Fundamental Concepts in the Embryogenesis of Dicotyledons: A Morphological Interpretation of Embryo Mutants.

Embryogenesis represents the critical stage in the develop? ment of a plant at which its basic organization and body plan have their inception. Therefore, in the effort to understand the regulation of plant development, molecular geneticists have recently focused considerable attention on the search for genes expressed during the embryogenesis of plant model systems (Mayer et al., 1991; Meinke, 1991b; Lindsey and Topping, 1993; West and Harada, 1993; Goldberg et al., 1994). This molecular approach toward plant embryogene? sis has been given even greater impetus by the significant advances made in the isolation of embryonic genes in ani? mal model systems, such as in Drosophila (Nusslein-Volhard and Wieschaus, 1980; Nusslein-Volhard, 1991) and zebrafish (Kane et al., 1996; Solnica-Krezel et al., 1996). As a consequence of these noteworthy successes in animal embryo? genesis, there has been a natural tendency for plant biologists to follow suit and interpret molecular genetic data in terms of animal developmental models (Jurgens et al., 1991; Mayer et al., 1991). One problem arising from this parallel strategy toward the study of plant and animal embryogenesis is that plant embryos bear a significantly different relationship to their adult forms than do animal embryos. This situation is further complicated by the history of the field of plant embryology, in which certain perspectives have led to less fundamental, even ambiguous characterizations of embryo morphogenesis. For example, traditional studies of plant embryogenesis have placed great emphasis on detailed characterizations of cell lineages in the earliest stages of plant embryogenesis (Johansen, 1950; Natesh and Rau, 1984). This kind of em? phasis was usually at the expense of the more general fea? tures of embryo morphogenesis, such as the time at which the meristems of the shoot and root are first organized and begin to function. We demonstrate that it is the morphology of the embryo that determines the cell lineage patterns rather than vice versa.