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September 22, 2019

Full-length RNA sequencing reveals unique transcriptome composition in bermudagrass.

Bermudagrass [Cynodon dactylon (L.) Pers.] is an important perennial warm-season turfgrass species with great economic value. However, the reference genome and transcriptome information are still deficient in bermudagrass, which severely impedes functional and molecular breeding studies. In this study, through analyzing a mixture sample of leaves, stolons, shoots, roots and flowers with single-molecule long-read sequencing technology from Pacific Biosciences (PacBio), we reported the first full-length transcriptome dataset of bermudagrass (C. dactylon cultivar Yangjiang) comprising 78,192 unigenes. Among the unigenes, 66,409 were functionally annotated, whereas 27,946 were found to have two or more isoforms. The annotated full-length unigenes provided many new insights into gene sequence characteristics and systematic phylogeny of bermudagrass. By comparison with transcriptome dataset in nine grass species, KEGG pathway analyses further revealed that C4 photosynthesis-related genes, notably the phosphoenolpyruvate carboxylase and pyruvate, phosphate dikinase genes, are specifically enriched in bermudagrass. These results not only explained the possible reason why bermudagrass flourishes in warm areas but also provided a solid basis for future studies in this important turfgrass species. Copyright © 2018 Elsevier Masson SAS. All rights reserved.

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September 22, 2019

Multiple regulatory networks are activated during cold stress in Medicago sativa L.

Cultivated alfalfa (Medicago sativa L.) is one of the most important perennial legume forages in the world, and it has considerable potential as a valuable forage crop for livestock. However, the molecular mechanisms underlying alfalfa responses to cold stress are largely unknown. In this study, the transcriptome changes in alfalfa under cold stress at 4 °C for 2, 6, 24, and 48 h (three replicates for each time point) were analyzed using the high-throughput sequencing platform, BGISEQ-500, resulting in the identification of 50,809 annotated unigenes and 5283 differentially expressed genes (DEGs). Metabolic pathway enrichment analysis demonstrated that the DEGs were involved in carbohydrate metabolism, photosynthesis, plant hormone signal transduction, and the biosynthesis of amino acids. Moreover, the physiological changes of glutathione and proline content, catalase, and peroxidase activity were in accordance with dynamic transcript profiles of the relevant genes. Additionally, some transcription factors might play important roles in the alfalfa response to cold stress, as determined by the expression pattern of the related genes during 48 h of cold stress treatment. These findings provide valuable information for identifying and characterizing important components in the cold signaling network in alfalfa and enhancing the understanding of the molecular mechanisms underlying alfalfa responses to cold stress.

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September 22, 2019

ISOdb: A comprehensive database of full-length isoforms generated by Iso-Seq.

The accurate landscape of transcript isoforms plays an important role in the understanding of gene function and gene regulation. However, building complete transcripts is very challenging for short reads generated using next-generation sequencing. Fortunately, isoform sequencing (Iso-Seq) using single-molecule sequencing technologies, such as PacBio SMRT, provides long reads spanning entire transcript isoforms which do not require assembly. Therefore, we have developed ISOdb, a comprehensive resource database for hosting and carrying out an in-depth analysis of Iso-Seq datasets and visualising the full-length transcript isoforms. The current version of ISOdb has collected 93 publicly available Iso-Seq samples from eight species and presents the samples in two levels: (1) sample level, including metainformation, long read distribution, isoform numbers, and alternative splicing (AS) events of each sample; (2) gene level, including the total isoforms, novel isoform number, novel AS number, and isoform visualisation of each gene. In addition, ISOdb provides a user interface in the website for uploading sample information to facilitate the collection and analysis of researchers’ datasets. Currently, ISOdb is the first repository that offers comprehensive resources and convenient public access for hosting, analysing, and visualising Iso-Seq data, which is freely available.

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September 22, 2019

Multiplex amplicon sequencing for microbe identification in community-based culture collections.

