Female Gametophyte Development
Ramin Yadegaria,1 and Gary N. Drewsb
a Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721-0036
b Department of Biology, University of Utah, Salt Lake City, Utah 84112
INTRODUCTION
Early in their evolution, plants acquired a life cycle that alternates
between a multicellular haploid organism, the gametophyte, and
a multicellular diploid organism, the sporophyte. Angiosperms
have both female and male gametophytes. The female gametophyte
is critical to many steps of the angiosperm reproductive
process, including pollen tube guidance, fertilization, the induction
of seed development upon fertilization, and maternal
control of seed development after fertilization. Genetic analysis
in Arabidopsis and maize has revealed mutants defective in
almost all stages of female gametophyte development, and
analysis of these mutants is beginning to reveal features of
the female gametophyte developmental program. In addition,
mutants defective in female gametophyte function have revealed
regulatory genes required for the induction of endosperm development.
From these studies, we are beginning to understand
the regulatory networks involved in female gametophyte
development and function.
Gametophytes and sporophytes differ morphologically and
functionally. The major function of diploid sporophyte generation
is to produce haploid spores, which are the products of meiosis.
Spores undergo cell proliferation and differentiation to develop
into gametophytes. The major function of gametophyte generation
is to produce haploid gametes. The fusion of egg and
sperm gives rise to the zygote, which is the beginning of diploid
sporophyte generation, thereby completing the life cycle (Gifford
and Foster, 1989).
During the angiosperm life cycle, the sporophyte produces
two types of spores, microspores and megaspores, that give rise
to male gametophytes and female gametophytes, respectively.
The angiosperm gametophytes develop within sporophytic
tissues that constitute the sexual organs of the flower. The male
gametophyte, also referred to as the pollen grain or microgametophyte,
develops within the stamen’s anther and is
composed of two sperm cells encased within a vegetative cell
(McCormick, 1993, 2004). The female gametophyte, also referred
to as the embryo sac or megagametophyte, develops
within the ovule, which is found within the carpel’s ovary. The
most common female gametophyte form, depicted in Figure 1,
consists of seven cells and four different cell types: three
antipodal cells, two synergid cells, one egg cell, and one central
cell (Maheshwari, 1950).
The angiosperm gametophytes are essential for the reproductive
process. During sexual reproduction in angiosperms, the
male gametophyte is transferred from the anther to the carpel’s
stigma, whereupon it forms a pollen tube that grows great
distances through the carpel’s internal tissues to deliver its two
sperm cells to the female gametophyte. One sperm cell fertilizes
the egg cell and the second sperm cell fuses with the central cell.
After fertilization, the ovule gives rise to a seed; the seed’s
embryo, endosperm, and seed coat are derived from the
fertilized egg cell, fertilized central cell, and ovule integuments,
respectively (Maheshwari, 1950).
The female gametophyte plays a critical role in essentially
every step of the reproductive process. During pollen tube
growth, the female gametophyte participates in directing the
pollen tube to the ovule (Higashiyama, 2002; Johnson and
Preuss, 2002; Higashiyama et al., 2003). During fertilization,
cytoskeletal components within the female gametophyte direct
the sperm cells to the egg cell and the central cell (Russell,
1992, 1993; Lord and Russell, 2002). Upon fertilization, female
gametophyte–expressed genes control the initiation of seed
development (Chaudhury et al., 2001). During seed development,
female gametophyte–expressed gene products play a role
in controlling embryo and endosperm development (Ray, 1997;
Chaudhury and Berger, 2001).
In this review, we describe angiosperm female gametophyte
structure and development, summarize the female gametophyte’s
reproductive functions, and discuss the molecular
and genetic approaches that are being used to understand
these processes at the molecular level. Comprehensive reviews
of the female gametophyte have been published previously
(Maheshwari, 1950; Willemse and van Went, 1984; Haig, 1990;
Huang and Russell, 1992; Russell, 2001).
DEVELOPMENT AND FERTILIZATION OF THE
FEMALE GAMETOPHYTE
Female gametophyte development occurs over two phases
referred to as megasporogenesis and megagametogenesis.
