The plant cell wall is believed to play an important role in resisting gravity force and supporting the plant body under 1 g on earth, as the bones and muscles in an animal body. However, no direct evidence supporting this idea has been provided by space experiments. We analyzed the cell wall properties of rice coleoptiles and Arabidopsis hypocotyls grown under microgravity conditions during the Space Shuttle STS-95 mission. In space, elongation growth of both organs was stimulated and the cell wall extensibility increased. The increase in the wall extensibility in space was almost completely attributable to the increase in the irreversible extensibility. Also, the levels and the molecular size of the certain matrix polysaccharides, such as (1→3), (1→4)-β - glucans in rice coleoptiles and xyloglucans in Arabidopsis hypocotyls, decreased in space. These modifications in the matrix polysaccharides could be involved in increasing the cell wall extensibility, leading to growth stimulation in space. The results suggest that plant seedlings modify the metabolism of the certain cell wall polysaccharides and thus regulate the cell wall extensibility and the growth rate, thereby adapting different gravity conditions.
