Identification of alternative splicing events by RNA sequencing in early growth tomato fruits

RNA-seq analysis reveals transcriptome reprogramming and alternative splicing during early response to salt stress in tomato root

Salt stress is one of the dominant abiotic stress conditions that cause severe damage to plant growth and, in turn, limiting crop productivity. It is therefore crucial to understand the molecular mechanism underlying plant root responses to high salinity as such knowledge will aid in efforts to develop salt-tolerant crops. Alternative splicing (AS) of precursor RNA is one of the important RNA processing steps that regulate gene expression and proteome diversity, and, consequently, many physiological and biochemical processes in plants, including responses to abiotic stresses like salt stress. In the current study, we utilized high-throughput RNA-sequencing to analyze the changes in the transcriptome and characterize AS landscape during the early response of tomato root to salt stress. Under salt stress conditions, 10,588 genes were found to be differentially expressed, including those involved in hormone signaling transduction, amino acid metabolism, and cell cycle regulation. More than 700 transcription factors (TFs), including members of the MYB, bHLH, and WRKY families, potentially regulated tomato root response to salt stress. AS events were found to be greatly enhanced under salt stress, where exon skipping was the most prevalent event. There were 3709 genes identified as differentially alternatively spliced (DAS), the most prominent of which were serine/threonine protein kinase, pentatricopeptide repeat (PPR)-containing protein, E3 ubiquitin-protein ligase. More than 100 DEGs were implicated in splicing and spliceosome assembly, which may regulate salt-responsive AS events in tomato roots. This study uncovers the stimulation of AS during tomato root response to salt stress and provides a valuable resource of salt-responsive genes for future studies to improve tomato salt tolerance.

Transcriptomic Analysis of Alternative Splicing Events during Different Fruit Ripening Stages of Coffea arabica L

To date, genomic and transcriptomic data on Coffea arabica L. in public databases are very limited, and there has been no comprehensive integrated investigation conducted on alternative splicing (AS). Previously, we have constructed and sequenced eighteen RNA-seq libraries of C. arabica at different ripening stages of fruit development. From this dataset, a total of 3824, 2445, 2564, 2990, and 3162 DSGs were identified in a comparison of different fruit ripening stages. The largest proportion of DSGs, approximately 65%, were of the skipped exon (SE) type. Biologically, 9 and 29 differentially expressed DSGs in the spliceosome pathway and carbon metabolism pathway, respectively, were identified. These DSGs exhibited significant variations, primarily in S1 vs. S2 and S5 vs. S6, and they involve many aspects of organ development, hormone transduction, and the synthesis of flavor components. Through the examination of research findings regarding the biological functions and biochemical pathways associated with DSGs and DEGs, it was observed that six DSGs significantly enriched in ABC transporters, namely, LOC113712394, LOC113726618, LOC113739972, LOC113725240, LOC113730214, and LOC113707447, were continually down-regulated at the fruit ripening stage. In contrast, a total of four genes, which were LOC113732777, LOC113727880, LOC113690566, and LOC113711936, including those enriched in the cysteine and methionine metabolism, were continually up-regulated. Collectively, our findings may contribute to the exploration of alternative splicing mechanisms for focused investigations of potential genes associated with the ripening of fruits in C. arabica.

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.

Alternative Splicing Variation: Accessing and Exploiting in Crop Improvement Programs

Alternative splicing (AS) is a gene regulatory mechanism modulating gene expression in multiple ways. AS is prevalent in all eukaryotes including plants. AS generates two or more mRNAs from the precursor mRNA (pre-mRNA) to regulate transcriptome complexity and proteome diversity. Advances in next-generation sequencing, omics technology, bioinformatics tools, and computational methods provide new opportunities to quantify and visualize AS-based quantitative trait variation associated with plant growth, development, reproduction, and stress tolerance. Domestication, polyploidization, and environmental perturbation may evolve novel splicing variants associated with agronomically beneficial traits. To date, pre-mRNAs from many genes are spliced into multiple transcripts that cause phenotypic variation for complex traits, both in model plant Arabidopsis and field crops. Cataloguing and exploiting such variation may provide new paths to enhance climate resilience, resource-use efficiency, productivity, and nutritional quality of staple food crops. This review provides insights into AS variation alongside a gene expression analysis to select for novel phenotypic diversity for use in breeding programs. AS contributes to heterosis, enhances plant symbiosis (mycorrhiza and rhizobium), and provides a mechanistic link between the core clock genes and diverse environmental clues.

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.

Genes Associated with Biological Nitrogen Fixation Efficiency Identified Using RNA Sequencing in Red Clover (Trifolium pratense L.)

Commonly studied in the context of legume-rhizobia symbiosis, biological nitrogen fixation (BNF) is a key component of the nitrogen cycle in nature. Despite its potential in plant breeding and many years of research, information is still lacking as to the regulation of hundreds of genes connected with plant-bacteria interaction, nodulation, and nitrogen fixation. Here, we compared root nodule transcriptomes of red clover (Trifolium pratense L.) genotypes with contrasting nitrogen fixation efficiency, and we found 491 differentially expressed genes (DEGs) between plants with high and low BNF efficiency. The annotation of genes expressed in nodules revealed more than 800 genes not yet experimentally confirmed. Among genes mediating nodule development, four nod-ule-specific cysteine-rich (NCR) peptides were confirmed in the nodule transcriptome. Gene duplication analyses revealed that genes originating from tandem and dispersed duplication are significantly over-represented among DEGs. Weighted correlation network analysis (WGCNA) organized expression profiles of the transcripts into 16 modules linked to the analyzed traits, such as nitrogen fixation efficiency or sample-specific modules. Overall, the results obtained broaden our knowledge about transcriptomic landscapes of red clover's root nodules and shift the phenotypic description of BNF efficiency on the level of gene expression in situ.

