Growth stimulation in inflorescences of an Arabidopsis tubulin mutant under microgravity conditions in space

Microgravity and evasion of plant innate immunity by human bacterial pathogens

Spaceflight microgravity and modeled-microgravity analogs (MMA) broadly alter gene expression and physiology in both pathogens and plants. Research elucidating plant and bacterial responses to normal gravity or microgravity has shown the involvement of both physiological and molecular mechanisms. Under true and simulated microgravity, plants display differential expression of pathogen-defense genes while human bacterial pathogens exhibit increased virulence, antibiotic resistance, stress tolerance, and reduced LD50 in animal hosts. Human bacterial pathogens including Salmonella enterica and E. coli act as cross-kingdom foodborne pathogens by evading and suppressing the innate immunity of plants for colonization of intracellular spaces. It is unknown if evasion and colonization of plants by human pathogens occurs under microgravity and if there is increased infection capability as demonstrated using animal hosts. Understanding the relationship between microgravity, plant immunity, and human pathogens could prevent potentially deadly outbreaks of foodborne disease during spaceflight. This review will summarize (1) alterations to the virulency of human pathogens under microgravity and MMA, (2) alterations to plant physiology and gene expression under microgravity and MMA, (3) suppression and evasion of plant immunity by human pathogens under normal gravity, (4) studies of plant-microbe interactions under microgravity and MMA. A conclusion suggests future study of interactions between plants and human pathogens under microgravity is beneficial to human safety, and an investment in humanity's long and short-term space travel goals.

Comprehensive analyses of plant hormones in etiolated pea and maize seedlings grown under microgravity conditions in space: Relevance to the International Space Station experiment "Auxin Transport"

Functional relationships between endogenous levels of plant hormones in the growth and development of shoots in etiolated Alaska pea and etiolated Golden Cross Bantam maize seedlings under different gravities were investigated in the "Auxin Transport" experiment aboard the International Space Station (ISS). Comprehensive analyses of 31 species of plant hormones of pea and maize seedlings grown under microgravity (μg) in space and 1 g conditions were conducted. Principal component analysis (PCA) and a multiple regression analysis with the dataset from the plant hormone analysis of the etiolated pea seedlings grown under μg and 1 g conditions in the presence and absence of 2,3,5-triiodobenzoic acid (TIBA) revealed endogenous levels of auxin correlated positively with bending and length of epicotyls. Endogenous cytokinins correlated negatively with them. These results suggest an interaction of auxin and cytokinins in automorphogenesis and growth inhibition of etiolated Alaska pea epicotyls grown under μg conditions in space. Less polar auxin transport with reduced endogenous levels of auxin increased endogenous levels of cytokinins, resulting in changing the growth direction of epicotyls and inhibiting growth. On the other hand, almost no close relationship between endogenous plant hormone levels and growth and development in etiolated maize seedlings grown was observed under μg conditions in space, as per Schulze et al. (1992). However, endogenous levels of IAA in the seedlings grown under μg conditions in space were significantly higher than those grown on Earth, similar to the cases of polar auxin transport already reported.

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.

The genomic basis of the plant island syndrome in Darwin's giant daisies

The repeated, rapid and often pronounced patterns of evolutionary divergence observed in insular plants, or the ‘plant island syndrome’, include changes in leaf phenotypes, growth, as well as the acquisition of a perennial lifestyle. Here, we sequence and describe the genome of the critically endangered, Galápagos-endemic species Scalesia atractyloides Arnot., obtaining a chromosome-resolved, 3.2-Gbp assembly containing 43,093 candidate gene models. Using a combination of fossil transposable elements, k-mer spectra analyses and orthologue assignment, we identify the two ancestral genomes, and date their divergence and the polyploidization event, concluding that the ancestor of all extant Scalesia species was an allotetraploid. There are a comparable number of genes and transposable elements across the two subgenomes, and while their synteny has been mostly conserved, we find multiple inversions that may have facilitated adaptation. We identify clear signatures of selection across genes associated with vascular development, growth, adaptation to salinity and flowering time, thus finding compelling evidence for a genomic basis of the island syndrome in one of Darwin’s giant daisies.

