Genome-Wide Profiling of Circular RNAs in the Rapidly Growing Shoots of Moso Bamboo (Phyllostachys edulis)

Expression and function analysis of phenylalanine ammonia-lyase genes involved in Bamboo lignin biosynthesis

Bamboo, renowned as the fastest-growing plant globally, matures within an astonishingly short period of 40-50 days from shoots, reaching heights of 10-20 meters. Moreover, it can be harvested for various uses within 3-5 years. Bamboo exhibits exceptional mechanical properties, characterized by high hardness and flexibility, largely attributed to its lignin content. Phenylalanine ammonia-lyase (PAL) catalyzes the crucial initial step in lignin biosynthesis, but its precise role in bamboo lignification processes remains elusive. Thus, elucidating the functions of PAL genes in bamboo lignification processes is imperative for understanding its rapid growth and mechanical strength. Here, we systematically identified and classified PAL genes in Moso bamboo, ensuring nomenclature consistency across prior studies. Subsequently, we evaluated PAL gene expression profiles using publicly available transcriptome data. The downregulation of PePALs expression in Moso bamboo through in planta gene editing resulted in a decrease in PAL activity and a subsequent reduction in lignin content. In contrast, overexpression of PePAL led to enhanced PAL activity and an increase in lignin content. These findings highlight the critical role of PAL in the lignin biosynthesis process of Moso bamboo, which will help to unravel the mechanism underpinning bamboo's rapid growth and mechanical strength, with a specific emphasis on elucidating the functions of PAL genes.

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.

Phylogenomic profiles of whole-genome duplications in Poaceae and landscape of differential duplicate retention and losses among major Poaceae lineages

Poaceae members shared a whole-genome duplication called rho. However, little is known about the evolutionary pattern of the rho-derived duplicates among Poaceae lineages and implications in adaptive evolution. Here we present phylogenomic/phylotranscriptomic analyses of 363 grasses covering all 12 subfamilies and report nine previously unknown whole-genome duplications. Furthermore, duplications from a single whole-genome duplication were mapped to multiple nodes on the species phylogeny; a whole-genome duplication was likely shared by woody bamboos with possible gene flow from herbaceous bamboos; and recent paralogues of a tetraploid Oryza are implicated in tolerance of seawater submergence. Moreover, rho duplicates showing differential retention among subfamilies include those with functions in environmental adaptations or morphogenesis, including ACOT for aquatic environments (Oryzoideae), CK2β for cold responses (Pooideae), SPIRAL1 for rapid cell elongation (Bambusoideae), and PAI1 for drought/cold responses (Panicoideae). This study presents a Poaceae whole-genome duplication profile with evidence for multiple evolutionary mechanisms that contribute to gene retention and losses.

The emerging role of circRNAs on skeletal muscle development in economical animals

CircRNAs are a novel type of closed circular molecules formed through a covalent bond lacking a 5'cap and 3' end tail, which mainly arise from mRNA precursor. They are widely distributed in plants and animals and are characterized by stable structure, high conservativeness in cells or tissues, and showed the expression specificity at different stages of development in different tissues. CircRNAs have been gradually attracted wide attention with the development of RNA sequencing, which become a new research hotspot in the field of RNA. CircRNAs play an important role in gene expression regulation. Presently, the related circRNAs research in the regulation of animal muscle development is still at the initial stage. In this review, the formation, properties, biological functions of circRNAs were summarized. The recent research progresses of circRNAs in skeletal muscle growth and development from economic animals including livestock, poultry and fishes were introduced. Finally, we proposed a prospective for further studies of circRNAs in muscle development, and we hope our research could provide new ideas, some theoretical supports and helps for new molecular genetic markers exploitation and animal genetic breeding in future.

Comprehensive genome-wide analysis of the DREB gene family in Moso bamboo (Phyllostachys edulis): evidence for the role of PeDREB28 in plant abiotic stress response

Dehydration response element binding (DREB) proteins are vital for plant abiotic stress responses, but the understanding of DREBs in bamboo, an important sustainable non-timber forest product, is limited. Here we conducted a comprehensive genome-wide analysis of the DREB gene family in Moso bamboo, representing the most important running bamboo species in Asia. In total, 44 PeDREBs were identified, and information on their gene structures, protein motifs, phylogenetic relationships, and stress-related cis-regulatory elements (CREs) was provided. Based on the bioinformatical analysis, we further analyzed PeDREBs from the A5 group and found that four of five PeDREB transcripts were induced by salt, drought, and cold stresses, and their proteins could bind to stress-related CREs. Among these, PeDREB28 was selected as a promising candidate for further functional characterization. PeDREB28 is localized in nucleus, has transcriptional activation activity, and could bind to the DRE- and coupling element 1- (CE1) CREs. Overexpression of PeDREB28 in Arabidopsis and bamboo improved plant abiotic stress tolerance. Transcriptomic analysis showed that broad changes due to the overexpression of PeDREB28. Furthermore, 628 genes that may act as the direct PeDREB28 downstream genes were identified by combining DAP-seq and RNA-seq analysis. Moreover, we confirmed that PeDREB28 could bind to the promoter of pyrabactin-resistance-like gene (DlaPYL3), which is a homolog of abscisic acid receptor in Arabidopsis, and activates its expression. In summary, our study provides important insights into the DREB gene family in Moso bamboo, and contributes to their functional verification and genetic engineering applications in the future.

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.

