Unexpected consequences of a sudden and massive transposon amplification on rice gene expression

Genomic impact of stress-induced transposable element mobility in Arabidopsis

Transposable elements (TEs) have long been known to be major contributors to plant evolution, adaptation and crop domestication. Stress-induced TE mobilization is of particular interest because it may result in novel gene regulatory pathways responding to stresses and thereby contribute to stress adaptation. Here, we investigated the genomic impacts of stress induced TE mobilization in wild type Arabidopsis plants. We find that the heat-stress responsive ONSEN TE displays an insertion site preference that is associated with specific chromatin states, especially those rich in H2A.Z histone variant and H3K27me3 histone mark. In order to better understand how novel ONSEN insertions affect the plant's response to heat stress, we carried out an in-depth transcriptomic analysis. We find that in addition to simple gene knockouts, ONSEN can produce a plethora of gene expression changes such as: constitutive activation of gene expression, alternative splicing, acquisition of heat-responsiveness, exonisation and genesis of novel non-coding and antisense RNAs. This report shows how the mobilization of a single TE-family can lead to a rapid rise of its copy number increasing the host's genome size and contribute to a broad range of transcriptomic novelty on which natural selection can then act.

Extensively Current Activity of Transposable Elements in Natural Rice Accessions Revealed by Singleton Insertions

Active transposable elements (TEs) have drawn more attention as they continue to create new insertions and contribute to genetic diversity of the genome. However, only a few have been discovered in rice up to now, and their activities are mostly induced by artificial treatments (e.g., tissue culture, hybridization etc.) rather than under normal growth conditions. To systematically survey the current activity of TEs in natural rice accessions and identify rice accessions carrying highly active TEs, the transposon insertion polymorphisms (TIPs) profile was used to identify singleton insertions, which were unique to a single accession and represented the new insertion of TEs in the genome. As a result, 10,924 high-confidence singletons from 251 TE families were obtained, covering all investigated TE types. The number of singletons varied substantially among different superfamilies/families, perhaps reflecting distinct current activity. Particularly, eight TE families maintained potentially higher activity in 3,000 natural rice accessions. Sixty percent of rice accessions were detected to contain singletons, indicating the extensive activity of TEs in natural rice accessions. Thirty-five TE families exhibited potentially high activity in at least one rice accession, and the majority of them showed variable activity among different rice groups/subgroups. These naturally active TEs would be ideal candidates for elucidating the molecular mechanisms underlying the transposition and activation of TEs, as well as investigating the interactions between TEs and the host genome.

Elevated transcription of transposable elements is accompanied by het-siRNA-driven de novo DNA methylation in grapevine embryogenic callus

Background

Somatic variation is a valuable source of trait diversity in clonally propagated crops. In grapevine, which has been clonally propagated worldwide for centuries, important phenotypes such as white berry colour are the result of genetic changes caused by transposable elements. Additionally, epiallele formation may play a role in determining geo-specific (‘terroir’) differences in grapes and thus ultimately in wine. This genomic plasticity might be co-opted for crop improvement via somatic embryogenesis, but that depends on a species-specific understanding of the epigenetic regulation of transposable element (TE) expression and silencing in these cultures. For this reason, we used whole-genome bisulphite sequencing, mRNA sequencing and small RNA sequencing to study the epigenetic status and expression of TEs in embryogenic callus, in comparison with leaf tissue.

Results

We found that compared with leaf tissue, grapevine embryogenic callus cultures accumulate relatively high genome-wide CHH methylation, particularly across heterochromatic regions. This de novo methylation is associated with an abundance of transcripts from highly replicated TE families, as well as corresponding 24 nt heterochromatic siRNAs. Methylation in the TE-specific CHG context was relatively low over TEs located within genes, and the expression of TE loci within genes was highly correlated with the expression of those genes.

Conclusions

This multi-‘omics analysis of grapevine embryogenic callus in comparison with leaf tissues reveals a high level of genome-wide transcription of TEs accompanied by RNA-dependent DNA methylation of these sequences in trans. This provides insight into the genomic conditions underlying somaclonal variation and epiallele formation in plants regenerated from embryogenic cultures, which is an important consideration when using these tissues for plant propagation and genetic improvement.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07973-9.

Recent amplification of microsatellite-associated miniature inverted-repeat transposable elements in the pineapple genome

BACKGROUND

Miniature inverted-repeat transposable elements (MITEs) are non-autonomous DNA transposable elements that play important roles in genome organization and evolution. Genome-wide identification and characterization of MITEs provide essential information for understanding genome structure and evolution.

RESULTS

We performed genome-wide identification and characterization of MITEs in the pineapple genome. The top two MITE families, accounting for 29.39% of the total MITEs and 3.86% of the pineapple genome, have insertion preference in (TA) n dinucleotide microsatellite regions. We therefore named these MITEs A. comosus microsatellite-associated MITEs (Ac-mMITEs). The two Ac-mMITE families, Ac-mMITE-1 and Ac-mMITE-2, shared sequence similarity in the terminal inverted repeat (TIR) regions, suggesting that these two Ac-mMITE families might be derived from a common or closely related autonomous elements. The Ac-mMITEs are frequently clustered via adjacent insertions. Among the 21,994 full-length Ac-mMITEs, 46.1% of them were present in clusters. By analyzing the Ac-mMITEs without (TA) n microsatellite flanking sequences, we found that Ac-mMITEs were likely derived from Mutator-like DNA transposon. Ac-MITEs showed highly polymorphic insertion sites between cultivated pineapples and their wild relatives. To better understand the evolutionary history of Ac-mMITEs, we filtered and performed comparative analysis on the two distinct groups of Ac-mMITEs, microsatellite-targeting MITEs (mt-MITEs) that are flanked by dinucleotide microsatellites on both sides and mutator-like MITEs (ml-MITEs) that contain 9/10 bp TSDs. Epigenetic analysis revealed a lower level of host-induced silencing on the mt-MITEs in comparison to the ml-MITEs, which partially explained the significantly higher abundance of mt-MITEs in pineapple genome. The mt-MITEs and ml-MITEs exhibited differential insertion preference to gene-related regions and RNA-seq analysis revealed their differential influences on expression regulation of nearby genes.

CONCLUSIONS

Ac-mMITEs are the most abundant MITEs in the pineapple genome and they were likely derived from Mutator-like DNA transposon. Preferential insertion in (TA) n microsatellite regions of Ac-mMITEs occurred recently and is likely the result of damage-limiting strategy adapted by Ac-mMITEs during co-evolution with their host. Insertion in (TA) n microsatellite regions might also have promoted the amplification of mt-MITEs. In addition, mt-MITEs showed no or negligible impact on nearby gene expression, which may help them escape genome control and lead to their amplification.

Phenotypic and Transcriptomic Responses to Stress Differ According to Population Geography in an Invasive Species

Adaptation to rapid environmental changes must occur within a short-time scale. In this context, studies of invasive species may provide insights into the underlying mechanisms of rapid adaptation as these species have repeatedly encountered and adapted to novel environmental conditions. We investigated how invasive and noninvasive genotypes of Drosophila suzukii deal with oxidative stress at the phenotypic and molecular levels. We also studied the impact of transposable element (TE) insertions on the gene expression in response to stress. Our results show that flies from invasive areas (France and the United States) live longer in natural conditions than the ones from native Japanese areas. As expected, lifespan for all genotypes was significantly reduced following exposure to paraquat, but this reduction varied among genotypes (genotype-by-environment interaction) with invasive genotypes appearing more affected by exposure than noninvasive ones. A transcriptomic analysis of genotypes upon paraquat treatment detected many genes differentially expressed (DE). Although a small core set of genes were DE in all genotypes following paraquat exposure, much of the response of each genotype was unique. Moreover, we showed that TEs were not activated after oxidative stress and DE genes were significantly depleted of TEs. In conclusion, it is likely that transcriptomic changes are involved in the rapid adaptation to local environments. We provide new evidence that in the decade since the invasion from Asia, the sampled genotypes in Europe and the United States of D. suzukii diverged from the ones from the native area regarding their phenotypic and genomic response to oxidative stress.

