Microgametogenesis- Brief Idea

Microgametogenesis as well as male gametophyte to explore the beauty of sexuality of angiosperms along with the different attributes .

1. Welcome to
Flower Development: Microgametogenesis
By N.Sannigrahi, Associate Professor, Department of
Botany, Nistarini College, Purulia, 723101(W.B) India
The sexual reproduction needs two non-identical
gametes for fertilization for the zygote followed by the
embryo –the baby plant. Pollen grains represent the
highly reduced haploid(N) male gametophyte generation
in flowering plants, consisting of two or three cells when
released from anthers. The role is to deliver twin sperm
cells to the embryo sac to undergo fusion with the egg
and the central cell. Initially the meiocytes differentiate
and become selectively isolated from the mother plant
through callose secretion by the meiocyte cytoplasm. By
series of changes, the male gametophyte development
takes place.

3. Where & When?
•Microspore or pollen grain , the 1
st
cell of the male
gametophyte contains one haploid(N) nucleus.
During the early stage of development, it remains
within the microsporangia.The cell undergoes
unequal division and forms a small generative cell &
large vegetative or tube cell. Initially the generative
cell remains lying at one corner of the spore wall.
Within a short time, it gets detached and becomes
ellipsoid or fusiform in shape and remains suspended
in the cytoplasm of the vegetative cell. But at the
onset of the pollen germination, the next stages
happens to be in a complete desired shape.

4. Pollen -Pistil Interaction
•Pollen grains are deposited on the stigma either due to
the closeness of the anthers to the stigma or by the
pollinating agents-biotic or abiotic.A successful
pollination brings about sequential event as in the
pistil that ultimately ends up by the discharge of the
male gametes in the embryo sac by the following
pathways.
•Pollination→ Pollen adhesion→ Pollen hydration→
Pollen germination→ Pollen tube entry into stigma→
Pollen tube growth through style→ Pollen tube entry
into the ovule and embryo sac→ Double fertilization

5. Pollen Germination
•When a pollen grains fall on a receptive stigma, the stored
RNA, protein & bioactive small molecules allow rapid
germination and growth of the tube that penetrates and
grows within a style.
•Pollen attachment and hydration of the takes place
provides suitable conditions for the recovery of the
membrane integrity of the vegetative cell.
•The stigmatic surface provides the essential prerequisites
for a successful germination that are absent in pollen. The
mechanisms differ with respect to the nature of the
stigma-dry or wet.
•Intraspecific incompatibility is a well designed genetic
mechanisms by which certain plants recognize and reject
their own pollen thus facing out breeding.

6. Flow Chart of pollen pathway
•Pollen touches down on stigma papilla→ Acceptance
→Hydration , Germination, tube growth→
Acceptance →Tube penetrates stigma cuticle
→hydrolyzing enzymes activated→ Acceptance
→Tube growth through style→ Acceptance →Tube
penetrates into the embryo sac, fertilization
The above flow chart indicates how the
compatible pollen grains make an avenue until
fertilization takes place.
•In course of the pollen attachment with the stigma, a
diverse mechanisms operated having biochemical &
genetically mechanisms in this regard.

7. Bio-chemical mechanisms of pollen development
•Pollen germination & tube growth are processes
fundamental for reproduction of flowering plants.
Germinating and mature pollens were compared via gel-
based proteomic tools(Zhu et al, 2007).Proteins present at
higher amounts in mature pollen were mainly associated
with tube wall modification, actin cytoskeleton
organisation,energy metabolism, signaling, protein
folding and degredation.Specifically, pectin
methylestyerase, inorganic pyrophosphatage, glucose 1
phosphate uridylyltransferase were profoundly up
regulated, demonstrating their role in pollen tube growth.
Proteins present in declining amounts included
detoxifying enzymes as well as few allergens and
metabolic enzymes.

8. Birds’ eye view of Microgemetogenesis

9. Method of Development
•The first division of the microspores give rise to two
unequal cells:
• The larger one is vegetative cell which develops into
pollen tube
•The smaller one is the generative cell which with the
subsequent mitotic division transforms male gametes.
•Geitler (1935) observed that the metaphase spindle
usually shows pronounced asymmetry, the wall ward pole
being blunt and the free pole acute
•The cause of the asymmetry has been attributed in the
time of the development of the spindle poles
•Generative pole develops more slowly than the vegetative
because of the smaller amount of cytoplasm is associated
with the former

10. Recognition of the pollen by the stigma
•The physiological mechanisms operates to ensure that
only intraspecific pollen germinate successfully by a self
incompatibility system that allow the recognition reaction
system sets in almost after the pollen comes in contact
with the stigma.
•The pollen grains following in contact with the stigma
synthesis nearly forty new proteins with highly
phosphorylated.The protein phosphorylation may be
responsible for signal transduction in a compatible
association( Hiscock et al, 1995).There is a brief Ca+2
peaks in the stigmatic papillae following its contact with
the compatible pollen grains. Thus, there is a definite
participation of calcium ion in pollen signal perception.

11. Mode of development
•However, the microspores of the same tetrad show same stage
of development but a complete synchronization is observed in
massulae and pollinia , where microspores are united together
as in Mimosaceae, Asclepiadaceae & Orchidaceae.
•The generative cell is either cut off inner side or on the outer
side or on the radial wall or in a corner
•The generative cell soon loses contact with the wall of the
microspores and come to lie freely in the vegetative cell,
•Immediately after the detachment, the generative cell is
spherical,
•However, variation in shapes like elliptical, lenticular , spindle
shaped or sometimes vermiform have also been observed in
different species,
•The generative cell contains all usual organelles except plastid
with few exceptions
•The nucleus is smaller but contains higher amount of DNA.

12. microgametogenesis

13. Mode of Development
•The generative cell either divides when the pollens
are still within anther or it may takes place after the
discharge of the pollen
•The two male gametes or sperms thus formed are
discrete cells and their cytoplasmic sheath persists
throughout their course in the pollen tube
•The tube cell , which enclose the generative cell, will
produce the pollen tube , a structure essential for the
delivery of male gamete, sperm

14. Diagrammatic view

15. Conclusion
•Microgametogenesis or male gametophyte development
is an integral part of the alternation of generations of
angiosperms. The generative cell leads to form the
gametes but the vegetative cell having tube nucleus does
not play any crucial role.Intially tube nucleus thought to
be the responsible for the direction of the growth of the
pollen tube but recent investigation confirms the role of
tube nucleus. It is purely a non-functional and vestigial
structure and play some accessory role in the
development of the male gametophyte development.
Thus, angiospermous male gametophyte are “lean”,
apparently requiring a minimum number of cells and
nuclei; they function to deliver sperm cells to the female
gametophyte as a part of siphonogamy.

16. Thank you for your participation