Exploration of plant growth and development using the European Modular Cultivation System facility on the International Space Station

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.

Lipid Rafts and Plant Gravisensitivity

The necessity to include plants as a component of a Bioregenerative Life Support System leads to investigations to optimize plant growth facilities as well as a better understanding of the plant cell membrane and its numerous activities in the signaling, transport, and sensing of gravity, drought, and other stressors. The cell membrane participates in numerous processes, including endo- and exocytosis and cell division, and is involved in the response to external stimuli. Variable but stabilized microdomains form in membranes that include specific lipids and proteins that became known as (detergent-resistant) membrane microdomains, or lipid rafts with various subclassifications. The composition, especially the sterol-dependent recruitment of specific proteins affects endo- and exo-membrane domains as well as plasmodesmata. The enhanced saturated fatty acid content in lipid rafts after clinorotation suggests increased rigidity and reduced membrane permeability as a primary response to abiotic and mechanical stress. These results can also be obtained with lipid-sensitive stains. The linkage of the CM to the cytoskeleton via rafts is part of the complex interactions between lipid microdomains, mechanosensitive ion channels, and the organization of the cytoskeleton. These intricately linked structures and functions provide multiple future research directions to elucidate the role of lipid rafts in physiological processes.

Red Light Enhances Plant Adaptation to Spaceflight and Mars g-Levels

Understanding how plants respond and adapt to extraterrestrial conditions is essential for space exploration initiatives. Deleterious effects of the space environment on plant development have been reported, such as the unbalance of cell growth and proliferation in the root meristem, or gene expression reprogramming. However, plants are capable of surviving and completing the seed-to-seed life cycle under microgravity. A key research challenge is to identify environmental cues, such as light, which could compensate the negative effects of microgravity. Understanding the crosstalk between light and gravity sensing in space was the major objective of the NASA-ESA Seedling Growth series of spaceflight experiments (2013–2018). Different g-levels were used, with special attention to micro-g, Mars-g, and Earth-g. In spaceflight seedlings illuminated for 4 days with a white light photoperiod and then photostimulated with red light for 2 days, transcriptomic studies showed, first, that red light partially reverted the gene reprogramming induced by microgravity, and that the combination of microgravity and photoactivation was not recognized by seedlings as stressful. Two mutant lines of the nucleolar protein nucleolin exhibited differential requirements in response to red light photoactivation. This observation opens the way to directed-mutagenesis strategies in crop design to be used in space colonization. Further transcriptomic studies at different g-levels showed elevated plastid and mitochondrial genome expression in microgravity, associated with disturbed nucleus–organelle communication, and the upregulation of genes encoding auxin and cytokinin hormonal pathways. At the Mars g-level, genes of hormone pathways related to stress response were activated, together with some transcription factors specifically related to acclimation, suggesting that seedlings grown in partial-g are able to acclimate by modulating genome expression in routes related to space-environment-associated stress.

The International Space Station Environment Triggers Molecular Responses in Aspergillus niger

Due to immense phenotypic plasticity and adaptability, Aspergillus niger is a cosmopolitan fungus that thrives in versatile environments, including the International Space Station (ISS). This is the first report of genomic, proteomic, and metabolomic alterations observed in A. niger strain JSC-093350089 grown in a controlled experiment aboard the ISS. Whole-genome sequencing (WGS) revealed that ISS conditions, including microgravity and enhanced irradiation, triggered non-synonymous point mutations in specific regions, chromosomes VIII and XII of the JSC-093350089 genome when compared to the ground-grown control. Proteome analysis showed altered abundance of proteins involved in carbohydrate metabolism, stress response, and cellular amino acid and protein catabolic processes following growth aboard the ISS. Metabolome analysis further confirmed that space conditions altered molecular suite of ISS-grown A. niger JSC-093350089. After regrowing both strains on Earth, production of antioxidant—Pyranonigrin A was significantly induced in the ISS-flown, but not the ground control strain. In summary, the microgravity and enhanced irradiation triggered unique molecular responses in the A. niger JSC-093350089 suggesting adaptive responses.