Transcriptomic and splicing changes underlying tomato responses to combined water and nutrient stress

Tomato is a horticultural crop of high economic and nutritional value. Suboptimal environmental conditions, such as limited water and nutrient availability, cause severe yield reductions. Thus, selection of genotypes requiring lower inputs is a goal for the tomato breeding sector. We screened 10 tomato varieties exposed to water deficit, low nitrate or a combination of both. Biometric, physiological and molecular analyses revealed different stress responses among genotypes, identifying T270 as severely affected, and T250 as tolerant to the stresses applied. Investigation of transcriptome changes caused by combined stress in roots and leaves of these two genotypes yielded a low number of differentially expressed genes (DEGs) in T250 compared to T270, suggesting that T250 tailors changes in gene expression to efficiently respond to combined stress. By contrast, the susceptible tomato activated approximately one thousand and two thousand genes in leaves and roots respectively, indicating a more generalized stress response in this genotype. In particular, developmental and stress-related genes were differentially expressed, such as hormone responsive factors and transcription factors. Analysis of differential alternative splicing (DAS) events showed that combined stress greatly affects the splicing landscape in both genotypes, highlighting the important role of AS in stress response mechanisms. In particular, several stress and growth-related genes as well as transcription and splicing factors were differentially spliced in both tissues. Taken together, these results reveal important insights into the transcriptional and post-transcriptional mechanisms regulating tomato adaptation to growth under reduced water and nitrogen inputs.

Comparative Analysis of Environment-Responsive Alternative Splicing in the Inflorescences of Cultivated and Wild Tomato Species

Cultivated tomato (Solanum lycopersicum) is bred for fruit production in optimized environments, in contrast to harsh environments where their ancestral relatives thrive. The process of domestication and breeding has profound impacts on the phenotypic plasticity of plant development and the stress response. Notably, the alternative splicing (AS) of precursor message RNA (pre-mRNA), which is one of the major factors contributing to transcriptome complexity, is responsive to developmental cues and environmental change. To determine a possible association between AS events and phenotypic plasticity, we investigated environment-responsive AS events in the inflorescences of cultivated tomato and its ancestral relatives S. pimpinellifolium. Despite that similar AS frequencies were detected in the cultivated tomato variety Moneymaker and two S. pimpinellifolium accessions under the same growth conditions, 528 genes including splicing factors showed differential splicing in the inflorescences of plants grown in open fields and plastic greenhouses in the Moneymaker variety. In contrast, the two S. pimpinellifolium accessions, LA1589 and LA1781, had 298 and 268 genes showing differential splicing, respectively. Moreover, seven heat responsive genes showed opposite expression patterns in response to changing growth conditions between Moneymaker and its ancestral relatives. Accordingly, there were eight differentially expressed splice variants from genes involved in heat response in Moneymaker. Our results reveal distinctive features of AS events in the inflorescences between cultivated tomato and its ancestral relatives, and show that AS regulation in response to environmental changes is genotype dependent.

Integrated omics analysis identified genes and their splice variants involved in fruit development and metabolites production in Capsicum species

To date, several transcriptomic studies during fruit development have been reported; however, no comprehensive integrated study on expression diversity, alternative splicing, and metabolomic profiling was reported in Capsicum. This study analyzed RNA-seq data and untargeted metabolomic profiling from early green (EG), mature green (MG), and breaker (Br) fruit stages from two Capsicum species, i.e., C. annuum (Cann) and C. frutescens (Cfrut) from Northeast India. A total of 117,416 and 96,802 alternatively spliced events (AltSpli-events) were identified from Cann and Cfrut, respectively. Among AltSpli-events, intron retention (IR; 32.2% Cann and 25.75% Cfrut) followed by alternative acceptor (AA; 15.4% Cann and 18.9% Cfrut) were the most abundant in Capsicum. Around 7600 genes expressed in at least one fruit stage of Cann and Cfrut were AltSpli. The study identified spliced variants of genes including transcription factors (TFs) potentially involved in fruit development/ripening (Aux/IAA 16-like, ETR, SGR1, ARF, CaGLK2, ETR, CaAGL1, MADS-RIN, FUL1, SEPALLATA1), carotenoid (PDS, CA1, CCD4, NCED3, xanthoxin dehydrogenase, CaERF82, CabHLH100, CaMYB3R-1, SGR1, CaWRKY28, CaWRKY48, CaWRKY54), and capsaicinoids or flavonoid biosynthesis (CaMYB48, CaWRKY51), which were significantly differentially spliced (DS) between consecutive Capsicum fruit stages. Also, this study observed that differentially expressed isoforms (DEiso) from 38 genes with differentially spliced events (DSE) were significantly enriched in various metabolic pathways such as starch and sucrose metabolism, amino acid metabolism, cysteine cutin suberin and wax biosynthesis, and carotenoid biosynthesis. Furthermore, the metabolomic profiling revealed that metabolites from aforementioned pathways such as carbohydrates (mainly sugars such as D-fructose, D-galactose, maltose, and sucrose), organic acids (carboxylic acids), and peptide groups significantly altered during fruit development. Taken together, our findings could help in alternative splicing-based targeted studies of candidate genes involved in fruit development and ripening in Capsicum crop.