Testing the Genomic Shock Hypothesis Using Transposable Element Expression in Yeast Hybrids

Transposable element (TE) insertions are a source of structural variation and can cause genetic instability and gene expression changes. A host can limit the spread of TEs with various repression mechanisms. Many examples of plant and animal interspecific hybrids show disrupted TE repression leading to TE propagation. Recent studies in yeast did not find any increase in transposition rate in hybrids. However, this does not rule out the possibility that the transcriptional or translational activity of TEs increases following hybridization because of a disruption of the host TE control mechanisms. Thus, whether total expression of a TE family is higher in hybrids than in their parental species remains to be examined. We leveraged publically available RNA-seq and ribosomal profiling data on yeast artificial hybrids of the Saccharomyces genus and performed differential expression analysis of their LTR retrotransposons (Ty elements). Our analyses of total mRNA levels show that Ty elements are generally not differentially expressed in hybrids, even when the hybrids are exposed to a low temperature stress condition. Overall, only 2/26 Ty families show significantly higher expression in the S. cerevisiae × S. uvarum hybrids while there are 3/26 showing significantly lower expression in the S. cerevisiae x S. paradoxus hybrids. Our analysis of ribosome profiling data of S. cerevisiae × S. paradoxus hybrids shows similar translation efficiency of Ty in both parents and hybrids, except for Ty1_cer showing higher translation efficiency. Overall, our results do not support the hypothesis that hybridization could act as a systematic trigger of TE expression in yeast and suggest that the impact of hybridization on TE activity is strain and TE specific.

DNA methyltransferase CHROMOMETHYLASE3 prevents ONSEN transposon silencing under heat stress

DNA methylation plays crucial roles in transposon silencing and genome integrity. CHROMOMETHYLASE3 (CMT3) is a plant-specific DNA methyltransferase responsible for catalyzing DNA methylation at the CHG (H = A, T, C) context. Here, we identified a positive role of CMT3 in heat-induced activation of retrotransposon ONSEN. We found that the full transcription of ONSEN under heat stress requires CMT3. Interestingly, loss-of-function CMT3 mutation led to increased CHH methylation at ONSEN. The CHH methylation is mediated by CMT2, as evidenced by greatly reduced CHH methylation in cmt2 and cmt2 cmt3 mutants coupled with increased ONSEN transcription. Furthermore, we found more CMT2 binding at ONSEN chromatin in cmt3 compared to wild-type accompanied with an ectopic accumulation of H3K9me2 under heat stress, suggesting a collaborative role of H3K9me2 and CHH methylation in preventing heat-induced ONSEN activation. In summary, this study identifies a non-canonical role of CMT3 in preventing transposon silencing and provides new insights into how DNA methyltransferases regulate transcription under stress conditions.

DNA methylation is generally known to silence transposon and maintain genome integrity. Environmental stress has been reported to release the transcriptional silencing of some transposable elements. DNA methylation is involved in the transcriptional restriction of heat-induced Copia-type retrotransposon ONSEN in Arabidopsis when subjected to heat stress. Here, we identified a non-canonical and positive role of the DNA methyltransferase CMT3 in ONSEN reactivation under heat stress. We showed that CMT3 prevents CMT2-mediated CHH methylation and H3K9me2 accumulation under heat at ONSEN chromatin to modulate ONSEN transcription. Our work revealed the molecular mechanism of CMT3 in heat-induced ONSEN activation and sheds new light on the survival mechanism of certain transposons in the host genome under stress conditions.

Predicting transcriptional responses to cold stress across plant species

The same gene is often regulated differently in response to stress in even closely related plant species. Directly measuring stress-responsive gene expression can be financially and logistically challenging in nonmodel species. Here, we show that models trained using data on which genes respond to cold in one species can predict which genes will respond to cold in related species, even when the training and target species vary in their degree of tolerance to cold. The prediction models we used require only genomic sequence and gene models. As a result, data from well-studied model species may be used to predict which genes will respond to stress in less-studied species with sequenced genomes.

Although genome-sequence assemblies are available for a growing number of plant species, gene-expression responses to stimuli have been cataloged for only a subset of these species. Many genes show altered transcription patterns in response to abiotic stresses. However, orthologous genes in related species often exhibit different responses to a given stress. Accordingly, data on the regulation of gene expression in one species are not reliable predictors of orthologous gene responses in a related species. Here, we trained a supervised classification model to identify genes that transcriptionally respond to cold stress. A model trained with only features calculated directly from genome assemblies exhibited only modest decreases in performance relative to models trained by using genomic, chromatin, and evolution/diversity features. Models trained with data from one species successfully predicted which genes would respond to cold stress in other related species. Cross-species predictions remained accurate when training was performed in cold-sensitive species and predictions were performed in cold-tolerant species and vice versa. Models trained with data on gene expression in multiple species provided at least equivalent performance to models trained and tested in a single species and outperformed single-species models in cross-species prediction. These results suggest that classifiers trained on stress data from well-studied species may suffice for predicting gene-expression patterns in related, less-studied species with sequenced genomes.

Transposable elements expression in Rhinella marina (cane toad) specimens submitted to immune and stress challenge

Transposable elements (TEs) are important components of eukaryotic genomes and compose around 30% of the genome of Rhinella marina, an invasive toad species. Considering the possible role of TEs in the adaptation of populations, we have analyzed the expression of TEs in publicly available spleen tissue transcriptomic data generated for this species after immune and stress challenge. By analyzing the transcriptome assembly, we detected a high number of TE segments. Moreover, some distinct TE families were differentially expressed in some conditions. Our result shows that several TEs are capable of being transcribed in R. marina and they could help to generate a rapid response of specimens to the environment. Also, we can suggest that these TEs could be activated in the germinative cells as well producing variability to be selected and shaped by the evolutionary processes behind the success of this invasive species. Thus, the TEs are important targets for investigation in the context of R. marina adaptation.

Assessing the regulatory potential of transposable elements using chromatin accessibility profiles of maize transposons

Transposable elements (TEs) have the potential to create regulatory variation both through the disruption of existing DNA regulatory elements and through the creation of novel DNA regulatory elements. In a species with a large genome, such as maize, many TEs interspersed with genes create opportunities for significant allelic variation due to TE presence/absence polymorphisms among individuals. We used information on putative regulatory elements in combination with knowledge about TE polymorphisms in maize to identify TE insertions that interrupt existing accessible chromatin regions (ACRs) in B73 as well as examples of polymorphic TEs that contain ACRs among four inbred lines of maize including B73, Mo17, W22, and PH207. The TE insertions in three other assembled maize genomes (Mo17, W22, or PH207) that interrupt ACRs that are present in the B73 genome can trigger changes to the chromatin, suggesting the potential for both genetic and epigenetic influences of these insertions. Nearly 20% of the ACRs located over 2 kb from the nearest gene are located within an annotated TE. These are regions of unmethylated DNA that show evidence for functional importance similar to ACRs that are not present within TEs. Using a large panel of maize genotypes, we tested if there is an association between the presence of TE insertions that interrupt, or carry, an ACR and the expression of nearby genes. While most TE polymorphisms are not associated with expression for nearby genes, the TEs that carry ACRs exhibit enrichment for being associated with higher expression of nearby genes, suggesting that these TEs may contribute novel regulatory elements. These analyses highlight the potential for a subset of TEs to rewire transcriptional responses in eukaryotic genomes.

Population genomics of transposable element activation in the highly repressive genome of an agricultural pathogen

The activity of transposable elements (TEs) can be an important driver of genetic diversity with TE-mediated mutations having a wide range of fitness consequences. To avoid deleterious effects of TE activity, some fungi have evolved highly sophisticated genomic defences to reduce TE proliferation across the genome. Repeat-induced point mutation (RIP) is a fungal-specific TE defence mechanism efficiently targeting duplicated sequences. The rapid accumulation of RIPs is expected to deactivate TEs over the course of a few generations. The evolutionary dynamics of TEs at the population level in a species with highly repressive genome defences is poorly understood. Here, we analyse 366 whole-genome sequences of Parastagonospora nodorum, a fungal pathogen of wheat with efficient RIP. A global population genomics analysis revealed high levels of genetic diversity and signs of frequent sexual recombination. Contrary to expectations for a species with RIP, we identified recent TE activity in multiple populations. The TE composition and copy numbers showed little divergence among global populations regardless of the demographic history. Miniature inverted-repeat transposable elements (MITEs) and terminal repeat retrotransposons in miniature (TRIMs) were largely underlying recent intra-species TE expansions. We inferred RIP footprints in individual TE families and found that recently active, high-copy TEs have possibly evaded genomic defences. We find no evidence that recent positive selection acted on TE-mediated mutations rather that purifying selection maintained new TE insertions at low insertion frequencies in populations. Our findings highlight the complex evolutionary equilibria established by the joint action of TE activity, selection and genomic repression.

