This document discusses the processes of reproduction in crop plants, focusing on both sexual and asexual reproduction. Key stages such as sporogenesis, gametogenesis, fertilization, and seed development are explained in detail, highlighting the roles of various structures like the ovule, integuments, and endosperm. The document also covers embryogeny and different forms of endosperm development, emphasizing their significance in plant growth and reproduction.

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PLANT SCIENCE: Seed Science and Technology
TITBITS 2: Sporogenesis, Gametogenesis, Fertilization & Seed
Development
Prepared by
Dr. K. Vanangamudi
Formerly Dean (Agriculture),
Dean Adhiparasakthi Agricultural College,
Professor & Head,
Seed Science & Technology, TNAU, Coimbatore

2.  Reproduction - process by which living organisms give rise to offspring of
similar kind (species).
 In crop plants, mode of reproduction is of two types, viz.
1) Sexual reproduction
2) Asexual reproduction
Sexual reproduction
 Multiplication of plants through embryos which have developed by fusion of male
and female gametes
Ovule development
 Development of ovule occurs within the ovary, which provides a location for
nurture and development of the female gametophyte
 Ovule growth begins as a small outgrowth within the nucellus.
 Secondary outgrowths,orcollars(integuments),soonappear aroundtheperiphery
of the nucellar outgrowths and envelop it.
 Consist of the inner and outer integuments and ultimately become the testa
(seed coat) of the mature ovule.
Funicle, hilum, chalaza and raphe
 Developing ovule is commonly attached to placenta by the funiculus.
 Scar on the ovule made where the funiculus detaches at maturity - hilum.
 Point where the integuments meets at the nucellar apex is the micropyle, and
the region of integumentary origin and attachment, usually opposite the
micropyle, is the chalaza.
 Between chalaza and hilum of many species is an area known as the raphe.

3. Nucellus
 Nucellus provides tissue for the origin and nature of the female gametophyte,
from archesporial cell to the mature megagametophyte.
 Originates from ovary tissues and provides site of archesporial cell origin.
Integuments
 Nature and thickness of the integuments vary considerably among species,
depending on their role in contributing to embryo sac and ovule development.
Micropyle
 Integumentary pore or opening in the ovule through which the pollen tube
grows to fertilize the egg cell of the female gametophyte.
Epistase
 Development of well-defined nucellar or integumentary tissue in the micropylar
region of the seed of certain species.
 Cells adjacent to the micropyle may show a marked elongation.
SEXUAL REPRODUCTION
Sporogenesis
 Production of microspores and megaspores
 In anthers, microspores are formed through microsporogensis
 In ovules, the megaspores are formed through megasporogenesis.
Gametogenesis
 Production of male and female gametes in the microspores and megaspores.
 Microsporogenesis
 Sporophytic cells in the pollen sacs of anther which undergo meiotic division to
form haploid i.e., microspores are called microspore (MMC) or pollen mother
cell (PMC)
 Each PMC produce four microspores and each microspore after thickening of
the wall transforms into pollen grain.

4.  Microgametogenesis
 Production of male gametes or sperm.
 On maturation of the pollen, microspore nucleus divides mitotically to produce a
generative and a vegetative or tube nucleus.
 Pollen is generally released in this binucleate stage.
 Reach of pollen over the stigma - pollination.
 Generative nucleus undergoes another mitotic division to produce two male
gametes or sperm nuclei.
 Pollen along with pollen tube possessing a pair of sperm nuclei -
microgametophyte.
 Pollen tube enters the embryo sac through micropyle and discharges the two
sperm nuclei.
 Megasporogenesis
 In angiosperms, seeds originate from meristematic tissue of the ovary wall
called ovule primordial.
 From nucellus, one cell develops special characteristics that distinguish it from
adjacent cells.
 As this cell increases in size, its nucleus becomes larger and its cytoplasm grows
denser in preparation for cell division (archesporial cell).
 First division results in a megaspore mother cell and a parietal cell. Usually the
parietal cell remains undivided and soon deteriorates; however, in some
species, it undergoes further division and contributes to seed formation.

5.  A diploid (2N) megaspore mother cell undergoes a two-step cell division called
as meiosis which (Fig. 2) resulted in four megaspores (haploid (1N) cells), each
having one half the chromosome complement of the mother plant.
 Normally, only one megaspore is functional, while the other three
degenerate.
 Megagametogenesis
 Nucleus of the functional megaspore undergoes three mitotic divisions to produce
eight or more nuclei.
 The megaspore nucleus divides thrice to produce eight nuclei.
 Three of these nuclei move to one pole and produce a central egg cell and two
synergid cells on either side.
 Another three nuclei migrate to opposite pole to develop into three antipodal
cells.
 Two polar nuclei remaining in the center, fuse to form the secondary nuclei.
 Megaspore develops into a mature female gametophyte - megagametophyte or
embryo sac.
 Development of embryo sac from a megaspore - megagametogeneis.
Fertilization
 Fusion of one of two sperms with egg cell producing a diploid zygote
 Fusion of the remaining sperm with secondary nucleus leading to formation of a
triploid primary endosperm nucleus - triple fusion.

