Global identification of alternative splicing via comparative analysis of SMRT- and Illumina-based RNA-seq in strawberry

FvUVI4 inhibits cell division and cell expansion to modulate fruit development in Fragaria vesca

Fruit development is mainly regulated by cell division and expansion. As a negative regulator of the anaphase-promoting complex/cyclosome, UVI4 plays important roles in plant growth and development via coordinating cell cycle. However, currently there is no report on UVI4's functions in regulating fruit development in strawberry. Here, Fragaria vesca homolog FvUVI4 is identified and localizes in the nucleus. FvUVI4 has high gene expression in roots, leaves, flower, buds and green fruits, and low expression in petiole, stem, white and yellow fruit. Fruit development of F. vesca 'Hawaii4' is regulated by endoreduplication, and the expression of FvUVI4 is negatively correlated with fruit cell size. Overexpression of FvUVI4 inhibits endoreduplication of leaves, flowers and fruits in both Arabidopsis and F. vesca 'Hawaii4', thereby limiting cell expansion and decreasing cell area. Overexpression of FvUVI4 also inhibits mitotic cell cycle leading to decreased cell number, and ultimately affects the growth of leaves, petals and seeds or fruits. Arabidopsis uvi4 mutants obtained via CRISPR-Cas9 technology display opposite growth phenotypes to Arabidopsis and F. vesca 'Hawaii4' overexpression lines, which can be restored by overexpression of FvUVI4 in Arabidopsis uvi4 mutants. In conclusion, our study indicates that FvUVI4 inhibits cell expansion and cell division to modulate receptacle development in woodland strawberry.

Comprehensive deciphering the alternative splicing patterns involved in leaf morphogenesis of Liriodendron chinense

Alternative splicing (AS), a pivotal post-transcriptional regulatory mechanism, profoundly amplifies diversity and complexity of transcriptome and proteome. Liriodendron chinense (Hemsl.) Sarg., an excellent ornamental tree species renowned for its distinctive leaf shape, which resembles the mandarin jacket. Despite the documented potential genes related to leaf development of L. chinense, the underlying post-transcriptional regulatory mechanisms remain veiled. Here, we conducted a comprehensive analysis of the transcriptome to clarify the genome-wide landscape of the AS pattern and the spectrum of spliced isoforms during leaf developmental stages in L. chinense. Our investigation unveiled 50,259 AS events, involving 10,685 genes (32.9%), with intron retention as the most prevalent events. Notably, the initial stage of leaf development witnessed the detection of 804 differentially AS events affiliated with 548 genes. Although both differentially alternative splicing genes (DASGs) and differentially expressed genes (DEGs) were enriched into morphogenetic related pathways during the transition from fishhook (P2) to lobed (P7) leaves, there was only a modest degree of overlap between DASGs and DEGs. Furthermore, we conducted a comprehensively AS analysis on homologous genes involved in leaf morphogenesis, and most of which are subject to post-transcriptional regulation of AS. Among them, the AINTEGUMENTA-LIKE transcript factor LcAIL5 was characterization in detailed, which experiences skipping exon (SE), and two transcripts displayed disparate expression patterns across multiple stages. Overall, these findings yield a comprehensive understanding of leaf development regulation via AS, offering a novel perspective for further deciphering the mechanism of plant leaf morphogenesis.

Exploring the potential of endophyte-plant interactions for improving crop sustainable yields in a changing climate

Climate change poses a major threat to global food security, significantly reducing crop yields as cause of abiotic stresses, and for boosting the spread of new and old pathogens and pests. Sustainable crop management as a route to mitigation poses the challenge of recruiting an array of solutions and tools for the new aims. Among these, the deployment of positive interactions between the micro-biotic components of agroecosystems and plants can play a highly significant role, as part of the agro-ecological revolution. Endophytic microorganisms have emerged as a promising solution to tackle this challenge. Among these, Arbuscular Mycorrhizal Fungi (AMF) and endophytic bacteria and fungi have demonstrated their potential to alleviate abiotic stresses such as drought and heat stress, as well as the impacts of biotic stresses. They can enhance crop yields in a sustainable way also by other mechanisms, such as improving the nutrient uptake, or by direct effects on plant physiology. In this review we summarize and update on the main types of endophytes, we highlight several studies that demonstrate their efficacy in improving sustainable yields and explore possible avenues for implementing crop-microbiota interactions. The mechanisms underlying these interactions are highly complex and require a comprehensive understanding. For this reason, omic technologies such as genomics, transcriptomics, proteomics, and metabolomics have been employed to unravel, by a higher level of information, the complex network of interactions between plants and microorganisms. Therefore, we also discuss the various omic approaches and techniques that have been used so far to study plant-endophyte interactions.

Enhancing transcriptome analysis in medicinal plants: multiple unigene sets in Astragalus membranaceus

Astragalus membranaceus is a medicinal plant mainly used in East Asia and contains abundant secondary metabolites. Despite the importance of this plant, the available genomic and genetic information is still limited. De novo transcriptome construction is recognized as an essential method for transcriptome research when reference genome information is incomplete. In this study, we constructed three individual transcriptome sets (unigene sets) for detailed analysis of the phenylpropanoid biosynthesis pathway, a major metabolite of A. membranaceus. Set-1 was a circular consensus sequence (CCS) generated using PacBio sequencing (PacBio-seq). Set-2 consisted of hybridized assembled unigenes with Illumina sequencing (Illumina-seq) reads and PacBio CCS using rnaSPAdes. Set-3 unigenes were assembled from Illumina-seq reads using the Trinity software. Construction of multiple unigene sets provides several advantages for transcriptome analysis. First, it provides an appropriate expression filtering threshold for assembly-based unigenes: a threshold transcripts per million (TPM) ≥ 5 removed more than 88% of assembly-based unigenes, which were mostly short and low-expressing unigenes. Second, assembly-based unigenes compensated for the incomplete length of PacBio CCSs: the ends of the 5`/3` untranslated regions of phenylpropanoid-related unigenes derived from set-1 were incomplete, which suggests that PacBio CCSs are unlikely to be full-length transcripts. Third, more isoform unigenes could be obtained from multiple unigene sets; isoform unigenes missing in Set-1 were detected in set-2 and set-3. Finally, gene ontology and Kyoto Encyclopedia of Genes and Genomes analyses showed that phenylpropanoid biosynthesis and carbohydrate metabolism were highly activated in A. membranaceus roots. Various sequencing technologies and assemblers have been developed for de novo transcriptome analysis. However, no technique is perfect for de novo transcriptome analysis, suggesting the need to construct multiple unigene sets. This method enables efficient transcript filtering and detection of longer and more diverse transcripts.

Responsive Alternative Splicing Events of Opisthopappus Species against Salt Stress

Salt stress profoundly affects plant growth, prompting intricate molecular responses, such as alternative splicing (AS), for environmental adaptation. However, the response of AS events to salt stress in Opisthopappus (Opisthopappus taihangensis and Opisthopappus longilobus) remains unclear, which is a Taihang Mountain cliff-dwelling species. Using RNA-seq data, differentially expressed genes (DEGs) were identified under time and concentration gradients of salt stress. Two types of AS, skipped exon (SE) and mutually exclusive exons (MXE), were found. Differentially alternative splicing (DAS) genes in both species were significantly enriched in "protein phosphorylation", "starch and sucrose metabolism", and "plant hormone signal transduction" pathways. Meanwhile, distinct GO terms and KEGG pathways of DAS occurred between two species. Only a small subset of DAS genes overlapped with DEGs under salt stress. Although both species likely adopted protein phosphorylation to enhance salt stress tolerance, they exhibited distinct responses. The results indicated that the salt stress mechanisms of both Opisthopappus species exhibited similarities and differences in response to salt stress, which suggested that adaptive divergence might have occurred between them. This study initially provides a comprehensive description of salt responsive AS events in Opisthopappus and conveys some insights into the molecular mechanisms behind species tolerance on the Taihang Mountains.

Long read sequencing to reveal the full complexity of a plant transcriptome by targeting both standard and long workflows

BACKGROUND

Long read sequencing allows the analysis of full-length transcripts in plants without the challenges of reliable transcriptome assembly. Long read sequencing of transcripts from plant genomes has often utilized sized transcript libraries. However, the value of including libraries of differing sizes has not been established.

METHODS

A comprehensive transcriptome of the leaves of Jojoba (Simmondsia chinensis) was generated from two different PacBio library preparations: standard workflow (SW) and long workflow (LW).

RESULTS

The importance of using both transcript groups in the analysis was demonstrated by the high proportion of unique sequences (74.6%) that were not shared between the groups. A total of 37.8% longer transcripts were only detected in the long dataset. The completeness of the combined transcriptome was indicated by the presence of 98.7% of genes predicted in the jojoba male reference genome. The high coverage of the transcriptome was further confirmed by BUSCO analysis showing the presence of 96.9% of the genes from the core viridiplantae_odb10 lineage. The high-quality isoforms post Cd-Hit merged dataset of the two workflows had a total of 167,866 isoforms. Most of the transcript isoforms were protein-coding sequences (71.7%) containing open reading frames (ORFs) ≥ 100 amino acids (aa). Alternative splicing and intron retention were the basis of most transcript diversity when analysed at the whole genome level and by specific analysis of the apetala2 gene families.

