Movement of the Organelles in Mierogravity

As said before, the nucleus occupies the proximal end of the statocyte in most cases examined (Perbal and Driss-Ecole 1989, Sievers and Volkmann 1972). Sievers and Heyder-Caspers (1983) observed sedimentation of the amyloplasts and of the nucleus toward the distal cell pole and a lateral displacement of the endoplasmic reticulum after centrifugation for 20 min at 50 g. Statocytes re-established the structural polarity within 1 h after cessation of centrifugation. In lentil roots, centrifugal forces were applied toward the tip to move the nucleus and it was shown (Lorenzi and Perbal 1990) that at 19 g (20 min) this organelle sedimented in the oldest statocytes, whereas at 40 g (20 min) all nuclei had settled down on the amyloplasts. These results showed that the displacement of the nucleus needed a very strong centrifugal force.

In mierogravity, the location of the nucleus was different from that observed in 1 g on the ground (Perbal and Driss-Ecole 1989). But the 1-g control in space was similar to the mierogravity sample. According to Lorenzi and Perbal (1990) this could be due to the fact that the 1-g space control was exposed to mierogravity for about 15 min before fixation, because it was not possible to fix the seedlings on the centrifuge.

Experiments carried out on a slowly rotating clinostat demonstrated that simulation of mierogravity also displaced the nucleus toward the cell center (Lorenzi and Perbal 1990). Cytochalasin B or D, which inhibits polymerization of actin filaments, caused sedimentation of the nucleus on the amyloplasts in 1 g (Figure 6-10B). This displacement of the nucleus in statocytes after cytochalasin B application indicated that an inhibition of polymerization of actin monomers affected the position of the nucleus (Lorenzi and Perbal 1990).

It is well known that the nucleus is surrounded by a kind of basket of actin filaments, which is involved in the positioning of this organelle. It has been suggested that actin filaments are associated with both the nucleus and the plasma membrane and generated tension between them (Figure 6-12). The results obtained by Lorenzi and Perbal (1990) were in good agreement with this hypothesis since the displacement of the nucleus in mierogravity could be induced by a relaxation of the cytoskeleton.

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