Embryonic control of epidermal cell patterning in the root and hypocotyl of Arabidopsis

A position-dependent pattern of epidermal cell types is produced during the development of the Arabidopsis seedling root and hypocotyl. To understand the origin and regulation of this patterning mechanism, we have examined th e embryonic expression of the GLABRA2 (GL2) gene, which encodes a cell-type -specific transcription factor. Using in situ RNA hybridization and a sensi tive GL2::GFP reporter, we discovered that a position-dependent pattern of GL2 expression is established within protodermal cells at the heart stage a nd is maintained throughout the remainder of embryogenesis. In addition, we show that an exceptional GL2 expression character and epidermal cell patte rn arises during development of the root-hypocotyl junction, which represen ts an anatomical transition zone. Furthermore, we find that two of the gene s regulating seedling epidermal patterning, TRANSPARENT TESTA GLABRA (TTG) and WEREWOLF (WER), also control the embryonic GL2 pattern, whereas the CAP RICE (CPC) and GL2 genes are not required to establish this pattern. These results indicate that position-dependent patterning of epidermal cell types begins at an early stage of embryogenesis, before formation of the apical meristems and shortly after the cellular anatomy of the protoderm and outer ground tissue layer is established. Thus, epidermal cell specification in the Arabidopsis seedling relies on the embryonic establishment of a pattern ing mechanism that is perpetuated postembryonically.