CONCLUSION

This suggests the need to specifically target the capture of longer transcripts to provide more comprehensive genome coverage in plant transcriptome analysis and reveal the high level of alternative splicing.

Comprehensive Profiling of Alternative Splicing and Alternative Polyadenylation during Fruit Ripening in Watermelon (Citrullus lanatus)

Fruit ripening is a highly complicated process that is accompanied by the formation of fruit quality. In recent years, a series of studies have demonstrated post-transcriptional control play important roles in fruit ripening and fruit quality formation. Till now, the post-transcriptional mechanisms for watermelon fruit ripening have not been comprehensively studied. In this study, we conducted PacBio single-molecule long-read sequencing to identify genome-wide alternative splicing (AS), alternative polyadenylation (APA) and long non-coding RNAs (lncRNAs) in watermelon fruit. In total, 6,921,295 error-corrected and mapped full-length non-chimeric (FLNC) reads were obtained. Notably, more than 42,285 distinct splicing isoforms were derived from 5,891,183 intron-containing full-length FLNC reads, including a large number of AS events associated with fruit ripening. In addition, we characterized 21,506 polyadenylation sites from 11,611 genes, 8703 of which have APA sites. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that fructose and mannose metabolism, starch and sucrose metabolism and carotenoid biosynthesis were both enriched in genes undergoing AS and APA. These results suggest that post-transcriptional regulation might potentially have a key role in regulation of fruit ripening in watermelon. Taken together, our comprehensive PacBio long-read sequencing results offer a valuable resource for watermelon research, and provide new insights into the molecular mechanisms underlying the complex regulatory networks of watermelon fruit ripening.

Current insights into posttranscriptional regulation of fleshy fruit ripening

Fruit ripening is a complicated process that is accompanied by the formation of fruit quality. It is not only regulated at the transcriptional level via transcription factors or DNA methylation but also fine-tuned after transcription occurs. Here, we review recent advances in our understanding of key regulatory mechanisms of fleshy fruit ripening after transcription. We mainly highlight the typical mechanisms by which fruit ripening is controlled, namely, alternative splicing, mRNA N6-methyladenosine RNA modification methylation, and noncoding RNAs at the posttranscriptional level; regulation of translation efficiency and upstream open reading frame-mediated translational repression at the translational level; and histone modifications, protein phosphorylation, and protein ubiquitination at the posttranslational level. Taken together, these posttranscriptional regulatory mechanisms, along with transcriptional regulation, constitute the molecular framework of fruit ripening. We also critically discuss the potential usage of some mechanisms to improve fruit traits.

Full-Length Transcriptome Sequencing Reveals the Molecular Mechanism of Metasequoia glyptostroboides Seed Responding to Aging

Metasequoia glyptostroboides, Hu and W. C. Cheng, as the only surviving relict species of the Taxodiaceae Metasequoia genus, is a critically endangered and protected species in China. There is a risk of extinction due to the low vigor of M. glyptostroboides seeds, and the physiological mechanism of seed aging in M. glyptostroboides is not yet clear. In order to investigate the physiological and molecular mechanisms underlying the aging process of M. glyptostroboides seeds, we analyzed the antioxidant system and transcriptome at 0, 2, 4, 6, and 8 days after artificial accelerated aging treatment at 40 °C and 100% relative humidity. It was found that the germination percentage of fresh dried M. glyptostroboides seeds was 54 ± 5.29%, and significantly declined to 9.33 ± 1.88% after 6 days of aging, and then gradually decreased until the seed died on day 8. Superoxide dismutase (SOD) activity, ascorbic acid (AsA), glutathione (GSH) content and superoxide anion (O₂^·-) content and production rate significantly decreased, while malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) content and glutathione peroxidase (GPX) and catalase (CAT) activity gradually increased during the aging process. A total of 42,189 unigenes were identified in the whole transcriptome, and 40,446 (95.86%) unigenes were annotated in at least one protein database. A total of 15,376 differentially expressed genes (DEGs) were obtained; KEGG enrichment analysis results revealed that seed aging may be mainly involved in the protein-processing pathways in endoplasmic reticulum, oxidative phosphorylation, and ascorbate and aldarate metabolism. Weighted gene co-expression network analysis (WGCNA) revealed that the dark magenta, orange, and medium purple modules were highly correlated with physiological indicators such as SOD, CAT, and GSH and further identified 40 hub genes such as Rboh, ACO, HSF, and CML as playing important roles in the antioxidant network of M. glyptostroboides seeds. These findings provide a broader perspective for studying the regulatory mechanism of seed aging and a large number of potential target genes for the breeding of other endangered gymnosperms.

De novo full-length transcriptome analysis of two ecotypes of Phragmites australis (swamp reed and dune reed) provides new insights into the transcriptomic complexity of dune reed and its long-term adaptation to desert environments

BACKGROUND

The extremely harsh environment of the desert is changing dramatically every moment, and the rapid adaptive stress response in the short term requires enormous energy expenditure to mobilize widespread regulatory networks, which is all the more detrimental to the survival of the desert plants themselves. The dune reed, which has adapted to desert environments with complex and variable ecological factors, is an ideal type of plant for studying the molecular mechanisms by which Gramineae plants respond to combinatorial stress of the desert in their natural state. But so far, the data on the genetic resources of reeds is still scarce, therefore most of their research has focused on ecological and physiological studies.

RESULTS

In this study, we obtained the first De novo non-redundant Full-Length Non-Chimeric (FLNC) transcriptome databases for swamp reeds (SR), dune reeds (DR) and the All of Phragmites australis (merged of iso-seq data from SR and DR), using PacBio Iso-Seq technology and combining tools such as Iso-Seq3 and Cogent. We then identified and described long non-coding RNAs (LncRNA), transcription factor (TF) and alternative splicing (AS) events in reeds based on a transcriptome database. Meanwhile, we have identified and developed for the first time a large number of candidates expressed sequence tag-SSR (EST-SSRs) markers in reeds based on UniTransModels. In addition, through differential gene expression analysis of wild-type and homogenous cultures, we found a large number of transcription factors that may be associated with desert stress tolerance in the dune reed, and revealed that members of the Lhc family have an important role in the long-term adaptation of dune reeds to desert environments.

CONCLUSIONS

Our results provide a positive and usable genetic resource for Phragmites australis with a widespread adaptability and resistance, and provide a genetic database for subsequent reeds genome annotation and functional genomic studies.

Transcriptomic analysis reveals mechanisms for the different drought tolerance of sweet potatoes

Drought is a common environmental stress with great negative impacts on plant growth, development and geographical distribution as well as agriculture and food production. Sweet potato is characterized by starchy, fresh and pigmented tuber, and is regarded as the seventh most important food crop. However, there has been no comprehensive study of the drought tolerance mechanism of different sweet potato cultivars to date. Here, we studied the mechanism for drought response of seven sweet potato drought-tolerant cultivars using the drought coefficients, physiological indicators and transcriptome sequencing. The seven sweet potato cultivars were classified into four groups of drought tolerance performance. A large number of new genes and transcripts were identified, with an average of about 8000 new genes per sample. Alternative splicing events in sweet potato, which were dominated by first exon and last exon alternative splicing, were not conserved among different cultivars and not significantly affected by drought stress. Furthermore, different drought-tolerance mechanisms were revealed through differentially expressed gene analysis and functional annotation. Two drought-sensitive cultivars, Shangshu-9 and Xushu-22, mainly resisted drought stress by up-regulating plant signal transduction. The other drought-sensitive cultivar Jishu-26 responded to drought stress by down-regulating isoquinoline alkaloid biosynthesis and nitrogen/carbohydrate metabolism. In addition, the drought-tolerant cultivar Chaoshu-1 and drought-preferred cultivar Z15-1 only shared 9% of differentially expressed genes, as well as many opposite metabolic pathways in response to drought. They mainly regulated flavonoid and carbohydrate biosynthesis/metabolism in response to drought, while Z15-1 increased photosynthesis and carbon fixation capacity. The other drought-tolerant cultivar Xushu-18 responded to drought stress by regulating the isoquinoline alkaloid biosynthesis and nitrogen/carbohydrate metabolism. The extremely drought-tolerant cultivar Xuzi-8 was almost unaffected by drought stress and responded to drought environment only by regulating the cell wall. These findings provide important information for the selection of sweet potatoes for specific purposes.

Integrated single-molecule real-time sequencing and RNA sequencing reveal the molecular mechanisms of salt tolerance in a novel synthesized polyploid genetic bridge between maize and its wild relatives

BACKGROUND

Tripsacum dactyloides (2n = 4x = 72) and Zea perennis (2n = 4x = 40) are tertiary gene pools of Zea mays L. and exhibit many abiotic adaptations absent in modern maize, especially salt tolerance. A previously reported allopolyploid (hereafter referred to as MTP, 2n = 74) synthesized using Zea mays, Tripsacum dactyloides, and Zea perennis has even stronger salt tolerance than Z. perennis and T. dactyloides. This allopolyploid will be a powerful genetic bridge for the genetic improvement of maize. However, the molecular mechanisms underlying its salt tolerance, as well as the key genes involved in regulating its salt tolerance, remain unclear.

