Abstract
We conducted the Resist Tubule space experiment to clarify the role of the cell wall in the resistance of inflorescence stems of Arabidopsis (Arabidopsis thaliana) to gravitational acceleration in the Cell Biology Experiment Facility on the Kibo Module of the International Space Station. The cell wall rigidity of inflorescence stems was lower under microgravity conditions than at 1g, particularly in the basal region, which plays a central role in supporting the entire plant in the presence of gravity. In the basal region under microgravity conditions, the levels of matrix and cellulosic polysaccharides per unit length decreased in correlation with a decline in cell wall rigidity. The expression of cellulose synthase genes CESA4, CESA7, and CESA8, and certain xylan synthase genes tended toward suppression in the basal region under microgravity conditions. The downregulation of the expression of these genes may cause a decrease in cell wall polysaccharide levels, thereby maintaining a soft and extensile cell wall under microgravity conditions. In addition, the levels of UV-absorbing compounds decreased in pace with the cellulose levels in the basal region. Combined with the data of the hypergravity experiments, these results suggest that secondary walls in the basal supporting region play an important role in the resistance of Arabidopsis inflorescence stems to gravity in the range of 1 g to hypergravity.