More than 15 different patterns of female gametophyte development
have been described. As summarized in Figure 2 and
discussed below, the different patterns arise mainly from
variations in cytokinesis during meiosis, number and pattern of
mitotic divisions, and pattern of cellularization. The developmental
pattern exhibited by most species is referred to as the
Polygonum type (Figure 2, top row) because it was first
described in Polygonum divaricatum (Strasburger, 1879;
Maheshwari, 1950). The Polygonum-type female gametophyte
1 To whom correspondence should be addressed. E-mail yadegari@
ag.arizona.edu; fax 520-621-7186.
Article, publication date, and citation information can be found at
www.plantcell.org/cgi/doi/10.1105/tpc.018192.
The Plant Cell, Vol. 16, S133–S141, Supplement 2004, www.plantcell.org ª 2004 American Society of Plant Biologists
is exhibited by >70% of flowering plants and is the pattern found
in many economically and biologically important groups, including
Brassicaceae (e.g., Arabidopsis, Capsella, Brassica),
Gramineae (e.g., maize, rice, wheat), Malvaceae (e.g., cotton),
Leguminoseae (e.g., beans, soybean), and Solanaceae (e.g.,
pepper, tobacco, tomato, potato, petunia) (Maheshwari, 1950;
Willemse and van Went, 1984; Haig, 1990; Huang and Russell,
1992).
Megasporogenesis
During megasporogenesis, the diploid megaspore mother cell
undergoes meiosis and gives rise to four haploid nuclei.
Angiosperms exhibit three main patterns of megasporogenesis,
referred to as monosporic, bisporic, and tetrasporic. These
three patterns are summarized in Figure 2. The three types differ
mainly in whether cell plate formation occurs after these
divisions, thus determining the number of meiotic products that
contribute to the formation of the mature female gametophyte.
In the monosporic pattern, both meiotic divisions are accompanied
by cell plate formation, resulting in four one-nucleate
megaspores. Subsequently, three megaspores, generally the
micropylar-most megaspores, undergo cell death. In the bisporic
pattern, cell plates form after meiosis I but not meiosis II.
The result is two two-nucleate megaspores, one of which
degenerates. In the tetrasporic pattern, cell plates fail to form
after both meiotic divisions, resulting in one four-nucleate megaspore.
Thus, these three patterns give rise to a single functional
megaspore that contains one (monosporic), two (bisporic), or
four (tetrasporic) meiotic nuclei. The monosporic pattern is the
most common form and is represented within the Polygonum
pattern (Maheshwari, 1950; Willemse and van Went, 1984; Haig,
1990; Huang and Russell, 1992).
Megagametogenesis
During megagametogenesis, the functional megaspore gives
rise to the mature female gametophyte. Initially, the megaspore
undergoes one or more rounds of mitosis without cytokinesis,
resulting in a multinucleate coenocyte. Subsequently, cell walls
form around these nuclei, resulting in a cellularized female
gametophyte. For example, in the Polygonum-type pattern
summarized in Figure 3, a single nucleus undergoes two rounds
of mitosis, producing a four-nucleate cell with two nuclei at each
pole. During a third mitosis, phragmoplasts and cell plates form
between sister and nonsister nuclei, and soon thereafter, the
female gametophyte cells become completely surrounded by
cell walls. During cellularization, two nuclei, one from each pole
(the polar nuclei), migrate toward the center of the developing
female gametophyte and fuse together either before or upon
fertilization of the central cell. These events result in a sevencelled
structure consisting of three antipodal cells, one central
cell, two synergid cells, and one egg cell (Figure 3). The monosporic,
Polygonum type of female gametophyte is typically a
seven-celled structure at maturity. However, this structure may
be modified by cell death or cell proliferation events in various
species. For example, in Arabidopsis, the antipodal cells
undergo cell death immediately before fertilization (Figure 3),
whereas in grasses (e.g., maize), the antipodal cells proliferate
(Maheshwari, 1950; Willemse and van Went, 1984; Haig, 1990;
Huang and Russell, 1992; Drews et al., 1998).
Throughout development, the female gametophyte exhibits
a polarity along its chalazal-micropylar axis. During Polygonumtype
megasporogenesis, the chalazal-most megaspore survives
and the other three megaspores undergo cell death (Figure 3).
