution to Chilling Tolerance of Tea Plant

Chilling stress threatens the yield and distribution pattern of global crops, including the tea plant (Camellia sinensis), one of the most important cash crops around the world. Circular RNA (circRNA) plays roles in regulating plant growth and biotic/abiotic stress responses. Understanding the evolutionary characteristics of circRNA and its feedbacks to chilling stress in the tea plant will help to elucidate the vital roles of circRNAs. In the current report, we systematically identified 2702 high-confidence circRNAs under chilling stress in the tea plant, and interestingly found that the generation of tea plant circRNAs was associated with the length of their flanking introns. Repetitive sequences annotation and DNA methylation analysis revealed that the longer flanking introns of circRNAs present more repetitive sequences and higher methylation levels, which suggested that repeat-elements-mediated DNA methylation might promote the circRNAs biogenesis in the tea plant. We further detected 250 differentially expressed circRNAs under chilling stress, which were functionally enriched in GO terms related to cold/stress responses. Constructing a circRNA-miRNA-mRNA interaction network discovered 139 differentially expressed circRNAs harboring potential miRNA binding sites, which further identified 14 circRNAs that might contribute to tea plant chilling responses. We further characterized a key circRNA, CSS-circFAB1, which was significantly induced under chilling stress. FISH and silencing experiments revealed that CSS-circFAB1 was potentially involved in chilling tolerance of the tea plant. Our study emphasizes the importance of circRNA and its preliminary role against low-temperature stress, providing new insights for tea plant cold tolerance breeding.

Identification, biogenesis, function, and mechanism of action of circular RNAs in plants

Circular RNAs (circRNAs) are a class of single-stranded, closed RNA molecules with unique functions that are ubiquitously expressed in all eukaryotes. The biogenesis of circRNAs is regulated by specific cis-acting elements and trans-acting factors in humans and animals. circRNAs mainly exert their biological functions by acting as microRNA sponges, forming R-loops, interacting with RNA-binding proteins, or being translated into polypeptides or proteins in human and animal cells. Genome-wide identification of circRNAs has been performed in multiple plant species, and the results suggest that circRNAs are abundant and ubiquitously expressed in plants. There is emerging compelling evidence to suggest that circRNAs play essential roles during plant growth and development as well as in the responses to biotic and abiotic stress. However, compared with recent advances in human and animal systems, the roles of most circRNAs in plants are unclear at present. Here we review the identification, biogenesis, function, and mechanism of action of plant circRNAs, which will provide a fundamental understanding of the characteristics and complexity of circRNAs in plants.

CircRNA: a rising star in plant biology

Circular RNAs (circRNAs) are covalently closed single-stranded RNA molecules, which are widespread in eukaryotic cells. As regulatory molecules, circRNAs have various functions, such as regulating gene expression, binding miRNAs or proteins, and being translated into proteins, which are important for cell proliferation and cell differentiation, individual growth and development, as well as many other biological processes. However, compared with that in animal models, studies of circRNAs in plants lags behind and, particularly, the regulatory mechanisms of biogenesis and molecular functions of plant circRNAs remain elusive. Recent studies have shown that circRNAs are wide spread in plants with tissue- or development-specific expression patterns and are responsive to a variety of environmental stresses. In this review, we summarize these advances, focusing on the regulatory mechanisms of biogenesis, molecular and biological functions of circRNAs, and the methods for investigating circRNAs. We also discuss the challenges and the prospects of plant circRNA studies.

In-Plant Persistence and Systemic Transport of Nicotiana benthamiana Retrozyme RNA