Microbiome analysis using metagenomic sequencing has revealed a vast microbial diversity associated with plants. Identifying the molecular functions associated with microbiome-plant interaction is a significant challenge concerning the development of microbiome-derived technologies applied to agriculture. An alternative to accelerate the discovery of the microbiome benefits to plants is to construct microbial culture collections concomitant with accessing microbial community structure and abundance. However, traditional methods of isolation, cultivation, and identification of microbes are time-consuming and expensive. Here we describe a method for identification of microbes in culture collections constructed by picking colonies from primary platings that may contain single or multiple microorganisms, which we named community-based culture collections (CBC). A multiplexing 16S rRNA gene amplicon sequencing based on two-step PCR amplifications with tagged primers for plates, rows, and columns allowed the identification of the microbial composition regardless if the well contains single or multiple microorganisms. The multiplexing system enables pooling amplicons into a single tube. The sequencing performed on the PacBio platform led to recovery near-full-length 16S rRNA gene sequences allowing accurate identification of microorganism composition in each plate well. Cross-referencing with plant microbiome structure and abundance allowed the estimation of diversity and abundance representation of microorganism in the CBC.

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September 22, 2019

Generation and comparative analysis of full-length transcriptomes in sweetpotato and its putative wild ancestor I. trifida.

Sweetpotato [Ipomoea batatas (L.) Lam.] is one of the most important crops in many developing countries and provides a candidate source of bioenergy. However, neither high-quality reference genome nor large-scale full-length cDNA sequences for this outcrossing hexaploid are still lacking, which in turn impedes progress in research studies in sweetpotato functional genomics and molecular breeding. In this study, we apply a combination of second- and third-generation sequencing technologies to sequence full-length transcriptomes in sweetpotato and its putative ancestor I. trifida. In total, we obtained 53,861/51,184 high-quality transcripts, which includes 34,963/33,637 putative full-length cDNA sequences, from sweetpotato/I. trifida. Amongst, we identified 104,540/94,174 open reading frames, 1476/1475 transcription factors, 25,315/27,090 simple sequence repeats, 417/531 long non-coding RNAs out of the sweetpotato/I. trifida dataset. By utilizing public available genomic contigs, we analyzed the gene features (including exon number, exon size, intron number, intron size, exon-intron structure) of 33,119 and 32,793 full-length transcripts in sweetpotato and I. trifida, respectively. Furthermore, comparative analysis between our transcript datasets and other large-scale cDNA datasets from different plant species enables us assessing the quality of public datasets, estimating the genetic similarity across relative species, and surveyed the evolutionary pattern of genes. Overall, our study provided fundamental resources of large-scale full-length transcripts in sweetpotato and its putative ancestor, for the first time, and would facilitate structural, functional and comparative genomics studies in this important crop.

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September 22, 2019

A survey of the sorghum transcriptome using single-molecule long reads.

Alternative splicing and alternative polyadenylation (APA) of pre-mRNAs greatly contribute to transcriptome diversity, coding capacity of a genome and gene regulatory mechanisms in eukaryotes. Second-generation sequencing technologies have been extensively used to analyse transcriptomes. However, a major limitation of short-read data is that it is difficult to accurately predict full-length splice isoforms. Here we sequenced the sorghum transcriptome using Pacific Biosciences single-molecule real-time long-read isoform sequencing and developed a pipeline called TAPIS (Transcriptome Analysis Pipeline for Isoform Sequencing) to identify full-length splice isoforms and APA sites. Our analysis reveals transcriptome-wide full-length isoforms at an unprecedented scale with over 11,000 novel splice isoforms. Additionally, we uncover APA of ~11,000 expressed genes and more than 2,100 novel genes. These results greatly enhance sorghum gene annotations and aid in studying gene regulation in this important bioenergy crop. The TAPIS pipeline will serve as a useful tool to analyse Iso-Seq data from any organism.

