- 1 Pre-fertilization Structures in Plants – Flower Structure and Male and Female Gametes Production
The Biology Topics of ecology involve studying the relationships between living organisms and their environment.
Pre-fertilization Structures in Plants – Flower Structure and Male and Female Gametes Production
The floral primordial development depends on several hormonal actions and structural changes are initiated which lead to differentiation and further development. In the case of flowers, the reproductive whorls are androecium and gynoecium. Androecium consists of a whole of stamens which represents the male reproductive organ. Whereas, gynoecium consists of pistil and embryosac which represents the female reproductive organ.
The androecium is the third set of floral organs. It is made up of male reproduction units. It consists of stamen, microsporangium (anther), and pollen grain.
The stamen (plural ‘stamina’ or ‘stamens’ from Latin stamen meaning ‘thread of the warp’) is the pollen-producing reproductive organ of a flower. Stamens typically consist of a stalk called the filament (from Latin Slum, meaning ‘thread’), and an anther (from ancient Greek anthera, meaning ‘flowery’) which contains microsporangia. Anthers are most commonly two-lobed and are attached to the filament either at the base or in the middle portion. The sterile tissue between the lobes is called the connective.
A typical anther contains four microsporangia. The microsporangia form sacs or pockets (locules) in the anther. The two separate locules on each side of an anther may fuse into a single locule. Each microsporangium is lined with a nutritive tissue layer called the tapetum and initially contains diploid pollen mother cells. These undergo meiosis to form haploid spores. The spores may remain attached to each other in a tetrad or separate after meiosis. Each microspore then divides mitotically to form an immature microgametophyte called a pollen grain.
In the typical flower (that is, in the majority of flowering plant species) each flower has both carpels and stamens. In some species, however, the flowers are unisexual with only carpels or stamens, (monoecious = both types of flowers found on the same plant; dioecious = the two types of flower found only on different plants). A flower with only stamens is called androecious. A flower with only carpels is called gynoecious.
A flower having only functional stamens and lacking functional carpels is called a staminate flower, or (inaccurately) male. A plant with only functional carpels is called pistillate, or (inaccurately) female. An abortive or rudimentary stamen is called a staminodium or staminode, such as in Scrophularia nodus.
Cohesion of Stamens
Stamens of a flower may remain united among themselves. It is called the cohesion of stamens. The union may involve only in the filaments, or only in the anthers, or along their length including both filaments and anthers. The cohesion of stamen may be of three types-
- Adelphous: The stamens are fused by their filaments only. The anthers are free. These may be-
- Monadelphous stamen: Stamens are united together into a single bundle by their filaments only, e.g., China rose.
- Diadelphous stamen: When the stamens are united together into two bundles by their filaments, e.g., Pea.
- Polyadelphous stamen: When the stamens are united into three or more bundles by their filaments, e.g., Silk cotton.
- Syngenesious: When the stamens are united by their anthers only, the filaments are free, e.g., Helianthus annum, Tagetes patula, etc. This is characteristically shown by the family Compositae.
- Synandrous: When the stamens are united both by their filaments and by their anthers, e.g., Cucurbita maxima.
Adhesion of Stamens
When the stamens remain united with members of different floral whorls, i.e., stamens with petals, stamens with carpels, etc. the phenomenon is called adhesion of stamens. Adhesion of stamens may be of the following types-
- Epipetalous: When the stamens are united with petals the condition is known as epipetalous. Generally, the filaments remain united with the petals and anthers remain free, e.g., Datura metel, Ocimum sanctum, etc.
- Epiphytlous: In this type, the stamens are united with the perianth as in tuberose (Polianthes tuberosa).
- Gynandrous: In this type, the stamens adhere to the carpels, When the union between stamens and carpels is complete, the condition is called gynostegium (Calotropis) or gynostemium (Orchid).
On the basis of length stamens are classified as flowers:
- Didymous Stamens: Here the stamens are of two equal pairs.
- Didynamous Stamens: In this type, stamens occur in two pairs, a long pair, and a shorter pair, e.g., Ocimum sanctum.
- Tetradynamous Stamens: In this type, stamens occur as a set of six stamens with four long and two shorter ones, e.g., Brassica.
Microsporangium produces spores that give rise to male gametophytes. Microsporangium are notable in spike mosses, and a minority of ferns. In Gymnosperms and Angiosperms (flowering plants), the microsporangium produces the microsporocyte, also known as the microspore mother cell, which then creates four microspores through meiosis. The microspores divide to create pollen grains. The anther is tetragonal in a structure consisting of four microsporangia which develop into pollen sacs, located at the corners, of each lobe.