RESULTS

Single-molecule real-time sequencing and RNA sequencing were used to identify the genes involved in salt tolerance and reveal the underlying molecular mechanisms. Based on the SMRT-seq results, we obtained 227,375 reference unigenes with an average length of 2300 bp; most of the unigenes were annotated to Z. mays sequences (76.5%) in the NR database. Moreover, a total of 484 and 1053 differentially expressed genes (DEGs) were identified in the leaves and roots, respectively. Functional enrichment analysis of DEGs revealed that multiple pathways responded to salt stress, including "Flavonoid biosynthesis," "Oxidoreductase activity," and "Plant hormone signal transduction" in the leaves and roots, and "Iron ion binding," "Acetyl-CoA carboxylase activity," and "Serine-type carboxypeptidase activity" in the roots. Transcription factors, such as those in the WRKY, B3-ARF, and bHLH families, and cytokinin negatively regulators negatively regulated the salt stress response. According to the results of the short time series-expression miner analysis, proteins involved in "Spliceosome" and "MAPK signal pathway" dynamically responded to salt stress as salinity changed. Protein-protein interaction analysis revealed that heat shock proteins play a role in the large interaction network regulating salt tolerance.

CONCLUSIONS

Our results reveal the molecular mechanism underlying the regulation of MTP in the response to salt stress and abundant salt-tolerance-related unigenes. These findings will aid the retrieval of lost alleles in modern maize and provide a new approach for using T. dactyloides and Z. perennis to improve maize.

Development and validation of SSR markers related to flower color based on full-length transcriptome sequencing in Chrysanthemum

Chrysanthemum (Chrysanthemum moriforlium Ramat.) is one of the most popular flowers worldwide, with very high ornamental and economic values. However, the limitations of available DNA molecular markers and the lack of full genomic sequences hinder the study of genetic diversity and the molecular breeding of chrysanthemum. Here, we developed simple sequence repeat (SSR) from the full-length transcriptome sequences of chrysanthemum cultivar 'Hechengxinghuo'. A total of 11,699 SSRs with mono-, di-, tri-, tetra-, penta- and hexanucleotide repeats were identified, of which eight out of eighteen SSR loci identified based on sixteen transcripts participated in carotenoid metabolism or anthocyanin synthesis were validated as polymorphic SSR markers. These SSRs were used to classify 117 chrysanthemum accessions with different flower colors at the DNA and cDNA levels. The results showed that four SSR markers of carotenoid metabolic pathway divided 117 chrysanthemum accessions into five groups at cDNA level and all purple chrysanthemum accessions were in the group III. Furthermore, the SSR marker CHS-3, LCYE-1 and 3MaT may be related to green color and the PSY-1b marker may be related to yellow color. Overall, our work may be provide a novel method for mining SSR markers associated with specific traits.

A survey of the full-length transcriptome of Gracilariopsis lemaneiformis using single-molecule long-read sequencing

BACKGROUND

Posttranscriptional processing of precursor mRNAs contributes to transcriptome and protein diversity and gene regulatory mechanisms in eukaryotes. However, this posttranscriptional mechanism has not been studied in the marine macroalgae Gracilariopsis lemaneiformis, which is the most cultivated red seaweed species in China.

RESULTS

In the present study, third-generation sequencing (Pacific Biosciences single-molecule real-time long-read sequencing, SMRT-Seq) was used to sequence the full-length transcriptome of G. lemaneiformis to identify alternatively spliced transcripts and alternative polyadenylation (APA) sites in this species. RNAs were isolated from G. lemaneiformis under various treatments including abiotic stresses and exogenous phytohormones, and then equally pooled for SMRT-Seq. In summary, 346,544 full-length nonchimeric reads were generated, from which 13,630 unique full-length transcripts were obtained in G. lemaneiformis. Compared with the known splicing events in the gene models, more than 3000 new alternative splicing (AS) events were identified in the SMRT-Seq reads. Additionally, 810 genes were found to have poly (A) sites and 91 microRNAs (miRNAs), 961 long noncoding RNAs and 1721 novel genes were identified in G. lemaneiformis. Moreover, validation experiments showed that abiotic stresses and phytohormones could induce some specific AS events, especially intron retain isoforms, cause some alterations to the relative ratios of transcripts annotated to the same gene, and generate novel 3' ends because of differential APA. The growth of G. lemaneiformis was inhibited by Cu stress, while this inhibition was alleviated by ACC treatment. RNA-Seq analysis further revealed that 211 differential alternative splicing (DAS) events and 142 DAS events was obtained in CK vs Cu and Cu vs Cu + ACC, respectively, suggesting that AS of functional genes could be regulated by Cu stress and ACC. Compared with Cu stress, the expression of transcripts with DAS events mainly involved in the carbon fixation in photosynthetic organisms and oxidative phosphorylation pathway was upregulated in Cu + ACC treatment, revealing that ACC alleviated the growth inhibition by Cu stress by increasing carbon fixation and oxidative phosphorylation.

CONCLUSIONS

Our results provide the first comprehensive picture of the full-length transcriptome and posttranscriptional mechanism in red macroalgae, including transcripts that appeared in the presence of common abiotic stresses and phytohormones, which will improve the gene annotations of Gracilariopsis and contribute to the study of gene regulation in this important cultivated seaweed.

Evolution of isoform-level gene expression patterns across tissues during lotus species divergence

Both gene duplication and alternative splicing (AS) drive the functional diversity of gene products in plants, yet the relative contributions of the two key mechanisms to the evolution of gene function are largely unclear. Here, we studied AS in two closely related lotus plants, Nelumbo lutea and Nelumbo nucifera, and the outgroup Arabidopsis thaliana, for both single-copy and duplicated genes. We show that most splicing events evolved rapidly between orthologs and that the origin of lineage-specific splice variants or isoforms contributed to gene functional changes during species divergence within Nelumbo. Single-copy genes contain more isoforms, have more AS events conserved across species, and show more complex tissue-dependent expression patterns than their duplicated counterparts. This suggests that expression divergence through isoforms is a mechanism to extend the expression breadth of genes with low copy numbers. As compared to isoforms of local, small-scale duplicates, isoforms of whole-genome duplicates are less conserved and display a less conserved tissue bias, pointing towards their contribution to subfunctionalization. Through comparative analysis of isoform expression networks, we identified orthologous genes of which the expression of at least some of their isoforms displays a conserved tissue bias across species, indicating a strong selection pressure for maintaining a stable expression pattern of these isoforms. Overall, our study shows that both AS and gene duplication contributed to the diversity of gene function during the evolution of lotus.

RNA-Seq and Iso-Seq Reveal the Important Role of COMT and CCoAOMT Genes in Accumulation of Scopoletin in Noni (Morinda citrifolia)

Scopoletin, the main component of clinical drugs and the functional component of health products, is highly abundant in noni fruit (Morinda citrifolia). Multiple enzyme genes regulate scopoletin accumulation. In the present study, differentially expressed genes of noni were analyzed by RNA sequencing (RNA-Seq) and the full-length genes by isoform-sequencing (Iso-Seq) to find the critical genes in the scopoletin accumulation mechanism pathway. A total of 32,682 full-length nonchimeric reads (FLNC) were obtained, out of which 16,620 non-redundant transcripts were validated. Based on KEGG (Kyoto Encyclopedia of Genes and Genomes) annotation and differential expression analysis, two differentially expressed genes, caffeic acid 3-O-methyltransferase (COMT) and caffeoyl-CoA O-methyltransferase (CCoAOMT), were found in the scopoletin accumulation pathway of noni. Real-time quantitative polymerase chain reaction (q-PCR), phylogenetic tree analysis, gene expression analysis, and the change in scopoletin content confirmed that these two proteins are important in this pathway. Based on these results, the current study supposed that COMT and CCoAOMT play a significant role in the accumulation of scopoletin in noni fruit, and COMT (gene number: gene 7446, gene 8422, and gene 6794) and CCoAOMT (gene number: gene 12,084) were more significant. These results provide the importance of COMT and CCoAOMT and a basis for further understanding the accumulation mechanism of scopoletin in noni.

Full-length fruit transcriptomes of southern highbush (Vaccinium sp.) and rabbiteye (V. virgatum Ait.) blueberry

Background

Blueberries (Vaccinium sp.) are native to North America and breeding efforts to improve blueberry fruit quality are focused on improving traits such as increased firmness, enhanced flavor and greater shelf-life. Such efforts require additional genomic resources, especially in southern highbush and rabbiteye blueberries.

