Comparative cross-species alternative splicing in plants

Nucleotide-level distance metrics to quantify alternative splicing implemented in TranD

Advances in affordable transcriptome sequencing combined with better exon and gene prediction has motivated many to compare transcription across the tree of life. We develop a mathematical framework to calculate complexity and compare transcript models. Structural features, i.e. intron retention (IR), donor/acceptor site variation, alternative exon cassettes, alternative 5'/3' UTRs, are compared and the distance between transcript models is calculated with nucleotide level precision. All metrics are implemented in a PyPi package, TranD and output can be used to summarize splicing patterns for a transcriptome (1GTF) and between transcriptomes (2GTF). TranD output enables quantitative comparisons between: annotations augmented by empirical RNA-seq data and the original transcript models; transcript model prediction tools for longread RNA-seq (e.g. FLAIR versus Isoseq3); alternate annotations for a species (e.g. RefSeq vs Ensembl); and between closely related species. In C. elegans, Z. mays, D. melanogaster, D. simulans and H. sapiens, alternative exons were observed more frequently in combination with an alternative donor/acceptor than alone. Transcript models in RefSeq and Ensembl are linked and both have unique transcript models with empirical support. D. melanogaster and D. simulans, share many transcript models and long-read RNAseq data suggests that both species are under-annotated. We recommend combined references.

Gene expression and alternative splicing contribute to adaptive divergence of ecotypes

Regulation of gene expression is a critical link between genotype and phenotype explaining substantial heritable variation within species. However, we are only beginning to understand the ways that specific gene regulatory mechanisms contribute to adaptive divergence of populations. In plants, the post-transcriptional regulatory mechanism of alternative splicing (AS) plays an important role in both development and abiotic stress response, making it a compelling potential target of natural selection. AS allows organisms to generate multiple different transcripts/proteins from a single gene and thus may provide a source of evolutionary novelty. Here, we examine whether variation in alternative splicing and gene expression levels might contribute to adaptation and incipient speciation of dune-adapted prairie sunflowers in Great Sand Dunes National Park, Colorado, USA. We conducted a common garden experiment to assess transcriptomic variation among ecotypes and analyzed differential expression, differential splicing, and gene coexpression. We show that individual genes are strongly differentiated for both transcript level and alternative isoform proportions, even when grown in a common environment, and that gene coexpression networks are disrupted between ecotypes. Furthermore, we examined how genome-wide patterns of sequence divergence correspond to divergence in transcript levels and isoform proportions and find evidence for both cis and trans-regulation. Together, our results emphasize that alternative splicing has been an underappreciated mechanism providing source material for natural selection at short evolutionary time scales.

Unbiased Subgenome Evolution in Allotetraploid Species of Ephedra and Its Implications for the Evolution of Large Genomes in Gymnosperms

The evolutionary dynamics of polyploid genomes and consequences of polyploidy have been studied extensively in angiosperms but very rarely in gymnosperms. The gymnospermous genus Ephedra is characterized by a high frequency of polyploidy, and thus provides an ideal system to investigate the evolutionary mode of allopolyploid genomes and test whether subgenome dominance has occurred in gymnosperms. Here, we sequenced transcriptomes of two allotetraploid species of Ephedra and their putative diploid progenitors, identified expressed homeologs, and analyzed alternative splicing and homeolog expression based on PacBio Iso-Seq and Illumina RNA-seq data. We found that the two subgenomes of the allotetraploids had similar numbers of expressed homeologs, similar percentages of homeologs with dominant expression, and approximately equal numbers of isoforms with alternative splicing, showing an unbiased subgenome evolution as in a few polyploid angiosperms, with a divergence of the two subgenomes at ∼8 Ma. In addition, the nuclear DNA content of the allotetraploid species is almost equal to the sum of two putative progenitors, suggesting limited genome restructuring after allotetraploid speciation. The allopolyploid species of Ephedra might have undergone slow diploidization, and the unbiased subgenome evolution implies that the formation of large genomes in gymnosperms could be attributed to even and slow fractionation following polyploidization.

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.

Single-molecule Real-time (SMRT) Isoform Sequencing (Iso-Seq) in Plants: The Status of the Bioinformatics Tools to Unravel the Transcriptome Complexity

Background:

The advent of the Single-Molecule Real-time (SMRT) Isoform Sequencing (Iso-Seq) has paved the way to obtain longer full-length transcripts. This method was found to be much superior in identifying full-length splice variants and other post-transcriptional events as compared to the Next Generation Sequencing (NGS)-based short read sequencing (RNA-Seq). Several different bioinformatics tools to analyze the Iso-Seq data have been developed and some of them are still being refined to address different aspects of transcriptome complexity. However, a comprehensive summary of the available tools and their utility is still lacking.

Objective:

Here, we summarized the existing Iso-Seq analysis tools and presented an integrated bioinformatics pipeline for Iso-Seq analysis, which overcomes the limitations of NGS and generates long contiguous Full-Length Non-Chimeric (FLNC) reads for the analysis of posttranscriptional events.

Results:

In this review, we summarized recent applications of Iso-Seq in plants, which include improved genome annotations, identification of novel genes and lncRNAs, identification of fulllength splice isoforms, detection of novel Alternative Splicing (AS) and Alternative Polyadenylation (APA) events. In addition, we also discussed the bioinformatics pipeline for comprehensive Iso-Seq data analysis, including how to reduce the error rate in the reads and how to identify and quantify post-transcriptional events. Furthermore, the visualization approach of Iso-Seq was discussed as well. Finally, we discussed methods to combine Iso-Seq data with RNA-Seq for transcriptome quantification.

Conclusion:

Overall, this review demonstrates that the Iso-Seq is pivotal for analyzing transcriptome complexity and this new method offers unprecedented opportunities to comprehensively understand transcripts diversity.

Evolution of Alternative Splicing in Eudicots

Alternative pre-mRNA splicing (AS) is prevalent in plants and is involved in many interactions between plants and environmental stresses. However, the patterns and underlying mechanisms of AS evolution in plants remain unclear. By analyzing the transcriptomes of four eudicot species, we revealed that the divergence of AS is largely due to the gains and losses of AS events among orthologous genes. Furthermore, based on a subset of AS, in which AS can be directly associated with specific transcripts, we found that AS that generates transcripts containing premature termination codons (PTC), are likely more conserved than those that generate non-PTC containing transcripts. This suggests that AS coupled with nonsense-mediated decay (NMD) might play an important role in affecting mRNA levels post-transcriptionally. To understand the mechanisms underlying the divergence of AS, we analyzed the key determinants of AS using a machine learning approach. We found that the presence/absence of alternative splice site (SS) within the junction, the distance between the authentic SS and the nearest alternative SS, the size of exon-exon junctions were the major determinants for both alternative 5' donor site and 3' acceptor site among the studied species, suggesting a relatively conserved AS mechanism. The comparative analysis further demonstrated that variations of the identified AS determinants significantly contributed to the AS divergence among closely related species in both Solanaceae and Brassicaceae taxa. Together, these results provide detailed insights into the evolution of AS in plants.