Many island plant species share a syndrome of characteristic phenotype and life history. Cerca et al. find the genomic basis of the plant island syndrome in one of Darwin’s giant daisies, while separating ancestral genomes in a chromosome-resolved polyploid assembly.

Transcriptomic Analysis of the Interaction Between FLOWERING LOCUS T Induction and Photoperiodic Signaling in Response to Spaceflight

Spaceflight has an impact on the growth and development of higher plants at both the vegetative stage and reproductive stage. A great deal of information has been available on the vegetative stage in space, but relatively little is known about the influence of spaceflight on plants at the reproductive stage. In this study, we constructed transgenic Arabidopsis thaliana plants expressing the flowering control gene, FLOWERING LOCUS T (FT), together with the green fluorescent protein gene (GFP) under control of a heat shock-inducible promoter (HSP17.4), by which we induced FT expression inflight through remote controlling heat shock (HS) treatment. Inflight photography data showed that induction of FT expression in transgenic plants in space under non-inductive short-day conditions could promote flowering and reduce the length of the inflorescence stem in comparison with that of wild-type plants under the same conditions. Whole-genome microarray analysis of gene expression changes in leaves of wild-type and these transgenic plants grown under the long-day and short-day photoperiod conditions in space indicated that the function of the photoperiod-related spaceflight responsive genes is mainly involved in protein synthesis and post-translation protein modulation, notably protein phosphorylation. In addition, changes of the circadian component of gene expression in response to spaceflight under different photoperiods indicated that roles of the circadian oscillator could act as integrators of spaceflight response and photoperiodic signals in Arabidopsis plants grown in space.

Suppression of Cortical Microtubule Reorientation and Stimulation of Cell Elongation in Arabidopsis Hypocotyls under Microgravity Conditions in Space

How microgravity in space influences plant cell growth is an important issue for plant cell biology as well as space biology. We investigated the role of cortical microtubules in the stimulation of elongation growth in Arabidopsis (Arabidopsis thaliana) hypocotyls under microgravity conditions with the Resist Tubule space experiment. The epidermal cells in the lower half of the hypocotyls of wild-type Columbia were longer in microgravity than at on-orbit 1 g, which precipitated an increase in the entire hypocotyl length. In the apical region, cortical microtubules adjacent to the outer tangential wall were predominantly transverse to the long axis of the cell, whereas longitudinal microtubules were predominant in the basal region. In the 9th to 12th epidermal cells (1 to 3 mm) from the tip, where the modification of microtubule orientation from transverse to longitudinal directions (reorientation) occurred, cells with transverse microtubules increased, whereas those with longitudinal microtubules decreased in microgravity, and the average angle with respect to the transverse cell axis decreased, indicating that the reorientation was suppressed in microgravity. The expression of tubulin genes was suppressed in microgravity. These results suggest that under microgravity conditions, the expression of genes related to microtubule formation was downregulated, which may cause the suppression of microtubule reorientation from transverse to longitudinal directions, thereby stimulating cell elongation in Arabidopsis hypocotyls.

How plants grow under gravity conditions besides 1 g: perspectives from hypergravity and space experiments that employ bryophytes as a model organism

Plants have evolved and grown under the selection pressure of gravitational force at 1 g on Earth. In response to this selection pressure, plants have acquired gravitropism to sense gravity and change their growth direction. In addition, plants also adjust their morphogenesis in response to different gravitational forces in a phenomenon known as gravity resistance. However, the gravity resistance phenomenon in plants is poorly understood due to the prevalence of 1 g gravitational force on Earth: not only it is difficult to culture plants at gravity > 1 g(hypergravity) for a long period of time but it is also impossible to create a < 1 genvironment (μg, micro g) on Earth without specialized facilities. Despite these technical challenges, it is important to understand how plants grow in different gravity conditions in order to understand land plant adaptation to the 1 g environment or for outer space exploration. To address this, we have developed a centrifugal device for a prolonged duration of plant culture in hypergravity conditions, and a project to grow plants under the μg environment in the International Space Station is also underway. Our plant material of choice is Physcomitrium (Physcomitrella) patens, one of the pioneer plants on land and a model bryophyte often used in plant biology. In this review, we summarize our latest findings regarding P. patens growth response to hypergravity, with reference to our on-going "Space moss" project. In our ground-based hypergravity experiments, we analyzed the morphological and physiological changes and found unexpected increments of chloroplast size and photosynthesis rate, which might underlie the enhancement of growth and increase in the number of gametophores and rhizoids. We further discussed our approaches at the cellular level and compare the gravity resistance in mosses and that in angiosperms. Finally, we highlight the advantages and perspectives from the space experiments and conclude that research with bryophytes is beneficial to comprehensively and precisely understand gravitational responses in plants.

