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Clinostats are instruments that continuously rotate biological specimens along an axis, thereby averaging their orientation relative to gravity over time. Our previous experiments indicated that low-speed clinorotation may itself trigger directional root tip curvature. In this project, we have investigated the root curvature response to low-speed clinorotation using Arabidopsis thaliana and Brachypodium distachyon seedlings as models. We show that low-speed clinorotation triggers root tip curvature in which direction is dictated by gravitropism during the first half-turn of clinorotation. We also show that the angle of root tip curvature is modulated by the speed of clinorotation. Arabidopsis mutations affecting gravity susception (pgm) or gravity signal transduction (arg1, toc132) are shown to affect the root tip curvature response to low-speed clinorotation. Furthermore, low-speed vertical clinorotation triggers relocalization of the PIN3 auxin efflux facilitator to the lateral membrane of Arabidopsis root cap statocytes, and creates a lateral gradient of auxin across the root tip. Together, these observations support a role for gravitropism in modulating root curvature responses to clinorotation. Interestingly, distinct Brachypodium distachyon accessions display different abilities to develop root tip curvature responses to low-speed vertical clinorotation, suggesting the possibility of using genome-wide association studies to further investigate this process. [ABSTRACT FROM AUTHOR]

The gravitropic curvature of seedlings of lentil (Lens culinaris L. cv. Verte du Puy) grown in microgravity and stimulated on the 1 g centrifuge for 5 to 60 min was followed by time lapse photography in near weightlessness in the frame of the IML 1 Mission of Spacelab. In microgravity, the root tip could overshoot the direction of the 1 g acceleration after bending, whereas roots stimulated on the ground did not reach the direction of the gravity vector. On earth, there is, therefore, a regulation (inhibition of root curvature), which is gravity dependent. In space, the initial rate of curvature as well as the amplitude of curvature varied as a function of the quantity of stimulation (Q, in gmin). For a given quantity of stimulation, the rate of curvature remained constant for 80 min. The bending has thus a certain inertia, which is linked to the mechanism of differential growth. The presentation time (Tp) of the lentil root was calculated by extrapolation to zero curvature of the regression line representing either the initial rate of curvature or the amplitude of curvature at 2 h after the end of the stimulation. Tp was estimated to 27 and 26 s, respectively. These results confirm the values of Tp obtained by clinostats, and they also lead to a reconsideration of the causes of the kinetics of root curvature. [ABSTRACT FROM AUTHOR]

Space experiments have offered a unique opportunity to analyse the mechanism of gravisensing in plant roots. It has been shown that the strict structural polarity of statocytes observed on the ground is perturbed in microgravity: the amyloplasts move towards the proximal half of the cell and, at least in some cases, the nucleus becomes located further away from the (proximal) plasma membrane. It has thus been demonstrated that the amyloplasts do not move freely in the cytoplasm. Experiments using cytochalasin B (or D) have indicated that these organelles are attached to the actin network, probably by motor proteins. These findings have led to a new hypothesis on gravisensing the basis of which is that the tension in the actin filaments resulting from interaction with the statoliths would be transmitted to stretch-activated ion channels located in the plasma membrane (Sievers et al., 1991, In: Lloyd (ed) The cytoskeletal basis of plant growth and form, Academic Press, London New York, pp 169–182. Recently, it has been shown that the sensitivity of roots grown under 1 g conditions in orbit is less than that of roots grown in microgravity or under simulated weightlessness on clinostats. Since the location of the amyloplasts in microgravity is different from that in 1 g, the greater sensitivity observed could be due to different tensions in the actin network. [ABSTRACT FROM AUTHOR]