Transcriptome Analysis Explored the Differential Genes’ Expression During the Development of the Stropharia rugosoannulata Fruiting Body

Stropharia rugosoannulata (S. rugosoannulata) is a fungus with great edible and nutritional values; however, the development mechanism of its fruiting body has not been studied. Thus, this study aimed to analyze the differentially expressed genes (DEGs) in four stages; primordia stage (Sra1), young mushroom stage (Sra2), picking stage (Sra3), and opening umbrella stage (Sra4). Therefore, total RNA was extracted for further RNA-sequencing analysis. In three pairwise comparison groups (PCGs), Sra1 vs. Sra2, Sra2 vs. Sra3, and Sra3 vs. Sra4, a total of 3,112 DEGs were identified among the three PCGs. A GO analysis of the DEGs showed that there were 21 terms significantly enriched in Sra1 vs. Sra2 PCG. There was no significantly enriched GO term in the other two PCGs. Furthermore, KEGG pathway analysis showed that these DEGs were mainly enriched in glucose and amino acid metabolisms. Moreover we found that intron retention (IR) and the alternative 3′ splice site (A3SS) accounted for more than 80%. The development of the S. rugosoannulata fruiting body mainly involved glucose and amino acid metabolisms. IR and A3SS were the two main types of ASE, which played an important role in the development and maturation of the S. rugosoannulata fruiting body.

Bulk RNA-Seq Analysis Reveals Differentially Expressed Genes Associated with Lateral Branch Angle in Peanut

Lateral branch angle (LBA), or branch habit, is one of the most important agronomic traits in peanut. To date, the underlying molecular mechanisms of LBA have not been elucidated in peanut. To acquire the differentially expressed genes (DEGs) related to LBA, a TI population was constructed through the hybridization of a bunch-type peanut variety Tifrunner and prostrate-type Ipadur. We report the identification of DEGs related to LBA by sequencing two RNA pools, which were composed of 45 F₃ lines showing an extreme opposite bunch and prostrate phenotype. We propose to name this approach Bulk RNA-sequencing (BR-seq) as applied to several plant species. Through BR-seq analysis, a total of 3083 differentially expressed genes (DEGs) were identified, including 13 gravitropism-related DEGs, 22 plant hormone-related DEGs, and 55 transcription factors-encoding DEGs. Furthermore, we also identified commonly expressed alternatively spliced (AS) transcripts, of which skipped exon (SE) and retained intron (RI) were most abundant in the prostrate and bunch-type peanut. AS isoforms between prostrate and bunch peanut highlighted important clues to further understand the post-transcriptional regulatory mechanisms of branch angle regulation. Our findings provide not only important insights into the landscape of the regulatory pathway involved in branch angle formation but also present practical information for peanut molecular breeding in the future.

Succession of Fungal Communities at Different Developmental Stages of Cabernet Sauvignon Grapes From an Organic Vineyard in Xinjiang

Fungi present on grape surface considerably impact grape growth and quality. However, information of the fungal community structures and dynamics on the worldwide cash crop, the Cabernet Sauvignon grape, from the budding to ripening stages remains limited. Here, we investigated the succession of fungal communities on Cabernet Sauvignon grapes from an organic vineyard in Xinjiang, China at different developmental stages via high-throughput sequencing combined with multivariate data analysis. In total, 439 fungal amplicon sequence variants (ASVs) from six phyla were identified. The fungal communities differed over the budding to the berry stages. Moreover, Aspergillus, Malassezia, Metschnikowia, and Udeniomyces were predominant during the unripe stage, whereas Erysiphe, Cryptococcus, Vishniacozyma, and Cladosporium were dominant in the ripe stages. Notably, Vishniacozyma was the most abundant genus, conserved in all development stages. Moreover, network analysis resulted in 171 edges—96 negative and 75 positive. Moreover, fungal genera such as Vishniacozyma, Sporobolomyces, Aspergillus, Alternaria, Erysiphe, Toxicodendron, and Metschnikowia were present in the hubs serving as the main connecting nodes. Extensive mutualistic interactions potentially occur among the fungi on the grape surface. In conclusion, the current study expounded the characteristics of the Cabernet Sauvignon grape fungal community during the plant growth process, and the results provided essential insights into the potential impacts of fungal communities on grape growth and health.

Fruit Colour and Novel Mechanisms of Genetic Regulation of Pigment Production in Tomato Fruits

Fruit colour represents a genetic trait with ecological and nutritional value. Plants mainly use colour to attract animals and favour seed dispersion. Thus, in many species, fruit colour coevolved with frugivories and their preferences. Environmental factors, however, represented other adaptive forces and further diversification was driven by domestication. All these factors cooperated in the evolution of tomato fruit, one of the most important in human nutrition. Tomato phylogenetic history showed two main steps in colour evolution: the change from green-chlorophyll to red-carotenoid pericarp, and the loss of the anthocyanic pigmentation. These events likely occurred with the onset of domestication. Then spontaneous mutations repeatedly occurred in carotenoid and phenylpropanoid pathways, leading to colour variants which often were propagated. Introgression breeding further enriched the panel of pigmentation patterns. In recent decades, the genetic determinants underneath tomato colours were identified. Novel evidence indicates that key regulatory and biosynthetic genes undergo mechanisms of gene expression regulation that are much more complex than what was imagined before: post-transcriptional mechanisms, with RNA splicing among the most common, indeed play crucial roles to fine-tune the expression of this trait in fruits and offer new substrate for the rise of genetic variables, thus providing further evolutionary flexibility to the character.