Conducting Plant Experiments in Space and on the Moon

The growth and development of plants during spaceflight have important implications for both basic and applied research supported by NASA and other international space agencies. While there have been many reviews of plant space biology, this chapter attempts to fill a gap in the literature on the actual process and methods of performing plant research in the spaceflight environment. One of the authors (JZK) has been a principal investigator on eight spaceflight projects. These experiences include using the U.S. Space Shuttle, the former Russian Space Station Mir, and the International Space Station, utilizing the Space Shuttle and Space X as launch vehicles. While there are several ways to fly an experiment into space and to obtain a spaceflight opportunity, this review focuses on using the NASA peer-reviewed sciences approach to get an experiment manifested for flight. Three narratives for the implementation of plant space biology experiments are considered from rapid turn around of a few months to a project with new hardware development that lasted 6 years. The many challenges of spaceflight research include logistical and resource constraints such as crew time, power, cold stowage, data downlinks, among others. Additional issues considered are working at NASA centers, hardware development, safety concerns, and the engineering versus science culture in space agencies. The difficulties of publishing the results from spaceflight research based on such factors as the lack of controls, limited sample size, and the indirect effects of the spaceflight environment also are summarized. Lessons learned from these spaceflight experiences are discussed in the context of improvements for future space-based research projects with plants. We also will consider new opportunities for Moon-based research via NASA's Artemis lunar exploration program.

Use of Reduced Gravity Simulators for Plant Biological Studies

Simulated microgravity and partial gravity research on Earth is a necessary complement to space research in real microgravity due to limitations of access to spaceflight. However, the use of ground-based facilities for reduced gravity simulation is far from simple. Microgravity simulation usually results in the need to consider secondary effects that appear in the generation of altered gravity. These secondary effects may interfere with gravity alteration in the changes observed in the biological processes under study. In addition to microgravity simulation, ground-based facilities are also capable of generating hypergravity or fractional gravity conditions whose effects on biological systems are worth being tested and compared with the results of microgravity exposure. Multiple technologies (2D clinorotation, random positioning machines, magnetic levitators, or centrifuges) and experimental hardware (different containers and substrates for seedlings or cell cultures) are available for these studies. Experimental requirements should be collectively and carefully considered in defining the optimal experimental design, taking into account that some environmental parameters, or life-support conditions, could be difficult to be provided in certain facilities. Using simulation facilities will allow us to anticipate, modify, or redefine the findings provided by the scarce available spaceflight opportunities.

Plant biology for space exploration - Building on the past, preparing for the future

A review of past insights of space experiments with plants outlines basic space and gravity effects as well as gene expression. Efforts to grow plants in space gradually incorporated basic question on plant productivity, stress response and cultivation. The prospect of extended space missions as well as colonization of the Moon and Mars require better understanding and therefore research efforts on biomass productivity, substrate and water relations, atmospheric composition, pressure and temperature and substrate and volume (growth space) requirements. The essential combination of using plants not only for food production but also for regeneration of waste, and recycling of carbon and oxygen production requires integration of complex biological and engineering aspects. We combine a historical account of plant space research with considerations for future research on plant cultivation, selection, and productivity based on space-related environmental conditions.

Synthetic biological circuit tested in spaceflight

Synthetic biology has potential spaceflight applications yet few if any studies have attempted to translate Earth-based synthetic biology tools into spaceflight. An exogenously inducible biological circuit for protein production in Arabidopsis thaliana, pX7-AtPDSi (Guo et al. 2003), was flown to ISS and functionally investigated. Seedlings were grown in a custom built 1.25 U plant greenhouse. Images recorded during the experiment show that leaves of pX7-AtPDSi seedlings photobleached as designed while wild type Col-0 leaves did not, which reveals that the synthetic circuit led to protein production during spaceflight. Polymerase chain reaction analysis post-flight also confirms that the Cre/LoxP (recombination system) portions of the circuit were functional in spaceflight. The subcomponents of the biological circuit, estrogen-responsive transcription factor XVE, Cre/LoxP DNA recombination system, and RNAi post-transcriptional gene silencing system now have flight heritage and can be incorporated in future designs for space applications. To facilitate future plant studies in space, the full payload design and manufacturing files are made available.

