Pollen wall morphogenesis and anther Dehiscence

Pollen wall development is important phenomenon for determining the further morphology of pollen grain .

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

5. Formation of Ubisch bodies and development of pollen wall.(after Echlin, 1968)

6. 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 : T he 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

7. 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.

8. The exposed surface-details of the pollen wall constitute the sculpturing. Some of the more important types are: P silate (smooth) F oveolate (pitted) F ossulate (grooved) S cabrate (very fine projections) V errucate (warty), B aculate (rod like elements), P ilate (rod-like elements with swollen tips) G emmate (sessile pilar), E chinate (spiny), R ugulate (elongate elements irregularly distributed tangentially over the surface) S triate (elongate, more or less parallel elements distributed tangentially over the surface), P unctate (minute perforations) and R eticulate (elements forming an open network). Exine Sculpturing

9. An aperture is any weak area on the pollen surface which is directly or indirectly associated with its germination. L ong 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

10. NPC refers to Number (N) Position (P) Character (C) of apertures. treme = 'aperture' NPC-System C atatreme - aperture on proximal face,Anatreme - on the distal face zonotreme - Equator Pantotreme - uniformly distributed . pollen have an aperture-like thin area, or leptoma . Pollen with one leptoma are called monolept .

11. 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

12. 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.

13. 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).

14. 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

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

16. 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.

17. 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

18. Key developmental events in anther dehiscence.