Using RNA-seq to characterize pollen-stigma interactions for pollination studies

Insects are essential for the reproduction of pollinator-dependent crops and contribute to the pollination of 87% of wild plants and 75% of the world’s food crops. Understanding pollen flow dynamics between plants and pollinators is thus essential to manage and conserve wild plants and ensure yields are maximized in food crops. However, the determination of pollen transfer in the field is complex and laborious. We developed a field experiment in a pollinator-dependent crop and used high throughput RNA sequencing (RNA-seq) to quantify pollen flow by measuring changes in gene expression between pollination treatments across different apple (Malus domestica Borkh.) cultivars. We tested three potential molecular indicators of successful pollination and validated these results with field data by observing single and multiple visits by honey bees (Apis mellifera) to apple flowers and measured fruit set in a commercial apple orchard. The first indicator of successful outcrossing was revealed via differential gene expression in the cross-pollination treatments after 6 h. The second indicator of successful outcrossing was revealed by the expression of specific genes related to pollen tube formation and defense response at three different time intervals in the stigma and the style following cross-pollination (i.e. after 6, 24, and 48 h). Finally, genotyping variants specific to donor pollen could be detected in cross-pollination treatments, providing a third indicator of successful outcrossing. Field data indicated that one or five flower visits by honey bees were insufficient and at least 10 honey bee flower visits were required to achieve a 25% probability of fruit set under orchard conditions. By combining the genotyping data, the differential expression analysis, and the traditional fruit set field experiments, it was possible to evaluate the pollination effectiveness of honey bee visits under orchards conditions. This is the first time that pollen-stigma-style mRNA expression analysis has been conducted after a pollinator visit (honey bee) to a plant (in vivo apple flowers). This study provides evidence that mRNA sequencing can be used to address complex questions related to stigma–pollen interactions over time in pollination ecology.

Identification and Regulation of Tomato Serine/Arginine-Rich Proteins Under High Temperatures

Alternative splicing is an important mechanism for the regulation of gene expression in eukaryotes during development, cell differentiation or stress response. Alterations in the splicing profiles of genes under high temperatures that cause heat stress (HS) can impact the maintenance of cellular homeostasis and thermotolerance. Consequently, information on factors involved in HS-sensitive alternative splicing is required to formulate the principles of HS response. Serine/arginine-rich (SR) proteins have a central role in alternative splicing. We aimed for the identification and characterization of SR-coding genes in tomato (Solanum lycopersicum), a plant extensively used in HS studies. We identified 17 canonical SR and two SR-like genes. Several SR-coding genes show differential expression and altered splicing profiles in different organs as well as in response to HS. The transcriptional induction of five SR and one SR-like genes is partially dependent on the master regulator of HS response, HS transcription factor HsfA1a. Cis-elements in the promoters of these SR genes were predicted, which can be putatively recognized by HS-induced transcription factors. Further, transiently expressed SRs show reduced or steady-state protein levels in response to HS. Thus, the levels of SRs under HS are regulated by changes in transcription, alternative splicing and protein stability. We propose that the accumulation or reduction of SRs under HS can impact temperature-sensitive alternative splicing.

Alternative splicing profiling provides insights into the molecular mechanisms of peanut peg development

BACKGROUND

The cultivated peanut (Arachis hypogaea) is one of the most important oilseed crops worldwide, and the generation of pegs and formation of subterranean pods are essential processes in peanut reproductive development. However, little information has been reported about alternative splicing (AS) in peanut peg formation and development.

RESULTS

Herein, we presented a comprehensive full-length (FL) transcriptome profiling of AS isoforms during peanut peg and early pod development. We identified 1448, 1102, 832, and 902 specific spliced transcripts in aerial pegs, subterranean pegs, subterranean unswollen pegs, and early swelling pods, respectively. A total of 184 spliced transcripts related to gravity stimulation, light and mechanical response, hormone mediated signaling pathways, and calcium-dependent proteins were identified as possibly involved in peanut peg development. For aerial pegs, spliced transcripts we got were mainly involved in gravity stimulation and cell wall morphogenetic processes. The genes undergoing AS in subterranean peg were possibly involved in gravity stimulation, cell wall morphogenetic processes, and abiotic response. For subterranean unswollen pegs, spliced transcripts were predominantly related to the embryo development and root formation. The genes undergoing splice in early swelling pods were mainly related to ovule development, root hair cells enlargement, root apex division, and seed germination.

CONCLUSION

This study provides evidence that multiple genes are related to gravity stimulation, light and mechanical response, hormone mediated signaling pathways, and calcium-dependent proteins undergoing AS express development-specific spliced isoforms or exhibit an obvious isoform switch during the peanut peg development. AS isoforms in subterranean pegs and pods provides valuable sources to further understand post-transcriptional regulatory mechanisms of AS in the generation of pegs and formation of subterranean pods.

Transcription of lncRNA ACoS-AS1 is essential to trans-splicing between SlPsy1 and ACoS-AS1 that causes yellow fruit in tomato

Phytoene synthase (PSY) has been considered as an important regulatory enzyme in carotenoids biosynthesis pathway. Previous study finds that the yellow fruit in Solanum lycopersicum var. cerasiforme accession PI 114490 is caused by loss-of-function of SlPSY1 due to trans-splicing between SlPsy1 and an unknown gene transcribed from neighbour opposite strand DNA of SlPsy1. The genomic DNA sequences of SlPsy1 between red and yellow-fruited tomato lines have one single-nucleotide polymorphism (SNP) in the fourth intron and one SSR in the intergenic region. In the current study, the cause of trans-splicing event was further investigated. The data showed that the previously defined unknown gene was a putative long non-coding RNA ACoS-AS1 with three variants in many yellow-fruited tomato lines. The intronic SNP and intergenic SSR were tightly associated with trans-splicing event SlPsy1-ACoS-AS1. However, transgenic tomato lines carrying the genomic DNA of SlPsy1 from PI 114490 did not generate transcripts of ACoS-AS1and SlPsy1-ACoS-AS1 suggesting that only the intronic SNP could not cause the trans-splicing event. Over-expression of SlPsy1-ACoS-AS1 in red-fruited tomato line M82 did not have any phenotype change while over-expression of wild type SlPsy1 resulted in altered leaf colour. Sub-cellular localization analysis showed that SlPSY1-ACoS-AS1 could not enter plastids where SlPSY1 has its enzyme activity. Mutation of ACoS-AS1 in PI 114490 generated by CRISPR/Cas9 techniques resulted in red fruits implying that ACoS-AS1 was essential to trans-splicing event SlPsy1-ACoS-AS1. The results obtained here will extend knowledge to understand the mechanism of trans-splicing event SlPsy1-ACoS-AS1 and provide additional information for the regulation of carotenoids biosynthesis.