Proteomic characterization of Aspergillus fumigatus isolated from air and surfaces of the International Space Station

The on-going Microbial Observatory Experiments on the International Space Station (ISS) revealed the presence of various microorganisms that may be affected by the distinct environment of the ISS. The low-nutrient environment combined with enhanced irradiation and microgravity may trigger changes in the molecular suite of microorganisms leading to increased virulence and resistance of microbes. Proteomic characterization of two Aspergillus fumigatus strains, ISSFT-021 and IF1SW-F4, isolated from HEPA filter debris and cupola surface of the ISS, respectively, is presented, along with a comparison to well-studied clinical isolates Af293 and CEA10. In-depth analysis highlights variations in the proteome of both ISS-isolated strains when compared to the clinical strains. Proteins that showed increased abundance in ISS isolates were overall involved in stress responses, and carbohydrate and secondary metabolism. Among the most abundant proteins were Pst2 and ArtA involved in oxidative stress response, PdcA and AcuE responsible for ethanol fermentation and glyoxylate cycle, respectively, TpcA, TpcF, and TpcK that are part of trypacidin biosynthetic pathway, and a toxin Asp-hemolysin. This report provides insight into possible molecular adaptation of filamentous fungi to the unique ISS environment.

Reduction of geomagnetic field (GMF) to near null magnetic field (NNMF) affects Arabidopsis thaliana root mineral nutrition

The Earth magnetic field (or geomagnetic field, GMF) is a natural component of our planet and variations of the GMF are perceived by plants with a still uncharacterized magnetoreceptor. The purpose of this work was to assess the effect of near null magnetic field (NNMF, ∼40 nT) on Arabidopsis thaliana Col0 root ion modulation. A time-course (from 10 min to 96 h) exposure of Arabidopsis to NNMF was compared to GMF and the content of some cations (NH₄⁺, K⁺, Ca²⁺ and Mg²⁺) and anions (Cl^-, SO₄⁼, NO₃^- and PO₄⁼) was evaluated by capillary electrophoresis. The expression of several cation and anion channel- and transporter-related genes was assessed by gene microarray. A few minutes after exposure to NNMF, Arabidopsis roots responded with a significant change in the content and gene expression of all nutrient ions under study, indicating the presence of a plant magnetoreceptor that responds immediately to MF variations by modulating channels, transporters and genes involved in mineral nutrition. The response of Arabidopsis to reduced MF was a general reduction of plant ion uptake and transport. Our data suggest the importance to understand the nature and function of the plant magnetoreceptor for future space programs involving plant growth in environments with a reduced MF.

Simulated microgravity and the antagonistic influence of strigolactone on plant nutrient uptake in low nutrient conditions

Human-assisted space exploration will require efficient methods of food production. Large-scale farming in presence of an Earth-like atmosphere in space faces two main challenges: plant yield in microgravity and plant nutrition in extraterrestrial soils, which are likely low in nutrients compared to terrestrial farm lands. We propose a plant-fungal symbiosis (i.e. mycorrhiza) as an efficient tool to increase plant biomass production in extraterrestrial environments. We tested the mycorrhization of Solanaceae on the model plant Petunia hybrida using the arbuscular mycorrhizal fungus Rhizophagus irregularis under simulated microgravity (s0-g) conditions obtained through a 3-D random positioning machine. Our results show that s0-g negatively affects mycorrhization and plant phosphate uptake by inhibiting hyphal elongation and secondary branching. However, in low nutrient conditions, the mycorrhiza can still support plant biomass production in s0-g when colonized plants have increased SL root exudation. Alternatively, s0-g in high nutrient conditions boosts tissue-specific cell division and cell expansion and overall plant size in Petunia, which has been reported for other plants species. Finally, we show that the SL mimic molecule rac-GR24 can still induce hyphal branching in vitro under simulated microgravity. Based on these results, we propose that in nutrient limited conditions strigolactone root exudation can challenge the negative microgravity effects on mycorrhization and therefore might play an important role in increasing the efficiency of future space farming.

