Pattern formation in plant embryogenesis: a reassessment

Gerd JurgensÈ Pattern formation in embryogenesis generates the basic body plan of ¯owering plants by establishing the diversity of position-dependent cell fates characteristic of the seedling. The concept of pattern formation, which was originally based on the analysis of Arabidopsis pattern mutants, is re-examined in the light of recent data that address the origin of the axis of polarity, the origin of the primary meristems as elements of the apical-basal pattern and the formation of the radial pattern of tissue layers. The available evidence from genetic, molecular and experimental approaches supports the notion that embryonic pattern formation occurs in steps involving communication between clonally unrelated cells. Key words: embryo r Arabidopsis r pattern formation r cell fate Q1998 Academic Press Ltd STARTING FROM THE reproductive single-cell stage, plant embryogenesis establishes the multicellular body organisation of a juvenile form, the seedling, that bears little resemblance to the adult. It is only during post-embryonic development that the meristems of the shoot and the root give rise to most structures of the adult plant. Nevertheless, the primary meristems are embedded in the body organisation of the ma-ture plant embryo or seedling, constituting the termi-nal elements of an apical]basal pattern along the main axis of polarity. The two meristems are sepa-rated from one another by a linear array of juvenile .structures consisting of cotyledons embryonic leaves, . hypocotyl embryonic stem and radicle embryonic.root. Another, radial pattern is apparent as a con-centric arrangement of cell layers in the stem]root axis of the seedling. These layers represent, from th