During cell differentiation, the nuclei at the micropylar end
become specified to develop into the egg cell, the micropylar
polar nucleus, and the synergid cells; the chalazal nuclei develop
into the three antipodal cells and the chalazal polar nucleus
(Figure 3). Furthermore, all of the cells within the female
gametophyte differentiate into polar structures. For example, in
many species, the egg cell’s nucleus is located toward the
chalazal end and its vacuole occupies the micropylar end; by
contrast, the synergid and central cells have the opposite polarity
(Figure 3) (Willemse and van Went, 1984; Huang and Russell,
1992; Christensen et al., 1997). Thus, the establishment of
polarity within the female gametophyte corresponds to the
asymmetric development of the surrounding ovule layers,
suggesting that female gametophyte polarity is regulated, at
least in part, by the surrounding sporophytic tissues. Sporophytic
factors that influence female gametophyte development
have yet to be identified.
Fertilization and Early Seed Development
Soon after pollen is transferred from anther to stigma, the male
gametophyte forms a pollen tube that grows via a tip-growth
process through the carpel’s sporophytic tissue to reach the
female gametophyte. The pollen tube enters the female
gametophyte by growing into one of the two synergid cells
through a structurally elaborated portion of the micropylar cell
wall known as the filiform apparatus. The penetrated synergid
Figure 1. The Arabidopsis Female Gametophyte.
(A) Ovule.
(B) Female gametophyte.
The view in (B) is perpendicular to that in (A). The mature female
gametophyte in Arabidopsis is 105 mm long and 25 mm wide. The
gray areas represent cytoplasm, the white areas represent vacuoles, and
the black areas represent nuclei. ac, antipodal cells; cc, central cell; ch,
chalazal region of the ovule; ec, egg cell; f, funiculus; mp, micropyle; sc,
synergid cell; sn, secondary nucleus.
S134 The Plant Cell
cell undergoes cell death soon before or upon pollen tube arrival.
Immediately after arrival, pollen tube growth ceases, an aperture
forms at or near the pollen tube tip, and the contents of the pollen
tube, including the two sperm cells, are released rapidly into the
degenerating synergid cytoplasm. Double fertilization occurs
when the two sperm cells migrate to the egg and central cells and
their plasma membranes fuse with the respective target cell
to transport the sperm nuclei for karyogamy (van Went and
Willemse, 1984; Russell, 1992, 1996).
ANALYSIS OF THE FEMALE GAMETOPHYTE
Identification of Mutations That Affect Female
Gametophyte Development and Function
Because of their two-staged life cycle, plants possess two broad
classes of mutations: sporophytic mutations and gametophytic
mutations. Sporophytic mutations affect the diploid sporophyte
phase of the plant life cycle and exhibit Mendelian 1:2:1
segregation patterns. Gametophytic mutations, by contrast,
affect the haploid gametophyte phase of the plant life cycle and
are not transmitted through egg and/or sperm. As a consequence,
gametophytic mutations exhibit non-Mendelian segregation
patterns and can only be transmitted from generation to
generation as heterozygotes. For example, in a self cross of
a heterozygous individual (e.g., genotype A/a), female gametophyte–specific
mutations (i.e., mutations that affect the female
gametophyte but not the male gametophyte) segregate 1:1 for
A/A:A/a progeny (Moore et al., 1997; Drews et al., 1998; Drews
and Yadegari, 2002; Page and Grossniklaus, 2002).
Sporophytic and gametophytic mutations affect different
aspectsoffemalegametophytedevelopment.Sporophyticmutations
affect those aspects that occur during the diploid phase,
including megaspore mother cell development, meiosis, and
control of female gametophyte development by the surrounding
sporophytic tissue (e.g., female gametophyte polarity;
discussed above). Sporophytic mutations that affect these
processes are identified in screens for female-sterile mutants
(Chaudhury et al., 1998; Gasser et al., 1998; Grossniklaus and
Schneitz, 1998; Schneitz et al., 1998; Schneitz, 1999).
Gametophytic mutations affect those aspects of female
gametophyte development that occur after meiosis, including
megagametogenesis and functioning of the mature female
gametophyte (pollen tube guidance, fertilization, induction of
seed development, or maternal control of seed development).
Gametophytic mutants typically are identified using two criteria:
reduced seed set and segregation distortion. Reduced seed set
results because on a plant heterozygous for a female gametophyte
mutation, approximately half of the female gametophytes
are mutant and nonfunctional; thus, they fail to undergo normal
seed development. Segregation distortion results because, as
described above, gametophytic mutations are transmitted to
subsequent generations at reduced frequency (Moore et al.,
1997; Drews et al., 1998; Drews and Yadegari, 2002; Page and
Grossniklaus, 2002).
