Genome-wide landscape of alternative splicing events in Brachypodium distachyon

Alternative Splicing Reveals Acute Stress Response of Litopenaeus vannamei at High Alkalinity

Alkalinity is regarded as one of the primary stressors for aquatic animals in saline-alkaline water. Alternative splicing (AS) can significantly increase the diversity of transcripts and play key roles in stress response; however, the studies on AS under alkalinity stress of crustaceans are still limited. In the present study, we devoted ourselves to the study of AS under acute alkalinity stress at control (50 mg/L) and treatment groups (350 mg/L) by RNA-seq in pacific white shrimp (Litopenaeus vannamei). We identified a total of 10,556 AS events from 4865 genes and 619 differential AS (DAS) events from 519 DAS genes in pacific white shrimp. Functional annotation showed that the DAS genes primarily involved in spliceosome. Five splicing factors (SFs), U2AF1, PUF60, CHERP, SR140 and SRSF2 were significantly up-regulated and promoted AS. Furthermore, alkalinity activated the Leukocyte transendothelial migration, mTOR signaling pathway and AMPK signaling pathway, which regulated MAPK1, EIF3B and IGFP-RP1 associated with these pathways. We also studied three SFs (HSFP1, SRSF2 and NHE-RF1), which underwent AS to form different transcript isoforms. The above results demonstrated that AS was a regulatory mechanism in pacific white shrimp in response to acute alkalinity stress. SFs played vital roles in AS of pacific white shrimp, such as HSFP1, SRSF2 and NHE-RF1. DAS genes were significantly modified in immunity of pacific white shrimp to cope with alkalinity stress. This is the first study on the response of AS to acute alkalinity stress, which provided scientific basis for AS mechanism of crustaceans response to alkalinity stress.

Effects of miR-722 on gene expression and alternative splicing in the liver of half-smooth tongue sole after infection with Vibrio anguillarum

MicroRNAs play crucial roles in various biological processes, including but not limited to differentiation, development, disease, and immunity. However, their immunoregulatory roles in half-smooth tongue sole are lacking. Our previous studies indicated that miR-722 could target C5aR1 to modulate the complement pathway to alleviate inflammatory response and even affect the mortality after the bacterial infection with Vibrio anguillarum. Driven by the purpose of revealing the underlying mechanisms, in this study, we investigated the effects of miR-722 on the gene expression and alternative splicing (AS) in the liver of half-smooth tongue sole after Vibrio anguillarum infection, with the approach of miR-722 overexpression/silencing and subsequent RNA-seq. Among the different comparisons, the I group (miR-722 inhibitor and V. anguillarum) versus blank control (PBS) exhibited the highest number of differentially expressed genes (DEGs), suggesting that the immune response was overactivated after inhibiting the miR-722. In addition, enrichment analyses were performed to reveal the functions of DEGs and differential AS (DAS) genes, reflecting the enrichment of RNA splicing and immune-related pathways including NF-κB and T cell receptor signaling pathway. Comparing the M group (miR-722 mimic and V. anguillarum) with the negative control (random sequence and V. anguillarum), two immune-related genes, cd48 and mapk8, were differentially expressed, of which mapk8 was also differentially spliced, indicating their importance in the immune response. Furthermore, representative gene analysis was performed, suggesting their corresponding functional changes due to AS. To verify the RNA-seq data, quantitative real-time PCR was employed with twenty pairs of primers for DEGs and DAS events. Overall, our results demonstrated that miR-722 could mediate the transcriptome-wide changes of gene expression and AS in half-smooth tongue sole, and provided insights into the regulatory role of miR-722 in immune responses, laying the foundation for further functional analyses and practical applications in aquaculture.

The Responses of Alternative Splicing during Heat Stress in the Pacific White Shrimp Litopenaeus vannamei

Heat tolerance is increasingly becoming a crucial trait for aquaculture species in the face of rapidly changing climate conditions. Alternative splicing (AS) is a vital mechanism within cells that modulates gene abundance and functional diversity, enabling organisms to effectively respond to diverse stressful conditions, including thermal stress. However, it is still uncertain whether AS contributes to heat tolerance in shrimp. In this study, we conducted an extensive transcriptome analysis on the Pacific white shrimp, Litopenaeus vannamei, revealing a total of 1267, 987, and 130 differential AS events (DAS) in the gill, hepatopancreas, and muscle, respectively, following exposure to heat stress. Among all of the DAS events, exon skipping (ES) was the predominant form of splicing modification observed. Interestingly, a minor portion of DAS genes exhibited overlap across the three tissues, implying that heat stress exerts unique effects on various tissue types. Moreover, the functional enrichment analysis demonstrated that commonly identified DAS genes were primarily associated with the "spliceosome" pathway, indicating that the AS of splicing-related genes played a crucial role in the response to heat stress. Our findings also revealed that heat stress tended to induce longer mRNA isoforms through differential alternative 3' splice site (A3SS) events. Notably, A3SS events exhibited the highest proportion of maintained open reading frames (ORFs) under heat stress. Interestingly, we observed a limited overlap between the genes exhibiting DAS and those showing differential gene expression (DEG), indicating that AS may function as a distinct regulatory mechanism independent of transcriptional regulation in response to heat stress. This is the first comprehensive study on AS in crustacea species under heat stress, which broadens our understanding of the regulatory mechanisms governing the crustaceans' response to environmental stress, providing valuable insights for the aquaculture breeding of shrimp and other aquatic animals.

Effect of selenium nanoparticles on alternative splicing in heat-stressed rainbow trout primary hepatocytes

Alternative splicing (AS) is a ubiquitous post-transcriptional regulatory mechanism in eukaryotes that generates multiple mRNA isoforms from a single gene, increasing diversity of mRNAs and proteins that are essential for eukaryotic developmental processes and responses to environmental stress. Results showed that a total of 37,463 AS events were identified in rainbow trout hepatocytes. In addition, a total of 364 differential alternative splicing (DAS) events were identified in hepatocytes under selenium nanoparticles (SeNPs) and 3632 DAS events were identified under a combination of SeNPs and heat stress (24 °C). Gene Ontology (GO) enrichment showed that some subcategories "immune effector processes", "response to stimuli" and "antioxidant activity" were associated with immunity, abiotic stimuli and antioxidants. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed that differentially expressed genes (DEGs) were significantly enriched in spliceosomes by adding SeNPs in heat-stressed hepatocytes. Splicing factor family (SRSF3, SRSF7, SRSF9, U2AF1 and U2AF2) and pre-RNA splicing factors (ACIN1 and PPRF18) were significantly upregulated and promoted AS. Furthermore, addition of SeNPs activated the phosphatidylinositol signaling system and upregulated the related genes PI4KA, DGKH, ITPK1 and Ocrl, and thus attenuated the inflammatory response to heat stress and enhanced resistance to heat stress by activating the adherent plaque kinase-PI3K-Akt signaling pathway and calcium channels. Those findings suggested that AS could be an essential regulatory mechanism in adaptation of rainbow trout to heat-stressed environments.

Relevance and Regulation of Alternative Splicing in Plant Heat Stress Response: Current Understanding and Future Directions

Alternative splicing (AS) is a major mechanism for gene expression in eukaryotes, increasing proteome diversity but also regulating transcriptome abundance. High temperatures have a strong impact on the splicing profile of many genes and therefore AS is considered as an integral part of heat stress response. While many studies have established a detailed description of the diversity of the RNAome under heat stress in different plant species and stress regimes, little is known on the underlying mechanisms that control this temperature-sensitive process. AS is mainly regulated by the activity of splicing regulators. Changes in the abundance of these proteins through transcription and AS, post-translational modifications and interactions with exonic and intronic cis-elements and core elements of the spliceosomes modulate the outcome of pre-mRNA splicing. As a major part of pre-mRNAs are spliced co-transcriptionally, the chromatin environment along with the RNA polymerase II elongation play a major role in the regulation of pre-mRNA splicing under heat stress conditions. Despite its importance, our understanding on the regulation of heat stress sensitive AS in plants is scarce. In this review, we summarize the current status of knowledge on the regulation of AS in plants under heat stress conditions. We discuss possible implications of different pathways based on results from non-plant systems to provide a perspective for researchers who aim to elucidate the molecular basis of AS under high temperatures.