Metabolomics Analysis in Different Development Stages on SP0 Generation of Rice Seeds After Spaceflight

Spaceflight is a special abiotic stress condition. In recent years, it has been confirmed that the spaceflight caused the stress response of rice seeds, and the protein level, transcription level, and methylation level will change during the planting process after returning to the ground. However, the changes at the metabolome level are not very clear. In this study, two kinds of rice seeds, Dongnong423 (DN3) and Dongnong416 (DN6), were carried on the ShiJian-10 retractable satellite (SJ-10) for 12.5 days in orbit, returned to the ground and planted in the field until the three-leaf (TLP) and tillering stage (TS). The results of antioxidant enzyme activity, soluble sugar, and electron leakage rate revealed that the spaceflight caused the stress response of rice. The TLP and TS of DN3 identified 110 and 57 different metabolites, respectively, while the TLP and TS of DN6 identified 104 and 74 different metabolites, respectively. These metabolites included amino acids, sugars, fatty acids, organic acids and secondary metabolites. We used qRT-PCR technology to explore the changes of enzyme genes in the tricarboxylic acid cycle (TCA) and amino acid metabolism pathway. Combined with the results of metabolomics, we determined that during the TLP, the TCA cycle rate of DN3 was inhibited and amino acid metabolism was activated, while the TCA cycle rate of DN6 was activated and amino acid metabolism was inhibited. In TS, the TCA cycle rate of DN3 was inhibited, and amino acid metabolism was not significantly changed, while the TCA cycle rate of DN6 was activated and amino acid metabolism was inhibited. These results suggested that the response mechanisms of the two different rice strains to spaceflight stress are different, and these differences may be reflected in energy consumption and compound biosynthesis of rice in different growth and development stages. This study provided new insights for further exploring the effects of spaceflight.

Biology and crop production in Space environments: Challenges and opportunities

Long-term manned space-exploration missions and the permanence of human colonies on orbital stations or planetary habitats will require the regeneration of resources onboard or in-situ. Bioregenerative Life Support Systems (BLSSs) are artificial environments where different compartments, involving both living organisms and physical-chemical processes, are integrated to achieve a safe, self-regulating, and chemically balanced Earth-like environment to support human life. Higher plants are key elements of such systems and Space greenhouses represent the producers' compartment. Growing plants in Space requires the knowledge of their growth responses not only to all environmental factors acting on Earth, but also to specific Space constraints such as altered gravity, ionizing radiations and confined volume. Moreover, cultivation techniques need to be adjusted considering such limitations. The type and intensity of environmental factors to be taken into account depend on the mission scenarios. Here, we summarize constraints and opportunities of cultivating higher plants in Space to regenerate resources and produce fresh food onboard. Both biological and agro-technological issues are considered briefly going through experiments both ground-based on Earth and in Space.

Vegetative and reproductive growth of Arabidopsis under microgravity conditions in space

We have performed a seed-to-seed experiment in the cell biology experiment facility (CBEF) installed in the Kibo (Japanese Experiment Module) in the International Space Station. The CBEF has a 1 × g compartment on a centrifuge and a microgravity compartment, to investigate the effects of microgravity on the vegetative and reproductive growth of Arabidopsis thaliana (L.) Heynh. Seeds germinated irrespective of gravitational conditions after water supply on board. Thereafter, seedlings developed rosette leaves. The time of bolting was slightly earlier under microgravity than under space 1 × g. Microgravity enhanced the growth rate of peduncles as compared with space 1 × g or ground control. Plants developed flowers, siliques and seeds, completing their entire life cycle during 62-days cultivation. Although the flowering time was not significantly affected under microgravity, the number of flowers in a bolted plant significantly increased under microgravity as compared with space 1 × g or ground control. Microscopic analysis of reproductive organs revealed that the longitudinal length of anthers was significantly shorter under microgravity when compared with space 1 × g, while the length of pistils and filaments was not influenced by the gravitational conditions. Seed mass significantly increased under microgravity when compared with space 1 × g. In addition, seeds produced in space were found not to germinate on the ground. These results indicate that microgravity significantly influenced the reproductive development of Arabidopsis plants even though Earth's gravitational environment is not absolutely necessary for them to complete their life cycle.