Era-like GTP protein gene expression in rice

The mutations are genetic changes in the genome sequences and have a significant role in biotechnology, genetics, and molecular biology even to find out the genome sequences of a cell DNA along with the viral RNA sequencing. The mutations are the alterations in DNA that may be natural or spontaneous and induced due to biochemical reactions or radiations which damage cell DNA. There is another cause of mutations which is known as transposons or jumping genes which can change their position in the genome during meiosis or DNA replication. The transposable elements can induce by self in the genome due to cellular and molecular mechanisms including hypermutation which caused the localization of transposable elements to move within the genome. The use of induced mutations for studying the mutagenesis in crop plants is very common as well as a promising method for screening crop plants with new and enhanced traits for the improvement of yield and production. The utilization of insertional mutations through transposons or jumping genes usually generates stable mutant alleles which are mostly tagged for the presence or absence of jumping genes or transposable elements. The transposable elements may be used for the identification of mutated genes in crop plants and even for the stable insertion of transposable elements in mutated crop plants. The guanine nucleotide-binding (GTP) proteins have an important role in inducing tolerance in rice plants to combat abiotic stress conditions.

Amplification dynamics of miniature inverted-repeat transposable elements and their impact on rice trait variability

Transposable elements (TEs) are a rich source of genetic variability. Among TEs, miniature inverted-repeat TEs (MITEs) are of particular interest as they are present in high copy numbers in plant genomes and are closely associated with genes. MITEs are deletion derivatives of class II transposons, and can be mobilized by the transposases encoded by the latter through a typical cut-and-paste mechanism. However, MITEs are typically present at much higher copy numbers than class II transposons. We present here an analysis of 103 109 transposon insertion polymorphisms (TIPs) in 738 Oryza sativa genomes representing the main rice population groups. We show that an important fraction of MITE insertions has been fixed in rice concomitantly with its domestication. However, another fraction of MITE insertions is present at low frequencies. We performed MITE TIP-genome-wide association studies (TIP-GWAS) to study the impact of these elements on agronomically important traits and found that these elements uncover more trait associations than single nucleotide polymorphisms (SNPs) on important phenotypes such as grain width. Finally, using SNP-GWAS and TIP-GWAS we provide evidence of the replicative amplification of MITEs.

Genome-wide identification of loci modifying spike-branching in tetraploid wheat

Genetic modification of spike architecture is essential for improving wheat yield. Newly identified loci for the 'Miracle wheat' phenotype on chromosomes 1AS and 2BS have significant effects on spike traits. The wheat (Triticum ssp.) inflorescence, also known as a spike, forms an unbranched inflorescence in which the inflorescence meristem generates axillary spikelet meristems (SMs) destined to become sessile spikelets. Previously, we identified the putatively causative mutation in the branched head^t (bh^t) gene (TtBH-A1) of tetraploid wheat (T. turgidum convar. compositum (L.f.) Filat.) responsible for the loss of SM identity, converting the non-branching spike to a branched wheat spike. In the current study, we performed whole-genome quantitative trait loci (QTL) analysis using 146 recombinant inbred lines (RILs) derived from a cross between spike-branching wheat ('Miracle wheat') and an elite durum wheat cultivar showing broad phenotypic variation for spike architecture. Besides the previously found gene at the bh^t-A1 locus on the short arm of chromosome 2A, we also mapped two new modifier QTL for spike-branching on the short arm of chromosome 1A, termed bh^t-A2, and 2BS. Using biparental mapping population and GWAS in 302 diverse accessions, the 2BS locus was highly associated with coding sequence variation found at the homoeo-allele of TtBH-B1 (bh^t-B1). Thus, RILs that combined both bh^t-A1 and bh^t-B1 alleles showed additive genetic effects leading to increased penetrance and expressivity of the supernumerary spikelet and/or mini-spike formation.

A Predictive Approach to Infer the Activity and Natural Variation of Retrotransposon Families in Plants

Plant genomes harbor a particularly rich landscape of repetitive sequences. Transposable elements (TEs) represent a major fraction of this diversity and are intimately linked with plasticity and evolution of genomes across the tree of life (Fedoroff, Science 338:758-767, 2012). Amplification of Long Terminal Repeats (LTR) retrotransposons have shaped the genomic landscape by reshuffling genomic regions, altering gene expression, and providing new regulatory sequences, some of which have been instrumental for crop domestication and breeding (Lisch, Nat Rev Genet 14:49-61, 2013; Vitte et al., Brief Funct Genomics 13:276-295, 2014). While many retrotransposon families are still active within plant genomes, the repetitive nature of retrotransposons has hindered accurate annotation and kingdom-wide predictive assessment of their activity and molecular evolution. While it is natural for the first approach towards a genome annotation to characterize all regions of the genome and associate them with known structures such as particular genes, transposable elements, or other types of non-coding regions, such efforts can result in a large proportion of false-positive annotations when seeking for active loci. To overcome this issue, the next round of annotation efforts needs to include functional annotations based on rigorously defined sequence structures and protein domain compositions. In the context of retrotransposons, such a functional annotation can enable efforts to mobilize particular retrotransposon families in species living today and harness their mutagenic potency for crop improvement (Paszkowski, Curr Opin Biotechnol 32:200-206, 2015). For this purpose, we present a predictive analytical approach to infer the activity and natural variation of retrotransposon families in plants. This is achieved by applying a combination of software and molecular biology tools we developed for functional annotation, activity monitoring, and the assessment of the population structure of particular retrotransposon families in multiple plant species.

Single-molecule long-read sequencing reveals extensive genomic and transcriptomic variation between maize and its wild relative teosinte (Zea mays ssp. parviglumis)

Teosinte (Zea mays ssp. parviglumis), the wild progenitor of maize (Zea mays L.), is an important germplasm resource for improvement of modern maize lines. However, we have limited genetic and genomic information about teosinte and lack state-of-the-art tools to annotate transcriptomes assembled by single-molecule long-read sequencing without a reference genome. Here, we employed single-molecule long-read sequencing of cDNA libraries from five tissues of the teosinte inbred line TIL11 and identified 70,044 nonredundant transcript isoforms. We devised a state-of-the-art, machine learning-based bioinformatics pipeline DenovoAS_Finder to annotate the TIL11 transcriptome without a complete reference genome with an accuracy of up to 91%, providing a robust gene classifier of complex genomes. Additionally, we constructed a draft TIL11 genome with 16,633 high-quality contigs and a N50 of 112 kb by Nanopore sequencing. Genes from families that expanded from teosinte to maize were significantly enriched in the gene ontology (GO) term "RNA modification pathway" and had more transcript isoforms in TIL11 than in the maize inbred line B73. Genes showed collinearity between TIL11 and B73, and intergenic regions were extensively altered by transposable elements. Our study furthers the understanding of maize domestication and provides a resource for the utilization of wild germplasm in maize breeding.

RNA-directed DNA methylation prevents rapid and heritable reversal of transposon silencing under heat stress in Zea mays

In large complex plant genomes, RNA-directed DNA methylation (RdDM) ensures that epigenetic silencing is maintained at the boundary between genes and flanking transposable elements. In maize, RdDM is dependent on Mediator of Paramutation1 (Mop1), a gene encoding a putative RNA dependent RNA polymerase. Here we show that although RdDM is essential for the maintenance of DNA methylation of a silenced MuDR transposon in maize, a loss of that methylation does not result in a restoration of activity. Instead, heritable maintenance of silencing is maintained by histone modifications. At one terminal inverted repeat (TIR) of this element, heritable silencing is mediated via histone H3 lysine 9 dimethylation (H3K9me2), and histone H3 lysine 27 dimethylation (H3K27me2), even in the absence of DNA methylation. At the second TIR, heritable silencing is mediated by histone H3 lysine 27 trimethylation (H3K27me3), a mark normally associated with somatically inherited gene silencing. We find that a brief exposure of high temperature in a mop1 mutant rapidly reverses both of these modifications in conjunction with a loss of transcriptional silencing. These reversals are heritable, even in mop1 wild-type progeny in which methylation is restored at both TIRs. These observations suggest that DNA methylation is neither necessary to maintain silencing, nor is it sufficient to initiate silencing once has been reversed. However, given that heritable reactivation only occurs in a mop1 mutant background, these observations suggest that DNA methylation is required to buffer the effects of environmental stress on transposable elements.

