Experimental Analysis of the Fertilization Process

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 syn- chrony, interaction of the MG with the FG, delivery of the male gametes,movementofspermcellstothefemalegametes,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