Alternative Splicing Enhances the Transcriptome Complexity of Liriodendron chinense

Alternative splicing (AS) plays pivotal roles in regulating plant growth and development, flowering, biological rhythms, signal transduction, and stress responses. However, no studies on AS have been performed in Liriodendron chinense, a deciduous tree species that has high economic and ecological value. In this study, we used multiple tools and algorithms to analyze transcriptome data derived from seven tissues via hybrid sequencing. Although only 17.56% (8,503/48,408) of genes in L. chinense were alternatively spliced, these AS genes occurred in 37,844 AS events. Among these events, intron retention was the most frequent AS event, producing 1,656 PTC-containing and 3,310 non-PTC-containing transcripts. Moreover, 183 long noncoding RNAs (lncRNAs) also underwent AS events. Furthermore, weighted gene coexpression network analysis (WGCNA) revealed that there were great differences in the activities of transcription and post-transcriptional regulation between pistils and leaves, and AS had an impact on many physiological and biochemical processes in L. chinense, such as photosynthesis, sphingolipid metabolism, fatty acid biosynthesis and metabolism. Moreover, our analysis showed that the features of genes may affect AS, as AS genes and non-AS genes had differences in the exon/intron length, transcript length, and number of exons/introns. In addition, the structure of AS genes may impact the frequencies and types of AS because AS genes with more exons or introns tended to exhibit more AS events, and shorter introns tended to be retained, whereas shorter exons tended to be skipped. Furthermore, eight AS genes were verified, and the results were consistent with our analysis. Overall, this study reveals that AS and gene interaction are mutual-on one hand, AS can affect gene expression and translation, while on the other hand, the structural characteristics of the gene can also affect AS. This work is the first to comprehensively report on AS in L. chinense, and it can provide a reference for further research on AS in L. chinense.

Alternative Splicing of Key Genes in LOX Pathway Involves Biosynthesis of Volatile Fatty Acid Derivatives in Tea Plant (Camellia sinensis)

Volatile fatty acid derivatives (VFADs) produced in tea plants (Camellia sinensis) not only have been shown to function as defense compounds but also impart a "fresh green" odor to green tea products; however, little is known about alternative splicing (AS) of genes in regulating the production of VFADs in plants. In this study, the contents of VFADs and corresponding transcriptome profiles were obtained in five different months (April, June, August, September, and October). Correlation analysis identified seven unique transcripts of enzyme-coding genes (CsLOX2, CsLOX4, CsADH4, CsADH8, and CsADH10), which are responsible for regulating VFAD biosynthesis; four AS transcripts of these genes (CsLOX2, CsLOX4, CsADH4, and CsADH8) were validated by RT-PCR. By employing the gene-specific antisense oligodeoxynucleotide-mediated reduction method, we found the expression levels of alternatively spliced transcripts of CsLOX4-iso1, CsLOX4-iso2, and CsADH4-iso3 were lower, and the contents of cis-3-hexenol were correspondingly reduced in the leaves of tea plant; this result suggested that the AS play important roles in regulating biosynthesis of VFADs in C. sinensis. Our results provide new insights into the important contribution of AS events in regulating the VFAD biosynthesis in tea plant.

Low temperature upregulates cwp expression and modifies alternative splicing patterns, increasing the severity of cwp-induced tomato fruit cuticular microfissures

The cwp (cuticular water permeability) gene controls the development of cuticular microfissuring and subsequent fruit dehydration in tomato. The gene underwent silencing in the evolution of the fleshy cultivated tomato but is expressed in the primitive wild tomato relatives. The introgression of the expressed allele from the wild S. habrochaites (cwp ^h ) into the cultivated tomato (Solanum lycopersicum) leads to the phenotype of fruit water loss during and following ripening. In this report, we show that low temperature impacts on the severity of the cuticular microfissure phenotype via a combination of effects on both expression and alternative splicing of cwp ^h . The cwp gene, comprising four exons and three introns, undergoes post-transcriptional alternative splicing processes, leading to seven alternative transcripts that differ in reading-frame lengths. Transgenic plants expressing each of the alternative transcripts identified the longest reading frame (VAR1) as the functional splice variant. Low temperature led to a strong upregulation of cwp ^h expression, compounded by an increase in the relative proportion of the functional VAR1 transcript, leading to increased severity of microfissuring of the cuticle. In summary, we demonstrate the molecular mechanism behind the horticultural phenomenon of the low-temperature effect on cuticular microfissures in the dehydrating tomato.

Identification of putative genes for polyphenol biosynthesis in olive fruits and leaves using full-length transcriptome sequencing

Olive (Olea europaea) is a rich source of valuable bioactive polyphenols, which has attracted widespread interest. In this study, we combined targeted metabolome, Pacbio ISOseq transcriptome, and Illumina RNA-seq transcriptome to investigate the association between polyphenols and gene expression in the developing olive fruits and leaves. A total of 12 main polyphenols were measured, and 122 transcripts of 17 gene families, 101 transcripts of 9 gene families, and 106 transcripts of 6 gene families that encode for enzymes involved in flavonoid, oleuropein, and hydroxytyrosol biosynthesis were separately identified. Additionally, 232 alternative splicing events of 18 genes related to polyphenol synthesis were analyzed. This is the first time that the third generations of full-length transcriptome technology were used to study the gene expression pattern of olive fruits and leaves. The results of transcriptome combined with targeted metabolome can help us better understand the polyphenol biosynthesis pathways in the olive.

