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THE ACTIN NETWORK IN LENTIL ROOT STATOCYTES.   D..Driss-Ecole and G. Perbal.   Laboratoire de Cytologie Expйrimentale et Morphogenиse Vйgйtale, Universitй Pierre et Marie Curie, 4 place Jussieu, F-75252 Paris Cedex 05, France.

Root statocytes are characterized by a strict structural polarity. A treatment by the actin-disrupting drug cytochalasin (B or D) demonstrates that the actin microfilaments (MFs) are involved in the distal positioning of ER and in the proximal location of the nucleus. We carried out a pre-embedding immunogold silver technique with a monoclonal anti-actin antibody and demonstrated that the amyloplasts (statoliths) are enmeshed in an actin web of short filaments arranged in different ways. Moreover myosin-related proteins were localized on the surface of statoliths by immunofluorescence.

Comparison of mean velocities of amyloplast movement in root transferred from 1 g centrifuge (root-tip-directed acceleration) to microgravity (S/MM-03 mission of Spacehab) and in inverted roots on earth allowed to determine the very low value of the force responsible for the movement of one amyloplast (0.016 pN). Taking into account the force produced by one molecule of myosin it can be hypothesized that there is a loss of a large part of energy during the basipetal movement in microgravity due to the fact that the MFs are not oriented parallel to the axis of the statocyte confirming our cytological observations. The analysis of the movement of the amyloplasts in microgravity showed that the amyloplasts can move individually.

These observations raise the possibility that in the root statocytes the actin cytoskeleton is made of an interconnected network of fibrous units organized in a random fashion close to the percolation model proposed by Forgacs. Myosin-coated statoliths can move unidirectionally along several and short actin filaments during the basipetal movement. In 1 g the link between MFs and molecules of myosin can be disrupted implying that amyloplasts will move by their own weight. 

(Supported by CNES 793/00/8103.)