The Egg Cell: Development and Role in Fertilization and Early Embryogenesis.

Flowering plant reproduction is unusual in many features, as evidenced by the different topics discussed in this issue of THE PLANT CELL. The egg cell of flowering plants in particu- lar displays several unique features. First, the egg cell is an integral part of the several thousands of cells forming the ovule, and it cannot be released without the aid of enzymes or microdissection. The egg cell appears to be part of a functional assemblage of cells that are fragile in isolation. Sur- rounding the egg cell in situ are the two synergids (collectively forming the egg apparatus) and the adjacent central cell, as shown in Figure 1. These are almost always derived from the same initial meiotic cell as the egg cell. The surrounding cells appear to provide nutrition for the egg cell and are involved in the positioning of the sperm cells to the precise site where gametic fusion occurs. Different developmental programs are invoked in both the egg and central cells by the fusion of each with a sperm cell in an event characteristic of flowering plants known as "dou- ble fertilization." During this event, one sperm nucleus fuses with the egg nucleus to form the zygote, whereas the other sperm nucleus fuses with the two (or more) central cell nuclei, resulting in the formation of the nutritive endosperm. The developmental potential, biochemical identity, and char-