Statocyte Polarity

The structural polarity of the root statocyte (see Figure 6-04A) has been intensively studied on the ground (Sievers and Volkmann 1972, Perbal and Driss-Ecole 1989) and can be considered to be similar from one species to another. The nucleus of gravisensing cells is always situated close to the proximal wall, whereas the amyloplasts are sedimented on large Endoplasmic Reticulum (ER) aggregates located along the distal wall. During differentiation of the statocyte, the endoplasmic reticulum migrates toward this wall, whereas the nucleus remains located very close to the plasma membrane lining the proximal wall (Sievers and Volkmann 1972).

Concomitantly, plastids synthesize starch and voluminous starch grains are formed in a dense stroma6. Amyloplasts have a high density (1.44 g/cm3) and therefore sediment within the cytoplasm, the density of which is 1.03 g/cm3 (for the density of the organelles, see review Sack 1991).

The development of the structural polarity of the root statocyte is genetically determined and does not depend upon gravity (Sievers and Braun 1996), since plants grown in mierogravity have distal ER tubules and a proximal nucleus in their statocytes (Figure 6-1 OA). However, a precise analysis of these two organelles in the lentil statocyte showed that their location was slightly different in the microgravity-grown sample and in the 1-g flight control (Perbal and Driss-Ecole 1989). In mierogravity, the majority of the ER aggregates were situated close to the distal wall of the statocytes, whereas some of these aggregates lined the longitudinal wall when root growth occurred on a 1-g centrifuge in space. This could well be linked to the sedimentation of the amyloplast, which could: a) exert a pressure on the ER aggregates and push them upward; or b) perturb the migration of the ER tubules.

One of the most intriguing phenomenon observed in mierogravity dealt with the position of the nucleus in the statocyte. In mierogravity, this organelle was closer to the longitudinal axis of the statocyte, in a more central position than in the statocytes differentiated on the 1 -g centrifuge (Perbal and Driss-Ecole 1989). Many observations (Perbal et al. 1987) indicated that there was a denser region between the plasma membrane and the nuclear envelope. This suggested that the nucleus could be attached to the cell periphery by actin filaments. This hypothesis was confirmed by a treatment by cytochalasin (B or D), which provoked sedimentation of the nucleus in 1 g (Figure 6-1 OB) and a more central distribution of this organelle in microgravity-grown lentil seedlings (Driss-Ecole et al. 2000a).

In the lentil statocyte and in mierogravity, the majority of the amyloplasts was located in the proximal part of the gravisensing cells around the nucleus. A 3D cell reconstruction done by Smith et al. (1997) has demonstrated that the distribution of the amyloplasts was not random in statocytes of Trifolium repens grown in mierogravity (Figure 6-11). These organelles were grouped near the cell center as on the clinostat, but the grouping was less dense in the latter case.

On lentil roots, it was also shown that clinorotation led to different amyloplast distribution when the roots were rotated with their axis parallel or perpendicular to the horizontal axis of the clinostat. The amyloplast distribution in the horizontal clinorotated statocytes was close to that observed in mierogravity (Lorenzi and Perbal 1990).

6 The non-membranous matrix material of a chloroplast.

Figure 6-11. Comparison of the position of the amvloplasts in root statocytes of white clover grown in the vertical position (1G), in microgravity (OG) or on the dinostat. These drawings represent 3D reconstruction of the statocytes from longitudinal sections. The limits of the protoplasm, the amyloplasts and the nucleus are represented. Adaptedfrom Smith et al. (1997).

Figure 6-11. Comparison of the position of the amvloplasts in root statocytes of white clover grown in the vertical position (1G), in microgravity (OG) or on the dinostat. These drawings represent 3D reconstruction of the statocytes from longitudinal sections. The limits of the protoplasm, the amyloplasts and the nucleus are represented. Adaptedfrom Smith et al. (1997).

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