Growth Regulation Mechanisms in Higher Plants under Microgravity Conditions. Changes in Cell Wall Metabolism.

Abstract

During Space Shuttle STS-95 mission, we cultivated seedlings of rice (Oryza sativa L. cv. Koshihikari and cv. Tan-ginbozu) and Arabidopsis (Arabidopsis thaliana L. cv. Columbia and cv. etr1-1) for 68.5, 91.5, and 136 hr on board, and then analyzed changes in the nature of their cell walls, growth, and morphogenesis under microgravity conditions. In space, elongation growth of both rice coleoptiles and Arabidopsis hypocotyls was stimulated. Also, the increase in the cell wall extensibility, especially that in the irreversible extensibility, was observed for such materials. The analyses of the amounts, the structure, and the physicochemical properties of the cell wall constituents indicated that the decreases in levels and molecular masses of cell wall polysaccharides were induced under microgravity conditions, which appeared to contribute to the increase in the wall extensibility. The activity of certain wall enzymes responsible for the metabolic turnover of the wall polysaccharides was increased in space. By the space flight, we also confirmed the occurrence of automorphogenesis of both seedlings under microgravity conditions; rice coleoptiles showed an adaxial bending, whereas Arabidopsis hypocotyls elongated in random directions. Furthermore, it was shown that spontaneous curvatures of rice coleoptiles in space were brought about uneven modifications of cell wall properties between the convex and the concave sides. 1)IntroductionPlants developed the cell wall with a complicated but well-organized structure during their evolution for more than 400 million years after they first went ashore. The plant cell wall plays an important role in supporting its body under 1 g on earth. At the same time, the cell wall encloses each cell and determines its size and shape. Growth and development of plant body are thus most directly controlled by the nature of the cell wall. Therefore, microgravity in space would greatly modify the properties of the cell wall, thereby influencing growth regulation mechanisms.The results obtained by our ground-based experiments with the water immersion method and the hypergravity conditions produced by centrifugation support such an idea 1)-3) . For instance, rice coleoptiles grow faster under water than in air. Air-bubbling partially suppresses coleoptile growth under water, suggesting that the stimulation of growth under water is caused by both gaseous factors and microgravity effect due to buoyancy. The rheological analysis of the cell wall properties of rice coleoptiles showed that the extensibility of the cell wall of water-grown coleoptiles was higher than that of air-grown ones. Submergence also causes diverse changes in the levels and the structure of the cell wall constituents, such as the decrease in the levels of cellulose, the matrix polysaccharides, the structural glycoproteins, and phenolics, that are supposed to be responsible for the increase in the cell wall extensibility of rice coleoptiles. On the contrary, hypergravity caused a suppression of elongation growth and...

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