Testing New Concepts for Crop Cultivation in Space: Effects of Rooting Volume and Nitrogen Availability

Long term human missions to the Moon and Mars, rely on life support systems for food production and regeneration of resources. In the EU H2020 TIME SCALE-project, an advanced life support system concept was developed to facilitate plant research and technology demonstration under different gravity conditions. Ground experiments assessed irrigation systems and effects of rooting- and nutrient solution volume. The maximal allowed volume for existing International Space Station research facilities (3.4 L) was able to support cultivation of two lettuce heads for at least 24 days. A smaller rooting volume (0.6 L) increased root biomass after 24 days, but induced a 5% reduction in total biomass at day 35. Regulating effects of nitrate supply on plant water fluxes in light and dark were also investigated. At low concentrations of nitrate in the nutrient solution, both transpiration and stomatal conductance increased rapidly with increasing nitrate concentration. During day-time this increase levelled off at high concentrations, while during nigh-time there was a distinct decline at supra optimal concentrations. Plants supplied with nitrate concentrations as low as 1.25 mM did not show visible signs of nutrient stress or growth reduction. These findings hold promise for both reducing the environmental impact of terrestrial horticulture and avoiding nutrient stress in small scale closed cultivation systems for space.

An innovative in vitro device providing continuous low doses of γ-rays mimicking exposure to the space environment: A dosimetric study

Astronauts are exposed to microgravity and chronic irradiation but experimental conditions combining these two factors are difficult to reproduce on earth. We have created an experimental device able to combine chronic irradiation and altered gravity that may be used for cell cultures or plant models in a ground based facility. Irradiation was provided by thorium nitrate powder, conditioned so as to constitute a sealed source that could be placed in an incubator. Cell plates or plant seedlings could be placed in direct contact with the source or at various distances above it. Moreover, a random positioning machine (RPM) could be positioned on the source to simulate microgravity. The activity of the source was established using the Bateman formula. The spectrum of the source, calculated according to the natural decrease of radioactivity and the gamma spectrometry, showed very good adequacy. The experimental fluence was close to the theoretical fluence evaluation, attesting its uniform distribution. A Monte Carlo model of the irradiation device was processed by GATE code. Dosimetry was performed with radiophotoluminescent dosimeters exposed for one month at different locations (x and y axes) in various cell culture conditions. Using the RPM placed on the source, we reached a mean absorbed dose of gamma rays of (0.33 ± 0.17) mSv per day. In conclusion, we have elaborated an innovative device allowing chronic radiation exposure to be combined with altered gravity. Given the limited access to the International Space Station, this device could be useful to researchers interested in the field of space biology.

Genetic dissection of the Arabidopsis spaceflight transcriptome: Are some responses dispensable for the physiological adaptation of plants to spaceflight?

Experimentation on the International Space Station has reached the stage where repeated and nuanced transcriptome studies are beginning to illuminate the structural and metabolic differences between plants grown in space compared to plants on the Earth. Genes that are important in establishing the spaceflight responses are being identified, their roles in spaceflight physiological adaptation are increasingly understood, and the fact that different genotypes adapt differently is recognized. However, the basic question of whether these spaceflight responses are actually required for survival has yet to be posed, and the fundamental notion that spaceflight responses may be non-adaptive has yet to be explored. Therefore the experiments presented here were designed to ask if portions of the plant spaceflight response can be genetically removed without causing loss of spaceflight survival and without causing increased stress responses. The CARA experiment compared the spaceflight transcriptome responses in the root tips of two Arabidopsis ecotypes, Col-0 and WS, as well as that of a PhyD mutant of Col-0. When grown with the ambient light of the ISS, phyD plants displayed a significantly reduced spaceflight transcriptome response compared to Col-0, suggesting that altering the activity of a single gene can actually improve spaceflight adaptation by reducing the transcriptome cost of physiological adaptation. The WS genotype showed an even simpler spaceflight transcriptome response in the ambient light of the ISS, more broadly indicating that the plant genotype can be manipulated to reduce the cost of spaceflight adaptation, as measured by transcriptional response. These differential genotypic responses suggest that genetic manipulation could further reduce, or perhaps eliminate the metabolic cost of spaceflight adaptation. When plants were germinated and then left in the dark on the ISS, the WS genotype actually mounted a larger transcriptome response than Col-0, suggesting that the in-space light environment affects physiological adaptation, which implies that manipulating the local habitat can also substantially impact the metabolic cost of spaceflight adaptation.

