The rhd6 Mutation of Arabidopsis thaliana Alters Root-Hair Initiation through an Auxin- and Ethylene-Associated Process

Root-hair initiation in Arabidopsis thaliana provides a model for studying cell polarity and its role in plant morphogenesis. Root hairs normally emerge at the apical end of root epidermal cells, implying that these cells are polarized. We have identified a mutant, rhd6, that displays three defects: (a) a reduction in the number of root hairs, (b) an overall basal shift in the site of root-hair emergence, and (c) a relatively high frequency of epidermal cells with multiple root hairs. These defects implicate the RHD6 gene in root-hair initiation and indicate that RHD6 is normally associated with the establishment of, or response to, root epidermal cell polarity. Similar alterations in the site of root-hair emergence, although less extreme, were also discovered in roots of the auxin-, ethylene-, abscisic acid-resistant mutant axr2 and the ethylene-resistant mutant etr1. All three rhd6 mutant phenotypes were rescued when either auxin (indoleacetic acid) or an ethylene precursor (1-aminocyclopropane-1-carboxylic acid) was included in the growth medium. The rhd6 root phenotypes could be phenocopied by treating wild-type seedlings with an inhibitor of the ethylene pathway (aminoethoxyvinylglycine). These results indicate that RHD6 is normally involved in directing the selection or assembly of the root-hair initiation site through a process involving auxin and ethylene.