Cell Growth and Organ Differentiation in Cultured Tobacco Cells under Spaceflight Condition.

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Abstract
The responses of cultured tobacco cells to microgravity were examined. Cultured tobacco cells grew and regenerated shoots under microgravity conditions. However, such growth, especially of stems, was much more heterogeneous than that in the ground control, and the increase in fresh weight of the flight samples was less than that of ground control. In addition, multiple shoot formation was not observed in flight samples. Microscopic observation showed that the meristem of regenerating shoot under microgravity was smaller than that in the ground control. Electron microscopy showed that chloroplasts in the ground control were slightly more developed than those in flight samples and extensive arrays of microtubules were more evident in ground control than in flight samples. Analyses of enzymes involved in primary and secondary metabolism indicated that callus grown on shoot regeneration medium under microgravity conditions had a much lower activity of caffeic acid O-methyltransferase, which is involved in lignin biosynthesis, than those grown on Earth. In addition, two-dimensional polyacrylamide-gel electrophoresis analysis of 35S-labeled proteins suggested that gene expression in the flight samples may be similar to that in the ground control.