Long-read sequencing of extrachromosomal circular DNA and genome assembly of a Solanum lycopersicum breeding line revealed active LTR retrotransposons originating from S. Peruvianum L. introgressions.

Transposable elements (TEs) are a major force in the evolution of plant genomes. Differences in the transposition activities and landscapes of TEs can vary substantially, even in closely related species. Interspecific hybridization, a widely employed technique in tomato breeding, results in the creation of novel combinations of TEs from distinct species. The implications of this process for TE transposition activity have not been studied in modern cultivars. In this study, we used nanopore sequencing of extrachromosomal circular DNA (eccDNA) and identified two highly active Ty1/Copia LTR retrotransposon families of tomato (Solanum lycopersicum), called Salsa and Ketchup. Elements of these families produce thousands of eccDNAs under controlled conditions and epigenetic stress. EccDNA sequence analysis revealed that the major parts of eccDNA produced by Ketchup and Salsa exhibited low similarity to the S. lycopersicum genomic sequence. To trace the origin of these TEs, whole-genome nanopore sequencing and de novo genome assembly were performed. We found that these TEs occurred in a tomato breeding line via interspecific introgression from S. peruvianum. Our findings collectively show that interspecific introgressions can contribute to both genetic and phenotypic diversity not only by introducing novel genetic variants, but also by importing active transposable elements from other species.

Cas9-targeted Nanopore sequencing rapidly elucidates the transposition preferences and DNA methylation profiles of mobile elements in plants.

Transposable element insertions (TEIs) are an important source of genomic innovation by contributing to plant adaptation, speciation, and the production of new varieties. The often large, complex plant genomes make identifying TEIs from short reads difficult and expensive. Moreover, rare somatic insertions that reflect mobilome dynamics are difficult to track using short reads. To address these challenges, we combined Cas9-targeted Nanopore sequencing (CANS) with the novel pipeline NanoCasTE to trace both genetically inherited and somatic TEIs in plants. We performed CANS of the EVADÉ (EVD) retrotransposon in wild-type Arabidopsis thaliana and rapidly obtained up to 40× sequence coverage. Analysis of hemizygous T-DNA insertion sites and genetically inherited insertions of the EVD transposon in the ddm1 (decrease in DNA methylation 1) genome uncovered the crucial role of DNA methylation in shaping EVD insertion preference. We also investigated somatic transposition events of the ONSEN transposon family, finding that genes that are downregulated during heat stress are preferentially targeted by ONSENs. Finally, we detected hypomethylation of novel somatic insertions for two ONSENs. CANS and NanoCasTE are effective tools for detecting TEIs and exploring mobilome organization in plants in response to stress and in different genetic backgrounds, as well as screening T-DNA insertion mutants and transgenic plants.

Composition and Structure of Arabidopsis thaliana Extrachromosomal Circular DNAs Revealed by Nanopore Sequencing.

Extrachromosomal circular DNAs (eccDNAs) are enigmatic DNA molecules that have been detected in a range of organisms. In plants, eccDNAs have various genomic origins and may be derived from transposable elements. The structures of individual eccDNA molecules and their dynamics in response to stress are poorly understood. In this study, we showed that nanopore sequencing is a useful tool for the detection and structural analysis of eccDNA molecules. Applying nanopore sequencing to the eccDNA molecules of epigenetically stressed Arabidopsis plants grown under various stress treatments (heat, abscisic acid, and flagellin), we showed that TE-derived eccDNA quantity and structure vary dramatically between individual TEs. Epigenetic stress alone did not cause eccDNA up-regulation, whereas its combination with heat stress triggered the generation of full-length and various truncated eccDNAs of the ONSEN element. We showed that the ratio between full-length and truncated eccDNAs is TE- and condition-dependent. Our work paves the way for further elucidation of the structural features of eccDNAs and their connections with various biological processes, such as eccDNA transcription and eccDNA-mediated TE silencing.

Epigenetic Stress and Long-Read cDNA Sequencing of Sunflower (Helianthus annuus L.) Revealed the Origin of the Plant Retrotranscriptome.

