The remarkable biology of pollen.

To reproduce, higher plants utilize a unique multicellular microorganism: the male gametophyte, or pollen grain. The independent lifetime of flowering plant gametophytes is greatly abbreviated compared to that of gametophytes of more primitive plants, yet the angiosperm pollen grain must be able to survive at least briefly free from the sporophytic plant and perform a number of specialized functions before fertilization is accomplished. Although the pollen grain is a rather simple twoor threecelled organism, cytogenetic and mutagenesis experiments indicate that higher plants make a significant investment in genetic material devoted to gametophyte production (Carlson, 1977; Birchler and Schwartz, 1979; Kindiger et al., 1991). Molecular studies identifying antherand pollen-specific genes support the notion that the construction of a functional male gametophyte requires a rather large pool of such genes (Kamalay and Goldberg, 1980; Willing and Mascarenhas, 1984; McCormick, 1991). This is not so surprising given the specialized structures, mechanisms for rapid growth, and cell-cell communication systems during pollen-pistil interactions that have evolved to allow for both the survival of the gametophyte free of the sporophyte and the efficient delivery of sperm to the embryo sac. Studies of certain aspects of the molecular mechanisms of gametophyte development and function have progressed very rapidly. Examples of these include the identification of cis-acting elements determining tissue-specific gene expression in pollen and tapetal cells (Ursin et al., 1989; Koltunow et al., 1990; McCormick et al., 1991) and of mechanisms of self-incompatibility in pollen-pistil interactions (McClure et al., 1989, 1990; Thorsness et al., 1991). The intent of this article is to focus on a few less well studied but intriguing aspects of pollen development and function, with an emphasis on pollen development in maize. For more general reviews of microsporogenesis, the reader is referred to Giles and Prakash (1987) and Mascarenhas (1989). For recent reviews of gene expression during pollen development, see Mascarenhas (1990) and McCormick (1991). OVERVIEW OF POLLEN DEVELOPMENT