Retrozymes are nonautonomous retrotransposons with hammerhead ribozymes in their long terminal repeats (LTRs). Retrozyme transcripts can be self-cleaved by the LTR ribozyme, circularized, and can undergo RNA-to-RNA replication. Here, we demonstrate that the Nicotiana benthamiana genome contains hundreds of retrozyme loci, of which nine represent full-length retrozymes. The LTR contains a promoter directing retrozyme transcription. Although retrozyme RNA is easily detected in plants, the LTR region is heavily methylated, pointing to its transcriptional silencing, which can be mediated by 24 nucleotide-long retrozyme-specific RNAs identified in N. benthamiana. A transcriptome analysis revealed that half of the retrozyme-specific RNAs in plant leaves have no exact matches to genomic retrozyme loci, containing up to 13% mismatches with the closest genomic sequences, and could arise as a result of many rounds of RNA-to-RNA replication leading to error accumulation. Using a cloned retrozyme copy, we show that retrozyme RNA is capable of replication and systemic transport in plants. The presented data suggest that retrozyme loci in the N. benthamiana genome are transcriptionally inactive, and that circular retrozyme RNA can persist in cells due to its RNA-to-RNA replication and be transported systemically, emphasizing functional and, possibly, evolutionary links of retrozymes to viroids-noncoding circular RNAs that infect plants.

Genome-wide circular RNA profiling and competing endogenous RNA regulatory network analysis provide new insights into the molecular mechanisms underlying early somatic embryogenesis in Dimocarpus longan Lour

Circular RNAs (circRNAs) are widely involved in plant growth and development. However, the function of circRNAs in plant somatic embryogenesis (SE) remains elusive. Here, by using high-throughput sequencing, a total of 5029 circRNAs were identified in the three stages of longan (Dimocarpus longan Lour.) early SE. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that differentially expressed (DE) circRNA host genes were enriched in the 'non-homologous end-joining' (NHEJ) and 'butanoate metabolism' pathways. In addition, the reactive oxygen species (ROS) content during longan early SE was determined. The results indicated that ROS-induced DNA double-strand breaks may not depend on the NHEJ repair pathway. Correlation analyses of the levels of related metabolites (glutamate, γ-aminobutyrate and pyruvate) and the expression levels of circRNAs and their host genes involved in butanoate metabolism were performed. The results suggested that circRNAs may act as regulators of the expression of cognate mRNAs, thereby affecting the accumulation of related compounds. A competing endogenous RNA (ceRNA) network of DE circRNAs, DE mRNAs, DE long noncoding RNAs (lncRNAs) and DE microRNAs (miRNAs) was constructed. The results showed that the putative targets of the noncoding RNA (ncRNAs) were significantly enriched in the KEGG pathways 'mitogen-activated protein kinase signaling' and 'nitrogen metabolism'. Furthermore, the expression patterns of the candidate circRNAs, lncRNAs, miRNAs and mRNAs confirmed the negative correlation between miRNAs and ceRNAs. In addition, two circRNA overexpression vectors were constructed to further verify the ceRNA network correlations in longan early SE. Our study revealed the potential role of circRNAs in longan early SE, providing new insights into the intricate regulatory mechanism underlying plant SE.

Transcriptome analysis revealed the expression levels of genes related to abscisic acid and auxin biosynthesis in grapevine (Vitis vinifera L.) under root restriction

The root system is essential for the stable growth of plants. Roots help anchor plants in the soil and play a crucial role in water uptake, mineral nutrient absorption and endogenous phytohormone formation. Root-restriction (RR) cultivation, a powerful technique, confines plant roots to a specific soil space. In the present study, roots of one-year-old "Muscat Hamburg" grapevine under RR and control (nR) treatments harvested at 70 and 125 days after planting were used for transcriptome sequencing, and in total, 2031 (nR7 vs. nR12), 1445 (RR7 vs. RR12), 1532 (nR7 vs. RR7), and 2799 (nR12 vs. RR12) differentially expressed genes (DEGs) were identified. Gene Ontology (GO) enrichment analysis demonstrated that there were several genes involved in the response to different phytohormones, including abscisic acid (ABA), auxin (IAA), ethylene (ETH), gibberellins (GAs), and cytokinins (CTKs). Among them, multiple genes, such as PIN2 and ERF113, are involved in regulating vital plant movements by various phytohormone pathways. Moreover, following RR cultivation, DEGs were enriched in the biological processes of plant-type secondary cell wall biosynthesis, the defense response, programmed cell death involved in cell development, and the oxalate metabolic process. Furthermore, through a combined analysis of the transcriptome and previously published microRNA (miRNA) sequencing results, we found that multiple differentially expressed miRNAs (DEMs) and DEG combinations in different comparison groups exhibited opposite trends, indicating that the expression levels of miRNAs and their target genes were negatively correlated. Furthermore, RR treatment indeed significantly increased the ABA content at 125 days after planting and significantly decreased the IAA content at 70 days after planting. Under RR cultivation, most ABA biosynthesis-related genes were upregulated, while most IAA biosynthesis-related genes were downregulated. These findings lay a solid foundation for further establishing the network through which miRNAs regulate grapevine root development through target genes and for further exploring the molecular mechanism through which endogenous ABA and IAA regulate root architecture development in grapevine.

