Plants show two principal responses to gravity: one is gravimorphogenesis and the other is gravity resistance. Gravitropism is a typical response of gravimorphogenesis, which enables the plant to orient their photosynthetic leaves to sunlight and to develop a root system for anchoring and absorbing water and minerals. Gravity resistance is a response that enables the plant to resist the gravitational force by developing a tough body. Plants increase the rigidity of their body locally in gravimorphogenesis and entirely in gravity resistance. The cell wall provides the protoplasts with the structural rigidity and is the major source of mechanical strength of a plant body. Thus, the cell wall plays an important role in both gravity responses, as do the bones and muscles in an animal body. Plant cell walls consist of fibrillar cellulose, a variety of matrix polysaccharides, structural proteins, and phenolic substances such as lignin. These cell wall constituents are differently involved in determining the rigidity of the cell wall in gravity responses, depending on the level, structure, and interactions with other constituents. Functions of the cell wall in gravity responses are sustained by various constituents of the plasma membrane including arabinogalactan-proteins, and the cytoskeleton such as microtubules. Also, a great number of cell wall-related genes are involved in gravity responses, via regulation of the metabolism of cell wall constituents. Hypergravity produced by centrifugation has been used to clarify the mechanism of gravity responses, in particular that of gravity resistance. The study with hypergravity has unveiled the fundamental roles of the cell wall in gravity responses. However, it is uncertain whether the knowledge obtained is applicable to responses of plants to 1 G gravity, as to those to hypergravity. To clarify this point, space experiments using plant materials have been carried out or are now underway (Table 1). Out of these experiments, the Cell Wall and the Resist Wall experiments have been just conducted in the European Modular Cultivation System of the Columbus Module on the International Space Station (ISS). The outline and scientific significance of the Cell Wall and the Resist Wall experiments has already been described in the previous Issue (Biol. Sci. Space, 2007). Other experiments are ready to be carried out in the Kibo Module on ISS within a couple of years. In the present Issue, five articles review the function of the cell wall in gravity responses of plants, which has been clarified by both ground-based and space experiments.

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The Japan Aerospace Exploration Agency (JAXA) recently conducted two space-based plant physiology experiments utilizing the European Modular Cultivation System (EMCS) facility of the European Space Agency (ESA). These experiments were named Cell Wall and Resist Wall (CWRW), and were designed to investigate the formation of plant cell walls and gravity resistance in plants. The CWRW experiments were taken aboard the International Space Station (ISS) by space shuttle mission STS-123 (1J/A) on March 11, 2008 and performed between March 30 and May 23, 2008. However, a number of failures in the EMCS environmental control system resulted in the experiments being performed differently than planned. On June 14, 2008, Arabidopsis plants grown in the CWRW experiments were recovered to Earth in the space shuttle mission STS-124 (1J) and are currently being analyzed. In this article, we elaborate on the timeline of the CWRW experiments from selection to performance. We also describe experiment unique equipments, the onboard operations by the ISS crew, the process by which the experiments were monitored from the ground and brief information about plants germination and growth stage under microgravity conditions in space. We conclude with lessons learned for future plant physiology experiments conducted in space.

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The characteristic growth pattern of vascular plants largely depends on the intrinsic properties of their cell walls, which are flexible, but strong enough to support the plant body. The plant body is composed of various tissues each with a specific cell wall type. Different sets of enzymes are required for the construction of these individual cell wall types. The cell wall type-specific enzyme-set hypothesis has been described to explain the mechanisms underlying cell wall construction. This hypothesis suggests that specific sets of transcription factors are required for the construction of each of the cell-wall types. Recent reverse genetic studies investigating secondary wall formation in Arabidopsis thaliana have demonstrated the existence of a hierarchical transcriptional network that governs the regulation of secondary wall formation in cell wall types. The examination of the effects of mechanical stimuli on the expression of genes encoding a particular set of cell wall-related enzymes and transcriptional factors has shown that A. thaliana is able to perceive subtle changes in self-weight of the aerial portions, and use this information as a signal to regulate formation of cell walls in the supporting tissues. However, the mechanisms by which mechanical signals are perceived via sensors presumably located at the cell surface remain unknown. In addition, the pathways through which the signal is transmitted and integrated into the transcriptional network that governs the coordinated actions of cell wall-related genes are also yet to be described. Current reverse genetic approaches based on comprehensive expression analysis of cell wall-related genes may aid in the elucidation of the regulatory mechanisms underlying supporting tissue formation via mechanical signals. Such information may contribute not only to a further understanding of the molecular basis underlying evolution of the plant vascular system, but may also provide us with the knowledge required for the future development and utilization of plant cell walls as a sustainable resource.

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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.

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Gramineous plants, such as rice, wheat, and maize, are essential crops. The cell wall composition of gramineous plants is distinguished from that of dicotyledons, such as Arabidopsis, pea, and mung bean. In cell walls of gramineous plants, arabinoxylans and β-glucans are the major matrix polysaccharides and they make network structure within cell wall architecture. Gravitational stimuli affect the metabolism of β-glucans in gramineous shoots; hypergravity suppressed the β-glucan breakdown, when it inhibited shoot elongation. The opposite results were obtained under microgravity conditions in space. On the other hand, the arabinoxylan and diferulic acid (DFA) contents increased under continuous hypergravity conditions. Since arabinoxylans are cross-linked by DFA-bridges, continuous hypergravity may stimulate the formation of arabinoxylan-DFA network within cell walls. These findings suggest that the β-glucan metabolism is primarily involved in the mechanism of growth regulation, while the arabinoxylan-DFA network has a load-bearing function against the gravitational force. The modification of these wall constituents may contribute to the capacity of gramineous plants to sustain their structure and growth under altered gravity conditions.

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The cell walls of land plants developed under the influence of earth's gravity. Arabinogalactan-proteins (AGPs) are a family of proteoglycans that localize on plasma membranes and in cell walls of higher plants. Recent studies have revealed that the expression levels of genes encoding the core proteins of AGPs are modified by hypergravity, indicating the involvement of AGPs in gravity resistance. A BLAST search in the genome databases of various organisms for genes encoding proteins related to fasciclin-like AGPs (FLAs), found FLAs in land plants including a moss, Physcomitrella patens subsp. patens, but not in the green algae, Chlamydomonas reinhardtii or Volvox carteri. On the other hand, the backbone structure of arabinogalactan moieties of AGPs, β-1,3:1,6-galactan, is widely distributed among organisms and has been confirmed in a species of Chlorellaceae, a snail and a mammal. These facts suggest that acquisition of some AGPs similar to those currently found, and FLAs in particular, was important in the evolution of the resistance of plants to gravitational force. By studying the molecular functions of AGPs under diverse gravitational conditions, we should be able to deepen our understanding of the evolutional process that turned aquatic organisms into terrestrial plants.

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Microbiology at the high altitude atmosphere is important for assessing the chances and limits of microbial transfer from the earth to extraterrestrial bodies. Among the microorganisms isolated from the high-atmospheric samples, spore formers and vegetative Deinococci were highly resistant against harsh environment at high altitude. From limited knowledge available to date, it is suggested that terrestrial microorganisms may have had chances to be ejected and transferred to outer space. Survival of these organisms during their space travel and proliferation on other planets might be also feasible. Directed Panspermia from Earth to extraterrestrial bodies is discussed on the basis of findings reported in literatures.

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