Female gametophyte
• Thefemale gametophyte organ is commonly known as the
embryo sac. The female gametophyte in angiosperms is
essential for plant reproduction because it contains the egg cell
and central cell, which, when fertilised, give rise to the embryo
and endosperm of the seed, respectively.
• The female gametophyte develops early in ovule development
when a diploid megaspore mother cell undergoes meiosis. One
of the haploid megaspores produced develops into the female
gametophyte.
3.
Female Gametophyte Developmentin
the Ovule
• In angiosperms, ovules are located within the ovary of the
gynoecium. Ovules are the sites of megasporogenesis.
• Upon fertilization of the female gamete, or egg, by a sperm
cell, embryogenesis is initiated and the ovule develops into a
seed. Simultaneously, the ovary enlarges and becomes a fruit.
• Ovule primordia arise in a specialized ovary tissue called the
placenta. The type of placentation within the ovary determines
the positions and arrangement of the seeds within the fruit.
4.
Ovule development
• TheArabidopsis gynoecium is an important model system for
studying ovule development.
• The gynoecium of Arabidopsis, as in many members of the
Brassicaceae (mustard family), consists of two fused carpels,
referred to as valves, separated by a medial partition called the
septum.
• The edges of valves and the septum are joined at a strip of tissue
called the replum, which plays an important role in the
dehiscence of the dry fruit.
Types of ovuleDescription Example
1. Orthotropous ovule erect or straight, chalaza,
micropyle and funiculus
lie on the same line
Gymnosperm,
Polygonum
2. Anatropous ovule Ovule bent at 180
degree, funiculus and
micropyle lie close to
each other
Mostly Angiosperm
3. Hemi-anatropous or
hemitropous ovule
ovule is placed at right
angles to funiculus
Ranunculaceae
4. Campylotropous
ovule
Nucellus curved and
micropylar end is bend
down slightly
Leguminosae
5. Amphitropous ovule Nucellus and embryo sac
both are curved and latter
appears horse shoe shape
Allismaceae, and
Butomaceae.
6. Circinotropous ovule Ovule bent at 360 degree Opuntia
9.
• According toone classification scheme, there are more
than 15 different patterns of embryo sac development in
angiosperms.
• The most common pattern was first described in the
genus Polygonum (“knotweed”) and is therefore called
the Polygonum type of embryo sac.
• Female gametophyte development in plants is divided
into two phases:
• 1. Megasporogenesis 2. Megagametogenesis.
10.
Megasporogenesis
• During megasporogenesis,the diploid megaspore mother
cell (MMC) goes through meiosis, which results in the
formation of four haploid nuclei.
• Angiosperms have three primary megasporogenesis patterns:
monosporic, bisporic, and tetrasporic. In these cases, ovules
typically differentiate a single MMC in the micropylar
region of the nucellus. This MMC is large, with dense
cytoplasm and a prominent nucleus.
11.
The archesporial cellwithin the nucellus differentiates into
the megaspore mother cell (MMC), the cell that undergoes
meiosis.
In the Polygonum type of embryo sac, meiosis of the diploid
MMC produces four haploid megaspores. Three of the
megaspores, usually those at the micropylar end of the
nucellus, subsequently undergo programmed cell death,
leaving only one functional megaspore.
Functional megaspores then undergo three rounds of free
nuclear mitotic divisions (mitoses without cytokinesis) to
produce a syncytium.
12.
• The resultis an eight-nucleate, immature embryo sac.
Four of the nuclei then migrate to the chalazal pole,
and the other four migrate to the micropylar pole.
• Three of the nuclei at each pole undergo
cellularization, while the remaining two nuclei, called
polar nuclei, migrate toward the central region of the
embryo sac, which also contains a large vacuole.
• The three cells at the chalazal end of the embryo sac
are termed the antipodal cells.
13.
• The eggcell (the female gamete that combines with a
sperm cell to form the zygote) and the two synergid cells
are located at the micropylar end of the embryo sac and
are collectively referred to as the egg apparatus. An
additional feature is the presence of a filiform apparatus
at the extreme micropylar end of each synergid.
