Considering further human activity in space, it is necessary to study the biological effects of combined microgravity and space radiation; however, many aspects of these combined effects remain unclear. In the field of space biology, it is difficult to investigate relative biological effectiveness (RBE) and combined effects because the ability to conduct and replicate space experiments is restricted. Therefore, a new three-dimensional (3D) clinostat synchronized X-irradiation system with a high-speed shutter was fabricated following the development of a heavy-ion irradiation system. This study showed that the system could simultaneously irradiate rotating samples using the 3D clinostat, with the samples in a horizontal position. The samples were completely irradiated because of the flatness and symmetry of the irradiation fields. Doses were virtually identical under both standing and rotation conditions, with the difference being <1% under the assumption of X-irradiation at a dose of 1 Gy. Our new device could accurately synchronize X-irradiation and simulated microgravity at the ground level. The device is expected to greatly contribute to space radiation research as a valuable platform for studies concerning RBE and the combined effects of radiation under microgravity.

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The present experiment aims to clarify the roles of 65 kDa microtubule-associated protein-1 (MAP65-1) and basic proline-rich protein1 (BPP1), which are involved in the maintenance of transverse microtubule orientation, in gravity resistance, using green fluorescent protein (GFP)-expressing Arabidopsis lines. Hypergravity at 300 G inhibited elongation growth and promoted lateral expansion of epidermal cells in the subapical region of hypocotyls in GFP-MAP65-1 line expressing by native promoter and BPP1-GFP line expressing by a constitutive cauliflower mosaic virus 35S promoter. In BPP1-GFP line, hypergravity showed smaller effects on modification of growth anisotropy than wild type. Also, hypergravity induced reorientation of cortical microtubules from transverse to longitudinal directions in both lines. However, in BPP1-GFP line, hypergravity showed smaller effects on reorientation of cortical microtubules. When the expression levels of MAP65-1 were determined by analyzing GFP fluorescence in hypocotyls of GFP-MAP65-1 line, hypergravity decreased the levels of MAP65-1 in the subapical region, where hypergravity modified growth anisotropy and orientation of cortical microtubules. These results indicate that the regulation of levels of MAP65-1 and BPP1 is involved in the hypergravity-induced reorientation of cortical microtubules, which may lead to modification of growth anisotropy, thereby developing a tough body against the gravitational force in Arabidopsis hypocotyls.

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