THE ROOT-HAIRLESS-1 GENE ENCODES A NUCLEAR-PROTEIN REQUIRED FOR ROOT HAIR INITIATION IN ARABIDOPSIS

The epidermis of Arabidopsis wild-type primary roots, in which some ce lls grow hairs and others remain hairless in a position-dependent mann er, has become an established model system to study cell differentiati on. Here we present a molecular analysis of the RHL1 (ROOT HAIRLESS 1) gene that, if mutated, prevents the formation of hairs on primary roo ts and causes a seedling lethal phenotype. We have cloned the RHL1 gen e by use of a T-DNA-tagged mutant and found that it encodes a protein that appears to be plant specific. The predicted RHL1 gene product is a small hydrophilic protein (38.9 kD) containing putative nuclear loca lization signals and shows no significant homology to any known amino acid sequence. We demonstrate that a 78-amino-acid sequence at its ami no terminus is capable of directing an RHL1-GFP fusion protein to the nucleus. The RHL1 transcript is present throughout the wild-type plant and in suspension culture cells, but in very low amounts, suggesting a regulatory function for the RHL1 protein. Structural evidence sugges ts a role for the RHL1 gene product in the nucleolus. We have examined the genetic relationship between RHL1 and GL2, an inhibitor of root h air initiation in non-hair cells. Our molecular and genetic data with double mutants, together with the expression analysis of a GL2 promote r-GUS reporter gene construct, indicate that the RHL1 gene acts indepe ndently of GL2.