Epigenomics in an extraterrestrial environment: organ-specific alteration of DNA methylation and gene expression elicited by spaceflight in Arabidopsis thaliana

Determinants of rapid adaptation in species with large variance in offspring production

The speed of population adaptation to changing biotic and abiotic environments is determined by the interaction between genetic drift, positive selection and linkage effects. Many marine species (fish, crustaceans), invertebrates and pathogens of humans and crops, exhibit sweepstakes reproduction characterized by the production of a very large amount of offspring (fecundity phase) from which only a small fraction may survive to the next generation (viability phase). Using stochastic simulations, we investigate whether the occurrence of sweepstakes reproduction affects the efficiency of a positively selected unlinked locus, and thus, the speed of adaptation since fecundity and/or viability have distinguishable consequences on mutation rate, probability and fixation time of advantageous alleles. We observe that the mean number of mutations at the next generation is always the function of the population size, but the variance increases with stronger sweepstakes reproduction when mutations occur in the parents. On the one hand, stronger sweepstakes reproduction magnifies the effect of genetic drift thus increasing the probability of fixation of neutral allele and decreasing that of selected alleles. On the other hand, the time to fixation of advantageous (as well as neutral) alleles is shortened by stronger sweepstakes reproduction. Importantly, fecundity and viability selection exhibit different probabilities and times to fixation of advantageous alleles under intermediate and weak sweepstakes reproduction. Finally, alleles under both strong fecundity and viability selection display a synergistic efficiency of selection. We conclude that measuring and modelling accurately fecundity and/or viability selection are crucial to predict the adaptive potential of species with sweepstakes reproduction.

Spaceflight impacts xyloglucan oligosaccharide abundance in Arabidopsis thaliana root cell walls

Over the course of more than a decade, space biology investigations have consistently indicated that cell wall remodeling occurs in a variety of spaceflight-grown plants. Here, we describe a mass spectrometric method to study the fundamental composition of xyloglucan, the most abundant hemicellulose in dicot cell walls, in space-grown plants. Four representative Arabidopsis root samples, from a previously conducted spaceflight experiment - Advanced Plant EXperiment - 04 (APEX-04), were used to investigate changes in xyloglucan oligosaccharides abundances in spaceflight-grown plants compared to ground controls. In situ localized enzymatic digestions and surface sampling mass spectrometry analysis provided spatial resolution of the changes in xyloglucan oligosaccharides abundances. Overall, the results showed that oligosaccharide XXLG/XLXG and XXFG branching patterns were more abundant in the lateral roots of spaceflight-grown plants, while XXXG, XLFG, and XLFG/XLFG were more abundant in the lateral roots of ground control plants. In the primary roots, XXFG had a higher abundance in ground controls than in spaceflight plants. This methodology of analyzing the basic components of the cell wall in this paper highlights two important findings. First, that are differences in the composition of xyloglucan oligosaccharides in spaceflight root cell walls compared to ground controls and, second, most of these differences are observed in the lateral roots. Thus, the methodology described in this paper provides insights into spaceflight cell wall modifications for future investigations.

Single-molecule long-read methylation profiling reveals regional DNA methylation regulated by Elongator Complex Subunit 2 in Arabidopsis roots experiencing spaceflight

BACKGROUND

The Advanced Plant Experiment-04 - Epigenetic Expression (APEX-04-EpEx) experiment onboard the International Space Station examined the spaceflight-altered cytosine methylation in two genetic lines of Arabidopsis thaliana, wild-type Col-0 and the mutant elp2-5, which is deficient in an epigenetic regulator Elongator Complex Subunit 2 (ELP2). Whole-genome bisulfite sequencing (WGBS) revealed distinct spaceflight associated methylation differences, presenting the need to explore specific space-altered methylation at single-molecule resolution to associate specific changes over large regions of spaceflight related genes. To date, tools of multiplexed targeted DNA methylation sequencing remain limited for plant genomes.

RESULTS

To provide methylation data at single-molecule resolution, Flap-enabled next-generation capture (FENGC), a novel targeted multiplexed DNA capture and enrichment technique allowing cleavage at any specified sites, was applied to survey spaceflight-altered DNA methylation in genic regions of interest. The FENGC capture panel contained 108 targets ranging from 509 to 704 nt within the promoter or gene body regions of gene targets derived from spaceflight whole-genome data sets. In addition to genes with significant changes in expression and average methylation levels between spaceflight and ground control, targets with space-altered distributions of the proportion of methylated cytosines per molecule were identified. Moreover, trends of co-methylation of different cytosine contexts were exhibited in the same DNA molecules. We further identified significant DNA methylation changes in three previously biological process-unknown genes, and loss-of-function mutants of two of these genes (named as EMO1 and EMO2 for ELP2-regulated Methylation in Orbit 1 and 2) showed enhanced root growth rate.

CONCLUSIONS

FENGC simplifies and reduces the cost of multiplexed, targeted, single-molecule profiling of methylation in plants, providing additional resolution along each DNA molecule that is not seen in population-based short-read data such as WGBS. This case study has revealed spaceflight-altered regional modification of cytosine methylation occurring within single DNA molecules of cell subpopulations, which were not identified by WGBS. The single-molecule survey by FENGC can lead to identification of novel functional genes. The newly identified EMO1 and EMO2 are root growth regulators which may be epigenetically involved in plant adaptation to spaceflight.

Conserved plant transcriptional responses to microgravity from two consecutive spaceflight experiments

Introduction

Understanding how plants adapt to the space environment is essential, as plants will be a valuable component of long duration space missions. Several spaceflight experiments have focused on transcriptional profiling as a means of understanding plant adaptation to microgravity. However, there is limited overlap between results from different experiments. Differences in experimental conditions and hardware make it difficult to find a consistent response across experiments and to distinguish the primary effects of microgravity from other spaceflight effects.

Methods

Plant Signaling (PS) and Plant RNA Regulation (PRR) were two separate spaceflight experiments conducted on the International Space Station utilizing the European Modular Cultivation System (EMCS). The EMCS provided a lighted environment for plant growth with centrifugal capabilities providing an onboard 1 g control.