Development of Pollen Sac
A very young anther consists of actively dividing meristematic cells surrounded by a layer of the epidermis. It then becomes two-lobed. Each anther lobe develops two pollen sacs. Thus, a bilobed anther develops four pollen sacs situated at four corners of the anther. Development of pollen sacs begins with the differentiation of archesporial cells in the hypodermal region below the epidermis at four corners of the young anther.
The archesporial cell divide by periclinal division to give a sub-epidermal primary parietal layer and a primary sporogenous layer. The cells of the primary parietal layer divide by successive periclinal and anticlinal divisions to form concentric layers of the pollen sac wall. The wall layers from the periphery to centre consist of:
- A single layer of the epidermis becomes stretched and shrivels off at maturity.
- A single layer of endothecium. The cells of endothecium possess fibrous thickening. They remain thin-walled and constitute stomium (line of dehiscence) in the shallow groove in between the two microsporangia of the anther lobe.
- One to three middle layers. Cells of these layers generally disintegrate in the mature anther.
- A single layer of the tapetum. The tapetal cells may be uni-, bi-, or multinucleate and possess dense cytoplasm. The cells of the primary sporogenous layer divide further and give rise to diploid sporogenous tissue.
According to this process, the cells of the sporogenous tissue, undergo meiotic cell division to form the microspore tetrads.
As the flower develops, four groups of sporogenous cell form with the anther. Each cell of the sporogenous tissue is able to form a microspore tetrad. These fertile sporogenous cells are surrounded by layers of sterile cells which either develop into pollen sac walls or the nutritive tapetal cells. The sporogenous cells or the Pollen Mother Cells (PMC) then undergo meiotic division to form four haploid microspores. These microspores are arranged in a cluster of four cells i.e., microspores tetrad by callose walls. The callose wall is broken down by an enzyme called callase. The microspores become free and develop into pollen grains.
Dehiscence of Anther
When the anthers mature it dries up and the dead cells of endothecium contract and shortened. The anther lobe wall ruptures and the exposed spores are dispersed by different agencies of pollination.
Pollen is a fine to coarse powder containing the micro-gametophytes of seed plants, which produce the male gametes (sperm cells). Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the female cone of coniferous plants. When pollen lands on a compatible pistil or female cone (i.e., when pollination has occurred), it germinates and produces a pollen tube that transfers the sperm to the ovule (or female gametophyte). Individual pollen grains are small enough to require magnification to see detail. The study of pollen is called palynology and is highly useful in paleoecology, paleontology, archeology, and forensics.
Pollen grains come in a wide variety of shapes (most often spherical), sizes, and surface markings characteristic of the species. Pollen grains of pines, firs, and spruce are winged. The smallest pollen grain, that of the forget-me-not (Myosotis sp.) is around 6 mm (0.006 mm) in diameter. Wind-borne pollen grains can be as large as about 90-100 mm.
The Structure and Formation of Pollen
Pollen itself is not the male gamete. Each pollen grain contains a vegetative (non-reproductive) cell, only a single cell in most flowering plants but several in other seed plants, and the generative (reproductive) cell containing two nuclei; a tube nucleus (that produces the pollen tube) and a generative nucleus (that divides to form the two sperm cells). The group of cells is surrounded by a cellulose-rich cell wall called the intine and a resistant outer wall composed largely of sporopollenin called the exine.
The pollen is eventually released when the anther forms openings (dehiscence). These may consist of longitudinal slits, pores, as in the heath family (Ericaceae), or valves, as in the barberry family (Berberidaceae). In some plants, notably members of Orchidaceae and Asclepiadaceae, the pollen remains in masses called pollinia, which are adapted to attach to particular pollinating agents such as birds or insects. More commonly, mature pollen grains separate and are dispersed by wind or water, pollinating insects, birds, or other pollination vectors.
Pollen of angiosperms must be transported to the stigma, the receptive surface of the carpel, of a compatible flower, for successful pollination to occur. After arriving, the pollen grain (an immature microgametophyte) typically completes its development. It may grow a pollen tube and undergo mitosis to produce two sperm nuclei. Stamens can be free or fused in various ways. A column formed from the fusion of multiple filaments is known as an Androphore.