Most plant genomes are mostly transposable elements (TEs), most of which are held in check by modifications of both DNA and histones. The bulk of silenced TEs are associated with methylated DNA and histone H3 lysine 9 dimethylation (H3K9me2). In contrast, epigenetically silenced genes are often associated with histone lysine 27 trimethylation (H3K27me3). Although stress can affect each of these modifications, plants are generally competent to rapidly reset them following that stress. Here we demonstrate that although DNA methylation is not required to maintain silencing of the MuDR element, it is essential for preventing heat-induced, stable and heritable changes in both H3K9me2 and H3K27me3 at this element, and for concomitant changes in transcriptional activity. These finding suggest that RdDM acts to buffer the effects of heat on silenced transposable elements, and that a loss of DNA methylation under conditions of stress can have profound and long-lasting effects on epigenetic silencing in maize.

Full-length LTR retroelements in Capsicum annuum revealed a few species-specific family bursts with insertional preferences

Capsicum annuum is a species that has undergone an expansion of the size of its genome caused mainly by the amplification of repetitive DNA sequences, including mobile genetic elements. Based on information obtained from sequencing the genome of pepper, the estimated fraction of retroelements is approximately 81%, and previous results revealed an important contribution of lineages derived from Gypsy superfamily. However, the dynamics of the retroelements in the C. annuum genome is poorly understood. In this way, the present work seeks to investigate the phylogenetic diversity and genomic abundance of the families of autonomous (complete and intact) LTR retroelements from C. annuum and inspect their distribution along its chromosomes. In total, we identified 1151 structurally full-length retroelements (340 Copia; 811 Gypsy) grouped in 124 phylogenetic families in the base of their retrotranscriptase. All the evolutive lineages of LTR retroelements identified in plants were present in pepper; however, three of them comprise 83% of the entire LTR retroelements population, the lineages Athila, Del/Tekay, and Ale/Retrofit. From them, only three families represent 70.8% of the total number of the identified retroelements. A massive family-specific wave of amplification of two of them occurred in the last 0.5 Mya (GypsyCa_16; CopiaCa_01), whereas the third is more ancient and occurred 3.0 Mya (GypsyCa_13). Fluorescent in situ hybridization performed with family and lineage-specific probes revealed contrasting patterns of chromosomal affinity. Our results provide a database of the populations LTR retroelements specific to C. annuum genome. The most abundant families were analyzed according to chromosome insertional preferences, suppling useful tools to the design of retroelement-based markers specific to the species.

A Global Landscape of Miniature Inverted-Repeat Transposable Elements in the Carrot Genome

Miniature inverted-repeat transposable elements (MITEs) are the most abundant group of Class II mobile elements in plant genomes. Their presence in genic regions may alter gene structure and expression, providing a new source of functional diversity. Owing to their small size and lack of coding capacity, the identification of MITEs has been demanding. However, the increasing availability of reference genomes and bioinformatic tools provides better means for the genome-wide identification and analysis of MITEs and for the elucidation of their contribution to the evolution of plant genomes. We mined MITEs in the carrot reference genome DH1 using MITE-hunter and developed a curated carrot MITE repository comprising 428 families. Of the 31,025 MITE copies spanning 10.34 Mbp of the carrot genome, 54% were positioned in genic regions. Stowaways and Tourists were frequently present in the vicinity of genes, while Mutator-like MITEs were relatively more enriched in introns. hAT-like MITEs were relatively more frequently associated with transcribed regions, including untranslated regions (UTRs). Some carrot MITE families were shared with other Apiaceae species. We showed that hAT-like MITEs were involved in the formation of new splice variants of insertion-harboring genes. Thus, carrot MITEs contributed to the accretion of new diversity by altering transcripts and possibly affecting the regulation of many genes.

Jumpstarting evolution: How transposition can facilitate adaptation to rapid environmental changes

Because of their ability to replicate across genomes, transposable elements (TEs) represent major generators of large-effect mutations. As a result, chromatin-based mechanisms have evolved to control the mutational potential of TEs at multiple levels, from the epigenetic silencing of TE sequences, through the modulation of their integration space, up to the alleviation of the impact of new insertions. Although most TE insertions are highly deleterious, some can provide key adaptive variation. Together with their remarkable sensitivity to the environment and precise integration preferences, the unique characteristics of TEs place them as potent genomic engines of adaptive innovation. Herein, we review recent works exploring the regulation and impact of transposition in nature and discuss their implications for the evolutionary response of species to drastic environmental changes.

DNA hypomethylation in tetraploid rice potentiates stress-responsive gene expression for salt tolerance

Polyploidy is a prominent feature for genome evolution in many animals and all flowering plants. Plant polyploids often show enhanced fitness in diverse and extreme environments, but the molecular basis for this remains elusive. Soil salinity presents challenges for many plants including agricultural crops. Here we report that salt tolerance is enhanced in tetraploid rice through lower sodium uptake and correlates with epigenetic regulation of jasmonic acid (JA)-related genes. Polyploidy induces DNA hypomethylation and potentiates genomic loci coexistent with many stress-responsive genes, which are generally associated with proximal transposable elements (TEs). Under salt stress, the stress-responsive genes including those in the JA pathway are more rapidly induced and expressed at higher levels in tetraploid than in diploid rice, which is concurrent with increased jasmonoyl isoleucine (JA-Ile) content and JA signaling to confer stress tolerance. After stress, elevated expression of stress-responsive genes in tetraploid rice can induce hypermethylation and suppression of the TEs adjacent to stress-responsive genes. These induced responses are reproducible in a recurring round of salt stress and shared between two japonica tetraploid rice lines. The data collectively suggest a feedback relationship between polyploidy-induced hypomethylation in rapid and strong stress response and stress-induced hypermethylation to repress proximal TEs and/or TE-associated stress-responsive genes. This feedback regulation may provide a molecular basis for selection to enhance adaptation of polyploid plants and crops during evolution and domestication.

Datura genome reveals duplications of psychoactive alkaloid biosynthetic genes and high mutation rate following tissue culture

BACKGROUND

Datura stramonium (Jimsonweed) is a medicinally and pharmaceutically important plant in the nightshade family (Solanaceae) known for its production of various toxic, hallucinogenic, and therapeutic tropane alkaloids. Recently, we published a tissue-culture based transformation protocol for D. stramonium that enables more thorough functional genomics studies of this plant. However, the tissue culture process can lead to undesirable phenotypic and genomic consequences independent of the transgene used. Here, we have assembled and annotated a draft genome of D. stramonium with a focus on tropane alkaloid biosynthetic genes. We then use mRNA sequencing and genome resequencing of transformants to characterize changes following tissue culture.

RESULTS

Our draft assembly conforms to the expected 2 gigabasepair haploid genome size of this plant and achieved a BUSCO score of 94.7% complete, single-copy genes. The repetitive content of the genome is 61%, with Gypsy-type retrotransposons accounting for half of this. Our gene annotation estimates the number of protein-coding genes at 52,149 and shows evidence of duplications in two key alkaloid biosynthetic genes, tropinone reductase I and hyoscyamine 6 β-hydroxylase. Following tissue culture, we detected only 186 differentially expressed genes, but were unable to correlate these changes in expression with either polymorphisms from resequencing or positional effects of transposons.

CONCLUSIONS

We have assembled, annotated, and characterized the first draft genome for this important model plant species. Using this resource, we show duplications of genes leading to the synthesis of the medicinally important alkaloid, scopolamine. Our results also demonstrate that following tissue culture, mutation rates of transformed plants are quite high (1.16 × 10- 3 mutations per site), but do not have a drastic impact on gene expression.