Identification and genetic analysis of alternative splicing of long non-coding RNAs in tomato initial flowering stage

Alternative splicing (AS) is a key modulator of development in many eukaryotic organisms. Long non-coding RNAs (lncRNAs) are a class of non-coding RNAs that play essential regulatory roles in various developmental processes and stress responses. However, the functions of AS lncRNAs during the initial flowering of tomato are largely unknown. This study was designed to investigate the AS pattern of lncRNAs in tomato flower, leaf, and root tissues at the initial flowering stage. Using RNA-Seq, we found that 72.55% of lncRNAs underwent AS in these tissues, yielding a total of 16,995 AS events. Among them, the main type of AS event is alternative first exon (AFE), followed by retained intron (RI). We performed candidate target genes analysis on tissue-specific AS lncRNA, and the results indicated that the candidate target genes of these lncRNAs may be involved in the regulation of circadian rhythm, plant immunity, cellulose synthesis and phosphate-containing compound metabolic process. Moreover, a total of 73,085 putative SNPs and 15,679 InDels were detected, and the potential relationship between the AS of lncRNAs and interesting SNP and InDel loci, as well as their numbers, revealed their effects on tomato genetic diversity and genomic stability. Our data provide new insights into the complexity of the transcriptome and the regulation of AS.

Comparative transcriptomics analysis uncovers alternative splicing events and molecular markers in cabbage (Brassica oleracea L.)

Alternative splicing (AS) events were identified and verified in cabbage by comparative transcriptome analysis. The corresponding markers were developed and the germplasm resources were identified. Alternative splicing (AS) is a central regulatory mechanism that greatly contributes to plant gene expression and transcriptome diversity. A large body of evidence has shown that AS complexity is relevant for plant development, evolution, complexity, and adaptation. Both insertion/deletion (InDel) and single nucleotide polymorphism (SNP) are typically co-dominant inheritance markers and have abundant polymorphisms. These have been widely used for marker-assisted selection, genetic mapping, and germplasm identification in plants. However, little is known about the molecular mechanisms underlying AS events and the development of markers including SNP and InDel from the cabbage transcriptome. In this study, three cabbage transcriptome datasets were collected and aligned to the cabbage reference genome to analyze AS events and marker development. 31,524 AS events were identified from three cabbage genotypes, accounting for 20.8% of the total cabbage genes. Alternative 3' splice site donor (A3SS) was the most frequent type of the four main AS events in cabbage. 70,475 InDels and 706,269 SNPs were identified with average frequencies of 1 InDel/6.9 kb and 1 SNP/0.7 kb, respectively. 71,942 potential SSRs were identified in 53,129 assembled unigenes with a density of 1 SSR/6.8 kb. The ratio of SNPs with synonymous/non-synonymous mutations was 1:0.65. 142 InDels and 36 SNPs were randomly selected and validated via Sanger sequencing and polymorphism was found among 66.2% of the InDels and 78.6% of the SNPs. Furthermore, 35 informative InDel markers were successfully used for genetic diversity analysis on 36 cabbage accessions. These results facilitate understanding of the molecular regulation mechanism underlying AS events in cabbage. They also provide molecular marker resource data for genetic mapping construction and germplasm identification, and facilitate the genetic improvement of cabbage via breeding.

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.

Tomato lncRNA23468 functions as a competing endogenous RNA to modulate NBS-LRR genes by decoying miR482b in the tomato-Phytophthora infestans interaction

Our previous studies indicated that tomato miR482b could negatively regulate the resistance of tomato to Phytophthora infestans and the expression of miR482b was decreased after inoculation with P. infestans. However, the mechanism by which the accumulation of miR482b is suppressed remains unclear. In this study, we wrote a program to identify 89 long noncoding RNA (lncRNA)-originated endogenous target mimics (eTMs) for 46 miRNAs from our RNA-Seq data. Three tomato lncRNAs, lncRNA23468, lncRNA01308 and lncRNA13262, contained conserved eTM sites for miR482b. When lncRNA23468 was overexpressed in tomato, miR482b expression was significantly decreased, and the expression of the target genes, NBS-LRRs, was significantly increased, resulting in enhanced resistance to P. infestans. Silencing lncRNA23468 in tomato led to the increased accumulation of miR482b and decreased accumulation of NBS-LRRs, as well as reduced resistance to P. infestans. In addition, the accumulation of both miR482b and NBS-LRRs was not significantly changed in tomato plants that overexpressed lncRNA23468 with a mutated eTM site. Based on the VIGS system, a target gene of miR482b, Solyc02g036270.2, was silenced. The disease symptoms of the VIGS-Solyc02g036270.2 tomato plants were in accordance with those of tomato plants in which lncRNA23468 was silenced after inoculation with P. infestans. More severe disease symptoms were found in the modified plants than in the control plants. Our results demonstrate that lncRNAs functioning as eTMs may modulate the effects of miRNAs in tomato and provide insight into how the lncRNA23468-miR482b-NBS-LRR module regulates tomato resistance to P. infestans.

Genome-wide identification of long non-coding RNAs in tomato plants irradiated by neutrons followed by infection with Tomato yellow leaf curl virus

Long non-coding RNAs (lncRNAs) play an important role in regulating many biological processes. In this study, tomato seeds were first irradiated by neutrons. Eight tomato mutants were then selected and infected by Tomato yellow leaf curl virus (TYLCV). RNA sequencing followed by bioinformatics analyses identified 1,563 tomato lncRNAs. About half of the lncRNAs were derived from intergenic regions, whereas antisense lncRNAs accounted for 35%. There were fewer lncRNAs identified in our study than in other studies identifying tomato lncRNAs. Functional classification of 794 lncRNAs associated with tomato genes showed that many lncRNAs were associated with binding functions required for interactions with other molecules and localized in the cytosol and membrane. In addition, we identified 19 up-regulated and 11 down-regulated tomato lncRNAs by comparing TYLCV infected plants to non-infected plants using previously published data. Based on these results, the lncRNAs identified in this study provide important resources for characterization of tomato lncRNAs in response to TYLCV infection.