Results

We generated the first full-length fruit transcriptome for the southern highbush and rabbiteye blueberry using the cultivars, Suziblue and Powderblue, respectively. The transcriptome was generated using the Pacific Biosciences single-molecule long-read isoform sequencing platform with cDNA pooled from seven stages during fruit development and postharvest storage. Raw reads were processed through the Isoseq pipeline and full-length transcripts were mapped to the ‘Draper’ genome with unmapped reads collapsed using Cogent. Finally, we identified 16,299 and 15,882 non-redundant transcripts in ‘Suziblue’ and ‘Powderblue’ respectively by combining the reads mapped to Northern Highbush blueberry ‘Draper’ genome and Cogent analysis. In both cultivars, > 80% of sequences were longer than 1,000 nt, with the median transcript length around 1,700 nt. Functionally annotated transcripts using Blast2GO were > 92% in both ‘Suziblue’ and ‘Powderblue’ with overall equal distribution of gene ontology (GO) terms in the two cultivars. Analyses of alternative splicing events indicated that around 40% non-redundant sequences exhibited more than one isoform. Additionally, long non-coding RNAs were predicted to represent 5.6% and 7% of the transcriptomes in ‘Suziblue’ and ‘Powderblue’, respectively. Fruit ripening is regulated by several hormone-related genes and transcription factors. Among transcripts associated with phytohormone metabolism/signaling, the highest number of transcripts were related to abscisic acid (ABA) and auxin metabolism followed by those for brassinosteroid, jasmonic acid and ethylene metabolism. Among transcription factor-associated transcripts, those belonging to ripening-related APETALA2/ethylene-responsive element-binding factor (AP2/ERF), NAC (NAM, ATAF1/2 and CUC2), leucine zipper (HB-zip), basic helix-loop-helix (bHLH), MYB (v-MYB, discovered in avian myeloblastosis virus genome) and MADS-Box gene families, were abundant.

Further we measured three fruit ripening quality traits and indicators [ABA, and anthocyanin concentration, and texture] during fruit development and ripening. ABA concentration increased during the initial stages of fruit ripening and then declined at the Ripe stage, whereas anthocyanin content increased during the final stages of fruit ripening in both cultivars. Fruit firmness declined during ripening in ‘Powderblue’. Genes associated with the above parameters were identified using the full-length transcriptome. Transcript abundance patterns of these genes were consistent with changes in the fruit ripening and quality-related characteristics.

Conclusions

A full-length, well-annotated fruit transcriptome was generated for two blueberry species commonly cultivated in the southeastern United States. The robustness of the transcriptome was verified by the identification and expression analyses of multiple fruit ripening and quality–regulating genes. The full-length transcriptome is a valuable addition to the blueberry genomic resources and will aid in further improving the annotation. It will also provide a useful resource for the investigation of molecular aspects of ripening and postharvest processes.

Combination of long-read and short-read sequencing provides comprehensive transcriptome and new insight for Chrysanthemum morifolium ray-floret colorization

Chrysanthemum morifolium is one of the most popular ornamental plants globally. Owing to its large and complex genome (around 10 Gb, segmental hexaploid), it has been difficult to obtain comprehensive transcriptome, which will promote to perform new breeding technique, such as genome editing, in C. morifolium. In this study, we used single-molecule real-time (SMRT) sequencing and RNA-seq technologies, combined them with an error-correcting process, and obtained high-coverage ray-floret transcriptome. The SMRT-seq data increased the ratio of long mRNAs containing complete open-reading frames, and the combined dataset provided a more complete transcriptomic data than those produced from either SMRT-seq or RNA-seq-derived transcripts. We finally obtained 'Sei Arabella' transcripts containing 928,645 non-redundant mRNA, which showed 96.6% Benchmarking Universal Single-Copy Orthologs (BUSCO) score. We also validated the reliability of the dataset by analyzing a mapping rate, annotation and transcript expression. Using the dataset, we searched anthocyanin biosynthesis gene orthologs and performed a qRT-PCR experiment to assess the usability of the dataset. The assessment of the dataset and the following analysis indicated that our dataset is reliable and useful for molecular biology. The combination of sequencing methods provided genetic information and a way to analyze the complicated C. morifolium transcriptome.

Single-molecule Real-time (SMRT) Sequencing Facilitates Transcriptome Research and Genome Annotation of the Fish Sillago sinica

As a newly described Sillaginidae species, Chinese sillago (Sillago sinica) needs a better understanding of gene annotation information. In this study, we reported the first full-length transcriptome data of S. sinica using the PacBio isoform sequencing Iso-seq and a description of transcriptome structure analysis. A total of 454,979 high-quality full-length transcripts were obtained by single-molecule real-time (SMRT) sequencing, which was corrected by Illumina sequencing data. After that, 66,948 non-redundant full-length transcripts were generated after mapping to the reference genome of S. sinica, including 49 fusion isoforms and 9,250 novel isoforms. 63,459 isoforms were successfully annotated by one of the Nr, Nt, SwissProt, Pfam, KOG, GO, and KEGG databases. Additionally, 30,987 alternative polyadenylation (APA) sites, 451,867 alternative splicing (AS) events, 21,928 long non-coding RNAs (lncRNAs) and 12,911 transcription factors (TFs) were identified. The full-length transcripts of S. sinica would provide a precious resource for characterizing the transcriptome of S. sinica and for the further study of gene function and regulatory mechanism of this species.

Construction of drought stress regulation networks in potato based on SMRT and RNA sequencing data

BACKGROUND

Potato (Solanum tuberosum) is the fourth most important food crop in the world and plays an important role in food security. Drought stress has a significantly negative impact on potato growth and production. There are several publications involved drought stress in potato, this research contributes to enrich the knowledge.

RESULTS

In this study, next-generation sequencing (NGS) and single-molecule real-time (SMRT) sequencing technology were used to study the transcription profiles in potato in response to 20%PEG6000 simulates drought stress. The leaves of the variety "Désirée" from in vitro plantlets after drought stress at six time points from 0 to 48 hours were used to perform NGS and SMRT sequencing. According to the sequencing data, a total of 12,798 differentially expressed genes (DEGs) were identified in six time points. The real-time (RT)-PCR results are significantly correlated with the sequencing data, confirming the accuracy of the sequencing data. Gene ontology and KEGG analysis show that these DEGs participate in response to drought stress through galactose metabolism, fatty acid metabolism, plant-pathogen interaction, glutathione metabolism and other pathways. Through the analysis of alternative splicing of 66,888 transcripts, the functional pathways of these transcripts were enriched, and 51,098 transcripts were newly discovered from alternative splicing events and 47,994 transcripts were functionally annotated. Moreover, 3445 lncRNAs were predicted and enrichment analysis of corresponding target genes was also performed. Additionally, Alternative polyadenylation was analyzed by TADIS, and 26,153 poly (A) sites from 13,010 genes were detected in the Iso-Seq data.

CONCLUSION

Our research greatly enhanced potato drought-induced gene annotations and provides transcriptome-wide insights into the molecular basis of potato drought resistance.

Comparative full-length transcriptome analysis by Oxford Nanopore Technologies reveals genes involved in anthocyanin accumulation in storage roots of sweet potatoes (Ipomoea batatas L.)

Background

Storage roots of sweet potatoes (Ipomoea batatas L.) with different colors vary in anthocyanin content, indicating different economically agronomic trait. As the newest DNA/RNA sequencing technology, Oxford Nanopore Technologies (ONT) have been applied in rapid transcriptome sequencing for investigation of genes related to nutrient metabolism. At present, few reports concern full-length transcriptome analysis based on ONT for study on the molecular mechanism of anthocyanin accumulation leading to color change of tuberous roots of sweet potato cultivars.

Results

The storage roots of purple-fleshed sweet potato (PFSP) and white-fleshed sweet potato (WFSP) at different developmental stages were subjected to anthocyanin content comparison by UV-visible spectroscopy as well as transcriptome analysis at ONT MinION platform. UV-visible spectrophotometric measurements demonstrated the anthocyanin content of PFSP was much higher than that of WFSP. ONT RNA-Seq results showed each sample generated average 2.75 GB clean data with Full-Length Percentage (FL%) over 70% and the length of N50 ranged from 1,192 to 1,395 bp, indicating reliable data for transcriptome analysis. Subsequent analysis illustrated intron retention was the most prominent splicing event present in the resulting transcripts. As compared PFSP with WFSP at the relative developmental stages with the highest (PH vs. WH) and the lowest (PL vs. WL) anthocyanin content, 282 and 216 genes were up-regulated and two and 11 genes were down-regulated respectively. The differential expression genes involved in flavonoid biosynthesis pathway include CCoAOMT, PpLDOX, DFR, Cytochrome P450, CHI, and CHS. The genes encoding oxygenase superfamily were significantly up-regulated when compared PFSP with WFSP at the relative developmental stages.

Conclusions

Comparative full-length transcriptome analysis based on ONT serves as an effective approach to detect the differences in anthocyanin accumulation in the storage roots of different sweet potato cultivars at transcript level, with noting that some key genes can now be closely related to flavonoids biosynthesis. This study helps to improve understanding of molecular mechanism for anthocyanin accumulation in sweet potatoes and also provides a theoretical basis for high-quality sweet potato breeding.