Multi-omics of Circular RNAs and Their Responses to Hormones in Moso Bamboo (Phyllostachys edulis)

Circular RNAs (circRNAs) are endogenous non-coding RNAs with covalently closed structures, which have important functions in plants. However, their biogenesis, degradation, and function upon treatment with gibberellins (GAs) and auxins (1-naphthaleneacetic acid, NAA) remain unknown. Here, we systematically identified and characterized the expression patterns, evolutionary conservation, genomic features, and internal structures of circRNAs using RNase R-treated libraries from moso bamboo (Phyllostachys edulis) seedlings. Moreover, we investigated the biogenesis of circRNAs dependent on both cis- and trans-regulation. We explored the function of circRNAs, including their roles in regulating microRNA (miRNA)-related genes and modulating the alternative splicing of their linear counterparts. Importantly, we developed a customized degradome sequencing approach to detect miRNA-mediated cleavage of circRNAs. Finally, we presented a comprehensive view of the participation of circRNAs in the regulation of hormone metabolism upon treatment of bamboo seedlings with GA and NAA. Collectively, our study provides insights into the biogenesis, function, and miRNA-mediated degradation of circRNAs in moso bamboo.

Identification and characterization of CircRNA-associated CeRNA networks in moso bamboo under nitrogen stress

BACKGROUND

Nitrogen is a macronutrient element for plant growth and development. Circular RNAs (circRNAs) serve as pivotal regulators for the coordination between nutrient supply and plant demand. Moso bamboo (Phyllostachys edulis) is an excellent plant with fast growth, and the mechanism of the circRNA-target module in response to nitrogen remains unclear.

RESULTS

Deep small RNA sequencing results of moso bamboo seedlings under different concentrations of KNO₃ (N0 = 0 mM, N6 = 6 mM, N18 = 18 mM) were used to identify circRNAs. A total of 549 circRNAs were obtained, of which 309 were generated from corresponding parental coding genes including 66 new ones. A total of 536 circRNA-parent genes were unevenly distributed in 24 scaffolds and were associated with root growth and development. Furthermore, 52 differentially expressed circRNAs (DECs) were obtained, including 24, 33 and 15 DECs from three comparisons of N0 vs. N6, N0 vs. N18 and N6 vs. N18, respectively. Based on integrative analyses of the identified DECs, differentially expressed mRNAs (DEGs), and miRNAs (DEMs), a competitive endogenous RNA (ceRNA) network was constructed, including five DECs, eight DEMs and 32 DEGs. A regulatory module of PeSca_6:12,316,320|12,372,905-novel_miR156-PH02Gene35622 was further verified by qPCR and dual-luciferase reporter assays.

CONCLUSION

The results indicated that circRNAs could participate in multiple biological processes as miRNA sponges, including organ nitrogen compound biosynthesis and metabolic process regulation in moso bamboo. Our results provide valuable information for further study of circRNAs in moso bamboo under fluctuating nitrogen conditions.

Development of an efficient expression system with large cargo capacity for interrogation of gene function in bamboo based on bamboo mosaic virus

Bamboo is one of the fastest growing plants among monocotyledonous species and is grown extensively in subtropical regions. Although bamboo has high economic value and produces much biomass quickly, gene functional research is hindered by the low efficiency of genetic transformation in this species. We therefore explored the potential of a bamboo mosaic virus (BaMV)-mediated expression system to investigate genotype-phenotype associations. We determined that the sites between the triple gene block proteins (TGBps) and the coat protein (CP) of BaMV are the most efficient insertion sites for the expression of exogenous genes in both monopodial and sympodial bamboo species. Moreover, we validated this system by individually overexpressing the two endogenous genes ACE1 and DEC1, which resulted in the promotion and suppression of internode elongation, respectively. In particular, this system was able to drive the expression of three 2A-linked betalain biosynthesis genes (more than 4 kb in length) to produce betalain, indicating that it has high cargo capacity and may provide the prerequisite basis for the development of a DNA-free bamboo genome editing platform in the future. Since BaMV can infect multiple bamboo species, we anticipate that the system described in this study will greatly contribute to gene function research and further promote the molecular breeding of bamboo.

Identification and Functional Prediction of CircRNAs in Leaves of F1 Hybrid Poplars with Different Growth Potential and Their Parents

Circular RNAs (CircRNAs) regulate plant growth and development; however, their role in poplar heterosis is unclear. We identified 3722 circRNAs in poplar leaves, most of which were intergenic (57.2%) and exonic (40.2%). The expression of circRNAs in F1 hybrids with high growth potential was higher than that in those with low growth potential. Non-additive expression of circRNAs and single-parent expression of circRNAs (SPE-circRNAs) might regulate poplar heterosis through microRNA sponging and protein translation, respectively. DECs among F1 hybrids with different growth potentials might regulate the growth potential of poplar via microRNA sponging. Correlation analysis between circRNA expression and its parent gene expression showed that SPE-M circRNA (circRNAs expressed by male parent only) might regulate poplar heterosis by inhibiting parent gene expression, while other circRNAs might regulate poplar heterosis by enhancing parent gene expression. Weighted correlation network analysis of gene/circRNA expression showed that circRNAs mainly regulate poplar heterosis via carbohydrate metabolism, amino acid metabolism, energy metabolism, and material transport. In addition, we identified seven circRNAs that positively or negatively regulate poplar heterosis. Thus, non-additively expressed circRNAs and SPE circRNAs are involved in regulating poplar heterosis, and DECs among F1 hybrids with different growth potentials were involved in regulating poplar growth potential.