Results and discussion

An RNA-Seq analysis of shoot samples from PS and PRR revealed a significant overlap of genes differentially expressed in microgravity between the two experiments. Relative to onboard 1 g controls, genes involved in transcriptional regulation, shoot development, and response to auxin and light were upregulated in microgravity in both experiments. Conversely, genes involved in defense response, abiotic stress, Ca++ signaling, and cell wall modification were commonly downregulated in both datasets. The downregulation of stress responses in microgravity in these two experiments is interesting as these pathways have been previously observed as upregulated in spaceflight compared to ground controls. Similarly, we have observed many stress response genes to be upregulated in the 1 g onboard control compared to ground reference controls; however these genes were specifically downregulated in microgravity. In addition, we analyzed the sRNA landscape of the 1 g and microgravity (μ g) shoot samples from PRR. We identified three miRNAs (miR319c, miR398b, and miR8683) which were upregulated in microgravity, while several of their corresponding target genes were found to be downregulated in microgravity. Interestingly, the downregulated target genes are enriched in those encoding chloroplast-localized enzymes and proteins. These results uncover microgravity unique transcriptional changes and highlight the validity and importance of an onboard 1 g control.

Transcriptomic dynamics in the transition from ground to space are revealed by Virgin Galactic human-tended suborbital spaceflight

The Virgin Galactic Unity 22 mission conducted the first astronaut-manipulated suborbital spaceflight experiment. The experiment examined the operationalization of Kennedy Space Center Fixation Tubes (KFTs) as a generalizable approach to preserving biology at various phases of suborbital flight. The biology chosen for this experiment was Arabidopsis thaliana, ecotype Col-0, because of the plant history of spaceflight experimentation within KFTs and wealth of comparative data from orbital experiments. KFTs were deployed as a wearable device, a leg pouch attached to the astronaut, which proved to be operationally effective during the course of the flight. Data from the inflight samples indicated that the microgravity period of the flight elicited the strongest transcriptomic responses as measured by the number of genes showing differential expression. Genes related to reactive oxygen species and stress, as well as genes associated with orbital spaceflight, were highly represented among the suborbital gene expression profile. In addition, gene families largely unaffected in orbital spaceflight were diversely regulated in suborbital flight, including stress-responsive transcription factors. The human-tended suborbital experiment demonstrated the operational effectiveness of the KFTs in suborbital flight and suggests that rapid transcriptomic responses are a part of the temporal dynamics at the beginning of physiological adaptation to spaceflight.

Arabidopsis telomerase takes off by uncoupling enzyme activity from telomere length maintenance in space

Spaceflight-induced changes in astronaut telomeres have garnered significant attention in recent years. While plants represent an essential component of future long-duration space travel, the impacts of spaceflight on plant telomeres and telomerase have not been examined. Here we report on the telomere dynamics of Arabidopsis thaliana grown aboard the International Space Station. We observe no changes in telomere length in space-flown Arabidopsis seedlings, despite a dramatic increase in telomerase activity (up to 150-fold in roots), as well as elevated genome oxidation. Ground-based follow up studies provide further evidence that telomerase is induced by different environmental stressors, but its activity is uncoupled from telomere length. Supporting this conclusion, genetically engineered super-telomerase lines with enhanced telomerase activity maintain wildtype telomere length. Finally, genome oxidation is inversely correlated with telomerase activity levels. We propose a redox protective capacity for Arabidopsis telomerase that may promote survivability in harsh environments.

Functional Meta-Analysis of the Proteomic Responses of Arabidopsis Seedlings to the Spaceflight Environment Reveals Multi-Dimensional Sources of Variability across Spaceflight Experiments

The human quest for sustainable habitation of extraterrestrial environments necessitates a robust understanding of life's adaptability to the unique conditions of spaceflight. This study provides a comprehensive proteomic dissection of the Arabidopsis plant's responses to the spaceflight environment through a meta-analysis of proteomics data from four separate spaceflight experiments conducted on the International Space Station (ISS) in different hardware configurations. Raw proteomics LC/MS spectra were analyzed for differential expression in MaxQuant and Perseus software. The analysis of dissimilarities among the datasets reveals the multidimensional nature of plant proteomic responses to spaceflight, impacted by variables such as spaceflight hardware, seedling age, lighting conditions, and proteomic quantification techniques. By contrasting datasets that varied in light exposure, we elucidated proteins involved in photomorphogenesis and skotomorphogenesis in plant spaceflight responses. Additionally, with data from an onboard 1 g control experiment, we isolated proteins that specifically respond to the microgravity environment and those that respond to other spaceflight conditions. This study identified proteins and associated metabolic pathways that are consistently impacted across the datasets. Notably, these shared proteins were associated with critical metabolic functions, including carbon metabolism, glycolysis, gluconeogenesis, and amino acid biosynthesis, underscoring their potential significance in Arabidopsis' spaceflight adaptation mechanisms and informing strategies for successful space farming.

Glycome profiling and immunohistochemistry uncover changes in cell walls of Arabidopsis thaliana roots during spaceflight

A large and diverse library of glycan-directed monoclonal antibodies (mAbs) was used to determine if plant cell walls are modified by low-gravity conditions encountered during spaceflight. This method called glycome profiling (glycomics) revealed global differences in non-cellulosic cell wall epitopes in Arabidopsis thaliana root extracts recovered from RNA purification columns between seedlings grown on the International Space Station-based Vegetable Production System and paired ground (1-g) controls. Immunohistochemistry on 11-day-old seedling primary root sections showed that ten of twenty-two mAbs that exhibited spaceflight-induced increases in binding through glycomics, labeled space-grown roots more intensely than those from the ground. The ten mAbs recognized xyloglucan, xylan, and arabinogalactan epitopes. Notably, three xylem-enriched unsubstituted xylan backbone epitopes were more intensely labeled in space-grown roots than in ground-grown roots, suggesting that the spaceflight environment accelerated root secondary cell wall formation. This study highlights the feasibility of glycomics for high-throughput evaluation of cell wall glycans using only root high alkaline extracts from RNA purification columns, and subsequent validation of these results by immunohistochemistry. This approach will benefit plant space biological studies because it extends the analyses possible from the limited amounts of samples returned from spaceflight and help uncover microgravity-induced tissue-specific changes in plant cell walls.

