Perspectives on Genetic Analysis of Plant Embryogenesis.

Embryogenesis plays a central role in the life cycle of flowering plants. Ever since the process of double fertilization was documented at the turn of the century, plant embryologists have attempted to characterize the cellular and biochemical changes that occur within developing seeds (Maheshwari, 1950; Raghavan, 1976). For many years, the emphasis was on comparative morphology and the analysis of cell division patterns during early stages of embryo development (Johansen, 1950; Wardlaw, 1955). Attention then shifted to experimental studies of somatic embryogenesis, embryo culture, and haploid embryos derived from microspores (Raghavan, 1986). Manipulation of zygotic embryos was limited by the presence of surrounding maternal tissues. Electron microscopy provided additional details on cellular changes associated with embryogenesis but generally failed to identify the mechanisms responsible. Plant embryologists believed that genetic factors played an important role in morphogenesis, but mutants with altered patterns of embryo development were largely ignored (Meinke, 1986). Recent advances in molecular biology have led to a renewed interest in plant embryology and the underlying patterns of gene expression that occur throughout seed development (Goldberg et al., 1989). Many genes transcribed during embryonic maturation have now been examined at the molecular level. Genetics provides a complementary approach to the study of plant embryo development by allowing the identification of genes with essential functions during this critical stage of the life cycle (Meinke, 1986). The purpose of this review will be to explore the potential benefits and limitations of mutant analysis in relation to plant embryo development. Additional information can be obtained through recent reviews on plant development (Steeves and Sussex, 1989; Lyndon, 1990), experimental plant embryogenesis (Johri, 1984; Raghavan, 1986; Williams and Maheswaran, 1986), and plant developmental genetics (Sheridan, 1988; Meinke, 1991 a, 1991 b). OVERVIEW OF PLANT EMBRY OGENESIS