| Arabidopsis thaliana roots develop in a wavy growth pattern on tilted agar surfaces. This wavy root growth behavior is the result of repetitive alternations in the direction of root coiling, and is associated with similar reversions in the polarity of root axial rotation. Due to an intrinsic bias toward counterclockwise coiling and left-handed axial rotation, the roots of most A. thaliana ecotypes consistently skew to the right on non-horizontal agar surfaces, away from the gravity vector. The process by which roots are capable of reversing their handedness of root tip rotation in response to mechanical stimuli remains unclear. To further characterize the mechanisms underlying root waving, we have identified two mutants, wvd2-1 and wvd6 (an allele of rhd3), which display distinct altered root growth phenotypes on inclined surfaces.; The roots of wvd2-1 seedlings skew to the left and do not wave on inclined agar surfaces. These phenotypes arise from transcriptional activation of the novel WAVE-DAMPENED2 (WVD2) gene by the Cauliflower Mosiac Virus 35S promoter in mutant plants. Seedlings overexpressing WVD2 exhibit constitutive right-handed helical root growth. Moreover, the anisotropic expansion of cells is impaired, resulting in the formation of shorter and stockier organs. In roots, the phenotype is accompanied by a change in the arrangement of cortical microtubules within peripheral cap cells and cells at the basal end of the elongation zone. WVD2 is predicted to encode a small, highly hydrophilic protein containing the novel KLEEK motif. Simultaneous co-suppression of WVD2 and two WVD2-like genes in Arabidopsis results in an increased rightward root slanting phenotype.; The roots of wvd6 exhibit strong suppression of both waving and skewing. The mutation involves a T-DNA-induced reciprocal translocation event that disrupts RHD3, a gene previously implicated in the regulation of root elongation. Interestingly, wvd6 roots are resistant to the skew-inducing effects of propyzamide, indicating that they are strongly impaired in their ability to undergo helical growth. We hypothesize that the decreased helical growth phenotype of wvd6 may derive from a defect in the vesicular trafficking of components that regulate rotational polarity to the plasma membrane and/or cell wall. |