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.

Overexpression of DsEXLA2 gene from Dendrocalamus sinicus accelerates the plant growth rate of Arabidopsis

Expansins play crucial roles in cell wall loosening and a range of life activities involving cell wall modification. Nevertheless, the biological functions of expansin genes during fast growth of bamboo remain unclear. In this study, Dendrocalamus sinicus, the largest and fastest growing bamboo species in the world, was used as the research material, and the full length of DsEXLA2 was cloned. Bioinformatics analysis revealed that DsEXLA2 contained expansin family typical domains (DPBB_1 and Pollen_allerg_1, CDRC motif) and amino acid sequence was highly conserved among different species. The expression level of DsEXLA2 increased from top section to basal section in different internodes. Subcellular localization verified that DsEXLA2 protein was located in the cell wall. Further genetic transformation studies in Arabidopsis indicated that compared with the wild type, DsEXLA2 overexpressed transgenic plants exhibited higher plant height, thicker stem, larger leaf, and less epidermal hair number and smaller stomatal aperture in the prophase and metaphase of growth. In addition, the cellulose content in the stem of transgenic plants was increased, and cell wall was thickened significantly. Moreover, a total of 1656 differentially expressed genes (DEGs) were identified by RNA-seq. The upregulated genes were predominantly enriched in the plant-pathogen interaction, MAPK signaling pathway-plant, plant hormone signal transduction, lipid metabolism and amino acid metabolism, while the downregulated genes were mainly enriched in energy metabolism, carbohydrate metabolism, plant hormone signal transduction and ribosome. These data implied that overexpression of DsEXLA2 gene accelerates the plant growth rate of Arabidopsis. This study is helpful to reveal the molecular mechanism of DsEXLA2 in culm growth and development of D. sinicus, and to understand the rapid growth of bamboos.

Genome-Wide Identification of NAP1 and Function Analysis in Moso Bamboo (Phyllostachys edulis)

The nucleosome assembly protein 1 (NAP1) family is the main histone chaperone of histone H2A–H2B. To explore the function of NAP1 family genes in moso bamboo (Phyllostachys edulis), characterized by extremely rapid growth and a long flowering cycle, we originally conducted a genome-wide analysis of the PheNAP1 gene. The phylogenetic relationship, gene expression pattern, DNA methylation, and histone modification were analyzed. Eventually, 12 PheNAP1 genes were recognized from the Phyllostachys edulis genome, divided into two sorts: the NRP subfamily (four members) and the NAP subfamily (eight members). Highly conserved motifs exist in each subfamily, which are distinct between subfamilies. PheNAP1 was distributed homogeneously on 10 out of 24 chromosomes, and gene duplication contributed significantly to the enhancement of the PheNAP1 gene in the genome. Cis-acting element analysis showed that PheNAP1 family genes are involved in light, hormone, and abiotic stress responses and may play an important role in the rapid growth and flowering. PheNAP1 exhibited the highest expression level in fast-growing shoots, indicating it is closely associated with the rapid growth of moso bamboo. Besides, PheNAP1 can rescue the early-flowering phenotype of nrp1-1 nrp2-2, and it affected the expression of genes related to the flowering pathway, like BSU1, suggesting the vital role that PheNAP1 may take in the flowering process of moso bamboo. In addition, histone modification results showed that PheNAP1 could bind to phosphorylation-, acetylation-, and methylation-modified histones to further regulate gene expression. A sketch appears: that PheNAP1 can accompany histones to regulate fast-growth- and flowering-related genes in moso bamboo. The consequences of this study enrich the understanding of the epigenetic regulation mechanism of bamboo plants and lays a foundation for further studies on the role of the NAP1 gene in Phyllostachys edulis and the function of chromatin regulation in forest growth and development.

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 expression analysis of the glycosyltransferase GT43 family members in bamboo reveal their potential function in xylan biosynthesis during rapid growth

Background

Xylan is one of the most abundant hemicelluloses and can crosslink cellulose and lignin to increase the stability of cell walls. A number of genes encoding glycosyltransferases play vital roles in xylan biosynthesis in plants, such as those of the GT43 family. However, little is known about glycosyltransferases in bamboo, especially woody bamboo which is a good substitute for timber.

Results

A total of 17 GT43 genes (PeGT43–1 ~ PeGT43–17) were identified in the genome of moso bamboo (Phyllostachys edulis), which belong to three subfamilies with specific motifs. The phylogenetic and collinearity analyses showed that PeGT43s may have undergone gene duplication, as a result of collinearity found in 12 pairs of PeGT43s, and between 17 PeGT43s and 10 OsGT43s. A set of cis-acting elements such as hormones, abiotic stress response and MYB binding elements were found in the promoter of PeGT43s. PeGT43s were expressed differently in 26 tissues, among which the highest expression level was found in the shoots, especially in the rapid elongation zone and nodes. The genes coexpressed with PeGT43s were annotated as associated with polysaccharide metabolism and cell wall biosynthesis. qRT–PCR results showed that the coexpressed genes had similar expression patterns with a significant increase in 4.0 m shoots and a peak in 6.0 m shoots during fast growth. In addition, the xylan content and structural polysaccharide staining intensity in bamboo shoots showed a strong positive correlation with the expression of PeGT43s. Yeast one-hybrid assays demonstrated that PeMYB35 could recognize the 5′ UTR/promoter of PeGT43–5 by binding to the SMRE cis-elements.

