橋本 博文

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.

To investigate microbial viability and DNA damage, dried cell pellets of the radiation-resistant bacterium Deinococcus radiodurans were exposed to various space environmental conditions at the Exposure Facility of the International Space Station (ISS) as part of the Tanpopo mission. Mutation analysis was done by sequencing the rpoB gene encoding RNA polymerase β-subunit of the rifampicin-resistant mutants. Samples included bacteria exposed to the space environment with and without exposure to UV radiation as well as control samples held in the ISS cabin and at ground. The mutation sites of the rpoB gene obtained from the space-exposed and ISS/ground control samples were similar to the rpoB mutation sites previously reported in D. radiodurans. Most mutations were found at or near the rifampicin binding site in the RNA polymerase β-subunit. Mutation sites found in UV-exposed samples were mostly shared with non-exposed and ISS/ground control samples. These results suggest that most mutations found in our experiments were induced during procedures that were applied across all treatments: preparation, transfer from our laboratory to the ISS, return from the ISS, and storage before analysis. Some mutations may be enhanced by specific factors in the space experiments, but the mutations were also found in the spontaneous and control samples. Our experiment suggests that the dried cells of the microorganism D. radiodurans can travel without space-specific deterioration that may induce excess mutations relative to travel at Earth's surface. However, upon arrival at a recipient location, they must still be able to survive and repair the general damage induced during travel.

「たんぽぽ計画」では国際宇宙ステーション（ISS）曝露部で微生物を採集することによって，この高度での微生物の存在可能性を検討する．また，微生物が宇宙空間で，どの程度の時間生存できるのかを，微生物を宇宙環境に曝露することによって調べる．生命の起原以前に，宇宙空間で合成された有機物が宇宙塵とともに地球に飛来した可能性がある．そこで，ISS上で宇宙塵の採集を行い，有機物を解析する．地球周辺には，スペースデブリが多量に蓄積している．本計画ではそのモニターも行う．これらの実験のために0.01g/cm³という超低密度のエアロゲルを開発した．これは今後の宇宙における様々な高速衝突微粒子採集に利用可能である．曝露試料は1年間，2年間，3年間曝露の後に地上に持ち帰り解析する．すでに，1年目と2年目の試料が地上に帰還して分析が行われている．1年間曝露した微生物の生存が確認された．

A wide variety of organic compounds have been found in space, and their relevance to the origin of life is discussed. Interplanetary dust particles (IDPs) are most promising carriers of extraterrestrial organic compounds, but presence of bioorganic compounds are controversial since they are so small and were collected in the terrestrial biosphere. In addition, IDPs are directly exposed to cosmic and solar radiation. Thus, it is important to evaluate the stability of organics in IDPs in space environment. We are planning a novel astrobiology mission named Tanpopo by utilizing the Exposed Facility of Japan Experimental Module (JEM/EF) of the International Space Station (ISS). Two types of experiments will be done: Capture experiments and exposure experiments. In the exposure experiments, organics and microbes will be exposed to the space environments to examine possible alteration of organic compounds and survivability of microbes. Selected targets for the exposure experiments of organic compounds are as follows: Amino acids (glycine and isovaline), their possible precursors (hydantoin and 5-ethyl-5-methyl hydantoin) and complex precursors "CAW" synthesized from a mixture of carbon monoxide, ammonia and water by proton irradiation. In addition to them, powder of the Murchison meteorite will be exposed to examine possible alteration of meteoritic organics in space. We will show the results of preparatory experiments on ground by using a UV lamp, a ⁶⁰Co source, synchrotron facilities, and a heavy ion irradiation facility.

Gravity resistance is a principal graviresponse in plants. In resistance to hypergravity, the gravity signal may be perceived by the mechanoreceptors located on the plasma membrane, and then transformed and transduced via the structural continuum or physiological continuity of cortical microtubules-plasma membrane-cell wall, leading to an increase in the cell wall rigidity as the final response. The Resist Tubule experiment, which will be conducted in the Kibo Module on the International Space Station, aims to confirm that this hypothesis is applicable to resistance to 1 G gravity. There are two major objectives in the Resist Tubule experiment. One is to quantify the contributions of cortical microtubules to gravity resistance using Arabidopsis tubulin mutants with different degrees of defects. Another objective is to analyze the modifications to dynamics of cortical microtubules and membrane rafts under microgravity conditions on-site by observing green fluorescent protein (GFP)-expressing Arabidopsis lines with the fluorescence microscope in the Kibo. We have selected suitable mutants, developed necessary hardware, and fixed operation procedure for the experiment.

JAXAが検討している火星探査において，生命探査を行う意義について述べ，具体的方法を提案した。

第25回宇宙利用シンポジウム(2009年1月14日-15日, 宇宙航空研究開発機構宇宙科学研究本部相模原キャンパス)形態: カラー図版あり資料番号: AA0064297069