Analysis of Transcriptome and Epitranscriptome in Plants Using PacBio Iso-Seq and Nanopore-Based Direct RNA Sequencing

Nanopore sequencing from Oxford Nanopore Technologies (ONT) and Pacific BioSciences (PacBio) single-molecule real-time (SMRT) long-read isoform sequencing (Iso-Seq) are revolutionizing the way transcriptomes are analyzed. These methods offer many advantages over most widely used high-throughput short-read RNA sequencing (RNA-Seq) approaches and allow a comprehensive analysis of transcriptomes in identifying full-length splice isoforms and several other post-transcriptional events. In addition, direct RNA-Seq provides valuable information about RNA modifications, which are lost during the PCR amplification step in other methods. Here, we present a comprehensive summary of important applications of these technologies in plants, including identification of complex alternative splicing (AS), full-length splice variants, fusion transcripts, and alternative polyadenylation (APA) events. Furthermore, we discuss the impact of the newly developed nanopore direct RNA-Seq in advancing epitranscriptome research in plants. Additionally, we summarize computational tools for identifying and quantifying full-length isoforms and other co/post-transcriptional events and discussed some of the limitations with these methods. Sequencing of transcriptomes using these new single-molecule long-read methods will unravel many aspects of transcriptome complexity in unprecedented ways as compared to previous short-read sequencing approaches. Analysis of plant transcriptomes with these new powerful methods that require minimum sample processing is likely to become the norm and is expected to uncover novel co/post-transcriptional gene regulatory mechanisms that control biological outcomes during plant development and in response to various stresses.

Genome-wide analyses supported by RNA-Seq reveal non-canonical splice sites in plant genomes

BACKGROUND

Most eukaryotic genes comprise exons and introns thus requiring the precise removal of introns from pre-mRNAs to enable protein biosynthesis. U2 and U12 spliceosomes catalyze this step by recognizing motifs on the transcript in order to remove the introns. A process which is dependent on precise definition of exon-intron borders by splice sites, which are consequently highly conserved across species. Only very few combinations of terminal dinucleotides are frequently observed at intron ends, dominated by the canonical GT-AG splice sites on the DNA level.

RESULTS

Here we investigate the occurrence of diverse combinations of dinucleotides at predicted splice sites. Analyzing 121 plant genome sequences based on their annotation revealed strong splice site conservation across species, annotation errors, and true biological divergence from canonical splice sites. The frequency of non-canonical splice sites clearly correlates with their divergence from canonical ones indicating either an accumulation of probably neutral mutations, or evolution towards canonical splice sites. Strong conservation across multiple species and non-random accumulation of substitutions in splice sites indicate a functional relevance of non-canonical splice sites. The average composition of splice sites across all investigated species is 98.7% for GT-AG, 1.2% for GC-AG, 0.06% for AT-AC, and 0.09% for minor non-canonical splice sites. RNA-Seq data sets of 35 species were incorporated to validate non-canonical splice site predictions through gaps in sequencing reads alignments and to demonstrate the expression of affected genes.

CONCLUSION

We conclude that bona fide non-canonical splice sites are present and appear to be functionally relevant in most plant genomes, although at low abundance.

Functions for fission yeast splicing factors SpSlu7 and SpPrp18 in alternative splice-site choice and stress-specific regulated splicing

Budding yeast spliceosomal factors ScSlu7 and ScPrp18 interact and mediate intron 3'ss choice during second step pre-mRNA splicing. The fission yeast genome with abundant multi-intronic transcripts, degenerate splice signals and SR proteins is an apt unicellular fungal model to deduce roles for core spliceosomal factors in alternative splice-site choice, intron retention and to study the cellular implications of regulated splicing. From our custom microarray data we deduce a stringent reproducible subset of S. pombe alternative events. We examined the role of factors SpSlu7 or SpPrp18 for these splice events and investigated the relationship to growth phase and stress. Wild-type log and stationary phase cells showed ats1+ exon 3 skipped and intron 3 retained transcripts. Interestingly the non-consensus 5'ss in ats1+ intron 3 caused SpSlu7 and SpPrp18 dependent intron retention. We validated the use of an alternative 5'ss in dtd1+ intron 1 and of an upstream alternative 3'ss in DUF3074 intron 1. The dtd1+ intron 1 non-canonical 5'ss yielded an alternative mRNA whose levels increased in stationary phase. Utilization of dtd1+ intron 1 sub-optimal 5' ss required functional SpPrp18 and SpSlu7 while compromise in SpSlu7 function alone hampered the selection of the DUF3074 intron 1 non canonical 3'ss. We analysed the relative abundance of these splice isoforms during mild thermal, oxidative and heavy metal stress and found stress-specific splice patterns for ats1+ and DUF3074 intron 1 some of which were SpSlu7 and SpPrp18 dependent. By studying ats1+ splice isoforms during compromised transcription elongation rates in wild-type, spslu7-2 and spprp18-5 mutant cells we found dynamic and intron context-specific effects in splice-site choice. Our work thus shows the combinatorial effects of splice site strength, core splicing factor functions and transcription elongation kinetics to dictate alternative splice patterns which in turn serve as an additional recourse of gene regulation in fission yeast.

Identification and analysis of alternative splicing events in Phaseolus vulgaris and Glycine max

BACKGROUND

The vast diversification of proteins in eukaryotic cells has been related with multiple transcript isoforms from a single gene that result in alternative splicing (AS) of primary transcripts. Analysis of RNA sequencing data from expressed sequence tags and next generation RNA sequencing has been crucial for AS identification and genome-wide AS studies. For the identification of AS events from the related legume species Phaseolus vulgaris and Glycine max, 157 and 88 publicly available RNA-seq libraries, respectively, were analyzed.

RESULTS

We identified 85,570 AS events from P. vulgaris in 72% of expressed genes and 134,316 AS events in 70% of expressed genes from G. max. These were categorized in seven AS event types with intron retention being the most abundant followed by alternative acceptor and alternative donor, representing ~75% of all AS events in both plants. Conservation of AS events in homologous genes between the two species was analyzed where an overrepresentation of AS affecting 5'UTR regions was observed for certain types of AS events. The conservation of AS events was experimentally validated for 8 selected genes, through RT-PCR analysis. The different types of AS events also varied by relative position in the genes. The results were consistent in both species.

CONCLUSIONS

The identification and analysis of AS events are first steps to understand their biological relevance. The results presented here from two related legume species reveal high conservation, over ~15-20 MY of divergence, and may point to the biological relevance of AS.

Genome-Wide Study of the Tomato SlMLO Gene Family and Its Functional Characterization in Response to the Powdery Mildew Fungus Oidium neolycopersici

The MLO (Mildew Locus O) gene family encodes plant-specific proteins containing seven transmembrane domains and likely acting in signal transduction in a calcium and calmodulin dependent manner. Some members of the MLO family are susceptibility factors toward fungi causing the powdery mildew disease. In tomato, for example, the loss-of-function of the MLO gene SlMLO1 leads to a particular form of powdery mildew resistance, called ol-2, which arrests almost completely fungal penetration. This type of penetration resistance is characterized by the apposition of papillae at the sites of plant-pathogen interaction. Other MLO homologs in Arabidopsis regulate root response to mechanical stimuli (AtMLO4 and AtMLO11) and pollen tube reception by the female gametophyte (AtMLO7). However, the role of most MLO genes remains unknown. In this work, we provide a genome-wide study of the tomato SlMLO gene family. Besides SlMLO1, other 15 SlMLO homologs were identified and characterized with respect to their structure, genomic organization, phylogenetic relationship, and expression profile. In addition, by analysis of transgenic plants, we demonstrated that simultaneous silencing of SlMLO1 and two of its closely related homologs, SlMLO5 and SlMLO8, confer higher level of resistance than the one associated with the ol-2 mutation. The outcome of this study provides evidence for functional redundancy among tomato homolog genes involved in powdery mildew susceptibility. Moreover, we developed a series of transgenic lines silenced for individual SlMLO homologs, which lay the foundation for further investigations aimed at assigning new biological functions to the MLO gene family.