6. Seed development
 Embryo sac may be ellipsoidal, elongated or variously bent in shape.
 Longitudinal axis extends from chalaza to micropyle and through antipodal cells,
the endosperm nucleus and the egg apparatus.
 Morphologically, micropyle is at upper end of the embryo sac.
 As embryo sac is growing, it requires a constant nutritive supply, which is provided
through the chalaza, establishing a polar gradient from the antipodal to the
micropylar end.
 Also obtained from the nucellus and integumentary layers directly through the
wall of the embryo sac.
Embryogeny

7.  After sexual fusion or syngamy, a brief period of reorganization occurs, during
which the large vacuole gradually disappears, with the zygote cytoplasm
becoming more homogeneous and the nucleus large.
 Lines of polarity in preparation for future division and growth already exist in
embryo sac having been established in the unfertilized egg.
 The still undivided zygote typically elongates along the horizontal axis, and
small vacuoles become evenly distributed through the cytoplasm.
1. Globular stage
 Globular stage of embryo development precedes the cotyledon development.
 Embryo is spherical in shape. Suspensor also develops along with embryo.
2. Heart stage
 Globular embryo develops into two lobed form and looks like a heart.
 Suspensor acts as an “umbilical cord” providing nutrients to the embryo.
3. Torpedo stage
 Cotyledons and axis elongate, enlarge and grow longer.
 Embryo may remain straight or became curved.
4. Mature embryo
 As the embryo mature, cotyledons curve and provide nutrition to plant, enough to
get plant started with photosynthesis.
 Suspensor disappears when cotyledons enlarge and mature.
 Endosperm has been used for nutrients for the development and maturation
of cotyledons and embryonic axis.

8. Endosperm development
 Endosperm is the principle nutritive support for the embryo of many species
(especially monocotyledons) during both seed development and germination.
 Provide nutrition for the developing embryo; therefore, its composition is
compatible with the embryo’s needs.
 But, endosperm must also draw its nutritive support from embryo sac and
surrounding tissues.
 Net effect is to surround embryo with a rich nutritive tissue from which it can
draw for development and growth.
 Creates competition for nutrients, both within and outside the embryo sac.
Types of endosperm development
 Division of the primary endosperm nucleus yields micropylar and chalazal
chambers
 When only one develops, the other is crushed and soon degenerates.
 Classification depending on the sequence of nuclear division and cell wall
formation.
Cellular endosperm
 Each nuclear division is accompanied by cell wall formation.
Nuclear endosperm
 Characterized by nuclear divisions unaccompanied by cell wall formation.
 Nuclei may remain free or may later be separated by cell walls that form in one of
the three ways:
(1) one to three layers of cell wall may form around the periphery, with free nuclei
inside
(2) A cell wall may form in the micropylar area, with the rest remaining in a free-
cell state, or
(3) Entire endosperm may be filled with walled cells.

9. Helobial endosperm
 Intermediate between nuclear and cellular types.
 Free nuclear divisions occur, but cell wall formation accompanies nuclear division
in some parts of the endosperm as well.
Endosperm haustoria
 Nutrient gathering outgrowths called haustoria.
 May develop at either the micropylar and chalazal ends and reach into the
nucellar, integumentry, or even ovary tissue.
 Branch into several prominent lobes, or diverticulae.
 When local food supply is exhausted, haustoria lobes terminate and become
crushed by further endosperm and embryo growth.
The mature endosperm
 Monocotyledonous endosperms usually reach their maximum morphological
development at physiological maturity.
 In dicotyledonous species, endosperm may not develop or may be used up by
developing embryo.
 Seed with little or no endosperm are exalbuminous.
 Well-developed endosperm (or perisperm) are known as albuminous.
 Some species have a well developed chalazosperm, in which both the nucellus
and endosperm disappear during development and chalazal tissue proliferates
and forms storage tissue.
 Outermost layer of the endosperm is known as the aleurone layer.
 Become thickened and filled with protein granules.
 Functions: both as storage tissue and for secretion of hydrolytic enzymes,
which upon activation during germination help to break down storage tissue.

10. Life cycle of plant