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September 22, 2019

Assessment of an organ-specific de novo transcriptome of the nematode trap-crop, Solanum sisymbriifolium

Solanum sisymbriifolium, also known as “Litchi Tomato” or “Sticky Nightshade,” is an undomesticated and poorly researched plant related to potato and tomato. Unlike the latter species, S. sisymbriifolium induces eggs of the cyst nematode, Globodera pallida, to hatch and migrate into its roots, but then arrests further nematode maturation. In order to provide researchers with a partial blueprint of its genetic make-up so that the mechanism of this response might be identified, we used single molecule real time (SMRT) sequencing to compile a high quality de novo transcriptome of 41,189 unigenes drawn from individually sequenced bud, root, stem, and leaf RNA populations. Functional annotation and BUSCO analysis showed that this transcriptome was surprisingly complete, even though it represented genes expressed at a single time point. By sequencing the 4 organ libraries separately, we found we could get a reliable snapshot of transcript distributions in each organ. A divergent site analysis of the merged transcriptome indicated that this species might have undergone a recent genome duplication and re-diploidization. Further analysis indicated that the plant then retained a disproportionate number of genes associated with photosynthesis and amino acid metabolism in comparison to genes with characteristics of R-proteins or involved in secondary metabolism. The former processes may have given S. sisymbriifolium a bigger competitive advantage than the latter did. Copyright © 2018 Wixom et al.

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September 22, 2019

Genome analysis of Taraxacum kok-saghyz Rodin provides new insights into rubber biosynthesis

The Russian dandelion Taraxacum kok-saghyz Rodin (TKS), a member of the Composite family and a potential alternative source of natural rubber (NR) and inulin, is an ideal model system for studying rubber biosynthesis. Here we present the draft genome of TKS, the first assembled NR-producing weed plant. The draft TKS genome assembly has a length of 1.29 Gb, containing 46,731 predicted protein-coding genes and 68.56% repeats, in which the LTR-RT elements predominantly contribute to the genome enlargement. We analyzed the heterozygous regions/genes, suggesting its possible involvement in inbreeding depression. Through comparative studies between rubber-producing and non-rubber-producing plants, we found that enzymes of the mevalonate (MVA) pathway and rubber elongation might be critical for rubber biosynthesis, and several key isoforms have been isolated showing predominantly expressed in the latex, indicating their crucial functions in rubber biosynthesis. Moreover, for two important families in rubber elongation, the CPT/CPTL and REF/SRPP families, diverse evolutionary tracks have been revealed. These results provide valuable resources and new insights into the mechanism of NR biosynthesis, and facilitate the development of alternative NR producing crops.

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September 22, 2019

Genome and secretome analysis of Pochonia chlamydosporia provide new insight into egg-parasitic mechanisms.

Pochonia chlamydosporia infects eggs and females of economically important plant-parasitic nematodes. The fungal isolates parasitizing different nematodes are genetically distinct. To understand their intraspecific genetic differentiation, parasitic mechanisms, and adaptive evolution, we assembled seven putative chromosomes of P. chlamydosporia strain 170 isolated from root-knot nematode eggs (~44?Mb, including 7.19% of transposable elements) and compared them with the genome of the strain 123 (~41?Mb) isolated from cereal cyst nematode. We focus on secretomes of the fungus, which play important roles in pathogenicity and fungus-host/environment interactions, and identified 1,750 secreted proteins, with a high proportion of carboxypeptidases, subtilisins, and chitinases. We analyzed the phylogenies of these genes and predicted new pathogenic molecules. By comparative transcriptome analysis, we found that secreted proteins involved in responses to nutrient stress are mainly comprised of proteases and glycoside hydrolases. Moreover, 32 secreted proteins undergoing positive selection and 71 duplicated gene pairs encoding secreted proteins are identified. Two duplicated pairs encoding secreted glycosyl hydrolases (GH30), which may be related to fungal endophytic process and lost in many insect-pathogenic fungi but exist in nematophagous fungi, are putatively acquired from bacteria by horizontal gene transfer. The results help understanding genetic origins and evolution of parasitism-related genes.

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September 22, 2019

Analysis of the hybrid genomes of two field isolates of the soil-borne fungal species Verticillium longisporum.