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.

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.

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.

Comprehensive analysis of alternative splicing in Digitalis purpurea by strand-specific RNA-Seq

Digitalis purpurea (D. purpurea) is one of the most important medicinal plants and is well known in the treatment of heart failure because of the cardiac glycosides that are its main active compounds. However, in the absence of strand specific sequencing information, the post-transcriptional mechanism of gene regulation in D. purpurea thus far remains unknown. In this study, a strand-specific RNA-Seq library was constructed and sequenced using Illumina HiSeq platforms to characterize the transcriptome of D. purpurea with a focus on alternative splicing (AS) events and the effect of AS on protein domains. De novo RNA-Seq assembly resulted in 48,475 genes. Based on the assembled transcripts, we reported a list of 3,265 AS genes, including 5,408 AS events in D. purpurea. Interestingly, both glycosyltransferases and monooxygenase, which were involved in the biosynthesis of cardiac glycosides, are regulated by AS. A total of 2,422 AS events occurred in coding regions, and 959 AS events were located in the regions of 882 unique protein domains, which could affect protein function. This D. purpurea transcriptome study substantially increased the expressed sequence resource and presented a better understanding of post-transcriptional regulation to further facilitate the medicinal applications of D. purpurea for human health.

Identification, characterization, and utilization of single copy genes in 29 angiosperm genomes

Background

Single copy genes are common across angiosperm genomes. With the sufficiently high quality sequenced genomes, the identification of large-scale single copy genes among multiple species is possible. Although some characteristics have been reported, our study provides novel insights into single copy genes.

Results

We identified single copy genes across 29 angiosperm genomes. A significant negative correlation was found between the number of duplicate blocks and the number of single copy genes. We found that a considerable number of single copy genes are located in organelles, showing a preference for binding and catalytic activity. The analysis of effective number of codons (Nc) illustrates that single copy genes have a stronger codon bias than non-single copy genes in eudicots. The relative high expression level of single copy genes was partially confirmed by the RNA-seq data, rather than the Codon Adaptation Index (CAI). Unlike in most other species, a strongly negatively correlation occurs between Nc and GC3 among single copy genes in grass genomes. When compared to all non-single copy genes, single copy genes indicate more conservation (as indicated by Ka and Ks values). But our alternative splicing (AS) results reveal that selective constraints are weaker in single copy genes than in low copy family genes (1–10 in-paralogs) and stronger than high copy family genes (>10 in-paralogs). Using concatenated shared single copy genes, we obtained a well-resolved phylogenetic tree. With the addition of intron sequences, the branch support is improved, but striking incongruences are also evident. Therefore, it is noteworthy that inclusion of intron sequences seems more appropriate for the phylogenetic reconstruction at lower taxonomic levels.

Conclusions

Our analysis provides insight into the evolutionary characteristics of single copy genes across 29 angiosperm genomes. The results suggest that there are key differences in evolutionary constraints between single copy genes and non-single copy genes. And to some extent, these evolutionary constraints show some species-specific differences, especially between eudicots and monocots. Our preliminary evidence also suggests that the concatenated shared single copy genes are well suited for use in resolving phylogenetic relationships.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-504) contains supplementary material, which is available to authorized users.

Genome-wide identification and phylogenetic analysis of Family-1 UDP glycosyltransferases in maize (Zea mays)

Family-1 UDP glycosyltransferases (UGTs) from plants transfer sugar moieties from activated sugar donors to a wide range of small molecules, and control many metabolic processes during plant growth and development. Here, we report a genome-wide analysis of maize that identified 147 Family-1 glycosyltransferases based on their conserved PSPG motifs. Phylogenetic analysis of these genes with 18 Arabidopsis UGTs and two rice UGTs clustered them into 17 groups (A-Q). The patterns of intron gain/loss events, as well as their positions within UGTs from the same group, further aided elucidation of their divergence and evolutionary relationships between UGTs. Expression analysis of the maize UGT genes using both online microarray data and quantitative real-time PCR verification indicates that UGT genes are widely expressed in various tissues and likely play important roles in plant growth and development. Our study provides useful information on the Family-1 UGTs in maize, and will facilitate their further characterization to better understand their functions.

Single-nucleotide resolution mapping of the Gossypium raimondii transcriptome reveals a new mechanism for alternative splicing of introns

Alternative splicing (AS) is a vital genetic mechanism that enhances the diversity of eukaryotic transcriptomes. Here, we generated 8.3 Gb high-quality RNA-sequencing data from cotton (Gossypium raimondii) and performed a systematic, comparative analysis of AS events. We mapped 85% of the RNA-sequencing data onto the reference genome and identified 154368 splice junctions with 16437 as events in 10197 genes. Intron retention constituted the majority (40%) of all AS events in G. raimondii. Comparison across 11 eukaryote species showed that intron retention is the most common AS type in higher plants. Although transposable elements (TEs) were found in only 2.9% of all G. raimondii introns, they are present in 43% of the retained introns, suggesting that TE-insertion may be an important mechanism for intron retention during AS. The majority of the TE insertions are concentrated 0-40 nt upstream of the 3'-splice site, substantially altering the distribution of branch points from preferred positions and reducing the efficiency of intron splicing by decreasing RNA secondary structure flexibility. Our data suggest that TE-insertion-induced changes in branch point-site distribution are important for intron retention-type AS. Our findings may help explain the vast differences in intron-retention frequencies between vertebrates and higher plants.

A deep survey of alternative splicing in grape reveals changes in the splicing machinery related to tissue, stress condition and genotype

BACKGROUND

Alternative splicing (AS) significantly enhances transcriptome complexity. It is differentially regulated in a wide variety of cell types and plays a role in several cellular processes. Here we describe a detailed survey of alternative splicing in grape based on 124 SOLiD RNAseq analyses from different tissues, stress conditions and genotypes.

RESULTS

We used the RNAseq data to update the existing grape gene prediction with 2,258 new coding genes and 3,336 putative long non-coding RNAs. Several gene structures have been improved and alternative splicing was described for about 30% of the genes. A link between AS and miRNAs was shown in 139 genes where we found that AS affects the miRNA target site. A quantitative analysis of the isoforms indicated that most of the spliced genes have one major isoform and tend to simultaneously co-express a low number of isoforms, typically two, with intron retention being the most frequent alternative splicing event.

CONCLUSIONS

As described in Arabidopsis, also grape displays a marked AS tissue-specificity, while stress conditions produce splicing changes to a minor extent. Surprisingly, some distinctive splicing features were also observed between genotypes. This was further supported by the observation that the panel of Serine/Arginine-rich splicing factors show a few, but very marked differences between genotypes. The finding that a part the splicing machinery can change in closely related organisms can lead to some interesting hypotheses for evolutionary adaptation, that could be particularly relevant in the response to sudden and strong selective pressures.

