The document discusses pollen wall morphogenesis and anther dehiscence, detailing the development of pollen grains from pollen mother cells and the construction of complex pollen walls comprising intine and exine layers. It elaborates on the process of anther dehiscence, which involves specialized cells and various types of dehiscing mechanisms for pollen release. Additionally, it highlights the significance of pollen structure for taxonomic classification and the interaction of allergens present in pollen.

1. Pollen wall morphogenesis
and anther dehiscence
Presentation by - Deepanshi patel
Department of Botany
University of Allahabad

2. Pollen
Pollen is a powdery substance produced by seed plants. It
consists of pollen grains (highly reduced
microgametophytes), which produce male gametes (sperm
cells). The pollen grains are produced within the anther of
the flower. Pollen mother cells originate from the
sporogenous tissue of the anther which later divide
meiotically to form four pollen grains called tetrad.
The pollen grains do not remain united at maturity, and are
dissociated into single pollen grain called monad.
Sometimes rarer types like dyads (two pollen grains),
Octads (eight pollen grains) and Polyads (many pollen
grains) are also observed
Fig. 4.1 Pollen units (A = Monad, B = Dyads, C
Tetrahedral tetrad, D-Tetragonal tetrad, E =
Rhomboidal tetrad, F = Decussate tetrad, G =
T-Shaped tetrad, H= Linear tetrad, I
Cryptotetrad, J = Polyads, K = Pollinia)

3. Schematic diagram illustrating pollen development.

4. Development of pollen wall
1. 1st layer of Pollen Wall is of cellulose-Primexine. Deposited between callose wall and sporę
plasmalemma
2. Below plasmalemma are plates of endoplasmic reticulum. Here, primexine is discontinuous-Germpores
3. Once primexine has reached a certain thickness, additional gaps appear in it
4. Columns of convoluted lamellae are deposited in these gaps on plasmalemma-Probacula
5. Precursors of sporopollenin are synthesized by spore cytoplasm, which are polymerised and deposited
on the surface of these lamellae-Bacula
6. Lower ends of bacula spread side ways into cellulosic layers of primexine and form the Foot layer
7. Foot layer is like a floor on which the columns or bacula are raised
8. Tops of bacula columns can also spread side ways in all directions to form Tectum
9. Tectum is responsible for intricate pattern of exine
10. All these stages of wall development occur while the spore is enclosed in callose wall
11. With maturity of tetrads, the callose is degraded abd spores released in anther cavity
12. Free from callose pollen grains now synthesize Intine and innermost layer of exine-the endexine

7. The pollen wall is the most complex wall than any other plant cell wall
that enables the pollen to withstand physical abrasion, dessication and
UV-B radiation.
The wall of the pollen grain comprises of two distinct
layers:
INTINE :
● The intine is the inner, more or less uniform layer. It is
pectocellulosic in nature.
● The inner layer is laid by the cells themselves, the outer wall is
deposited by the tapetum.
● The inner wall consists of cellulose and hemicellulose,callose is
always present .
● Callose is a polysaccharide. It has two distinctive properties ;
(1) High impermeability
(2) Rapid synthesis and easy degradation.
POLLEN WALL

8. EXINE :
The exine layer is highly sculptured and ornamented.
The different sculpturing patterns of the exine have long
been used for taxonomic classification and for forensic
identification.
The exine is made up of a complex and stable
biopolymer, sporopollenin that is resistant to non-
oxidative, physical, biological and chemical degradation
processes (Blackmore, 2007).
The exine is interrupted by one or more circular or
elongate sites called the apertures, through which the
pollen tube emerges.
Sometimes the grain is covered by a liquid, fatty
substance, so-called "pollenkit". Intine, exine and
cytoplasm can all three contain allergens that may
cause hay fever.

