STATOCYTE POLARITY AND GRAVISENSITIVITY IN SEEDLING ROOTS GROWN IN MICROGRAVITY

Space experiments have offered a unique opportunity to analyse the mec hanism of gravisensing in plant roots. It has been shown that the stri ct structural polarity of statocytes observed on the ground is perturb ed in microgravity: the amyloplasts move towards the proximal half of the cell and, at least in some cases, the nucleus becomes located furt her away from the (proximal) plasma membrane. It has thus been demonst rated that the amyloplasts do not move freely in the cytoplasm. Experi ments 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 whi ch is that the tension in the actin filaments resulting from interacti on with the statoliths would be transmitted to stretch-activated ion c hannels located in the plasma membrane (Sievers et al., 1991, In: Lloy d (ed) The cytoskeletal basis of plant growth and form, Academic Press , London New York, pp 169-182. Recently, it has been shown that the se nsitivity of roots grown under 1 g conditions in orbit is less than th at 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.