Transcriptome reprogramming, epigenetic modifications and alternative splicing orchestrate the tomato root response to the beneficial fungus Trichoderma harzianum

Beneficial interactions of rhizosphere microorganisms are widely exploited for plant biofertilization and mitigation of biotic and abiotic constraints. To provide new insights into the onset of the roots-beneficial microorganisms interplay, we characterised the transcriptomes expressed in tomato roots at 24, 48 and 72 h post inoculation with the beneficial fungus Trichoderma harzianum T22 and analysed the epigenetic and post-trascriptional regulation mechanisms. We detected 1243 tomato transcripts that were differentially expressed between Trichoderma-interacting and control roots and 83 T. harzianum transcripts that were differentially expressed between the three experimental time points. Interaction with Trichoderma triggered a transcriptional response mainly ascribable to signal recognition and transduction, stress response, transcriptional regulation and transport. In tomato roots, salicylic acid, and not jasmonate, appears to have a prominent role in orchestrating the interplay with this beneficial strain. Differential regulation of many nutrient transporter genes indicated a strong effect on plant nutrition processes, which, together with the possible modifications in root architecture triggered by ethylene/indole-3-acetic acid signalling at 72 h post inoculation may concur to the well-described growth-promotion ability of this strain. Alongside, T. harzianum-induced defence priming and stress tolerance may be mediated by the induction of reactive oxygen species, detoxification and defence genes. A deeper insight into gene expression and regulation control provided first evidences for the involvement of cytosine methylation and alternative splicing mechanisms in the plant-Trichoderma interaction. A model is proposed that integrates the plant transcriptomic responses in the roots, where interaction between the plant and beneficial rhizosphere microorganisms occurs.

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.

Alternative Splicing as a Regulator of Early Plant Development

Most plant genes are interrupted by introns and the corresponding transcripts need to undergo pre-mRNA splicing to remove these intervening sequences. Alternative splicing (AS) is an important posttranscriptional process that creates multiple mRNA variants from a single pre-mRNA molecule, thereby enhancing the coding and regulatory potential of genomes. In plants, this mechanism has been implicated in the response to environmental cues, including abiotic and biotic stresses, in the regulation of key developmental processes such as flowering, and in circadian timekeeping. The early plant development steps - from embryo formation and seed germination to skoto- and photomorphogenesis - are critical to both execute the correct body plan and initiate a new reproductive cycle. We review here the available evidence for the involvement of AS and various splicing factors in the initial stages of plant development, while highlighting recent findings as well as potential future challenges.

Transcriptome analysis of alternative splicing in peanut (Arachis hypogaea L.)

BACKGROUND

Alternative splicing (AS) represents a mechanism widely used by eukaryotes for the post-transcriptional regulation of genes. The detailed exploration of AS in peanut has not been documented.

RESULTS

The strand-specific RNA-Seq technique was exploited to characterize the distribution of AS in the four samples of peanut (FH1-seed1, FH1-seed2, FH1-root and FH1-leaf). AS was detected as affecting around 37.2% of the full set of multi-exon genes. Some of these genes experienced AS throughout the plant, while in the case of others, the effect was organ-specific. Overall, AS was more frequent in the seed than in either the root or leaf. The predominant form of AS was intron retention, and AS in transcription start site and transcription terminal site were commonly identified in all the four samples. It is interesting that in genes affected by AS, the majority experienced only a single type of event. Not all of the in silico predicted transcripts appeared to be translated, implying that these are either degraded or sequestered away from the translation machinery. With respect to genes involved in fatty acid metabolism, about 61.6% were shown to experience AS.

CONCLUSION

Our report contributes significantly in AS analysis of peanut genes in general, and these results have not been mentioned before. The specific functions of different AS forms need further investigation.

Alternative splicing in tomato pollen in response to heat stress

Alternative splicing (AS) is a key control mechanism influencing signal response cascades in different developmental stages and under stress conditions. In this study, we examined heat stress (HS)-induced AS in the heat sensitive pollen tissue of two tomato cultivars. To obtain the entire spectrum of HS-related AS, samples taken directly after HS and after recovery were combined and analysed by RNA-seq. For nearly 9,200 genes per cultivar, we observed at least one AS event under HS. In comparison to control, for one cultivar we observed 76% more genes with intron retention (IR) or exon skipping (ES) under HS. Furthermore, 2,343 genes had at least one transcript with IR or ES accumulated under HS in both cultivars. These genes are involved in biological processes like protein folding, gene expression and heat response. Transcriptome assembly of these genes revealed that most of the alternative spliced transcripts possess truncated coding sequences resulting in partial or total loss of functional domains. Moreover, 141 HS specific and 22 HS repressed transcripts were identified. Further on, we propose AS as layer of stress response regulating constitutively expressed genes under HS by isoform abundance.

CRISPR/Cas9-mediated mutagenesis of lncRNA1459 alters tomato fruit ripening

With the development of high-throughput sequencing, many long non-coding RNAs (lncRNAs) have been found to play important roles in diverse biological processes. However, the biological functions of most plant lncRNAs are still unknown. We have previously discovered a tomato ripening-related lncRNA, lncRNA1459. Here, we cloned the full-length lncRNA1459, giving two transcript isoforms. In addition, lncRNA1459 exhibited a specific location in the nucleus. Furthermore, in order to fully identify the function of lncRNA1459 in tomato ripening, loss-of-function mutants of lncRNA1459 were developed using clustered regularly interspaced short palindromic repeats (CRISPR)/-associated protein 9 (Cas9)-induced genome editing technology. Compared with wild-type fruits, the tomato ripening process was significantly repressed in lncRNA1459 mutants. Ethylene production and lycopene accumulation were largely repressed in lncRNA1459 mutants. Additionally, genes related to ethylene and carotenoid biosynthesis were distinctly downregulated in lncRNA1459 mutants compared with wild-type fruits. Moreover, expression of numerous ripening-related genes was changed significantly when lncRNA1459 was knocked out. Expression of potential tomato ripening-related lncRNAs was also specifically changed after knocking out lncRNA1459. Taken together, these results provide insight into the role of lncRNA1459 in tomato fruit ripening.

