This PPT explores the structure of pollen wall, origin, function and different attributes along with male germ unit(MGU).

1. Welcome to
Pollen wall structure & Male Germ Unit(MGU)
By N.Sannigrahi, Associate Professor, Department of
Botany, Nistarini College, Purulia, 723101(W.B) India

2. Pollen wall

3. POLLEN GRAIN-MORPHOLOGY
• Pollen grain is a tough, resistant outer coat termed exine with
elaborative sculpturing. The exine & intine-the two layers are
the most important in this regard although the exine does not
develop certain regions which define the positions of
germination apertures with elongate furrows and circular pores
or both. Aperture Number, Position & Characters(NPC) play
an important role in taxonomy.
• Not only the protection, the exine sculpturing plays a role I
attachment to the insect pollinators and adhesion to the
stigmatic surfaces.
• Beneath the exine, the presence of one or two basal nexine
layers, a second major wall layers surrounds the pollen grain
termed intine composed of pectin & Cellulose although the
exine is made up of Sporopollenin (bio-polymer containing
fatty acids, Phenylpropanoids, Phenolois & Carotinoids).

4. Synthesis of Pollen Wall Layers
• The Exine is developed through the contribution of the early
microspores cytoplasm and the inner anther cell layer, the tapetum
having a role in sporopollenin.
• The intine wall synthesis under the control of microspore cytoplasm
and involves gametophyte gene expression from the haploid
microspore nucleus.
• The Pollen wall formation can be summarized under two models-
Primexine model & Undulation model.
• Heslop -Harrison (1971) considers primexine as the blue print for
exine. The primexine has a matrix presumably made up of cellulose
micro fibrils , is deposited in between the spore and the callose wall.
Scott(1994) believes that “ the primexine acts as a loose scaffold to
which the sporopollenine monomers(Fatty acids & phenols) are
covalently attached by the localized action of super oxide radicals
generated at the plasmalemma”.

5. Pollen wall
• POLLENWALL:
• Wall of the pollen is stratified.
• Comprising two layers-outer Exine & inner Intine.
• Exine consists of many sub layers, composed of sporopollenin,
derived from oxidative polymerization, resistant to physical &
biological decomposition preserved for long periods during
fossilization, performs protective during its journey from
anther to the stigmatic surface.
• Intine, a pectocellulosic layer is microfibrilar with micro fibrils
oriented in plane parallel to the surface, presence of beads,
ribbons, plate of enzymatic protein particularly at the vicinity
of germ pores.
• .Both the layers extend regulative role –stability & biological
attributes.

6. POLLEN WALL

7. DEVELOPMENT OF POLLEN WALL
• At the initial stage, microspores tetrad surrounded by common
callose wall, individual spore without wall & is separated from the
outer spores of the tetrad by callose partitions,
• Enclosed by callose wall, the individual spore starts wall formation.
• 2 phases in the pollen wall formation
• In the first phase, the wall materials derived from cytoplasm of the
spore wall & this is during the tetrad stage.
• In the 2nd phase, after the release of the spores through enzymatic
degradation of the callose wall, the wall materials are contributed by
the tapetum in addition to the spore cytoplasm.
• The 1st layer of the pollen wall is cellulosic & is known as
primexine. Cellulosic micro fibrils of this layer are deposited by the
convoluted plasma lemma of the spore and the callose wall.
• The cellulosic primexine is discontinuous in these origins and the
gaps thus created mark in the position of germ pores.

8. Continuation
• When the cellulosic primexine attains a degree of thickness,
additional gaps appear in it, columns of convoluted lamellae
are deposited in these gaps at the surface of the plasmalemma.
• These columns are called probacula.The cytoplasm of the
spore starts synthesizing the precursors of the sporepollenin,
polymerized & deposited on the surface of the lamellae. The
columns are called bacula.
• The distribution and orientation of the bacula very according
to the pattern of the mature exine.
• Later, the lower ends of the bacula sideways to the cellulosic
layer premexine and combination to foot layer.
• The foot-layers forms a sort of floor on which are raised the
columns & bacula.
• The top of the bacula-columns may also spread side ways in
all directions to form the tapetum.

9. Origin & development

10. Continuation--------
• To this extent, exine is formed inside the callose wall of the tetrads
• The pattern of the mature exine has already been laid down before
the spores are set free.
• With the passage of time, the callose is gradually digested and the
individual spores lies free within the anther locule
• In the free state, pollen grains synthesize the intine and the
innermost layer of exine
• The formation of the intine by the dint of dictyosomes.It usually
starts to develop at the vacuolated stage, beneath the aperatures.It
increases thickness under the pores and later on starts to develop
under the interaperatuural parts as a part of thin layer. Golgi bodies
are frequent during intine synthesis. The intine beneath the pore
become comparatively very thick and is provided with fibrillar
material and radially arranged membranous units.

11. POLLEN KIT
• POLLEN KIT:
Pollen kit , an oily layer found on the outside of the mature pollen
grains of many entomophilies species,
Pollen kit materials is contributed by tapetal cell comprising of
lipid and carotenoids.
Carotenoids responsible for its yellow or orange color and
contains some proteins making sticky.
The biological function of pollen kit is obscure but some
suggestions as far as its function is concerned-
Acting as insect attractant
Protecting pollen from UV
May function as pollen-borne substances involved in saprophytic
incompatibility

12. SPOROPOLLENIN
• Pollen grains consists of three substances-
• The inside cell is filled with living cytoplasm that deteriorates
rapidly during fossilization.
• The inner layer of the cell wall mainly composed of pectin and
cellulose also undergo degradation.
• The outer wall , the exine mainly composed of sporopollenin ,
an N-free polymeric substances belong to trepans having
chemical formulae-C90H130-158O24-44.
• It is extremely unsaturated and undergoes corrosion by
oxygen. But is resistant to strongly alkaline substances and
organic acids. It is thus most resistance substances of the plant
world.

13. Pollen kit

14. Layers of pollen wall in image

15. MALE GERM UNIT(MGU)

16. Diagram of MGU

17. Male Germ Unit(MGU)
• The MGU consists of a stable association between vegetative
nucleus and germ cells(Generative cells or sperm cell pair)[Dumas
et al, 1984;Mogensen, 1992].The reproductive unit is transported
within pollen tube and delivered to the embryo sac. Generative and
sperm cells coated in myosin can be transported along the actin
cytoskeleton in vitro or injected into alga cells. The sperm cells may
involve an acto-myosin like motility similar to animal cells. The
proposed function of the male germ unit may be to ensure the
synchrony of the fusion of two sperm cell fusion events, with egg
cell & central cell, and the subsequent development of the resulting
zygote and endosperm. In certain species of Plumbago, the sperm
cells are dimorphic and preferentially fuse with either egg cell or
central cell , further illustrating the tightly controlled recognition
process involved in double fertilization(Russell,1985).

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