Except in the case of some submerged aquatic plants, the mature pollen grain has a double wall, a thin delicate wall of unaltered cellulose i.e., the endospore or intine, and a tough outer cuticularized exospore or exine. The exine often bears spines or warts or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. In some flowering plants, germination of the pollen grain often begins before it leaves the microsporangium, with the generative cell forming the two sperm cells.
Development of Male Gametogenesis
The process of development of male gametophytes from microspores or pollen grains is called micro gametogenesis. Most of the development of male gametophytes is completed inside the microsporangium or pollen sac and a part of development occurs on the stigma after pollination.
Microspore or the pollen grain is the first cell of the male gametophyte. It contains only one haploid nucleus. The cell undergoes unequal division and forms a small generative cell and a large vegetative or tube cell. The pollen grains are generally discharged from the anther at this two-celled stage. In some plants, the generative cell divides further to give rise to two male gametes before their discharge. Further development occurs on the stigma after pollination.
The pollen grain absorbs water and nutrients from the stigmatic secretion through its germ pore. The tube cell enlarges and comes out the pollen grain through the germ pore to form a pollen tube. The pollen tube secretes some hydrolytic enzymes to create a passage through the style. The tube nucleus descends to the tip of the pollen tube. The generative cell divides into two nonmotile male gametes if it is not already divided. Each male gamete is lenticular to spherical in outline. The tube nucleus may degenerate completely. A pollen grain with a pollen tube carrying male gametes represents a mature male gametophyte. It is a 3-celled structure bearing one tube cell and two male gametes.
Characteristics of Development of Male Gametophyte
- A diploid microsporocyte present inside each microsporangium divides by meiosis to produce four haploid microspores.
- Microspores undergo mitosis to produce a male gametophyte with only two cells: a generative cell and a tube cell.
- A pollen grain is composed of the generative cell and the tube cell surrounded by the spore wall. A spore wall is characterized by the deposition of materials in a unique species-specific pattern.
- Male gametophyte maturation occurs within each pollen grain when its generative cell divides to form two sperm cells, which usually occurs after pollen lands on a stigma, followed by the growth of the pollen tube.
- A pollen tube, which is a long cellular protuberance capable of rapid growth (about 1 cm or more/hour), delivers sperm to the female gametophyte.
The gynoecium is the fourth set of floral organs. It is made up of female reproductive units which consist of the pistil, megasporangium (Ovule), and embryo sac.
The gynoecium is characterized by the female reproductive part of the flower. The pistil, centrally located, typically consists of a swollen base, the ovary, which contains the potential seeds, or ovules; a stalk, or style, arising from the ovary; and a pollen-receptive tip, the stigma, variously shaped and often sticky. When gynoecium contains a single pistil, then it is characterized as a monocarpellary and if gynoecium has more than one pistil then it is called a multicarpellary.
Each pistil has three different parts style, stigma, and ovary. Pistils are of two types-
- Syncarpous pistil: Pistils may be fused together e.g., Pap aver.
- Apocarpous pistil: Pistils may be free e.g., Michelia.
The stigma serves as a proper landing platform for pollen grains. The style is the elongated slender part. The basal bulged part of the pistil is recognized as the ovary. The placenta is placed inside of the ovarian cavity. Megasporangia arise from the placenta and are called ovules.
Differences between Syncarpous and Apocarpous Gynoecium:
|Syncarpous Gynoecium||Apocarpous Gynoecium|
|1. In this type of gynoecium, carpels are fused.||1. In this type of gynoecium, carpels are separated from each other.|
|2. Here uni or multilocular ovary is present.||2. Here, an unilocular ovary is present.|
|3. Fruit is a simple type.||3. Fruit is an aggregate type.|
Depending on the insertion of floral leaves – carpels, stamens, petals, and sepals on the thalamus, the flowers are of three types:
1. Hypogynous: The thalamus is convex or conical, the ovary develops at its top, while stamens, petals, and sepals are borne successively below. Such a flower is called hypogynous and the ovary is described as superior, e.g., Hibiscus rosa-sinensis.
2. Perigynous: The thalamus is a cup-shaped structure, the ovary is located in the centre of the concave thalamus. The other floral parts are inserted on the rim or margin of the thalamus. The ovary in this position is called half inferior, e.g., Pisum, Rosa.
3. Epigynous: The cup-shaped thalamus grows upwards and fuses with the wall of the ovary and bears sepals, petals, and stamens on the top of the ovary. The ovary in such a case is called inferior, e.g., Cucurbita, Pyrus, etc.