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.

Analysis of Whole Transcriptome RNA-seq Data Reveals Many Alternative Splicing Events in Soybean Roots under Drought Stress Conditions

Alternative splicing (AS) is a common post-transcriptional regulatory mechanism that modulates gene expression to increase proteome diversity. Increasing evidence indicates that AS plays an important role in regulating plant stress responses. However, the mechanism by which AS coordinates with transcriptional regulation to regulate drought responses in soybean remains poorly understood. In this study, we performed a genome-wide analysis of AS events in soybean (Glycine max) roots grown under various drought conditions using the high-throughput RNA-sequencing method, identifying 385, 989, 1429, and 465 AS events that were significantly differentially spliced under very mild drought stress, mild drought stress, severe drought stress, and recovery after severe drought conditions, respectively. Among them, alternative 3' splice sites and skipped exons were the major types of AS. Overall, 2120 genes that experienced significant AS regulation were identified from these drought-treated root samples. Gene Ontology term analysis indicated that the AS regulation of binding activity has vital roles in the drought response of soybean root. Notably, the genes encoding splicing regulatory factors in the spliceosome pathway and mRNA surveillance pathway were enriched according to the Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis. Splicing regulatory factor-related genes in soybean root also responded to drought stress and were alternatively spliced under drought conditions. Taken together, our data suggest that drought-responsive AS acts as a direct or indirect mode to regulate drought response of soybean roots. With further in-depth research of the function and mechanism of AS in the process of abiotic stress, these results will provide a new strategy for enhancing stress tolerance of plants.

SMRT- and Illumina-based RNA-seq analyses unveil the ginsinoside biosynthesis and transcriptomic complexity in Panax notoginseng

Panax notoginseng is one of the most widely used traditional Chinese herbs with particularly valued roots. Triterpenoid saponins are mainly specialized secondary metabolites, which medically act as bioactive components. Knowledge of the ginsenoside biosynthesis in P. notoginseng, which is of great importance in the industrial biosynthesis and genetic breeding program, remains largely undetermined. Here we combined single molecular real time (SMRT) and Second-Generation Sequencing (SGS) technologies to generate a widespread transcriptome atlas of P. notoginseng. We mapped 2,383 full-length non-chimeric (FLNC) reads to adjacently annotated genes, corrected 1,925 mis-annotated genes and merged into 927 new genes. We identified 8,111 novel transcript isoforms that have improved the annotation of the current genome assembly, of which we found 2,664 novel lncRNAs. We characterized more alternative splicing (AS) events from SMRT reads (20,015 AS in 6,324 genes) than Illumina reads (18,498 AS in 9,550 genes), which contained a number of AS events associated with the ginsenoside biosynthesis. The comprehensive transcriptome landscape reveals that the ginsenoside biosynthesis predominantly occurs in flowers compared to leaves and roots, substantiated by levels of gene expression, which is supported by tissue-specific abundance of isoforms in flowers compared to roots and rhizomes. Comparative metabolic analyses further show that a total of 17 characteristic ginsenosides increasingly accumulated, and roots contained the most ginsenosides with variable contents, which are extraordinarily abundant in roots of the three-year old plants. We observed that roots were rich in protopanaxatriol- and protopanaxadiol-type saponins, whereas protopanaxadiol-type saponins predominated in aerial parts (leaves, stems and flowers). The obtained results will greatly enhance our understanding about the ginsenoside biosynthetic machinery in the genus Panax.

Genome-wide identification, molecular evolution, and expression analysis provide new insights into the APETALA2/ethylene responsive factor (AP2/ERF) superfamily in Dimocarpus longan Lour

Background

The APETALA2/ethylene responsive factor (AP2/ERF) superfamily members are transcription factors that regulate diverse developmental processes and stress responses in plants. They have been identified in many plants. However, little is known about the AP2/ERF superfamily in longan (Dimocarpus longan Lour.), which is an important tropical/subtropical evergreen fruit tree that produces a variety of bioactive compounds with rich nutritional and medicinal value. We conducted a genome-wide analysis of the AP2/ERF superfamily and its roles in somatic embryogenesis (SE) and developmental processes in longan.

Results

A genome-wide survey of the AP2/ERF superfamily was carried out to discover its evolution and function in longan. We identified 125 longan AP2/ERF genes and classified them into the ERF (101 members), AP2 (19 members), RAV (four members) families, and one Soloist. The AP2 and Soloist genes contained one to ten introns, whereas 87 genes in the ERF and RAV families had no introns. Hormone signaling molecules such as methyl jasmonate (MeJA), abscisic acid (ABA), gibberellin, auxin, and salicylic acid (SA), and stress response cis-acting element low-temperature (55) and defense (49) boxes also were identified. We detected diverse single nucleotide polymorphisms (SNPs) between the ‘Hong He Zi’ (HHZ) and ‘SI JI MI’ (SJM) cultivars. The number of insertions and deletions (InDels) was far fewer than SNPs. The AP2 family members exhibited more alternative splicing (AS) events in different developmental processes of longan than members of the other families. Expression pattern analysis revealed that some AP2/ERF members regulated early SE and developmental processes in longan seed, root, and flower, and responded to exogenous hormones such as MeJA, SA, and ABA, and 2,4-D, a synthetic auxin. Protein interaction predictions indicated that the Baby Boom (BBM) transcription factor, which was up-regulated at the transcriptional level in early SE, may interact with the LALF/AGL15 network.

Conclusions

The comprehensive analysis of molecular evolution and expression patterns suggested that the AP2/ERF superfamily may plays an important role in longan, especially in early SE, and in seed, root, flower, and young fruit. This systematic analysis provides a foundation for further functional characterization of the AP2/ERF superfamily with the aim of longan improvement.

Genome-Wide Analysis of Alternative Splicing Provides Insights Into Stress Response of the Pacific White Shrimp Litopenaeus vanname

Alternative splicing (AS) can enhance transcript diversity dramatically and play an important role in stress adaptation. Limited researches of AS have been reported in the Pacific white shrimp (Litopenaeus vannamei), which is an important aquaculture species in the world. Here, we performed a genome-wide identification of AS events in L. vannamei based on eight transcriptomes. We identified 38,781 AS events in the shrimp genome, and some of them were validated by polymerase chain reaction experiments. These AS events correspond to 9,209 genes, accounting for 36% of protein-coding genes in the shrimp genome. The number of AS events increased after virus or bacteria infection and low salinity stress. Type 1 AS genes (AS was initially activated) were mainly enriched in substance and energy metabolism, such as carbon metabolism and amino metabolism. However, type 2 AS genes (AS events changed) displayed specific enrichment under different stress challenges. Specifically, type 2 AS genes under biotic stresses were mainly enriched in the pathogenic pathway and immune network, and the AS genes under low salinity stress were significantly enriched for betalain biosynthesis. In summary, our study indicates that AS events are complex in shrimp and may be related to stress adaptation. These results will provide valuable resource for functional genomic studies on crustaceans.

