A Common Position-Dependent Mechanism Controls Cell-Type Patterning and GLABRA2 Regulation in the Root and Hypocotyl Epidermis of Arabidopsis1

Abstract

A position-dependent pattern of epidermal cell types is produced during root development in Arabidopsis thaliana. This pattern is reflected in the expression pattern of GLABRA2 (GL2), a homeobox gene that regulates cell differentiation in the root epidermis. GL2 promoter::GUS fusions were used to show that the TTG gene, a regulator of root epidermis development, is necessary for maximal GL2 activity but is not required for the pattern of GL2 expression. Furthermore, GL2-promoter activity is influenced by expression of the myc-like maize R gene (35S::R) in Arabidopsis but is not affected by gl2 mutations. A position-dependent pattern of cell differentiation and GL2-promoter activity was also discovered in the hypocotyl epidermis that was analogous to the pattern in the root. Non-GL2-expressing cell files in the hypocotyl epidermis located outside anticlinal cortical cell walls exhibit reduced cell length and form stomata. Like the root, the hypocotyl GL2 activity was shown to be influenced by ttg and 35S::R but not by gl2. The parallel pattern of cell differentiation in the root and hypocotyl indicates that TTG and GL2 participate in a common position-dependent mechanism to control cell-type patterning throughout the apicalbasal axis of the Arabidopsis seedling.

Root Hairs

“…As described above, GL2 expression initiates during the early heart stage of embryogenesis, where it rapidly assumes its N-cell-specific expression pattern (Lin and Schiefelbein, 2001). The embryonic and post-embryonic GL2 gene expression is influenced by the WER, GL3/EGL3, and TTG genes; wer mutations nearly abolish GL2 promoter activity, while ttg and gl3 egl3 mutations reduce GL2 promoter activity (Hung et al, 1998;Lee and Schiefelbein, 1999;Lin and Schiefelbein, 2001;Bernhardt et al, 2003). Because appropriate cell position-dependent GL2 expression is present in the ttg and gl3 egl3 mutants, but not the wer mutant, it implies that WER is most critical for conveying positional information for GL2 expression.…”

Tissue-Specific Profiling Reveals Transcriptome Alterations inArabidopsisMutants Lacking Morphological Phenotypes

“…Qualitative histochemical GUS staining of seedling roots was performed as previously described (Hung et al, 1998;Wang et al, 2002).…”

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

“…The bHLH transcription factors GLABRA3 (GL3) and ENHANCER OF GLABRA3 (EGL3) also function to specify hairless cells in a redundant manner, and ectopic root hairs form from N cells in gl3 egl3 double mutants (Payne et al, 2000;Bernhardt et al, 2003;Zhang et al, 2003). GL2 expression is promoted by WER, TTG1, GL3, and EGL3 in N cells (Hung et al, 1998;Lee and Schiefelbein, 1999;Bernhardt et al, 2003). GL3 and EGL3 interact with WER and TTG1, and the WER protein binds to two MYB binding sites in the GL2 promoter, suggesting that a transcriptional complex consisting of WER, GL3/EGL3, and TTG1 directly regulates GL2 transcription (Payne et al, 2000;Bernhardt et al, 2003;Zhang et al, 2003;Koshino-Kimura et al, 2005).…”