Biological Sciences in Space

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: In this review, we attempted to summarize the effect of gravity on growth of woody plants, broad leaved trees, on earth. It is well known that in tilted broad leaved trees, tension wood formed in the secondary xylem causes negative gravitropism. Gibberellin has been shown to induce tension wood in weeping branch, causing its upright growth. Recent study has shown that seedling of Japanese cherry tree grew on three dimensional clinostat, a device that simulates microgravity, grew at random angles, and that the formation of secondary xylem, as supporting tissue for upright growth, decreased. In the decreased xylem formation, the inhibition of the differentiation and development of fiber cell was clearly observed. These results suggest that in attitude control and morphogenesis of stem in woody plant, secondary xylem formation seriously relates to gravity on earth. In woody plant, the mechanism of gravity perception and the following signal transduction have not yet been elucidated, although the recent study reported the possibility that endodermal starch sheath cells and plant hormones may play some role in the mechanism. Space experiment is expected to study these problem.

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