A position-dependent pattern of epidermal cell types is produced during the development of the Arabidopsis

A fundamental feature of development in multicellular organisms is the specification and patterning of distinct cell types. An important issue in cell specification relates to the developmental origin of the mechanisms that establish celltype patterns. Of particular interest is the relationship between the origin of cell patterning mechanisms and the development of the anatomy and identity of the resident tissues and organs during embryogenesis (Davidson et al., 1998; Simpson et al., 1999). The development of a position-dependent pattern of epidermal cell types in the Arabidopsis root and hypocotyl provides a simple model system to explore the control of cell patterning in plants. Epidermal cells in contact with two underlying cortical cells (i.e. cells over an anticlinal cortical cell wall; the ‘H’ cell position) preferentially differentiate as root hair cells in the root and stomata in the hypocotyl, whereas cells in contact with a single cortical cell (i.e. cells over a periclinal cortical cell wall; the ‘N’ cell position) differentiate into non-hair cells in the root and non-stomatal cells in the hypocotyl (Dolan et al., 1993; Dolan et al., 1994; Galway et al., 1994; Berger et al., 1998b; Hung et al., 1998). This cell type pattern implies that the underlying cortical cells provide, or influence the presentation of, an inductive signal that determines epidermal cell fate. Several genes have been identified that influence the specification of the epidermal cell types in the root and hypocotyl of Arabidopsis. The TRANSPARENT TESTA GLABRA (TTG), WEREWOLF (WER) and GLABRA2 (GL2) genes are each required for proper specification of the N cell (non-hair or non-stomatal cell) fate (Galway et al., 1994; Masucci et al., 1996; Hung et al., 1998; Lee and Schiefelbein, 1999). The WD40 protein encoded by TTG (Walker et al., 1999) and the myb transcription factor encoded by WER (Lee and Schiefelbein, 1999) are positive regulators of the GL2 gene (Hung et al., 1998; Lee and Schiefelbein, 1999). The GL2 gene encodes a homeodomain-leucine zipper (HD-Zip) protein (Rerie et al., 1994; DiCristina et al., 1996) and is preferentially expressed in the N-cell position in the developing epidermis of both the root and hypocotyl (Masucci et al., 1996; Hung et al., 1998). In contrast to these three genes, the CAPRICE (CPC) gene is required to specify the hair cell fate in the root epidermis and encodes a small myb protein that negatively regulates GL2 (Wada et al., 1997). Thus, epidermal cell fate appears to be largely determined by the regulated expression of the downstream N-cell-specific transcription factor GL2. The similar pattern of epidermal cell types in the root and hypocotyl implies that the patterning mechanism originates during embryogenesis, because the epidermal tissue of the root and hypocotyl is derived from the same set of embryonic protodermal cells (Scheres et al., 1994). To define the 3697 Development 128, 3697-3705 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV0380