Electron microscopic analysis of gravisensing Chara rhizoids developed under microgravity conditions

Tip-growing, unicellular Chara rhizoids that react gravitropically on Earth developed in microgravity, In microgravity, they grew out from the nodes o f the green thallus in random orientation. Development and morphogenesis fo llowed an endogenous program that is not affected by the gravitational fiel d. The cell shape, the polar cytoplasmic organization, and the polar distri bution of cell organelles, except for the statoliths, were not different fr om controls that had grown on earth (ground controls), The ultrastructure o f the organelles and the microtubules were well preserved. Microtubules wer e excluded from the apical zone in both ground controls as well as microgra vity-grown rhizoids. The statoliths (vesicles containing BaSO4 crystals in a matrix) in microgravity-grown rhizoids were spread over a larger area (up to 50 mu m basal to the tip) than the statoliths of ground controls (10-30 mu m), Some statoliths were even located in the subapical zone close to mi crotubules, which was not observed in ground controls. The crystals in stat oliths from microgravity-gown rhizoids appeared more loosely arranged in th e vesicle matrix compared with ground controls. The chemical composition of the crystals was identified as BaSO4 by X-ray microanalysis, There is evid ence that the amount of BaSO4 in statoliths of rhizoids developed in microg ravity is lower than in ground controls, indicating that the gravisensitivi ty of microgravity-developed rhizoids might be reduced compared with ground controls. Lack of gravity, however, does not affect the process of tip gro wth and does not inhibit the development of the structures needed for the g ravity-sensing machinery.