Conclusions

PeGT43s were found to be adapted to the requirement of xylan biosynthesis during rapid cell elongation and cell wall accumulation, as evidenced by the expression profile of PeGT43s and the rate of xylan accumulation in bamboo shoots. Yeast one-hybrid analysis suggested that PeMYB35 might be involved in xylan biosynthesis by regulating the expression of PeGT43–5 by binding to its 5′ UTR/promoter. Our study provides a comprehensive understanding of PeGT43s in moso bamboo and lays a foundation for further functional analysis of PeGT43s for xylan biosynthesis during rapid growth.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08192-y.

New Genes Interacted with Recent Whole Genome Duplicates in the Fast Stem Growth of Bamboos

As drivers of evolutionary innovations, new genes allow organisms to explore new niches. However, clear examples of this process remain scarce. Bamboos, the unique grass lineage diversifying into the forest, have evolved with a key innovation of fast growth of woody stem, reaching up to 1 m/day. Here, we identify 1,622 bamboo-specific orphan genes that appeared in recent 46 million years, and 19 of them evolved from noncoding ancestral sequences with entire de novo origination process reconstructed. The new genes evolved gradually in exon−intron structure, protein length, expression specificity, and evolutionary constraint. These new genes, whether or not from de novo origination, are dominantly expressed in the rapidly developing shoots, and make transcriptomes of shoots the youngest among various bamboo tissues, rather than reproductive tissue in other plants. Additionally, the particularity of bamboo shoots has also been shaped by recent whole-genome duplicates (WGDs), which evolved divergent expression patterns from ancestral states. New genes and WGDs have been evolutionarily recruited into coexpression networks to underline fast-growing trait of bamboo shoot. Our study highlights the importance of interactions between new genes and genome duplicates in generating morphological innovation.

Integrative lncRNA landscape reveals lncRNA-coding gene networks in the secondary cell wall biosynthesis pathway of moso bamboo (Phyllostachys edulis)

Background

LncRNAs are extensively involved in plant biological processes. However, the lack of a comprehensive lncRNA landscape in moso bamboo has hindered the molecular study of lncRNAs. Moreover, the role of lncRNAs in secondary cell wall (SCW) biosynthesis of moso bamboo is elusive.

Results

For comprehensively identifying lncRNA throughout moso bamboo genome, we collected 231 RNA-Seq datasets, 1 Iso-Seq dataset, and 1 full-length cDNA dataset. We used a machine learning approach to improve the pipeline of lncRNA identification and functional annotation based on previous studies and identified 37,009 lncRNAs in moso bamboo. Then, we established a network of potential lncRNA-coding gene for SCW biosynthesis and identified SCW-related lncRNAs. We also proposed that a mechanism exists in bamboo to direct phenylpropanoid intermediates to lignin or flavonoids biosynthesis through the PAL/4CL/C4H genes. In addition, we identified 4 flavonoids and 1 lignin-preferred genes in the PAL/4CL/C4H gene families, which gained implications in molecular breeding.

Conclusions

We provided a comprehensive landscape of lncRNAs in moso bamboo. Through analyses, we identified SCW-related lncRNAs and improved our understanding of lignin and flavonoids biosynthesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07953-z.

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.

NGS Methodologies and Computational Algorithms for the Prediction and Analysis of Plant Circular RNAs

Circular RNAs (circRNAs) are a class of single-stranded RNAs derived from exonic, intronic, and intergenic regions from precursor messenger RNAs (pre-mRNA), where a noncanonical back-splicing event occurs, in which the 5' and 3' ends are attached by covalent bond. CircRNAs participate in the regulation of gene expression at the transcriptional and posttranscriptional level primarily as miRNA and RNA-binding protein (RBP) sponges, but also involved in the regulation of alternative RNA splicing and transcription. CircRNAs are widespread and abundant in plants where they have been involved in stress responses and development. Through the analysis of all publications in this field in the last five years, we can summarize that the identification of these molecules is carried out through next generation sequencing studies, where samples have been previously treated to eliminate DNA, rRNA, and linear RNAs as a means to enrich circRNAs. Once libraries are prepared, they are sequenced and subsequently studied from a bioinformatics point of view. Among the different tools for identifying circRNAs, we can highlight CIRI as the most used (in 60% of the published studies), as well as CIRCExplorer (20%) and find_circ (20%). Although it is recommended to use more than one program in combination, and preferably developed specifically to treat with plant samples, this is not always the case. It should also be noted that after identifying these circular RNAs, most of the authors validate their findings in the laboratory in order to obtain bona fide results.

Circular RNAs acting as ceRNAs mediated by miRNAs may be involved in the synthesis of soybean fatty acids

Soybean oil is composed of fatty acids and glycerol. The content and composition of fatty acids partly determine the quality of soybean seeds. Circular RNAs (circRNAs) are endogenous non-coding RNAs that competitively bind to microRNAs (miRNAs) through miRNA recognition elements, thereby acting as sponges to regulate the expression of target genes. Although circRNAs have been identified previously in soybean, only their expression has been investigated without exploration of the competitive endogenous RNAs (ceRNAs) network of circRNAs-miRNAs-mRNAs. In this study, circRNAs in immature pods of a low linolenic acid soybean Mutant 72' (MT72) and the wild-type control 'Jinong 18' (JN18) were systematically identified and analyzed at 30 and 40 days after flowering using high-throughput sequencing technology. We identified 6377 circRNAs, of which 114 were differentially expressed. Gene ontology and KEGG pathway analyses of targeted mRNAs in the ceRNAs network indicated that the differentially expressed circRNAs may be involved in fatty acid transport, suggesting that circRNAs may play a post-transcriptional regulatory role in soybean oil synthesis. This study provides a foundation for future exploration of the function of circRNAs in soybean and presents novel insights to guide further studies of plant circRNAs.

