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Frontiers in Fertilization.

Fertilization, a process central to sexual reproduction, takes place deep within maternal tissues of higher plants, hardly an accessible location. But the mysteries associated with the fusion of sperm and egg may soon be exposed: a number of researchers have devoted considerable effort to isolating plant gametes, and routine in vitro fertilization may not be far off. The potential payoffs of in vitro fertilization systems for plants are enormous. In addition to providing information about the basic biology of fertilization and early development, the ability to control gamete fusion in vitro may find important practical applications. This technology might, for example, break down species barriers, serving as a tool for the development of new types of crop plants. In animals, fertilization, both in vivo and in vitro, involves the individual gametes. In plants, by contrast, not only thegametes themselves, but also other cells of both the male and female gametophytes, are intimately involved in plant fertilization. These cells facilitate fertilization in vivo by allowing the sperm and egg cells to gain access to one another. Sperm cells, for example, do not make their way to the egg as individuals. Instead, they are enclosed completely within the cytoplasm of the vegetative cell, and they migrate toward the egg within the confines of the pollen tube, which grows down through the style toward the ovule. The female gametophyte, or embryo sac, contains not only an egg cell but also a central cell and, typically, synergids and several other accessory cells. The role of one of the synergids is to allow the pollen tube to enter the embryo sac, which it does by degenerating, usually before the pollen tube reaches the embryo sac. The pollen tube can then enter the embryo sac through the degenerating synergid. After penetrating the embryo sac, the pollen tube riiptures, discharging the two sperm cells: one fuses with the egg to produce the zygote, and the other fuses with the central cell to produce the endosperm.