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A major problem in plant development is to unravel the mechanisms operating during embryogenesis that enable a plant to specify its body plan and tissue differentiation patterns. Although progress with a variety of animal systems has been spectacular in this regard (1), a detailed understanding of the events that govern plant embryo formation has yet to be realized. One obstacle in achieving this goal is the location of embryos within the plant and their relative inaccessibility to experimental manipulation, particularly at the early stages of embryogenesis. In flowering plants, reproductive processes occur within floral organs (Fig. 1) (2). (Fig. 1 omitted) The egg cell is present in the ovule, a multicellular structure that is buried beneath several cell layers of the pistil, the female reproductive organ (2-4). Because egg cell formation, fertilization, and embryogenesis occur within the pistil, it has been difficult to dissect the major events that take place during the early stages of higher plant development.
Recently, it has become feasible to isolate plant eggs and fertilize them in vitro in order to investigate the initial events of plant embryogenesis (5). In addition, genetic approaches have been used to identify genes required for various embryogenic processes, including pattern formation (6, 7). Genetic manipulation of Arabidopsis thaliana, by both chemical mutagenesis (8-12) and insertional mutagenesis (13-15), has identified a large number of mutants that are blocked at different stages of embryogenesis. In this review we outline the major insights that have been derived from studies of Arabidopsis embryo mutants, and we summarize gene transcription experiments in other plants that provide new information about the processes regulating higher plant embryogenesis. Both the genetic and molecular approaches suggest that a plant embryo has a modular structure and consists of several regions that form autonomously during embryogenesis.
EMBRYOS BEGIN THE DIPLOID PHASE OF THE HIGHER PLANT LIFE CYCLE
The flowering plant life cycle is divided into haploid and diploid generations that are dependent on each other (Fig. 1) (2, 16-18). The haploid, or gametophytic, generation begins after meiosis with spores that undergo mitosis and differentiate into either a pollen grain (male gametophyte) or an embryo sac (female gametophyte) (3, 19-20). The pollen grain contains two sperm cells, whereas the embryo sac contains a single egg (Fig. 1)....