MStoCIRC: A powerful tool for downstream analysis of MS/MS data to predict translatable circRNAs

CircRNAs are formed by a non-canonical splicing method and appear circular in nature. CircRNAs are widely distributed in organisms and have the features of time- and tissue-specific expressions. CircRNAs have attracted increasing interest from scientists because of their non-negligible effects on the growth and development of organisms. The translation capability of circRNAs is a novel and valuable direction in the functional research of circRNAs. To explore the translation potential of circRNAs, some progress has been made in both experimental identification and computational prediction. For computational prediction, both CircCode and CircPro are ribosome profiling-based software applications for predicting translatable circRNAs, and the online databases riboCIRC and TransCirc analyze as many pieces of evidence as possible and list the predicted translatable circRNAs of high confidence. Simultaneously, mass spectrometry in proteomics is often recognized as an efficient method to support the identification of protein and peptide sequences from diverse complex templates. However, few applications fully utilize mass spectrometry to predict translatable circRNAs. Therefore, this research aims to build up a scientific analysis pipeline with two salient features: 1) it starts with the data analysis of raw tandem mass spectrometry data; and 2) it also incorporates other translation evidence such as IRES. The pipeline has been packaged into an analysis tool called mass spectrometry to translatable circRNAs (MStoCIRC). MStoCIRC is mainly implemented by Python3 language programming and could be downloaded from GitHub (https://github.com/QUMU00/mstocirc-master). The tool contains a main program and several small, independent function modules, making it more multifunctional. MStoCIRC can process data efficiently and has obtained hundreds of translatable circRNAs in humans and Arabidopsis thaliana.

Discovery of cold-resistance genes in Vitis amurensis using bud-based quantitative trait locus mapping and RNA-seq

Background

In cold regions, low temperature is the main limiting factor affecting grape production. As an important breeding resource, V. amurensis Rupr. has played a crucial role in the discovery of genes which confer cold resistance in grapes. Thus far, many cold-resistance genes have been reported based on the study of V. amurensis. In order to identify more candidate genes related to cold resistance in V. amurensis, QTL mapping and RNA-seq was conducted based on the hybrid population and different cold-resistance cultivars in this study.

Results

In this study, highly cold-resistant grape cultivar ‘Shuangyou’ (SY) which belongs to V. amurensis, and cold-sensitive cultivar ‘Red Globe’ (RG) which belongs to Vitis vinifera L. were used to identify cold resistance genes. Cold-resistance quantitative trait locus (QTL) mapping was performed based on genetic population construction through interspecific crossing of ‘Shuangyou’ and ‘Red Globe’. Additionally, transcriptome analysis was conducted for the dormant buds of these two cultivars at different periods. Based on transcriptome analysis and QTL mapping, many new structural genes and transcription factors which relate to V. amurensis cold resistance were discovered, including CORs (VaCOR413IM), GSTs (VaGST-APIC, VaGST-PARB, VaGSTF9 and VaGSTF13), ARFs (VaIAA27 and VaSAUR71), ERFs (VaAIL1), MYBs (VaMYBR2, VaMYBLL and VaMYB3R-1) and bHLHs (VaICE1 and VabHLH30).

Conclusions

This discovery of candidate cold-resistance genes will provide an important theoretical reference for grape cold-resistance mechanisms, research, and cold-resistant grape cultivar breeding in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08788-y.

