lefty1 and lefty2 are semi-dominant left-handed helical growth mutants of Arabidopsis thaliana, which result from identical dominant-negative amino acid substitutions in alpha-tubulin 6 and alpha-tubulin 4, respectively. Here we characterized the expression patterns of the affected tubulin genes and the phenotypes of the lefty double mutant to address increasing effects of microtubule defects on cell morphogenesis. Both tubulin genes were expressed ubiquitously in examined tissue and cell types, and the alpha-tubulin 2/4/6 subfamily transcripts predominated over other alpha-tubulin transcripts in Arabidopsis seedlings. The lefty double mutant seedlings showed helical growth in hypocotyls and radial cell expansion in the root elongation zone where mutant cortical microtubule arrays were more fragmented and less well aligned than wild-type arrays. Branching of leaf trichomes was highly reduced. In adult mutant plants, anisotropic growth of anther filament cells was severely impaired. These results suggest that left-handed twisted elongation is an intermediate state that leads to full isotropic expansion as the cortical microtubules are increasingly destabilized.