11 Pattern Formation in the Embryo

Abstract

Constructing a multicellular organism takes more than cell division and cell differentiation. A characteristic body organization has to be established such that tissues and organs are arranged in a structurally and functionally meaningful context, or pattern. To generate the body pattern, the cells of the developing organism have to exchange, and respond to, information about their relative positions. Although in animals pattern formation is largely confined to embryogenesis, flowering plants establish their body pattern during two distinct phases of the life cycle. Starting from a single cell, the zygote, embryogenesis lays down a primary body organization that is expressed in the seedling. Although dicot and monocot types can be distinguished, the body organization of the seedling is remarkably uniform across flowering plant species. Postembryonically, localized growth centers, or meristems, which occupy the opposite ends of the seedling axis, add new structures to the seedling body, producing species-specific adult forms (for review, see Steeves and Sussex 1989; Sussex 1989; Medford 1992). Since the meristems themselves originate in the embryo, the structurally simple seedling is a good choice for analyzing how essential features of the plant body organization are generated de novo.

The body organization of the seedling can formally be described as the superimposition of two patterns, an apical-basal pattern along the main axis of polarity and a radial pattern perpendicular to the axis (Fig. 1). The apical-basal pattern consists of a series of distinct elements which are, from top to bottom, shoot meristem, cotyledons, hypocotyl, embryonic root (radicle), and root...