Non-target Site Herbicide Resistance Is Conferred by Two Distinct Mechanisms in Black-Grass (Alopecurus myosuroides)

Non-target site resistance (NTSR) to herbicides in black-grass (Alopecurus myosuroides) results in enhanced tolerance to multiple chemistries and is widespread in Northern Europe. To help define the underpinning mechanisms of resistance, global transcriptome and biochemical analysis have been used to phenotype three NTSR black-grass populations. These comprised NTSR1 black-grass from the classic Peldon field population, which shows broad-ranging resistance to post-emergence herbicides; NTSR2 derived from herbicide-sensitive (HS) plants repeatedly selected for tolerance to pendimethalin; and NTSR3 selected from HS plants for resistance to fenoxaprop-P-ethyl. NTSR in weeds is commonly associated with enhanced herbicide metabolism catalyzed by glutathione transferases (GSTs) and cytochromes P450 (CYPs). As such, the NTSR populations were assessed for their ability to detoxify chlorotoluron, which is detoxified by CYPs and fenoxaprop-P-ethyl, which is acted on by GSTs. As compared with HS plants, enhanced metabolism toward both herbicides was determined in the NTSR1 and NTSR2 populations. In contrast, the NTSR3 plants showed no increased detoxification capacity, demonstrating that resistance in this population was not due to enhanced metabolism. All resistant populations showed increased levels of AmGSTF1, a protein functionally linked to NTSR and enhanced herbicide metabolism. Enhanced AmGSTF1 was associated with increased levels of the associated transcripts in the NTSR1 and NTSR2 plants, but not in NTSR3, suggestive of both pre- and post-transcriptional regulation. The related HS, NTSR2, and NTSR3 plants were subject to global transcriptome sequencing and weighted gene co-expression network analysis to identify modules of genes with coupled regulatory functions. In the NTSR2 plants, modules linked to detoxification were identified, with many similarities to the transcriptome of NTSR1 black-grass. Critical detoxification genes included members of the CYP81A family and tau and phi class GSTs. The NTSR2 transcriptome also showed network similarities to other (a)biotic stresses of plants and multidrug resistance in humans. In contrast, completely different gene networks were activated in the NTSR3 plants, showing similarity to the responses to cold, osmotic shock and fungal infection determined in cereals. Our results demonstrate that NTSR in black-grass can arise from at least two distinct mechanisms, each involving complex changes in gene regulatory networks.

Miniature inverted repeat transposable elements cis-regulate circular RNA expression and promote ethylene biosynthesis, reducing heat tolerance in Populus tomentosa

Transposable elements (TEs) and their reverse complementary sequence pairs (RCPs) are enriched around loci that produce circular RNAs (circRNAs) in plants. However, the function of these TE-RCP pairs in modulating circRNA expression remains elusive. Here, we identified 4609 circRNAs in poplar (Populus tomentosa) and showed that miniature inverted repeat transposable elements (MITEs)-RCPs were enriched in circRNA flanking regions. Moreover, we used expression quantitative trait nucleotide (eQTN) mapping to decipher the cis-regulatory role of MITEs. eQTN results showed that 14 single-nucleotide polymorphisms (SNPs) were significantly associated with Circ_0000408 and Circ_0003418 levels and the lead associated SNPs were located in MITE-RCP regions, indicating that MITE-RCP sequence variations affect exon circularization. Overexpression and knockdown analysis showed that Circ_0003418 positively modulated its parental gene, which encodes the RING-type E3 ligase XBAT32, and specifically increased the expression of the PtoXBAT32.5 transcript variant, which lacks the E3 ubiquitin ligase domain. Under heat stress, PtoXBAT32.5 expression was induced with up-regulation of Circ_0003418, resulting in increased production of ethylene and peroxidation of membrane lipids. Our findings thus reveal the cis-regulatory mechanism by which a MITE-RCP pair affects circRNA abundance in poplar and indicate that Circ_0003418 is a negative regulator of poplar heat tolerance via the ubiquitin-mediated protein modification pathway.

Mobility connects: transposable elements wire new transcriptional networks by transferring transcription factor binding motifs

Transposable elements (TEs) constitute major fractions of plant genomes. Their potential to be mobile provides them with the capacity to cause major genome rearrangements. Those effects are potentially deleterious and enforced the evolution of epigenetic suppressive mechanisms controlling TE activity. However, beyond their deleterious effects, TE insertions can be neutral or even advantageous for the host, leading to long-term retention of TEs in the host genome. Indeed, TEs are increasingly recognized as major drivers of evolutionary novelties by regulating the expression of nearby genes. TEs frequently contain binding motifs for transcription factors and capture binding motifs during transposition, which they spread through the genome by transposition. Thus, TEs drive the evolution and diversification of gene regulatory networks by recruiting lineage-specific targets under the regulatory control of specific transcription factors. This process can explain the rapid and repeated evolution of developmental novelties, such as C₄ photosynthesis and a wide spectrum of stress responses in plants. It also underpins the convergent evolution of embryo nourishing tissues, the placenta in mammals and the endosperm in flowering plants. Furthermore, the gene regulatory network underlying flower development has also been largely reshaped by TE-mediated recruitment of regulatory elements; some of them being preserved across long evolutionary timescales. In this review, we highlight the potential role of TEs as evolutionary toolkits in plants by showcasing examples of TE-mediated evolutionary novelties.

Hybridization History and Repetitive Element Content in the Genome of a Homoploid Hybrid, Yucca gloriosa (Asparagaceae)

Hybridization in plants results in phenotypic and genotypic perturbations that can have dramatic effects on hybrid physiology, ecology, and overall fitness. Hybridization can also perturb epigenetic control of transposable elements, resulting in their proliferation. Understanding the mechanisms that maintain genomic integrity after hybridization is often confounded by changes in ploidy that occur in hybrid plant species. Homoploid hybrid species, which have no change in chromosome number relative to their parents, offer an opportunity to study the genomic consequences of hybridization in the absence of change in ploidy. Yucca gloriosa (Asparagaceae) is a young homoploid hybrid species, resulting from a cross between Yucca aloifolia and Yucca filamentosa. Previous analyses of ∼11 kb of the chloroplast genome and nuclear-encoded microsatellites implicated a single Y. aloifolia genotype as the maternal parent of Y. gloriosa. Using whole genome resequencing, we assembled chloroplast genomes from 41 accessions of all three species to re-assess the hybrid origins of Y. gloriosa. We further used re-sequencing data to annotate transposon abundance in the three species and mRNA-seq to analyze transcription of transposons. The chloroplast phylogeny and haplotype analysis suggest multiple hybridization events contributing to the origin of Y. gloriosa, with both parental species acting as the maternal donor. Transposon abundance at the superfamily level was significantly different between the three species; the hybrid was frequently intermediate to the parental species in TE superfamily abundance or appeared more similar to one or the other parent. In only one case-Copia LTR transposons-did Y. gloriosa have a significantly higher abundance relative to either parent. Expression patterns across the three species showed little increased transcriptional activity of transposons, suggesting that either no transposon release occurred in Y. gloriosa upon hybridization, or that any transposons that were activated via hybridization were rapidly silenced. The identification and quantification of transposon families paired with expression evidence paves the way for additional work seeking to link epigenetics with the important trait variation seen in this homoploid hybrid system.

Identification of Key Tissue-Specific, Biological Processes by Integrating Enhancer Information in Maize Gene Regulatory Networks

Enhancers are key players in the spatio-temporal coordination of gene expression during numerous crucial processes, including tissue differentiation across development. Characterizing the transcription factors (TFs) and genes they connect, and the molecular functions underpinned is important to better characterize developmental processes. In plants, the recent molecular characterization of enhancers revealed their capacity to activate the expression of several target genes. Nevertheless, identifying these target genes at a genome-wide level is challenging, particularly for large-genome species, where enhancers and target genes can be hundreds of kilobases away. Therefore, the contribution of enhancers to plant regulatory networks remains poorly understood. Here, we investigate the enhancer-driven regulatory network of two maize tissues at different stages: leaves at seedling stage (V2-IST) and husks (bracts) at flowering. Using systems biology, we integrate genomic, epigenomic, and transcriptomic data to model the regulatory relationships between TFs and their potential target genes, and identify regulatory modules specific to husk and V2-IST. We show that leaves at the V2-IST stage are characterized by the response to hormones and macromolecules biogenesis and assembly, which are regulated by the BBR/BPC and AP2/ERF TF families, respectively. In contrast, husks are characterized by cell wall modification and response to abiotic stresses, which are, respectively, orchestrated by the C2C2/DOF and AP2/EREB families. Analysis of the corresponding enhancer sequences reveals that two different transposable element families (TIR transposon Mutator and MITE Pif/Harbinger) have shaped part of the regulatory network in each tissue, and that MITEs have provided potential new TF binding sites involved in husk tissue-specificity.

