Imprinting and Seed Development

Abstract

Fertilization Seeds consist of three genetically distinct components: embryo, endosperm, and seed coat. In plants, mitosis follows meiosis to produce the haploid phase of the plant life cycle, the male and female gametophytes. The angiosperm female gametophyte, the site of fertilization, is completely embedded within the maternal sporophytic tissues of the ovule. The most prevalent type of mature female gametophyte is a seven-celled organism consist- ing of three antipodal cells, two synergid cells, an egg cell, and a diploid central cell. The function of the antipodal cells is unknown. The micropylar synergid cells help attract the pollen tube to the female gametophyte (Higashiyama et al., 2001). The egg cell lies adjacent to the synergid cells and is the progenitor of the embryo. The large central cell, the progenitor of the endosperm, contains two polar nuclei. These fuse before, or at the time of, fertilization to form a diploid central cell nucleus. The male gametophyte, or pollen, develops in the anther from microspores. A mature male gametophyte consists of two haploid sperm cells encased by a haploid vegetative cell. Seed development begins upon double fertilization. The pollen tube, formed from the vegetative cell of the male gametophyte, enters the female gametophyte through the micropylar end and releases two sperm into a synergid cell. One sperm fertilizes the egg cell and the other fertilizes the central cell. The resulting embryo and endosperm are genetically identical except for ploidy level: the embryo is diploid and the endosperm is triploid. Fertilization also initiates changes in maternal tissues. The ovary develops into a fruit and the ovule integuments differentiate to form the protective seed coat.