A Global Analysis of Alternative Splicing of Dichocarpum Medicinal Plants, Ranunculales

Background: The multiple isoforms are often generated from a single gene via Alternative Splicing (AS) in plants, and the functional diversity of the plant genome is significantly increased. Despite well-studied gene functions, the specific functions of isoforms are little known, therefore, the accurate prediction of isoform functions is exceedingly wanted. Methods: Here we perform the first global analysis of AS of Dichocarpum, a medicinal genus of Ranunculales, by utilizing full-length transcriptome datasets of five Chinese endemic Dichocarpum taxa. Multiple software were used to identify AS events, the gene function was annotated based on seven databases, and the protein-coding sequence of each AS isoform was translated into an amino acid sequence. The self-developed software DIFFUSE was used to predict the functions of AS isoforms. Results: Among 8,485 genes with AS events, the genes with two isoforms were the most (6,038), followed by those with three isoforms and four isoforms. Retained intron (RI, 551) was predominant among 1,037 AS events, and alternative 3' splice sites and alternative 5' splice sites were second. The software DIFFUSE was effective in predicting functions of Dichocarpum isoforms, which have not been unearthed. When compared with the sequence alignment-based database annotations, DIFFUSE performed better in differentiating isoform functions. The DIFFUSE predictions on the terms GO:0003677 (DNA binding) and GO: 0010333 (terpene synthase activity) agreed with the biological features of transcript isoforms. Conclusion: Numerous AS events were for the first time identified from full-length transcriptome datasets of five Dichocarpum taxa, and functions of AS isoforms were successfully predicted by the self-developed software DIFFUSE. The global analysis of Dichocarpum AS events and predicting isoform functions can help understand the metabolic regulations of medicinal taxa and their pharmaceutical explorations.

A survey of transcriptome complexity using full-length isoform sequencing in the tea plant Camellia sinensis

Tea is one of the most popular beverages and its leaves are rich in catechins, contributing to the diverse flavor as well as beneficial for human health. However, the study of the post-transcriptional regulatory mechanism affecting the synthesis of catechins remains insufficient. Here, we sequenced the transcriptome using PacBio sequencing technology and obtained 63,111 full-length high-quality isoforms, including 1302 potential novel genes and 583 highly reliable fusion transcripts. We also identified 1204 lncRNAs with high quality, containing 188 known and 1016 novel lncRNAs. In addition, 311 mis-annotated genes were corrected based on the high-quality Isoseq reads. A large number of alternative splicing (AS) events (3784) and alternative polyadenylation (APA) genes (18,714) were analyzed, accounting for 8.84% and 43.7% of the total annotated genes, respectively. We also found that 2884 genes containing AS and APA features exhibited higher expression levels than other genes. These genes are mainly involved in amino acid biosynthesis, carbon fixation in photosynthetic organisms, phenylalanine, tyrosine, tryptophan biosynthesis, and pyruvate metabolism, suggesting that they play an essential role in the catechins content of tea polyphenols. Our results further improved the level of genome annotation and indicated that post-transcriptional regulation plays a crucial part in synthesizing catechins.

Combination analysis of single-molecule long-read and Illumina sequencing provides insights into the anthocyanin accumulation mechanism in an ornamental grass, Pennisetum setaceum cv. Rubrum

Combination analysis of single-molecule long-read and Illumina sequencing provide full-length transcriptome information and shed new light on the anthocyanin accumulation mechanism of Pennisetum setaceum cv. 'Rubrum'. Pennisetum setaceum cv. 'Rubrum' is an ornamental grass with purple leaves widely used in landscaping. However, the current next-generation sequencing (NGS) transcriptome information of this species is not satisfactory due to the difficulties in obtaining full-length transcripts. Furthermore, the molecular mechanisms of anthocyanin accumulation in P. setaceum have not been thoroughly studied. In this study, we used PacBio full-length transcriptome sequencing (SMRT) combined with NGS technology to build and improve the transcriptomic datasets and reveal the molecular mechanism of anthocyanin accumulation in P. setaceum cv. 'Rubrum'. Therefore, 280,413 full-length non-chimeric reads sequences were obtained using the SMRT technology. We obtained 97,450 high-quality non-redundant transcripts and identified 5352 alternative splicing events. In addition, 93,066 open reading frames (ORFs), including 57,457 full ORFs and 2910 long non-coding RNA (lncRNAs) were screened out. Furthermore, 10,795 differentially expressed genes were identified using NGS. We also explored key genes, synthesis pathways, and detected lncRNA involved in anthocyanin accumulation, providing new insights into anthocyanin accumulation in P. setaceum cv. 'Rubrum'. To our best knowledge, we provided the full-length transcriptome information of P. setaceum cv. 'Rubrum' for the first time. The results of this study will provide baseline information for gene function studies and pave the way for future P. setaceum cv. 'Rubrum' breeding projects.

Integration of full-length transcriptomes and anthocyanin metabolite analysis for understanding fruit coloration mechanism in Schisandra chinensis

Coloration directly affects the commercial value of Schisandra chinensis fruits. The composition and content of anthocyanin determine the S. chinensis fruit coloration. However, the molecular mechanism of anthocyanin biosynthesis and regulation in this fruit remains unknown. In this study, we performed integrative full-length transcriptomics and targeted metabolomics analyses in S. chinensis fruits at four different developmental stages to elucidate the coloration mechanism. Cyanidin3-O-xyl-rutinoside is the key anthocyanin, which is responsible for the reddening of S. chinensis fruits, and its accumulation gradually accelerated from the 80th day after fluorescence. Overall, 122,289 unigenes with an average length of 2592 bp and an N50 of 4232 bp were obtained through single-molecule real-time sequencing; a total of 16,456 differentially expressed genes were identified. Moreover, 10 full-length structural genes related to anthocyanin biosynthesis were found to be significantly differentially expressed with fruit ripening. Moreover, 10 glycosyltransferases (GTs) that may possess the activities of anthocyanidin 3-O-glucosyltransferase, anthocyanidin 3-O-glucoside rhamnosyltransferase, and xylosyltransferases, which are involved in the final three steps for cyanidin3-O-xyl-rutinoside synthesis, were identified through phylogenetic analysis. Based on these findings, we constructed the complete anthocyanin biosynthetic pathway in S. chinensis fruits; five ScMYBs, three ScbHLHs, and two ScWD40s potentially involved in regulating anthocyanin biosynthesis in S. chinensis fruits were also selected. Our study provides the foundation for further research on the molecular mechanism of anthocyanin biosynthesis and regulation for improving the quality of S. chinensis fruits. The results of full-length transcriptomes would provide researchers with novel insights into the molecular cloning of enzymes and their activity.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01179-3.

Integrative Analysis of Long- and Short-Read Transcriptomes Identify the Regulation of Terpenoids Biosynthesis Under Shading Cultivation in Oenanthe javanica

Water dropwort (Oenanthe javanica) is a popular vegetable with high nutritional value and distinctive flavor. The flavor is mainly correlate with the biosynthesis of terpenoids. Shading cultivation was used to improve the flavor in the production of water dropwort. However, the changes of terpenoids and the genes involved in terpenoids biosynthesis under shading treatment remains unclear. In this study, the long- and short-reads transcriptomes of water dropwort were constructed. In total, 57,743 non-redundant high-quality transcripts were obtained from the transcriptome. 28,514 SSRs were identified from non-redundant transcripts and the mono-nucleotide repeats were the most abundant SSRs. The lncRNAs of water dropwort were recognized and their target genes were predicted. The volatile compound contents in petioles and leaf blades of water dropwort were decreased after the shading treatment. The DEGs analysis was performed to identify the terpenoids biosynthesis genes. The results indicated that 5,288 DEGs were differentially expressed in petiole, of which 22 DEGs were enriched in the terpenoids backbone biosynthesis pathway. A total of 12 DEGs in terpenoids biosynthesis pathway were selected and further verified by qRT-PCR assay, demonstrating that the terpenoids biosynthesis genes were down-regulated under shading treatment. Here, the full-length transcriptome was constructed and the regulatory genes related to terpenoids biosynthesis in water dropwort were also investigated. These results will provide useful information for future researches on functional genomics and terpenoids biosynthesis mechanism in water dropwort.

Transcriptome Analysis of Otodectes cynotis in Different Developmental Stages

The mite Otodectes cynotis is distributed worldwide and parasitism the ear canals of cats and dogs, causing otitis externa. Molecular biology of O. cynotis is poorly understood, with only a few genes being deposited in public databases. In the present study, we aimed to perform transcriptome analysis of O. cynotis using SMRT and Illumina sequencing of RNA from different development stages. SMRT-Seq of O. cynotis demonstrated 5,431 final transcripts, including 406 long non-coding RNAs and 2,698 differentially expressed genes (DEGs), including 1,357 up-regulated genes and 1,341 down-regulated genes between adult mites and nymph/larva. A total of 397 putative allergen genes were detected, 231 of which were DEGs. Among them, 77 were homologous of known mite allergens. The expression level of allergen genes hints at the pathogenicity of mites in different life stages, and the protein interaction network analysis could identify possible key genes in the pathogenic mechanism. Intriguingly, Gene Ontology analysis showed that most of the (DEGs) were associated with the terms hydrolase activity and proteolysis. Kyoto Encyclopedia of genes and genomes (KEGG) analysis identified drug metabolism-cytochrome P450 signal pathway as one of the top pathways. SMRT-Seq of the full-length transcriptome of O. cynotis was performed first, and a valuable resource was acquired through the combination analysis with the Illumina sequencing data. The results of our analyses provide new information for further research into Otodectes cynotis.