Microgravity Affects the Level of Matrix Polysaccharide 1,3:1,4-β-Glucans in Cell Walls of Rice Shoots by Increasing the Expression Level of a Gene Involved in Their Breakdown

The plant cell wall provides each cell with structural support and mechanical strength, and thus, it plays an important role in supporting the plant body against the gravitational force. We investigated the effects of microgravity on the composition of cell wall polysaccharides and on the expression levels of genes involved in cell wall metabolism using rice shoots cultivated under artificial 1 g and microgravity conditions on the International Space Station. The bulk amount of the cell wall obtained from microgravity-grown shoots was comparable with that from 1 g-grown shoots. However, the analysis of sugar constituents of matrix polysaccharides showed that microgravity specifically reduced the amount of glucose (Glc)-containing polysaccharides such as 1,3:1,4-β-glucans, in shoot cell walls. The expression level of a gene for endo-1,3:1,4-β-glucanase, which hydrolyzes 1,3:1,4-β-glucans, largely increased under microgravity conditions. However, the expression levels of genes involved in the biosynthesis of 1,3:1,4-β-glucans were almost the same under both gravity conditions. On the contrary, microgravity scarcely affected the level and the metabolism of arabinoxylans. These results suggest that a microgravity environment promotes the breakdown of 1,3:1,4-β-glucans, which, in turn, causes the reduced level of these polysaccharides in growing rice shoots. Changes in 1,3:1,4-β-glucan level may be involved in the modification of mechanical properties of cell walls under microgravity conditions in space.

Polar auxin transport is essential to maintain growth and development of etiolated pea and maize seedlings grown under 1 g conditions: Relevance to the international space station experiment

We conducted "Auxin Transport" space experiments in 2016 and 2017 in the Japanese Experiment Module (JEM) on the International Space Station (ISS), with the principal objective being integrated analyses of the growth and development of etiolated pea (Pisum sativum L. cv Alaska) and maize (Zea mays L. cv Golden Cross Bantam) seedlings under true microgravity conditions in space relative to auxin dynamics. Etiolated pea seedlings grown under microgravity conditions in space for 3 days showed automorphogenesis. Epicotyls and roots bent ca. 45° and 20° toward the direction away from the cotyledons, respectively, whereas those grown under artificial 1 g conditions produced by a centrifuge in the Cell Biology Experimental Facility (CBEF) in space showed negative and positive gravitropic response in epicotyls and in roots, respectively. On the other hand, the coleoptiles of 4-day-old etiolated maize seedlings grew almost straight, but the mesocotyls curved and grew toward a random direction under microgravity conditions in space. In contrast, the coleoptiles and mesocotyls of etiolated maize seedlings grown under 1 g conditions on Earth were almost straight and grew upward or toward the direction against the gravity vector. The polar auxin transport activity in etiolated pea epicotyls and in maize shoots was significantly inhibited and enhanced, respectively, under microgravity conditions in space as compared with artificial 1 g conditions in space or 1 g conditions on Earth. An inhibitor of polar auxin transport, 2,3,5-triiodobenzoic acid (TIBA) substantially affected the growth direction and polar auxin transport activity in etiolated pea seedlings grown under both artificial 1 g and microgravity conditions in space. These results strongly suggest that adequate polar auxin transport is essential for gravitropic response in plants. Possible mechanisms enhancing polar auxin transport in etiolated maize seedlings grown under microgravity conditions in space are also proposed.

