| In higher plants, columella cells of the root cap, proposed to be sites of gravity perception, are characterized by a developmental polarity and contain starch-filled plastids which sediment in response to gravity. The results of several space experiments provide an insight on the gravity signal transduction mechanism in roots. On shuttle mission STS-69, analysis by electron microscopy revealed that plastid location in the columella cells depends on their density both under 1g or in microgravity. On STS-54, electron micrographs of columella cells revealed that microfilaments anchor both the nucleus and the ER to the plasma membrane and play an important role in the maintenance of the cellular polarity. Comparisons with clinorotated controls suggest a tumbling movement of the amyloplasts during clinorotation and an upward movement in microgravity. Cortical microtubules along the plasma membrane were visualised in microgravity samples (STS-57). On STS-60 and during clinorotation, fixation of the seedlings with a calcium precipitating agent suggested a calcium redistribution in the columella cells. Experiments also suggest a strong interaction between phytohormones in producing "spaceflight effects". When soybean were grown in BRIC canisters, clinorotation decreased shoot length but increased root length. Increased ethylene production may be due to an auxin redistribution. Space-grown soybean seedlings (BRIC-01 & -03) exhibited greater root length and produced twice as much ethylene. Analysis of cotyledon cells and lateral root columella cells by electron microscopy revealed that space samples had larger amyloplast starch grains, an increased lipid bodies number and size, and thicker cell walls.; It is evident from these results that microgravity affects etiolated seedlings at a variety of physiological levels: morphological, metabolic and ultrastructural. Spaceflight-induced repositioning of amyloplasts may trigger a cytoskeletal rearrangement, in turn inducing a calcium redistribution in the columella cells. Such a calcium influx may target calcium-dependent proteins, influence the auxin distribution at the tissue level, and thereby may increase ethylene production. These two last factors would undoubtedly affect plant morphology and metabolism. |