Preliminary Results about Lamb Meat Tenderness Based on the Study of Novel Isoforms and Alternative Splicing Regulation Pathways Using Iso-seq, RNA-seq and CTCF ChIP-seq Data

Tenderness is an important indicator of meat quality. Novel isoforms associated with meat tenderness and the role of the CCCTC-binding factor (CTCF) in regulating alternative splicing to produce isoforms in sheep are largely unknown. The current project studied six sheep from two crossbred populations (Dorper × Hu × Hu, DHH and Dorper × Dorper × Hu, DDH) with divergent meat tenderness. Pooled Iso-seq data were used to annotate the sheep genomes. Then, the updated genome annotation and six RNA-seq data were combined to identify differentially expressed isoforms (DEIs) in muscles between DHH and DDH. These data were also combined with peaks detected from CTCF ChIP-seq data to investigate the regulatory role of CTCF for the alternative splicing. As a result, a total of 624 DEIs were identified between DDH and DHH. For example, isoform 7.524.18 transcribed from CAPN3 may be associated with meat tenderness. In addition, a total of 86 genes were overlapped between genes with transcribed DEIs and genes in differential peaks identified by CTCF ChIP-seq. Among these overlapped genes, ANKRD23 produces different isoforms which may be regulated by CTCF via methylation. As preliminary research, our results identified novel isoforms associated with meat tenderness and revealed the possible regulating mechanisms of alternative splicing to produce isoforms.

Full-length transcriptome reconstruction reveals genetic differences in hybrids of Oryza sativa and Oryza punctata with different ploidy and genome compositions

BACKGROUND

Allopolyploid breeding is an efficient technique for improving the low seed setting rate of autotetraploids in plant breeding and one of the most promising breeding methods. However, there have been few comprehensive studies of the posttranscriptional mechanism in allopolyploids.

RESULTS

By crossing cultivated rice (Oryza sativa, genome AA) with wild rice (Oryza punctata, genome BB), we created hybrid rice lines with different ploidy and genome compositions [diploid hybrid F01 (AB), allotetraploid hybrid F02 (AABB) and F03 (AAAB)]. The genetic differences of the hybrids and the mechanism of allopolyploid breeding dominance were revealed through morphological and cytological observations and single molecule real-time sequencing techniques. The tissues and organs of allotetraploid hybrid F02 exhibited "gigantism" and the highest levels of fertility. The numbers of non-redundant transcripts, gene loci and new isoforms in the polyploid rice lines were higher and the isoform lengths greater than those of the diploid line. Moreover, alternative splicing (AS) events occurred twice as often in the polyploid rice lines than the diploid line. During these events, intron retention dominated. Furthermore, a large number of new genes and isoforms specific to the lines of different ploidy were discovered.

CONCLUSIONS

The results indicated that alternative polyadenylation (APA) and AS events contributed to the complexity and superiority of polyploids in the activity of translation regulators, nucleic acid binding transcription factor activities and the regulation of molecular function. Therefore, these APA and AS events in allopolyploid rice were found to play a role in regulation. Our study provides new germplasm for polyploid rice breeding and reveals complex regulatory mechanisms that may be related to heterosis and fertility.

EST-SSR Marker Development and Full-Length Transcriptome Sequence Analysis of Tiger Lily (Lilium lancifolium Thunb)

The fast advancement and deployment of sequencing technologies after the Human Genome Project have greatly increased our knowledge of the eukaryotic genome sequences. However, due to technological concerns, high-quality genomic data has been confined to a few key organisms. Moreover, our understanding of which portions of genomes make up genes and which transcript isoforms synthesize these genes is scarce. Therefore, the current study has been designed to explore the reliability of the tiger lily (Lilium lancifolium Thunb) transcriptome. The PacBio-SMRT was used for attaining the complete transcriptomic profile. We obtained a total of 815,624 CCS (Circular Consensus Sequence) reads with an average length of 1295 bp. The tiger lily transcriptome has been sequenced for the first time using third-generation long-read technology. Furthermore, unigenes (38,707), lncRNAs (6852), and TF members (768) were determined based on the transcriptome data, followed by evaluating SSRs (3319). It has also been revealed that 105 out of 128 primer pairs effectively amplified PCR products. Around 15,608 transcripts were allocated to 25 distinct KOG Clusters, and 10,706 unigenes were grouped into 52 functional categories in the annotated transcripts. Until now, no tiger lily lncRNAs have been discovered. Results of this study may serve as an extensive set of reference transcripts and help us learn more about the transcriptomes of tiger lilies and pave the path for further research.

Alternative Splicing Dynamics During the Lifecycle of Salvia miltiorrhiza Root Revealed the Fine Tuning in Root Development and Ingredients Biosynthesis

Alternative splicing (AS) is an essential post-transcriptional process that enhances the coding and regulatory potential of the genome, thereby strongly influencing multiple plant physiology processes, such as metabolic biosynthesis. To explore how AS affects the root development and synthesis of tanshinones and phenolic acid pathways in Salvia miltiorrhiza roots, we investigated the dynamic landscape of AS events in S. miltiorrhiza roots during an annual life history. Temporal profiling represented a distinct temporal variation of AS during the entire development stages, showing the most abundant AS events at the early seedling stage (ES stage) and troughs in 45 days after germination (DAG) and 120 DAG. Gene ontology (GO) analysis indicated that physiological and molecular events, such as lateral root formation, gravity response, RNA splicing regulation, and mitogen-activated protein kinase (MAPK) cascade, were greatly affected by AS at the ES stage. AS events were identified in the tanshinones and phenolic acids pathways as well, especially for the genes for the branch points of the pathways as SmRAS and SmKSL1. Fifteen Ser/Arg-rich (SR) proteins and eight phosphokinases (PKs) were identified with high transcription levels at the ES stage, showing their regulatory roles for the high frequency of AS in this stage. Simultaneously, a co-expression network that includes 521 highly expressed AS genes, SRs, and PKs, provides deeper insight into the mechanism for the variable programming of AS.

Transcriptomic Complexity in Strawberry Fruit Development and Maturation Revealed by Nanopore Sequencing

The use of alternative transcription start or termination sites (aTSS or aTTS) as well as alternative splicing (AS) produce diverse transcript isoforms, playing indispensable roles in the plant development and environmental adaptations. Despite the advances in the finding of the genome-wide alternatively spliced genes in strawberry, it remains unexplored how AS responds to the developmental cues and what relevance do these outcomes have to the gene function. In this study, we have systematically investigated the transcriptome complexity using long-read Oxford Nanopore Technologies along the four successive developmental stages. The full-length cDNA sequencing results unraveled thousands of previously unexplored transcript isoforms raised from aTSS, aTTS, and AS. The relative contributions of these three processes to the complexity of strawberry fruit transcripts were compared. The aTSS and aTTS were more abundant than the AS. Differentially expressed transcripts unraveled the key transitional role of the white fruit stage. Isoform switches of transcripts from 757 genes were observed. They were associated with protein-coding potential change and domain gain or loss as the main consequences. Those genes with switched isoforms take part in the key processes of maturation in the late stages. A case study using yeast two hybrid analysis supported the functional divergence of the two isoforms of the B-box protein 22. Our results provided a new comprehensive overview of the dynamic transcriptomic landscape during strawberry fruit development and maturation.

Temporal regulation of alternative splicing events in rice memory under drought stress

Plant adaptation to drought stress is essential for plant survival and crop yield. Recently, harnessing drought memory, which is induced by repeated stress and recovery cycles, was suggested as a means to improve drought resistance at the transcriptional level. However, the genetic mechanism underlying drought memory is unclear. Here, we carried out a quantitative analysis of alternative splicing (AS) events in rice memory under drought stress, generating 12 transcriptome datasets. Notably, we identified exon skipping (ES) as the predominant AS type (>80%) in differential alternative splicing (DAS) in response to drought stress. Applying our analysis pipeline to investigate DAS events following drought stress in six other plant species revealed variable ES frequencies ranging from 9.94% to 60.70% depending on the species, suggesting that the relative frequency of DAS types in plants is likely to be species-specific. The dinucleotide sequence at AS splice sites in rice following drought stress was preferentially GC-AG and AT-AC. Since U12-type splicing uses the AT-AC site, this suggests that drought stress may increase U12-type splicing, and thus increase ES frequency. We hypothesize that multiple isoforms derived from exon skipping may be induced by drought stress in rice. We also identified 20 transcription factors and three highly connected hub genes with potential roles in drought memory that may be good targets for plant breeding.

A high-quality genome and comparison of short- versus long-read transcriptome of the palaearctic duck Aythya fuligula (tufted duck)

BACKGROUND

The tufted duck is a non-model organism that experiences high mortality in highly pathogenic avian influenza outbreaks. It belongs to the same bird family (Anatidae) as the mallard, one of the best-studied natural hosts of low-pathogenic avian influenza viruses. Studies in non-model bird species are crucial to disentangle the role of the host response in avian influenza virus infection in the natural reservoir. Such endeavour requires a high-quality genome assembly and transcriptome.

