Green beginnings - pattern formation in the early plant embryo.

Embryogenesis in plants transforms the zygote into a relatively simple structure, the seedling, which contains all tissues and organs that later form the mature plant body. Despite a profound diversity in cell division patterns among plant species, embryogenesis yields remarkably homologous seedling architectures. In this review, we describe the formative events during plant embryogenesis and discuss the molecular mechanisms that regulate these processes, focusing on Arabidopsis. Even though only a relatively small number of factors are known that regulate each patterning step, a picture emerges where locally acting transcription factors and intercellular signaling contribute to the specification and spatio-temporal coordination of the various cell types in the embryo. Notably, several patterning processes are controlled by the plant hormone auxin. Most regulators that were identified in Arabidopsis have orthologs in other sequenced plant genomes, and several of these are expressed in similar patterns. Therefore, it appears that robust conserved mechanisms may underlie pattern formation in plant embryos.