Female gametophyte development.

Early in their evolution, plants acquired a life cycle that alternates between a multicellular haploid organism, the gametophyte, and a multicellular diploid organism, the sporophyte. Angiosperms have both female and male gametophytes. The female gametophyte is critical to many steps of the angiosperm reproductive process, including pollen tube guidance, fertilization, the induction of seed development upon fertilization, and maternal control of seed development after fertilization. Genetic analysis in Arabidopsis and maize has revealed mutants defective in almost all stages of female gametophyte development, and analysis of these mutants is beginning to reveal features of the female gametophyte developmental program. In addition, mutants defective in female gametophyte function have revealed regulatory genes required for the induction of endosperm development. From these studies, we are beginning to understand the regulatory networks involved in female gametophyte development and function. Gametophytes and sporophytes differ morphologically and functionally. Themajor function of diploid sporophyte generation is to produce haploid spores, which are the products of meiosis. Spores undergo cell proliferation and differentiation to develop into gametophytes. The major function of gametophyte generation is to produce haploid gametes. The fusion of egg and sperm gives rise to the zygote, which is the beginning of diploid sporophyte generation, thereby completing the life cycle (Gifford and Foster, 1989). During the angiosperm life cycle, the sporophyte produces two types of spores, microspores andmegaspores, that give rise to male gametophytes and female gametophytes, respectively. The angiosperm gametophytes develop within sporophytic tissues that constitute the sexual organs of the flower. The male gametophyte, also referred to as the pollen grain or microgametophyte, develops within the stamen’s anther and is composed of two sperm cells encased within a vegetative cell (McCormick, 1993, 2004). The female gametophyte, also referred to as the embryo sac or megagametophyte, develops within the ovule, which is found within the carpel’s ovary. The most common female gametophyte form, depicted in Figure 1, consists of seven cells and four different cell types: three antipodal cells, two synergid cells, one egg cell, and one central cell (Maheshwari, 1950). The angiosperm gametophytes are essential for the reproductive process. During sexual reproduction in angiosperms, the male gametophyte is transferred from the anther to the carpel’s stigma, whereupon it forms a pollen tube that grows great distances through the carpel’s internal tissues to deliver its two sperm cells to the female gametophyte. One sperm cell fertilizes the egg cell and the second sperm cell fuses with the central cell. After fertilization, the ovule gives rise to a seed; the seed’s embryo, endosperm, and seed coat are derived from the fertilized egg cell, fertilized central cell, and ovule integuments, respectively (Maheshwari, 1950). The female gametophyte plays a critical role in essentially every step of the reproductive process. During pollen tube growth, the female gametophyte participates in directing the pollen tube to the ovule (Higashiyama, 2002; Johnson and Preuss, 2002; Higashiyama et al., 2003). During fertilization, cytoskeletal components within the female gametophyte direct the sperm cells to the egg cell and the central cell (Russell, 1992, 1993; Lord and Russell, 2002). Upon fertilization, female gametophyte–expressed genes control the initiation of seed development (Chaudhury et al., 2001). During seed development, female gametophyte–expressed gene products play a role in controlling embryo and endosperm development (Ray, 1997; Chaudhury and Berger, 2001). In this review, we describe angiosperm female gametophyte structure and development, summarize the female gametophyte’s reproductive functions, and discuss the molecular and genetic approaches that are being used to understand these processes at the molecular level. Comprehensive reviews of the female gametophyte have been published previously (Maheshwari, 1950; Willemse and van Went, 1984; Haig, 1990; Huang and Russell, 1992; Russell, 2001).