Differences among Hypogynous, Perigynous, and Epigynous Flowers:
|Hypogynous Flower||Perigynous Flower||Epigynous Flower|
|1. Thalamus is convex or conical.||1. Thalamus is plane or slightly concave.||1. Thalamus is deeply cup-shaped.|
|2. Ovary is situated at the apex of the thalamus. Other whorls are placed below.||2. Ovary is situated at the centre. Other whorls are placed at its level or slightly below.||2. Ovary is situated at the centre of the deeply lobed thalamus. Other whorls are placed above the level of the ovary.|
|3. Ovary is superior. Other members are inferior.||3. Ovary is superior or half inferior. Other members are inferior.||3. Ovary is inferior. Other members are superior.|
|4. Thalamus is free from the ovary wall.||4. Thalamus is free from the ovary wall.||4. Thalamus is fused with the ovary wall.|
Types of Ovary
Depending on the relative position of the ovary and other floral members on the thalamus, the ovary may be of the following types:
- Superior ovary: The ovary is situated at the top of the convex or conical thalamus, as a result, there is no attachment of the ovary with other floral whorls, e.g., Hibiscus rosa-sinensis.
- Inferior ovary: The ovary wall is fused with the raised margin of the cup-shaped thalamus both basally and laterally, As a result, the ovary is situated below other floral whorls, e.g., Cucurbita, Pyrus, etc.
- Half-inferior ovary: The ovary is situated at the centre of the slightly concave thalamus as a result the position of the ovary is intermediate between the inferior or superior conditions, e.g., Pea, Rose, etc.
Inside the ovary, ovules are borne on the ridge of soft parenchymatous tissue called the placenta.
The position and arrangement or distribution of placenta-bearing ovules inside the ovary are called placentation.
Various types of Placentations are as follows:
- Marginal: It is found in the monocarpellary, unilocular superior ovary. A single longitudinal placenta having one or two alternate rows of ovules occurs along with the wall of the ovary called ventral suture, e.g., Pea, Bean, etc.
- Parietal: It occurs in bi or multi-carpellary syncarpous but unilocular ovaries, the ovary becomes false partition walled called replum. Two or more longitudinal placentae develop along the wall of the ovary. The number of placentae corresponds to the number of fusing carpels, e.g., Cucurbita, Brassica, etc.
- Axile: It is found in bi or multi-carpellary syncarpous ovaries having two or more chambers. Placenta occurs in the central region where the septa meet so that an axile column bearing ovules is formed, e.g., Hibiscus, Datura, etc.
- Basal: The placenta is at the base of the ovary. It occurs in monocarpellary or syncarpous pistils with unilocular ovaries. It bears a single placenta at the base with generally a single ovule. In basal placentation, ovules are attached to the base of the ovary, e.g., Helianthus.
- Free central: It is found in polycarpellary and syncarpous pistils with unilocular ovaries. The ovules are borne around a central column, which is not connected with the ovary wall by any septum, e.g., Morus indica, Primula, etc.
- Superficial: It occurs in multi-carpellary syncarpous pistils. The ovules are borne on placentae which develop all around the inner surface of the ovary including the septa if present, e.g., Nymphaea.
The typical angiosperm ovule is a structure, attached to the placenta by a stalk, called a funicle. The specific region where the ovule fuses with the funicle i.e., called as hilum. Hilum plays a connection between the ovule and the funicle. The protective envelope of each ovule is called an integument which encircles the ovule except at the tip where a small opening is formed i.e., called a micropyle. Chalaza is the opposite part of the micropylar end, representing the basal part of the ovule.
The ovule is the structure that gives rise to and contains the female reproductive cells in seed plants. Ovule, that develops into a seed when fertilized. A mature ovule consists of a food tissue covered by one or two future seed coats, known as integuments. A small opening (the micropyle) in the integuments permits the pollen tube to enter and discharge its sperm nuclei into the embryo sac, a large integument permits the pollen tube to enter and discharge its sperm nuclei into the embryo sac, a large oval cell in which fertilization and development occur.
Each ovule is attached by its base to the stalk (funiculus) that bears it. It consists of three parts: The integument, forming its outer layer, the nucellus (or remnant of the megasporangium), and the female gametophyte (formed from a haploid megaspore) in its center. The female gametophyte – specifically termed a megagametophyte is also called the embryo sac in angiosperms. The megagametophyte produces an egg cell for the purpose of fertilization.
Another way that plants differ with regard to their ovules is the place where the ovules are found. Specifically, in gymnosperms, such as conifers, the ovules are found on the scales of female cones, while in angiosperms, which are flowering plants, the ovules are found inside of the ovary within the carpel.