Genome-wide identification and comparative analysis of alternative splicing across four legume species

MAIN CONCLUSION

Alternative splicing EVENTS were genome-wide identified for four legume species, and nitrogen fixation-related gene families and evolutionary analysis was also performed. Alternative splicing (AS) is a key regulatory mechanism that contributes to transcriptome and proteome diversity. Investigation of the genome-wide conserved AS events across different species will help with the understanding of the evolution of the functional diversity in legumes, allowing for genetic improvement. Genome-wide identification and characterization of AS were performed using the publically available mRNA, EST, and RNA-Seq data for four important legume species. A total of 15,165 AS genes in Glycine max, 6077 in Cicer arietinum, 7240 in Medicago truncatula, and 7358 in Lotus japonicus were identified. Intron retention (IntronR) was the dominant AS type among the identified events, with IntronR occurring from 53.76% in M. truncatula to 43.91% in C. arietinum. We identified 1159 AS genes that were conserved among four species. Furthermore, nine nitrogen fixation-related gene families with 237 genes were identified, and 80 of them were AS, accounting for the 43.48% in G. max and 27.78% in C. arietinum. An evolutionary analysis showed that these AS genes tended to be located adjacent to each other in the evolutionary tree and are unbalanced in the distribution in the sub-family. This study provides a foundation for future studies on transcription complexity, evolution, and the role of AS on plant functional regulation.

Differential Alternative Splicing Genes in Response to Boron Deficiency in Brassica napus

Alternative splicing (AS) can increase transcriptome diversity, protein diversity and protein yield, and is an important mechanism to regulate plant responses to stress. Oilseed rape (Brassica napus L.), one of the main oil crops in China, shows higher sensitivity to boron (B) deficiency than other species. Here, we demonstrated AS changes that largely increased the diversity of the mRNA expressed in response to B deficiency in B. napus. Each gene had two or more transcripts on average. A total of 33.3% genes in both Qingyou10 (QY10, B-efficient cultivar) and Westar10 (W10, B-inefficient cultivar) showed AS in both B conditions. The types of AS events were mainly intron retention, 3′ alternative splice site, 5′ alternative splice site and exon skipping. The tolerance ability of QY10 was higher than that of W10, possibly because there were far more differential alternative splicing (DAS) genes identified in QY10 at low B conditions than in W10. The number of genes with both DAS and differentially expressed (DE) was far lower than that of the genes that were either with DAS or DE in QY10 and W10, suggesting that the DAS and DE genes were independent. Four Serine/Arginine-rich (SR) splicing factors, BnaC06g14780D, BnaA01g14750D, BnaA06g15930D and BnaC01g41640D, underwent differentially alternative splicing in both cultivars. There existed gene–gene interactions between BnaC06g14780D and the genes associated with the function of B in oilseed rape at low B supply. This suggests that oilseed rape could regulate the alterative pre-mRNA splicing of SR protein related genes to increase the plant tolerance to B deficiency.

Computational analysis of alternative splicing in plant genomes

Computational analyses play crucial roles in characterizing splicing isoforms in plant genomes. In this review, we provide a survey of computational tools used in recently published, genome-scale splicing analyses in plants. We summarize the commonly used software and pipelines for read mapping, isoform reconstruction, isoform quantification, and differential expression analysis. We also discuss methods for analyzing long reads and the strategies to combine long and short reads in identifying splicing isoforms. We review several tools for characterizing local splicing events, splicing graphs, coding potential, and visualizing splicing isoforms. We further discuss the procedures for identifying conserved splicing isoforms across plant species. Finally, we discuss the outlook of integrating other genomic data with splicing analyses to identify regulatory mechanisms of AS on genome-wide scale.

Expanding Alternative Splicing Identification by Integrating Multiple Sources of Transcription Data in Tomato

Tomato (Solanum lycopersicum) is an important vegetable and fruit crop. Its genome was completely sequenced and there are also a large amount of available expressed sequence tags (ESTs) and short reads generated by RNA sequencing (RNA-seq) technologies. Mapping transcripts including mRNA sequences, ESTs, and RNA-seq reads to the genome allows identifying pre-mRNA alternative splicing (AS), a post-transcriptional process generating two or more RNA isoforms from one pre-mRNA transcript. We comprehensively analyzed the AS landscape in tomato by integrating genome mapping information of all available mRNA and ESTs with mapping information of RNA-seq reads which were collected from 27 published projects. A total of 369,911 AS events were identified from 34,419 genomic loci involving 161,913 transcripts. Within the basic AS events, intron retention is the prevalent type (18.9%), followed by alternative acceptor site (12.9%) and alternative donor site (7.3%), with exon skipping as the least type (6.0%). Complex AS types having two or more basic event accounted for 54.9% of total AS events. Within 35,768 annotated protein-coding gene models, 23,233 gene models were found having pre-mRNAs generating AS isoform transcripts. Thus the estimated AS rate was 65.0% in tomato. The list of identified AS genes with their corresponding transcript isoforms serves as a catalog for further detailed examination of gene functions in tomato biology. The post-transcriptional information is also expected to be useful in improving the predicted gene models in tomato. The sequence and annotation information can be accessed at plant alternative splicing database (http://proteomics.ysu.edu/altsplice).

Global dissection of alternative splicing uncovers transcriptional diversity in tissues and associates with the flavonoid pathway in tea plant (Camellia sinensis)

BACKGROUND

Alternative splicing (AS) regulates mRNA at the post-transcriptional level to change gene function in organisms. However, little is known about the AS and its roles in tea plant (Camellia sinensis), widely cultivated for making a popular beverage tea.

RESULTS

In our study, the AS landscape and dynamics were characterized in eight tissues (bud, young leaf, summer mature leaf, winter old leaf, stem, root, flower, fruit) of tea plant by Illumina RNA-Seq and confirmed by Iso-Seq. The most abundant AS (~ 20%) was intron retention and involved in RNA processes. The some alternative splicings were found to be tissue specific in stem and root etc. Thirteen co-expressed modules of AS transcripts were identified, which revealed a similar pattern between the bud and young leaves as well as a distinct pattern between seasons. AS events of structural genes including anthocyanidin reductase and MYB transcription factors were involved in biosynthesis of flavonoid, especially in vegetative tissues. The AS isoforms rather than the full-length ones were the major transcripts involved in flavonoid synthesis pathway, and is positively correlated with the catechins content conferring the tea taste. We propose that the AS is an important functional mechanism in regulating flavonoid metabolites.

CONCLUSION

Our study provides the insight into the AS events underlying tea plant's uniquely different developmental process and highlights the important contribution and efficacy of alternative splicing regulatory function to biosynthesis of flavonoids.

Identification and characterization of evolutionarily conserved alternative splicing events in a mangrove genus Sonneratia

Alternative splicing (AS), which produces multiple mRNA transcripts from a single gene, plays crucial roles in plant growth, development and environmental stress responses. Functional significances of conserved AS events among congeneric species have not been well characterized. In this study, we performed transcriptome sequencing to characterize AS events in four common species of Sonneratia, a mangrove genus excellently adaptive to intertidal zones. 7,248 to 12,623 AS events were identified in approximately 25% to 35% expressed genes in the roots of the four species. The frequency of AS events in Sonneratia was associated with genomic features, including gene expression level and intron/exon number and length. Among the four species, 1,355 evolutionarily conserved AS (ECAS) events were identified from 1,170 genes. Compared with non-ECAS events, ECAS events are of shorter length and less possibility to introduce premature stop codons (PTCs) and frameshifts. Functional annotations of the genes containing ECAS events showed that four of the 26 enriched Gene Ontology (GO) terms are involved in proton transport, signal transduction and carbon metabolism, and 60 genes from another three GO terms are implicated in responses to osmotic, oxidative and heat stresses, which may contribute to the adaptation of Sonneratia species to harsh intertidal environments.

