Abstract
In single-celled rhizoids of the green algae Chara, positively gravitropic growth is governed by statoliths kept in a dynamically stable position 10–25 μ m above the cell tip by a complex interaction of gravity and actomyosin forces. Any deviation of the tube-like cells from the tip-downward orientation causes statoliths to sediment onto the gravisensitive subapical cell flank which initiates a gravitropic curvature response. Microgravity experiments have shown that abolishing the net tip-directed gravity force results in an actomyosin-mediated axial displacement of statoliths away from the cell tip. The present study was performed to critically assess the quality of microgravity simulation provided by different operational modes of a Random Positioning Machine (RPM) running with one axis (2D mode) or two axes (3D mode) and different rotational speeds (2D), speed ranges and directions (3D). The effects of 2D and 3D rotation were compared with data from experiments in real microgravity conditions (MAXUS sounding rocket missions). Rotational speeds in the range of 60–85 rpm in 2D and 3D modes resulted in a similar kinetics of statolith displacement as compared to real microgravity data, while slower clinorotation (2–11 rpm) caused a reduced axial displacement and a more dispersed arrangement of statoliths closer to the cell tip. Increasing the complexity of rotation by adding a second rotation axis in case of 3D clinorotation did not increase the quality of microgravity simulation, however, increased side effects such as the level of vibrations resulting in a more dispersed arrangement of statoliths. In conclusion, fast 2D clinorotation provides the most appropriate microgravity simulation for investigating the graviperception mechanism in Chara rhizoids, whereas slower clinorotation speeds and rotating samples around two axes do not improve the quality of microgravity simulation.
Highlights
The International Space Station (ISS), satellites, sounding rockets, parabolic plane flights and drop towers are research facilities that provide years, month, minutes or only seconds of a real-microgravity environment that has been intensively used in the last decades for gravitational biology research
All rhizoids which were clinorotated for 20 min in the different modes (n = 11 each) continued to grow straight without showing any curvature response which indicates that sedimentation of statoliths triggering a gravitropic growth correction did not occur during any of the clinostat treatments
The statoliths complex was rearranged from a flat disk-like shape into an axially elongated shape further away from the cell tip
Summary
The International Space Station (ISS), satellites, sounding rockets, parabolic plane flights and drop towers are research facilities that provide years, month, minutes or only seconds of a real-microgravity (μg) environment that has been intensively used in the last decades for gravitational biology research. Different μg simulation facilities like clinostats, Random Positioning Machine (RPM), rotating wall vessels and magnetic levitation are frequently used as attractive alternatives for complementing and preparing real-microgravity experiments in the Microgravity Sci. Technol. They enable with some limitations, such as sample diameter, biological studies of the impact of altered gravity conditions on Earth (Herranz et al 2013; Brungs et al 2016). Due to the physical constraints only small samples placed precisely along the rotation axis will experience acceleration forces which are thought to be below the threshold of the biological system. Specimen larger than a few millimeters or samples not placed very close to the center of the rotation axis will experience centrifugal forces in the periphery
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