The formation of the megaspores from the megaspore mother cell (MMC), is called as megasporogenesis. Initially, a hypodermal cell of the nucellus gets differentiated to form the archesporial cell which again divides transversely to give the primary parietal cell and primary sporogenous cell. This primary sporogenous cell then acts as the Megaspore Mother cell (MMC). The megaspore mother cell undergoes meiotic cell division and produces four megaspores out of which three degenerate and only one remains functional. The diagrammatic representation has been shown in the following figure.
Components of Ovules
The ovule is made up of the nucellus, the integuments that form the outermost layer, and the female gametophyte (called an embryo sac in flowering plants), which is found at the very center.
The Nucellus: The nucellus is the largest part of the ovule. It houses the embryo sac as well as nutritive tissue and actually remains present in some flowering plants after fertilization as a source of nutrients for the embryo.
The Integuments: The integument is the tough outer protective layer of the ovule. Gymnosperms, such as pine trees and spruce trees, usually have one integument in an ovule, so we call them unitegmic. On the other hand, angiosperms, like maples and daisies, typically have two integuments, and we call them bitegmic. The integument encloses the nucellus except for a small gap, which is called the micropyle.
Embryo Sac (Female Gametophyte): in the case of flowering plants, meiosis results in the formation of four megaspores, out of which three degenerate and only one remains functional. The functional one develops into an embryo sac or female gametophyte. This is known as monosporic development the embryo sac is formed from a single megaspore.
The nucleus of the functional megaspore divides through mitotic division, forming two nuclei that move to opposite poles, ultimately forming a 2-nucleate embryo sac. This undergoes further mitotic nuclear division, forming 4-nucleate and 8-nucleate stages. At this 8-nucleate stage, the organization of the typical female gametophyte or embryo sac is reached.
The typical embryo sac has the following characteristic parts – The egg apparatus, which consists of two synergids and one egg cell. Three cells at the chalazal end, are characterized as antipodals. Finally, the typical embryo sac is formed at the 8-nucleate and 7-celled stages during maturity.
Functions of Synergids:
- These cells help to absorb nutrients from the nucellus.
- It secretes substances for attracting pollen tubes and forms a seat for pollen tube discharge.
Functions of Antipodal Cells:
- It secretes different types of enzymes, lipids, proteins, and starch.
- It also tubes a part in absorbing nourishment from the nucellus.
Types of Embryo Sac
1. Monosporic Type: Develops from one megaspore, e.g., Polygonum.
Development of Monosporic Embryo sac (Polygonum Type): The normal type of embryo sac development has been studied in Polygonum by Strasburger. Since this embryo sac develops from one megaspore, that is why this is a monosporic embryo sac. It develops from a chalazal functional megaspore (4th from micropyle). The nucleus of a functional megaspore divides into three mitotic divisions to form 8 nuclei, out of which:
- Three of them get organized as cells at the micropylar end forming egg apparatus. One is an egg cell (n) and two are synergids (n) or cooperative cells.
- Three cells a chalazal end form antipodals (n) or vegetative cells of the female gametophyte.
- Two nuclei (one from each pole) in the centre are called polar nuclei (n).
- As the embryo sac matures these polar nuclei get fused to form a secondary nucleus (2n) or definitive nucleus.
- This constitutes an embryo sac a 7-celled and 8-nucleated structure.
2. Bisporic Type: Develops from the cell which is to form two megaspores, e.g., Allium.
Development of Bi-sporadic Embryo sac (Allium Type): In this type female gametophyte develops from one of two haploid dead cells, after the first meiotic division of the megaspore mother cell (MMC). One of the functional dead cells by the second meiotic division gives rise to two free megaspore nuclei which take part in the development of eight nucleate female gametophytes by the two successive mitotic divisions.
3. Tetrasporic Type: Develops from the megaspore mother cell soon after meiosis in its nucleus, e.g., Adoxa.
Development of Tetrasporic Embryo sac (Adoxa Type): In this type, all four megaspore nuclei take part in the formation of the gametophyte. After the meiotic division of the megaspore mother cell (MMC), all four free megaspore nuclei lie within the young embryo sac and undergo only one mitotic division to form the 8-nucleated mature embryo sac.
