Figure 1. Angiosperm life cycle.

The plant life cycle alternates between a diploid sporophyte and a haploid gametophyte generation (Figure 1). Specialized cells of the diploid sporophyte undergo meiosis to produce male or female spores. The sexually dimorphic spores divide to develop into haploid organisms, the gametophytes, which produce the gametes--the sperm and egg cells. In flowering plants, the female gametophyte, called the embryo sac, consists of four cell types: the egg cell, the central cell, the synergids, and the antipodal cells (Figure 2). The male gametophyte (the pollen grain) contains a vegetative cell, which performs the metabolic functions of the pollen grain and delivers the gametes to the embryo sac, and two sperm cells that fertilize the egg and central cell to produce the embryo and endosperm, respectively. The embryo develops into a new plant to continue the life cycle. The endosperm produces the nutritive material used by the embryo and seedling and comprises the bulk of the grain weight in cereals such as wheat, rice, and maize. The embryo and endosperm together make up the seed. The gametophytes of plants have active haploid genomes, in contrast to animals, where the products of meiosis are genetically inactive.

Figure 2. Male and female gametophyte development. a=antipodal cells; cc=central cell; pn=polar nuclei; e=egg cell; sy=synergid; end=endosperm; emb=embryo; vn= vegetative cell nucleus; v= vegetative cell; va=vacuole gc=generative cell; sp=sperm cells; pt=pollen tube. FG1-FG8=female gametophyte stage 1 to 8. FM=free microspore; VM=vacuolated microspore; BC=bicellular pollen; MP=mature pollen; GP=germinating pollen; dm=degenerated microspores.

Maize, like the majority of Angiosperm species, undergoes the Polygonum type of embryo sac development. Three of the spores produced by meiosis abort, and the chalazal most megaspore undergoes three rounds of free nuclear division to produce an eight nucleate syncytium. Cellularization produces a seven-celled embryo sac consisting of: two synergids, which attract the male gametophyte; the egg cell; the homodiploid central cell; and three antipodal cells (Figure 3). In maize the antipodal cells persist and continue to divide.

Figure 3. Maize embryo sac. Confocal image of cellularized embryo sac. The embryo sac is imbedded within the nucellus (n) of the ovule. Antipodal cells (a) have already divided a few times. The egg (e) and one of the synergids (s) are in the plane of focus. The synergid has not yet degenerated. The central cell has a large vacuole and two polar nuclei adjacent to the egg.

The male gametophyte undergoes one round of division to produce the vegetative cell and the generative cell. The generative cell undergoes a second round of division to produce the two sperm cells, which are both contained within the cytoplasm of the vegetative cell. Upon interaction between the pollen grain and the female stigma, the pollen grain germinates and the vegetative cell produces a pollen tube that grows through the style and is guided to a synergid of the embryo sac. Interaction with the synergid causes pollen tube rupture and release of the two sperm cells, which then fuse with the egg and central cell to achieve fertilization.

Fertilization of the embryo sac leads to rapid development of the embryo and endosperm. By four days after pollination (DAP) a small proembryo of approximately 24 cells has formed consisting of the large basal cells of the suspensor and the smaller apical cells of the embryo proper. By eight DAP the embryo has entered the transition stage with a long suspensor and a globular embryo proper. The scutellum (cotyledon) enlarges rapidly after the transition stage, and the coleoptile and shoot apical meristem (SAM) form on the face of the scutellum by approximately 10 DAP.

Maize endosperm development begins rapidly after fertilization in comparison to the embryo (Figure 4). It begins with a period of free nuclear divisions to produce a syncytium of over 250 nuclei located at the periphery of the cell. This period is followed by cellularization at approximately three to four DAP, when the embryo is still in the proembryo phase. The endosperm differentiates into at least four distinct domains: the aleurone, the basal endosperm transfer layer (BETL), the starchy endosperm, and the embryo surrounding region (ESR).

Figure 4. Endosperm development in maize. The growth of the embryo (gray) is shown for comparison. A syncytial phase is followed by cellularization. A phase of mitotic growth is followed by cycles of endoreduplication and eventually programmed cell death. Blue indicates cells that have undergone programmed cell death.