Transposable elements (TEs) contribute not only to genome diversity but also to transcriptome diversity in plants. To unravel the sources of LTR retrotransposon (RTE) transcripts in sunflower, we exploited a recently developed transposon activation method ('TEgenesis') along with long-read cDNA Nanopore sequencing. This approach allows for the identification of 56 RTE transcripts from different genomic loci including full-length and non-autonomous RTEs. Using the mobilome analysis, we provided a new set of expressed and transpositional active sunflower RTEs for future studies. Among them, a Ty3/Gypsy RTE called SUNTY3 exhibited ongoing transposition activity, as detected by eccDNA analysis. We showed that the sunflower genome contains a diverse set of non-autonomous RTEs encoding a single RTE protein, including the previously described TR-GAG (terminal repeat with the GAG domain) as well as new categories, TR-RT-RH, TR-RH, and TR-INT-RT. Our results demonstrate that 40% of the loci for RTE-related transcripts (nonLTR-RTEs) lack their LTR sequences and resemble conventional eucaryotic genes encoding RTE-related proteins with unknown functions. It was evident based on phylogenetic analysis that three nonLTR-RTEs encode GAG (HadGAG1-3) fused to a host protein. These HadGAG proteins have homologs found in other plant species, potentially indicating GAG domestication. Ultimately, we found that the sunflower retrotranscriptome originated from the transcription of active RTEs, non-autonomous RTEs, and gene-like RTE transcripts, including those encoding domesticated proteins.

Transposons Hidden in Arabidopsis thaliana Genome Assembly Gaps and Mobilization of Non-Autonomous LTR Retrotransposons Unravelled by Nanotei Pipeline.

Long-read data is a great tool to discover new active transposable elements (TEs). However, no ready-to-use tools were available to gather this information from low coverage ONT datasets. Here, we developed a novel pipeline, nanotei, that allows detection of TE-contained structural variants, including individual TE transpositions. We exploited this pipeline to identify TE insertion in the Arabidopsis thaliana genome. Using nanotei, we identified tens of TE copies, including ones for the well-characterized ONSEN retrotransposon family that were hidden in genome assembly gaps. The results demonstrate that some TEs are inaccessible for analysis with the current A. thaliana (TAIR10.1) genome assembly. We further explored the mobilome of the ddm1 mutant with elevated TE activity. Nanotei captured all TEs previously known to be active in ddm1 and also identified transposition of non-autonomous TEs. Of them, one non-autonomous TE derived from (AT5TE33540) belongs to TR-GAG retrotransposons with a single open reading frame (ORF) encoding the GAG protein. These results provide the first direct evidence that TR-GAGs and other non-autonomous LTR retrotransposons can transpose in the plant genome, albeit in the absence of most of the encoded proteins. In summary, nanotei is a useful tool to detect active TEs and their insertions in plant genomes using low-coverage data from Nanopore genome sequencing.

Searching for a Needle in a Haystack: Cas9-Targeted Nanopore Sequencing and DNA Methylation Profiling of Full-Length Glutenin Genes in a Big Cereal Genome.

Sequencing and epigenetic profiling of target genes in plants are important tasks with various applications ranging from marker design for plant breeding to the study of gene expression regulation. This is particularly interesting for plants with big genome size for which whole-genome sequencing can be time-consuming and costly. In this study, we asked whether recently proposed Cas9-targeted nanopore sequencing (nCATS) is efficient for target gene sequencing for plant species with big genome size. We applied nCATS to sequence the full-length glutenin genes (Glu-1Ax, Glu-1Bx and Glu-1By) and their promoters in hexaploid triticale (X Triticosecale, AABBRR, genome size is 24 Gb). We showed that while the target gene enrichment per se was quite high for the three glutenin genes (up to 645×), the sequencing depth that was achieved from two MinION flowcells was relatively low (5-17×). However, this sequencing depth was sufficient for various tasks including detection of InDels and single-nucleotide variations (SNPs), read phasing and methylation profiling. Using nCATS, we uncovered SNP and InDel variation of full-length glutenin genes providing useful information for marker design and deciphering of variation of individual Glu-1By alleles. Moreover, we demonstrated that glutenin genes possess a 'gene-body' methylation epigenetic profile with hypermethylated CDS part and hypomethylated promoter region. The obtained information raised an interesting question on the role of gene-body methylation in glutenin gene expression regulation. Taken together, our work disclosures the potential of the nCATS approach for sequencing of target genes in plants with big genome size.

Nanopore RNA Sequencing Revealed Long Non-Coding and LTR Retrotransposon-Related RNAs Expressed at Early Stages of Triticale SEED Development.