Brassica plant species are attacked by a number of pathogens; among them, the ones with a soil-borne lifestyle have become increasingly important. Verticillium stem stripe caused by Verticillium longisporum is one example. This fungal species is thought to be of a hybrid origin, having a genome composed of combinations of lineages denominated A and D. In this study we report the draft genomes of 2 V. longisporum field isolates sequenced using the Illumina technology. Genomic characterization and lineage composition, followed by selected gene analysis to facilitate the comprehension of its genomic features and potential effector categories were performed.The draft genomes of 2 Verticillium longisporum single spore isolates (VL1 and VL2) have an estimated ungapped size of about 70 Mb. The total number of protein encoding genes identified in VL1 was 20,793, whereas 21,072 gene models were predicted in VL2. The predicted genome size, gene contents, including the gene families coding for carbohydrate active enzymes were almost double the numbers found in V. dahliae and V. albo-atrum. Single nucleotide polymorphisms (SNPs) were frequently distributed in the two genomes but the distribution of heterozygosity and depth was not independent. Further analysis of potential parental lineages suggests that the V. longisporum genome is composed of two parts, A1 and D1, where A1 is more ancient than the parental lineage genome D1, the latter being more closer related to V. dahliae. Presence of the mating-type genes MAT1-1-1 and MAT1-2-1 in the V. longisporum genomes were confirmed. However, the MAT genes in V. dahliae, V. albo-atrum and V. longisporum have experienced extensive nucleotide changes at least partly explaining the present asexual nature of these fungal species.The established draft genome of V. longisporum is comparatively large compared to other studied ascomycete fungi. Consequently, high numbers of genes were predicted in the two V. longisporum genomes, among them many secreted proteins and carbohydrate active enzyme (CAZy) encoding genes. The genome is composed of two parts, where one lineage is more ancient than the part being more closely related to V. dahliae. Dissimilar mating-type sequences were identified indicating possible ancient hybridization events.

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September 22, 2019

Extreme haplotype variation in the desiccation-tolerant clubmoss Selaginella lepidophylla.

Plant genome size varies by four orders of magnitude, and most of this variation stems from dynamic changes in repetitive DNA content. Here we report the small 109?Mb genome of Selaginella lepidophylla, a clubmoss with extreme desiccation tolerance. Single-molecule sequencing enables accurate haplotype assembly of a single heterozygous S. lepidophylla plant, revealing extensive structural variation. We observe numerous haplotype-specific deletions consisting of largely repetitive and heavily methylated sequences, with enrichment in young Gypsy LTR retrotransposons. Such elements are active but rapidly deleted, suggesting “bloat and purge” to maintain a small genome size. Unlike all other land plant lineages, Selaginella has no evidence of a whole-genome duplication event in its evolutionary history, but instead shows unique tandem gene duplication patterns reflecting adaptation to extreme drying. Gene expression changes during desiccation in S. lepidophylla mirror patterns observed across angiosperm resurrection plants.

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September 22, 2019

Fluorescently-tagged human eIF3 for single-molecule spectroscopy.

Human translation initiation relies on the combined activities of numerous ribosome-associated eukaryotic initiation factors (eIFs). The largest factor, eIF3, is an ~800 kDa multiprotein complex that orchestrates a network of interactions with the small 40S ribosomal subunit, other eIFs, and mRNA, while participating in nearly every step of initiation. How these interactions take place during the time course of translation initiation remains unclear. Here, we describe a method for the expression and affinity purification of a fluorescently-tagged eIF3 from human cells. The tagged eIF3 dodecamer is structurally intact, functions in cell-based assays, and interacts with the HCV IRES mRNA and the 40S-IRES complex in vitro. By tracking the binding of single eIF3 molecules to the HCV IRES RNA with a zero-mode waveguides-based instrument, we show that eIF3 samples both wild-type IRES and an IRES that lacks the eIF3-binding region, and that the high-affinity eIF3-IRES interaction is largely determined by slow dissociation kinetics. The application of single-molecule methods to more complex systems involving eIF3 may unveil dynamics underlying mRNA selection and ribosome loading during human translation initiation.© The Author(s) 2017. Published by Oxford University Press on behalf of Nucleic Acids Research.

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September 22, 2019

MUMmer4: A fast and versatile genome alignment system.