Genome-wide profiling of circular RNAs, alternative splicing, and R-loops in stem-differentiating xylem of Populus trichocarpa

Circular RNAs (circRNAs) are a recently discovered type of non-coding RNA derived from pre-mRNAs. R-loops consist of a DNA:RNA hybrid and the associated single-stranded DNA. In Arabidopsis thaliana, circRNA:DNA R-loops regulate alternative splicing (AS) of SEPALLATA3 (SEP3). However, the occurrence and functions of circRNAs and R-loops in Populus trichocarpa are largely unexplored. Here, we performed circRNA-enriched sequencing in the stem-differentiating xylem (SDX) of P. trichocarpa and identified 2,742 distinct circRNAs, including circ-CESA4, circ-IRX7, and circ-GUX1, which are generated from genes involved in cellulose, and hemicellulose biosynthesis, respectively. To investigate the roles of circRNAs in modulating alternative splicing (AS), we detected 7,836 AS events using PacBio Iso-Seq and identified 634 circRNAs that overlapped with 699 AS events. Furthermore, using DNA:RNA hybrid immunoprecipitation followed by sequencing (DRIP-seq), we identified 8,932 R-loop peaks that overlapped with 181 circRNAs and 672 AS events. Notably, several SDX-related circRNAs overlapped with R-loop peaks, pointing to their possible roles in modulating AS in SDX. Indeed, overexpressing circ-IRX7 increased the levels of R-loop structures and decreased the frequency of intron retention in linear IRX7 transcripts. This study provides a valuable R-loop atlas resource and uncovers the interplay between circRNAs and AS in SDX of P. trichocarpa.

Whole-genome characterization of chronological age-associated changes in methylome and circular RNAs in moso bamboo (Phyllostachys edulis) from vegetative to floral growth

In mammals, DNA methylation is associated with aging. However, age-related DNA methylation changes during phase transitions largely remain unstudied in plants. Moso bamboo (Phyllostachys edulis) requires a very long time to transition from the vegetative to the floral phase. To comprehensively investigate the association of DNA methylation with aging, we present here single-base-resolution DNA methylation profiles using both high-throughput bisulfite sequencing and single-molecule nanopore-based DNA sequencing, covering the long period of vegetative growth and transition to flowering in moso bamboo. We discovered that CHH methylation gradually accumulates from vegetative to reproductive growth in a time-dependent fashion. Differentially methylated regions, correlating with chronological aging, occurred preferentially at both transcription start sites and transcription termination sites. Genes with CG methylation changes showed an enrichment of Gene Ontology (GO) categories in 'vegetative to reproductive phase transition of meristem'. Combining methylation data with mRNA sequencing revealed that DNA methylation in promoters, introns and exons may have different roles in regulating gene expression. Finally, circular RNA (circRNA) sequencing revealed that the flanking introns of circRNAs are hypermethylated and enriched in long terminal repeat (LTR) retrotransposons. Together, the observations in this study provide insights into the dynamic DNA methylation and circRNA landscapes, correlating with chronological age, which paves the way to study further the impact of epigenetic factors on flowering in moso bamboo.

Genome-wide identification and expression analysis of dirigent-jacalin genes from plant chimeric lectins in Moso bamboo (Phyllostachys edulis)

Dirigent-jacalin (D-J) genes belong to the plant chimeric lectin family, and play vital roles in plant growth and resistance to abiotic and biotic stresses. To explore the functions of the D-J family in the growth and development of Moso bamboo (Phyllostachys edulis), their physicochemical properties, phylogenetic relationships, gene and protein structures, and expression patterns were analyzed in detail. Four putative PeD-J genes were identified in the Moso bamboo genome, and microsynteny and phylogenetic analyses indicated that they represent a new branch in the evolution of plant lectins. PeD-J proteins were found to be composed of a dirigent domain and a jacalin-related lectin domain, each of which contained two different motifs. Multiple sequence alignment and homologous modeling analysis indicated that the three-dimensional structure of the PeD-J proteins was significantly different compared to other plant lectins, primarily due to the tandem dirigent and jacalin domains. We surveyed the upstream putative promoter regions of the PeD-Js and found that they mainly contained cis-acting elements related to hormone and abiotic stress response. An analysis of the expression patterns of root, leaf, rhizome and panicle revealed that four PeD-J genes were highly expressed in the panicle, indicating that they may be required during the formation and development of several different tissue types in Moso bamboo. Moreover, PeD-J genes were shown to be involved in the rapid growth and development of bamboo shoots. Quantitative Real-time PCR (qRT PCR) assays further verified that D-J family genes were responsive to hormones and stresses. The results of this study will help to elucidate the biological functions of PeD-Js during bamboo growth, development and stress response.

