Cell wall changes involved in the automorphic curvature of rice coleoptiles under microgravity conditions in space

Seedlings of rice (Oryza sativa L. cv. Koshihikari and cv. Tan-ginbozu) were cultivated on board the Space Shuttle STS-95 mission and changes in the morphology and the cell wall properties of coleoptiles were analyzed. In space, rice coleoptiles showed a spontaneous (automorphic) curvature toward the caryopsis in the elongating region. The angle of automorphic curvature was larger in Koshihikari than in a gibberellin-deficient dwarf cultivar, Tan-ginbozu, and the angle gradually decreased during the growth of coleoptiles in both cultivars. The more quickly expanding convex side of the bending region of the rice coleoptiles showed a greater extensibility of the cell wall than the opposite side. There was a significant correlation between the angle of curvature and the difference in the cell wall extensibility between the convex and the concave sides. Both the levels of the cell wall polysaccharides per unit length of coleoptile and the ratio of high-molecular-mass polysaccharides in the hemicellulose fraction were lower in the convex side than the concave one. Also, the activity of (1-->3),(1-->4)-beta-glucanases in the cell wall was higher in the convex side than the concave one. These results suggest that the uneven modifications of cell wall metabolism bring about the difference in the levels and the molecular size of the cell wall polysaccharides, thereby causing the difference in capacity of the cell wall to expand between the dorsal and the ventral sides, leading to the automorphic curvature of rice coleoptiles in space. The data also suggest the involvement of gibberellins in inducing the automorphic curvature under microgravity conditions.