To investigate how roots respond to directional cues, we characterized a T-DNA-tagged Arabidopsis mutant named sku5 in which the roots skewed and looped away from the normal downward direction of growth on inclined agar surfaces. sku5 roots and etiolated hypocotyls were slightly shorter than normal and exhibited a counterclockwise (lefthanded) axial rotation bias. The surface-dependent skewing phenotype disappeared when the roots penetrated the agar surface, but the axial rotation defect persisted, revealing that these two directional growth processes are separable. The SKU5 gene belongs to a 19-member gene family designated SKS ( SKU5 Similar) that is related structurally to the multiple-copper oxidases ascorbate oxidase and laccase. However, the SKS proteins lack several of the conserved copper binding motifs characteristic of copper oxidases, and no enzymatic function could be assigned to the SKU5 protein. Analysis of plants expressing SKU5 reporter constructs and protein gel blot analysis showed that SKU5 was expressed most strongly in expanding tissues. SKU5 was glycosylated and modified by glycosyl phosphatidylinositol and localized to both the plasma membrane and the cell wall. Our observations suggest that SKU5 affects two directional growth processes, possibly by participating in cell wall expansion.

Wild-type Arabidopsis roots develop a wavy pattern of growth on tilted agar surfaces. For many Arabidopsis ecotypes, roots also grow askew on such surfaces, typically slanting to the right of the gravity vector. We identified a mutant, wvd2-1, that displays suppressed root waving and leftward root slanting under these conditions. These phenotypes arise from transcriptional activation of the novel WAVE-DAMPENED2 (WVD2) gene by the cauliflower mosaic virus 35S promoter in mutant plants. Seedlings overexpressing WVD2 exhibit constitutive right-handed helical growth in both roots and etiolated hypocotyls, whereas the petioles of WVD2-overexpressing rosette leaves exhibit left-handed twisting. 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 transcripts are detectable by reverse transcriptase-polymerase chain reaction in multiple organs of wild-type plants. Its predicted gene product contains a conserved region named "KLEEK," which is found only in plant proteins. The Arabidopsis genome possesses seven other genes predicted to encode KLEEK-containing products. Overexpression of one of these genes, WVD2-LIKE 1, which encodes a protein with regions of similarity to WVD2 extending beyond the KLEEK domain, results in phenotypes that are highly similar to wvd2-1. Silencing of WVD2 and its paralogs results in enhanced root skewing in the wild-type direction. Our observations suggest that at least two members of this gene family may modulate both rotational polarity and anisotropic cell expansion during organ growth.The primary roots of Arabidopsis possess an intrinsic handedness to their growth, consistently forming counterclockwise coils as they elongate upon a horizontal surface of hard agar (Mirza, 1987). However, when the surface is positioned vertically, the downward growth behavior of roots dictated by gravitropism conflicts with this counterclockwise coiling tendency, resulting in a net direction of growth that is to the right of the gravity vector. It should be noted that, in this report, we follow the convention used in Simmons et al. (1995) and Rutherford and Masson (1996) to describe the direction of root coiling (clockwise or counterclockwise) and slanting (skewing: leftward or rightward of the vertical axis), as viewed through the agar medium.While slanting on vertical surfaces, root tips of wild-type Arabidopsis seedlings also exhibit a largely left-handed rotation around the net axis of growth, resulting in a moderate, left-handed twisting of the discrete cell files that make up the root epidermal layer. This left-handed preference in root tip rotation is associated with and may be responsible for the counterclockwise bias in root coiling and, by extension, rightward root slanting on vertical surfaces (Simmons et al., 1995; Rutherford and Masson, 1996). Most mu...

Wild-type Arabidopsis (Arabidopsis thaliana L. Heynh.) roots growing on a tilted surface of impenetrable hard-agar media adopt a wave-like pattern and tend to skew to the right of the gravity vector (when viewed from the back of the plate through the medium). Reversible root-tip rotation often accompanies the clockwise and counterclockwise curves that form each wave. These rotations are manifested by epidermal cell file rotation (CFR) along the root. Loss-of-function alleles of ROOT HAIR DEFECTIVE3 (RHD3), a gene previously implicated in the control of vesicle trafficking between the endoplasmic reticulum and the Golgi compartments, resulted in an almost complete suppression of epidermal CFR, root skewing, and waving on hard-agar surfaces. Several other root hair defective mutants (rhd2-1, rhd4-1, and rhd6-1) did not exhibit dramatic alterations in these root growth behaviors, suggesting that a generalized defect in root hair formation is not responsible for the surfacedependent phenotypes of rhd3. However, similar alterations in root growth behavior were observed in a variety of mutants characterized by defects in cell expansion (cob-1, cob-2, eto1-1, eto2-1, erh2-1, and erh3-1). The erh2-1 and rhd3-1 mutants differed from other anisotropic cell expansion mutants, though, by an inability to respond to low doses of the microtubule-binding drug propyzamide, which normally causes enhanced left-handed CFR and right skewing. We hypothesize that RHD3 may control epidermal CFR, root skewing, and waving on hard-agar surfaces by regulating the traffic of wall-or plasma membraneassociated determinants of anisotropic cell expansion.The primary roots of Arabidopsis (Arabidopsis thaliana L. Heynh.) seedlings display different growth behaviors depending on the conditions to which they are exposed. If grown within a homogenous environment (such as liquid or penetrable agar media), primary roots will grow downward in response to gravity (Blancaflor and Masson, 2003). When forced to grow on the surface of impenetrable hard-agar media, they will display more complex growth behaviors with characteristics dictated by the angle of the surface (Okada and Shimura, 1990). On surfaces tilted slightly forward, the roots of commonly used ecotypes will skew their growth to the right of the vertical when viewed through the agar medium, as per the established convention (Rutherford and Masson, 1996;Simmons et al., 1996). If the surface is tilted backward, roots will form waves in addition to skewing. This pattern results from the root tip pressing on the surface when gravitropism directs downward growth Shimura, 1990, 1992;Rutherford and Masson, 1996;Simmons et al., 1996;Thompson and Holbrook, 2004). Root waving is usually accompanied by a reversible rotation of the tip about its axis, which manifests itself by a twisting of epidermal cell files (cell file rotation [CFR]) along the root (Okada and Shimura, 1990). While curving to the right, roots tend to display left-handed CFR. By contrast, roots curving to the left display right-handed C...

No abstract