Genome-wide survey of cold stress regulated alternative splicing in Arabidopsis thaliana with tiling microarray

Alternative splicing plays a major role in expanding the potential informational content of eukaryotic genomes. It is an important post-transcriptional regulatory mechanism that can increase protein diversity and affect mRNA stability. Alternative splicing is often regulated in a tissue-specific and stress-responsive manner. Cold stress, which adversely affects plant growth and development, regulates the transcription and splicing of plant splicing factors. This can affect the pre-mRNA processing of many genes. To identify cold regulated alternative splicing we applied Affymetrix Arabidopsis tiling arrays to survey the transcriptome under cold treatment conditions. A novel algorithm was used for detection of statistically relevant changes in intron expression within a transcript between control and cold growth conditions. A reverse transcription polymerase chain reaction (RT-PCR) analysis of a number of randomly selected genes confirmed the changes in splicing patterns under cold stress predicted by tiling array. Our analysis revealed new types of cold responsive genes. While their expression level remains relatively unchanged under cold stress their splicing pattern shows detectable changes in the relative abundance of isoforms. The majority of cold regulated alternative splicing introduced a premature termination codon (PTC) into the transcripts creating potential targets for degradation by the nonsense mediated mRNA decay (NMD) process. A number of these genes were analyzed in NMD-defective mutants by RT-PCR and shown to evade NMD. This may result in new and truncated proteins with altered functions or dominant negative effects. The results indicate that cold affects both quantitative and qualitative aspects of gene expression.

Genome-wide landscape of alternative splicing events in Brachypodium distachyon

Recently, Brachypodium distachyon has emerged as a model plant for studying monocot grasses and cereal crops. Using assembled expressed transcript sequences and subsequent mapping to the corresponding genome, we identified 1219 alternative splicing (AS) events spanning across 2021 putatively assembled transcripts generated from 941 genes. Approximately, 6.3% of expressed genes are alternatively spliced in B. distachyon. We observed that a majority of the identified AS events were related to retained introns (55.5%), followed by alternative acceptor sites (16.7%). We also observed a low percentage of exon skipping (5.0%) and alternative donor site events (8.8%). The 'complex event' that consists of a combination of two or more basic splicing events accounted for ∼14.0%. Comparative AS transcript analysis revealed 163 and 39 homologous pairs between B. distachyon and Oryza sativa and between B. distachyon and Arabidopsis thaliana, respectively. In all, we found 16 AS transcripts to be conserved in all 3 species. AS events and related putative assembled transcripts annotation can be systematically browsed at Plant Alternative Splicing Database (http://proteomics.ysu.edu/altsplice/plant/).

Removal of retained introns regulates translation in the rapidly developing gametophyte of Marsilea vestita

The utilization of stored RNA is a driving force in rapid development. Here, we show that retention and subsequent removal of introns from pre-mRNAs regulate temporal patterns of translation during rapid and posttranscriptionally controlled spermatogenesis of the fern Marsilea vestita. Analysis of RNAseq-derived transcriptomes revealed a large subset of intron-retaining transcripts (IRTs) that encode proteins essential for gamete development. Genomic and IRT sequence comparisons show that other introns have been previously removed from the IRT pre-mRNAs. Fully spliced isoforms appear at distinct times during development in a spliceosome-dependent and transcription-independent manner. RNA interference knockdowns of 17/17 IRTs produced anomalies after the time points when those transcripts would normally be spliced. Intron retention is a functional mechanism for forestalling precocious translation of transcripts in the male gametophyte of M. vestita. These results have broad implications for plant gene regulation, where intron retention is widespread.

Estimation of Gene Expression at Isoform Level from mRNA-Seq Data by Bayesian Hierarchical Modeling

mRNA-Seq is a precise and highly reproducible technique for measurement of transcripts levels and yields sequence information of a transcriptome at a single nucleotide base-level thus enabling us to determine splice junctions and alternative splicing events with high confidence. Often analysis of mRNA-Seq data does not attempt to quantify the expressions at isoform level. In this paper our objective would be use the mRNA-Seq data to infer expression at isoform level, where splicing patterns of a gene is assumed to be known. A Bayesian latent variable based modeling framework is proposed here, where the parameterization enables us to infer at various levels. For example, expression variability of an isoform across different conditions; the model parameterization also allows us to carry out two-sample comparisons, e.g., using a Bayesian t-test, in addition simple presence or absence of an isoform can also be estimated by the use of the latent variables present in the model. In this paper we would carry out inference on isoform expression under different normalization techniques, since it has been recently shown that one of the most prominent sources of variation in differential call using mRNA-Seq data is the normalization method used. The statistical framework is developed for multiple isoforms and easily extends to reads mapping to multiple genes. This could be achieved by slight conceptual modifications in definitions of what we consider as a gene and what as an exon. Additionally proposed framework can be extended by appropriate modeling of the design matrix to infer about yet unknown novel transcripts. However such attempts should be made judiciously since the input date used in the proposed model does not use reads from splice junctions.

Fine mapping of a major QTL for flag leaf width in rice, qFLW4, which might be caused by alternative splicing of NAL1

Leaf width is an important agricultural trait in rice. QTL mapping in a recombinant inbred line population derived from the cross between the javanica cultivar D50 (narrow-leaved) and the indica cultivar HB277 (wide-leaved) identified five QTLs controlling flag leaf width. Fine mapping of the major QTL qFLW4 narrowed its location to a 74.8 kb interval between the SSR loci RM17483 and RM17486, a region which also contains the gene NAL1 (Narrow leaf 1). There was no difference in the level of NAL1 expression between cvs. D50 and HB277, but an analysis of the NAL1 transcripts showed that while most (if not all) of those produced in cv. D50 were full-length, two-thirds of those in HB277 were non-functional due to either loss or gain of sequence. The inference was that NAL1 is probably synonymous with qFLW4, and that the functional difference between the two alleles was due to alternative splicing. The analysis of expression of other known genes involved in the determination of leaf width provided no evidence of their having any clear functional association with qFLW4/NAL1.

Alternative splicing: enhancing ability to cope with stress via transcriptome plasticity

Alternative splicing is a mechanism for the regulation of gene expression that is widespread in higher eukaryotes. Genome-wide approaches, based on comparison of expressed and genomic sequences, on tiling arrays, and on next-generation sequencing, have provided growing knowledge of the extent, distribution and association of alternative splicing with stress-related genes in plants. The functional meaning of alternative splicing in response to stress has been defined for many genes, and in particular for those involved in the regulation of the stress responses, such as protein kinases, transcription factors, splicing regulators and pathogen-resistance genes. The production of proteins with diverse domain rearrangements from the same gene is the main alternative splicing mechanism for pathogen-resistance genes. The plant response to abiotic stress is also characterized by a second mechanism, which consists of the expression of alternative transcripts that are targeted to nonsense-mediated decay. These quantitatively regulate stress-related gene expression. Many alternative splicing events are well conserved among plant species, and also across kingdoms, especially those observed in response to stress, for genes encoding splicing regulators, and other classes of RNA-binding proteins. Nevertheless, non-conserved events indicate that alternative splicing represents an evolutionary strategy that rapidly increases genome plasticity and develops new gene functions, along with other mechanisms such as gene duplication. Finally, the study of the naturally occurring variability of alternative splicing and the identification of genomic regions involved in the regulation of alternative splicing in crops are proposed as strategies for selecting genotypes with superior performance under adverse environmental conditions.

Intron retention and 3'-UTR analysis of Arabidopsis Dicer-like 2 transcripts

Arabidopsis thaliana Dicer-like protein 2 (AtDCL2) plays an essential role in the RNA interference pathway. The function of AtDCL2 and other DCLs has been much studied but little has been done to characterize the DCLs transcripts before they are translated into proteins. Here, we investigated AtDCL2 transcripts and showed that all 21 introns of AtDCL2 except intron 9, 18, 20 and 21 could be retained although spliced sequences usually predominated. Intron 10 was more frequently retained and transient expression assays in Nicotiana benthamiana leaves showed that when AG/C at the 3' splicing site of the intron was changed to AG/G, the intron was more frequently spliced out. Conversely, a high retention of intron 18 was obtained if the AG/G at the 3' splicing site was changed to AG/C. These results suggest that the sequence at the 3' splicing site affects the efficiency of intron splicing. The 3'-UTRs of AtDCL2 had lengths between 54 and 154 nts, and the different 3'-UTRs differentially affected the transcriptional levels of fused GFP expressed transiently in N. benthamiana. Further comparisons and mutation experiments suggested that a putative SBF-1 binding site and an AU-rich element in the 3'-UTR both down-regulated expression of the upstream GFP fused to the 3'-UTR. Conversely, a second poly(A) consensus signal sequence in one 3'-UTR up-regulated gene expression. Our results provide insight into the character of AtDCL2 transcripts and demonstrate the potential complexity of factors that affect the frequency and patterns of alternative splicing.