SeagrassDB: An open-source transcriptomics landscape for phylogenetically profiled seagrasses and aquatic plants

Seagrasses and aquatic plants are important clades of higher plants, significant for carbon sequestration and marine ecological restoration. They are valuable in the sense that they allow us to understand how plants have developed traits to adapt to high salinity and photosynthetically challenged environments. Here, we present a large-scale phylogenetically profiled transcriptomics repository covering seagrasses and aquatic plants. SeagrassDB encompasses a total of 1,052,262 unigenes with a minimum and maximum contig length of 8,831 bp and 16,705 bp respectively. SeagrassDB provides access to 34,455 transcription factors, 470,568 PFAM domains, 382,528 prosite models and 482,121 InterPro domains across 9 species. SeagrassDB allows for the comparative gene mining using BLAST-based approaches and subsequent unigenes sequence retrieval with associated features such as expression (FPKM values), gene ontologies, functional assignments, family level classification, Interpro domains, KEGG orthology (KO), transcription factors and prosite information. SeagrassDB is available to the scientific community for exploring the functional genic landscape of seagrass and aquatic plants at: http://115.146.91.129/index.php.

Transgene Pyramiding of Salt Responsive Protein 3-1 (SaSRP3-1) and SaVHAc1 From Spartina alterniflora L. Enhances Salt Tolerance in Rice

The transgenic technology using a single gene has been widely used for crop improvement. But the transgenic pyramiding of multiple genes, a promising alternative especially for enhancing complexly inherited abiotic stress tolerance, has received little attention. Here, we developed and evaluated transgenic rice lines with a single Salt Responsive Protein 3-1 (SaSRP3-1) gene as well as pyramids with two-genes SaSRP3-1 and Vacuolar H⁺-ATPase subunit c1 (SaVHAc1) derived from a halophyte grass Spartina alterniflora L. for salt tolerance at seedling, vegetative, and reproductive stages. The overexpression of this novel gene SaSRP3-1 resulted in significantly better growth of E. coli with the recombinant plasmid under 600 mM NaCl stress condition compared with the control. During early seedling and vegetative stages, the single gene and pyramided transgenic rice plants showed enhanced tolerance to salt stress with minimal wilting and drying symptoms, improved shoot and root growth, and significantly higher chlorophyll content, relative water content, and K⁺/Na⁺ ratio than the control plants. The salt stress screening during reproductive stage revealed that the transgenic plants with single gene and pyramids had better grain filling, whereas the pyramided plants showed significantly higher grain yield and higher grain weight compared to control plants. Our study demonstrated transgenic pyramiding as a viable approach to achieve higher level of salt tolerance in crop plants.

Transcriptome analysis of Brachypodium during fungal pathogen infection reveals both shared and distinct defense responses with wheat

Fusarium crown rot (FCR) of wheat and barley, predominantly caused by the fungal pathogen Fusarium pseudograminearum, is a disease of economic significance. The quantitative nature of FCR resistance within cultivated wheat germplasm has significantly limited breeding efforts to enhanced FCR resistance in wheat. In this study, we characterized the molecular responses of Brachypodium distachyon (Brachypodium hereafter) to F. pseudograminearum infection using RNA-seq to determine whether Brachypodium can be exploited as a model system towards better understanding of F. pseudograminearum-wheat interaction. The transcriptional response to infection in Brachypodium was strikingly similar to that previously reported in wheat, both in shared expression patterns of wheat homologs of Brachypodium genes and functional overlap revealed through comparative gene ontology analysis in both species. Metabolites produced by various biosynthetic pathways induced in both wheat and Brachypodium were quantified, revealing a high degree of overlap between these two species in metabolic response to infection but also showed Brachypodium does not produce certain defence-related metabolites found in wheat. Functional analyses of candidate genes identified in this study will improve our understanding of resistance mechanisms and may lead to the development of new strategies to protect cereal crops from pathogen infection.

Domestication reduces alternative splicing expression variations in sorghum

Domestication is known to strongly reduce genomic diversity through population bottlenecks. The resulting loss of polymorphism has been thoroughly documented in numerous cultivated species. Here we investigate the impact of domestication on the diversity of alternative transcript expressions using RNAseq data obtained on cultivated and wild sorghum accessions (ten accessions for each pool). In that aim, we focus on genes expressing two isoforms in sorghum and estimate the ratio between expression levels of those isoforms in each accession. Noticeably, for a given gene, one isoform can either be overexpressed or underexpressed in some wild accessions, whereas in the cultivated accessions, the balance between the two isoforms of the same gene appears to be much more homogenous. Indeed, we observe in sorghum significantly more variation in isoform expression balance among wild accessions than among domesticated accessions. The possibility exists that the loss of nucleotide diversity due to domestication could affect regulatory elements, controlling transcription or degradation of these isoforms. Impact on the isoform expression balance is discussed. As far as we know, this is the first time that the impact of domestication on transcript isoform balance has been studied at the genomic scale. This could pave the way towards the identification of key domestication genes with finely tuned isoform expressions in domesticated accessions while being highly variable in their wild relatives.

Evolutionarily Conserved Alternative Splicing Across Monocots

One difficulty when identifying alternative splicing (AS) events in plants is distinguishing functional AS from splicing noise. One way to add confidence to the validity of a splice isoform is to observe that it is conserved across evolutionarily related species. We use a high throughput method to identify junction-based conserved AS events from RNA-Seq data across nine plant species, including five grass monocots (maize, sorghum, rice, Brachpodium, and foxtail millet), plus two nongrass monocots (banana and African oil palm), the eudicot Arabidopsis, and the basal angiosperm Amborella In total, 9804 AS events were found to be conserved between two or more species studied. In grasses containing large regions of conserved synteny, the frequency of conserved AS events is twice that observed for genes outside of conserved synteny blocks. In plant-specific RS and RS2Z subfamilies of the serine/arginine (SR) splice-factor proteins, we observe both conservation and divergence of AS events after the whole genome duplication in maize. In addition, plant-specific RS and RS2Z splice-factor subfamilies are highly connected with R2R3-MYB in STRING functional protein association networks built using genes exhibiting conserved AS. Furthermore, we discovered that functional protein association networks constructed around genes harboring conserved AS events are enriched for phosphatases, kinases, and ubiquitylation genes, which suggests that AS may participate in regulating signaling pathways. These data lay the foundation for identifying and studying conserved AS events in the monocots, particularly across grass species, and this conserved AS resource identifies an additional layer between genotype to phenotype that may impact future crop improvement efforts.

Unraveling the Transcriptional Basis of Temperature-Dependent Pinoxaden Resistance in Brachypodium hybridum

Climate change endangers food security and our ability to feed the ever-increasing human population. Weeds are the most important biotic stress, reducing crop-plant productivity worldwide. Chemical control, the main approach for weed management, can be strongly affected by temperature. Previously, we have shown that temperature-dependent non-target site (NTS) resistance of Brachypodium hybridum is due to enhanced detoxification of acetyl-CoA carboxylase inhibitors. Here, we explored the transcriptional basis of this phenomenon. Plants were characterized for the transcriptional response to herbicide application, high-temperature and their combination, in an attempt to uncover the genetic basis of temperature-dependent pinoxaden resistance. Even though most of the variance among treatments was due to pinoxaden application (61%), plants were able to survive pinoxaden application only when grown under high-temperatures. Biological pathways and expression patterns of members of specific gene families, previously shown to be involved in NTS metabolic resistance to different herbicides, were examined. Cytochrome P450, glucosyl transferase and glutathione-S-transferase genes were found to be up-regulated in response to pinoxaden application under both control and high-temperature conditions. However, biological pathways related to oxidation and glucose conjugation were found to be significantly enriched only under the combination of pinoxaden application and high-temperature. Analysis of reactive oxygen species (ROS) was conducted at several time points after treatment using a probe detecting H2O2/peroxides. Comparison of ROS accumulation among treatments revealed a significant reduction in ROS quantities 24 h after pinoxaden application only under high-temperature conditions. These results may indicate significant activity of enzymatic ROS scavengers that can be correlated with the activation of herbicide-resistance mechanisms. This study shows that up-regulation of genes related to metabolic resistance is not sufficient to explain temperature-dependent pinoxaden resistance. We suggest that elevated activity of enzymatic processes at high-temperature may induce rapid and efficient pinoxaden metabolism leading to temperature-dependent herbicide resistance.