Current Knowledge about the Impact of Microgravity on Gene Regulation

Microgravity (µg) has a massive impact on the health of space explorers. Microgravity changes the proliferation, differentiation, and growth of cells. As crewed spaceflights into deep space are being planned along with the commercialization of space travelling, researchers have focused on gene regulation in cells and organisms exposed to real (r-) and simulated (s-) µg. In particular, cancer and metastasis research benefits from the findings obtained under µg conditions. Gene regulation is a key factor in a cell or an organism’s ability to sustain life and respond to environmental changes. It is a universal process to control the amount, location, and timing in which genes are expressed. In this review, we provide an overview of µg-induced changes in the numerous mechanisms involved in gene regulation, including regulatory proteins, microRNAs, and the chemical modification of DNA. In particular, we discuss the current knowledge about the impact of microgravity on gene regulation in different types of bacteria, protists, fungi, animals, humans, and cells with a focus on the brain, eye, endothelium, immune system, cartilage, muscle, bone, and various cancers as well as recent findings in plants. Importantly, the obtained data clearly imply that µg experiments can support translational medicine on Earth.

Brachypodium distachyon Seedlings Display Accession-Specific Morphological and Transcriptomic Responses to the Microgravity Environment of the International Space Station

Plants have been recognized as key components of bioregenerative life support systems for space exploration, and many experiments have been carried out to evaluate their adaptability to spaceflight. Unfortunately, few of these experiments have involved monocot plants, which constitute most of the crops used on Earth as sources of food, feed, and fiber. To better understand the ability of monocot plants to adapt to spaceflight, we germinated and grew Brachypodium distachyon seedlings of the Bd21, Bd21-3, and Gaz8 accessions in a customized growth unit on the International Space Station, along with 1-g ground controls. At the end of a 4-day growth period, seedling organ’s growth and morphologies were quantified, and root and shoot transcriptomic profiles were investigated using RNA-seq. The roots of all three accessions grew more slowly and displayed longer root hairs under microgravity conditions relative to ground control. On the other hand, the shoots of Bd21-3 and Gaz-8 grew at similar rates between conditions, whereas those of Bd21 grew more slowly under microgravity. The three Brachypodium accessions displayed dramatically different transcriptomic responses to microgravity relative to ground controls, with the largest numbers of differentially expressed genes (DEGs) found in Gaz8 (4527), followed by Bd21 (1353) and Bd21-3 (570). Only 47 and six DEGs were shared between accessions for shoots and roots, respectively, including DEGs encoding wall-associated proteins and photosynthesis-related DEGs. Furthermore, DEGs associated with the “Oxidative Stress Response” GO group were up-regulated in the shoots and down-regulated in the roots of Bd21 and Gaz8, indicating that Brachypodium roots and shoots deploy distinct biological strategies to adapt to the microgravity environment. A comparative analysis of the Brachypodium oxidative-stress response DEGs with the Arabidopsis ROS wheel suggests a connection between retrograde signaling, light response, and decreased expression of photosynthesis-related genes in microgravity-exposed shoots. In Gaz8, DEGs were also found to preferentially associate with the “Plant Hormonal Signaling” and “MAP Kinase Signaling” KEGG pathways. Overall, these data indicate that Brachypodium distachyon seedlings exposed to the microgravity environment of ISS display accession- and organ-specific responses that involve oxidative stress response, wall remodeling, photosynthesis inhibition, expression regulation, ribosome biogenesis, and post-translational modifications. The general characteristics of these responses are similar to those displayed by microgravity-exposed Arabidopsis thaliana seedlings. However, organ- and accession-specific components of the response dramatically differ both within and between species. These results suggest a need to directly evaluate candidate-crop responses to microgravity to better understand their specific adaptability to this novel environment and develop cultivation strategies allowing them to strive during spaceflight.

Utilizing the KSC Fixation Tube to Conduct Human-Tended Plant Biology Experiments on a Suborbital Spaceflight

Suborbital spaceflights now enable human-tended research investigating short-term gravitational effects in biological systems, eliminating the need for complex automation. Here, we discuss a method utilizing KSC Fixation Tubes (KFTs) to both carry biology to suborbital space as well as fix that biology at certain stages of flight. Plants on support media were inserted into the sample side of KFTs preloaded with RNAlater in the fixation chamber. The KFTs were activated at various stages of a simulated flight to fix the plants. RNA-seq analysis conducted on tissue samples housed in KFTs, showed that plants behaved consistently in KFTs when compared to petri-plates. Over the time course, roots adjusted to hypoxia and leaves adjusted to changes in photosynthesis. These responses were due in part to the environment imposed by the encased triple containment of the KFTs, which is a requirement for flight in human spacecraft. While plants exhibited expected reproducible transcriptomic alteration over time in the KFTs, responses to clinorotation during the simulated flight suggest that transcriptomic responses to suborbital spaceflight can be examined using this approach.

Recent transcriptomic studies to elucidate the plant adaptive response to spaceflight and to simulated space environments

Discovering the adaptation mechanisms of plants to the space environment is essential for supporting human space exploration. Transcriptomic analyses allow the identification of adaptation response pathways by detecting changes in gene expression at the global genome level caused by the main factors of the space environment, namely altered gravity and cosmic radiation. This article reviews transcriptomic studies carried out from plants grown in spaceflights and in different ground-based microgravity simulators. Despite differences in plant growth conditions, these studies have shown that cell wall remodeling, oxidative stress, defense response, and photosynthesis are common altered processes in plants grown under spaceflight conditions. European scientists have significantly contributed to the acquisition of this knowledge, e.g., by showing the role of red light in the adaptation response of plants (EMCS experiments) and the mechanisms of cellular response and adaptation mostly affecting cell cycle regulation, using cell cultures in microgravity simulators.

