During plant embryo development, a single cell, the zygote, gives rise to a multicellular organism that contains diverse cell types and tissues organized properly into a basic body plan. The body organization, which is most easily recognized at the seedling stage, consists of two superimposed patterns, one along the apical-basal axis and the other perpendicular to this axis. The basic apical-basal pattern is a linear array of a few elements only: the shoot meristem, the cotyledons, the hypocotyl and the root system, including the root meristem. The radial pattern is comprised of the primary tissues; that is, the epidermis, the parenchymal ground tissue and the vascular system. Once the seedling organization is established, the root and shoot meristems are responsible for further plant development by tip growth (Steeves & Sussex 1989). The simplicity of the Arabidopsis seedling facilitates the analysis of pattern formation during embryogenesis. Depending on the experimental approach used, different levels of pattern formation are recognized; morphological patterns are the consequence of patterns formed at the cellular level which, in turn, may reflect the distribution patterns of molecules. The genetic control of basic body formation in the embryo has been extensively studied in Drosophila where relatively few genes are specifically involved in establishing the basic body organization (Ingham 1988; Niisslein-Volhard 1991). Although no comparable analysis has been done in any plant species yet, embryo development has been genetically studied (Sheridan & Clark 1993; Nagato et al. 1989; Meinke 1985). Progress has recently been made in the genetic dissection of pattern formation in the Arabidopsis embryo, making the isolation of relevant genes feasible (Mayer et al. 1991).