Weak Effect of Gypsy Retrotransposon Bursts on Sonneratia alba Salt Stress Gene Expression

Transposable elements (TEs) are an important source of genetic diversity and can be co-opted for the regulation of host genes. However, to what extent the pervasive TE colonization of plant genomes has contributed to stress adaptation remains controversial. Plants inhabiting harsh environments in nature provide a unique opportunity to answer this question. We compared TE compositions and their evolutionary dynamics in the genomes of two mangrove species: the pioneer Sonneratia alba and its less salt-tolerant relative S. caseolaris. Age distribution, strength of purifying selection and the removal rate of LTR (long terminal repeat) retrotransposons were estimated. Phylogenetic analysis of LTR retrotransposons and their distribution in the genome of S. alba were surveyed. Small RNA sequencing and whole-genome bisulfite sequencing was conducted using leaves of S. alba. Expression pattern of LTR retrotransposons and their nearby genes were examined using RNA-seq data of S. alba under different salt treatments. S. alba possesses more TEs than S. caseolaris. Particularly, many more young Gypsy LTR retrotransposons have accumulated in S. alba than in S. caseolaris despite an increase in purifying selection against TE insertions. The top two most abundant Gypsy families in S. alba preferentially insert in gene-poor regions. They are under relaxed epigenetic repression, probably due to the presence of CHROMO domains in their 3'-ends. Although a considerable number of TEs in S. alba showed differential expression under salt stress, only four copies were significantly correlated with their nearby genes in expression levels. One such TE-gene pair involves Abscisic acid 8'-hydroxylase 3 functioning in abscisic acid catabolism. This study sheds light on the evolutionary dynamics and potential function of TEs in an extremophile. Our results suggest that the conclusion on co-option of TEs should be cautious even though activation of TEs by stress might be prevalent.

Development of mPing-based activation tags for crop insertional mutagenesis

Modern plant breeding increasingly relies on genomic information to guide crop improvement. Although some genes are characterized, additional tools are needed to effectively identify and characterize genes associated with crop traits. To address this need, the mPing element from rice was modified to serve as an activation tag to induce expression of nearby genes. Embedding promoter sequences in mPing resulted in a decrease in overall transposition rate; however, this effect was negated by using a hyperactive version of mPing called mmPing20. Transgenic soybean events carrying mPing-based activation tags and the appropriate transposase expression cassettes showed evidence of transposition. Expression analysis of a line that contained a heritable insertion of the mmPing20F activation tag indicated that the activation tag induced overexpression of the nearby soybean genes. This represents a significant advance in gene discovery technology as activation tags have the potential to induce more phenotypes than the original mPing element, improving the overall effectiveness of the mutagenesis system.

A transposon surveillance mechanism that safeguards plant male fertility during stress

Although plants are able to withstand a range of environmental conditions, spikes in ambient temperature can impact plant fertility causing reductions in seed yield and notable economic losses^1,2. Therefore, understanding the precise molecular mechanisms that underpin plant fertility under environmental constraints is critical to safeguarding future food production³. Here, we identified two Argonaute-like proteins whose activities are required to sustain male fertility in maize plants under high temperatures. We found that MALE-ASSOCIATED ARGONAUTE-1 and -2 associate with temperature-induced phased secondary small RNAs in pre-meiotic anthers and are essential to controlling the activity of retrotransposons in male meiocyte initials. Biochemical and structural analyses revealed how male-associated Argonaute activity and its interaction with retrotransposon RNA targets is modulated through the dynamic phosphorylation of a set of highly conserved, surface-located serine residues. Our results demonstrate that an Argonaute-dependent, RNA-guided surveillance mechanism is critical in plants to sustain male fertility under environmentally constrained conditions, by controlling the mutagenic activity of transposons in male germ cells.

Evolutionary Dynamics of Transposable Elements Following a Shared Polyploidization Event in the Tribe Andropogoneae

Both polyploidization and transposable element (TE) activity are known to be major drivers of plant genome evolution. Here, we utilize the Zea-Tripsacum clade to investigate TE activity and accumulation after a shared polyploidization event. Comparisons of TE evolutionary dynamics in various Zea and Tripsacum species, in addition to two closely related diploid species, Urelytrum digitatum and Sorghum bicolor, revealed variation in repeat content among all taxa included in the study. The repeat composition of Urelytrum is more similar to that of Zea and Tripsacum compared to Sorghum, despite the similarity in genome size with the latter. Although LTR-retrotransposons were abundant in all species, we observed an expansion of the copia superfamily, specifically in Z. mays and T. dactyloides, species that have adapted to more temperate environments. Additional analyses of the genomic distribution of these retroelements provided evidence of biased insertions near genes involved in various biological processes including plant development, defense, and macromolecule biosynthesis. Specifically, copia insertions in Zea and T. dactyloides were significantly enriched near genes involved in abiotic stress response, suggesting independent evolution post Zea-Tripsacum divergence. The lack of copia insertions near the orthologous genes in S. bicolor suggests that duplicate gene copies generated during polyploidization may offer novel neutral sites for TEs to insert, thereby providing an avenue for subfunctionalization via TE insertional mutagenesis.

A Field Guide to Eukaryotic Transposable Elements

Transposable elements (TEs) are mobile DNA sequences that propagate within genomes. Through diverse invasion strategies, TEs have come to occupy a substantial fraction of nearly all eukaryotic genomes, and they represent a major source of genetic variation and novelty. Here we review the defining features of each major group of eukaryotic TEs and explore their evolutionary origins and relationships. We discuss how the unique biology of different TEs influences their propagation and distribution within and across genomes. Environmental and genetic factors acting at the level of the host species further modulate the activity, diversification, and fate of TEs, producing the dramatic variation in TE content observed across eukaryotes. We argue that cataloging TE diversity and dissecting the idiosyncratic behavior of individual elements are crucial to expanding our comprehension of their impact on the biology of genomes and the evolution of species.

Genomic diversity generated by a transposable element burst in a rice recombinant inbred population

Genomes of all characterized higher eukaryotes harbor examples of transposable element (TE) bursts-the rapid amplification of TE copies throughout a genome. Despite their prevalence, understanding how bursts diversify genomes requires the characterization of actively transposing TEs before insertion sites and structural rearrangements have been obscured by selection acting over evolutionary time. In this study, rice recombinant inbred lines (RILs), generated by crossing a bursting accession and the reference Nipponbare accession, were exploited to characterize the spread of the very active Ping/mPing family through a small population and the resulting impact on genome diversity. Comparative sequence analysis of 272 individuals led to the identification of over 14,000 new insertions of the mPing miniature inverted-repeat transposable element (MITE), with no evidence for silencing of the transposase-encoding Ping element. In addition to new insertions, Ping-encoded transposase was found to preferentially catalyze the excision of mPing loci tightly linked to a second mPing insertion. Similarly, structural variations, including deletion of rice exons or regulatory regions, were enriched for those with break points at one or both ends of linked mPing elements. Taken together, these results indicate that structural variations are generated during a TE burst as transposase catalyzes both the high copy numbers needed to distribute linked elements throughout the genome and the DNA cuts at the TE ends known to dramatically increase the frequency of recombination.

A small targeting domain in Ty1 integrase is sufficient to direct retrotransposon integration upstream of tRNA genes

Integration of transposable elements into the genome is mutagenic. Mechanisms targeting integrations into relatively safe locations, hence minimizing deleterious consequences for cell fitness, have emerged during evolution. In budding yeast, integration of the Ty1 LTR retrotransposon upstream of RNA polymerase III (Pol III)-transcribed genes requires interaction between Ty1 integrase (IN1) and AC40, a subunit common to Pol I and Pol III. Here, we identify the Ty1 targeting domain of IN1 that ensures (i) IN1 binding to Pol I and Pol III through AC40, (ii) IN1 genome-wide recruitment to Pol I- and Pol III-transcribed genes, and (iii) Ty1 integration only at Pol III-transcribed genes, while IN1 recruitment by AC40 is insufficient to target Ty1 integration into Pol I-transcribed genes. Swapping the targeting domains between Ty5 and Ty1 integrases causes Ty5 integration at Pol III-transcribed genes, indicating that the targeting domain of IN1 alone confers Ty1 integration site specificity.