Sporophytic Mutations That Affect Megagametogenesis
As discussed above, the sporophytic tissue surrounding the
female gametophyte may play a role in controlling megagametogenesis.
Consistent with this notion, megagametogenesis is
affected in most sporophytic ovule-development mutants
(Chaudhury et al., 1998; Gasser et al., 1998; Grossniklaus and
Schneitz, 1998; Schneitz et al., 1998; Schneitz, 1999). Mutants
Figure 2. Patterns of Female Gametophyte Development Exhibited by Angiosperms.
Genera exhibiting these patterns are indicated in parentheses. More comprehensive descriptions of the variation among angiosperms can be found in
several reviews (Maheshwari, 1950; Willemse and van Went, 1984; Haig, 1990; Huang and Russell, 1992; Russell, 2001). In this figure, the chalazal end
of the female gametophyte is up and the micropylar end is down. FG, female gametophyte.
Female Gametophyte Development S135
defective specifically in the sporophytic control of megagametogenesis
should exhibit defects in megagametogenesis but not in
the sporophytic parts of the ovule. Mutants with this phenotype
have been reported (Schneitz et al., 1997). However, the genes
affected in these mutants have not been identified; thus, the
molecular basis of these sporophytic effects remains to be
determined.
Gametophytic Mutations That Affect Megagametogenesis
During the last few years, many gametophytic mutants affected
in female gametophyte development have been identified and
analyzed (Christensen et al., 2002; Drews and Yadegari, 2002;
Grini et al., 2002; Huck et al., 2003; Rotman et al., 2003). We
have identified and analyzed >150 mutants, allowing genetic
dissection of the female gametophyte developmental pathway
(Christensen et al., 2002; D. Otsuga, C. Dever, N. Huefner, L.A.
Ogden, L.G. Jones, and G.N. Drews, unpublished data). Most
of these mutants are affected in megagametogenesis. As
summarized in Figure 3, the megagametogenesis mutants fall
into six phenotypic categories corresponding to key developmental
events involved in the formation of a mature female
gametophyte. Although the genes affected for most of the
megagametogenesis mutants are not known yet, the use of
insertional mutagens (e.g., T-DNA or transposons) should
facilitate their rapid identification.
Certain key features of megagametogenesis mutant screens
will enable a comprehensive analysis of gametophyte generation
and also will contribute to an understanding of important cellular
and developmental processes throughout the plant life cycle.
First, these screens have yielded mutants with defects throughout
megagametogenesis. Based on these phenotypes, the
affected genes can be ordered, to some extent, within the
developmental program. In addition, many mutations exhibit
defects in specific cellular processes, including mitosis, vacuole
formation, cell wall formation, nuclear fusion, and cell death.
Significantly, many of these mutations would not be identified
in typical sporophytic screens because they cannot become
homozygous during sporophyte generation. Therefore, genetic
analysis of these key processes likely will rely on the characterization
of defective gametophytes.
Pollen Tube Guidance
During the final stages of pollen tube growth, the pollen tube
grows toward an ovule and then up the surface of the funiculus
until it enters the micropyle to penetrate the female gametophyte
(Higashiyama, 2002; Johnson and Preuss, 2002; Higashiyama
et al., 2003). This directed growth pattern suggests that the ovule
and female gametophyte play a role in guiding pollen tube
growth. Many studies suggest that pollen tube guidance is
controlled by both sporophytically expressed and gametophytically
expressed factors (Palanivelu and Preuss, 2000; FranklinTong,
2002; Higashiyama, 2002; Johnson and Preuss, 2002;
Higashiyama et al., 2003). Recently, g-aminobutyric acid was
identified as a sporophytic signal that regulates pollen tube
growth and guidance (Palanivelu et al., 2003).
To determine whether the female gametophyte plays a role in
pollen tube guidance, several groups analyzed pollen tube
growth patterns in Arabidopsis mutants defective in embryo sac
development. Such studies have shown that ovules lacking
female gametophytes fail to attract pollen tubes, suggesting very
strongly that the embryo sac is the source of an attractant that
guides the pollen tube to the ovule (Hulskamp et al., 1995; Ray
et al., 1997; Couteau et al., 1999; Shimizu and Okada, 2000).
