1. Dynamique du Cytosquelette, Laboratoire d'Enzymologie et Biochimie Structurales, Centre National de la Recherche Scientifique, 91198 Gif-sur-Yvette Cedex, France; Laboratory of Plant Cell Biology, Institute of Molecular Agrobiology, National University of Singapore, Singapore 118240; and Laboratory of Plant Molecular Biology, Rockefeller University, New York 10021