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Botany
Plant Development
Fertilization
1. Details of Module and its Structure
Module Detail
Subject Name Botany
Paper Name Plant Development Module Name/Title Fertilization Module Id
Pre-requisites
Objectives To study Fertilization in angiosperms and its significance
Keywords Callose plugs, Double fertilization, Endosperm, Filiform apparatus, Generative cell, Micropyle, Pollen tube, Pollen-pistil interaction, Polyspermy, Sporopollenin, Synergids, Tip oriented growth, Vegetative cell
Structure of Module / Syllabus of a module (Define Topic / Sub-topic of module )
Fertilization
Botany
Plant Development
Fertilization
2. 2. Development Team
Role Name Affiliation
National Coordinator <National Coordinator Name> Subject Coordinator <Prof. Sujata Bhargava> Paper Coordinator
Botany
Plant Development
Fertilization
TABLE OF CONTENTS (for textual content)
Fertilization Contents
Introduction
1. Pollen-pistil interaction
a. Pollen germination and pollen-tube growth 2. Tip oriented growth
a. Composition of pollen-tube and its wall 3. Pollen-tube guidance
4. Role of synergids 5. Double fertilization
a. Repulsion signaling and prevention of polyspermy b. Significance of double fertilization
c. Post fertilization changes Introduction:
Fertilization involves the fusion of a male gamete with the female gamete. In angiosperms the female gametophyte is positioned deep in the ovarian cavity, the embryo sac far away from the stigma. There is a characteristic distribution of the cells within the embryo sac (Fig. 1). Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell. Other three cells are at the chalazal end and are called as the antipodals. The large central cell has two polar nuclei. Thus, at maturity, a typical angiospermic embryo sac is 8-nucleated and 7-celled. However, a diverse variation in the distribution of cells also exists throughout the angiospermic families.
Mature pollen grains have a bilayered cover: the intine, which is pollen grain-derived, and contains cellulose, pectin and enzymatic proteins; and the outer exine, which consists of sporopollenin (derived by oxidative polymerization of carotenoids) synthesized and secreted by the anther (Fig. 2). Sporopollenin is resistant to physical and biological decomposition. Because of this property pollen grains are tolerant of desiccation and can be transported over long distances by wind or animal pollinators. Due to the presence of sporopollenin the pollen grain walls are often preserved for long periods in fossil deposits. Lipids and proteins are additionally embedded along the exine surface in a layer known as the tryphine.
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mitotic division of the generative cells may produce two sperm cells. They are non-motile and thus require transportation to the egg apparatus via the pollen-tube to execute double fertilization.
Fig. 1. An organized embryo sac.
(From Bhojwani and Bhatnagar’s The Embryology of Angiosperms, 4th Edition, 1999; Vicas Publishing House Pvt Ltd)
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(From, http://www.yourarticlelibrary.com/wp-content/uploads/2013/10/clip_image01015.jpg) Pollen-tubes emerge at the germ pore on the pollen grains. For this, the pollen grains germinate on the stigma and put forth pollen-tubes which grow through the style and find their way into the ovules, where they discharge the sperms in the vicinity of the egg (Fig. 3).
Polarized outgrowth of a pollen-tube from each grain, the directional elongation of the pollen-tubes to reach the ovary, the guided entrance of each pollen-tube into an ovule to penetrate the embryo sac (the female gametophyte), and the release of the two sperm cells from the pollen-tube to fuse with the egg cell (forming zygote) and polars or the secondary nucleus (forming primary endosperm nucleus), accomplishes double fertilization and triple fusion, a phenomenon unique to the angiosperms.
1. Pollen-pistil interaction
Plant sexual reproduction relies on intimate interactions between the pollen and the pistil. Pollen grains interact with several different diploid tissues and haploid cells in the pistil to lead successful sexual reproduction in flowering plants. Fertilization depends on further cellular activities in the pollen on encountering the pistil. For a compatible pollination to occur, pollen-pistil interactions must involve recognition signals stimulating pollen and pistil responses, aid nutrition for pollen cellular activities and provide directional signal cascades to guide the homing of pollen-tubes to the ovules. Besides this, there exist multiple mechanisms to arrest incompatible pollen grains, their tubes and the sperm cells, thus preventing cross fertilization between incompatible species and self fertilization in self-incompatible plants.
a. Pollen germination and pollen-tube growth
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(A)
(B)
Fig. 3. Growth of the pollen tube on and in the stigma, of spinach. after Wilms, 1980 (A), a part enlarged.