Analysis of pollen tube growth patterns in Arabidopsis mutants
defective in female gametophyte development suggests that
guidance by the female gametophyte involves multiple steps. In
mutants in which female gametophyte development is affected
severely (e.g., mutants in categories 1 to 3 in Figure 3), pollen
tubes fail to grow along the funiculus (Hulskamp et al., 1995; Ray
et al., 1997). By contrast, in mutants in which female gametophyte
development is affected less severely (e.g., magatama
Figure 3. Female Gametophyte Development in Arabidopsis.
The steps are described in the text and by Christensen et al. (1997). Category designations show the developmental stage affected in the female
gametophyte mutants. Phenotypic categories are as follows: category 1, megaspores fail to undergo cell death; category 2, megaspores do not
progress beyond stage FG1; category 3, pleiotropic defects during the nuclear division phase; category 4, failure to cellularize or abnormal cell shape;
category 5, polar nuclei fail to fuse; category 6, antipodal cells fail to undergo cell death; category 7 (not shown), morphologically wild-type female
gametophytes at the terminal stage. The gray areas represent cytoplasm, the white areas represent vacuoles, and the black areas represent nuclei. In
this figure, the chalazal end of the female gametophyte is up and the micropylar end is down. ac, antipodal cells; cc, central cell; ec, egg cell; fm,
functional megaspore; m, megaspore; mmc, megaspore mother cell; pn, polar nuclei; sc, synergid cell; sn, secondary nucleus.
S136 The Plant Cell
mutants, which exhibit delayed development and have unfused
polar nuclei at the time of pollination), pollen tubes grow along the
funiculus but do not enter the micropyle (Shimizu and Okada,
2000). These observations suggest that guidance by the female
gametophyte involves at least two phases: guidance from the
placenta to the funiculus (funiculus guidance phase) and
guidance from the funiculus to the micropyle (micropyle
guidance phase) (Shimizu and Okada, 2000).
To identify the specific cells within the female gametophyte
that are the source of the pollen tube attractant, Higashiyama
and colleagues (1998) performed laser ablation experiments in
an in vitro pollen germination system using ovules from Torenia
fournieri. In this species, much of the embryo sac protrudes
from the ovule integuments and is accessible to experimental
manipulation (Tiwari, 1982). Control ovules (no cells ablated) and
ovules in which the egg cell and central cell were ablated
attracted pollen tubes, whereas ovules with ablated synergid
cells failed to attract pollen tubes (Higashiyama et al., 2001).
These studies identify the synergid cells as the source of the
pollen tube attractant. Studies in the in vitro T. fournieri system
also indicate that the effective distance of pollen tube attraction
is 100 to 200 mm and that the attraction signal is species specific
(Higashiyama et al., 1998, 2003; Higashiyama, 2002).
What is the biochemical nature of the synergid cell guidance
cue? Calcium has been proposed to be a guidance signal
because it can attract pollen tubes in vitro in some species
(Mascarenhas, 1962; Mascarenhas and Machlis, 1962, 1964;
Reger et al., 1992), is present in high concentrations in synergid
cells (Jensen, 1965; Chaubal and Reger, 1990, 1992a, 1992b,
1993; Tian and Russell, 1997), and is necessary for pollen tube
growth (Pierson et al., 1994, 1996; Holdaway-Clarke et al., 1997;
Li et al., 1999). However, the addition of calcium to the medium
in the in vitro T. fournieri system does not affect pollen tube
attraction, indicating that calcium is not a universal attractant
(Higashiyama, 2002; Higashiyama et al., 2003). The T. fournieri in
vitro pollen tube guidance assay potentially could facilitate the
biochemical isolation of the guidance cue, although an inherent
difficulty of this approach is likely to be the limitations on the
amount of the target tissue, the synergid cells. Alternatively,
screens for female gametophyte mutants with defects specifically
in pollen tube guidance may facilitate the identification of
the attractant(s). Female gametophyte mutants defective specifically
in pollen tube guidance have yet to be reported. Thus,
the chemical nature of the synergid cell guidance cue remains to
be determined.
Fertilization and Synergid Cell Death
The pollen tube enters the female gametophyte by growing into
one of the synergid cells. Soon thereafter, the pollen tube ceases
growth, ruptures at or near its tip, and releases its contents. The
synergid cell penetrated by the pollen tube undergoes cell death,
either before or upon pollen tube arrival; thus, the pollen tube
contents are released into the degenerating cytoplasm of the
synergid cell (van Went and Willemse, 1984; Willemse and
van Went, 1984; Russell, 1992, 1996; Higashiyama, 2002; Lord
and Russell, 2002; Raghavan, 2003). The time course of these
events has been determined in Arabidopsis (Faure et al., 2002).
