1. Megasporogenesis and microsporogenesis
megasporocyte microsporocyte
Generates megaspore and microspores
meiosis
REVIEW

2. Examples of angiosperm diversity in pollen and stigma structures
A.Hydrated Lilium longiflorum pollen grain. Red stained has
several fissures at the single aperture . Green coating (green) forms
droplets on the surfaces of the ornamented exine walls
B.Multiapertured pollen of Passiflora fanguinolenta
C.Polyad pollen of Acacia retinoides
D.Several pollen types
E.A portion of stigma of Torenia fourmiere
F.Stigma papillae of Arabidopsis
D

3. The exine protects the spore against dessication,
mechanical pressure and ultraviolet radiation. Sometimes
the exine layer is covered by sticky substances (pollenkitt,
tryphine, elastoviscin and sporopollenin viscin threads),
which are also produced by the tapetum.
Pollenkitt- an adhesive material facilitates the attachment of
pollen grains to insects, and in this way also zoophilic
pollination.
It also plays an important role in the adhesion of pollen
grains to the female stigma and in the recognition between
pollen and pistil. Also substances responsible for
pollen allergy are often products originating from the
tapetum.
Pollen grains

4. A. Pollen tetrad from Arabidopsis mutant quartet. Four complete pollen grains connected by
exine bridges
B. Fused pollen grains of Arabidopsis mutant tes/stud. Cytokinesis does not occur after
meiosis resulting in multinucleate cytoplasm
c. Natural pollen tetrads from Drocera binata

5. The exine protects the spore against dessication,
mechanical pressure and ultraviolet radiation. Sometimes
the exine layer is covered by sticky substances (pollenkitt,
tryphine, elastoviscin and sporopollenin viscin threads),
which are also produced by the tapetum.
Pollenkitt- an adhesive material facilitates the attachment of
pollen grains to insects, and in this way also zoophilic
pollination.
It also plays an important role in the adhesion of pollen
grains to the female stigma and in the recognition between
pollen and pistil. Also substances responsible for
pollen allergy are often products originating from the
tapetum.
Pollen grains

6. Pollen outer surface
Outer exine- multilayered ,
sporopollenin, interrupted
by apertures
Inner intine- sometimes
multilayered with cellulose
Pollen coat- lipids, proteins
pigments, aromatic
compounds fill the
sculptured cavities of exine.,
Pollen-stigma interaction
3 strata ( relative amount
varying between species
C

7. Pollen tube
Stigma- receptive part of pistil,
bind pollen ,mediate tube
migration into style:
Receptive stigma are of 2
types:
Wet- covered with surface
cells , often lyse & release
secretions. Pollen simply sinks
in the viscuous fluid.

8. Stigma papillae
from Arabidopsis
Dry- intact surface cells,
protrusion as papillae and
cells covered with cell wall,
waxy cuticle and
proteinaceous pellicle..
Never has copious surface
secretion. Possibilility for
pollen attraction and
retention is adherence by
chemical bonds.
Ex. A chemical bond forms
where the exine contacts the
pellicle or where tapetal
coatings on exine flows on
the stigma surface.

9. stigma
Pollen-stigma functions
1. Initial adhesion: exine-mediated
May depend on biophysical or
chemical interactions between
stigma surface and exine polymers
2.Mobilization of pollen coat
leads to mixing of lipids and
proteins and form “foot” of
contact on stigma surface.
3. Later stage: Proteins and lipids
on pollen coat and proteins on
stigma surface contribute to
adhesion, requires protein-
protein interaction
A. Pollen adhesion

10. D. Pollen coat has mobilized to the site of contact between pollen
and the stigma forming a foot between the 2 surfaces

11. B. Pollen hydration: Activating metabolism.
Pollen highly dessicated: 15 to 35 % water content when released
from anthers.
On wet stigmas water immediately surrounds pollen grains
On dry stigmas pollen mobilize their lipid coat to form an
interface between the two surfaces. This interface
will promote water flow.
water
nutrients, etc.
Transported rapidly into the grain from
stigma exudate (wet stigma )
or from stigma papillae from dry stigma
Aquaporins expression in the stigma :
water channels involved in the rapid but
regulated water release from stigma to
pollen
Aquaporins are
transport
proteins that facilitate
water transport
across membranes

12. C. Pollen polarization and germination: preparing for
pollen tube growth
Hydration transforms a pollen from non-polar cell to highly
polarized cell. Within minutes after hydration pollen organizes
its cytoplasm and cytoskeleton structures to support
extension of a single tube:
 Formation of filamentous cytoskeletal structures that
wrap around the nuclei
 Actin cytoskeleton polarization toward the site of tube
emergence
 Reorientation of large vegetative nucleus so that it enters
the extending tube before the generative cells
 Assembly of mitochondria and polysaccharide particles at
the site of elongating tube tip
 Selection of pollen plasma membrane for secretory vesicle
targeting and deposition of callose at site of tube
emergence

