Anther development: basic principles and practical applications.

Male reproductive processes in flowering plants take place in the stamen (Esau, 1977). This sporophytic organ system contains diploid cells that undergo meiosis and produce haploid male spores, or microspores. Microspores divide mitotically and differentiate into multicellular male gametophytes, or pollen grains, that contain the sperm cells. Figures 1 and 2 show that the stamen consists of two morphologically distinct partsthe anther and the filament. The filament is a tube of vascular tissue that anchors the stamen to the flower and serves as a conduit for water and nutrients. By contrast, the anther contains the reproductive and nonreproductive tissues that are responsible for producing and releasing pollen grains so that pollination and fertilization processes can occur within the flower. Figure 1 shows that anther development can be divided into two general phases. During phase 1, the morphology of the anther is established, cell and tissue differentiation occur, and microspore mother cells undergo meiosis. At the end of phase 1, the anther contains most of its specialized cells and tissues, and tetrads of microspores are present within the pollen sacs (Figure 1). During phase 2, pollen grains differentiate, the anther enlarges and is pushed upward in the flower by filament extension, and tissue degeneration, dehiscence, and pollen grain release occur (Figure 1). The cellular processes that regulate anther cell differentiation, establish anther tissue patterns, and cause the anther to switch from a histospecification program (phase l) to a cell degeneration and dehiscence program (phase 2) are not known. The developmental events leading to anther formation and pollen release are exquisitely timed and choreographed (Koltunow et al., 1990; Scott et al., 1991). Cell differentiation and dehiscence events occur in a precise chronological order that correlates with floral bud size (Koltunow et al., 1990; Scott et al., 1991). This permits the mechanisms responsible for cell-type differentiation, tissue degeneration, and cellspecific gene activation within the anther to be explored with relative Base. During the past few years, there has been an explosive burst of interest in anther biology, both as a system to dissect plant developmental processes at the molecular and genetic levels (Koltunow et al., 1990; Gasser, 1991; McCormick,