9. The exposed surface-details of the pollen wall
constitute the sculpturing.
Some of the more important types are:
1. Psilate (smooth)
2. Foveolate (pitted)
3. Fossulate (grooved)
4. Scabrate (very fine projections)
5. Verrucate (warty),
6. Baculate (rod like elements),
7. Pilate (rod-like elements with swollen tips)
8. Gemmate (sessile pilar),
9. Echinate (spiny),
10. Rugulate (elongate elements irregularly
distributed tangentially over the surface)
11. Striate (elongate, more or less parallel elements
distributed tangentially over the surface),
12. Punctate (minute perforations) and
13. Reticulate (elements forming an open network).
Exine Sculpturing

10. An aperture is any weak area on the pollen surface which is
directly or indirectly associated with its germination.
Long apertures are called colpi, and short ones pores.
The apertures may be simple or compound.
Pollen grains with simple apertures are either colpate (with
colpi) or porate (with pores).
A compound aperture consists of a central region called oral,
and an outer region called colpal in colporate pollen (with
compound colpi), and poral in pororate pollen (with compound
pores).
Pollen Aperture

11. NPC refers to
Number (N)
Position (P)
Character (C) of apertures.
treme = 'aperture'
NPC-System
Catatreme - aperture on proximal
face,Anatreme - on the distal face
zonotreme- Equator
Pantotreme - uniformly distributed .

12. POLLEN POLARITY
Pollen polarity refers to the position of one or more apertures
● The center of the tetrad is the proximal pole,
● That away from the tetrad center is the distal pole,
● Center - Equator
The three general types of pollen polarity are
(1) isopolar, in which the two polar hemispheres are the same but can be
distinguished from the equatorial region;
(2) heteropolar, in which the two polar hemispheres are different, because of
differential displacement of one or more apertures; and
(3) apolar, in which polar and equatorial regions cannot be distinguished after
pollen grain separation from the tetrad

14. Anther Dehiscence
Anther dehiscence is a multistage process involving localized differentiation and
degeneration, combined with changes in structure and water status of the anther
(Wilson et al., 2011).
Anther dehiscence involves three types of specialized cells:
(i) stomium,
(ii) septum
(iii) endothecium.
The stomium differentiates before the microspore mother cells enter meiosis. It
comprises of small specialized epidermal cells and, at anther maturation, splits to
facilitate anther dehiscence.

15. Four clusters of archesporial cells (Ar) in the anthers divide to form the primary parietal layer (PP)
and the primary sporogenous layer (Sp). The PP layer then goes through a further division to form
two secondary parietal layers, the inner secondary parietal layer (ISP) and the outer secondary
parietal layer (OSP). The OSP then divides again and differentiates to form the endothecium layer
(En), whereas the ISP divides and develops to form the tapetum (T) and middle cell layer (M).

16. E, epidermis; En,
endothecium; ML, middle
layer; T, tapetum; Sm,
septum; St, stomium; MMC,
microspore mother cells;
Ms, microsporocytes; Tds,
tetrads; Msp, microspores;
PG, pollen grain
Recap…!!
Fig: Anther morphology and key
events of anther development

17. 1. longitudinal Dehiscing- along long axis of theca.
1. poricidal Dehiscing -through a pore at apex of
theca.
1. transverse Dehiscing - at right angles to long axis
of theca.
1. valvular Dehiscing - through a pore covered by a
flap of tissue.
Types of anther dehiscence

18. The septum, that separates the two lobes of an anther, breaks down at a later
stage and the two sporangia of an anther lobe become joined to forms a single
locule
The endothecium is the hypodermal layer of the anther wall, which after the
release of microspores from the tetrads
It undergoes expansion and deposition of ligno-cellulosic secondary thickening
that arise from the inner tangential walls and run outward and upward ending near
the outer wall of each cell
The outer tangential wall remains thin. The thickening may be annular-rib type,
helical-rib type, reticulate-rib type or palmate-rib type depending on the species.

20. Degeneration of cells in the anther
Enzymatic breakdown of the septum
Several hydrolytic enzymes and proteins linked to cell wall loosening are thought to be
involved, including polygalacturonases (PGs),b-1,4-glucanases, and expansins
(Bonghi et al., 1993; Taylor et al., 1993)
Programmed cell death (PCD) of the septum and stomium
The anther septum and stomium go through a process of degeneration and cell death
to facilitate pollen release, and this is also thought to be via a PCD-related process
(Kuriyama and Fukuda, 2002; Sanders et al., 2005).
Regulation of endothecium secondary thickening
Opening the anther
Dehydration of the anther wall

21. Key developmental events in
anther dehiscence.