13. Germination of pollen tube
Pollen tubes
extend up to sev.
cm to reach
embryo sac. Cell
wall lacks
cellulose but has
another
polysaccharide-
callose- , a glucan
Callose –synthesized by Golgi and transported
to the extreme tip of pollen tube by Golgi-derived
vesicles .Fusion of vesicles with plasma
membrane expand the cell membrane of
elongating tube
Content of vesicles expand the wall of elongating tube

14. Cytoplasmic domain
A Clear zone--------------------devoid of large organelles, no organized
movement
B Subapical domain
C. Nuclear domain D. vacuolar domain
Pollen tube elongation up to many cm thru female tissues
Pollen tubes
are polarized cells.
Internal
differentiation with
distinct intracellular
zones

15. A. Tip domain- rich in Golgi vesicles
B. Sub-apical domain- with metabolically active organelles:
mitochondria, dictyosomes, ER, vesicles
C. Nuclear domain: large organelles and male germ unit
D. Vacuolar domain. Enlarges as the tube grows.

16. • Secretory vesicles originate from the Golgi network and are
transported over the actin cytoskeleton into the growing tip where
they fuse with the tip membrane expanding the wall and the plasma
membrane.
• Besides pollen tube wall proteins, these transport vesicles contain
pectins as part of the primary tube tip wall.
• Total cytoplasmic vol. does not increase as pollen tube grows; bulk
of cytoplasm is in close proximity to growing tip
Germinating
pollen

17.  Cytoplasm continues to move with
the tip as tube elongates
 Cytoskeleton continually transports organelles, the
generative or sperm cells and vegetative
nucleus toward growing tip
 Shows “reverse fountain” cytoplasmic streaming-forward
movement of organelles thru cortical region of the tube,
undergoes a turnover in the subapical domain moving back
centrally, away from the tip of the tube.

18. • Golgi/dictyosome-derived vesicles containing callose
(not cellulose) transported to tip by microfilaments.
Vesicles fuse with plasma membrane at tip of tube so
expands the wall
contents of vesicles expand the plasma membrane
as the tube grows
vesicles

20. pollination

21. POLLEN TUBE GUIDANCE ON THE PISTIL
1. THRU PAPILLA
CELL WALL OF DRY
STIGMA OR
INTERCELLULAR
SPACES
2. BETWEEN CELLS
IN SECRETORY
REGIONS OF WET
STIGMAS.
12hrs 24 hrs 6d

22. A. Porogamy (pollen directed to the micropyle by molecular
signal
B. Chalazogamy ( pollen adhere and grow up the surface of
chalazal region. C. Mesogamy (pollen directed to the
micropyle by molecular signals from synergids)

23. porogamy
chalazogamy Mesogamy
(thru funiculus)

24. Entry of pollen tube in the embryo sac is under chemotropic guidance
(synergids have filiform apparatus and secrete some chemicals). Once
inside the cytoplasm of the synergid, pollen
tube growth stops, tip ruptures, release the 2 sperm. One of the
synergid cells begin to degenerate as pollen tube enters it.
Release of pollen tube contents into the synergid : A.
Showing two male gametes and a degenerating vegetative
nucleus near the filiform apparatus, B. Release of male
gametes inside the synergid, C. Movement of male
gametes towards the egg nucleus and into the central cell.

25. Pollen tube discharge: includes 2 sperms, the veg, nucleus and a
fair amount of cytoplasm. A portion of cytoplasm is retained in
the pollen tube.
No mixing between cytoplasm released by the pollen tube and
that of the synergid. They remain as two separate entities.

26. Tube enters at the apex of the filiform apparatus
and after growing, it arrives in the cytoplasm of the
synergid. The penetrated synergid starts
degenerating before the arrival of the pollen tube,
but after pollination. The process of discharge
takes place in seconds.
Degenerating
synergid
filiform-
specialized
region of the
synergid cell wall

27. Synergids “ synergos”- working together
Coined by Eduard Strasburger 19th
century botanist (Vesque, 1878)
filiform apparatus
Increases surface area of plasma membrane in the region, also
associated with elaborate ER
Synergids begin to degenerate as pollen tube enters it or shortly
Pollen tube guidance (shown by laser ablation techniques to
selectively remove different cells within the female gametophyte)
Mediates transport of molecules into and out of synergids
Necessary for the cessation of pollen tube growth and release
of sperm cells.
MYB98 protein- functions as regulator of genes expressed in synergids
required for formation of filiform apparatus, Mutants myb98 show
defects in pollen tube guidance and dev of filiform apparatus

28. Megaspore mother cell devs. from surrounding nucellar
tissue and undergoes meiotic division to form megaspore.
Nucellus considered as a megasporangium
funiculus
chalaza- region where integuments fuse with funiculus

29. Pollination
Double fertilization