THE ARABIDOPSIS ATHB-10 (GLABRA2) IS AN HD-ZIP PROTEIN REQUIRED FOR REGULATION OF ROOT HAIR DEVELOPMENT

Homeodomain-leucine zipper (HD-Zip) proteins are putative transcriptio n factors identified only in plants. Related Arabidopsis homeobox gene s, isolated by virtue of sequence conservation within the helix-3 regi on of the homeodomain, fall into four families based on sequence simil arity. This paper reports the characterization of Athb-10, a 747 amino acid protein belonging to the fourth HD-ZIP family. The studies indic ate that, although less conserved, the leucine zipper of Athb-10 can f unctionally replace that of Athb-2 in an in vitro DNA-binding assay. G ene mapping experiments and sequence comparison analysis revealed that Athb-10 corresponds to GLABRA2, a homeodomain protein involved in tri chome development. The mRMA expression analysis revealed that Athb-10/ GLABRA2 is expressed not only in trichome-bearing organs, but also in the root. The analysis of wild-type and mutant plants showed that the Athb-10/GLABRA2 gene expression in the aerial part of the plant and in the root is affected by mutations at the TTG locus. Morphological ana lysis of the gl2-1 mutant revealed that the gene is necessary not only for local outgrowth of the trichome, but also for the regulation of r oot hair development in a subset of epidermal cells. Interestingly, th e development of root hair cells in a position normally occupied by no n-hair cells is dependent upon the ethylene regime in which the gl2-1 plants are grown. Sequence analysis of the gl2-1 allele revealed that the mutant gene encodes a truncated protein that might still retain a partial activity responsible for the formation of aborted trichomes an d for the ethylene-dependent regulation of root hair formation.