Functional alterations of root meristematic cells of Arabidopsis thaliana induced by a simulated microgravity environment

Environmental gravity modulates plant growth and development, and these processes are influenced by the balance between cell proliferation and differentiation in meristems. Meristematic cells are characterized by the coordination between cell proliferation and cell growth, that is, by the accurate regulation of cell cycle progression and the optimal production of biomass for the viability of daughter cells after division. Thus, cell growth is correlated with the rate of ribosome biogenesis and protein synthesis. We investigated the effects of simulated microgravity on cellular functions of the root meristem in a sequential study. Seedlings were grown in a clinostat, a device producing simulated microgravity, for periods between 3 and 10days. In a complementary study, seedlings were grown in a Random Positioning Machine (RPM) and sampled sequentially after similar periods of growth. Under these conditions, the cell proliferation rate and the regulation of cell cycle progression showed significant alterations, accompanied by a reduction of cell growth. However, the overall size of the root meristem did not change. Analysis of cell cycle phases by flow cytometry showed changes in their proportion and duration, and the expression of the cyclin B1 gene, a marker of entry in mitosis, was decreased, indicating altered cell cycle regulation. With respect to cell growth, the rate of ribosome biogenesis was reduced under simulated microgravity, as shown by morphological and morphometric nucleolar changes and variations in the levels of the nucleolar protein nucleolin. Furthermore, in a nucleolin mutant characterized by disorganized nucleolar structure, the microgravity treatment intensified disorganization. These results show that, regardless of the simulated microgravity device used, a great disruption of meristematic competence was the first response to the environmental alteration detected at early developmental stages. However, longer periods of exposure to simulated microgravity do not produce an intensification of the cellular damages or a detectable developmental alteration in seedlings analyzed at further stages of their growth. This suggests that the secondary response to the gravity alteration is a process of adaptation, whose mechanism is still unknown, which eventually results in viable adult plants.

Clinorotation impacts root apex respiration and the ultrostructure of mitochondria

Mitochondrial respiration in plants provides energy for biosynthesis, and its balance with photosynthesis determines the rate of plant biomass accumulation. However, there are very limited data on the influence of altered gravity on the functional status of plant mitochondria. In the given paper, we presented the results of our investigations of root respiration, the mitochondrion ultrastructure, and AOX expression of pea 1-, 3- and 5-day old seedlings grown under slow horizontal clinorotation by using an inhibitor analysis, electron microscopy, and quantitative real-time RT-PCR. It was in the first time shown that enhancement of the respiration rate in root apices of pea etiolated seedlings at the 5th day of clinorotation does not connected with increasing of both alternative oxidize capacity and AOX expression. We assumed this phenomenon is provided by more intensive oxidation of respiratory substrates. At the structural level, mitochondria in cells of the distal elongation zone were the most sensitive to clinorotation that confirms the special physiological status of this zone. The performed investigation revealed an enough resistance of plant mitochondria to the influence of altered gravity that, on our opinion, is one of components providing plant adaptation to microgravity in space flight.

The Utilization of Plant Facilities on the International Space Station-The Composition, Growth, and Development of Plant Cell Walls under Microgravity Conditions

In the preparation for missions to Mars, basic knowledge of the mechanisms of growth and development of living plants under microgravity (micro-g) conditions is essential. Focus has centered on the g-effects on rigidity, including mechanisms of signal perception, transduction, and response in gravity resistance. These components of gravity resistance are linked to the evolution and acquisition of responses to various mechanical stresses. An overview is given both on the basic effect of hypergravity as well as of micro-g conditions in the cell wall changes. The review includes plant experiments in the US Space Shuttle and the effect of short space stays (8-14 days) on single cells (plant protoplasts). Regeneration of protoplasts is dependent on cortical microtubules to orient the nascent cellulose microfibrils in the cell wall. The space protoplast experiments demonstrated that the regeneration capacity of protoplasts was retarded. Two critical factors are the basis for longer space experiments: a. the effects of gravity on the molecular mechanisms for cell wall development, b. the availability of facilities and hardware for performing cell wall experiments in space and return of RNA/DNA back to the Earth. Linked to these aspects is a description of existing hardware functioning on the International Space Station.

