Abstract
The patterning of root-hair and non-hair epidermal cells in the Arabidopsis root is governed by a network of transcriptional regulators. The central MYB-bHLH-WD40 (MBW) transcriptional complex includes the WD40-repeat protein TRANSPARENT TESTA GLABRA1 (TTG1). To clarify the role of TTG1, we describe the identification and analysis of two new ttg1 mutants. Each of these mutants contains a single nucleotide change in the TTG1 gene, which causes a single amino-acid substitution in the predicted TTG1 protein and alters root-hair pattern formation. Surprisingly, these new ttg1 mutants exhibit decreased root-hair formation, particularly in the caprice (cpc) mutant background, rather than increased root-hair formation as reported for strong ttg1 mutants. We show that the unique phenotype of these mutants is due to differential effects of the altered TTG1 proteins on target gene expression, associated with a weakened ability to interact with its GLABRA3 bHLH partner. These findings demonstrate the crucial role of TTG1 for the appropriate balance of target gene activation to achieve the proper pattern of epidermal cell types during Arabidopsis root development.
Highlights
The development of the Arabidopsis root epidermis has been widely used as a simple model for understanding cell-type specification and pattern formation in plants (Duckett et al, 1994; Schiefelbein et al, 2009)
We observed a 3:1 segregation ratio of the cpc-like phenotype to the double mutant-like phenotype among offspring from a selfpollinated cpc-1/cpc-1 ttg1-23/+ population, indicating that the ttg1-23 mutation is recessive. These results indicate that the new cpc-1 enhancing mutation acts at an early developmental stage to cause a change in root epidermal cell fate
The cpc-1 ttg1-9, cpc-1 ttg1-23, and cpc-1 ttg1-24 double mutants produce a similar number of trichomes and fraction of trichome clusters, but significantly less than the cpc-1 mutant (Table 2). These results show that the ttg1-9, ttg1-23, and ttg124 mutations generate similar effects on epidermal cell differentiation, which are distinct from the effects of the strong ttg1-1 and ttg1-13 mutations
Summary
The development of the Arabidopsis root epidermis has been widely used as a simple model for understanding cell-type specification and pattern formation in plants (Duckett et al, 1994; Schiefelbein et al, 2009). The basic component of this network is a central complex containing a WD40-repeat protein encoded by TRANSPARENT TESTA GLABRA 1 (TTG1) (Galway et al, 1994; Walker et al, 1999), bHLH proteins encoded by the functionally redundant GLABRA3 and ENHANCER OF GLABRA3 (GL3/EGL3) (Payne et al, 2000; Bernhardt et al, 2003, 2005), and an R2R3-type. MYB protein encoded by WEREWOLF (WER) (Lee and Schiefelbein, 1999). This MYB-bHLH-WD40 (MBW) complex preferentially accumulates in non-hair cells where it directly promotes transcription of GLABRA 2 (GL2) (Di Cristina et al, 1996; Masucci et al, 1996). Null mutations of WER, GL3/EGL3, and TTG1, or GL2 cause a lack of non-hair cells and exhibit a hairy root phenotype
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