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First published online 19 November 2003
doi: 10.1242/dev.00880

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The bHLH genes GLABRA3 (GL3) andENHANCER OF GLABRA3 (EGL3) specify epidermal cell fate in the Arabidopsis root

¹ Department of Molecular, Cellular, and Developmental Biology, University of Michigan, 830 North University Avenue, Ann Arbor, MI 48109, USA
² Department of Biology, Yonsei University, Sinchon 134, Seoul 120-749, Korea
³ Institute for Cellular and Molecular Biology, University of Texas, Austin, TX 78712, USA

View larger version (52K): [in a new window]   Fig. 1. GL3 and EGL3 RNA accumulates in all major Arabidopsis organs. Reverse transcriptase (RT)-PCR analysis using GL3-, EGL3-or UBQ10-specific gene primers. Total RNA was isolated from the following tissues of wild-type (Columbia) plants: roots, hypocotyl and cotyledons, rosette leaves, cauline leaves, stems and flowers. 500 ng template RNA was used for each RT-PCR reaction.

View larger version (53K): [in a new window]   Fig. 2. The 35S::GL3 and 35S::EGL3 transgenes promote non-hair cell specification. Root phenotypes of four-day-old seedlings bearing the 35S::GL3 and 35S::EGL3 transgenes in the wild-type or the ttg-1 mutant backgrounds. The top set of panels shows upper regions of the roots (near the root-hypocotyl junction), and the bottom panels show lower regions of the roots (near the root apex).

View larger version (82K): [in a new window]   Fig. 3. The GL3 and EGL3 genes are required for non-hair cell specification. Root phenotypes of four-day-old seedlings bearing the indicated gl3 and/or egl3 mutations. In each composite, the top panels show upper regions of the roots, and the bottom panels show lower regions of the roots.

View larger version (67K): [in a new window]   Fig. 4 . The GL3 and EGL3 genes act early during epidermal cell fate specification. Transverse sections taken from the meristematic region of wild-type, gl3 egl3 and 35S::EGL3 roots indicate the relative vacuolation and cytoplasmic density in the H and N cell positions. Scale bar: 25 µm.

View larger version (44K): [in a new window]   Fig. 5. GL2 gene expression is regulated by GL3 and EGL3. Four-day-old roots from plants harboring the GL2::GUS transgene and the indicated mutation(s) were incubated with X-gluc. The gl3 egl3 root labeled `long stain' was incubated for an extended period of time (20 hours rather than 1 hour).

View larger version (40K): [in a new window]   Fig. 6. CPC gene expression is regulated by GL3 and EGL3. Seedling roots bearing the CPC::GUS reporter and the indicated mutation(s) were exposed to the X-gluc substrate for 12 hours. This reporter is also expressed in the developing stele near the root tip, but this expression is not associated with the role of CPC in epidermal development (Wada et al., 2002).

View larger version (56K): [in a new window]   Fig. 7. The proposed role of GL3 and EGL3 in epidermal cell specification. In the N cell position, there is a relatively high level of WER relative to CPC, which enables a WER-GL3/EGL3 complex to form and promote GL2 and CPC transcription. In the H position, a relatively high level of CPC leads predominantly to formation of the inactive CPC-GL3/EGL3 complex. Proteins shown in white ovals are at low concentrations. See text for detailed discussion.