Hormones act downstream of TTG and GL2 to promote root hair outgrowth during epidermis development in the Arabidopsis root.

The Arabidopsis root produces a position-dependent pattern of hair-bearing and hairless cell types during epidermis development. Five loci (TRANSPARENT TESTA GLABRA [TTG], GLABRA2 [GL2], ROOT HAIR DEFECTIVE6 [RHD6], CONSTITUTIVE TRIPLE RESPONSE1 [CTR1], and AUXIN RESISTANT2 [AXR2]) and the plant hormones ethylene and auxin have been reported to affect the production of root hair and hairless cells in the Arabidopsis root. In this study, genetic, molecular, and physiological tests were employed to define the roles of these loci and hormones. Epistasis tests and reporter gene studies indicated that the hairless cell-promoting genes TTG and GL2 are likely to act early to negatively regulate the ethylene and auxin pathways. Studies of the developmental timing of the hormone effects indicated that ethylene and auxin pathways promote root hair outgrowth after cell-type differentiation has been initiated. The genetic analysis of ethylene-and auxin-related mutations showed that root hair formation is influenced by a network of hormone pathways, including a partially redundant ethylene signaling pathway. A model is proposed in which the patterning of root epidermal cells in Arabidopsis is regulated by the cell position-dependent action of the TTG/GL2 pathway, and the ethylene and auxin hormone pathways act to promote root hair outgrowth at a relatively late stage of differentiation.

The homeobox gene GLABRA2 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 development of the root epidermis has been investigated. The wild-type epidermis is composed of two cell types, root-hair cells and hairless cells, which are located at distinct positions within the root, implying that positional cues control cell-type differentiation. During the development of the root epidermis, the differentiating root-hair cells (trichoblasts) and the differentiating hairless cells (atrichoblasts) can be distinguished by their cytoplasmic density, vacuole formation, and extent of elongation. We have determined that mutations in the GL2 gene specifically alter the differentiation of the hairless epidermal cells, causing them to produce root hairs, which indicates that GL2 affects epidermal cell identity. Detailed analyses of these differentiating cells showed that, despite forming root hairs, they are similar to atrichoblasts of the wild type in their cytoplasmic characteristics, timing 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 differentiating hairless cells of the wild type, during a period in which epidermal cell identity is believed to be established. These results indicate that the GL2 homeodomain protein normally regulates a subset of the processes that occur during the differentiation of hairless epidermal cells of the Arabidopsis root. Specifically, GL2 appears to act in a cell-position-dependent manner to suppress hair formation in differentiating hairless cells.

The TTG gene is required to specify epidermal cell fate and cell patterning in the Arabidopsis root.

The control of cell fate was investigated in the root epidermis of Arabidopsis thaliana. Two distinct types of differentiated epidermal cells are normally present: root-hair-bearing cells and hairless cells. In wild-type Arabidopsis roots, epidermal cell fate was found to be correlated with cell position, with root-hair cells located over radial walls between cortical cells, and with hairless cells located directly over cortical cells. This normal positional relationship was absent in ttg (transparent testa glabrous) mutants (lacking trichomes, anthocyanins, and seed coat mucilage); epidermal cells in all positions differentiate into root-hair cells. The opposite condition was generated in roots of transgenic Arabidopsis expressing the maize R (R-Lc) gene product (a putative TTG homologue) under the control of a strong promoter (CaMV35S), which produced hairless epidermal cells in all positions. In both the ttg and R-expressing roots, epidermal cell differentiation was affected at an early stage, prior to the onset of cell elongation or root-hair formation. The ttg mutations were also associated with abnormalities in the morphology and organization of cells within and surrounding the root apical meristem. The results indicate that alterations in TTG activity cause developing epidermal cells to misinterpret their position and differentiate into inappropriate cell types. This suggests that, in wild-type roots, TTG provides, or responds to, positional signals to cause differentiating epidermal cells that lie over cortical cells to adopt a hairless cell fate.

The rhd6 Mutation of Arabidopsis thaliana Alters Root-Hair Initiation through an Auxin- and Ethylene-Associated Process.

Root-hair initiation in Arabidopsis thaliana provides a model for studying cell polarity and its role in plant morphogenesis. Root hairs normally emerge at the apical end of root epidermal cells, implying that these cells are polarized. We have identified a mutant, rhd6, that displays three defects: (a) a reduction in the number of root hairs, (b) an overall basal shift in the site of root-hair emergence, and (c) a relatively high frequency of epidermal cells with multiple root hairs. These defects implicate the RHD6 gene in root-hair initiation and indicate that RHD6 is normally associated with the establishment of, or response to, root epidermal cell polarity. Similar alterations in the site of root-hair emergence, although less extreme, were also discovered in roots of the auxin-, ethylene-, abscisic acid-resistant mutant axr2 and the ethylene-resistant mutant etr1. All three rhd6 mutant phenotypes were rescued when either auxin (indoleacetic acid) or an ethylene precursor (1-aminocyclopropane-1-carboxylic acid) was included in the growth medium. The rhd6 root phenotypes could be phenocopied by treating wild-type seedlings with an inhibitor of the ethylene pathway (aminoethoxyvinylglycine). These results indicate that RHD6 is normally involved in directing the selection or assembly of the root-hair initiation site through a process involving auxin and ethylene.

Identification of two regions from the Drosophila decapentaplegic gene required for embryonic midgut development and larval viability.

The Drosophila decapentaplegic (dpp) gene, a member of the transforming growth factor-beta family, is required for dorsal/ventral pattern formation and midgut and imaginal disk development. We have identified a 3-kb upstream regulatory region necessary for dpp expression in the visceral mesoderm of the gastric caeca primordia and a second 2.5-kb upstream regulatory region necessary for dpp expression in the midgut visceral mesoderm corresponding to a portion of abdominal segments 1 and 2 (parasegment 7). These regulatory regions act over a distance of up to 10-kb on all four of the dpp promoters examined. Absence of dpp expression in the gastric caeca primordia caused defective development of the gastric caeca and a concomitant partial reduction in larval and pupal viability. Absence of dpp expression in the visceral mesoderm of parasegment 7 caused a reduction in the length of the central portion of the larval gut and a change in the morphology of the midgut cells in this region but had little effect on the survival of the animals to the adult stage. However, a larval lethal phenotype was observed when both the central portion of the larval midgut and the gastric caeca were defective.

Pattern-specific expression of the Drosophila decapentaplegic gene in imaginal disks is regulated by 3' cis-regulatory elements.

The pattern of structures on most of the adult cuticle of Drosophila is determined in the larval imaginal disks. The Drosophila growth factor homolog decapentaplegic (dpp) is believed to participate in pattern formation in imaginal disks, primarily along what will become the proximal-to-distal axis of adult appendages. We report that dpp expression in wing, leg, and eye-antennal imaginal disks is localized to a band of cells along the presumptive proximal-to-distal axis. The pattern and level of dpp expression in imaginal disks is affected by mutant lesions that remove 3' cis-regulatory sequences. We demonstrate that one portion of the 3' cis-regulatory region contains regulatory elements sufficient to activate gene expression in a subset of the cells that normally express dpp in the imaginal disks, allowing rescue of dpp mutant phenotypes. We propose that the complete dpp expression pattern is generated by an array of 3' regulatory elements that differ in their potency in specific disks and in certain positions within a disk. The identification of the factors that activate these elements should provide clues as to how positional information is encoded in imaginal disks.