Deciphering the plant splicing code: experimental and computational approaches for predicting alternative splicing and splicing regulatory elements

Extensive alternative splicing (AS) of precursor mRNAs (pre-mRNAs) in multicellular eukaryotes increases the protein-coding capacity of a genome and allows novel ways to regulate gene expression. In flowering plants, up to 48% of intron-containing genes exhibit AS. However, the full extent of AS in plants is not yet known, as only a few high-throughput RNA-Seq studies have been performed. As the cost of obtaining RNA-Seq reads continues to fall, it is anticipated that huge amounts of plant sequence data will accumulate and help in obtaining a more complete picture of AS in plants. Although it is not an onerous task to obtain hundreds of millions of reads using high-throughput sequencing technologies, computational tools to accurately predict and visualize AS are still being developed and refined. This review will discuss the tools to predict and visualize transcriptome-wide AS in plants using short-reads and highlight their limitations. Comparative studies of AS events between plants and animals have revealed that there are major differences in the most prevalent types of AS events, suggesting that plants and animals differ in the way they recognize exons and introns. Extensive studies have been performed in animals to identify cis-elements involved in regulating AS, especially in exon skipping. However, few such studies have been carried out in plants. Here, we review the current state of research on splicing regulatory elements (SREs) and briefly discuss emerging experimental and computational tools to identify cis-elements involved in regulation of AS in plants. The availability of curated alternative splice forms in plants makes it possible to use computational tools to predict SREs involved in AS regulation, which can then be verified experimentally. Such studies will permit identification of plant-specific features involved in AS regulation and contribute to deciphering the splicing code in plants.

Alternative splicing expands the repertoire of dominant JAZ repressors of jasmonate signaling

Jasmonates (JAs) are fatty acid-derived signaling compounds that control diverse aspects of plant growth, development and immunity. The F-box protein COI1 functions both as a receptor for jasmonoyl-l-isoleucine (JA-Ile) and as the component of an E3-ubiquitin ligase complex (SCF(COI1) ) that targets JAZ transcriptional regulators for degradation. A key feature of JAZ proteins is the C-terminal Jas motif that mediates the JA-Ile-dependent interaction with COI1. Here, we show that most JAZ genes from evolutionarily diverse plants contain a conserved intron that splits the Jas motif into 20 N-terminal and seven C-terminal (X(5) PY) amino acid submotifs. In most members of the Arabidopsis JAZ family, alternative splicing events involving retention of this intron generate proteins that are truncated before the X(5) PY sequence. In vitro pull-down and yeast two-hybrid assays indicate that these splice variants have reduced capacity to form stable complexes with COI1 in the presence of the bioactive stereoisomer of the hormone (3R,7S)-JA-Ile. cDNA overexpression studies showed that some, but not all, truncated splice variants are dominant repressors of JA signaling. We also show that strong constitutive expression of an intron-containing JAZ10 genomic clone is sufficient to repress JA responses. These findings provide evidence for functional differences between JAZ isoforms, and establish a direct link between the alternative splicing of JAZ pre-mRNA and the dominant repression of JA signal output. We propose that production of dominant JAZ repressors by alternative splicing reduces the negative consequences associated with inappropriate or hyperactivation of the JA response pathway.

Protein phosphatase complement in rice: genome-wide identification and transcriptional analysis under abiotic stress conditions and reproductive development

BACKGROUND

Protein phosphatases are the key components of a number of signaling pathways where they modulate various cellular responses. In plants, protein phosphatases constitute a large gene family and are reportedly involved in the regulation of abiotic stress responses and plant development. Recently, the whole complement of protein phosphatases has been identified in Arabidopsis genome. While PP2C class of serine/threonine phosphatases has been explored in rice, the whole complement of this gene family is yet to be reported.

RESULTS

In silico investigation revealed the presence of 132-protein phosphatase-coding genes in rice genome. Domain analysis and phylogenetic studies of evolutionary relationship categorized these genes into PP2A, PP2C, PTP, DSP and LMWP classes. PP2C class represents a major proportion of this gene family with 90 members. Chromosomal localization revealed their distribution on all the 12 chromosomes, with 42 genes being present on segmentally duplicated regions and 10 genes on tandemly duplicated regions of chromosomes. The expression profiles of 128 genes under salinity, cold and drought stress conditions, 11 reproductive developmental (panicle and seed) stages along with three stages of vegetative development were analyzed using microarray expression data. 46 genes were found to be differentially expressing in 3 abiotic stresses out of which 31 were up-regulated and 15 exhibited down-regulation. A total of 82 genes were found to be differentially expressing in different developmental stages. An overlapping expression pattern was found for abiotic stresses and reproductive development, wherein 8 genes were up-regulated and 7 down-regulated. Expression pattern of the 13 selected genes was validated employing real time PCR, and it was found to be in accordance with the microarray expression data for most of the genes.

CONCLUSIONS

Exploration of protein phosphatase gene family in rice has resulted in the identification of 132 members, which can be further divided into different classes phylogenetically. Expression profiling and analysis indicate the involvement of this large gene family in a number of signaling pathways triggered by abiotic stresses and their possible role in plant development. Our study will provide the platform from where; the expression pattern information can be transformed into molecular, cellular and biochemical characterization of members belonging to this gene family.

Protein phosphatase complement in rice: genome-wide identification and transcriptional analysis under abiotic stress conditions and reproductive development

Background

Protein phosphatases are the key components of a number of signaling pathways where they modulate various cellular responses. In plants, protein phosphatases constitute a large gene family and are reportedly involved in the regulation of abiotic stress responses and plant development. Recently, the whole complement of protein phosphatases has been identified in Arabidopsis genome. While PP2C class of serine/threonine phosphatases has been explored in rice, the whole complement of this gene family is yet to be reported.

Results

In silico investigation revealed the presence of 132-protein phosphatase-coding genes in rice genome. Domain analysis and phylogenetic studies of evolutionary relationship categorized these genes into PP2A, PP2C, PTP, DSP and LMWP classes. PP2C class represents a major proportion of this gene family with 90 members. Chromosomal localization revealed their distribution on all the 12 chromosomes, with 42 genes being present on segmentally duplicated regions and 10 genes on tandemly duplicated regions of chromosomes. The expression profiles of 128 genes under salinity, cold and drought stress conditions, 11 reproductive developmental (panicle and seed) stages along with three stages of vegetative development were analyzed using microarray expression data. 46 genes were found to be differentially expressing in 3 abiotic stresses out of which 31 were up-regulated and 15 exhibited down-regulation. A total of 82 genes were found to be differentially expressing in different developmental stages. An overlapping expression pattern was found for abiotic stresses and reproductive development, wherein 8 genes were up-regulated and 7 down-regulated. Expression pattern of the 13 selected genes was validated employing real time PCR, and it was found to be in accordance with the microarray expression data for most of the genes.