Mitochondrion-encoded circular RNAs are widespread and translatable in plants

Nucleus-encoded circular RNAs (ncircRNAs) have been widely detected in eukaryotes, and most circRNA identification algorithms are designed to identify them. However, using these algorithms, few mitochondrion-encoded circRNAs (mcircRNAs) have been identified in plants, and the role of plant mcircRNAs has not yet been addressed. Here, we developed a circRNA identification algorithm, mitochondrion-encoded circRNA identifier, based on common features of plant mitochondrial genomes. We identified 7,524, 9,819, 1,699, 1,821, 1,809, and 5,133 mcircRNAs in maize (Zea mays), Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), tomato (Solanum lycopersicum), cucumber (Cucumis sativus), and grape (Vitis vinifera), respectively. These mcircRNAs were experimentally validated. Plant mcircRNAs had distinct characteristics from ncircRNAs, and they were more likely to be derived from RNA degradation but not intron backsplicing. Alternative circularization was prevalent in plant mitochondria, and most parental genomic regions hosted multiple mcircRNA isoforms, which have homogenous 5' termini but heterogeneous 3' ends. By analysis of mitopolysome and mitoribosome profiling data, 1,463 mcircRNAs bound to ribosomes were detected in maize and Arabidopsis. Further analysis of mass spectrometry-based proteomics data identified 358 mcircRNA-derived polypeptides. Overall, we developed a computational pipeline that efficiently identifies plant mcircRNAs, and we demonstrated mcircRNAs are widespread and translated in plants.

Identification and Characterization of Circular RNAs in Brassica rapa in Response to Plasmodiophora brassicae

Plasmodiophora brassicae is a soil-borne pathogen that attacks the roots of cruciferous plants and causes clubroot disease. CircRNAs are noncoding RNAs, widely existing in plant and animal species. Although knowledge of circRNAs has been updated continuously and rapidly, information about circRNAs in the regulation of clubroot disease resistance is extremely limited in Brassica rapa. Here, Chinese cabbage (BJN 222) containing clubroot resistance genes (CRa) against P. brassicae Pb4 was susceptible to PbE. To investigate the mechanism of cicRNAs responsible for clubroot disease resistance in B. rapa, circRNA-seq was performed with roots of 'BJN 222' at 0, 8, and 23 days post-inoculated (dpi) with Pb4 and PbE. A total of 231 differentially expressed circRNAs were identified between the groups. Based on the differentially expressed circRNAs, the circRNA-miRNA-mRNA network was constructed using the target genes directly or indirectly related to plant resistance. Upregulated novel_circ_000495 suppressed the expression of miR5656-y, leading to the upregulation of Bra026508, which might cause plant resistance. Our results provide new insights into clubroot resistance mechanisms and lay a foundation for further studies exploring complex gene regulation networks in B. rapa.

Biogenesis, Functions, Interactions, and Resources of Non-Coding RNAs in Plants

Plant transcriptomes encompass a large number of functional non-coding RNAs (ncRNAs), only some of which have protein-coding capacity. Since their initial discovery, ncRNAs have been classified into two broad categories based on their biogenesis and mechanisms of action, housekeeping ncRNAs and regulatory ncRNAs. With advances in RNA sequencing technology and computational methods, bioinformatics resources continue to emerge and update rapidly, including workflow for in silico ncRNA analysis, up-to-date platforms, databases, and tools dedicated to ncRNA identification and functional annotation. In this review, we aim to describe the biogenesis, biological functions, and interactions with DNA, RNA, protein, and microorganism of five major regulatory ncRNAs (miRNA, siRNA, tsRNA, circRNA, lncRNA) in plants. Then, we systematically summarize tools for analysis and prediction of plant ncRNAs, as well as databases. Furthermore, we discuss the silico analysis process of these ncRNAs and present a protocol for step-by-step computational analysis of ncRNAs. In general, this review will help researchers better understand the world of ncRNAs at multiple levels.