(& From Bhojwani and Bhatnagar’s The Embryology of Angiosperms, 4th Edition, 1999; Vicas Publishing House Pvt Ltd) Web source http://www.google.co.in/imgres?imgurl=https%3A%2F%2Fclassconnection.s3.amazonaws.com%2F959%2Fflas hcards%2F1239959%2Fpng%2Fpollen_tube1333260027626.png&imgrefurl=https%3A%2F%2Fwww.studyblu e.com%2Fnotes%2Fnote%2Fn%2Fchapter-38-reproduction-in-flowering-plants%2Fdeck%2F5231028&h=323&w=263&tbnid=8skA8DBascVypM%3A&zoom=1&docid=8VaVx8hZ6Td E_M&ei=TTOtVPnlHYiW8QX7lIDwCw&tbm=isch&client=firefox-a&ved=0CCcQMygLMAs&iact=rc&uact=3&dur=2118&page=1&start=0&ndsp=18
2. Tip oriented growth
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germination. It leads to the rupture of exine. It commences in a few seconds to a few minutes. This induces the inactive cytoplasm of the pollen grains. Many cellular organelles, both in number and arrangements, undergo considerable changes (Fig. 4), like:
(1) Formation of a thin callosic layer (related to dictyosome activity) at the site of germ pore from where the germ-tube originates.
(2) Production of many vesicles (small ones concerned with the formation of pecto-cellulosic wall, and large ones concerned with the formation of callosic wall layer. (3) Cisternae or RER arranged in stacks becomes free.
(4) Accumulation of ribosomes into polysomes. (5) Formation of lamellae inside the plastids.
Activation of pollen grain does not induce any change in mitochondria, lipid bodies, generative cell and vegetative nucleus. Almost entire content of the grain moves into the tube (Fig. 5). Pollen-tube growth is a tip-growth process. It involves the continuous deposition of cell membrane and cell wall materials at the tube apex; these materials are synthesized in the slightly distally located organelles and transported to the apex via secretory vesicles. An intracellular gradient of increasing calcium ion (Ca2+) concentration from the back to the tip and an influx of Ca2+ across the tip of in vitro grown pollen-tubes are essential for this tip-growth process. Both pollen and pistils have evolved the ability to produce flavonoid to ensure successful pollen germination. Each pollen-tube penetrates the cuticle of the stigmatic papillar cells and invades the extracellular matrix (ECM) of the underlying secretory cells.
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Fig. 4 Fine structure of an elongating pollen tube of lily. The cap block shows many Golgi vesicles that have migrated to the pollen tube tip. The remaining part of the tube contains ER, mitochondria, Golgi apparatus, starch grains and lipid particles (after Iwanami et al., 1988)
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Fig. 5 A germinated pollen grain showing that in a growing pollen tube the cytoplasm is confined to the apical portion. B. Enlarged apical portion of pollen tube after Iwanami, 1959. (From Bhojwani and Bhatnagar’s The Embryology of Angiosperms, 4th Edition, 1999; Vicas Publishing House Pvt Ltd)
a. Composition of pollen-tube and its wall:
At full maturity the pollen-tube shows four distinct zones:
(1) Apical zone or growth region (2-4 µm long), is swollen and contains abundant vesicles (small and large) but is devoid of cell organelles
(2) Sub-apical zone, has granular cytoplasm rich in cell organelles. The vesicles are produced in this region
(3) Nuclear zone, houses the vegetative nucleus and generative cell and later two sperms (4) Vacuolization zone, forms a transition between the active and the inactive cytoplasm of
the pollen-tube. It is highly vacuolated and ends with callosic plate.
Fig. 6. Callose plug in a pollen tube.
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Fig. 7. Entry of pollen tube in the ovule; A. Porogamy, B. Chalazogamy, C. Mesogamy (http://www.2classnotes.com/digital_notes_print.asp?p=Entry_of_the_pollen_tube)
The pollen-tube wall is three layered structure, with an outer pectin stratum, a middle pecto-cellulosic stratum and inner callosic. Normally, pollen germination and tube growth are restricted to the pistil of mature flowers.
