The TTG Gene Is Required to Specify Epidermal Cell Fate and Cell Patterning in the Arabidopsis Root

Abstract

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 ROOT HAIR DEFECTIVE3 gene encodes an evolutionarily conserved protein with GTP-binding motifs and is required for regulated cell enlargement in Arabidopsis.

“…As one test, we analyzed the distribution of cell types in the root epidermis, which normally produces a position-dependent pattern of roothair-bearing cells and hairless cells (Dolan et al 1993;Galway et al 1994). More than 95% (56 of 58) of the To further test the possible role of RHD3 in cell type specification in the root epidermis, the relationship between the RHD3 gene and the early-acting TTG and GL2 genes was explored.…”

“…These mutations define genes that influence early stages of epidermal cell type specification (e.g., TTG and GL2; Galway et al 1994;, genes that affect the initiation of root hairs [e.g., ROOT HAIR DEFEC-TIVES {RHD6h Masucci and Schiefelbein 1994], and genes that affect root hair enlargement (e.g., RHDl-RHD4; Schiefelbein and Somerville 1990). Mutations affecting the RHD3 gene are unique because they alter the enlargement of root hairs as well as the root proper,-the root hairs exhibit a short and wavy morphology, and the roots possess a reduced length (Schiefelbein and Somerville 1990).…”

Arabidopsis TRANSPARENT TESTA GLABRA2Is Directly Regulated by R2R3 MYB Transcription Factors and Is Involved in Regulation ofGLABRA2Transcription in Epidermal Differentiation

“…Several genes involved in controlling root hair patterning have been identified. WERE-WOLF (WER) encodes an R2R3 MYB transcription factor, TRANSPARENT TESTA GLABRA1 (TTG1) encodes a WD40 protein, and GLABRA2 (GL2) encodes a homeodomain-leucine zipper (HD-ZIP) transcription factor, all of which are required to specify hairless cells (Galway et al, 1994;Rerie et al, 1994;Di Cristina et al, 1996;Masucci et al, 1996;Lee and Schiefelbein, 1999;Walker et al, 1999). When these genes are mutated, ectopic root hairs are formed from N cells.…”

Root hair formation at the root-hypocotyl junction in CPC-LIKE MYB double and triple mutants ofArabidopsis

“…8 The WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1) also prompts trichome differentiation and inhibits root hair differentiation. 9,10 The R3-type MYB transcription factor CAPRICE (CPC), has been identified as a key regulator of epidermal cell fate determination in Arabidopsis. 11 .…”

“…18,[21][22][23][24] This transcriptional complex regulates GL2 gene expression in both root hair and trichome differentiation in Arabidopsis. 1,4,9,11,25 The TTG1-GL3/ EGL3-WER transcriptional complex promotes GL2 expression to induce a hairless cell differentiation. 26,27 CPC disrupts this transcriptional complex by competing with WER, thus negatively regulating the expression of GL2 and resulting in root hair formation.…”