Experimental analysis of the fertilization process.

In flowering plants, double fertilization is one of the defining features of reproductive development (Raghavan, 2003). Double fertilizationwas first discovered in 1898 byNawaschin. It involves a complex series of interactions between essentially three plants—the male gametophyte (MG), the female gametophyte (FG), and the sporophyte—culminating in the fusion of sexual cells and nuclei and the formation of an embryo and endosperm through separate, presumably parallel, fusion and activation steps. In this review, we focus on recent advances toward understanding the mechanisms that control maturational synchrony, interaction of the MG with the FG, delivery of the male gametes,movement of spermcells to the female gametes, fusion of the gametes, and cytological changes before and after fertilization. The mature MG or pollen is composed of three cells: one vegetative cell that encloses two sperm cells or male gametes. The vegetative cell coordinates the delivery of the two male gametes to the FG. After landing on the stigma of an appropriate flower, a cascade of events leads to the establishment of polarity in the MG and the formation of a pollen tube. Interactions between the sporophytic tissues of the stigma, the style, and the MG guide the tube—with the two sperm cells inside—to the ovary. Within the ovary, the tube grows toward a receptive ovule, enters the ovule through the micropyle, and delivers the sperm cells within the FG to effect double fertilization. The genetic programs that govern the development of the MG and its interactions with the sporophyte are reviewed elsewhere in this issue (Edlund et al., 2004; McCormick, 2004; Kao and Tsukamoto, 2004; Sanchez et al., 2004). The FG, or embryo sac, constitutes the structural setting for double fertilization. The FG is embedded in several diploid cell layers of the ovule and typically consists of seven cells: three antipodal cells at the chalazal end; a large central cell—precursor of the endosperm; at the micropylar end, two synergids presumed to govern the chemical attraction and delivery mechanism; and the egg cell—precursor of the zygote. Other variants on this pattern also occur (Huang and Russell, 1992). Together, the egg and synergids form the egg apparatus. Genetic programs that control the development of the FG are reviewed elsewhere in this issue (Yadegari and Drews, 2004). Upon arrival at and penetration into the micropyle, the pollen tube delivers its two sperm cells into one of the two synergids by entering near and sometimes penetrating the filiform apparatus of the receptive synergid and subsequently rupturing. Inside the synergid, a mixture of cytoplasm of the two former cells (i.e., the tube and synergid) forms themilieu in which the nonmotile sperm cells are conveyed to the egg and central cell, where they fuse and start the development of the zygote and the endosperm, respectively. Despite a large body of knowledge concerning the events surrounding fertilization (reviewed recently by Drews and Yadegari, 2002; Lord and Russell, 2002; Willemse and van Lammeren, 2002; Higashiyama et al., 2003; Raghavan, 2003), the cellular mechanisms that control these events are still poorly understood. However, in recent years, the rediscovery of Torenia as a model system, improvements in the experimental manipulation of male and female gametes, advances in microscope techniques, and new molecular and genetic approaches have provided the means to obtain deeper knowledge of the underlying control mechanisms that govern the identity and behavior of the major participants in the fertilization process.