Development of the leaf epidermis.

The leaf epidermis is essential to plant survival not only because of its protective role at the interface with the plant's environment but also because of crucial developmental functions. The protoderm is set aside early in embryogenesis, possibly in the zygote. Epidermal identity is determined by the interactions of a complex set of factors, including developmental phase of the plant, regional identity within the leaf, and axiality. For the most part, these characteristics appear to be specified by internal tissues. On the other hand, the epidermis has a key role in regulating organ growth and expansion; thus interactions between the epidermis and internal tissues regulate the overall leaf architecture. Overlying this is the specification of different cell types within the epidermis. Some aspects of this appear to involve interactions with internal tissues but the patterning of many epidermal cell types seems to occur within the two-dimensional field of the epidermis itself and to require both cell signaling and cell lineage dependent mechanisms. Genetic analyses have provided much of the insight into the underlying principles that regulate epidermal development and a number of molecules important for various aspects of the process have been identified. Yet, for the most part, our understanding of the molecular basis for each component of epidermal development is still rudimentary and we have not yet scratched the surface of understanding how these pieces are integrated. The emerging technologies of functional genomics will provide powerful tools for solving these problems and the near future is likely to produce rapid progress.