DNA methylation is reconfigured at the onset of reproduction in rice shoot apical meristem

DNA methylation is an epigenetic modification that specifies the basic state of pluripotent stem cells and regulates the developmental transition from stem cells to various cell types. In flowering plants, the shoot apical meristem (SAM) contains a pluripotent stem cell population which generates the aerial part of plants including the germ cells. Under appropriate conditions, the SAM undergoes a developmental transition from a leaf-forming vegetative SAM to an inflorescence- and flower-forming reproductive SAM. While SAM characteristics are largely altered in this transition, the complete picture of DNA methylation remains elusive. Here, by analyzing whole-genome DNA methylation of isolated rice SAMs in the vegetative and reproductive stages, we show that methylation at CHH sites is kept high, particularly at transposable elements (TEs), in the vegetative SAM relative to the differentiated leaf, and increases in the reproductive SAM via the RNA-dependent DNA methylation pathway. We also show that half of the TEs that were highly methylated in gametes had already undergone CHH hypermethylation in the SAM. Our results indicate that changes in DNA methylation begin in the SAM long before germ cell differentiation to protect the genome from harmful TEs.

The shoot apical meristem of flowering plants transitions from forming leaves to floral organs. Here Higo et al. show that DNA methylation of many transposons that are hypermethylated in gametes is established in the SAM before flowering, suggesting it protects against harmful transposition long before germ cell differentiation.

The contribution of transposable elements to transcriptional novelty in plants: the FLC affair

Transposable elements (TEs) are repetitive DNA sequences with the ability to replicate across genomes and generate mutations with major transcriptional effects. Epigenetic silencing mechanisms that target TEs to limit their activity, including DNA methylation, add to the range of gene expression variants generated by TEs. Here, using the iconic gene flowering locus C (FLC) as a case study I discuss the multiple ways by which TEs can affect the expression of genes and contribute to the adaptation of plants to changing environments

Exploring the Biochemical Origin of DNA Sequence Variation in Barley Plants Regenerated via in Vitro Anther Culture

Tissue culture is an essential tool for the regeneration of uniform plant material. However, tissue culture conditions can be a source of abiotic stress for plants, leading to changes in the DNA sequence and methylation patterns. Despite the growing evidence on biochemical processes affected by abiotic stresses, how these altered biochemical processes affect DNA sequence and methylation patterns remains largely unknown. In this study, the methylation-sensitive Amplified Fragment Length Polymorphism (metAFLP) approach was used to investigate de novo methylation, demethylation, and sequence variation in barley regenerants derived by anther culture. Additionally, we used Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy to identify the spectral features of regenerants, which were then analyzed by mediation analysis. The infrared spectrum ranges (710-690 and 1010-940 cm-1) identified as significant in the mediation analysis were most likely related to β-glucans, cellulose, and S-adenosyl-L-methionine (SAM). Additionally, the identified compounds participated as predictors in moderated mediation analysis, explaining the role of demethylation of CHG sites (CHG_DMV) in in vitro tissue culture-induced sequence variation, depending on the duration of tissue culture. The data demonstrate that ATR-FTIR spectroscopy is a useful tool for studying the biochemical compounds that may affect DNA methylation patterns and sequence variation, if combined with quantitative characteristics determined using metAFLP molecular markers and mediation analysis. The role of β-glucans, cellulose, and SAM in DNA methylation, and in cell wall, mitochondria, and signaling, are discussed to highlight the putative cellular mechanisms involved in sequence variation.

Characterization of B-Genome Specific High Copy hAT MITE Families in Brassica nigra Genome

Miniature inverted-repeat transposable elements (MITEs) are non-autonomous class II transposons which have been shown to influence genome evolution. Brassica nigra L. (B-genome) is one of three Brassica diploids cultivated primarily as an oil crop, which harbors novel alleles important for breeding. Two new high copy hAT MITE families (BniHAT-1 and BniHAT-2) from the B-genome were characterized and their prevalence assessed in the genomes of the related diploids, rapa L. (A) and Brassica oleracea L. (C). Both novel MITE families were present at high copy numbers in the B-genome with 434 and 331 copies of BniHAT-1 and BniHAT-2, respectively. Yet less than 20 elements were identified in the genome assemblies of the A, and C -genomes, supporting B-genome specific proliferation of these MITE families. Although apparently randomly distributed across the genome, 68 and 70% of the B-genome MITEs were present within 2 kb flanking regions of annotated genes suggesting they might influence gene expression and/or function. In addition, MITE derived microRNAs and transcription factor binding sites suggested a putative role in gene regulation. Age of insertion analysis revealed that the major proliferation of these elements occurred during 2-3 million years ago. Additionally, site-specific polymorphism analyses showed that 44% MITEs were undergoing active amplification into the B-genome. Overall, this study provides a comprehensive analysis of two high copy MITE families, which were specifically amplified in the B-genome, suggesting a potential role in shaping the Brassica B-genome.

Transposable elements employ distinct integration strategies with respect to transcriptional landscapes in eukaryotic genomes

Transposable elements (TEs) are ubiquitous DNA segments capable of moving from one site to another within host genomes. The extant distributions of TEs in eukaryotic genomes have been shaped by both bona fide TE integration preferences in eukaryotic genomes and by selection following integration. Here, we compare TE target site distribution in host genomes using multiple de novo transposon insertion datasets in both plants and animals and compare them in the context of genome-wide transcriptional landscapes. We showcase two distinct types of transcription-associated TE targeting strategies that suggest a process of convergent evolution among eukaryotic TE families. The integration of two precision-targeting elements are specifically associated with initiation of RNA Polymerase II transcription of highly expressed genes, suggesting the existence of novel mechanisms of precision TE targeting in addition to passive targeting of open chromatin. We also highlight two features that can facilitate TE survival and rapid proliferation: tissue-specific transposition and minimization of negative impacts on nearby gene function due to precision targeting.

Adaptation to Extreme Antarctic Environments Revealed by the Genome of a Sea Ice Green Alga

The unicellular green alga Chlamydomonas sp. ICE-L thrives in polar sea ice, where it tolerates extreme low temperatures, high salinity, and broad seasonal fluctuations in light conditions. Despite the high interest in biotechnological uses of this species, little is known about the adaptations that allow it to thrive in this harsh and complex environment. Here, we assembled a high-quality genome sequence of ∼542 Mb and found that retrotransposon proliferation contributed to the relatively large genome size of ICE-L when compared to other chlorophytes. Genomic features that may support the extremophilic lifestyle of this sea ice alga include massively expanded gene families involved in unsaturated fatty acid biosynthesis, DNA repair, photoprotection, ionic homeostasis, osmotic homeostasis, and reactive oxygen species detoxification. The acquisition of multiple ice binding proteins through putative horizontal gene transfer likely contributed to the origin of the psychrophilic lifestyle in ICE-L. Additional innovations include the significant upregulation under abiotic stress of several expanded ICE-L gene families, likely reflecting adaptive changes among diverse metabolic processes. Our analyses of the genome, transcriptome, and functional assays advance general understanding of the Antarctic green algae and offer potential explanations for how green plants adapt to extreme environments.

Hybrid breeding in wheat: how shaping floral biology can offer new perspectives

Hybrid breeding in wheat (Triticum aestivum L.) has the potential to deliver major yield increases. This is a requisite to guarantee food security for increasing population demands and to counterbalance the effects of extreme environmental conditions. Successful hybrid breeding in wheat relies on forced outcrossing while preventing self-pollination. To achieve this, research has been directed towards identifying and improving fertility control systems. To maximise cross-pollination and seed set, however, fertility control systems need to be complemented by breeding phenotypically distinct male and female lines. This review summarises existing and novel male sterility systems for wheat hybridisation. We also consider the genetic resources that can be used to alter wheat's floral development and spike morphology, with a focus on the genetic variation already available. Exploiting these resources can lead to enhanced outcrossing, a key requirement in the progress towards hybrid wheat breeding.