The following diagram shows the different patterns of embryo sac development-
Differences among Monosporic, Bisporic, and Tetrasporic Embryos Sac:
|1. Only one of the four megaspore nuclei remains functional to form an embryo sac.||1. Two of the four megaspore nuclei remain functional to form an embryo sac.||1. All four megaspore nuclei remain functional to form an embryo sac.|
|2. Undergoes three mitotic divisions to reach the 8-nucleate stage.||2. Undergoes two mitotic divisions to reach the 8-nucleate stage.||2. Undergoes only one mitotic division to reach the 8-nucleate stage.|
|3. Found in Polygonum so called Polygonum type.||3. Found in Allium so called Allium type.||3. Found in Adoxa so called Adoxa type.|
Types of Ovule
The ovule is six types-
- Orthotropous (Atropous): In this type, the ovule is straight in the ovary. The nucellus and integument merge and funicle, chalaza, and micropyle all are aligned straight, e.g., Polygonum.
- Anatropous: In this type, the ovules become completely inverted so that the microphytes lie close to the hilum. The micropyle and chalaza lie in one vertical line, e.g., gram, pea.
- Hemianatropous: In this type, the ovules lie transversely at a right angle in relation to the funicle. The micropyle and chalaza lie in a transverse line. e.g., Flowers of Brassicaceae.
- Campylotnopoils: In this type, the body of the ovule is bent and the alignment between the chalaza and micropyle is lost. Funicle does not fuse with integuments, e.g., Mustard.
- Amphitropous: In this type, the body of the ovule is very much curved so that the hilum, chalaza, and micropyle lie in close, e.g., Capsella.
- Circinotropous: In this type, the funicle is so, long that it creates a nearly full circle around the ovule and the micropyle is pointing upwards, e.g., Opuntia.
Development of Female Gametophyte
The process of development of a female gametophyte or embryo sac from a megaspore is called megagametogenesis. Megaspore (n) is the first cell of the female gametophyte. The functional megaspore becomes enlarged at the expense of the nucellus and thus forms the female gametophyte, i.e., the embryo sac. Its single nucleus undergoes three successive mitotic divisions to produce 8 nuclei. Nuclei get organized into three groups- three-celled egg apparatus towards the micropylar end, two middle polar nuclei, and three antipodal cells towards the chalazal end. The two polar nuclei fuse together and form the 2n nucleus, the definitive nucleus or polar fusion nucleus.
In the egg apparatus, each nucleus is surrounded by a viscous mass of cytoplasm without any wall, of which the middle one is the largest and called egg, ovum, or oosphere and the rest two are the synergids. This type of embryo sac development is very common in angiosperms and is known as the normal type or Polygonum type. This type is also known as monosporic type. P. Maheshwari (1950) classified the female gametophyte into monosporic, bisporic, and tetrasporic embryo sacs depending upon the number of megaspore nuclei taking part in the development.
Characteristics of Development of Female Gametophyte:
- Female gametophyte development occurs inside the megasporangium within each ovule. Development begins, when one diploid cell in the megasporangium of the ovule, known as the megaspore mother cell, enlarges and divides by meiosis to form four haploid megaspore cells.
- Integuments are the protective layers of sporophytic tissue that develop into the seed coat. Each megasporangium is surrounded by the two integuments, except at a gap i.e., micropyle.
- The embryo sac or multicellular female gametophyte is formed following three mitotic divisions of the megaspore without cytokinesis.
- Cell fate of the female gametophyte nuclei is determined by an auxin gradient at the micropyle.
- Synergids are 2 cells at the micropylar end that flank the egg and are involved in attracting and guiding the pollen tube toward the embryo sac.
Differences between Microsporogenesis and Megasporogenesis:
|1. Due to this process, the haploid microspores (by meiosis) are formed from the diploid microspore mother cell.||1. Due to this process, the formation of a haploid megaspore (by meiosis) from a diploid megaspore mother cell takes place.|
|2. In spore tetrad, all four microspores are functional.||2. In a spore tetrad, only one megaspore is functional.|
|3. Here, numerous microspores or pollen grains are formed per microsporangia.||3. Here a single embryo sac with a single egg is formed.|
Differences between Microspore and Megaspore:
|1. Microspore is smaller in size and present in the anther.||1. Megaspore is larger in size and present inside the ovary.|
|2. In male gametophyte it is the first cell.||2. In female gametophytes it is the first cell.|
|3. In microspores, a male gamete is formed.||3. In a megaspore, the female gamete is formed.|
Outbreeding Devices (Contrivances for Cross Pollination)
There are several devices that ensure cross-pollination
1. Dieting (Unisexuality): In unisexuality, the stamens and carpels occur in different flowers, i.e., male and female flowers. The two types of flowers may be borne on the same plant (monoecious) or on different plants (dioecious).