FINDINGS

This study presents the first high-quality, chromosome-level reference genome assembly of the tufted duck using the Vertebrate Genomes Project pipeline. We sequenced RNA (complementary DNA) from brain, ileum, lung, ovary, spleen, and testis using Illumina short-read and Pacific Biosciences long-read sequencing platforms, which were used for annotation. We found 34 autosomes plus Z and W sex chromosomes in the curated genome assembly, with 99.6% of the sequence assigned to chromosomes. Functional annotation revealed 14,099 protein-coding genes that generate 111,934 transcripts, which implies a mean of 7.9 isoforms per gene. We also identified 246 small RNA families.

CONCLUSIONS

This annotated genome contributes to continuing research into the host response in avian influenza virus infections in a natural reservoir. Our findings from a comparison between short-read and long-read reference transcriptomics contribute to a deeper understanding of these competing options. In this study, both technologies complemented each other. We expect this annotation to be a foundation for further comparative and evolutionary genomic studies, including many waterfowl relatives with differing susceptibilities to avian influenza viruses.

Integrative analysis of Iso-Seq and RNA-seq data reveals transcriptome complexity and differentially expressed transcripts in sheep tail fat

Background

Nowadays, both customers and producers prefer thin-tailed fat sheep. To effectively breed for this phenotype, it is important to identify candidate genes and uncover the genetic mechanism related to tail fat deposition in sheep. Accumulating evidence suggesting that post-transcriptional modification events of precursor-messenger RNA (pre-mRNA), including alternative splicing (AS) and alternative polyadenylation (APA), may regulate tail fat deposition in sheep. Differentially expressed transcripts (DETs) analysis is a way to identify candidate genes related to tail fat deposition. However, due to the technological limitation, post-transcriptional modification events in the tail fat of sheep and DETs between thin-tailed and fat-tailed sheep remains unclear.

Methods

In the present study, we applied pooled PacBio isoform sequencing (Iso-Seq) to generate transcriptomic data of tail fat tissue from six sheep (three thin-tailed sheep and three fat-tailed sheep). By comparing with reference genome, potential gene loci and novel transcripts were identified. Post-transcriptional modification events, including AS and APA, and lncRNA in sheep tail fat were uncovered using pooled Iso-Seq data. Combining Iso-Seq data with six RNA-sequencing (RNA-Seq) data, DETs between thin- and fat-tailed sheep were identified. Protein protein interaction (PPI) network, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were implemented to investigate the potential functions of DETs.

Results

In the present study, we revealed the transcriptomic complexity of the tail fat of sheep, result in 9,001 potential novel gene loci, 17,834 AS events, 5,791 APA events, and 3,764 lncRNAs. Combining Iso-Seq data with RNA-Seq data, we identified hundreds of DETs between thin- and fat-tailed sheep. Among them, 21 differentially expressed lncRNAs, such as ENSOART00020036299, ENSOART00020033641, ENSOART00020024562, ENSOART00020003848 and 9.53.1 may regulate tail fat deposition. Many novel transcripts were identified as DETs, including 15.527.13 (DGAT2), 13.624.23 (ACSS2), 11.689.28 (ACLY), 11.689.18 (ACLY), 11.689.14 (ACLY), 11.660.12 (ACLY), 22.289.6 (SCD), 22.289.3 (SCD) and 22.289.14 (SCD). Most of the identified DETs have been enriched in GO and KEGG pathways related to extracellular matrix (ECM). Our result revealed the transcriptome complexity and identified many candidate transcripts in tail fat, which could enhance the understanding of molecular mechanisms behind tail fat deposition.

Alternative splicing regulation appears to play a crucial role in grape berry development and is also potentially involved in adaptation responses to the environment

BACKGROUND

Alternative splicing (AS) produces transcript variants playing potential roles in proteome diversification and gene expression regulation. AS modulation is thus essential to respond to developmental and environmental stimuli. In grapevine, a better understanding of berry development is crucial for implementing breeding and viticultural strategies allowing adaptation to climate changes. Although profound changes in gene transcription have been shown to occur in the course of berry ripening, no detailed study on splicing modifications during this period has been published so far. We report here on the regulation of gene AS in developing berries of two grapevine (Vitis vinifera L.) varieties, Gewurztraminer (Gw) and Riesling (Ri), showing distinctive phenotypic characteristics. Using the software rMATS, the transcriptomes of berries at four developmental steps, from the green stage to mid-ripening, were analysed in pairwise comparisons between stages and varieties.

RESULTS

A total of 305 differential AS (DAS) events, affecting 258 genes, were identified. Interestingly, 22% of these AS events had not been reported before. Among the 80 genes that underwent the most significant variations during ripening, 22 showed a similar splicing profile in Gw and Ri, which suggests their involvement in berry development. Conversely, 23 genes were subjected to splicing regulation in only one variety. In addition, the ratios of alternative isoforms were different in Gw and Ri for 35 other genes, without any change during ripening. This last result indicates substantial AS differences between the two varieties. Remarkably, 8 AS events were specific to one variety, due to the lack of a splice site in the other variety. Furthermore, the transcription rates of the genes affected by stage-dependent splicing regulation were mostly unchanged, identifying AS modulation as an independent way of shaping the transcriptome.

CONCLUSIONS

The analysis of AS profiles in grapevine varieties with contrasting phenotypes revealed some similarity in the regulation of several genes with developmental functions, suggesting their involvement in berry ripening. Additionally, many splicing differences were discovered between the two varieties, that could be linked to phenotypic specificities and distinct adaptive capacities. Together, these findings open perspectives for a better understanding of berry development and for the selection of grapevine genotypes adapted to climate change.

Single-Molecule Real-Time and Illumina Sequencing to Analyze Transcriptional Regulation of Flavonoid Synthesis in Blueberry

Blueberries (Vaccinium corymbosum) contain large amounts of flavonoids, which play important roles in the plant's ability to resist stress and can also have beneficial effects on human health when the fruits are eaten. However, the molecular mechanisms that regulate flavonoid synthesis in blueberries are still unclear. In this study, we combined two different transcriptome sequencing platforms, single-molecule real-time (SMRT) and Illumina sequencing, to elucidate the flavonoid synthetic pathways in blueberries. We analyzed transcript quantity, length, and the number of annotated genes. We mined genes associated with flavonoid synthesis (such as anthocyanins, flavonols, and proanthocyanidins) and employed fluorescence quantitative PCR to analyze the expression of these genes and their correlation with flavonoid synthesis. We discovered one R2R3 MYB transcription factor from the sequencing library, VcMYB1, that can positively regulate anthocyanin synthesis in blueberries. VcMYB1 is mainly expressed in colored (mature) fruits. Experiments showed that overexpression and transient expression of VcMYB1 promoted anthocyanin synthesis in Arabidopsis, tobacco (Nicotiana benthamiana) plants and green blueberry fruits. Yeast one-hybrid (Y1H) assay, electrophoretic mobility shift assay, and transient expression experiments showed that VcMYB1 binds to the MYB binding site on the promoter of the structural gene for anthocyanin synthesis, VcMYB1 to positively regulate the transcription of VcDFR, thereby promoting anthocyanin synthesis. We also performed an in-depth investigation of transcriptional regulation of anthocyanin synthesis. This study provides background information and data for studying the synthetic pathways of flavonoids and other secondary metabolites in blueberries.

Unveiling the transcriptomic complexity of Miscanthus sinensis using a combination of PacBio long read- and Illumina short read sequencing platforms

Background

Miscanthus sinensis Andersson is a perennial grass that exhibits remarkable lignocellulose characteristics suitable for sustainable bioenergy production. However, knowledge of the genetic resources of this species is relatively limited, which considerably hampers further work on its biology and genetic improvement.

Results

In this study, through analyzing the transcriptome of mixed samples of leaves and stems using the latest PacBio Iso-Seq sequencing technology combined with Illumina HiSeq, we report the first full-length transcriptome dataset of M. sinensis with a total of 58.21 Gb clean data. An average of 15.75 Gb clean reads of each sample were obtained from the PacBio Iso-Seq system, which doubled the data size (6.68 Gb) obtained from the Illumina HiSeq platform. The integrated analyses of PacBio- and Illumina-based transcriptomic data uncovered 408,801 non-redundant transcripts with an average length of 1,685 bp. Of those, 189,406 transcripts were commonly identified by both methods, 169,149 transcripts with an average length of 619 bp were uniquely identified by Illumina HiSeq, and 51,246 transcripts with an average length of 2,535 bp were uniquely identified by PacBio Iso-Seq. Approximately 96 % of the final combined transcripts were mapped back to the Miscanthus genome, reflecting the high quality and coverage of our sequencing results. When comparing our data with genomes of four species of Andropogoneae, M. sinensis showed the closest relationship with sugarcane with up to 93 % mapping ratios, followed by sorghum with up to 80 % mapping ratios, indicating a high conservation of orthologs in these three genomes. Furthermore, 306,228 transcripts were successfully annotated against public databases including cell wall related genes and transcript factor families, thus providing many new insights into gene functions. The PacBio Iso-Seq data also helped identify 3,898 alternative splicing events and 2,963 annotated AS isoforms within 10 function categories.

Conclusions

Taken together, the present study provides a rich data set of full-length transcripts that greatly enriches our understanding of M. sinensis transcriptomic resources, thus facilitating further genetic improvement and molecular studies of the Miscanthus species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07971-x.