Profiling of circular RNA N6 -methyladenosine in moso bamboo (Phyllostachys edulis) using nanopore-based direct RNA sequencing

N⁶ -methyladenosine (m⁶ A) is a prevalent modification in messenger RNAs and circular RNAs that play important roles in regulating various aspects of RNA metabolism. However, the occurrence of the m⁶ A modification in plant circular RNAs has not been reported. A widely used method to identify m⁶ A modifications relies on m⁶ A-specific antibodies followed by next-generation sequencing of precipitated RNAs (MeRIP-Seq). However, one limitation of MeRIP-Seq is that it does not provide the precise location of m⁶ A at single-nucleotide resolution. Although more recent sequencing techniques such as Nanopore-based direct RNA sequencing (DRS) can overcome such limitations, the technology does not allow sequencing of circular RNAs, as these molecules lack a poly(A) tail. Here, we developed a novel method to detect the precise location of m⁶ A modifications in circular RNAs using Nanopore DRS. We first enriched our samples for circular RNAs, which we then fragmented and sequenced on the Nanopore platform with a customized protocol. Using this method, we identified 470 unique circular RNAs from DRS reads based on the back-spliced junction region. Among exonic circular RNAs, about 10% contained m⁶ A sites, which mainly occurred around acceptor and donor splice sites. This study demonstrates the utility of our antibody-independent method in identifying total and methylated circular RNAs using Nanopore DRS. This method has the additional advantage of providing the exact location of m⁶ A sites at single-base resolution in circular RNAs or linear transcripts from non-coding RNA without poly(A) tails.

Comprehensive Identification and Expression Profiling of Circular RNAs During Nodule Development in Phaseolus vulgaris

Symbiotic nitrogen fixation by legume nodules provides an abundant nitrogen source for plants, and understanding this process is key for developing green agriculture. Circular RNA (circRNA), a type of endogenous RNA produced by reverse splicing of mRNA precursors, plays important regulatory roles in plants at the transcriptional and post-transcriptional levels. However, the relationship between circRNAs and legume-rhizobium is unknown. Here, we performed comprehensive identification and expression profiling of circRNAs during nodulation in common bean (Phaseolus vulgaris) compared to uninoculated roots of corresponding ages by constructing circRNA-seq and mRNA-seq libraries. We identified 8,842 high-confident circRNAs, 3,448 of which were specifically produced during symbiosis, with the highest number at the nitrogen-fixing stage. Significantly, more circRNAs were derived from exons than from intergenic regions or introns in all samples. The lengths and GC contents of the circRNAs were similar in roots and nodules. However, circRNAs showed specific spatiotemporal expression patterns during nodule and root development. GO and other functional annotation of parental genes of differentially expressed circRNAs indicated their potential involvement in different biological processes. The expression of major circRNAs during symbiosis is independent of parental genes' expression to a certain degree, while expression of the remaining minor circRNAs showed positive correlation to parental genes. Functional annotation of the targeted mRNAs in the circRNA-miRNA-mRNA network showed that circRNAs may be involved in transmembrane transport and positive regulation of kinase activity during nodulation and nitrogen fixation as miRNA sponges. Our comprehensive analysis of the expression profile of circRNAs and their potential functions suggests that circRNAs may function as new post-transcriptional regulators in legume-rhizobium symbiosis.

Transcriptome Reveals the Specificity of Phyllostachys edulis ‘Pachyloen’ Shoots at Different Developmental Stages

Phyllostachys edulis ‘Pachyloen’ can have a stalk wall thickness of up to 2.5 cm at a height of 1.3 m, which is 1.8 times that of normal Moso bamboo (Phyllostachys edulis); this serves as an excellent cultivar, comprising both wood and bamboo shoots. We collected bamboo shoot samples of Phyllostachys edulis ‘Pachyloen’ and Moso bamboo on a monthly basis from September to April and used transcriptome sequencing to explore the differences in their development. The results showed that there were 666–1839 Phyllostachys edulis ‘Pachyloen’-specific genes at different developmental stages enriched in 20 biological processes, 15 cellular components, 12 molecular functions, and 137 metabolic pathways, 52 of which were significant. Among these, 27 metabolic pathways such as tyrosine metabolism and their uniquely expressed genes were found to play important roles in the thickening of Phyllostachys edulis ‘Pachyloen’. This study provides insights into the mechanisms underlying the thickening of the culm wall of Phyllostachys edulis ‘Pachyloen’.

Genome-wide identification and characterization of UDP-glucose dehydrogenase family genes in moso bamboo and functional analysis of PeUGDH4 in hemicellulose synthesis

Uridine diphosphate glucose dehydrogenases (UGDHs) are critical for synthesizing many nucleotide sugars and help promote the carbohydrate metabolism related to cell wall synthesis. In plants, UGDHs are encoded by a small gene family. Genome-wide analyses of these genes have been conducted in Glycine max and Arabidopsis thaliana, however, the UGDH gene family has not been comprehensively and systematically investigated in moso bamboo (Phyllostachys edulis), which is a special woody grass monocotyledonous species. In this study, we identified nine putative PeUGDH genes. Furthermore, analysis of gene duplication events and divergences revealed that the expansion of the PeUGDH family was mainly due to segmental and tandem duplications approximately 4.76–83.16 million years ago. An examination of tissue-specific PeUGDH expression indicated that more than 77% of the genes were predominantly expressed in the stem. Based on relative expression levels among PeUGDH members in different tissues in moso bamboo, PeUGDH4 was selected for detailed analysis. The results of subcellular localization indicated that PeUGDH4-GFP fusion proteins was observed to be localized in the cytoplasm. The ectopic overexpression of PeUGDH4 in Arabidopsis significantly increased the contents of hemicellulose and soluble sugar, suggesting that PeUGDH4 acts as a key enzyme involved in bamboo cell wall synthesis.