2. Dichogamy: In some bisexual flowers, the stamens and carpels do not mature at the same time, thereby self-pollination is inhibited. It is of two types Protandry and Protogyny.
- Protandry: Anthers mature earlier than the stigma, i.e., Clerodendron, sunflower.
- Protogyny: Stigmas mature earlier so that they get pollinated before the anthers of the same flower develop pollen grains, e.g., Mirabilis jalapa.
3. Prepotency: Pollen grains of another flower germinate on the stigma more rapidly, than the pollen grains of the same flower, e.g., Apple, pear, grape.
4. Self Sterility: Pollen grains of a flower do not germinate on the stigma of the same flower due to the presence of a similar self-sterile gene (S1S3 in pistil and S1 or S3 in pollen grain), e.g., Tobacco, potato, crucifers.
5. Heterostyly: Flowers have different lengths of styles and stamens. This is of the following types:
- Diheterostyfy (Dimorphic heterostyly): There are two types of flowers:
- One type with long style, short stamens.
- Other types with a short style, long stamens, e.g., Jasminum, Primula.
- Triheterostyly (Trimorphic heterostyly): There are three types of flowers:
- Long style, stamens medium and short.
- Style medium, stamens long and short.
- Style short, stamens long, and medium, e.g., Lythrum, Oxalis.
In heterostyly, cross-pollination takes place between stamens and stigmas of the same length, a condition that is found in different flowers.
6. Herkogamy: It is a mechanical device to prevent self-pollination and promotes cross-pollination. In orchids and members of Asclepiadaceae like Calotropis procera, where the pollens are aggregated in pollinia, the pollination completely depends on the mercy of the insects. Due to the dextrose anthers in the Gloriosa superba of Liliaceae, the pollens become out of reach of their own stigma.
Plant sexual reproduction is based on the interaction between pollen and pistil. These largely extracellular interactions lead to the cellular activities within the pollen, enabling the transport of the male gamete by the growth of pollen tube to the ovules for fertilization. These studies together with the genetic analysis of sexual reproduction have revealed the mechanisms behind the pollen-pistil interaction. Pollen grains of a number of plants settle over a stigma. All of them do not germinate. Only the compatible pollen of the same species is able to germinate.
Germination depends on the compatibility-incompatibility reaction between proteins present over the pollen grains and the stigma. If the reaction is favorable, the pollen grains are able to absorb water and nutrients from the surface of the stigma. They germinate and produce pollen tubes. Pollen tubes grow through the style. Interaction between pollen tubes and pistil continues till the pollen tubes reach the ovule. The pollen tube secretes enzymes, which digest the reserve food materials in the tissues of stigma and style. The food materials are utilized by the pollen tube for its growth. The pollen tube grows chemotropically due to a concentration gradient of calcium, boron, and inositol complex.
Stages of Pollen-Pistil Interaction
- Pollen falls on the stigma of the flower.
- A continuous dialogue is initiated between the pollen and the pistil, i.e., the reactions occur between the chemicals produced by the pollen and the pistil.
- The stigma recognizes the pollen.
- The incompatible pollen is rejected by the stigma. So, it cannot germinate.
- Moisture and sugars present on the surface of the stigma induce the germination of compatible pollen.
- Intine grows out through the germ pore as a pollen tube which produces pectinase and other hydrolyzing enzymes.
- The tissue of the stigma and style are digested and the pollen tube elongates through the style and reaches the ovule.
- The elongation of the pollen tube is aided by the Calcium-Boron-Inositol-Sugar-Complex formed in the style.
- The pollen tube enters the embryo sac through one of the synergids at the micropyle.
- The pollen tube is guided into the embryo sac by the filiform apparatus present on the synergid.
Pollen-pistil interaction, therefore, depends on the compatibility-incompatibility reaction between proteins present over them. Knowledge about the mechanism of interaction can help to overcome incompatible reactions by supplying the chemicals that induce compatibility and allow the pollen to grow. Plant breeders can utilize the process to obtain hybrids between different species.
Pollen-pistil interactions are an essential prelude to fertilization in angiosperms and determine compatibility or incompatibility. This interaction has been studied at the molecular and cellular level in relatively few families. Self-incompatibility is best understood in pollen-pistil interaction at the molecular level where three different molecular mechanisms have been identified in just five families. Here we review the studies of pollen-pistil interaction and self-incompatibility in Asteraceae, an important and advanced angiosperms family that has been relatively studied in these areas of reproductive biology.