Identification and Characterization of circRNAs under Drought Stress in Moso Bamboo (Phyllostachys edulis)

Circular RNAs (circRNAs) are a class of endogenous noncoding RNAs formed by 3′-5′ ligation during splicing. They play an important role in the regulation of transcription and miRNA in eukaryotes. Drought is one of the detrimental abiotic stresses that limit plant growth and productivity. How circRNAs influence the response to drought stress in moso bamboo (Phyllostachys edulis) remains elusive. In this study, we investigate the expression pattern of circRNAs in moso bamboo at 6 h, 12 h, 24 h and 48 h after drought treatment by deep sequencing and bioinformatics analysis and identify 4931 circRNAs, 52 of which are differentially expressed (DEcircRNAs) in drought-treated and untreated moso bamboo. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses of the host genes that generate the DEcircRNAs indcate that these DEcircRNAs are predicted to be involved in biochemical processes in response to drought, such as ubiquitin-mediated proteolysis, calcium-dependent protein kinase phosphorylation, amino acid biosynthesis and plant hormone signal transduction including abscisic acid. In addition, some circRNAs are shown to act as sponges for 291 miRNAs. Taken together, our results characterize the transcriptome profiles of circRNAs in drought responses and provide new insights into resistance breeding of moso bamboo.

Identification and Characterization of Rice Circular RNAs Responding to Xanthomonas oryzae pv. oryzae Invasion

Emerging roles of circular RNAs (circRNAs) in various biological processes have advanced our knowledge of transcriptional and posttranscriptional gene regulation. To date, no research has been conducted to explore their roles in the rice-Xanthomonas oryzae pv. oryzae interaction. Therefore, we identified 3,517 circRNAs from rice leaves infected with the highly virulent X. oryzae pv. oryzae strain PXO99^A by using rRNA depleted RNA sequencing technique coupled with the CIRI2 and CIRCexplorer2 pipeline. Characterization analyses showed that these circRNAs were distributed across the whole genome of rice, and most circRNAs arose from exons (85.13%), ranged from 200 to 1,000 bp, and were with a noncanonical GT/AG (including CT/AC equivalent) splicing signal. Functional annotation and enrichment analysis of the host genes that produced the differentially expressed circRNAs (DEcircRNAs) suggested that these identified circRNAs might play an important role in reprogramming rice responses to PXO99^A invasion, mainly by mediating photorespiration and chloroplast, peroxisome, and diterpenoid biosynthesis. Moreover, 31 DEcircRNAs were predicted to act as microRNA decoys in rice. The expression profile of four DEcircRNAs were validated by quantitative real-time PCR with divergent primers, and the back-splicing sites of seven DEcircRNAs were verified by PCR analysis and Sanger sequencing. Collectively, these results inferred a potential functional role of circRNAs in the regulation of rice immunity and provide novel clues about the molecular mechanisms of rice-PXO99^A interaction.

Evaluation of CircRNA Sequence Assembly Methods Using Long Reads

The functional study on circRNAs has been increasing in the past decade due to its important roles in micro RNA sponge, protein coding, the initiation, and progression of diseases. The study of circRNA functions depends on the full-length sequences of circRNA, and current sequence assembly methods based on short reads face challenges due to the existence of linear transcript. Long reads produced by long-read sequencing techniques such as Nanopore technology can cover full-length sequences of circRNA and therefore can be used to evaluate the correctness and completeness of circRNA full sequences assembled from short reads of the same sample. Using long reads of the same samples, one from human and the other from mouse, we have comprehensively evaluated the performance of several well-known circRNA sequence assembly algorithms based on short reads, including circseq_cup, CIRI_full, and CircAST. Based on the F1 score, the performance of CIRI-full was better in human datasets, whereas in mouse datasets CircAST was better. In general, each algorithm was developed to handle special situations or circumstances. Our results indicated that no single assembly algorithm generated better performance in all cases. Therefore, these assembly algorithms should be used together for reliable full-length circRNA sequence reconstruction. After analyzing the results, we have introduced a screening protocol that selects out exonic circRNAs with full-length sequences consisting of all exons between back splice sites as the final result. After screening, CIRI-full showed better performance for both human and mouse datasets. The average F1 score of CIRI-full over four circRNA identification algorithms increased from 0.4788 to 0.5069 in human datasets, and it increased from 0.2995 to 0.4223 in mouse datasets.

