Microgravity Affects the Level of Matrix Polysaccharide 1,3:1,4-beta-Glucans in Cell Walls of Rice Shoots by Increasing the Expression Level of a Gene Involved in Their Breakdown

The plant cell wall provides each cell with structural support and mechanical strength, and thus, it plays an important role in supporting the plant body against the gravitational force. We investigated the effects of microgravity on the composition of cell wall polysaccharides and on the expression levels of genes involved in cell wall metabolism using rice shoots cultivated under artificial 1 g and microgravity conditions on the International Space Station. The bulk amount of the cell wall obtained from microgravity-grown shoots was comparable with that from 1 g-grown shoots. However, the analysis of sugar constituents of matrix polysaccharides showed that microgravity specifically reduced the amount of glucose (Glc)-containing polysaccharides such as 1,3:1,4-beta-glucans, in shoot cell walls. The expression level of a gene for endo-1,3:1,4-beta-glucanase, which hydrolyzes 1,3:1,4-beta-glucans, largely increased under microgravity conditions. However, the expression levels of genes involved in the biosynthesis of 1,3:1,4-beta-glucans were almost the same under both gravity conditions. On the contrary, microgravity scarcely affected the level and the metabolism of arabinoxylans. These results suggest that a microgravity environment promotes the breakdown of 1,3:1,4-beta-glucans, which, in turn, causes the reduced level of these polysaccharides in growing rice shoots. Changes in 1,3:1,4-beta-glucan level may be involved in the modification of mechanical properties of cell walls under microgravity conditions in space.