Recent advances in understanding female gametophyte development

Abstract

The haploid female gametophyte (embryo sac) is an essential reproductive unit of flowering plants, usually comprising four specialized cell types, including the female gametes (egg cell and central cell). The differentiation of these cells relies on spatial signals which pattern the gametophyte along a proximal-distal axis, but the molecular and genetic mechanisms by which cell identities are determined in the embryo sac have long been a mystery. Recent identification of key genes for cell fate specification and their relationship to hormonal signaling pathways that act on positional cues has provided new insights into these processes. A model for differentiation can be devised with egg cell fate as a default state of the female gametophyte and with other cell types specified by the action of spatially regulated factors. Cell-to-cell communication within the gametophyte is also important for maintaining cell identity as well as facilitating fertilization of the female gametes by the male gametes (sperm cells).

Keywords: embryo sac; female gametophyte.

Figure 1.. Arabidopsis female gametophyte development.

The progression of female gametophyte development is shown from left to right. After meiosis, a single haploid cell, usually the basal (chalazal) cell, will enlarge and form the functional megaspore while the remaining products of meiosis degenerate. This haploid megaspore will have three mitotic divisions accompanied by nuclear movement to create a defined pattern at each division. From stage FG4, the large vacuole (blue) separates the nuclei along the chalazal-micropylar axis. At FG5, the polar nuclei (red) migrate to meet each other and eventually fuse. At FG6/FG7, the mature female gametophyte has seven cells: two synergids, egg cell, central cell with large diploid nucleus (central cell nucleus, or CCN), and three antipodal cells (which are present through FG7 though much diminished ). Stages are numbered in accordance with Christensen et al. .

Figure 2.. Model for acquisition of cell identity in the female gametophyte.

RKD genes (expression pattern shown in blue) may act early to set up a default egg cell state in the gametophyte and continue to promote egg cell identity later. At the micropylar pole, auxin signaling, together with sporophytically active AMP1 (green) which could potentially affect the auxin:cytokinin balance, acts to specify synergid cell identity. The egg cell (with blue nucleus) maintains synergid identity by suppressing egg cell fate in the adjacent synergid cells (black nuclei). CKI1 (orange) represses micropylar fates in the chalazal domain and is needed to specify central cell identity (polar nuclei in red) in a pathway involving AHP proteins. CKI1, together with additional factors that may be provided from the chalaza, specifies antipodal cell fates (yellow nuclei). Note: The nuclei in this sketch are not drawn to scale. AC, antipodal cell; AHP, Arabidopsis phosphotransfer protein; AMP1, altered meristem program 1; CC, central cell; CK, cytokinin; CKI1, CYTOKININ INSENSITIVE 1; EC, egg cell; RKD, RWP-RK DOMAIN CONTAINING; SC, synergid cell.