The ROOT HAIR DEFECTIVE3 Gene Encodes an Evolutionarily Conserved Protein with GTP-binding Motifs and is Required for Regulated Cell Enlargement in Arabidopsis

In plants, morphogenesis is largely determined by the orientation and extent of cell enlargement. To define the molecular mechanisms regulating plant cell enlargement, we have conducted a molecular genetic analysis of the ROOT HAIR DEFECTIVE3 (RHD3) gene of Arabidopsis thaliana. Mutations affecting the RHD3 gene were found to alter cell size, but not cell number, in tissues throughout the plant. Genetic and physiological analyses suggest that the RHD3 gene is not required for proper cell type specification, and it is likely to act downstream of the hormones auxin and ethylene. The RHD3 gene was cloned by a T-DNA tagging method and confirmed by the molecular complementation of the rhd3 mutant phenotype and by the analyses of six rhd3 mutant alleles. Consistent with the global effects of the rhd3 mutations, the RHD3 gene is expressed in all major plant organs. The deduced RHD3 product is a novel 89-kD polypeptide with putative GTP-binding motifs near the amino terminus. RHD3-like genes were identified from a protozoan (Entamoeba histolytica), a fungus (Saccharomyces cerevisiae), and another plant species (Oryza sativa), with the sequence identity including the putative GTP-binding motifs. These results imply that the RHD3 protein is a member of a new class of GTP-binding proteins that is widespread in eukaryotes and required for regulated cell enlargement.