Gametes and Fertilization: Maize as a Model System for Experimental Embryogenesis in Flowering Plants.

Abstract

The involvement of each of the two sperm cells from a pollen tube in separate fusion events characterizes double fertilization: a process that is illustrated in Figure 1. This process, which initiates the development of both the embryo (via the fusion of sperm and egg) and the endosperm (via the fusion of sperm and central cell), was discovered in angiosperms nearly acentury ago (Nawaschin, 1898; Guignard, 1899; reviewed in Russell, 1992). The application of the transmission electron microscope to the study of flowering plant embryogenesis in the early 1960s by Jensen and coworkers (reviewed in Jensen, 1973, 1974) marked the beginning of great strides that have been made in our understanding of embryogenesis since Maheshwari’s (1950) comprehensive review. Jensen and Fisher (1968,1970) confirmed the cellular nature of sperm cells and noted that sperm cells remain connected after their formation from the generative cell division. Jensen (1973,1974) also showed that the two synergids that typically flank the egg are active, functional cells that play a significant role in the fertilization process. More recent studies, also employing transmission electron microscopy but extending its use to the creation of three-dimensional, computer-generated reconstructions, have led to the concept of the male germ unit, which views the sperm cells and vegetative nucleus as a functional assemblage required for normal gamete transport, recognition, and fusion (see below; Dumas et al., 1984; reviewed in Mogensen, 1992). Because the sperms of a pair are dimorphic in some species, it also became apparent that double fertilization may not be a random event, i.e., that each sperm may be preprogrammed for fusion with either the egg or the central cell (Russell, 1985). Although the process of double fertilization offers unique opportunities for studying gamete interactions and control mechanisms, it also presents special problems not encountered in animals and nonangiospermous plants. A major deterrent to physiological, molecular, and experimental studies of embryogenesis in angiosperms has been the inaccessibility of the gametes. During the past few years, however, it has become possible in some angiosperm species to isolate male and female sex cells in large enough quantities to allow for the initial phases of such analyses.