Little is known about how cell shape is controlled. We are using the morphogenesis of trichomes (plant hairs) on the plant Arabidopsis thaliana as a model to study how cell shape is controlled. Wild-type Arabidopsis trichomes are large, single epidermal cells with a stalk and three or four branches, whereas in zwichel (zwi) mutants the trichomes have a shortened stalk and only two branches. To further understand the role of the ZWI gene in trichome morphogenesis we have cloned the wild-type ZWICHEL (ZWI) gene by T-DNA tagging, and report here that it encodes a member of the kinesin superfamily of microtubule motor proteins. Kinesin proteins transport diverse cellular materials in a directional manner along microtubules. Kinesin-like proteins are characterized by a highly conserved ''head'' region that comprises the motor domain, and a nonconserved ''tail'' region that is thought to participate in recognition and binding of the appropriate cargo.Plant cell expansion is central to plant development. During plant cell differentiation, the relative rates of expansion of different regions of the cell wall determine the final shape of the cell. For most cell types the functional significance of cell shape is evident. For example, the special shape of stomatal guard cells permits the opening and closing of stomata, the shape of spongy mesophyll cells allows for efficient gas exchange, and the broadly expanded but thin shape of leaf epidermal cells increases the surface area of a leaf and allows penetration of light to the underlying photosynthetic mesophyll cells.Plant cell expansion results from a complex interplay of diverse processes. These processes involve the loosening of the cell wall, the maintenance of turgor pressure which is needed to drive cell expansion, and the deposition of new cell wall material (reviewed in refs.