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Cell wall, photosynthesis, and stress response are key in plant adaptation to space

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DNA methylation and alternative splicing are among the involved molecular mechanisms

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Light is an essential factor for plant development, even more in the space environment

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EMCS and simulation cell culture experiments are the main European contributions

Activating stress memory: eustressors as potential tools for plant breeding

Plants are continuously exposed to stress conditions, such that they have developed sophisticated and elegant survival strategies, which are reflected in their phenotypic plasticity, priming capacity, and memory acquisition. Epigenetic mechanisms play a critical role in modulating gene expression and stress responses, allowing malleability, reversibility, stability, and heritability of favourable phenotypes to enhance plant performance. Considering the urgency to improve our agricultural system because of going impacting climate change, potential and sustainable strategies rely on the controlled use of eustressors, enhancing desired characteristics and yield and shaping stress tolerance in crops. However, for plant breeding purposes is necessary to focus on the use of eustressors capable of establishing stable epigenetic marks to generate a transgenerational memory to stimulate a priming state in plants to face the changing environment.

DNA Methylation Dynamics Associated with Long-term Isolation of Simulated Space Travel

Long-term isolation is one of the risk factors that astronauts will encounter in spaceflight. At present, few researches have explored DNA methylation dynamics during long-term isolation. In this study, using time series DNA methylation data from “Mars-500” mission, we conducted a multi-step analysis to investigate the characteristics and dynamic patterns of DNA methylation as well as their functional insights during long-term isolation. The results showed that genome-wide methylation changes were minimal. In the six identified DNA methylation dynamic patterns, most of significantly fluctuating CpG sites could be returned to the baseline in post-isolation, and the remaining sites persistently decreased during isolation. Next, functional enrichment analysis of genes with each pattern revealed strong functional specificity. Some patterns were also significantly associated with nervous system diseases, digestive system diseases and cancers. In conclusion, the DNA methylation dynamics during long-term isolation have great functional significance, and might be helpful for protection of astronaut health.

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Six dynamic patterns of DNA methylation were identified during long-term isolation

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Most of significantly fluctuating methylation sites recovered in post-isolation

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Six patterns showed strong functional specificity

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Genes with decreased methylation levels might be associated with tumor

Latest knowledge about changes in the proteome in microgravity

INTRODUCTION

: A long-term stay of humans in space causes a large number of well-known health problems and changes in protists and plants. Deep space exploration will increase the time humans or rodents will spend in microgravity (µg). Moreover, they are exposed to cosmic radiation, hypodynamia, and isolation. OMICS investigations will increase our knowledge of the underlying mechanisms of µg-induced alterations in vivo and in vitro.

AREAS COVERED

: We summarize the findings over the recent 3 years on µg-induced changes in the proteome of protists, plants, rodent and human cells. Considering the thematic orientation of microgravity-related publications in that time frame, we focus on medicine-associated findings such as the µg-induced antibiotic resistance of bacteria, the myocardial consequences of µg-induced calpain activation and the role of MMP13 in osteoarthritis. All these point to the fact that µg is an extreme stressor that could not be evolutionarily addressed on Earth.

EXPERT COMMENTARY

: In conclusion, when interpreting µg-experiments, the direct, mostly unspecific stress response, must be distinguished from specific µg-effects. For this reason, recent studies often do not consider single protein findings but place them in the context of protein-protein interactions. This enables an estimation of functional relationships, especially if these are supported by epigenetic and transcriptional data (multi-omics).

Dynamic changes in DNA methylation occur in TE regions and affect cell proliferation during leaf-to-callus transition in Arabidopsis

Plant somatic cells can be reprogrammed into pluripotent cell mass, called callus, through a two-step in vitro tissue culture method. Incubation on callus-inducing medium triggers active cell proliferation to form a pluripotent callus. Notably, DNA methylation is implicated during callus formation, but a detailed molecular process regulated by DNA methylation remains to be fully elucidated. Here, we compared genome-wide DNA methylation profiles between leaf and callus tissues in Arabidopsis using whole-genome bisulphite-sequencing. Global distribution of DNA methylation showed that CHG methylation was increased, whereas CHH methylation was reduced especially around transposable element (TE) regions during the leaf-to-callus transition. We further analysed differentially expressed genes around differentially methylated TEs (DMTEs) during the leaf-to-callus transition and found that genes involved in cell cycle regulation were enriched and also constituted a coexpression gene network along with pluripotency regulators. In addition, a conserved DNA sequence analysis for upstream cis-elements led us to find a putative transcription factor associated with cell fate transition. CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) was newly identified as a regulator of plant regeneration, and consistently, the cca1lhy mutant displayed altered phenotypes in callus proliferation. Overall, these results suggest that DNA methylation coordinates cell cycle regulation during callus formation, and CCA1 may act as a key upstream coordinator at least in part in the processes.

Plant Gravitropism: From Mechanistic Insights into Plant Function on Earth to Plants Colonizing Other Worlds

Gravitropism, the growth of roots and shoots toward or away from the direction of gravity, has been studied for centuries. Such studies have not only led to a better understanding of the gravitropic process itself, but also paved new paths leading to deeper mechanistic insights into a wide range of research areas. These include hormone biology, cell signal transduction, regulation of gene expression, plant evolution, and plant interactions with a variety of environmental stimuli. In addition to contributions to basic knowledge about how plants function, there is accumulating evidence that gravitropism confers adaptive advantages to crops, particularly under marginal agricultural soils. Therefore, gravitropism is emerging as a breeding target for enhancing agricultural productivity. Moreover, research on gravitropism has spawned several studies on plant growth in microgravity that have enabled researchers to uncouple the effects of gravity from other tropisms. Although rapid progress on understanding gravitropism witnessed during the past decade continues to be driven by traditional molecular, physiological, and cell biological tools, these tools have been enriched by technological innovations in next-generation omics platforms and microgravity analog facilities. In this chapter, we review the field of gravitropism by highlighting recent landmark studies that have provided unique insights into this classic research topic while also discussing potential contributions to agriculture on Earth and beyond.