Upon germination, the pollen-tubes enter the secretory zone and converge from a relatively broad stigmatic region into the stylar transmitting tissue. Therefore, a directional bias is established early in the pollen-tube growth process. In a growing tube most of the cytoplasm is confined to the apical region, and a large vacuole fills the grain and the older region of the tube. To restrict the cytoplasm to the apical region of the growing tube, a series of callose plugs are formed at a regular distance behind the tip (Fig. 6). The plugs originate as a ring on the inner side of the wall. They gradually grow towards the centre reducing the lumen and, finally, sealing the tube. The partition in the pollen-tube prevents the backflow of cytoplasm and nuclei. The transparent apical zone is called ‘cap block’. It exists only as long as the tube is growing and disappears when the growth ceases. 3. Pollen-tube guidance
Inside the ovary, the pollen-tubes continue to elongate along the surface of the placenta on which ovules are attached. They navigate until gaining access into an ovule via the micropyle. This mode of penetration of ovule is called as porogamy and supposed to be the commonest form. Other forms of penetrations like chalazogamy (as in Casuarina) and mesogamy (through the integuments, as in Cucurbita or through the funiculus, as in Pistacia ) have also been reported (Fig. 7). The micropyles are covered with exudates that are enriched in glycosylated compounds and are believed to be important for attracting pollen-tubes into the ovules. It is known that high concentration of Ca2+ is present inside the synergid cells, which flank the egg cell at the micropylar end of the embryo sac.
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and the receptive pistil. Upon pollen-tube arrival, further signal transduction cascades are initiated leading to the death of synergid cells and pollen-tube rupture. Ultimately, if all goes right, fertilization occurs after successful gamete interaction and processes are activated to avoid polyspermy or to recover fertilization failures.
Fig. 8 Summary diagram to show sperm transfer in the embryo sac. (After Jensen, 1973)
(From Bhojwani and Bhatnagar’s The Embryology of Angiosperms, 4th Edition, 1999; Vicas Publishing House Pvt Ltd)
4. Role of synergids:
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5. Double fertilization
Pollen-tube does not enter directly into the egg cell, but it releases its male gametes in a synergid which later on come into the contact of egg cell. The other synergid cell is persistent. Antipodals degenerate. After the release, one male gamete moves towards the egg cell while the other approaches the central cells. The site where the male gametes attach with the egg cell or the central cells, the membrane of attachment sites disappears and the nuclei of male gametes get entry into egg cell and the central cells, respectively. The fusion of egg cell and the first male gamete forms diploid zygote. This process is called as true fertilization or syngamy, discovered by Edward Strasburger (1884). Fusion of the second male gamete with the polar nuclei forms primary endosperm nucleus. Since here occurs the sound fusion of three nuclei, this is why the process is called as triple fusion. Since both male gametes take part in the fertilization process, this is called as double fertilization, discovered by Sergius Nawaschin (1898).
a. Repulsion signaling and prevention of polyspermy
Once double fertilization has been successfully achieved, it is important to establish barriers to prevent the attraction of additional pollen-tubes and to prevent the release and fusion of multiple sperm with single gametes (polyspermy). Observations clearly indicate that repulsion molecules are secreted once the first pollen-tube enters the micropyle. Various mechanisms exist in angiosperms to efficiently prevent polyspermy. Once ovules are targeted by a single pollen-tube, supernumerary pollen-tubes are repelled. In the grasses, multiple pollen-tubes compete to approach female gametophyte. The fastest-growing pollen-tube penetrates the micropyle and releases its contents into the receptive synergid. Additional pollen-tubes seem to be repelled.
b. Significance of double fertilization:
Double fertilization ensures the formation of viable seeds as triploid endosperms, the nutritive tissue for the growing embryo is also formed simultaneously. The tissue of angiospermic endosperm is more vigorous and physiologically aggressive because it has both maternal as well as paternal origin.
c. Post-fertilization changes:
Following changes occur in flower and ovary after fertilization:
1. Diploid zygote forms a miniature embryo which comprises plumule, radical and cotyledons.
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3. Nucellus is completely used up till the development of embryo, but in some plants like betel (Piper betle), it persists longer in the form perisperm and provides nutrition.
4. Outer integument of the ovule forms seed cover or testa while the inner forms tegmen. Testa and tegmen, together are known as seed coat.
5. Ovule forms seed while ovary transforms into fruit.