Molecular Cytogenetics of Pisum sativum L. Grown under Spaceflight-Related Stress

The ontogenesis and reproduction of plants cultivated aboard a spacecraft occur inside the unique closed ecological system wherein plants are subjected to serious abiotic stresses. For the first time, a comparative molecular cytogenetic analysis of Pisum sativum L. (Fabaceae) grown on board the RS ISS during the Expedition-14 and Expedition-16 and also plants of their succeeding (F1 and F2) generations cultivated on Earth was performed in order to reveal possible structural chromosome changes in the pea genome. The karyotypes of these plants were studied by multicolour fluorescence in situ hybridization (FISH) with five different repeated DNA sequences (45S rDNA, 5S rDNA, PisTR-B/1, microsatellite motifs (AG)12, and (GAA)9) as probes. A chromosome aberration was revealed in one F1 plant. Significant changes in distribution of the examined repeated DNAs in karyotypes of the "space grown" pea plants as well as in F1 and F2 plants cultivated on Earth were not observed if compared with control plants. Additional oligo-(GAA)9 sites were detected on chromosomes 6 and 7 in karyotypes of F1 and F2 plants. The detected changes might be related to intraspecific genomic polymorphism or plant cell adaptive responses to spaceflight-related stress factors. Our findings suggest that, despite gradual total trace contamination of the atmosphere on board the ISS associated with the extension of the space station operating life, exposure to the space environment did not induce serious chromosome reorganizations in genomes of the "space grown" pea plants and generations of these plants cultivated on Earth.

Single-base resolution methylome analysis shows epigenetic changes in Arabidopsis seedlings exposed to microgravity spaceflight conditions on board the SJ-10 recoverable satellite

DNA methylation is a very important epigenetic modification that participates in many biological functions. Although many studies of DNA methylation have been reported in various plant species, few studies have assessed the global DNA methylation pattern in plants challenged by exposure to microgravity conditions. In this report, we mapped the Arabidopsis genome methylation pattern changes associated with microgravity conditions on board the Chinese recoverable scientific satellite SJ-10 at single-base resolution. Interestingly, we found epigenetic differences in Arabidopsis seedlings exposed to microgravity in that the Arabidopsis genome exhibits lower methylation levels in the CHG, CHH, and CpG contexts under microgravity conditions. Microgravity stimulation was related to altered methylation of a number of genes, including DNA methylation-associated genes, hormone signaling related genes, cell-wall modification genes and transposable elements (TEs). Relatively unstable DNA methylation of TEs was responsible for the induction of active transposons. These observations suggest that DNA demethylation within TEs may affect the transcription of transposons in response to microgravity conditions. In summary, the results of this investigation are beneficial for understanding the mechanism of plant adaptation to microgravity and improve strategies to allow plants to adapt to space.

Spaceflight alters the pattern of chemical tags that adorn DNA in plant seedlings. Weiming Cai and colleagues from the Shanghai Institute of Plant Physiology and Ecology, China, profiled the genome-wide epigenetic patterns of Arabidopsis thaliana seedlings that spent 60 h in the microgravity of low Earth orbit aboard the Shijian-10 recoverable satellite. They analyzed the distribution of methyl tags across the genome — an epigenetic mark that affects gene expression levels — and found that seedlings exposed to microgravity had lower methylation on average than control plants grown on the ground, although certain genes related to methylation, transcription factors and hormones tended to be more methylated. Epigenetic differences were also observed among genes involved in cell-wall modification and in transposable elements. The findings could help inform the design of plants optimized for growth in space.

Growth and cortical microtubule dynamics in shoot organs under microgravity and hypergravity conditions

The body shape of plants varied in proportion to the logarithm of the magnitude of gravity in the range from microgravity to hypergravity to resist the gravitational force. Here we discuss the roles of cortical microtubule and 65 kDa microtubule-associated protein-1 (MAP65-1) in gravity-induced modification of growth anisotropy. Microgravity stimulated elongation growth and suppressed lateral expansion in shoot organs, such as hypocotyls and epicotyls. On the other hand, hypergravity inhibited elongation growth and promoted lateral expansion in shoot organs. The number of cells with transverse microtubules was increased by microgravity, but decreased by hypergravity. Furthermore, the levels of MAP65-1, which is involved in the maintenance of the transverse microtubule orientation, were increased by microgravity, but decreased by hypergravity. Therefore, the regulation of orientation of cortical microtubules via changes in the levels of MAP65-1 may contribute to the modification of the body shape of plants to resist the gravitational force.