Detecting alternatively spliced transcript isoforms from single-molecule long-read sequences without a reference genome

Alternative splicing (AS) is a major source of transcript and proteome diversity, but examining AS in species without well-annotated reference genomes remains difficult. Research on both human and mouse has demonstrated the advantages of using Iso-Seq™ data for isoform-level transcriptome analysis, including the study of AS and gene fusion. We applied Iso-Seq™ to investigate AS in Amborella trichopoda, a phylogenetically pivotal species that is sister to all other living angiosperms. Our data show that, compared with RNA-Seq data, the Iso-Seq™ platform provides better recovery on large transcripts, new gene locus identification and gene model correction. Reference-based AS detection with Iso-Seq™ data identifies AS within a higher fraction of multi-exonic genes than observed for published RNA-Seq analysis (45.8% vs. 37.5%). These data demonstrate that the Iso-Seq™ approach is useful for detecting AS events. Using the Iso-Seq-defined transcript collection in Amborella as a reference, we further describe a pipeline for detection of AS isoforms from PacBio Iso-Seq™ without using a reference sequence (de novo). Results using this pipeline show a 66%-76% overall success rate in identifying AS events. This de novoAS detection pipeline provides a method to accurately characterize and identify bona fide alternatively spliced transcripts in any nonmodel system that lacks a reference genome sequence. Hence, our pipeline has huge potential applications and benefits to the broader biology community.

The developmental transcriptome atlas of the biofuel crop Camelina sativa

Camelina sativa is currently being embraced as a viable industrial bio-platform crop due to a number of desirable agronomic attributes and the unique fatty acid profile of the seed oil that has applications for food, feed and biofuel. The recent completion of the reference genome sequence of C. sativa identified a young hexaploid genome. To complement this work, we have generated a genome-wide developmental transcriptome map by RNA sequencing of 12 different tissues covering major developmental stages during the life cycle of C. sativa. We have generated a digital atlas of this comprehensive transcriptome resource that enables interactive visualization of expression data through a searchable database of electronic fluorescent pictographs (eFP browser). An analysis of this dataset supported expression of 88% of the annotated genes in C. sativa and provided a global overview of the complex architecture of temporal and spatial gene expression patterns active during development. Conventional differential gene expression analysis combined with weighted gene expression network analysis uncovered similarities as well as differences in gene expression patterns between different tissues and identified tissue-specific genes and network modules. A high-quality census of transcription factors, analysis of alternative splicing and tissue-specific genome dominance provided insight into the transcriptional dynamics and sub-genome interplay among the well-preserved triplicated repertoire of homeologous loci. The comprehensive transcriptome atlas in combination with the reference genome sequence provides a powerful resource for genomics research which can be leveraged to identify functional associations between genes and understand the regulatory networks underlying developmental processes.

Genome-Wide Analysis of Alternative Splicing Provides Insights into Stress Adaptation of the Pacific Oyster

Alternative splicing (AS) is thought to enhance transcriptome diversity dramatically and play an important role in stress adaptation. While well studied in vertebrates, AS remains poorly understood in invertebrates. Here, we used high-throughput RNA-sequencing data to perform a genome-wide survey of AS in the Pacific oyster (Crassostrea gigas), an economically important mollusk that is cultivated worldwide. This analysis identified 8223 AS events corresponding to 4480 genes in the Pacific oyster, suggesting that about 16 % of oyster multiexonic genes undergo AS. We observed that a majority of the identified AS events were related to skipped exons (37.8 %). Then Gene Ontology analysis was conducted to analyze the function of the genes that undergo AS and the genes that produce more than five AS isoforms. After that, the expression of AS isoforms facing temperature, salinity, and air exposure challenge were examined. To validate our bioinformatic-predicted results and examine whether AS affects stress adaptation, we selected heat-shock protein 60 (HSP60) and HSP90 genes, both of which experience AS, for reverse transcription PCR (RT-PCR). We also performed quantitative real-time PCR (qRT-PCR) to determine the relative expression of each AS isoform among different stress adapted populations. Our study indicates that AS events are likely complex in the Pacific oyster and may be related to stress adaptation. These results will complement the predicted gene database of C. gigas and provide an invaluable resource for future functional genomic studies on molluscs.

Abscisic acid metabolic genes of wheat (Triticum aestivum L.): identification and insights into their functionality in seed dormancy and dehydration tolerance

The three homeologues of wheat NCED2 were identified; the wheat NCED2A and CYP707A1B affect seed ABA level and dormancy but not leaf ABA level and transpirational water loss in Arabidopsis. Biosynthesis and catabolism of abscisic acid (ABA) in plants are primarily regulated by 9-cis-epoxycarotenoid dioxygenases (NCEDs) and ABA 8'-hydroxylase (ABA8'OH), respectively. The present study identified the complete coding sequences of a second NCED gene, designated as TaNCED2, and its homeologues (TaNCED2A, TaNCED2B and TaNCED2D) in hexaploid wheat, and characterized its functionality in seed dormancy and leaf dehydration tolerance using the TaNCED2A homeologue. The study also investigated the role of the B genome copy of the cytochrome P450 monooxygenase 707A1 (CYP707A1) gene of hexaploid wheat (TaCYP707A1B), which encodes ABA8'OH, in regulating the two traits as this has not been studied before. Ectopic expression of TaNCED2A and TaCYP707A1B in Arabidopsis resulted in altered seed ABA level and dormancy with no effect on leaf ABA content and transpirational water loss. To gain insights into the physiological roles of TaNCED2 and TaCYP707A1 in wheat, the study examined their spatiotemporal expression patterns and determined the genomic contributions of transcripts to their total expression.

Evolutionary Character of Alternative Splicing in Plants

Alternative splicing (AS) is one of the most important ways to enhance the functional diversity of genes. Huge amounts of data have been produced by microarray, expressed sequence tag, and RNA-seq, and plenty of methods have been developed specifically for this task. The most frequently asked questions in previous research were as follows. What is the content rate of AS genes among the whole gene set? How many AS types are presented in the genome, and which type is dominant? How about the conservation ability of AS among different species? Which kinds of isoforms from some genes have the environmental response to help individual adaptation? Based on this background, we collected analysis results from 17 species to try to map out the landscape of AS studies in plants. We have noted the shortages of previous results, and we appeal to all scientists working in the AS field to make a standard protocol so that analyses between different projects are comparable.

Global Identification of the Full-Length Transcripts and Alternative Splicing Related to Phenolic Acid Biosynthetic Genes in Salvia miltiorrhiza

Salvianolic acids are among the main bioactive components in Salvia miltiorrhiza, and their biosynthesis has attracted widespread interest. However, previous studies on the biosynthesis of phenolic acids using next-generation sequencing platforms are limited with regard to the assembly of full-length transcripts. Based on hybrid-seq (next-generation and single molecular real-time sequencing) of the S. miltiorrhiza root transcriptome, we experimentally identified 15 full-length transcripts and four alternative splicing events of enzyme-coding genes involved in the biosynthesis of rosmarinic acid. Moreover, we herein demonstrate that lithospermic acid B accumulates in the phloem and xylem of roots, in agreement with the expression patterns of the identified key genes related to rosmarinic acid biosynthesis. According to co-expression patterns, we predicted that six candidate cytochrome P450s and five candidate laccases participate in the salvianolic acid pathway. Our results provide a valuable resource for further investigation into the synthetic biology of phenolic acids in S. miltiorrhiza.