Synergid cell death may be a prerequisite for normal
fertilization in angiosperms. For example, the process of degeneration
itself could decrease resistance to both pollen tube
penetration and sperm cell migration during fertilization
(Willemse and van Went, 1984; Huang and Russell, 1992). In
addition, synergid degeneration generally is accompanied by
cytoskeletal reorganizations that may facilitate male gamete
transfer from the pollen tube to the egg and the central cell
(Russell, 1993; Fu et al., 2000).
In some species, synergid cell death appears to be a final step
of the megagametogenesis developmental program. By contrast,
in other species, including Arabidopsis, synergid degeneration
is not an inherent feature of the megagametogenesis
process per se, because synergid cell death does not occur if
pollination is prevented (van Went and Willemse, 1984; Willemse
and van Went, 1984; Russell, 1992; Christensen et al., 1997;
Faure, 2001). Two general, and not necessarily mutually exclusive,
mechanisms might be responsible for synergid cell death
in species such as Arabidopsis. First, synergid degeneration may
be a purely physical process (e.g., the release of pollen tube
contents into the synergid cell may physically rupture the
synergid membrane). Second, synergid cell death may be
a physiological process induced by pollen. Pollen could induce
synergid cell death via a diffusible signal (from the pollen tubes or
from female tissue) or through direct contact with the synergid
cell (van Went and Willemse, 1984; Willemse and van Went,
1984; Russell, 1992; Higashiyama, 2002). All of these mechanisms
may act in angiosperms. For example, in some species,
the synergid cell appears to be completely intact at the moment
of pollen tube discharge, suggesting degeneration via a physical
process. By contrast, in other species, synergid cell death
appears to be initiated before pollen tube arrival at the female
gametophyte, suggesting that a long-range, diffusible signal
induces synergid cell death (van Went and Willemse, 1984;
Willemse and van Went, 1984; Russell, 1992; Higashiyama,
2002). In Arabidopsis, the status of synergid cell death at the
moment of pollen tube arrival at the female gametophyte has not
been determined. However, as discussed below, analysis of
several female gametophyte mutants suggests that synergid cell
death in Arabidopsis is an induced, physiological process.
Because fertilization takes place within the embryo sac, it is
likely that many female gametophyte–expressed gene products
are necessary for this process. However, because the targets of
double fertilization are physically inaccessible, a molecular
understanding of the angiosperm fertilization process has been
lagging. The development of in vitro fertilization systems (Kranz
et al., 1991; Kranz and Lorz, 1994; Kranz and Dresselhaus, 1996)
should allow the manipulation and dissection of double fertilization
(Kranz, 2001; Lord and Russell, 2002). For example, the
in vitro fertilization system could facilitate the identification of all
metabolites, including mRNA, protein, and small molecules,
present within pure preparations of gametophytes before,
during, and after fertilization.
An alternative approach is the identification of mutants defective
in the fertilization process. During the last 2 years, several
female gametophyte mutants affected in the fertilization process
have been reported, including gametophytic factor2 (gfa2)
(Christensen et al., 2002), feronia (fer) (Huck et al., 2003), and
Female Gametophyte Development S137
sirene (srn) (Rotman et al., 2003). In all three mutants, embryo sac
development is normal (fer and srn) or essentially normal (gfa2
female gametophytes also have defects in fusion of the polar
nuclei) and mutant female gametophytes attract pollen tubes but
fail to become fertilized. In response to pollination, fer embryo
sacs undergo synergid cell death (Huck et al., 2003); by contrast,
gfa2 and srn embryo sacs fail to undergo synergid cell death after
pollination (Christensen et al., 2002; Rotman et al., 2003). The
gfa2 and srn mutations do not affect megaspore or antipodal cell
death, suggesting that synergid cell death has unique features.
The srn mutant has an additional defect also exhibited by the
fer mutant: wild-type pollen tubes enter mutant female gametophytes
but fail to cease growth, rupture, and release their
contents (Huck et al., 2003; Rotman et al., 2003). With srn, it is
unclear whether the synergid cell death defect is a secondary
consequence of the pollen tube tip rupture defect or vice versa.
Regardless, the phenotypes of these two mutants clearly
indicate that they are defective in some aspect of pollen–embryo
sac interaction.
