Visual examination of roots from 12,000 mutagenized Arabidopsis seedlings has led to the identification of more'. than 40 mutants impaired in root hair morphogenesis. Mutants from four phenotypic classes have been characterized in detail, and genetic tests show that these result from single nuclear recessive mutations in four different genes designated RHD1, RHDP, RHD3, and RHD4. The phenotypic analysis of the mutants and homozygous double mutants has led to a proposed model for root hair

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... pment and the stages at which the genes are normally required. The RHDl gene product appears to be necessary for proper initiation of root hairs, whereas the RHDS, RHD3, and RHD4 gene products are required for normal hair elongation. These results demonstrate that root hair development in Arabidopsis is amenable to genetic dissection and should prove to be a useful model system to study the molecular mechanisms governing cell differentiation in plants.