The MUMmer system and the genome sequence aligner nucmer included within it are among the most widely used alignment packages in genomics. Since the last major release of MUMmer version 3 in 2004, it has been applied to many types of problems including aligning whole genome sequences, aligning reads to a reference genome, and comparing different assemblies of the same genome. Despite its broad utility, MUMmer3 has limitations that can make it difficult to use for large genomes and for the very large sequence data sets that are common today. In this paper we describe MUMmer4, a substantially improved version of MUMmer that addresses genome size constraints by changing the 32-bit suffix tree data structure at the core of MUMmer to a 48-bit suffix array, and that offers improved speed through parallel processing of input query sequences. With a theoretical limit on the input size of 141Tbp, MUMmer4 can now work with input sequences of any biologically realistic length. We show that as a result of these enhancements, the nucmer program in MUMmer4 is easily able to handle alignments of large genomes; we illustrate this with an alignment of the human and chimpanzee genomes, which allows us to compute that the two species are 98% identical across 96% of their length. With the enhancements described here, MUMmer4 can also be used to efficiently align reads to reference genomes, although it is less sensitive and accurate than the dedicated read aligners. The nucmer aligner in MUMmer4 can now be called from scripting languages such as Perl, Python and Ruby. These improvements make MUMer4 one the most versatile genome alignment packages available.

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September 22, 2019

Genomic diversity in the endosymbiotic bacterium Rhizobium leguminosarum.

Rhizobium leguminosarum bv. viciae is a soil a-proteobacterium that establishes a diazotrophic symbiosis with different legumes of the Fabeae tribe. The number of genome sequences from rhizobial strains available in public databases is constantly increasing, although complete, fully annotated genome structures from rhizobial genomes are scarce. In this work, we report and analyse the complete genome of R. leguminosarum bv. viciae UPM791. Whole genome sequencing can provide new insights into the genetic features contributing to symbiotically relevant processes such as bacterial adaptation to the rhizosphere, mechanisms for efficient competition with other bacteria, and the ability to establish a complex signalling dialogue with legumes, to enter the root without triggering plant defenses, and, ultimately, to fix nitrogen within the host. Comparison of the complete genome sequences of two strains of R. leguminosarum bv. viciae, 3841 and UPM791, highlights the existence of different symbiotic plasmids and a common core chromosome. Specific genomic traits, such as plasmid content or a distinctive regulation, define differential physiological capabilities of these endosymbionts. Among them, strain UPM791 presents unique adaptations for recycling the hydrogen generated in the nitrogen fixation process.

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September 22, 2019

Comparative mapping of the ASTRINGENCY locus controlling fruit astringency in hexaploid persimmon (Diospyros kaki Thunb.) with the diploid D. lotus reference genome

Persimmon (Diospyros kaki) is a tree crop species that originated in East Asia, consists mainly of hexaploid individuals (2n = 6x = 90) with some nonaploid individuals. One of the unique characteristics of persimmon is the continuous accumulation of proanthocyanidins (PAs) in its fruit until the middle of fruit development, resulting in a strong astringent taste even at commercial fruit maturity. Among persimmon cultivars, pollination-constant and non-astringent (PCNA) types cease PA accumulation in early fruit development and become non-astringent at commercial maturity. PCNA is an allelic trait to non-PCNA and is controlled by a single locus called the ASTRINGENCY (AST) locus. Previous segregation analyses indicated that the AST locus shows hexasomic inheritance; a recessive allele, ast, at this locus confers PCNA. Here, we report a shuttle mapping approach to delimit the AST locus region in the hexaploid persimmon genome by using D. lotus, a diploid relative of D. kaki, as a reference. A D. lotus F1 population of 333 individuals and 296 D. kaki siblings segregating for the PCNA trait were used to map the AST region using haplotype-specific markers covering the AST region. This indicated that the AST locus is syntenic to an approximately 915-kb region of the D. lotus genome. In this 915-kb region, we found several candidates for AST that were revealed from the fruit transcriptome of a population segregating for the PCNA trait. These results could provide important clues for the isolation of AST in hexaploid persimmon.

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