Heat Stress Regulates the Expression of Genes at Transcriptional and Post-Transcriptional Levels, Revealed by RNA-seq in Brachypodium distachyon

Heat stress greatly affects plant growth/development and influences the output of crops. With the increased occurrence of extreme high temperature, the negative influence on cereal products from heat stress becomes severer and severer. It is urgent to reveal the molecular mechanism in response to heat stress in plants. In this research, we used RNA-seq technology to identify differentially expressed genes (DEGs) in leaves of seedlings, leaves and inflorescences at heading stage of Brachypodium distachyon, one model plant of grasses. Results showed many genes in responding to heat stress. Of them, the expression level of 656 DEGs were altered in three groups of samples treated with high temperature. Gene ontology (GO) analysis showed that the highly enriched DEGs were responsible for heat stress and protein folding. According to KEGG pathway analysis, the DEGs were related mainly to photosynthesis-antenna proteins, the endoplasmic reticulum, and the spliceosome. Additionally, the expression level of 454 transcription factors belonging to 49 gene families was altered, as well as 1,973 splicing events occurred after treatment with high temperature. This research lays a foundation for characterizing the molecular mechanism of heat stress response and identifying key genes for those responses in plants. These findings also clearly show that heat stress regulates the expression of genes not only at transcriptional level, but also at post-transcriptional level.

Genome-wide cataloging and analysis of alternatively spliced genes in cereal crops

Background

Protein functional diversity at the post-transcriptional level is regulated through spliceosome mediated pre-mRNA alternative splicing (AS) events and that has been widely demonstrated to be a key player in regulating the functional diversity in plants. Identification and analysis of AS genes in cereal crop plants are critical for crop improvement and understanding regulatory mechanisms.

Results

We carried out the comparative analyses of the functional landscapes of the AS using the consensus assembly of expressed sequence tags and available mRNA sequences in four cereal plants. We identified a total of 8,734 in Oryza sativa subspecies (ssp) japonica, 2,657 in O. sativa ssp indica, 3,971 in Sorghum bicolor, and 10,687 in Zea mays AS genes. Among the identified AS events, intron retention remains to be the dominant type accounting for 23.5 % in S. bicolor, and up to 55.8 % in O. sativa ssp indica. We identified a total of 887 AS genes that were conserved among Z. mays, S. bicolor, and O. sativa ssp japonica; and 248 AS genes were found to be conserved among all four studied species or ssp. Furthermore, we identified 53 AS genes conserved with Brachypodium distachyon. Gene Ontology classification of AS genes revealed functional assignment of these genes in many biological processes with diverse molecular functions.

Conclusions

AS is common in cereal plants. The AS genes identified in four cereal crops in this work provide the foundation for further studying the roles of AS in regulation of cereal plant growth and development. The data can be accessed at Plant Alternative Splicing Database (http://proteomics.ysu.edu/altsplice/).

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1914-5) contains supplementary material, which is available to authorized users.

Nonadditive gene expression in polyploids

Allopolyploidy involves hybridization and duplication of divergent parental genomes and provides new avenues for gene expression. The expression levels of duplicated genes in polyploids can show deviation from parental additivity (the arithmetic average of the parental expression levels). Nonadditive expression has been widely observed in diverse polyploids and comprises at least three possible scenarios: (a) The total gene expression level in a polyploid is similar to that of one of its parents (expression-level dominance); (b) total gene expression is lower or higher than in both parents (transgressive expression); and (c) the relative contribution of the parental copies (homeologs) to the total gene expression is unequal (homeolog expression bias). Several factors may result in expression nonadditivity in polyploids, including maternal-paternal influence, gene dosage balance, cis- and/or trans-regulatory networks, and epigenetic regulation. As our understanding of nonadditive gene expression in polyploids remains limited, a new generation of investigators should explore additional phenomena (i.e., alternative splicing) and use other high-throughput "omics" technologies to measure the impact of nonadditive expression on phenotype, proteome, and metabolome.

Full-length transcriptome sequences and splice variants obtained by a combination of sequencing platforms applied to different root tissues of Salvia miltiorrhiza and tanshinone biosynthesis

Danshen, Salvia miltiorrhiza Bunge, is one of the most widely used herbs in traditional Chinese medicine, wherein its rhizome/roots are particularly valued. The corresponding bioactive components include the tanshinone diterpenoids, the biosynthesis of which is a subject of considerable interest. Previous investigations of the S. miltiorrhiza transcriptome have relied on short-read next-generation sequencing (NGS) technology, and the vast majority of the resulting isotigs do not represent full-length cDNA sequences. Moreover, these efforts have been targeted at either whole plants or hairy root cultures. Here, we demonstrate that the tanshinone pigments are produced and accumulate in the root periderm, and apply a combination of NGS and single-molecule real-time (SMRT) sequencing to various root tissues, particularly including the periderm, to provide a more complete view of the S. miltiorrhiza transcriptome, with further insight into tanshinone biosynthesis as well. In addition, the use of SMRT long-read sequencing offered the ability to examine alternative splicing, which was found to occur in approximately 40% of the detected gene loci, including several involved in isoprenoid/terpenoid metabolism.

nagnag: Identification and quantification of NAGNAG alternative splicing using RNA-Seq data

Regulation of proteome diversity by alternative splicing has been widely demonstrated in plants and animals. NAGNAG splicing, which was recently defined as a tissue specific event, results in the production of two distinct isoforms that are distinguished by three nucleotides (NAG) as a consequence of the intron proximal or distal to the splice site. Since the NAGNAG mechanism is not well characterized, tools for the identification and quantification of NAGNAG splicing events remain under-developed. Here we report nagnag, an R-based NAGNAG splicing detection tool, which accurately identifies and quantifies NAGNAG splicing events using RNA-Seq. Overall, nagnag produces user-friendly visualization reports and highlights differences between the DNA/RNA/protein across the identified isoforms of the reported gene. The package is available on https://sourceforge.net/projects/nagnag/files/; or http://genome.sdau.edu.cn/research/software/nagnag.html.

Comparative transcriptomics uncovers alternative splicing changes and signatures of selection from maize improvement

Background

Alternative splicing (AS) is an important regulatory mechanism that greatly contributes to eukaryotic transcriptome diversity. A substantial amount of evidence has demonstrated that AS complexity is relevant to eukaryotic evolution, development, adaptation, and complexity. In this study, six teosinte and ten maize transcriptomes were sequenced to analyze AS changes and signatures of selection in maize domestication and improvement.

Results

In maize and teosinte, 13,593 highly conserved genes, including 12,030 multiexonic genes, were detected. By identifying AS isoforms from mutliexonic genes, we found that AS types were not significantly different between maize and teosinte. In addition, the two main AS types (intron retention and alternative acceptor) contributed to more than 60% of the AS events in the two species, but the average unique AS events per each alternatively spliced gene in maize (4.12) was higher than that in teosinte (2.26). Moreover, 94 genes generating 98 retained introns with transposable element (TE) sequences were detected in maize, which is far more than 9 retained introns with TEs detected in teosinte. This indicates that TE insertion might be an important mechanism for intron retention in maize. Additionally, the AS levels of 3864 genes were significantly different between maize and teosinte. Of these, 151 AS level-altered genes that are involved in transcriptional regulation and in stress responses are located in regions that have been targets of selection during maize improvement. These genes were inferred to be putatively improved genes.

Conclusions

We suggest that both maize and teosinte share similar AS mechanisms, but more genes have increased AS complexity during domestication from teosinte to maize. Importantly, a subset of AS level-increased genes that encode transcription factors and stress-responsive proteins may have been selected during maize improvement.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1582-5) contains supplementary material, which is available to authorized users.