The intergenic space of plant genomes encodes many functionally important yet unexplored RNAs. The genomic loci encoding these RNAs are often considered "junk", DNA as they are frequently associated with repeat-rich regions of the genome. The latter makes the annotations of these loci and the assembly of the corresponding transcripts using short RNAseq reads particularly challenging. Here, using long-read Nanopore direct RNA sequencing, we aimed to identify these "junk" RNA molecules, including long non-coding RNAs (lncRNAs) and transposon-derived transcripts expressed during early stages (10 days post anthesis) of seed development of triticale (AABBRR, 2n = 6x = 42), an interspecific hybrid between wheat and rye. Altogether, we found 796 lncRNAs and 20 LTR retrotransposon-related transcripts (RTE-RNAs) expressed at this stage, with most of them being previously unannotated and located in the intergenic as well as intronic regions. Sequence analysis of the lncRNAs provide evidence for the frequent exonization of Class I (retrotransposons) and class II (DNA transposons) transposon sequences and suggest direct influence of "junk" DNA on the structure and origin of lncRNAs. We show that the expression patterns of lncRNAs and RTE-related transcripts have high stage specificity. In turn, almost half of the lncRNAs located in Genomes A and B have the highest expression levels at 10-30 days post anthesis in wheat. Detailed analysis of the protein-coding potential of the RTE-RNAs showed that 75% of them carry open reading frames (ORFs) for a diverse set of GAG proteins, the main component of virus-like particles of LTR retrotransposons. We further experimentally demonstrated that some RTE-RNAs originate from autonomous LTR retrotransposons with ongoing transposition activity during early stages of triticale seed development. Overall, our results provide a framework for further exploration of the newly discovered lncRNAs and RTE-RNAs in functional and genome-wide association studies in triticale and wheat. Our study also demonstrates that Nanopore direct RNA sequencing is an indispensable tool for the elucidation of lncRNA and retrotransposon transcripts.

Genomic and Transcriptomic Survey Provides New Insight into the Organization and Transposition Activity of Highly Expressed LTR Retrotransposons of Sunflower (Helianthus annuus L.).

LTR retrotransposons (RTEs) play a crucial role in plant genome evolution and adaptation. Although RTEs are generally silenced in somatic plant tissues under non-stressed conditions, some expressed RTEs (exRTEs) escape genome defense mechanisms. As our understanding of exRTE organization in plants is rudimentary, we systematically surveyed the genomic and transcriptomic organization and mobilome (transposition) activity of sunflower (Helianthus annuus L.) exRTEs. We identified 44 transcribed RTEs in the sunflower genome and demonstrated their distinct genomic features: more recent insertion time, longer open reading frame (ORF) length, and smaller distance to neighboring genes. We showed that GAG-encoding ORFs are present at significantly higher frequencies in exRTEs, compared with non-expressed RTEs. Most exRTEs exhibit variation in copy number among sunflower cultivars and one exRTE Gagarin produces extrachromosomal circular DNA in seedling, demonstrating recent and ongoing transposition activity. Nanopore direct RNA sequencing of full-length RTE RNA revealed complex patterns of alternative splicing in RTE RNAs, resulting in isoforms that carry ORFs for distinct RTE proteins. Together, our study demonstrates that tens of expressed sunflower RTEs with specific genomic organization shape the hidden layer of the transcriptome, pointing to the evolution of specific strategies that circumvent existing genome defense mechanisms.

Functional Allium fistulosum Centromeres Comprise Arrays of a Long Satellite Repeat, Insertions of Retrotransposons and Chloroplast DNA.

The centromere is a unique part of the chromosome combining a conserved function with an extreme variability in its DNA sequence. Most of our knowledge about the functional centromere organization is obtained from species with small and medium genome/chromosome sizes while the progress in plants with big genomes and large chromosomes is lagging behind. Here, we studied the genomic organization of the functional centromere in Allium fistulosum and A. cepa, both species with a large genome (13 Gb and 16 Gb/1C, 2n = 2x = 16) and large-sized chromosomes. Using low-depth DNA sequencing for these two species and previously obtained CENH3 immunoprecipitation data we identified two long (1.2 Kb) and high-copy repeats, AfCen1K and AcCen1K. FISH experiments showed that AfCen1K is located in all centromeres of A. fistulosum chromosomes while no AcCen1K FISH signals were identified on A. cepa chromosomes. Our molecular cytogenetic and bioinformatics survey demonstrated that these repeats are partially similar but differ in chromosomal location, sequence structure and genomic organization. In addition, we could conclude that the repeats are transcribed and their RNAs are not polyadenylated. We also observed that these repeats are associated with insertions of retrotransposons and plastidic DNA and the landscape of A. cepa and A. fistulosum centromeric regions possess insertions of plastidic DNA. Finally, we carried out detailed comparative satellitome analysis of A. cepa and A. fistulosum genomes and identified a new chromosome- and A. cepa-specific tandem repeat, TR2CL137, located in the centromeric region. Our results shed light on the Allium centromere organization and provide unique data for future application in Allium genome annotation.