Use of microgravity simulators for plant biological studies

Simulated microgravity and partial gravity research on Earth is highly convenient for every space biology researcher due to limitations of access to spaceflight. However, the use of ground-based facilities for microgravity simulation is far from simple. Microgravity simulation usually results in the need to consider additional environmental parameters which appear as secondary effects in the generation of altered gravity. These secondary effects may interfere with gravity alteration in the changes observed in the biological processes under study. Furthermore, ground-based facilities are also capable of generating hypergravity or fractional gravity conditions, which are worth being tested and compared with the results of microgravity exposure. Multiple technologies (2D clinorotation, random positioning machines, magnetic levitators or centrifuges), experimental hardware (proper use of containers and substrates for the seedlings or cell cultures), and experimental requirements (some life support/environmental parameters are more difficult to provide in certain facilities) should be collectively considered in defining the optimal experimental design that will allow us to anticipate, modify, or redefine the findings provided by the scarce spaceflight opportunities that have been (and will be) available.

Spaceflight exploration in plant gravitational biology

Before there was access to space, all experiments on plant tropisms were conducted upon the background of gravity. The gravity vector could be disrupted, such as with clinorotation and random positioning machines, and by manipulating incident angles of root growth with respect to gravity, such as with Darwin's plants on slanted plates, but gravity could not be removed from the experimental equation. Access to microgravity through spaceflight has opened new doors to plant research. Here we provide an overview of some of the methodologies of conducting plant research in the unique spaceflight environment.

Effects of the Extraterrestrial Environment on Plants: Recommendations for Future Space Experiments for the MELiSSA Higher Plant Compartment

Due to logistical challenges, long-term human space exploration missions require a life support system capable of regenerating all the essentials for survival. Higher plants can be utilized to provide a continuous supply of fresh food, atmosphere revitalization, and clean water for humans. Plants can adapt to extreme environments on Earth, and model plants have been shown to grow and develop through a full life cycle in microgravity. However, more knowledge about the long term effects of the extraterrestrial environment on plant growth and development is necessary. The European Space Agency (ESA) has developed the Micro-Ecological Life Support System Alternative (MELiSSA) program to develop a closed regenerative life support system, based on micro-organisms and higher plant processes, with continuous recycling of resources. In this context, a literature review to analyze the impact of the space environments on higher plants, with focus on gravity levels, magnetic fields and radiation, has been performed. This communication presents a roadmap giving directions for future scientific activities within space plant cultivation. The roadmap aims to identify the research activities required before higher plants can be included in regenerative life support systems in space.

Microgravity induces changes in microsome-associated proteins of Arabidopsis seedlings grown on board the international space station

The "GENARA A" experiment was designed to monitor global changes in the proteome of membranes of Arabidopsis thaliana seedlings subjected to microgravity on board the International Space Station (ISS). For this purpose, 12-day-old seedlings were grown either in space, in the European Modular Cultivation System (EMCS) under microgravity or on a 1 g centrifuge, or on the ground. Proteins associated to membranes were selectively extracted from microsomes and identified and quantified through LC-MS-MS using a label-free method. Among the 1484 proteins identified and quantified in the 3 conditions mentioned above, 80 membrane-associated proteins were significantly more abundant in seedlings grown under microgravity in space than under 1 g (space and ground) and 69 were less abundant. Clustering of these proteins according to their predicted function indicates that proteins associated to auxin metabolism and trafficking were depleted in the microsomal fraction in µg space conditions, whereas proteins associated to stress responses, defence and metabolism were more abundant in µg than in 1 g indicating that microgravity is perceived by plants as a stressful environment. These results clearly indicate that a global membrane proteomics approach gives a snapshot of the cell status and its signaling activity in response to microgravity and highlight the major processes affected.