Epigenetic regulation of salinity stress responses in cereals

Cereals are important crops and are exposed to various types of environmental stresses that affect the overall growth and yield. Among the various abiotic stresses, salt stress is a major environmental factor that influences the genetic, physiological, and biochemical responses of cereal crops. Epigenetic regulation which includes DNA methylation, histone modification, and chromatin remodelling plays an important role in salt stress tolerance. Recent studies in rice genomics have highlighted that the epigenetic changes are heritable and therefore can be considered as molecular signatures. An epigenetic mechanism under salinity induces phenotypic responses involving modulations in gene expression. Association between histone modification and altered DNA methylation patterns and differential gene expression has been evidenced for salt sensitivity in rice and other cereal crops. In addition, epigenetics also creates stress memory that helps the plant to better combat future stress exposure. In the present review, we have discussed epigenetic influences in stress tolerance, adaptation, and evolution processes. Understanding the epigenetic regulation of salinity could help for designing salt-tolerant varieties leading to improved crop productivity.

MET1-Dependent DNA Methylation Represses Light Signaling and Influences Plant Regeneration in Arabidopsis

Plant somatic cells can be reprogrammed into a pluripotent cell mass, called callus, which can be subsequently used for de novo shoot regeneration through a two-step in vitro tissue culture method. MET1-dependent CG methylation has been implicated in plant regeneration in Arabidopsis, because the met1-3 mutant exhibits increased shoot regeneration compared with the wild-type. To understand the role of MET1 in de novo shoot regeneration, we compared the genome-wide DNA methylomes and transcriptomes of wild-type and met1-3 callus and leaf. The CG methylation patterns were largely unchanged during leaf-to-callus transition, suggesting that the altered regeneration phenotype of met1-3 was caused by the constitutively hypomethylated genes, independent of the tissue type. In particular, MET1-dependent CG methylation was observed at the blue light receptor genes, CRYPTOCHROME 1 (CRY1) and CRY2, which reduced their expression. Coexpression network analysis revealed that the CRY1 gene was closely linked to cytokinin signaling genes. Consistently, functional enrichment analysis of differentially expressed genes in met1-3 showed that gene ontology terms related to light and hormone signaling were overrepresented. Overall, our findings indicate that MET1-dependent repression of light and cytokinin signaling influences plant regeneration capacity and shoot identity establishment.

Epigenomic Regulators Elongator Complex Subunit 2 and Methyltransferase 1 Differentially Condition the Spaceflight Response in Arabidopsis

Background: Plants subjected to the novel environment of spaceflight show transcriptomic changes that resemble aspects of several terrestrial abiotic stress responses. Under investigation here is whether epigenetic modulations, similar to those that occur in terrestrial stress responses, have a functional role in spaceflight physiological adaptation. The Advanced Plant Experiment-04 - Epigenetic Expression experiment examined the role of cytosine methylation in spaceflight adaptation. The experiment was conducted onboard the International Space Station, and evaluated the spaceflight-altered, genome-wide methylation profiles of two methylation-regulating gene mutants [methyltransferase 1 (met1-7) and elongator complex subunit 2 (elp2-5)] along with a wild-type Col-0 control. Results: The elp2-5 plants suffered in their physiological adaptation to spaceflight in that their roots failed to extend away from the seed and the overall development of the plants was greatly impaired in space. The met1-7 plants suffered less, with their morphology affected by spaceflight in a manner similar to that of the Col-0 controls. The differentially expressed genes (DEGs) in spaceflight were dramatically different in the elp2-5 and met1-7 plants compared to Col-0, indicating that the disruptions in these mutants resulted in a reprogramming of their spaceflight responses, especially in elp2-5. Many of the genes comprising the spaceflight transcriptome of each genotype were differentially methylated in spaceflight. In Col-0 the majority of the DEGs were representative of the now familiar spaceflight response, which includes genes associated with cell wall remodeling, pathogen responses and ROS signaling. However, the spaceflight transcriptomes of met1-7 and elp2-5 each presented patterns of DEGs that are almost completely different than Col-0, and to each other. Further, the DEGs of the mutant genotypes suggest a more severe spaceflight stress response in the mutants, particularly in elp2-5. Conclusion: Arabidopsis physiological adaptation to spaceflight results in differential DNA methylation in an organ-specific manner. Disruption of Met1 methyltransferase function does not dramatically affect spaceflight growth or morphology, yet met1-7 reprograms the spaceflight transcriptomic response in a unique manner. Disruption of elp2-5 results in poor development in spaceflight grown plants, together with a diminished, dramatically reprogrammed transcriptomic response.

Histone deacetylase HDA-4-mediated epigenetic regulation in space-flown C. elegans

Epigenetic changes during long-term spaceflight are beginning to be studied by NASA's twin astronauts and other model organisms. Here, we evaluate the epigenetic regulation of gene expression in space-flown C. elegans by comparing wild type and histone deacetylase (hda)-4 mutants. Expression levels of 39 genes were consistently upregulated in all four generations of adult hda-4 mutants grown under microgravity compared with artificial Earth-like gravity (1G). In contrast, in the wild type, microgravity-induced upregulation of these genes occurred a little. Among these genes, 11 contain the domain of unknown function 19 (DUF-19) and are located in a cluster on chromosome V. When compared with the 1G condition, histone H3 trimethylation at lysine 27 (H3K27me3) increased under microgravity in the DUF-19 containing genes T20D4.12 to 4.10 locus in wild-type adults. On the other hand, this increase was also observed in the hda-4 mutant, but the level was significantly reduced. The body length of wild-type adults decreased slightly but significantly when grown under microgravity. This decrease was even more pronounced with the hda-4 mutant. In ground-based experiments, one of the T20D4.11 overexpressing strains significantly reduced body length and also caused larval growth retardation and arrest. These results indicate that under microgravity, C. elegans activates histone deacetylase HDA-4 to suppress overregulation of several genes, including the DUF-19 family. In other words, the expression of certain genes, including negative regulators of growth and development, is epigenetically fine-tuned to adapt to the space microgravity.