Conclusions

Exploration of protein phosphatase gene family in rice has resulted in the identification of 132 members, which can be further divided into different classes phylogenetically. Expression profiling and analysis indicate the involvement of this large gene family in a number of signaling pathways triggered by abiotic stresses and their possible role in plant development. Our study will provide the platform from where; the expression pattern information can be transformed into molecular, cellular and biochemical characterization of members belonging to this gene family.

Identification and characterization of NAGNAG alternative splicing in the moss Physcomitrella patens

BACKGROUND

Alternative splicing (AS) involving tandem acceptors that are separated by three nucleotides (NAGNAG) is an evolutionarily widespread class of AS, which is well studied in Homo sapiens (human) and Mus musculus (mouse). It has also been shown to be common in the model seed plants Arabidopsis thaliana and Oryza sativa (rice). In one of the first studies involving sequence-based prediction of AS in plants, we performed a genome-wide identification and characterization of NAGNAG AS in the model plant Physcomitrella patens, a moss.

RESULTS

Using Sanger data, we found 295 alternatively used NAGNAG acceptors in P. patens. Using 31 features and training and test datasets of constitutive and alternative NAGNAGs, we trained a classifier to predict the splicing outcome at NAGNAG tandem splice sites (alternative splicing, constitutive at the first acceptor, or constitutive at the second acceptor). Our classifier achieved a balanced specificity and sensitivity of >or= 89%. Subsequently, a classifier trained exclusively on data well supported by transcript evidence was used to make genome-wide predictions of NAGNAG splicing outcomes. By generation of more transcript evidence from a next-generation sequencing platform (Roche 454), we found additional evidence for NAGNAG AS, with altogether 664 alternative NAGNAGs being detected in P. patens using all currently available transcript evidence. The 454 data also enabled us to validate the predictions of the classifier, with 64% (80/125) of the well-supported cases of AS being predicted correctly.

CONCLUSION

NAGNAG AS is just as common in the moss P. patens as it is in the seed plants A. thaliana and O. sativa (but not conserved on the level of orthologous introns), and can be predicted with high accuracy. The most informative features are the nucleotides in the NAGNAG and in its immediate vicinity, along with the splice sites scores, as found earlier for NAGNAG AS in animals. Our results suggest that the mechanism behind NAGNAG AS in plants is similar to that in animals and is largely dependent on the splice site and its immediate neighborhood.

Unconstrained mining of transcript data reveals increased alternative splicing complexity in the human transcriptome

Mining massive amounts of transcript data for alternative splicing information is paramount to help understand how the maturation of RNA regulates gene expression. We developed an algorithm to cluster transcript data to annotated genes to detect unannotated splice variants. A higher number of alternatively spliced genes and isoforms were found compared to other alternative splicing databases. Comparison of human and mouse data revealed a marked increase, in human, of splice variants incorporating novel exons and retained introns. Previously unannotated exons were validated by tiling array expression data and shown to correspond preferentially to novel first exons. Retained introns were validated by tiling array and deep sequencing data. The majority of retained introns were shorter than 500 nt and had weak polypyrimidine tracts. A subset of retained introns matching small RNAs and displaying a high GC content suggests a possible coordination between splicing regulation and production of noncoding RNAs. Conservation of unannotated exons and retained introns was higher in horse, dog and cow than in rodents, and 64% of exon sequences were only found in primates. This analysis highlights previously bypassed alternative splice variants, which may be crucial to deciphering more complex pathways of gene regulation in human.

Large-scale analysis of full-length cDNAs from the tomato (Solanum lycopersicum) cultivar Micro-Tom, a reference system for the Solanaceae genomics

BACKGROUND

The Solanaceae family includes several economically important vegetable crops. The tomato (Solanum lycopersicum) is regarded as a model plant of the Solanaceae family. Recently, a number of tomato resources have been developed in parallel with the ongoing tomato genome sequencing project. In particular, a miniature cultivar, Micro-Tom, is regarded as a model system in tomato genomics, and a number of genomics resources in the Micro-Tom-background, such as ESTs and mutagenized lines, have been established by an international alliance.

RESULTS

To accelerate the progress in tomato genomics, we developed a collection of fully-sequenced 13,227 Micro-Tom full-length cDNAs. By checking redundant sequences, coding sequences, and chimeric sequences, a set of 11,502 non-redundant full-length cDNAs (nrFLcDNAs) was generated. Analysis of untranslated regions demonstrated that tomato has longer 5'- and 3'-untranslated regions than most other plants but rice. Classification of functions of proteins predicted from the coding sequences demonstrated that nrFLcDNAs covered a broad range of functions. A comparison of nrFLcDNAs with genes of sixteen plants facilitated the identification of tomato genes that are not found in other plants, most of which did not have known protein domains. Mapping of the nrFLcDNAs onto currently available tomato genome sequences facilitated prediction of exon-intron structure. Introns of tomato genes were longer than those of Arabidopsis and rice. According to a comparison of exon sequences between the nrFLcDNAs and the tomato genome sequences, the frequency of nucleotide mismatch in exons between Micro-Tom and the genome-sequencing cultivar (Heinz 1706) was estimated to be 0.061%.

CONCLUSION

The collection of Micro-Tom nrFLcDNAs generated in this study will serve as a valuable genomic tool for plant biologists to bridge the gap between basic and applied studies. The nrFLcDNA sequences will help annotation of the tomato whole-genome sequence and aid in tomato functional genomics and molecular breeding. Full-length cDNA sequences and their annotations are provided in the database KaFTom http://www.pgb.kazusa.or.jp/kaftom/ via the website of the National Bioresource Project Tomato http://tomato.nbrp.jp.

Large-scale analysis of full-length cDNAs from the tomato (Solanum lycopersicum) cultivar Micro-Tom, a reference system for the Solanaceae genomics

Background

The Solanaceae family includes several economically important vegetable crops. The tomato (Solanum lycopersicum) is regarded as a model plant of the Solanaceae family. Recently, a number of tomato resources have been developed in parallel with the ongoing tomato genome sequencing project. In particular, a miniature cultivar, Micro-Tom, is regarded as a model system in tomato genomics, and a number of genomics resources in the Micro-Tom-background, such as ESTs and mutagenized lines, have been established by an international alliance.

Results

To accelerate the progress in tomato genomics, we developed a collection of fully-sequenced 13,227 Micro-Tom full-length cDNAs. By checking redundant sequences, coding sequences, and chimeric sequences, a set of 11,502 non-redundant full-length cDNAs (nrFLcDNAs) was generated. Analysis of untranslated regions demonstrated that tomato has longer 5'- and 3'-untranslated regions than most other plants but rice. Classification of functions of proteins predicted from the coding sequences demonstrated that nrFLcDNAs covered a broad range of functions. A comparison of nrFLcDNAs with genes of sixteen plants facilitated the identification of tomato genes that are not found in other plants, most of which did not have known protein domains. Mapping of the nrFLcDNAs onto currently available tomato genome sequences facilitated prediction of exon-intron structure. Introns of tomato genes were longer than those of Arabidopsis and rice. According to a comparison of exon sequences between the nrFLcDNAs and the tomato genome sequences, the frequency of nucleotide mismatch in exons between Micro-Tom and the genome-sequencing cultivar (Heinz 1706) was estimated to be 0.061%.