Full-length transcriptome profiling reveals insight into the cold response of two kiwifruit genotypes (A. arguta) with contrasting freezing tolerances

BACKGROUND

Kiwifruit (Actinidia Lindl.) is considered an important fruit species worldwide. Due to its temperate origin, this species is highly vulnerable to freezing injury while under low-temperature stress. To obtain further knowledge of the mechanism underlying freezing tolerance, we carried out a hybrid transcriptome analysis of two A. arguta (Actinidi arguta) genotypes, KL and RB, whose freezing tolerance is high and low, respectively. Both genotypes were subjected to - 25 °C for 0 h, 1 h, and 4 h.

RESULTS

SMRT (single-molecule real-time) RNA-seq data were assembled using the de novo method, producing 24,306 unigenes with an N50 value of 1834 bp. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of DEGs showed that they were involved in the 'starch and sucrose metabolism', the 'mitogen-activated protein kinase (MAPK) signaling pathway', the 'phosphatidylinositol signaling system', the 'inositol phosphate metabolism', and the 'plant hormone signal transduction'. In particular, for 'starch and sucrose metabolism', we identified 3 key genes involved in cellulose degradation, trehalose synthesis, and starch degradation processes. Moreover, the activities of beta-GC (beta-glucosidase), TPS (trehalose-6-phosphate synthase), and BAM (beta-amylase), encoded by the abovementioned 3 key genes, were enhanced by cold stress. Three transcription factors (TFs) belonging to the AP2/ERF, bHLH (basic helix-loop-helix), and MYB families were involved in the low-temperature response. Furthermore, weighted gene coexpression network analysis (WGCNA) indicated that beta-GC, TPS5, and BAM3.1 were the key genes involved in the cold response and were highly coexpressed together with the CBF3, MYC2, and MYB44 genes.

CONCLUSIONS

Cold stress led various changes in kiwifruit, the 'phosphatidylinositol signaling system', 'inositol phosphate metabolism', 'MAPK signaling pathway', 'plant hormone signal transduction', and 'starch and sucrose metabolism' processes were significantly affected by low temperature. Moreover, starch and sucrose metabolism may be the key pathway for tolerant kiwifruit to resist low temperature damages. These results increase our understanding of the complex mechanisms involved in the freezing tolerance of kiwifruit under cold stress and reveal a series of candidate genes for use in breeding new cultivars with enhanced freezing tolerance.

Full-length transcriptome analysis of pecan (Carya illinoinensis) kernels

Pecan is rich in bioactive components such as fatty acids (FAs) and flavonoids and is an important nut type worldwide. Therefore, the molecular mechanisms of phytochemical biosynthesis in pecan are a focus of research. Recently, a draft genome and several transcriptomes have been published. However, the full-length mRNA transcripts remain unclear, and the regulatory mechanisms behind the quality components biosynthesis and accumulation have not been fully investigated. In this study, single-molecule long-read sequencing technology was used to obtain full-length transcripts of pecan kernels. In total, 37,504 isoforms of 16,702 genes were mapped to the reference genome. The numbers of known isoforms, new isoforms, and novel isoforms were 9013 (24.03%), 26,080 (69.54%), and 2411 (6.51%), respectively. Over 80% of the transcripts (30,751, 81.99%) had functional annotations. A total of 15,465 alternative splicing (AS) events and 65,761 alternative polyadenylation events were detected; wherein, the retained intron was the predominant type (5652, 36.55%) of AS. Furthermore, 1894 long noncoding RNAs and 1643 transcription factors were predicted using bioinformatics methods. Finally, the structural genes associated with FA and flavonoid biosynthesis were characterized. A high frequency of AS accuracy (70.31%) was observed in FA synthesis-associated genes. This study provides a full-length transcriptome data set of pecan kernels, which will significantly enhance the understanding of the regulatory basis of phytochemical biosynthesis during pecan kernel maturation.

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.

TASPERT: Target-Specific Reverse Transcript Pools to Improve HTS Plant Virus Diagnostics

High-throughput sequencing (HTS) is becoming the new norm of diagnostics in plant quarantine settings. HTS can be used to detect, in theory, all pathogens present in any given sample. The technique’s success depends on various factors, including methods for sample management/preparation and suitable bioinformatic analysis. The Limit of Detection (LoD) of HTS for plant diagnostic tests can be higher than that of PCR, increasing the risk of false negatives in the case of low titer of the target pathogen. Several solutions have been suggested, particularly for RNA viruses, including rRNA depletion of the host, dsRNA, and siRNA extractions, which increase the relative pathogen titer in a metagenomic sample. However, these solutions are costly and time-consuming. Here we present a faster and cost-effective alternative method with lower HTS-LoD similar to or lower than PCR. The technique is called TArget-SPecific Reverse Transcript (TASPERT) pool. It relies on pathogen-specific reverse primers, targeting all RNA viruses of interest, pooled and used in double-stranded cDNA synthesis. These reverse primers enrich the sample for only pathogens of interest. Evidence on how TASPERT is significantly superior to oligodT, random 6-mer, and 20-mer in generating metagenomic libraries containing the pathogen of interest is presented in this proof of concept.

TeaAS: a comprehensive database for alternative splicing in tea plants (Camellia sinensis)

Alternative splicing (AS) increases the diversity of transcripts and proteins through the selection of different splice sites and plays an important role in the growth, development and stress tolerance of plants. With the release of the reference genome of the tea plant (Camellia sinensis) and the development of transcriptome sequencing, researchers have reported the existence of AS in tea plants. However, there is a lack of a platform, centered on different RNA-seq datasets, that provides comprehensive information on AS.To facilitate access to information on AS and reveal the molecular function of AS in tea plants, we established the first comprehensive AS database for tea plants (TeaAS, http://www.teaas.cn/index.php ). In this study, 3.96 Tb reads from 66 different RNA-seq datasets were collected to identify AS events. TeaAS supports four methods of retrieval of AS information based on gene ID, gene name, annotation (non-redundant/Kyoto encyclopedia of genes and genomes/gene ontology annotation or chromosomal location) and RNA-seq data. It integrates data pertaining to genome annotation, type of AS event, transcript sequence, and isoforms expression levels from 66 RNA-seq datasets. The AS events resulting from different environmental conditions and that occurring in varied tissue types, and the expression levels of specific transcripts can be clearly identified through this online database. Moreover, it also provides two useful tools, Basic Local Alignment Search Tool and Generic Genome Browser, for sequence alignment and visualization of gene structure.The features of the TeaAS database make it a comprehensive AS bioinformatics platform for researchers, as well as a reference for studying AS events in woody crops. It could also be helpful for revealing the novel biological functions of AS in gene regulation in tea plants.

Transcriptome profiling of Capsicum annuum using Illumina- and PacBio SMRT-based RNA-Seq for in-depth understanding of genes involved in trichome formation

Trichomes, specialized epidermal cells located in aerial parts of plants, play indispensable roles in resisting abiotic and biotic stresses. However, the regulatory genes essential for multicellular trichrome development in Capsicum annuum L. (pepper) remain unclear. In this study, the transcript profiles of peppers GZZY-23 (hairy) and PI246331 (hairless) were investigated to gain insights into the genes responsible for the formation of multicellular trichomes. A total of 40,079 genes, including 4743 novel genes and 13,568 differentially expressed genes (DEGs), were obtained. Functional enrichment analysis revealed that the most noticeable pathways were transcription factor activity, sequence-specific DNA binding, and plant hormone signal transduction, which might be critical for multicellular trichome formation in hairy plants. We screened 11 DEGs related to trichome development; 151 DEGs involved in plant hormone signal transduction; 312 DEGs belonging to the MYB, bHLH, HD-Zip, and zinc finger transcription factor families; and 1629 DEGs predicted as plant resistance genes (PRGs). Most of these DEGs were highly expressed in GZZY-23 or trichomes. Several homologs of trichome regulators, such as SlCycB2, SlCycB3, and H, were considerably upregulated in GZZY-23, especially in the trichomes. The transcriptomic data generated in this study provide a basis for future characterization of trichome formation in pepper.

Active Microbiome Structure and Functional Analyses of Freshwater Benthic Biofilm Samples Influenced by RNA Extraction Methods

Advances in high-throughput sequencing technologies have enabled extensive studies of freshwater biofilms and significant breakthroughs in biofilm meta-omics. To date, however, no standardized protocols have been developed for the effective isolation of RNA from freshwater benthic biofilms. In this study, we compared column-based kit RNA extraction with five RNAzol-based extractions, differentiated by various protocol modifications. The RNA products were then evaluated to determine their integrity, purity and yield and were subjected to meta-transcriptomic sequencing and analysis. Significant discrepancies in the relative abundance of active communities and structures of eukaryotic, bacterial, archaebacterial, and viral communities were observed as direct outcomes of the tested RNA extraction methods. The column isolation-based group was characterized by the highest relative abundance of Archaea and Eukaryota, while the organic isolation-based groups commonly had the highest relative abundances of Prokaryota (bacteria). Kit extraction methods provided the best outcomes in terms of high-quality RNA yield and integrity. However, these methods were deemed questionable for studies of active bacterial communities and may contribute a significant degree of bias to the interpretation of downstream meta-transcriptomic analyses.