Potential evidence for transgenerational epigenetic memory in Arabidopsis thaliana following spaceflight

Plants grown in spaceflight exhibited differential methylation responses and this is important because plants are sessile, they are constantly exposed to a variety of environmental pressures and respond to them in many ways. We previously showed that the Arabidopsis genome exhibited lower methylation level after spaceflight for 60 h in orbit. Here, using the offspring of the seedlings grown in microgravity environment in the SJ-10 satellite for 11 days and returned to Earth, we systematically studied the potential effects of spaceflight on DNA methylation, transcriptome, and phenotype in the offspring. Whole-genome methylation analysis in the first generation of offspring (F₁) showed that, although there was no significant difference in methylation level as had previously been observed in the parent plants, some residual imprints of DNA methylation differences were detected. Combined DNA methylation and RNA-sequencing analysis indicated that expression of many pathways, such as the abscisic acid-activated pathway, protein phosphorylation, and nitrate signaling pathway, etc. were enriched in the F₁ population. As some phenotypic differences still existed in the F₂ generation, it was suggested that these epigenetic DNA methylation modifications were partially retained, resulting in phenotypic differences in the offspring. Furthermore, some of the spaceflight-induced heritable differentially methylated regions (DMRs) were retained. Changes in epigenetic modifications caused by spaceflight affected the growth of two future seed generations. Altogether, our research is helpful in better understanding the adaptation mechanism of plants to the spaceflight environment.

In order to investigate whether the effects of spaceflight on plants persist in future generations, Xu et al studied the offspring of Arabidopsis thaliana seedlings that had been grown in a microgravity environment for 11 days. They found that epigenetic modifications caused by spaceflight potentially affected the growth of two future seed generations, shedding light on the longevity of the effects of spaceflight on plants.

Relevance of the Unfolded Protein Response to Spaceflight-Induced Transcriptional Reprogramming in Arabidopsis

Plants are primary producers of food and oxygen on Earth and will likewise be indispensable to the establishment of large-scale sustainable ecosystems and human survival in space. To contribute to the understanding of how plants respond to spaceflight stress, we examined the significance of the unfolded protein response (UPR), a conserved signaling cascade that responds to a number of unfavorable environmental stresses, in the model plant Arabidopsis thaliana. To do so, we performed a large-scale comparative transcriptome profiling in wild type and various UPR-defective mutants during the SpaceX-CRS12 mission to the International Space Station. We established that orbital culture substantially alters the expression of hundreds of stress-related genes compared with ground control conditions. Although expression of those genes varied in the UPR mutants on the ground, it was largely similar across the genotypes in the spaceflight condition. Our results have yielded new information on how plants respond to growth in orbit and support the hypothesis that spaceflight induces the activation of signaling pathways that compensate for the loss of UPR regulators in the control of downstream transcriptional regulatory networks.

UVR8 interacts with de novo DNA methyltransferase and suppresses DNA methylation in Arabidopsis

DNA methylation is an important epigenetic gene regulatory mechanism conserved in eukaryotes. Emerging evidence shows DNA methylation alterations in response to environmental cues. However, the mechanism of how cells sense these signals and reprogramme the methylation landscape is poorly understood. Here, we uncovered a connection between ultraviolet B (UVB) signalling and DNA methylation involving UVB photoreceptor (UV RESISTANCE LOCUS 8 (UVR8)) and a de novo DNA methyltransferase (DOMAINS REARRANGED METHYLTRANSFERASE 2 (DRM2)) in Arabidopsis. We demonstrated that UVB acts through UVR8 to inhibit DRM2-mediated DNA methylation and transcriptional de-repression. Interestingly, DNA transposons with high DNA methylation are more sensitive to UVB irradiation. Mechanistically, UVR8 interacts with and negatively regulates DRM2 by preventing its chromatin association and inhibiting the methyltransferase activity. Collectively, this study identifies UVB as a potent inhibitor of DNA methylation and provides mechanistic insights into how signalling transduction cascades intertwine with chromatin to guide genome functions.

Whole-Genome DNA Methylation Analysis in Hydrogen Peroxide Overproducing Transgenic Tobacco Resistant to Biotic and Abiotic Stresses

Epigenetic regulation is a key component of stress responses, acclimatization and adaptation processes in plants. DNA methylation is a stable mark plausible for the inheritance of epigenetic traits, such that it is a potential scheme for plant breeding. However, the effect of modulators of stress responses, as hydrogen peroxide (H₂O₂), in the methylome status has not been elucidated. A transgenic tobacco model to the CchGLP gene displayed high H₂O₂ endogen levels correlated with biotic and abiotic stresses resistance. The present study aimed to determine the DNA methylation status changes in the transgenic model to obtain more information about the molecular mechanism involved in resistance phenotypes. The Whole-genome bisulfite sequencing analysis revealed a minimal impact of overall levels and distribution of methylation. A total of 9432 differential methylated sites were identified in distinct genome regions, most of them in CHG context, with a trend to hypomethylation. Of these, 1117 sites corresponded to genes, from which 83 were also differentially expressed in the plants. Several genes were associated with respiration, energy, and calcium signaling. The data obtained highlighted the relevance of the H₂O₂ in the homeostasis of the system in stress conditions, affecting at methylation level and suggesting an association of the H₂O₂ in the physiological adaptation to stress functional linkages may be regulated in part by DNA methylation.