The comparison of alternative splicing among the multiple tissues in cucumber

BACKGROUND

Alternative splicing (AS) is an important post-transcriptional process. It has been suggested that most AS events are subject to tissue-specific regulation. However, the global dynamics of AS in different tissues are poorly explored.

RESULTS

To analyse global changes in AS in multiple tissues, we identified the AS events and constructed a comprehensive catalogue of AS events within each tissue based on the genome-wide RNA-seq reads from ten tissues in cucumber. First, we found that 58% of the multi-exon genes underwent AS. We further obtained 565 genes with significantly more AS events compared with random genes. These genes were found significant enrichment in biological processes related to the regulation of actin filament length. Second, significantly different AS event profiles among ten tissues were found. The tissues with the same origin of development are more likely to have a relatively similar AS profile. Moreover, 7370 genes showed tissue-specific AS events and were highly enriched in biological processes related to the positive regulation of cellular component organization. Root-specificity AS genes were related to the cellular response to DNA damage stimulus. Third, the genes with different intron retention (IR) patterns among the ten tissues showed significant difference in GC percentages of the retained intron, and the number of exons and FPKM of the major transcripts.

CONCLUSIONS

Our study provided a comprehensive view of AS in multiple tissues. We revealed novel insights into the patterns of AS in multiple tissues and the tissue-specific AS in cucumber.

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.

Comparative transcriptomics uncovers alternative splicing and molecular marker development in radish (Raphanus sativus L.)

Background

Alternative splicing (AS) plays important roles in gene expression and proteome diversity. Single nucleotide polymorphism (SNP) and insertion/deletion (InDel) are abundant polymorphisms and co-dominant inheritance markers, which have been widely used in germplasm identification, genetic mapping and marker-assisted selection in plants. So far, however, little information is available on utilization of AS events and development of SNP and InDel markers from transcriptome in radish.

Results

In this study, three radish transcriptome datasets were collected and aligned to the reference radish genome. A total of 56,530 AS events were identified from three radish genotypes with intron retention (IR) being the most frequent AS type, which accounted for 59.4% of the total expressed genes in radish. In all, 22,412 SNPs and 9436 InDels were identified with an average frequency of 1 SNP/17.9 kb and 1 InDel/42.5 kb, respectively. A total of 43,680 potential SSRs were identified in 31,604 assembled unigenes with a density of 1 SSR/2.5 kb. The ratio of SNPs with nonsynonymous/synonymous mutations was 1.05:1. Moreover, 35 SNPs and 200 InDels were randomly selected and validated by Sanger sequencing, 83.9% of the SNPs and 70% of the InDels exhibited polymorphism among these three genotypes. In addition, the 15 SNPs and 125 InDels were found to be unevenly distributed on 9 linkage groups. Furthermore, 40 informative InDel markers were successfully used for the genetic diversity analysis on 32 radish accessions.

Conclusions

These results would not only provide new insights into transcriptome complexity and AS regulation, but also furnish large amount of molecular marker resources for germplasm identification, genetic mapping and further genetic improvement of radish in breeding programs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3874-4) contains supplementary material, which is available to authorized users.

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

Alternative splicing (AS) is a key post-transcriptional regulatory mechanism, yet little information is known about its roles in fruit crops. Here, AS was globally analyzed in the wild strawberry Fragaria vesca genome with RNA-seq data derived from different stages of fruit development. The AS landscape was characterized and compared between the single-molecule, real-time (SMRT) and Illumina RNA-seq platform. While SMRT has a lower sequencing depth, it identifies more genes undergoing AS (57.67% of detected multiexon genes) when it is compared with Illumina (33.48%), illustrating the efficacy of SMRT in AS identification. We investigated different modes of AS in the context of fruit development; the percentage of intron retention (IR) is markedly reduced whereas that of alternative acceptor sites (AA) is significantly increased post-fertilization when compared with pre-fertilization. When all the identified transcripts were combined, a total of 66.43% detected multiexon genes in strawberry undergo AS, some of which lead to a gain or loss of conserved domains in the gene products. The work demonstrates that SMRT sequencing is highly powerful in AS discovery and provides a rich data resource for later functional studies of different isoforms. Further, shifting AS modes may contribute to rapid changes of gene expression during fruit set.

Response of Gene Expression and Alternative Splicing to Distinct Growth Environments in Tomato

Phenotypic plasticity is the phenomenon that one particular genotype produces different phenotypes under different environmental conditions, but its underlying molecular and genetic mechanisms are poorly understood. Plastic traits may be under the control of genes whose expression is modulated by environmental cues. In this study, we investigated phenotypic plasticity in tomato (Solanum lycopersicum) and its ancestral species S. pimpinellifolium by comparing the global gene expression of young seedlings grown under two distinct growth conditions. Our results show that more than 7000 genes exhibited differential expression in response to environmental changes from phytotron to a plastic greenhouse, and 98 environmentally sensitive genes displayed the same patterns of expression response across the two tomato species. We also found that growth conditions had a remarkable impact on transcriptome complexity, attributable to alternative splicing (AS), in which 665 splice variants showed differential expression in response to the environmental changes. Moreover, more splice variants and AS events per gene were detected in plastic greenhouse-grown seedlings than their phytotron counterparts, and these seedlings also had higher percentages of intron retention events. The identification of the conserved environmentally-sensitive genes and the splice variants in this study will be useful for further analysis of gene regulation of environmental response in tomato and other crops.