The Maize Megagametophyte

Abstract

The life cycle of plants alternates between a diploid and a haploid generation. In flowering plants the haploid gametophytes are sexually dimorphic and produce the gametes, which fuse to produce the diploid sporophyte of the next generation. The megagametophyte of maize follows the Polygonum-type pattern of development:one of the four meiotic products, the functional megaspore, undergoes three free nuclear divisions to produce a polarized, eight-nucleate syncytium. Cellularization produces seven cells that differentiate into four cell types:two syn-ergids, three antipodals, and the two female gametes, the egg cell and the central cell. The position of the nuclei in the syncytial phase and the position and differentiation of cell types after cellularization follow stereotypical patterns, suggesting a tight genetic regulation of the cellular processes involved. Recent genetic evidence demonstrates that many of these cellular processes are regulated by the activity of the haploid genome of the megagametophyte itself, rather than the parental diploid genome from which it originates. The functions performed by the megagameto-phyte includes both basic cellular functions and functions that unique to the meg-agametophyte, such as pollen tube guidance and reception, as well as processes associated with double fertilization and the maternal control over seed development. In this chapter we describe the development and functions of the megagame-tophyte, and what is known the regulation of the underlying processes.

Nuclear behavior, cell polarity, and cell specification in the female gametophyte

“…2f, g) (Christensen et al 1997). By contrast, in maize, they meet in the center of the embryo sac, partially fuse, and then migrate to the micropylar end of the central cell where they typically stay unfused until fertilization (Evans and Grossniklaus 2009). After cellularization, the FG cells occupy specific positions within the FG and exhibit conspicuous cellular polarities, reflected by a characteristic nuclear and vacuole positioning.…”

“…Abbreviations: ap antipodal cells, ccn central cell nucleus, chz chalaza, cv central vacuole, ecn egg cell nucleus, f filiform apparatus, FG female gametophyte, mp micropyle, sn synergid nucleus simultaneously organized at both the micropylar and the chalazal poles, indicating the future cell plates (Huang and Sheridan 1994;Webb and Gunning 1994). The two polar nuclei will form the homodiploid (2n) nucleus of the central cell and exhibit exceptionally large nucleoli (Evans and Grossniklaus 2009), suggesting an increase in RNA metabolism. After cellularization, the two polar nuclei start to migrate from the opposite poles along the adaxial surface of the embryo sac toward each other (Fig.…”

Female gametophyte development and double fertilization in Balsas teosinte, Zea mays subsp. parviglumis (Poaceae)

“…More than 30 antipodal cells have been reported in mature Mexicana teosinte female gametophytes (Cooper 1937;Koul 1959). In maize, the number of antipodal cells usually varies from 20 to 48, although as many as 100 have been reported for some varieties (Hector, 1936;Randolph 1936;Stover 1937;Koul 1959;Diboll and Larson 1966;Diboll 1968;Kiesselbach 1980;Huang and Sheridan 1994;Evans and Grossniklaus 2009). In Tripsacum, however, four to six antipodal cells are typically formed during female gametophyte development (Burson et al 1990;Leblanc et al 1995).…”

Pattern formation in miniature: the female gametophyte of flowering plants

“…However, the mutant screens have been disappointing in terms of revealing the overall principles and mechanisms that govern embryo sac cell specification, or, more broadly, how the embryo sac is patterned. Indeed, a recent review has commented that "The signaling events underlying cell specification in the female gametophyte are, however, completely unknown" (Evans and Grossniklaus, 2009). This situation has now changed owing to some recent progress, which is discussed in the next section.…”