Seeds of a tropical tree species from Brazil, Astronium fraxinifolium, or zebrawood, were germinated, for the first time in microgravity, aboard the International Space Station for nine days. Following three days of subsequent growth under normal terrestrial gravitational conditions, greater root length and numbers of secondary roots was observed in the microgravity-treated seedlings compared to terrestrially germinated controls. Suppression subtractive hybridization of cDNA and EST analysis were used to detect differential gene expression in the microgravity-treated seedlings in comparison to those initially grown in normal gravity (forward subtraction). Despite their return to, and growth in normal gravity, the subtracted library derived from microgravity-treated seedlings was enriched in known microgravity stress-related ESTs, corresponding to large and small heat shock proteins, 14-3-3-like protein, polyubiquitin, and proteins involved in glutathione metabolism. In contrast, the reverse-subtracted library contained a comparatively greater variety of general metabolism-related ESTs, but was also enriched for peroxidase, possibly indicating the suppression of this protein in the microgravity-treated seedlings. Following continued growth for 30 days, higher concentrations of total chlorophyll were detected in the microgravity-exposed seedlings.
Research Containing: plant growth
Growth and Cell Wall Properties in Hypocotyls of Arabidopsis tua6 Mutant under Microgravity Conditions in Space
Seedlings of Arabidopsis α-tubulin 6 mutant (tua6) were cultivated under microgravity conditions in the European Modular Cultivation System on the International Space Station, and growth and cell wall properties of their hypocotyls were analyzed (the Resist Wall experiment). Seeds of tua6 mutant were shown to germinate and grow normally until the seedling stage under microgravity conditions, as at 1 G on the ground. The seedlings were naturally air-dried in orbit, which were then recovered and transported to earth. When the mechanical properties of the cell wall of rehydrated hypocotyls were examined with a tensile tester, the hypocotyls showed typical stress-strain and stress-relaxation curves, as normally fixed or frozen materials. Also, no prominent differences were detected in the extensibility or the stress-relaxation parameters of the cell wall between space-grown hypocotyls and ground controls, suggesting that tua6 hypocotyls formed the regular cell wall architecture under microgravity conditions. The results and lessons learned from the Resist Wall experiment are expected to provide the basis for the following space experiments to clarify the mechanism of gravity resistance in plants.
Gravity resistance is one of two principal gravity responses in plants, comparable to gravitropism. In the final step of gravity resistance, plants increase the rigidity of their cell walls via modifications to the metabolism. Various constituents of the plasma membrane and the cytoskeleton play an important role in sustaining functions of the cell wall in gravity resistance. Mechanoreceptors located on the plasma membrane are involved in the perception of gravity signal. The perceived signal may be, at least partly, transformed and transduced via membrane sterol rafts, depending on its magnitude. Cellulose synthases and proton pumps are responsible for modifications to the cell wall metabolism and the apoplastic environment, respectively. On the other hand, the reorientation of cortical microtubules contributes to modification of growth anisotropy, which is related to gravity resistance. Also, microtubule-associated proteins are important in maintenance of the structure and induction of the reorientation of cortical microtubules. Gravity resistance in plants is thus mediated by the structural continuum or physiological continuity of cortical microtubules-plasma membrane-cell wall.
Providing Optimal Root-Zone Fluid Fluxes: Effects of Hysteresis on Capillary-Dominated Water Distributions in Reduced Gravity
Management of water, air and nutrients in coarse-textured porous plant-growth substrates relies not only on the relative amounts of fluids but also on their distribution within porous media. Integration of plants in future life support systems for space exploration raises the question of how fluid distributions in porous plant-growth substrates are altered under reduced gravitational conditions. Central to addressing this issue is the behavior of the water retention characteristic (WRC). WRC encapsulates fluid-porous medium interactions and is key for control of water supply to plants. The hysteretic nature of WRC implies non-homogenous water distributions between its primary draining and wetting curves. During dynamic drainage and wetting cycles, considerable water content gradients develop at separations of only a few pore lengths. In the absence of a gravity force, these non-homogeneous distributions give rise to considerably different fluid distribution relative to terrestrial observations in the same porous media. Consequently, such altered fluid distributions may reduce connectivity and increase tortuosity of gaseous pathways sustaining diffusive gas transport compared to terrestrial conditions for similar air-filled contents. Such changes in fluid pathways may induce limitations to exchange of respiratory gases for plant roots, and highlight a range of other potential changes in microgravity behavior of capillary dominated processes important for distribution and transport of fluids in porous media.