GFA2 encodes a chaperone that functions in the mitochondrial
matrix, and the yeast ortholog is required for mitochondrial
function. These data suggest that synergid cell death requires
functional mitochondria, which also are required for cell death in
animals (Christensen et al., 2002). Molecular aspects of the
synergid cell death process remain to be determined. The
proteins encoded by FER and SRN are unknown.
Analysis of the gfa2, fer, and srn mutants reveals several
aspects of the fertilization process. First, gfa2 and srn do not
affect pollen tube attraction and should not prevent the physical
rupture of the synergid cell, suggesting that synergid cell death in
Arabidopsis is not a purely physical process. Second, gfa2 and
srn embryo sacs fail to undergo synergid cell death and yet
attract pollen tubes, suggesting that synergid cell death is not
required for pollen tube attraction. Third, the abnormal pollen
tube behavior within fer and srn female gametophytes suggests
that the presence of synergid cells per se does not ensure normal
pollen tube termination and discharge within the female gametophyte
(Christensen et al., 2002; Huck et al., 2003; Rotman
et al., 2003).
Control of Seed Development
Recent genetic data indicate that the female gametophyte
controls seed development at several levels. First, the female
gametophyte controls the initiation of seed development by
expressing a set of proteins that repress this process in the
absence of fertilization. Second, the female gametophyte
contains factors before fertilization that are required for embryo
and endosperm development after fertilization. Third, the female
gametophyte plays a role in controlling the imprinting of genes
required for seed development.
Female gametophyte mutations that affect the initiation of
seed development include fertilization-independent endosperm
(fie) (Ohad et al., 1996; Chaudhury et al., 1997), medea (mea)
(Chaudhury et al., 1997; Grossniklaus et al., 1998), and
fertilization-independent seed2 (fis2) (Chaudhury et al., 1997).
In all three mutants, endosperm development occurs in the
absence of fertilization. The FIE, MEA, and FIS2 proteins are
related to Polycomb group proteins involved in the heritable
silencing of homeotic gene expression in Drosophila and
mammals (Grossniklaus et al., 1998; Kiyosue et al., 1999; Luo
et al., 1999; Ohad et al., 1999). The FIE, MEA, and FIS2 genes are
expressed in the female gametophyte, primarily in the central
cell, before fertilization (Vielle-Calzada et al., 1999; Luo et al.,
2000; Spillane et al., 2000; Yadegari et al., 2000). Together, these
data suggest that the female gametophyte expresses a set of
proteins that repress endosperm development before fertilization
(Gehring et al., 2004). By this scenario, fertilization could lead
to the initiation of endosperm development by inactivating the
FIE/FIS2/MEA repressive complex (Ohad et al., 1999). How
fertilization accomplishes this task remains an unanswered
question.
The FIE/FIS2/MEA complex most likely represses endosperm
development by preventing the transcription of target genes
involved directly in this process. One such target gene,
PHERES1 (PHE1), was identified recently (Kohler et al., 2003).
PHE1 encodes a MADS domain–containing protein. Chromatin
immunoprecipitation assays have shown that MEA and FIE
interact directly with the PHE1 promoter. In the wild type, PHE1
expression occurs in the early stages of endosperm development
and is not detected in the female gametophyte before
fertilization. PHE1 also is expressed in preglobular-stage
embryos, suggesting a function in both embryo and endosperm.
However, PHE1 expression is upregulated strongly in mea
and fie seeds and is activated inappropriately in fie female
gametophytes. After fertilization, mea seeds exhibit endosperm
overproliferation and embryo abortion (Grossniklaus et al., 1998;
Kiyosue et al., 1999). These postfertilization defects are attributable,
in part, to the activity of PHE1, because reduced levels of
PHE1 expression partially rescue the mea seed phenotype
(Kohler et al., 2003).
After the initiation step, the female gametophyte controls seed
development by providing maternal cues required for this
process; specifically, the female gametophyte contains maternal
factors before fertilization that are required for embryo and
endosperm development after fertilization. Female gametophyte–expressed
genes required for embryo and endosperm
development are referred to as gametophytic maternal-effect
genes (Ray, 1997; Drews et al., 1998; Drews and Yadegari,
2002). This is in contrast to sporophytic maternal-effect genes
(e.g., Arabidopsis SHORT INTEGUMENTS1, barley SHRUNKEN
ENDOSPERM, and petunia FLORAL BINDING PROTEIN FBP7
and FBP11), which are expressed in sporophytic tissue (Felker
et al., 1985; Ray et al., 1996; Colombo et al., 1997).