SplicingTypesAnno: annotating and quantifying alternative splicing events for RNA-Seq data

Alternative splicing plays a key role in the regulation of the central dogma. Four major types of alternative splicing have been classified as intron retention, exon skipping, alternative 5 splice sites or alternative donor sites, and alternative 3 splice sites or alternative acceptor sites. A few algorithms have been developed to detect splice junctions from RNA-Seq reads. However, there are few tools targeting at the major alternative splicing types at the exon/intron level. This type of analysis may reveal subtle, yet important events of alternative splicing, and thus help gain deeper understanding of the mechanism of alternative splicing. This paper describes a user-friendly R package, extracting, annotating and analyzing alternative splicing types for sequence alignment files from RNA-Seq. SplicingTypesAnno can: (1) provide annotation for major alternative splicing at exon/intron level. By comparing the annotation from GTF/GFF file, it identifies the novel alternative splicing sites; (2) offer a convenient two-level analysis: genome-scale annotation for users with high performance computing environment, and gene-scale annotation for users with personal computers; (3) generate a user-friendly web report and additional BED files for IGV visualization. SplicingTypesAnno is a user-friendly R package for extracting, annotating and analyzing alternative splicing types at exon/intron level for sequence alignment files from RNA-Seq. It is publically available at https://sourceforge.net/projects/splicingtypes/files/ or http://genome.sdau.edu.cn/research/software/SplicingTypesAnno.html.

Genome-wide evolutionary characterization and analysis of bZIP transcription factors and their expression profiles in response to multiple abiotic stresses in Brachypodium distachyon

BACKGROUND

Plant basic leucine zipper (bZIP) transcription factors are one of the largest and most diverse gene families and play key roles in regulating diverse stress processes. Brachypodium distachyon is emerging as a widely recognized model plant for the temperate grass family and the herbaceous energy crops, however there is no comprehensive analysis of bZIPs in B. distachyon, especially those involved in stress tolerances.

RESULTS

In this study, 96 bZIP genes (BdbZIPs) were identified distributing unevenly on each chromosome of B. distachyon, and most of them were scattered in the low CpG content regions. Gene duplications were widespread throughout B. distachyon genome. Evolutionary comparisons suggested B. distachyon and rice's bZIPs had the similar evolutionary patterns. The exon splicing in BdbZIP motifs were more complex and diverse than those in other plant species. We further revealed the potential close relationships between BdbZIP gene expressions and items including gene structure, exon splicing pattern and dimerization features. In addition, multiple stresses expression profile demonstrated that BdbZIPs exhibited significant expression patterns responding to 14 stresses, and those responding to heavy metal treatments showed opposite expression pattern comparing to the treatments of environmental factors and phytohormones. We also screened certain up- and down-regulated BdbZIP genes with fold changes ≥2, which were more sensitive to abiotic stress conditions.

CONCLUSIONS

BdbZIP genes behaved diverse functional characters and showed discrepant and some regular expression patterns in response to abiotic stresses. Comprehensive analysis indicated these BdbZIPs' expressions were associated not only with gene structure, exon splicing pattern and dimerization feature, but also with abiotic stress treatments. It is possible that our findings are crucial for revealing the potentialities of utilizing these candidate BdbZIPs to improve productivity of grass plants and cereal crops.

Genome-wide evolutionary characterization and analysis of bZIP transcription factors and their expression profiles in response to multiple abiotic stresses in Brachypodium distachyon

BACKGROUND

Plant basic leucine zipper (bZIP) transcription factors are one of the largest and most diverse gene families and play key roles in regulating diverse stress processes. Brachypodium distachyon is emerging as a widely recognized model plant for the temperate grass family and the herbaceous energy crops, however there is no comprehensive analysis of bZIPs in B. distachyon, especially those involved in stress tolerances.

RESULTS

In this study, 96 bZIP genes (BdbZIPs) were identified distributing unevenly on each chromosome of B. distachyon, and most of them were scattered in the low CpG content regions. Gene duplications were widespread throughout B. distachyon genome. Evolutionary comparisons suggested B. distachyon and rice's bZIPs had the similar evolutionary patterns. The exon splicing in BdbZIP motifs were more complex and diverse than those in other plant species. We further revealed the potential close relationships between BdbZIP gene expressions and items including gene structure, exon splicing pattern and dimerization features. In addition, multiple stresses expression profile demonstrated that BdbZIPs exhibited significant expression patterns responding to 14 stresses, and those responding to heavy metal treatments showed opposite expression pattern comparing to the treatments of environmental factors and phytohormones. We also screened certain up- and down-regulated BdbZIP genes with fold changes ≥2, which were more sensitive to abiotic stress conditions.

CONCLUSIONS

BdbZIP genes behaved diverse functional characters and showed discrepant and some regular expression patterns in response to abiotic stresses. Comprehensive analysis indicated these BdbZIPs' expressions were associated not only with gene structure, exon splicing pattern and dimerization feature, but also with abiotic stress treatments. It is possible that our findings are crucial for revealing the potentialities of utilizing these candidate BdbZIPs to improve productivity of grass plants and cereal crops.

Differential gene expression and alternative splicing between diploid and tetraploid watermelon

The exploitation of synthetic polyploids for producing seedless fruits is well known in watermelon. Tetraploid progenitors of triploid watermelon plants, compared with their diploid counterparts, exhibit wide phenotypic differences. Although many factors modulate alternative splicing (AS) in plants, the effects of autopolyploidization on AS are still unknown. In this study, we used tissues of leaf, stem, and fruit of diploid and tetraploid sweet watermelon to understand changes in gene expression and the occurrence of AS. RNA-sequencing analysis was performed along with reverse transcription quantitative PCR and rapid amplification of cDNA ends (RACE)-PCR to demonstrate changes in expression and splicing. All vegetative tissues except fruit showed an increased level of AS in the tetraploid watermelon throughout the growth period. The ploidy levels of diploids and the tetraploid were confirmed using a ploidy analyser. We identified 5362 and 1288 genes that were up- and downregulated, respectively, in tetraploid as compared with diploid plants. We further confirmed that 22 genes underwent AS events across tissues, indicating possibilities of generating different protein isoforms with altered functions of important transcription factors and transporters. Arginine biosynthesis, chlorophyllide synthesis, GDP mannose biosynthesis, trehalose biosynthesis, and starch and sucrose degradation pathways were upregulated in autotetraploids. Phloem protein 2, chloroplastic PGR5-like protein, zinc-finger protein, fructokinase-like 2, MYB transcription factor, and nodulin MtN21 showed AS in fruit tissues. These results should help in developing high-quality seedless watermelon and provide additional transcriptomic information related to other cucurbits.

Deep transcriptome sequencing provides new insights into the structural and functional organization of the wheat genome

Background

Because of its size, allohexaploid nature, and high repeat content, the bread wheat genome is a good model to study the impact of the genome structure on gene organization, function, and regulation. However, because of the lack of a reference genome sequence, such studies have long been hampered and our knowledge of the wheat gene space is still limited. The access to the reference sequence of the wheat chromosome 3B provided us with an opportunity to study the wheat transcriptome and its relationships to genome and gene structure at a level that has never been reached before.

Results

By combining this sequence with RNA-seq data, we construct a fine transcriptome map of the chromosome 3B. More than 8,800 transcription sites are identified, that are distributed throughout the entire chromosome. Expression level, expression breadth, alternative splicing as well as several structural features of genes, including transcript length, number of exons, and cumulative intron length are investigated. Our analysis reveals a non-monotonic relationship between gene expression and structure and leads to the hypothesis that gene structure is determined by its function, whereas gene expression is subject to energetic cost. Moreover, we observe a recombination-based partitioning at the gene structure and function level.

