THE HOMEOBOX GENE GLABRA-2 IS REQUIRED FOR POSITION-DEPENDENT CELL-DIFFERENTIATION IN THE ROOT EPIDERMIS OF ARABIDOPSIS-THALIANA

The role of the Arabidopsis homeobox gene, GLABRA 2 (GL2), in the deve lopment of the root epidermis has been investigated, The wild-type epi dermis is composed of two cell types, root-hair cells and hairless cel ls, which are located at distinct positions within the root, implying that positional cues control cell-type differentiation. During the dev elopment of the root epidermis, the differentiating root-hair cells (t richoblasts) and the differentiating hairless cells (atrichoblasts) ca n be distinguished by their cytoplasmic density, vacuole formation, an d extent of elongation, We have determined that mutations in the GL2 g ene specifically alter the differentiation of the hairless epidermal c ells, causing them to produce root hairs, which indicates that GL2 aff ects epidermal cell identity, Detailed analyses of these differentiati ng cells showed that, despite forming root hairs, they are similar to atrichoblasts of the wild type in their cytoplasmic characteristics, t iming of vacuolation, and extent of cell elongation. The results of in situ nucleic acid hybridization and GUS reporter gene fusion studies show that the GL2 gene is preferentially expressed in the differentiat ing hairless cells of the wild type, during a period in which epiderma l cell identity is believed to be established, These results indicate that the GL2 homeodomain protein normally regulates a subset of the pr ocesses that occur during the differentiation of hairless epidermal ce lls of the Arabidopsis root, Specifically, GL2 appears to act in a cel l-position-dependent manner to suppress hair formation in differentiat ing hairless cells.