The Male Gametophyte of Flowering Plants. (1989)

Angiosperm plants produce flowers, very beautiful and intricate structures, within which their reproductive devel? opment takes place. In flowering plants, as in other groups of plants, a diploid, spore-producing generation (sporo? phyte) altemates with a haploid, gamete-producing gen? eration (gametophyte). Unlike some of the evolutionarily more primitive plants, the male and female gametophytes of angiosperms are reduced to microscopic structures that are dependent on the tissues of the sporophyte for their development. The flower contains specialized structures, the anthers and the pistil or gynoecium, in which the male and female gametophytes, respectively, are formed. The functions of the gametophytes are the production of the sperm cells and the female cells, and their union in fertilization. In flowering plants, the pollen grain is the male gametophyte and the embryo sac is the female gametophyte. The male gametophyte completes its early development within the anther. The sequential stages of pollen devel? opment are shown in Figure 1. Microsporocytes or pollen mother cells (Figures 1A and 1K) are produced in the sporogenous tissue within the anther. The two divisions of meiosis transform these cells into haploid microspores, each pollen mother cell producing first a dyad (Figures 1B and 1L) and, after the second meiotic division, a tetrad of microspores (Figures 1C and 1M). The tetrad and each microspore within the tetrad are surrounded by a callose (1,3-?-glucan) wall (Figure 1M). Upon release from the tetrad, the microspores increase rapidly in volume and undergo a change in shape (Figures 1D, 1E, 1N, and 10). This is followed by a period of slower growth until the maximum volume of the pollen grains is reached before anthesis. Following release of microspores from the te? trads, there is an extended interphase period that terminates with a very unequal division of the microspore (mi? crospore mitosis), forming a vegetative cell and a generative cell, both of which are included within the confines of the cell wall of the original microspore. The vegetative cell constitutes the bulk of the young pollen grain, while the generative cell, which inherits a very small amount of the microspore cytoplasm, lies within the vegetative cell (Fig? ures 1F and 1G). In several plants, such as corn, the generative cell undergoes a mitotic division within the pollen grain, forming two sperm cells (Figures 1H and 1R). In most pollens, however, the generative cell completes its division during the growth of the pollen tube in the style. At ma urity, the male gametophyte consists of three cells, the vegetative cell and the two sperm cells, which lie within the cytoplasm of the vegetative cell (Figures 11 and 1S). For a short while following anthesis (the rupture of the anther and release of pollen), the mature pollen grain exists as a free organism until it is transported by wind, insects, or other agents to the stigma of an appropriate pistil. It then begins another phase of its life and development. Each pollen grain germinates by the extrusion of a tube through a germ pore in the pollen wall (Figures 1J and 1T). The tube then grows down into the style and the vegetative nucleus, and, depending on the plant species, the gener? ative cell or sperm cells move out of the pollen grain and i to the tube. Germination and pollen tube growth are relatively rapid events in most plants, the period from pollination to fertilization ranging from 1 hr to around 48 hr. The rate of pollen tube growth varies in different plant species; extremely rapid rates as high as 35 mm/hr have been reported for some plants. The pollen tube grows through the transmitting tissue of the style, enters the micropyle of the ovule, and reaches the embryo sac. It penetrates one of the synergids of the embryo sac (nor? mally the one that has begun to degenerate), the tube rup ures, and the sperm cells together with some of the other tube contents are discharged into the synergid. The two sperm move by a presently unknown mechanism, one fusing with the egg cell to form the diploid zygote. The other sperm fuses with the normally diploid central cell, giving rise to the primary endosperm nucleus. This com? pletes the process of double fertilization. A major recent advance is the observation that the two sperm cells in a pollen grain are often morphologically different (Russell,