Changes in DNA Methylation in Arabidopsis thaliana Plants Exposed Over Multiple Generations to Gamma Radiation

Previous studies have found indications that exposure to ionising radiation (IR) results in DNA methylation changes in plants. However, this phenomenon is yet to be studied across multiple generations. Furthermore, the exact role of these changes in the IR-induced plant response is still far from understood. Here, we study the effect of gamma radiation on DNA methylation and its effect across generations in young Arabidopsis plants. A multigenerational set-up was used in which three generations (Parent, generation 1, and generation 2) of 7-day old Arabidopsis thaliana plants were exposed to either of the different radiation treatments (30, 60, 110, or 430 mGy/h) or to natural background radiation (control condition) for 14 days. The parental generation consisted of previously non-exposed plants, whereas generation 1 and generation 2 plants had already received a similar irradiation in the previous one or two generations, respectively. Directly after exposure the entire methylomes were analysed with UPLC-MS/MS to measure whole genome methylation levels. Whole genome bisulfite sequencing was used to identify differentially methylated regions (DMRs), including their methylation context in the three generations and this for three different radiation conditions (control, 30 mGy/h, and 110 mGy/h). Both intra- and intergenerational comparisons of the genes and transposable elements associated with the DMRs were made. Taking the methylation context into account, the highest number of changes were found for cytosines followed directly by guanine (CG methylation), whereas only limited changes in CHG methylation occurred and no changes in CHH methylation were observed. A clear increase in IR-induced DMRs was seen over the three generations that were exposed to the lowest dose rate, where generation 2 had a markedly higher number of DMRs than the previous two generations (Parent and generation 1). Counterintuitively, we did not see significant differences in the plants exposed to the highest dose rate. A large number of DMRs associated with transposable elements were found, the majority of them being hypermethylated, likely leading to more genetic stability. Next to that, a significant number of DMRs were associated with genes (either in their promoter-associated region or gene body). A functional analysis of these genes showed an enrichment for genes related to development as well as various stress responses, including DNA repair, RNA splicing, and (a)biotic stress responses. These observations indicate a role of DNA methylation in the regulation of these genes in response to IR exposure and shows a possible role for epigenetics in plant adaptation to IR over multiple generations.

Cuscuta chinensis flavonoids down-regulate the DNA methylation of the H19/Igf2 imprinted control region and estrogen receptor alpha promoter of the testis in bisphenol A exposed mouse offspring

Exposure to the emerging contaminant bisphenol A (BPA) is ubiquitous and associated with reproductive disorders. The BPA effect as an endocrine disruptor is widely known but other mechanisms underlying developmental disease, such as epigenetic modifications, still remain unclear. The objective of this study was to investigate whether Cuscuta chinensis flavonoids (CCFs) can be used as a dietary supplement to reverse BPA-induced epigenetic disorders, by analyzing the molecular processes related to BPA impairment of testicular development. BPA and different concentrations of CCFs were administered to the dams at gestation day (GD) 0.5-17.5. The testis and serum of male mice were collected at postnatal day (PND) 21 and PND 56 for the detection of related indicators. Our results showed that compared with the BPA group, CCFs could significantly increase the serum contents of testosterone (T), estradiol (E2) in males at PND 21 and PND 56, as well as the contents and transcription levels of DNA methyltransferase 3A (Dnmt3A), Dnmt3B in males at PND 21 and that of estrogen receptor alpha (ERα) at PND 56. The expressions of Dnmt1 and ERα at PND 21 and ERβ at both PND 21 and PND 56 in males were significantly decreased with the administration of different concentrations of CCFs (P < 0.01 or P < 0.05). CCFs also significantly inhibited the BPA-induced hypermethylated status of the ERα promoter and H19/Igf2 imprinting control region (ICR) in the testis at PND 56. These results indicated that CCFs could decrease the methylation levels of ERα and H19/Igf2 genes by inhibiting the expression of DNA methyltransferases (DNMTs), thereby decreasing the levels of reproductive hormones and receptors in adult males, and ultimately alleviating the negative effect of BPA on testicular development in male mice.

Exploration of space to achieve scientific breakthroughs

Living organisms adapt to changing environments using their amazing flexibility to remodel themselves by a process called evolution. Environmental stress causes selective pressure and is associated with genetic and phenotypic shifts for better modifications, maintenance, and functioning of organismal systems. The natural evolution process can be used in complement to rational strain engineering for the development of desired traits or phenotypes as well as for the production of novel biomaterials through the imposition of one or more selective pressures. Space provides a unique environment of stressors (e.g., weightlessness and high radiation) that organisms have never experienced on Earth. Cells in the outer space reorganize and develop or activate a range of molecular responses that lead to changes in cellular properties. Exposure of cells to the outer space will lead to the development of novel variants more efficiently than on Earth. For instance, natural crop varieties can be generated with higher nutrition value, yield, and improved features, such as resistance against high and low temperatures, salt stress, and microbial and pest attacks. The review summarizes the literature on the parameters of outer space that affect the growth and behavior of cells and organisms as well as complex colloidal systems. We illustrate an understanding of gravity-related basic biological mechanisms and enlighten the possibility to explore the outer space environment for application-oriented aspects. This will stimulate biological research in the pursuit of innovative approaches for the future of agriculture and health on Earth.

Rocket Science: The Effect of Spaceflight on Germination Physiology, Ageing, and Transcriptome of Eruca sativa Seeds

In the 'Rocket Science' project, storage of Eruca sativa (salad rocket) seeds for six months on board the International Space Station resulted in delayed seedling establishment. Here we investigated the physiological and molecular mechanisms underpinning the spaceflight effects on dry seeds. We found that 'Space' seed germination vigor was reduced, and ageing sensitivity increased, but the spaceflight did not compromise seed viability and the development of normal seedlings. Comparative analysis of the transcriptomes (using RNAseq) in dry seeds and upon controlled artificial ageing treatment (CAAT) revealed differentially expressed genes (DEGs) associated with spaceflight and ageing. DEG categories enriched by spaceflight and CAAT included transcription and translation with reduced transcript abundances for 40S and 60S ribosomal subunit genes. Among the 'spaceflight-up' DEGs were heat shock proteins (HSPs), DNAJ-related chaperones, a heat shock factor (HSFA7a-like), and components of several DNA repair pathways (e.g., ATM, DNA ligase 1). The 'response to radiation' category was especially enriched in 'spaceflight-up' DEGs including HSPs, catalases, and the transcription factor HY5. The major finding from the physiological and transcriptome analysis is that spaceflight causes vigor loss and partial ageing during air-dry seed storage, for which space environmental factors and consequences for seed storage during spaceflights are discussed.