Life in spaceflight demonstrates remarkable acclimation processes within the specialized habitats of vehicles subjected to the myriad of unique environmental issues associated with orbital trajectories. To examine the response processes that occur in plants in space, leaves and roots from Arabidopsis (Arabidopsis thaliana) seedlings from three GFP reporter lines that were grown from seed for 12 days on the International Space Station and preserved on orbit in RNAlater were returned to Earth and analyzed by using iTRAQ broad-scale proteomics procedures. Using stringent criteria, we identified over 1500 proteins, which included 1167 leaf proteins and 1150 root proteins we were able to accurately quantify. Quantification revealed 256 leaf proteins and 358 root proteins that showed statistically significant differential abundance in the spaceflight samples compared to ground controls, with few proteins differentially regulated in common between leaves and roots. This indicates that there are measurable proteomics responses to spaceflight and that the responses are organ-specific. These proteomics data were compared with transcriptome data from similar spaceflight samples, showing that there is a positive but limited relationship between transcriptome and proteome regulation of the overall spaceflight responses of plants. These results are discussed in terms of emergence understanding of plant responses to spaceflight particularly with regard to cell wall remodeling, as well as in the context of deriving multiple omics data sets from a single on-orbit preservation and operations approach.
MAIN CONCLUSION: Cyclic leaf ascent and descent occur in synchrony and phase congruence with the lunisolar tidal force under a broad range of conditions. Digitized records of the vertical leaf movements of Arabidopsis thaliana were collected under space flight conditions in the International Space Station (ISS). Oscillations of leaf movements with periods of 45 and 90 min were found under light-adapted conditions, whereas in darkness, the periods were 45, 90, and 135 min. To demonstrate the close relationship between these oscillations and cyclical variations of the lunisolar gravitational force, we estimated the oscillations of the in-orbit lunisolar tide as they apply to the ISS, with the aid of the Etide software application. In general, in-orbit lunisolar gravitational profiles exhibited a periodicity of 45 min. Alignment of these in-orbit oscillations with the oscillations of Arabidopsis leaf movement revealed high degrees of synchrony and a congruence of phase. These data corroborate previous results which suggested a correlative relationship and a possible causal link between leaf movement rhythms obtained on ground and the rhythmic variation of the lunisolar tidal force.
Is it possible to produce large quantities of high quality vegetables on a space station, about 320 kilometers away from earth? To answer this question, three twelfth-grade students from the agricultural program at the Edith Stein School in Ravensburg, Germany, set up a research project to examine how crops can be cultivated in the microgravity of space. The research trial is scheduled for take-off to the International Space Station (ISS) by the end of 2015, and will have the scientific and financial sponsorship of BASF. “We are excited about this project and about working with forward-thinking young people who strive for groundbreaking ideas and innovation. With our 100 years of experience in agriculture, it has been a thrilling challenge to investigate what could come next and how to achieve the ultimate goal of growing and reproducing plants on a space station,” said Dr. Harald Rang, Senior Vice President Research & Development, BASF Crop Protection.
The present suite of advanced space plant cultivation facilities require a significant level of resources to launch and maintain in flight. The facilities are designed to accommodate a broad size range of plant species and are, therefore, not configured to support the specific growth requirements of small plant species such as Arabidopsis thaliana at maximum efficiency with respect to mass and power. The facilities are equally not configured to support automated plant harvesting or tissue processing procedures, but rely on crew intervention and time. The recent reorganization of both spaceflight opportunities and allocation of limited in-flight resources demand that experiments be conducted with optimal efficiency. The emergence of A. thaliana as a dominant space flight model organism utilized in research on vegetative and reproductive phase biology provides strong justification for the establishment of a dedicated cultivation system for this species. This paper presents work on the design of a small plant cultivation facility directed at supporting research on the vegetative growth phase of A. thaliana . The design of the facility is based on the use of existing space flight hardware, and configured to support the fully automated germination of seed, cultivation of plants, and final termination of plant growth by chemical fixation and preservation of plant tissue.
Gravity control of growth form in Brassica rapa and Arabidopsis thaliana (Brassicaceae): Consequences for secondary metabolism
How gravity influences the growth form and flavor components of plants is of interest to the space program because plants could be used for food and life support during prolonged missions away from the planet, where that constant feature of Earth's environment does not prevail. We used plant growth hardware from prior experiments on the space shuttle to grow Brassica rapa and Arabidopsis thaliana plants during 16-d or 11-d hypergravity treatments on large-diameter centrifuge rotors. Both species showed radical changes in growth form, becoming more prostrate with increasing g-loads (2-g and 4-g). In Brassica, height decreased and stems thickened in a linear relationship with increasing g-load. Glucosinolates, secondary compounds that contribute flavor to Brassica, decreased by 140% over the range of micro to 4-g, while the structural secondary compound, lignin, remained constant at approximately 15% (w/w) cell wall dry mass. Stem thickening at 4-g was associated with substantial increases in cell size (47%, 226%, and 33% for pith, cortex, and vascular tissue), rather than any change in cell number. The results, which demonstrate the profound effect of gravity on plant growth form and secondary metabolism, are discussed in the context of similar thigmostresses such as touch and wind.