Biological Parts for Engineering Abiotic Stress Tolerance in Plants

It is vital to ramp up crop production dramatically by 2050 due to the increasing global population and demand for food. However, with the climate change projections showing that droughts and heatwaves becoming common in much of the globe, there is a severe threat of a sharp decline in crop yields. Thus, developing crop varieties with inbuilt genetic tolerance to environmental stresses is urgently needed. Selective breeding based on genetic diversity is not keeping up with the growing demand for food and feed. However, the emergence of contemporary plant genetic engineering, genome-editing, and synthetic biology offer precise tools for developing crops that can sustain productivity under stress conditions. Here, we summarize the systems biology-level understanding of regulatory pathways involved in perception, signalling, and protective processes activated in response to unfavourable environmental conditions. The potential role of noncoding RNAs in the regulation of abiotic stress responses has also been highlighted. Further, examples of imparting abiotic stress tolerance by genetic engineering are discussed. Additionally, we provide perspectives on the rational design of abiotic stress tolerance through synthetic biology and list various bioparts that can be used to design synthetic gene circuits whose stress-protective functions can be switched on/off in response to environmental cues.

Cryo-Treatment Enhances the Embryogenicity of Mature Somatic Embryos via the lncRNA-miRNA-mRNA Network in White Spruce

In conifers, somatic embryogenesis is uniquely initiated from immature embryos in a narrow time window, which is considerably hindered by the difficulty to induce embryogenic tissue (ET) from other tissues, including mature somatic embryos. In this study, the embryogenic ability of newly induced ET and DNA methylation levels was detected, and whole-transcriptome sequencing analyses were carried out. The results showed that ultra-low temperature treatment significantly enhanced ET induction from mature somatic embryos, with the induction rate from 0.4% to 15.5%, but the underlying mechanisms remain unclear. The newly induced ET showed higher capability in generating mature embryos than the original ET. DNA methylation levels fluctuated during the ET induction process. Here, WGCNA analysis revealed that OPT4, TIP1-1, Chi I, GASA5, GST, LAX3, WRKY7, MYBS3, LRR-RLK, PBL7, and WIN1 genes are involved in stress response and auxin signal transduction. Through co-expression analysis, lncRNAs MSTRG.505746.1, MSTRG.1070680.1, and MSTRG.33602.1 might bind to pre-novel_miR_339 to promote the expression of WRKY7 genes for stress response; LAX3 could be protected by lncRNAs MSTRG.1070680.1 and MSTRG.33602.1 via serving as sponges for novel_miR_495 to initiate auxin signal transduction; lncRNAs MSTRG.505746.1, MSTRG.1070680.1, and MSTRG.33602.1 might serve as sponges for novel_miR_527 to enhance the expression of Chi I for early somatic embryo development. This study provides new insight into the area of stress-enhanced early somatic embryogenesis in conifers, which is also attributable to practical applications.

Genome-Wide Identification of Circular RNAs Potentially Involved in the Biosynthesis of Secondary Metabolites in Salvia miltiorrhiza

Circular RNAs (circRNAs) play various roles in cellular functions. However, no studies have been reported on the potential involvement of circRNAs in the biosynthesis of secondary metabolites in plants. Here, we performed a genome-wide discovery of circRNAs from root, stem and leaf samples of Salvia miltiorrhiza using RNA-Seq. We predicted a total of 2,476 circRNAs with at least two junction reads using circRNA_finder and CIRI, of which 2,096, 151 and 229 were exonic, intronic and intergenic circRNAs, respectively. Sequence similarity analysis showed that 294 out of 2,476 circRNAs were conserved amongst multiple plants. Of the 55 predicted circRNAs, 31 (56%) were validated successfully by PCR and Sanger sequencing using convergent and divergent primer pairs. Alternative circularisation analysis showed that most parental genes produced two circRNAs. Functional enrichment analyses of the parental genes showed that the primary metabolism pathways were significantly enriched, particularly the carbon metabolism. Differential expression analysis showed that the expression profiles of circRNAs were tissue-specific. Co-expression analysis showed 275 circRNAs, and their parental genes had significantly positive correlations. However, 14 had significantly negative correlations. Weighted gene co-expression network analysis showed that nine circRNAs were co-expressed with four modules of protein-coding genes. Next, we found 416 exonic circRNAs with miRNA-binding sites, suggesting possible interactions between circRNAs and miRNAs. Lastly, we found six validated circRNAs, namely, SMscf2473-46693-46978, SMscf3091-29256-29724, SMscf16-111773-112193, SMscf432-13232-13866, SMscf7007-10563-10888 and SMscf1730-1749-2013, which were originated from the genes involved in the biosynthesis of secondary metabolites. Their parental genes were acetyl-CoA C-acetyltransferase 1 (SmAACT1), 1-deoxy-d-xylulose-5-phosphate synthase 2 (SmDXS2), 4-hydroxy-3-methylbut-2-enyl diphosphate reductase 1 (SmHDR1), kaurene synthase-like 2 (SmKSL2), DWF4 and CYP88A3, respectively. In particular, the correlation coefficient of SMscf2473-46693-46978 and SmDXS2 gene was 0.86 (p = 0.003), indicating a potential interaction between this pair of circRNA and its parent gene. Our results provided the first comprehensive catalogue of circRNAs in S. miltiorrhiza and identified one circRNA that might play important roles in the biosynthesis of secondary metabolites.