Due to the interaction between stigma and pollen grain, the unnecessary pollen grains are not germinated. Then it is called incompatibility. Incompatibility is divided into two types:
- Interspecific incompatibility: In this type of incompatibility, pollen grains of one species failed on the stigma of other species, then, the pollen grains are not germinated.
- Intraspecific incompatibility: When the pollen grains are not germinated in the different flowers of the same species.
This type of incompatibility, divided into two types are as follows:
1. Sporophytic Self-incompatibility (SSL):
This form of self-incompatibility has been studied intensively in members of the mustard family (Brassica), including turnips, rape, cabbage, broccoli, and cauliflower.
(i) Rejection of seif pollen is controlled by the diploid genotype of the sporophyte generation.
(ii) The control lies in the “S-locus”, which is actually a cluster of three tightly-linked loci:
- SLG (S-Locus Glycoprotein) encodes part of a receptor present in the cell wall of the stigma.
- SRK (S-Receptor Kinase), which encodes the other part of the receptor. Kinases attach phosphate groups to other proteins.
- SRK is a transmembrane protein embedded in the plasma membrane of the stigma cell.
- SCR (S-locus Cysteine-Rich protein), encodes a soluble ligand for the same receptor which is secreted by the pollen.
(iii) Because the plants cannot fertilize themselves, they tend to be heterozygous; that is, carry a pair of different S loci (here designated S1 and S2).
(iv) However, dozens of different S alleles may be present in the population of the species; that is; the S-locus in the species is extremely polymorphic (analogous to the major histocompatibility locus of vertebrates.
(v) The difference between the alleles is concentrated in certain “hypervariable regions” of the receptor (analogous to the hypervariable regions that provide the great binding diversity of antibodies.
- Pollen will not germinate on the stigma (diploid) of a flower that contains either of the two alleles in the sporophyte parent that produce the pollen.
- This holds true even though each pollen grain being haploid contains only one of the alleles.
- In the example shown here, the S2 pollen, which was produced by an S1S2 parent, cannot germinate on an S1S3 stigma.
- The S1S2 pollen-producing sporophyte synthesizes both SCR1 and SCR2 for incorporation in (and later release from) both S1 and S2 pollen grains.
- If either SCR molecule can bind to either receptor on the pistil, the kinase triggers a series of events that lead to the failure of the stigma to support the germination of the pollen grain.
- Among these events is the ubiquitination of proteins targeting them for destruction in proteasomes.
- If this path is not triggered (e.g., pollen from an S1S2 parent on an S3S4 stigma, the pollen germinates successfully.
2. Gametophytic Self-incompatibility (GSI):
This form of self-incompatibility is more common than SSI but not so well understood. It occurs in nearly one-half of all the families of angiosperms, including
- the Solanaceae [potatoes, tomatoes (wild, not cultivated), and tobacco]
- beets (Beta vulgaris)
- many types of grass
- The S loci are (as in SSI plants) extremely polymorphic; that is, there is an abundance of multiple alleles in the population.
- Incompatibility is controlled by the single S allele in the haploid pollen grain.
- Thus a pollen grain will grow in any pistil that does not contain the same allele (so, as shown here and in contrast to what happens in SSI, S2 pollen from an S1S2) parent will grow down an S1S3 style.
This appears to be the mechanism in the Petunia:
- All pollen grains- incompatible as well as compatible germinate forming pollen tubes that begin to grow down the style.
- However, the growth of incompatible pollen tubes stops in the style while compatible tubes go on to fertilize the egg in the ovary.
- The block within incompatible pollen tubes is created by an S-locus-encoded ribonuclease (S-RNase), which is
- synthesized within the style;
- enters the pollen tube and
- destroys its RNA molecules
- halting pollen tube growth.
- The RNase molecules contain a hypervariable region, each encoded by a different allele, which establishes each S specificity (S1, S2, S3, etc.).
- The pollen tube expresses a protein designated SLF that binds S-RNase. SLF also exists in different S-specificities (S1, S2, S3, etc.).
- Incompatible (“nonself’’) tubes, the SCF triggers the degradation (in proteasomes) of the S-RNase thus permitting RNAs in the pollen tube to survive and growth to continue.
- In incompatible (“self”) tubes the interaction of, for example, the S1 SCF with the S1 S-RNase blocks its degradation so the RNAs of the pollen tube are destroyed and growth is halted.