Modification of growth anisotropy and cortical microtubule dynamics in Arabidopsis hypocotyls grown under microgravity conditions in space

We carried out a space experiment, denoted as Aniso Tubule, to examine the effects of microgravity on the growth anisotropy and cortical microtubule dynamics in Arabidopsis hypocotyls, using lines in which microtubules are visualized by labeling tubulin or microtubule-associated proteins (MAPs) with green fluorescent protein (GFP). In all lines, GFP-tubulin6 (TUB6)-, basic proline-rich protein1 (BPP1)-GFP- and spira1-like3 (SP1L3)-GFP-expressing using a constitutive promoter, and spiral2 (SPR2)-GFP- and GFP-65 kDa MAP-1 (MAP65-1)-expressing using a native promoter, the length of hypocotyls grown under microgravity conditions in space was longer than that grown at 1 g conditions on the ground. In contrast, the diameter of hypocotyls grown under microgravity conditions was smaller than that of the hypocotyls grown at 1 g. The percentage of cells with transverse microtubules was increased under microgravity conditions, irrespective of the lines. Also, the average angle of the microtubules with respect to the transverse cell axis was decreased in hypocotyls grown under microgravity conditions. When GFP fluorescence was quantified in hypocotyls of GFP-MAP65-1 and SPR2-GFP lines, microgravity increased the levels of MAP65-1, which appears to be involved in the maintenance of transverse microtubule orientation. However, the levels of SPR2 under microgravity conditions were comparable to those at 1 g. These results suggest that the microgravity-induced increase in the levels of MAP65-1 is involved in increase in the transverse microtubules, which may lead to modification of growth anisotropy, thereby developing longer and thinner hypocotyls under microgravity conditions in space.

Persistence of plant hormone levels in rice shoots grown under microgravity conditions in space: its relationship to maintenance of shoot growth

We investigated the effects of microgravity environment on growth and plant hormone levels in dark-grown rice shoots cultivated in artificial 1 g and microgravity conditions on the International Space Station (ISS). Growth of microgravity-grown shoots was comparable to that of 1 g-grown shoots. Endogenous levels of indole-3-acetic acid (IAA) in shoots remained constant, while those of abscisic acid (ABA), jasmonic acid (JA), cytokinins (CKs) and gibberellins (GAs) decreased during the cultivation period under both conditions. The levels of auxin, ABA, JA, CKs and GAs in rice shoots grown under microgravity conditions were comparable to those under 1 g conditions. These results suggest microgravity environment in space had minimal impact on levels of these plant hormones in rice shoots, which may be the cause of the persistence of normal growth of shoots under microgravity conditions. Concerning ethylene, the expression level of a gene for 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, the key enzyme in ethylene biosynthesis, was reduced under microgravity conditions, suggesting that microgravity may affect the ethylene production. Therefore, ethylene production may be responsive to alterations of the gravitational force.

Genome-Wide Association Study Dissecting the Genetic Architecture Underlying the Branch Angle Trait in Rapeseed (Brassica napus L.)

The rapeseed branch angle is an important morphological trait because an adequate branch angle enables more efficient light capture under high planting densities. Here, we report that the average angle of the five top branches provides a reliable representation of the average angle of all branches. Statistical analyses revealed a significantly positive correlation between the branch angle and multiple plant-type and yield-related traits. The 60 K Brassica Infinium^® single nucleotide polymorphism (SNP) array was utilized to genotype an association panel with 520 diverse accessions. A genome-wide association study was performed to determine the genetic architecture of branch angle, and 56 loci were identified as being significantly associated with the branch angle trait via three models, including a robust, novel, nonparametric Anderson-Darling (A-D) test. Moreover, these loci explained 51.1% of the phenotypic variation when a simple additive model was applied. Within the linkage disequilibrium (LD) decay ranges of 53 loci, we observed plausible candidates orthologous to documented Arabidopsis genes, such as LAZY1, SGR2, SGR4, SGR8, SGR9, PIN3, PIN7, CRK5, TIR1, and APD7. These results provide insight into the genetic basis of the branch angle trait in rapeseed and might facilitate marker-based breeding for improvements in plant architecture.

