Loss-of-Function Mutations of ROOT HAIR DEFECTIVE3 Suppress Root Waving, Skewing, and Epidermal Cell File Rotation in Arabidopsis

Abstract

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...

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