Growth stimulation in inflorescences of an Arabidopsis tubulin mutant under microgravity conditions in space

Cortical microtubules are involved in plant resistance to hypergravity, but their roles in resistance to 1g gravity are still uncertain. To clarify this point, we cultivated an Arabidopsis α-tubulin 6 mutant (tua6) in the Cell Biology Experiment Facility on the Kibo Module of the International Space Station, and analyzed growth and cell wall mechanical properties of inflorescences. Growth of inflorescence stems was stimulated under microgravity conditions, as compared with ground and on-orbit 1g conditions. The stems were 10-45% longer and their growth rate 15-55% higher under microgravity conditions than those under both 1g conditions. The degree of growth stimulation tended to be higher in the tua6 mutant than the wild-type Columbia. Under microgravity conditions, the cell wall extensibility in elongating regions of inflorescences was significantly higher than the controls, suggesting that growth stimulation was caused by cell wall modifications. No clear differences were detected in any growth or cell wall property between ground and on-orbit 1g controls. These results support the hypothesis that cortical microtubules generally play an important role in plant resistance to the gravitational force.