Characterization and Function of Circular RNAs in Plants

CircRNAs are covalently closed-loop single-stranded RNA molecules ubiquitously expressing in eukaryotes. As an important member of the endogenous ncRNA family, circRNAs are associated with diverse biological processes and can regulate transcription, modulate alternative splicing, and interact with miRNAs or proteins. Compared to abundant advances in animals, studies of circRNAs in plants are rapidly emerging. The databases and analysis tools for plant circRNAs are constantly being developed. Large numbers of circRNAs have been identified and characterized in plants and proved to play regulatory roles in plant growth, development, and stress responses. Here, we review the biogenesis, characteristics, bioinformatics resources, and biological functions of plant circRNAs, and summarize the distinct circularization features and differentially expression patterns comparison with animal-related results.

Genome-wide identification of long non-coding RNAs and circular RNAs reveal their ceRNA networks in response to cucumber green mottle mosaic virus infection in watermelon

Long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs) play vital roles in plant defense responses against viral infections. However, there is no systematic understanding of lncRNAs and circRNAs and their competing endogenous RNA (ceRNA) networks in watermelon under cucumber green mottle mosaic virus (CGMMV) stress. Here, we present the characterization and expression profiles of lncRNAs and circRNAs in watermelon leaves 48-h post-inoculation (48 hpi) with CGMMV, with mock inoculation as a control. Deep sequencing analysis revealed 2373 lncRNAs and 606 circRNAs in the two libraries. Among them, 67 lncRNAs (40 upregulated and 27 downregulated) and 548 circRNAs (277 upregulated and 271 downregulated) were differentially expressed (DE) in the 48 hpi library compared with the control library. Furthermore, 263 cis-acting matched lncRNA-mRNA pairs were detected for 49 of the DE-lncRNAs. KEGG pathway analysis of the cis target genes of the DE-lncRNAs revealed significant associations with phenylalanine metabolism, the citrate cycle (TCA cycle), and endocytosis. Additionally, 30 DE-lncRNAs were identified as putative target mimics of 33 microRNAs (miRNAs), and 153 DE-circRNAs were identified as putative target mimics of 88 miRNAs. Furthermore, ceRNA networks of lncRNA/circRNA-miRNA-mRNA in response to CGMMV infection are described, with 12 DE-lncRNAs and 65 DE-circRNAs combining with 22 miRNAs and competing for the miRNA binding sites on 29 mRNAs. The qRT-PCR validation of selected lncRNAs and circRNAs showed a general correlation with the high-throughput sequencing results. This study provides a valuable resource of lncRNAs and circRNAs involved in the response to CGMMV infection in watermelon.

Systematic identification and analysis of heat-stress-responsive lncRNAs, circRNAs and miRNAs with associated co-expression and ceRNA networks in cucumber (Cucumis sativus L.)

Researchers have shown that long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs) act as competitive endogenous RNAs (ceRNAs) and are mutually regulated by competition for binding to common microRNA response elements (MREs). However, a comprehensive identification and analysis of lncRNAs and circRNAs as ceRNAs have not yet been completed in cucumber (Cucumis sativus L.) exposed to high-temperature stress. In our study, 32 663 coding transcripts, 2085 lncRNAs, 2477 circRNAs and 348 differentially expressed miRNAs were identified using RNA sequencing. In addition, six heat-stress-responsive miRNAs (five known and one novel miRNAs) and eight lncRNAs were selected for qPCR to confirm their expression profiles. By analyzing the cis effects of lncRNAs, we constructed a lncRNA-mRNA co-expression network. Based on the results, the corresponding lncRNAs play a regulatory role in the stress response in cucumber plants. In our study, the PatMatch software was used to predict the potential function of lncRNAs and circRNAs as ceRNAs. A total of 18 lncRNAs and seven circRNAs were predicted to bind to 114 differentially expressed miRNAs and compete with 359 mRNAs for miRNA binding sites. These mRNAs are predicted to be involved in various pathways, such as plant hormone signal transduction, plant-pathogen interaction and glutathione metabolism. Among them, TCONS_00031790, TCONS_00014332, TCONS_00014717, TCONS_00005674, novel_circ_001543 and novel_circ_000876 may interact with miR9748 by plant hormone signal transduction pathways in response to high-temperature stress. Moreover, indole-3-acetic acid (IAA) and 1-aminocyclopropane-l-carboxylic acid (ACC) levels decreased in the high-temperature treatment group, indicating that IAA and ethylene signaling might be involved in response to high-temperature stress. In this study, we conducted a full transcriptomic analysis in response to high-temperature stress in cucumber and, for the first time, integrated the potential ceRNA functions of lncRNAs/circRNAs. The results provide a basis for studying the potential functions of lncRNAs/circRNAs in response to high-temperature stress.

