Background: Cell position rather than cell lineage governs most aspects of development in plants. However, the nature and the origin of positional information remains elusive. Animal epidermal patterning relies in many cases on positional information provided by cell-cell communication. The epidermal layer of the Arabidopsis root is made of alternating files of two cell types and thus presents a simple pattern to study positional mechanisms.
Results: Clonal analysis of the root epidermis in combination with molecular and morphological markers has shown that cell fate is determined by position relative to the underlying cell layer, the cortex. The epidermal pattern appears to be organised during embryogenesis. Fate is not fixed in the developing root, though, as cells that move into a position previously occupied by neighbour cells ablated using laser microsurgery change fate. In contrast, cell fate is not altered when communication with living neighbour cells is impaired. Precise mapping of the influence of the position of extracellular cues on cell fate has shown that domains of positional information are organised with well-defined boundaries.
Conclusions: Cell-fate specification in the root epidermis relies on positional information that is organised in stable domains with sharp boundaries. The epidermal pattern is defined during embryogenesis and positional information remains active in the root until the initiation of cell morphogenesis. The origin of some positional cues might be extracellular.