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Electron microscopic analysis of gravisensing Chara rhizoids developed under microgravity conditions

Botanisches Institut, Universität Bonn, Bonn, Germany

¹Correspondence: Botanisches Institut, Universität Bonn, Venusbergweg 22, D-53115 Bonn, Germany. E-mail: unb13a{at}uni-bonn.de

Tip-growing, unicellular Chara rhizoids that react gravitropically on Earth developed in microgravity. In microgravity, they grew out from the nodes of the green thallus in random orientation. Development and morphogenesis followed an endogenous program that is not affected by the gravitational field. The cell shape, the polar cytoplasmic organization, and the polar distribution of cell organelles, except for the statoliths, were not different from controls that had grown on earth (ground controls). The ultrastructure of the organelles and the microtubules were well preserved. Microtubules were excluded from the apical zone in both ground controls as well as microgravity-grown rhizoids. The statoliths (vesicles containing BaSO₄ crystals in a matrix) in microgravity-grown rhizoids were spread over a larger area (up to 50 µm basal to the tip) than the statoliths of ground controls (10–30 µm). Some statoliths were even located in the subapical zone close to microtubules, which was not observed in ground controls. The crystals in statoliths from microgravity-grown rhizoids appeared more loosely arranged in the vesicle matrix compared with ground controls. The chemical composition of the crystals was identified as BaSO₄ by X-ray microanalysis. There is evidence that the amount of BaSO₄ in statoliths of rhizoids developed in microgravity is lower than in ground controls, indicating that the gravisensitivity of microgravity-developed rhizoids might be reduced compared with ground controls. Lack of gravity, however, does not affect the process of tip growth and does not inhibit the development of the structures needed for the gravity-sensing machinery.—Braun, M., Buchen, B., Sievers, A. Electron microscopic analysis of gravisensing Chara rhizoids developed under microgravity conditions.

Key Words: gravitropism • statolith • tip growth

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