Identification and Characterization of circRNAs Responsive to Methyl Jasmonate in Arabidopsis thaliana

Circular RNAs (circRNAs) are endogenous noncoding RNAs with covalently closed continuous loop structures that are formed by 3′–5′ ligation during splicing. These molecules are involved in diverse physiological and developmental processes in eukaryotic cells. Jasmonic acid (JA) is a critical hormonal regulator of plant growth and defense. However, the roles of circRNAs in the JA regulatory network are unclear. In this study, we performed high-throughput sequencing of Arabidopsis thaliana at 24 h, 48 h, and 96 h after methyl JA (MeJA) treatment. A total of 8588 circRNAs, which were distributed on almost all chromosomes, were identified, and the majority of circRNAs had lengths between 200 and 800 bp. We identified 385 differentially expressed circRNAs (DEcircRNAs) by comparing data between MeJA-treated and untreated samples. Gene Ontology (GO) enrichment analysis of the host genes that produced the DEcircRNAs showed that the DEcircRNAs are mainly involved in response to stimulation and metabolism. Additionally, some DEcircRNAs were predicted to act as miRNA decoys. Eight DEcircRNAs were validated by qRT-PCR with divergent primers, and the junction sites of five DEcircRNAs were validated by PCR analysis and Sanger sequencing. Our results provide insight into the potential roles of circRNAs in the MeJA regulation network.

Circular RNAs-The Road Less Traveled

Circular RNAs are the most recent addition in the non-coding RNA family, which has started to gain recognition after a decade of obscurity. The first couple of reports that emerged at the beginning of this decade and the amount of evidence that has accumulated thereafter has, however, encouraged RNA researchers to navigate further in the quest for the exploration of circular RNAs. The joining of 5′ and 3′ ends of RNA molecules through backsplicing forms circular RNAs during co-transcriptional or post-transcriptional processes. These molecules are capable of effectively sponging microRNAs, thereby regulating the cellular processes, as evidenced by numerous animal and plant systems. Preliminary studies have shown that circular RNA has an imperative role in transcriptional regulation and protein translation, and it also has significant therapeutic potential. The high stability of circular RNA is rendered by its closed ends; they are nevertheless prone to degradation by circulating endonucleases in serum or exosomes or by microRNA-mediated cleavage due to their high complementarity. However, the identification of circular RNAs involves diverse methodologies and the delineation of its possible role and mechanism in the regulation of cellular and molecular architecture has provided a new direction for the continuous research into circular RNA. In this review, we discuss the possible mechanism of circular RNA biogenesis, its structure, properties, degradation, and the growing amount of evidence regarding the detection methods and its role in animal and plant systems.

Regulatory networks of circRNAs related to transcription factors in Populus euphratica Oliv. heteromorphic leaves

Circular RNAs (circRNAs) are a novel class of non-coding RNAs that are characterized by a covalently closed circular structure. They have been widely found in Populus euphratica Oliv. heteromorphic leaves (P. hl). To study the role of circRNAs related to transcription factors (TFs) in the morphogenesis of P. hl, the expression profiles of circRNAs in linear, lanceolate, ovate, and broad-ovate leaves of P. euphratica were elucidated by strand-specific sequencing. We identified and characterized 22 circRNAs related to TFs in P. hl at the four developmental stages. Using the competing endogenous RNAs hypothesis as a guide, we constructed circRNA-miRNA-TF mRNA regulatory networks, which indicated that circRNAs antagonized microRNAs (miRNAs), thereby influencing the expression of the miRNA target genes and playing a significant role in transcriptional regulation. Gene ontology annotation of the target TF genes predicted that these circRNAs were associated mainly with the regulation of leaf development, leaf morphogenesis, signal transduction, and response to abiotic stress. These findings implied that the circRNAs affected the size and number of cells in P. hl by regulating the expression of TF mRNAs. Our results provide a basis for further studies of leaf development in poplar trees.

Genome-Wide Characterization and Gene Expression Analyses of GATA Transcription Factors in Moso Bamboo (Phyllostachys edulis)

Moso bamboo is well-known for its rapid-growth shoots and widespread rhizomes. However, the regulatory genes of these two processes are largely unexplored. GATA transcription factors regulate many developmental processes, but their roles in moso bamboo height control and rhizome development remains unexplored. Here, thirty-one bamboo GATA factors (PeGATAs) were identified, which are evolutionarily closer to rice than Arabidopsis, and their gene expression patterns were analyzed in bamboo development and phytohormone response with bioinformatics and molecular methods. Interestingly, PeGATAs could only be classified into three groups. Phytohormone responsive cis-elements were found in PeGATA promoters and the expression profiles showed that PeGATA genes might respond to gibberellin acid and abscisic acid but not to auxin at the transcriptional level. Furthermore, PeGATA genes have a tissue-specific expression pattern in bamboo rhizomes. Interestingly, most PeGATA genes were down-regulated during the rapid-growth of bamboo shoots. In addition, over-expressing one of the PeGATA genes, PeGATA26, significantly repressed the primary root length and plant height of transgenic Arabidopsis plants, which may be achieved by promoting the gibberellin acid turnover. Overall, our results provide insight into the function of GATA transcription factors in bamboo, and into genetic resources for engineering plant height.

Integration of Bioinformatic Predictions and Experimental Data to Identify circRNA-miRNA Associations

Circular RNAs (circRNAs) have recently emerged as a novel class of transcripts, characterized by covalently linked 3'-5' ends that result in the so-called backsplice junction. During the last few years, thousands of circRNAs have been identified in different organisms. Yet, despite their role as disease biomarker started to emerge, depicting their function remains challenging. Different studies have shown that certain circRNAs act as miRNA sponges, but any attempt to generalize from the single case to the "circ-ome" has failed so far. In this review, we explore the potential to define miRNA "sponging" as a more general function of circRNAs and describe the different approaches to predict miRNA response elements (MREs) in known or novel circRNA sequences. Moreover, we discuss how experiments based on Ago2-IP and experimentally validated miRNA:target duplexes can be used to either prioritize or validate putative miRNA-circRNA associations.