Conclusions

Our analysis provides new insights into the relationships between gene and genome structure and function. It reveals mechanisms conserved with other plant species as well as superimposed evolutionary forces that shaped the wheat gene space, likely participating in wheat adaptation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0601-9) contains supplementary material, which is available to authorized users.

Stress-induced endogenous siRNAs targeting regulatory intron sequences in Brachypodium

Exposure to abiotic stresses triggers global changes in the expression of thousands of eukaryotic genes at the transcriptional and post-transcriptional levels. Small RNA (smRNA) pathways and splicing both function as crucial mechanisms regulating stress-responsive gene expression. However, examples of smRNAs regulating gene expression remain largely limited to effects on mRNA stability, translation, and epigenetic regulation. Also, our understanding of the networks controlling plant gene expression in response to environmental changes, and examples of these regulatory pathways intersecting, remains limited. Here, to investigate the role of smRNAs in stress responses we examined smRNA transcriptomes of Brachypodium distachyon plants subjected to various abiotic stresses. We found that exposure to different abiotic stresses specifically induced a group of novel, endogenous small interfering RNAs (stress-induced, UTR-derived siRNAs, or sutr-siRNAs) that originate from the 3' UTRs of a subset of coding genes. Our bioinformatics analyses predicted that sutr-siRNAs have potential regulatory functions and that over 90% of sutr-siRNAs target intronic regions of many mRNAs in trans. Importantly, a subgroup of these sutr-siRNAs target the important intron regulatory regions, such as branch point sequences, that could affect splicing. Our study indicates that in Brachypodium, sutr-siRNAs may affect splicing by masking or changing accessibility of specific cis-elements through base-pairing interactions to mediate gene expression in response to stresses. We hypothesize that this mode of regulation of gene expression may also serve as a general mechanism for regulation of gene expression in plants and potentially in other eukaryotes.

Mutational bias is the driving force for shaping the synonymous codon usage pattern of alternatively spliced genes in rice (Oryza sativa L.)

Alternative splicing plays important roles in diverse aspects of plant development, metabolism, and stress responses. However, the regulatory mechanisms of alternative splicing of genes still remain incompletely elucidated, especially in plants. In this study, the synonymous codon usage pattern of alternatively spliced (AS) genes in rice was firstly explored using the combination of correspondence analysis (CA), internal CA, correlation and ANOVA analyses. The results show that alternatively and non-alternatively spliced (non-AS) genes have similar tendency for overall codon usage, but exhibit significant difference in 58 out of 64 codons. AS and non-AS genes are both under strong purifying selection, but the former ones have significant lower mutation rate and are prone to be enriched towards the chromosomal ends. In the group of AS genes, the variability in synonymous codon usage between genes is mainly due to the variations in GC content, CDS length, as well as gene functions. Mutational bias that accounts for 25.85 % of the total codon usage variability plays a major role in shaping the codon usage pattern of AS genes. In contrast, no obvious evidence is found for the contributions of translational selection, AS types, the conservation of AS events, and numbers of AS variants to the codon usage divergence between AS genes. These findings may be useful for further understanding the mechanisms of origination, differentiation and regulation of alternatively spliced genes in plants.

Genome-wide survey of Alternative Splicing in Sorghum Bicolor

Sorghum bicolor is a member of grass family which is an attractive model plant for genome study due to interesting genome features like low genome size. In this research, we performed comprehensive investigation of Alternative Splicing and ontology aspects of genes those have undergone these events in sorghum bicolor. We used homology based alignments between gene rich transcripts, represented by tentative consensus (TC) transcript sequences, and genomic scaffolds to deduce the structure of genes and identify alternatively spliced transcripts in sorghum. Using homology mapping of assembled expressed sequence tags with genomics data, we identified 2,137 Alternative Splicing events in S. bicolor. Our study showed that complex events and intron retention are the main types of Alternative Splicing events in S. bicolor and highlights the prevalence of splicing site recognition for definition of introns in this plant. Annotations of the alternatively spliced genes revealed that they represent diverse biological process and molecular functions, suggesting a fundamental role for Alternative Splicing in affecting the development and physiology of S. bicolor.

Deep transcriptome sequencing of rhizome and aerial-shoot in Sorghum propinquum

Transcriptomic data for Sorghum propinquum, the wild-type sorghum, are limited in public databases. S. propinquum has a subterranean rhizome and transcriptome data will help in understanding the molecular mechanisms underlying rhizome formation. We sequenced the transcriptome of S. propinquum aerial-shoot and rhizome using an Illumina platform. More than 70 % of the genes in the S. propinquum genome were expressed in aerial-shoot and rhizome. The expression patterns of 1963 and 599 genes, including transcription factors, were specific or enriched in aerial-shoot and rhizome respectively, indicating their possible roles in physiological processes in these tissues. Comparative analysis revealed several cis-elements, ACGT box, GCCAC, GATC and TGACG box, which showed significantly higher abundance in aerial-shoot-specific genes. In rhizome-specific genes MYB and ROOTMOTIFTAPOX1 motifs, and 10 promoter and cytokinin-responsive elements were highly enriched. Of the S. propinquum genes, 27.9 % were identified as alternatively spliced and about 60 % of the alternative splicing (AS) events were tissue-specific, suggesting that AS played a crucial role in determining tissue-specific cellular function. The transcriptome data, especially the co-localized rhizome-enriched expressed transcripts that mapped to the publicly available rhizome-related quantitative trait loci, will contribute to gene discovery in S. propinquum and to functional studies of the sorghum genome. Deep transcriptome sequencing revealed a clear difference in the expression patterns of genes between aerial-shoot and rhizome in S. propinquum. This data set provides essential information for future studies into the molecular genetic mechanisms involved in rhizome formation.

Comprehensive analysis of RNA-seq data reveals the complexity of the transcriptome in Brassica rapa

Background

The species Brassica rapa (2n=20, AA) is an important vegetable and oilseed crop, and serves as an excellent model for genomic and evolutionary research in Brassica species. With the availability of whole genome sequence of B. rapa, it is essential to further determine the activity of all functional elements of the B. rapa genome and explore the transcriptome on a genome-wide scale. Here, RNA-seq data was employed to provide a genome-wide transcriptional landscape and characterization of the annotated and novel transcripts and alternative splicing events across tissues.

Results

RNA-seq reads were generated using the Illumina platform from six different tissues (root, stem, leaf, flower, silique and callus) of the B. rapa accession Chiifu-401-42, the same line used for whole genome sequencing. First, these data detected the widespread transcription of the B. rapa genome, leading to the identification of numerous novel transcripts and definition of 5'/3' UTRs of known genes. Second, 78.8% of the total annotated genes were detected as expressed and 45.8% were constitutively expressed across all tissues. We further defined several groups of genes: housekeeping genes, tissue-specific expressed genes and co-expressed genes across tissues, which will serve as a valuable repository for future crop functional genomics research. Third, alternative splicing (AS) is estimated to occur in more than 29.4% of intron-containing B. rapa genes, and 65% of them were commonly detected in more than two tissues. Interestingly, genes with high rate of AS were over-represented in GO categories relating to transcriptional regulation and signal transduction, suggesting potential importance of AS for playing regulatory role in these genes. Further, we observed that intron retention (IR) is predominant in the AS events and seems to preferentially occurred in genes with short introns.

Conclusions

The high-resolution RNA-seq analysis provides a global transcriptional landscape as a complement to the B. rapa genome sequence, which will advance our understanding of the dynamics and complexity of the B. rapa transcriptome. The atlas of gene expression in different tissues will be useful for accelerating research on functional genomics and genome evolution in Brassica species.