• The fully cellularized embryo sac represents the mature
female gametophyte or embryo sac. At maturity, the
Polygonum-type embryo sac consists of seven cells and
eight nuclei.
Integument
Mostly an ovulehas either one or two integuments. Ovules
with one integument are called unitegmic, and those with
two integuments are known as bitegmic. The Sympetalae
predominantly show unitegmic condition. Bitegmic ovules
occur in Polypetalae and monocots (Table 6.1). In some
members of the Olacaceae (Liriosma, Olax imbricata,
Ptychopetalum) the ovules lack an integument and are
called ategmic (Davis, 1966).
17.
Ontogenetically, an ovulearises as a small mound of
homogeneous tissue on the placenta. At this stage the ovule
looks orthotropous and at the end stage looks anatropous.
Integuments arise close to the base of this tissue which
forms the nucellus in a mature ovule.
D
Fig. 6.3 A-D. Stages in ovule development in Aquilegia vulgaris. Arrows are
pointing at the primordia of integuments. E. Fully developed bitegmic,
anatropous ovule.
18.
ENDOTHELIUM: In mostplants belonging to the
Sympetalae with unitegmic, tenuinucellate ovules, the
nucellus degenerates at an early stage of ovule development,
and the innermost layer of the integument becomes
specialized to perform the nutritive function for the embryo
sac. This specialized tissue, present around the embryo sac,
is called endothelium. The endothelium is usually single-
layered. In Asteraceae it may become multilayered; ten to
twelve-layered endothelium.
19.
MICROPYLE: Depending uponthe presence or absence
of integuments, the micropyle may or may not be
organized. In bitegmic ovules the micropyle is generally
formed by either both the integuments or only the inner
integument. Only rarely does the outer integument alone
constitute the micropyle. When both the integuments are
involved the passage formed by the outer integument is
called exostome and that by the inner integument is
called endostome.
20.
OBTURATOR. Any ovularstructure associated with
directing the growth of pollen tube toward the micropyle is
generally referred to as obturator. They may originate from
placenta or funiculus, or both. The most common type of
obturator is one formed by local swelling of the funiculus
(Acanthaceae, Anacardiaceae, Lamiaceae, Magnoliaceae).
In Crinum the funiculus simply becomes knee-shaped and
functions as an obturator.
21.
NUCELLUS
Nucellus represents thewall of megasporangium. Each
ovule has only one nucellus. As an abnormality, however,
twin nucelli may occur in a common fold of integuments.
This has been observed in Aegle marmelos, Hydrocleis etc.
22.
HYPOSTASE AND EPISTASE:Hypostase is derived
from the nucellar cells below the embryo sac. It refers to a
group of cells present between the chalaza and embryosac.
While, the Epistase is formed by the nucellar epidermis
above the embryo sac. It forms a cap-like structure of
cutinized cells above embryosac at micropylar region. It is
distinguishable even during advanced stages of embryo
development.
23.
Embryo sac developmentinvolves hormonal signaling
• Three hormones auxin, cytokinin, and brassinosteroids
have been implicated in the regulation of various stages
of female gametophyte development in Arabidopsis.
• For example, two YUCCA genes, which encode flavin
monooxygenases involved in local auxin biosynthesis, are
expressed in the ovule, and the auxin efflux carrier PIN1
is expressed in the nucellus. Mutations in the latter have
been shown to cause defects in the development of the
female gametophyte, causing it to arrest at the one- or
two-nucleate stage.
24.
• These observationsare consistent with the role of auxin
as a cell fate determinant in female gametophytes.
Cytokinins synthesized in the chalazal region of the
nucellus have been implicated in megasporogenesis.
• Triple mutants lacking functional AHK receptors,
which are required for the cytokinin response, fail to
develop functional megaspores.
• Brassinosteroids have been shown to be required for
the initiation of mitotic divisions by the megaspore.
25.
The female gametophytesof Arabidopsis mutants with a
defective CYP85A1 gene, which codes for an enzyme that
regulates brassinosteroid biosynthesis in the embryo sac,
are arrested before the first nuclear mitotic division of the
haploid functional megaspore. In other words,
brassinosteroid biosynthesis inside the embryo sac is
required for the initiation of megagametophyte
development.
