Actin mediated statolith displacement in gravitropic chara rhizoids under altered gravitational stimulation: Suitability of 2D- and 3D clinorotation for simulation of microgravity

Gravity-depended statolith positioning in Chara rhizoids were investigated under altered gravity conditions on the Random Positioning Machine (RPM) by using this device in different operational modes (2D clinorotation and 3D clinorotation in real random mode). The objective of this study was to evaluate different techniques which are applied to simulate microgravity. A comparison of actin-dependent statolith displacement kinetics in Chara rhizoids during the different RPM operation modes with data from real microgravity conditions from Texus and Maxus missions allowed an assessment of the simulation quality. Displacement distance and velocity of statoliths correlated with the rotational speed of the RPM. Both, but necessarily fast 2D clinorotation and 3D rotation in real random mode induced in Chara rhizoids a displacement of the statoliths away from the tip with a comparable kinetic as in real microgravity. Slower rotational speeds in the 2D clinorotation and 3D real random mode caused less displacement of statoliths and different displacement kinetics. As a consequence, with respect to the displacement of statoliths in a unicellular system, the 2D clinorotation is more preferable than the 3D real random mode. In view to the displacement of statoliths in Chara rhizoids on a 3D clinostat, the results are matching with Hoson’s (Hoson et al. 1997) observations and extend them with respect to displacement kinetics. Furthermore, the results show that every alteration of gravity amount and direction affects the movement of statoliths. Therefore it was possible to show qualitative differences between the different RPM modes based on displacement kinetics and displacement distance of statoliths. The results show, that the rotational speed of a clinostat or an RPM has to be carefully selected to achieve a qualitative microgravity simulation. Beside the rotational speed, the selection of RPM operational mode has also to be considered. Although no significant differences in statolith displacement distance were observed between fast 2D clinorotation and 3D real random mode, the displacement kinetics were slightly different. These differences might be influenced by stronger vibrations during the respective operation mode. The additional rotational axis in 3D real random mode caused more intense vibration patterns. Vibration maxima appeared irregular in context to changing rotation directions. In contrast to this operation mode, the 2D clinorotation caused weaker vibration maxima and a more regular vibration pattern through the whole measured frequency spectrum. The differentiation between 2D mode performed with the inner frame and 2D mode performed with the outer frame showed also different vibration patterns. The inner frame caused lesser vibrations probable through a smaller mechanical slackness and is therefore more preferable. To conclude these observations, the 2D clinorotation with the inner frame is more preferable in view to a nearly stimulation-free environment and a qualitative good simulation. The additional rotation axis in 3D real random mode has no further advantage toward the 2D clinorotation with respect to the selected experimental parameters