Stress, Adaptation, and the Deep Genome: Why Transposons Matter

Stress is a common, if often unpredictable life event. It can be defined from an evolutionary perspective as a force an organism perceives it must adapt to. Thus stress is a useful tool to study adaptation and the adaptive capacity of organisms. The deep genome, long neglected as a pile of “junk” has emerged as a source of regulatory DNA and RNA as well as a potential stockpile of adaptive capacity at the organismal and species levels. Recent work on the regulation of transposable elements (TEs), the principle constituents of the deep genome, by stress has shown that these elements are responsive to host stress and other environmental cues. Further, we have shown that some are likely directly regulated by the glucocorticoid receptor (GR), one of the two major vertebrate stress steroid receptors in a fashion that appears adaptive. On the basis of this and other emerging evidence I argue that the deep genome may represent an adaptive toolkit for organisms to respond to their environments at both individual and evolutionary scales. This argues that genomes may be adapted for what Waddington called “trait adaptability” rather than being purely passive objects of natural selection and single nucleotide level mutation.

Soybean aphids adapted to host-plant resistance by down regulating putative effectors and up regulating transposable elements

In agricultural systems, crops equipped with host-plant resistance (HPR) have enhanced protection against pests, and are used as a safe and sustainable tool in pest management. In soybean, HPR can control the soybean aphid (Aphis glycines), but certain aphid populations have overcome this resistance (i.e., virulence). The molecular mechanisms underlying aphid virulence to HPR are unknown, but likely involve effector proteins that are secreted by aphids to modulate plant defenses. Another mechanism to facilitate adaptation is through the activity of transposable elements, which can become activated by stress. In this study, we performed RNA sequencing of virulent and avirulent soybean aphids fed susceptible or resistant (Rag1 + Rag2) soybean. Our goal was to better understand the molecular mechanisms underlying soybean aphid virulence. Our data showed that virulent aphids mostly down regulate putative effector genes relative to avirulent aphids, especially when aphids were fed susceptible soybean. Decreased expression of effectors may help evade HPR plant defenses. Virulent aphids also transcriptionally up regulate a diverse set of transposable elements and nearby genes, which is consistent with stress adaptation. Our work demonstrates two mechanisms of pest adaptation to resistance, and identifies effector gene targets for future functional testing.

Miniature inverted-repeat transposable elements (MITEs), derived insertional polymorphism as a tool of marker systems for molecular plant breeding

Plant molecular breeding is expected to give significant gains in cultivar development through development and utilization of suitable molecular marker systems for genetic diversity analysis, rapid DNA fingerprinting, identification of true hybrids, trait mapping and marker-assisted selection. Transposable elements (TEs) are the most abundant component in a genome and being used as genetic markers in the plant molecular breeding. Here, we review on the high copious transposable element belonging to class-II DNA TEs called "miniature inverted-repeat transposable elements" (MITEs). MITEs are ubiquitous, short and non-autonomous DNA transposable elements which have a tendency to insert into genes and genic regions have paved a way for the development of functional DNA marker systems in plant genomes. This review summarises the characteristics of MITEs, principles and methodologies for development of MITEs based DNA markers, bioinformatics tools and resources for plant MITE discovery and their utilization in crop improvement.

Transcriptional regulation of genes bearing intronic heterochromatin in the rice genome

Intronic regions of eukaryotic genomes accumulate many Transposable Elements (TEs). Intronic TEs often trigger the formation of transcriptionally repressive heterochromatin, even within transcription-permissive chromatin environments. Although TE-bearing introns are widely observed in eukaryotic genomes, their epigenetic states, impacts on gene regulation and function, and their contributions to genetic diversity and evolution, remain poorly understood. In this study, we investigated the genome-wide distribution of intronic TEs and their epigenetic states in the Oryza sativa genome, where TEs comprise 35% of the genome. We found that over 10% of rice genes contain intronic heterochromatin, most of which are associated with TEs and repetitive sequences. These heterochromatic introns are longer and highly enriched in promoter-proximal positions. On the other hand, introns also accumulate hypomethylated short TEs. Genes with heterochromatic introns are implicated in various biological functions. Transcription of genes bearing intronic heterochromatin is regulated by an epigenetic mechanism involving the conserved factor OsIBM2, mutation of which results in severe developmental and reproductive defects. Furthermore, we found that heterochromatic introns evolve rapidly compared to non-heterochromatic introns. Our study demonstrates that heterochromatin is a common epigenetic feature associated with actively transcribed genes in the rice genome.

Nanopore sequencing-based genome assembly and evolutionary genomics of circum-basmati rice

Background

The circum-basmati group of cultivated Asian rice (Oryza sativa) contains many iconic varieties and is widespread in the Indian subcontinent. Despite its economic and cultural importance, a high-quality reference genome is currently lacking, and the group’s evolutionary history is not fully resolved. To address these gaps, we use long-read nanopore sequencing and assemble the genomes of two circum-basmati rice varieties.

Results

We generate two high-quality, chromosome-level reference genomes that represent the 12 chromosomes of Oryza. The assemblies show a contig N50 of 6.32 Mb and 10.53 Mb for Basmati 334 and Dom Sufid, respectively. Using our highly contiguous assemblies, we characterize structural variations segregating across circum-basmati genomes. We discover repeat expansions not observed in japonica—the rice group most closely related to circum-basmati—as well as the presence and absence variants of over 20 Mb, one of which is a circum-basmati-specific deletion of a gene regulating awn length. We further detect strong evidence of admixture between the circum-basmati and circum-aus groups. This gene flow has its greatest effect on chromosome 10, causing both structural variation and single-nucleotide polymorphism to deviate from genome-wide history. Lastly, population genomic analysis of 78 circum-basmati varieties shows three major geographically structured genetic groups: Bhutan/Nepal, India/Bangladesh/Myanmar, and Iran/Pakistan.

Conclusion

The availability of high-quality reference genomes allows functional and evolutionary genomic analyses providing genome-wide evidence for gene flow between circum-aus and circum-basmati, describes the nature of circum-basmati structural variation, and reveals the presence/absence variation in this important and iconic rice variety group.

Genome-wide analysis of short interspersed nuclear elements provides insight into gene and genome evolution in citrus

Short interspersed nuclear elements (SINEs) are non-autonomous retrotransposons that are highly abundant, but not well annotated, in plant genomes. In this study, we identified 41,573 copies of SINEs in seven citrus genomes, including 11,275 full-length copies. The citrus SINEs were distributed among 12 families, with an average full-length rate of 0.27, and were dispersed throughout the chromosomes, preferentially in AT-rich areas. Approximately 18.4% of citrus SINEs were found in close proximity (≤1 kb upstream) to genes, indicating a significant enrichment of SINEs in promoter regions. Citrus SINEs promote gene and genome evolution by offering exons as well as splice sites and start and stop codons, creating novel genes and forming tandem and dispersed repeat structures. Comparative analysis of unique homologous SINE-containing loci (HSCLs) revealed chromosome rearrangements in sweet orange, pummelo, and mandarin, suggesting that unique HSCLs might be valuable for understanding chromosomal abnormalities. This study of SINEs provides us with new perspectives and new avenues by which to understand the evolution of citrus genes and genomes.

Genome Triplication Leads to Transcriptional Divergence of FLOWERING LOCUS C Genes During Vernalization in the Genus Brassica

The genus Brassica includes oil crops, vegetables, condiments, fodder crops, and ornamental plants. Brassica species underwent a whole genome triplication event after speciation between ancestral species of Brassica and closely related genera including Arabidopsis thaliana. Diploid species such as Brassica rapa and Brassica oleracea have three copies of genes orthologous to each A. thaliana gene, although deletion in one or two of the three homologs has occurred in some genes. The floral transition is one of the crucial events in a plant's life history, and time of flowering is an important agricultural trait. There is a variation in flowering time within species of the genus Brassica, and this variation is largely dependent on a difference in vernalization requirements. In Brassica, like in A. thaliana, the key gene of vernalization is FLOWERING LOCUS C (FLC). In Brassica species, the vernalization response including the repression of FLC expression by cold treatment and the enrichment of the repressive histone modification tri-methylated histone H3 lysine 27 (H3K27me3) at the FLC locus is similar to A. thaliana. B. rapa and B. oleracea each have four paralogs of FLC, and the allotetraploid species, Brassica napus, has nine paralogs. The increased number of paralogs makes the role of FLC in vernalization more complicated; in a single plant, paralogs vary in the expression level of FLC before and after vernalization. There is also variation in FLC expression levels between accessions. In this review, we focus on the regulatory circuits of the vernalization response of FLC expression in the genus Brassica.