NEW EMBO MEMBER'S REVIEW: Apical-basal pattern formation in Arabidopsis embryogenesis

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Abstract
Embryogenesis transforms the fertilized egg cell into a multicellular organism. In higher animals, the mature embryo is a miniature variant of the adult animal, and whatever changes may take place during post‐embryonic development, they occur within the confines of the body organization established during embryogenesis. By contrast, higher plant embryogenesis generates a juvenile form, the seedling, which lacks most species‐specific features of the adult plant. Post‐embryonic development originates from two primary meristems, stem‐cell systems that occupy opposite ends of the main body axis. The primary shoot meristem at the top end is the source of cells for new organs, such as leaves, and secondary shoot meristems, including flowers. The primary root meristem at the bottom end produces cells for extension growth of the primary root. In addition, root branches are initiated from specific cell groups within the primary root. These primordia recapitulate radial patterning and root meristem establishment as occurs in embryogenesis (Malamy and Benfey, 1997). Thus, the considerable increase in architectural complexity during post‐embryonic development is contingent upon the basic body organization laid down during embryogenesis. Formally, the seedling body organization can be viewed as a superimposition of two patterns: an apical–basal pattern along the main axis of polarity and a radial pattern across the axis (Figure 1). Apical–basal pattern elements are, from top to bottom: shoot meristem, one or two cotyledons (embryonic leaves), hypocotyl (embryonic stem), radicle (embryonic root) and root meristem. The radial pattern consists of tissue layers that are arranged concentrically from the periphery to the centre: epidermis, cortex and endodermis (both derived from ground tissue), pericycle and vascular tissue (xylem and phloem). The origin of seedling structures can be traced back to cell groups in the young embryo, due to the highly invariant pattern of cell division in Arabidopsis early embryogenesis (Figure 1; for …