Different α-tubulin cDNA sequences fused in an antisense orientation to a CaMV 35S promoter were introduced into Arabidopsis thaliana plants. Several independent transgenic lines that showed a moderate but clear reduction of α-tubulin gene expression (TUA6/AS lines) were obtained and phenotypically characterized. Although no apparent abnormalities were detected in the aerial parts of TUA6/AS plants, root development was severely affected. Cells in TUA6/AS root tips were found to contain aberrant microtubular structures, to expand abnormally and to be unable to undergo regular cell division. These cellular defects caused a dramatic radial expansion of the root tip and inhibited root elongation. In addition, TUA6/AS roots displayed ectopic formation of root hairs, root hair branching and a reduced ability to respond to gravitropic challenges. Our results contribute to an improved understanding of the different roles microtubules play during root development and demonstrate that reverse genetics is a powerful tool to analyze cytoskeletal functions during plant organogenesis.