Test of Arabidopsis Space Transcriptome: A Discovery Environment to Explore Multiple Plant Biology Spaceflight Experiments

Recent advances in the routine access to space along with increasing opportunities to perform plant growth experiments on board the International Space Station have led to an ever-increasing body of transcriptomic, proteomic, and epigenomic data from plants experiencing spaceflight. These datasets hold great promise to help understand how plant biology reacts to this unique environment. However, analyses that mine across such expanses of data are often complex to implement, being impeded by the sheer number of potential comparisons that are possible. Complexities in how the output of these multiple parallel analyses can be presented to the researcher in an accessible and intuitive form provides further barriers to such research. Recent developments in computational systems biology have led to rapid advances in interactive data visualization environments designed to perform just such tasks. However, to date none of these tools have been tailored to the analysis of the broad-ranging plant biology spaceflight data. We have therefore developed the Test Of Arabidopsis Space Transcriptome (TOAST) database (https://astrobiology.botany.wisc.edu/astrobotany-toast) to address this gap in our capabilities. TOAST is a relational database that uses the Qlik database management software to link plant biology, spaceflight-related omics datasets, and their associated metadata. This environment helps visualize relationships across multiple levels of experiments in an easy to use gene-centric platform. TOAST draws on data from The US National Aeronautics and Space Administration's (NASA's) GeneLab and other data repositories and also connects results to a suite of web-based analytical tools to facilitate further investigation of responses to spaceflight and related stresses. The TOAST graphical user interface allows for quick comparisons between plant spaceflight experiments using real-time, gene-specific queries, or by using functional gene ontology, Kyoto Encyclopedia of Genes and Genomes pathway, or other filtering systems to explore genetic networks of interest. Testing of the database shows that TOAST confirms patterns of gene expression already highlighted in the literature, such as revealing the modulation of oxidative stress-related responses across multiple plant spaceflight experiments. However, this data exploration environment can also drive new insights into patterns of spaceflight responsive gene expression. For example, TOAST analyses highlight changes to mitochondrial function as likely shared responses in many plant spaceflight experiments.

Root Skewing-Associated Genes Impact the Spaceflight Response of Arabidopsis thaliana

The observation that plant roots skew in microgravity recently refuted the long-held conviction that skewing was a gravity-dependent phenomenon. Further, spaceflight root skewing suggests that specific root morphologies and cell wall remodeling systems may be important aspects of spaceflight physiological adaptation. However, connections between skewing, cell wall modification and spaceflight physiology are currently based on inferences rather than direct tests. Therefore, the Advanced Plant Experiments-03-2 (APEX-03-2) spaceflight study was designed to elucidate the contribution of two skewing- and cell wall-associated genes in Arabidopsis to root behavior and gene expression patterns in spaceflight, to assess whether interruptions of different skewing pathways affect the overall spaceflight-associated process. SPIRAL1 is a skewing-related protein implicated in directional cell expansion, and functions by regulating cortical microtubule dynamics. SKU5 is skewing-related glycosylphosphatidylinositol-anchored protein of the plasma membrane and cell wall implicated in stress response signaling. These two genes function in different cellular pathways that affect skewing on the Earth, and enable a test of the relevance of skewing pathways to spaceflight physiological adaptation. In this study, both sku5 and spr1 mutants showed different skewing behavior and markedly different patterns of gene expression in the spaceflight environment. The spr1 mutant showed fewer differentially expressed genes than its Col-0 wild-type, whereas sku5 showed considerably more than its WS wild-type. Developmental age played a substantial role in spaceflight acclimation in all genotypes, but particularly in sku5 plants, where spaceflight 4d seedlings had almost 10-times as many highly differentially expressed genes as the 8d seedlings. These differences demonstrated that the two skewing pathways represented by SKU5 and SPR1 have unique and opposite contributions to physiological adaptation to spaceflight. The spr1 response is less intense than wild type, suggesting that the loss of SPR1 positively impacts spaceflight adaptation. Conversely, the intensity of the sku5 responses suggests that the loss of SKU5 initiates a much more complex, deeper and more stress related response to spaceflight. This suggests that proper SKU5 function is important to spaceflight adaptation.

Changes in DNA methylation pattern of apple long-term in vitro shoot culture and acclimatized plants

DNA methylation is a process of epigenetic modification that can alter the functionality of a genome. Using whole-genome bisulfite sequencing, this study quantify the level of DNA methylation in the epigenomes of two diploid apple (Malus x domestica) scion cultivars ('McIntosh' and 'Húsvéti rozmaring') derived from three environmental conditions: in vivo mother plants in an orchard, in vitro culture, and acclimatized in vitro plants. The global DNA methylation levels were not dependent on the source of plant material, and the average level of DNA methylation was 49.77%, 34.65% and 8.77% in CpG, CHG and CHH contexts, respectively. Significant differences in DNA methylation were identified in 586 (specifically 334, 201 and 131 in CpG, CHG and CHH contexts, respectively) out of 45,116 genes, including promoter and coding sequences. These were classified as differentially methylated genes (DMGs). This is a 1.3% difference in the level of DNA methylation of genes in response to a change in the environment. Differential methylation was visualised by MA plots and functional genomic maps were established for biological processes, molecular functions and cellular components. When the DMGs were considered, in vitro tissue culture resulted in the highest level of methylation, but it was lower in acclimatized in vitro plants which was similar to that in the mother tree. Methylation patterns of the two scions differed, indicating cultivar-specific epigenetic regulation of gene expression during adaptation to various environments. After selecting genes that displayed differences larger than ±10% in CpG and CHG contexts, or larger than ±1.35% in the CHH context from among the DMGs, they were annotated in Blast2 GO v5.1.12 for Gene Ontology. DMGs identified as MD07G1113000 (protein transport), MD08G1041600 (extracellular space), MD09G1054800 (phosphatidic acid binding), and MD10G1265800 (not annotated) were methylated in all three contexts in in vitro shoots. These DNA methylation results suggest that epigenetic changes may contribute to the adaptation of apple to environmental changes by modifying the epigenome and thereby gene expression.