Conclusion

The collection of Micro-Tom nrFLcDNAs generated in this study will serve as a valuable genomic tool for plant biologists to bridge the gap between basic and applied studies. The nrFLcDNA sequences will help annotation of the tomato whole-genome sequence and aid in tomato functional genomics and molecular breeding. Full-length cDNA sequences and their annotations are provided in the database KaFTom http://www.pgb.kazusa.or.jp/kaftom/ via the website of the National Bioresource Project Tomato http://tomato.nbrp.jp.

Genome-wide analysis of alternative splicing in Chlamydomonas reinhardtii

Background

Genome-wide computational analysis of alternative splicing (AS) in several flowering plants has revealed that pre-mRNAs from about 30% of genes undergo AS. Chlamydomonas, a simple unicellular green alga, is part of the lineage that includes land plants. However, it diverged from land plants about one billion years ago. Hence, it serves as a good model system to study alternative splicing in early photosynthetic eukaryotes, to obtain insights into the evolution of this process in plants, and to compare splicing in simple unicellular photosynthetic and non-photosynthetic eukaryotes. We performed a global analysis of alternative splicing in Chlamydomonas reinhardtii using its recently completed genome sequence and all available ESTs and cDNAs.

Results

Our analysis of AS using BLAT and a modified version of the Sircah tool revealed AS of 498 transcriptional units with 611 events, representing about 3% of the total number of genes. As in land plants, intron retention is the most prevalent form of AS. Retained introns and skipped exons tend to be shorter than their counterparts in constitutively spliced genes. The splice site signals in all types of AS events are weaker than those in constitutively spliced genes. Furthermore, in alternatively spliced genes, the prevalent splice form has a stronger splice site signal than the non-prevalent form. Analysis of constitutively spliced introns revealed an over-abundance of motifs with simple repetitive elements in comparison to introns involved in intron retention. In almost all cases, AS results in a truncated ORF, leading to a coding sequence that is around 50% shorter than the prevalent splice form. Using RT-PCR we verified AS of two genes and show that they produce more isoforms than indicated by EST data. All cDNA/EST alignments and splice graphs are provided in a website at http://combi.cs.colostate.edu/as/chlamy.

Conclusions

The extent of AS in Chlamydomonas that we observed is much smaller than observed in land plants, but is much higher than in simple unicellular heterotrophic eukaryotes. The percentage of different alternative splicing events is similar to flowering plants. Prevalence of constitutive and alternative splicing in Chlamydomonas, together with its simplicity, many available public resources, and well developed genetic and molecular tools for this organism make it an excellent model system to elucidate the mechanisms involved in regulated splicing in photosynthetic eukaryotes.

Genome-wide mapping of alternative splicing in Arabidopsis thaliana

Alternative splicing can enhance transcriptome plasticity and proteome diversity. In plants, alternative splicing can be manifested at different developmental stages, and is frequently associated with specific tissue types or environmental conditions such as abiotic stress. We mapped the Arabidopsis transcriptome at single-base resolution using the Illumina platform for ultrahigh-throughput RNA sequencing (RNA-seq). Deep transcriptome sequencing confirmed a majority of annotated introns and identified thousands of novel alternatively spliced mRNA isoforms. Our analysis suggests that at least approximately 42% of intron-containing genes in Arabidopsis are alternatively spliced; this is significantly higher than previous estimates based on cDNA/expressed sequence tag sequencing. Random validation confirmed that novel splice isoforms empirically predicted by RNA-seq can be detected in vivo. Novel introns detected by RNA-seq were substantially enriched in nonconsensus terminal dinucleotide splice signals. Alternative isoforms with premature termination codons (PTCs) comprised the majority of alternatively spliced transcripts. Using an example of an essential circadian clock gene, we show that intron retention can generate relatively abundant PTC(+) isoforms and that this specific event is highly conserved among diverse plant species. Alternatively spliced PTC(+) isoforms can be potentially targeted for degradation by the nonsense mediated mRNA decay (NMD) surveillance machinery or regulate the level of functional transcripts by the mechanism of regulated unproductive splicing and translation (RUST). We demonstrate that the relative ratios of the PTC(+) and reference isoforms for several key regulatory genes can be considerably shifted under abiotic stress treatments. Taken together, our results suggest that like in animals, NMD and RUST may be widespread in plants and may play important roles in regulating gene expression.

Functional diversity of human protein kinase splice variants marks significant expansion of human kinome

BACKGROUND

Protein kinases are involved in diverse spectrum of cellular processes. Availability of draft version of the human genomic data in the year 2001 enabled recognition of repertoire of protein kinases. However, over the years the human genomic data is being refined and the current release of human genomic data has helped us to recognize a larger repertoire of over 900 human protein kinases represented mainly by splice variants.

RESULTS

Many of these identified protein kinases are alternatively spliced products. Interestingly, some of the human kinase splice variants appear to be significantly diverged in terms of their functional properties as represented by incorporation or absence of one or more domains. Many sets of protein kinase splice variants have substantially different domain organization and in a few sets of splice variants kinase domains belong to different subfamilies of kinases suggesting potential participation in different signal transduction pathways.

CONCLUSIONS

Addition or deletion of a domain between splice variants of multi-domain kinases appears to be a means of generating differences in the functional features of otherwise similar kinases. It is intriguing that marked sequence diversity within the catalytic regions of some of the splice variant kinases result in kinases belonging to different subfamilies. These human kinase splice variants with different functions might contribute to diversity of eukaryotic cellular signaling.

Carrot alternative oxidase gene AOX2a demonstrates allelic and genotypic polymorphisms in intron 3

Single nucleotide polymorphisms (SNPs) and insertion-deletions (InDels) are becoming important genetic markers for major crop species. In this study, we focus on variations at genomic level of the Daucus carota L. AOX2a gene. The use of gene-specific primers designed in exon regions on the boundaries of introns permitted to recognize intron length polymorphism (ILP) in intron 3 AOX2a by simple polymerase chain reaction (PCR) assays. The length of intron 3 can vary in individual carrot plants. Thus, allelic variation can be used as a tool to discriminate between single plant genotypes. Using this approach, individual plants from cv. Rotin and from diverse breeding lines and cultivars were identified that showed genetic variability by AOX2a ILPs. Repetitive patterns of intron length variation have been observed which allows grouping of genotypes. Polymorphic and identical PCR fragments revealed underlying high levels of sequence polymorphism. Variability was due to InDel events and intron single nucleotide polymorphisms (ISNPs), with a repetitive deletion in intron 3 affecting a putative pre-miRNA site. The results suggest that high AOX2a gene diversity in D. carota can be explored for the development of functional markers related to agronomic traits.

Intron polymorphism pattern in AOX1b of wild St John's wort (Hypericum perforatum) allows discrimination between individual plants

The present paper deals with the analysis of natural polymorphism in a selected alternative oxidase (AOX) gene of the medicinal plant, St John's wort. Four partial AOX gene sequences were isolated from the genomic DNA of a wild plant of Hypericum perforatum L. Three genes belong to the subfamily AOX1 (HpAOX1a, b and c) and one to the subfamily AOX2 (HpAOX2). The partial sequence of HpAOX1b showed polymerase chain reaction (PCR) fragment size variation as a result of variable lengths in two introns. PCR performed by Exon Primed Intron Crossing (EPIC)-PCR displayed the same two-band pattern in six plants from a collection. Both fragments showed identical sequences for all exons. However, each of the two introns showed an insertion/deletion (InDel) in identical positions for all plants that counted for the difference in the two fragment sizes. The InDel in intron 1 influenced the predictability of a pre-microRNA site. The almost identical PCR fragment pattern was characterized by a high variability in the sequences. The InDels in both introns were linked to repetitive intron single nucleotide polymorphisms (ISNP)s. The polymorphic pattern obtained by InDels and ISNPs from both fragments together was appropriate to discriminate between all individual plants. We suggest that AOX sequence polymorphism in H. perforatum can be used for studies on gene diversity and biodiversity. Further, we conclude that AOX sequence polymorphism of individual plants should be considered in biological studies on AOX activity to exclude the influence of genetic diversity. The identified polymorphic fragments are available to be explored in future experiments as a potential source for functional marker development related to the characterization of origins/accessions and agronomic traits such as plant growth, development and yield stability.