Grapevine Virology in the Third-Generation Sequencing Era: From Virus Detection to Viral Epitranscriptomics

Among all economically important plant species in the world, grapevine (Vitis vinifera L.) is the most cultivated fruit plant. It has a significant impact on the economies of many countries through wine and fresh and dried fruit production. In recent years, the grape and wine industry has been facing outbreaks of known and emerging viral diseases across the world. Although high-throughput sequencing (HTS) has been used extensively in grapevine virology, the application and potential of third-generation sequencing have not been explored in understanding grapevine viruses and their impact on the grapevine. Nanopore sequencing, a third-generation technology, can be used for the direct sequencing of both RNA and DNA with minimal infrastructure. Compared to other HTS methods, the MinION nanopore platform is faster and more cost-effective and allows for long-read sequencing. Due to the size of the MinION device, it can be easily carried for field viral disease surveillance. This review article discusses grapevine viruses, the principle of third-generation sequencing platforms, and the application of nanopore sequencing technology in grapevine virus detection, virus-plant interactions, as well as the characterization of viral RNA modifications.

Circular RNAs Repertoire and Expression Profile during Brassica rapa Pollen Development

Circular RNAs (circRNAs) are covalently closed RNA molecules generated by the back-splicing of exons from linear precursor mRNAs. Though various linear RNAs have been shown to play important regulatory roles in many biological and developmental processes, little is known about the role of their circular counterparts. In this study, we performed high-throughput RNA sequencing to delineate the expression profile and potential function of circRNAs during the five stages of pollen development in Brassica rapa. A total of 1180 circRNAs were detected in pollen development, of which 367 showed stage-specific expression patterns. Functional enrichment and metabolic pathway analysis showed that the parent genes of circRNAs were mainly involved in pollen-related molecular and biological processes such as mitotic and meiotic cell division, DNA processes, protein synthesis, protein modification, and polysaccharide biosynthesis. Moreover, by predicting the circRNA–miRNA network from our differentially expressed circRNAs, we found 88 circRNAs with potential miRNA binding sites, suggesting their role in post-transcriptional regulation of the genes. Finally, we confirmed the back-splicing sites of nine selected circRNAs using divergent primers and Sanger sequencing. Our study presents the systematic analysis of circular RNAs during pollen development and forms the basis of future studies for unlocking complex gene regulatory networks underpinning reproduction in flowering plants.

Recent research progress on circular RNAs: Biogenesis, properties, functions, and therapeutic potential

Circular RNAs (circRNAs), an emerging family member of RNAs, have gained importance in research due to their new functional roles in cellular physiology and disease progression. circRNAs are usually available in a wide range of cells and have shown tissue-specific expression as well as developmental specific expression. circRNAs are characterized by structural stability, conservation, and high abundance in the cell. In this review, we discuss the different models of biogenesis. The properties of circRNAs such as localization, structure and conserved pattern, stability, and expression specificity are also been illustrated. Furthermore, we discuss the biological functions of circRNAs such as microRNA (miRNA) sponging, cell cycle regulation, cell-to-cell communication, transcription regulation, translational regulation, disease diagnosis, and therapeutic potential. Finally, we discuss the recent research progress and future perspective of circRNAs. This review provides an understanding of potential diagnostic markers and the therapeutic potential of circRNAs, which are emerging daily.