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.

The International Space Station (ISS) provides a precious opportunity to study plant growth and development under microgravity (micro-G) conditions. In this study, four lines of Arabidopsis seeds (wild type, wild-type MCA1-GFP, mca1-knockout, and MCA1-overexpressed) were cultured on a nylon lace mesh placed on Gelrite-solidified MS-medium in the Japanese experiment module KIBO on the ISS, and the entanglement of roots with the mesh was examined under micro-G and 1-G conditions. We found that root entanglement with the mesh was enhanced, and root coiling was induced under the micro-G condition. This behavior was less pronounced in mca1-knockout seedlings, although MCA1-GFP distribution at the root tip of the seedlings was nearly the same in micro-G-grown seedlings and the ground control seedlings. Possible involvement of MCA1 in the root entanglement is discussed.

In the “Space Seed” experiment performed in Kibo module of the International Space Station, growth direction of the inflorescence stem of arabidopsis was examined under space 1 G , μ G , and ground 1 G conditions in the presence of light. The stems grew almost upright (vertical to the surface of seedbed) under ground 1 G . Although the stems were primarily upright both under space 1 G and μ G , they tilted slightly. The tilting of the stems under space 1 G was indicated to be due to tilting of the artificial gravitational acceleration vectors produced on the centrifuge. The tilting of the stems under μ G was suggested to be due to the pressure of directional airflow produced by ventilation.