Genome-wide mapping of alternative splicing in Arabidopsis thaliana

Alternative splicing can enhance transcriptome plasticity and proteome diversity. In plants, alternative splicing can be manifested at different developmental stages, and is frequently associated with specific tissue types or environmental conditions such as abiotic stress. We mapped the Arabidopsis transcriptome at single-base resolution using the Illumina platform for ultrahigh-throughput RNA sequencing (RNA-seq). Deep transcriptome sequencing confirmed a majority of annotated introns and identified thousands of novel alternatively spliced mRNA isoforms. Our analysis suggests that at least approximately 42% of intron-containing genes in Arabidopsis are alternatively spliced; this is significantly higher than previous estimates based on cDNA/expressed sequence tag sequencing. Random validation confirmed that novel splice isoforms empirically predicted by RNA-seq can be detected in vivo. Novel introns detected by RNA-seq were substantially enriched in nonconsensus terminal dinucleotide splice signals. Alternative isoforms with premature termination codons (PTCs) comprised the majority of alternatively spliced transcripts. Using an example of an essential circadian clock gene, we show that intron retention can generate relatively abundant PTC(+) isoforms and that this specific event is highly conserved among diverse plant species. Alternatively spliced PTC(+) isoforms can be potentially targeted for degradation by the nonsense mediated mRNA decay (NMD) surveillance machinery or regulate the level of functional transcripts by the mechanism of regulated unproductive splicing and translation (RUST). We demonstrate that the relative ratios of the PTC(+) and reference isoforms for several key regulatory genes can be considerably shifted under abiotic stress treatments. Taken together, our results suggest that like in animals, NMD and RUST may be widespread in plants and may play important roles in regulating gene expression.

Alternative splicing studies of the reactive oxygen species gene network in Populus reveal two isoforms of high-isoelectric-point superoxide dismutase

Recent evidence has shown that alternative splicing (AS) is widely involved in the regulation of gene expression, substantially extending the diversity of numerous proteins. In this study, a subset of expressed sequence tags representing members of the reactive oxygen species gene network was selected from the PopulusDB database to investigate AS mechanisms in Populus. Examples of all known types of AS were detected, but intron retention was the most common. Interestingly, the closest Arabidopsis (Arabidopsis thaliana) homologs of half of the AS genes identified in Populus are not reportedly alternatively spliced. Two genes encoding the protein of most interest in our study (high-isoelectric-point superoxide dismutase [hipI-SOD]) have been found in black cottonwood (Populus trichocarpa), designated PthipI-SODC1 and PthipI-SODC2. Analysis of the expressed sequence tag libraries has indicated the presence of two transcripts of PthipI-SODC1 (hipI-SODC1b and hipI-SODC1s). Alignment of these sequences with the PthipI-SODC1 gene showed that hipI-SODC1b was 69 bp longer than hipI-SODC1s due to an AS event involving the use of an alternative donor splice site in the sixth intron. Transcript analysis showed that the splice variant hipI-SODC1b was differentially expressed, being clearly expressed in cambial and xylem, but not phloem, regions. In addition, immunolocalization and mass spectrometric data confirmed the presence of hipI-SOD proteins in vascular tissue. The functionalities of the spliced gene products were assessed by expressing recombinant hipI-SOD proteins and in vitro SOD activity assays.

Alternative splicing and differential expression of two transcripts of nicotine adenine dinucleotide phosphate oxidase B gene from Zea mays

With the exception of rice, little is known about the existence of respiratory burst oxidase homolog (rboh) gene in cereals. The present study reports the cloning and analysis of a novel rboh gene, termed ZmrbohB, from maize (Zea mays L.). The full-length cDNA of ZmrbohB encodes a 942 amino acid protein containing all of the respiratory burst oxidase homolog catalytically critical motifs. Alternative splicing of ZmrbohB has generated two transcript isoforms, ZmrbohB-alpha and -beta. Spliced transcript ZmrbohB-beta retains an unspliced intron 11 that carries a premature termination codon and probably leads to nonsense-mediated mRNA decay. Expression analysis showed that two splice isoforms were differentially expressed in various tissues and at different developmental stages, and the major product was ZmrbohB-alpha. The transcripts of ZmrbohB-alpha accumulated markedly when the maize seedlings were subjected to various abiotic stimuli, such as wounding, cold (4 degrees C), heat (40 degrees C), UV and salinity stress. In addition, several abiotic stimuli also affected the alternative splicing pattern of ZmrbohB except wounding. These results provide new insight into roles in the expression regulation of plant rboh genes and suggest that ZmrbohB gene may play a role in response to environmental stresses.

Expressed sequence tags: an overview

Expressed sequence tags (ESTs) are fragments of mRNA sequences derived through single sequencing reactions performed on randomly selected clones from cDNA libraries. To date, over 45 million ESTs have been generated from over 1400 different species of eukaryotes. For the most part, EST projects are used to either complement existing genome projects or serve as low-cost alternatives for purposes of gene discovery. However, with improvements in accuracy and coverage, they are beginning to find application in fields such as phylogenetics, transcript profiling and proteomics. This volume provides practical details on the generation and analysis of ESTs. Chapters are presented which cover creation of cDNA libraries; generation and processing of sequence data; bioinformatics analysis of ESTs; and their application to phylogenetics and transcript profiling.

WebScipio: an online tool for the determination of gene structures using protein sequences

Background

Obtaining the gene structure for a given protein encoding gene is an important step in many analyses. A software suited for this task should be readily accessible, accurate, easy to handle and should provide the user with a coherent representation of the most probable gene structure. It should be rigorous enough to optimise features on the level of single bases and at the same time flexible enough to allow for cross-species searches.

Results

WebScipio, a web interface to the Scipio software, allows a user to obtain the corresponding coding sequence structure of a here given a query protein sequence that belongs to an already assembled eukaryotic genome. The resulting gene structure is presented in various human readable formats like a schematic representation, and a detailed alignment of the query and the target sequence highlighting any discrepancies. WebScipio can also be used to identify and characterise the gene structures of homologs in related organisms. In addition, it offers a web service for integration with other programs.

Conclusion

WebScipio is a tool that allows users to get a high-quality gene structure prediction from a protein query. It offers more than 250 eukaryotic genomes that can be searched and produces predictions that are close to what can be achieved by manual annotation, for in-species and cross-species searches alike. WebScipio is freely accessible at .

Regulation of alternative splicing of pre-mRNAs by stresses

A substantial fraction (approximately 30%) of plant genes is alternatively spliced, but how alternative splicing is regulated remains unknown. Many plant genes undergo alternative splicing in response to a variety of stresses. Large-scale computational analyses and experimental approaches focused on select genes are beginning to reveal that alternative splicing constitutes an integral part of gene regulation in stress responses. Based on the studies discussed in this chapter, it appears that alternative splicing generates transcriptome/proteome complexity that is likely to be important for stress adaptation. However, the signaling pathways that relay stress conditions to splicing machinery and if and how the alternative spliced products confer adaptive advantages to plants are poorly understood.