It is suggested that the loss of the unequivocal one-g, otolith-mediated reference, which corresponds closely to the visual frame, results in a decalibration in the central vestibular system. During prolonged microgravity, it appears that a body coordinate system, presumably mediated by distributed proprioception, is used as frame-of-reference. The results provide evidence that under one-g conditions the gravitational reference is used to stabilise the internal spatial co-ordinate system in the vestibular and oculomotor systems. Presuming a reweighting of otolithic afferent information during prolonged microgravity, it was hypothesised that a corollary, or inverse, re-weighting of the proprioceptive afferents from the neck should become evident during head-to-trunk movement. In a second experiment, the role of cervical proprioception and its role in adaptive modification during spaceflight was examined. Analysis of the three-dimensional vestibulo-ocular response (3D-VOR) by way of 3x3 velocity gain matrices and estimation of minimal gain vectors demonstrates that the coordinate system of the 3D-VOR is re-oriented during the inflight period, and again after return to the one-g environment. The head movements were performed during visual fixation of space-fixed and imaginary targets. Eye and head movements were recorded by three-dimensional video-oculography (VOG) during active roll, pitch and yaw oscillations of the head, both in space and on earth. In the present experiments, the influence of this adaptive modification on the three-dimensional vestibulo-ocular reflex (3D-VOR) was examined. The flux of otolith-mediated, gravitoinertial information is radically altered in microgravity. In addition, actin-dependent relocalization of PIN3 in response to gravity provides a mechanism for redirecting auxin flux to trigger asymmetric growth. Our data indicate that PIN3 is a component of the lateral auxin transport system regulating tropic growth. In the root columella, PIN3 is positioned symmetrically at the plasma membrane but rapidly relocalizes laterally on gravity stimulation. PIN3 localizes to the plasma membrane and to vesicles that cycle in an actin-dependent manner. PIN3 is expressed in gravity-sensing tissues, with PIN3 protein accumulating predominantly at the lateral cell surface. Mutations in the Arabidopsis gene PIN3, a regulator of auxin efflux, alter differential growth. Here, we show that auxin accumulates asymmetrically during differential growth in an efflux-dependent manner. Physiological studies implicated a specific transport system that relocates auxin laterally, thereby effecting differential growth(4) however, neither the molecular components of this system nor the cellular mechanism of auxin redistribution on light or gravity perception have been identified. Long-standing models propose that plant growth responses to light or gravity are mediated by asymmetric distribution of the phytohormone auxin(1-3).
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