The clearest examples of gametophytic maternal-effect
mutants described to date include the Arabidopsis capulet1
(cap1) and cap2 mutants (Grini et al., 2002), the Arabidopsis
prolifera (prl) mutant (Springer et al., 1995, 2000), and the maize
maternal effect lethal1 (mel1) mutant (Evans and Kermicle, 2001).
With the exception of prl, which shows arrest predominantly at
the four-nucleate stage of development (Springer et al., 1995),
female gametophytes in these mutants appear normal, whereas
the development of the embryo or endosperm or both is affected
severely very early during seed development. For example,
embryos arising from cap1 female gametophytes exhibit defects
as early as the zygote stage and fail to progress beyond the
S138 The Plant Cell
one-cell proembryo stage (Grini et al., 2002). The CAP1, CAP2,
and MEL1 genes have not been isolated; thus, the molecular
basis for the gametophytic maternal effects observed in these
mutants remains to be determined. However, PRL encodes a
highly conserved homolog of the DNA replication licensing factor
Mcm7, whose accumulation in the female gametophyte appears
to be required for normal embryo development (Springer
et al., 1995, 2000).
A third level at which the female gametophyte influences seed
development is through the control of genomic imprinting. An
important aspect of seed development is the parent-specific
expression of genes required during this process. Parentspecific
expression is achieved by imprinting the alleles inherited
from the male or female gametophyte. For example, with FIS2
and MEA, the maternal alleles are active but the paternal alleles
are inactive during endosperm development (Kinoshita et al.,
1999; Luo et al., 2000).
The imprinting is achieved via epigenetic modification of the
maternal or paternal alleles, a process generally associated with
the methylation of cytosine residues within and flanking the
coding region of the target gene (Jaenisch and Bird, 2003). An
important question is when the inhibitory modifications are
established during development. With paternally imprinted
genes, the simplest model is that the inhibitory modifications
are established during the male gametophyte lineage. An
alternative possibility is that both alleles become modified at
some point during the life cycle and the inhibitory modifications
are removed during the female gametophyte lineage. The latter
possibility appears to be the case with the MEA gene. Recent
evidence suggests that inhibitory modifications of the maternal
allele of MEA are removed in the female gametophyte’s central
cell by a protein called DEMETER (DME). DME is a DNA
glycosylase/lyase related to the superfamily of base excision
DNA repair proteins (Choi et al., 2002). DME is expressed in the
female gametophyte’s central cell before fertilization. MEA
expression is reduced in dme female gametophytes and the
endosperm of seeds derived from dme embryo sacs. DME likely
activates the MEA gene by modifying chromatin structure
through the removal of inhibitory methylated cytosine residues
from its gene-regulatory sequences and counteracting MEA’s
imprinted/silenced state in the female gametophyte. In effect, the
female gametophyte marks the maternal allele of MEA and
presumably other regulatory genes for continued activity after
fertilization. The function of DME in derepressing gene expression
may not be unique, because mutations in a related gene,
termed REPRESSOR OF SILENCING1, have been shown to
cause transcriptional silencing of a transgene and a homologous
endogenous gene (Gong et al., 2002).
SUMMARY
The female gametophyte represents an essential portion of the
plant life cycle mediating several reproductive processes,
including pollen tube guidance, fertilization, the induction of
seed development, and maternal control of seed development.
During the past few years, genetic approaches in Arabidopsis
and maize have provided us with molecular and genetic information
about megagametogenesis and the induction of seed
development. However, we still know little about pollen tube
guidance, fertilization, and the maternal control of seed development
at the molecular level. In addition, a comprehensive
profile of the genes expressed in the female gametophyte has not
been obtained. In light of the fact that a large proportion of female
gametophytic genes are expected to have partially or completely
redundant functions (Drews and Yadegari, 2002), a combination
of forward- and reverse-genetics approaches will be required
to completely understand the development and reproductive
functions of the female gametophyte.
ACKNOWLEDGMENTS
We thank Mark Johnson for critical review of the manuscript. Our work
on female gametophyte development was supported by grants from the
National Science Foundation (IBN-9630371) and Ceres, Inc., to G.N.D.
and from the Department of Energy (DE-FG02-03ER15438) to R.Y.
Received October 13, 2003; accepted February 4, 2004.
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