Stamen structure and function.

Stamens are the male reproductive organs of flowering plants. They consist of an anther, the site of pollen development, and in most species a stalk-like filament, which transmits water and nutrients to the anther and positions it to aid pollen dispersal. Within the anther, male sporogenous cells differentiate and undergo meiosis to produce microspores, which give rise to pollen grains, whereas other cell types contribute to pollen maturation, protection, or release. Pollen development involves an array of extraordinary events, including cell division and differentiation independent of a conventional meristem, the transition from sporophytic to gametophytic generation, and modifications of cell division to produce structures that are unusual in plant development, including coenocytic tissues (the tapetum and the microsporocyte mass), and subsequently free cells (microspores) that give rise to self-contained units for genome dispersal (pollen grains). Reviews of stamen development and gene expression include articles by Bhandari (1984), Scott et al. (1991a), Goldberg et al. (1993), and Irish (1999). A change in emphasis will be apparent since anther developmentwas reviewed in the first PlantReproduction special issue of The Plant Cell (Goldberg et al., 1993). The earlier review drew largely on studies of tobacco, but this article focusesmore tightly on Arabidopsis, which has surpassed other species in the genetic tools publically available, including a sequenced genome, microarrays, well-characterized mutants associated with cloned genes, and knockout collections for reverse genetics. The advances of the last 10 years include an increased understanding of stamen specification, stamen-specific gene expression, internal patterning of the anther, regulation of male meiosis, and anther dehiscence. Stamen and pollen development are intimately linked. We discuss those aspects of pollen genesis that are essentially controlled by the sporophytic genome (differentiation of themale germline, meiosis, and pollen wall formation), but the biology of the male gametophyte, pollen tube growth, and fertilization are covered by other reviews in this volume (Edlund et al., 2004; Hanson and Bentolila, 2004; McCormick, 2004).