The <i>Arabidopsis</i> Athb‐10 (GLABRA2) is an HD‐Zip protein required for regulation of root hair development

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<jats:p>Homeodomain‐leucine zipper (HD‐Zip) proteins are putative transcription factors identified only in plants. Related <jats:italic>Arabidopsis</jats:italic> homeobox genes, isolated by virtue of sequence conservation within the helix‐3 region of the homeodomain, fall into four families based on sequence similarity. This paper reports the characterization of Athb‐10, a 747 amino acid protein belonging to the fourth HD‐ZIP family. The studies indicate that, although less conserved, the leucine zipper of Athb‐10 can functionally replace that of Athb‐2 in an <jats:italic>in vitro</jats:italic> DNA‐binding assay. Gene mapping experiments and sequence comparison analysis revealed that Athb‐10 corresponds to GLABRA2, a homeodomain protein involved in trichome development. The mRNA expression analysis revealed that <jats:italic>Athb‐10/GLABRA2</jats:italic> is expressed not only in trichome‐bearing organs, but also in the root. The analysis of wild‐type and mutant plants showed that the <jats:italic>Athb‐10/GLABRA2</jats:italic> gene expression in the aerial part of the plant and in the root is affected by mutations at the <jats:italic>TTG</jats:italic> locus. Morphological analysis of the <jats:italic>gl2‐1</jats:italic> mutant revealed that the gene is necessary not only for local outgrowth of the trichome, but also for the regulation of root hair development in a subset of epidermal cells. Interestingly, the development of root hair cells in a position normally occupied by non‐hair cells is dependent upon the ethylene regime in which the <jats:italic>gl2‐1</jats:italic> plants are grown. Sequence analysis of the <jats:italic>gl2‐1</jats:italic> allele revealed that the mutant gene encodes a truncated protein that might still retain a partial activity responsible for the formation of aborted trichomes and for the ethylene‐dependent regulation of root hair formation.</jats:p>

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