Hypergravity experiments to evaluate gravity resistance mechanisms in plants

Hypergravity generated by centrifugal acceleration is the only practical method to modify the magnitude of gravitational acceleration for a sufficient duration on Earth and has been used to analyze the nature and mechanism of graviresponse, particularly gravity resistance, in plants. Plant organs are generally resistant to gravitational acceleration. Hypergravity produced from centrifugation speeds in the range of 10-300 × g, which is easily produced by a benchtop centrifuge, is often used during plant experiments. After centrifugation, the plant material is fixed with suitable fixatives in appropriate sample storage containers such as the Chemical Fixation Bag. The material is then analyzed with a variety of methods, depending on the purpose of the experiment. Plant material fixed with the RNAlater(®) solution can be sequentially used for determining the mechanical properties of the cell wall, for RNA extraction (which is necessary for gene expression analysis), for estimating the enzyme activity of the cell wall proteins, and for determining the levels as well as the compositions of cell wall polysaccharides. The plant material can also be used directly for microscopic observation of cellular components such as cortical microtubules.

Role of the plant cell wall in gravity resistance

Gravity resistance, mechanical resistance to the gravitational force, is a principal graviresponse in plants, comparable to gravitropism. The cell wall is responsible for the final step of gravity resistance. The gravity signal increases the rigidity of the cell wall via the accumulation of its constituents, polymerization of certain matrix polysaccharides due to the suppression of breakdown, stimulation of cross-link formation, and modifications to the wall environment, in a wide range of situations from microgravity in space to hypergravity. Plants thus develop a tough body to resist the gravitational force via an increase in cell wall rigidity and the modification of growth anisotropy. The development of gravity resistance mechanisms has played an important role in the acquisition of responses to various mechanical stresses and the evolution of land plants.

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.

Plant Growth and Morphogenesis under Different Gravity Conditions: Relevance to Plant Life in Space

The growth and morphogenesis of plants are entirely dependent on the gravitational acceleration of earth. Under microgravity conditions in space, these processes are greatly modified. Recent space experiments, in combination with ground-based studies, have shown that elongation growth is stimulated and lateral expansion suppressed in various shoot organs and roots under microgravity conditions. Plant organs also show automorphogenesis in space, which consists of altered growth direction and spontaneous curvature in the dorsiventral (back and front) directions. Changes in cell wall properties are responsible for these modifications of growth and morphogenesis under microgravity conditions. Plants live in space with interesting new sizes and forms.

Plant biology in space: recent accomplishments and recommendations for future research

Gravity has shaped the evolution of life since its origin. However, experiments in the absence of this overriding force, necessary to precisely analyse its role, e.g. for growth, development, and orientation of plants and single cells, only became possible with the advent of spaceflight. Consequently, this research has been supported especially by space agencies around the world for decades, mainly for two reasons: first, to enable fundamental research on gravity perception and transduction during growth and development of plants; and second, to successfully grow plants under microgravity conditions with the goal of establishing a bioregenerative life support system providing oxygen and food for astronauts in long-term exploratory missions. For the second time, the International Space Life Sciences Working Group (ISLSWG), comprised of space agencies with substantial life sciences programmes in the world, organised a workshop on plant biology research in space. The present contribution summarises the outcome of this workshop. In the first part, an analysis is undertaken, if and how the recommendations of the first workshop held in Bad Honnef, Germany, in 1996 have been implemented. A chapter summarising major scientific breakthroughs obtained in the last 15 years from plant research in space concludes this first part. In the second part, recommendations for future research in plant biology in space are put together that have been elaborated in the various discussion sessions during the workshop, as well as provided in written statements from the session chairs. The present paper clearly shows that plant biology in space has contributed significantly to progress in plant gravity perception, transduction and responses - processes also relevant for general plant biology, including agricultural aspects. In addition, the interplay between light and gravity effects has increasingly received attention. It also became evident that plants will play a major role as components of bioregenerative life support and energy systems that are necessary to complement physico-chemical systems in upcoming long-term exploratory missions. In order to achieve major progress in the future, however, standardised experimental conditions and more advanced analytical tools, such as state-of-the-art onboard analysis, are required.