Monday, April 30, 2012

2. Sexual reproduction in flowering plants

All flowering plants (angiosperms) show sexual reproduction.
-    Several hormonal and structural changes are initiated which lead to the differentiation and further development of the floral primordium.
-    Inflorescences are formed which bear the floral buds and then the flowers.
STRUCTURE OF FLOWER (For figures see TB page: 20, 21)
-    Flowers are morphological & embryological structures and the sites of sexual reproduction.
-    In flower, the male (androecium) & female (gynoecium) reproductive structures differentiate and develop.
Androecium (whorl of Stamens)
Androecium consists of a whorl of stamens. Their number and length are variable in flowers of different species.
A stamen has 2 parts:
a.    Filament: The long and slender stalk. Its proximal end is attached to the thalamus or the petal of the flower.
b.   Anther: The terminal and typically bilobed structure. Each lobe has 2 thecae, i.e. they are dithecous. Often a longitudinal groove runs lengthwise separating the theca.
Transverse section of an anther:
-    The anther is a tetragonal structure consisting of four microsporangia located at the corners, two in each lobe.
-    The microsporangia develop further and become pollen sacs. They extend longitudinally all through the length of an anther and are packed with pollen grains.
Structure of microsporangium:
-    A typical microsporangium appears near circular in outline.
-    It is generally surrounded by four wall layers– the epidermis, endothecium, middle layers & tapetum.
-    The outer three layers perform the function of protection and help indehiscence of anther to release the pollen.
-    The tapetum (innermost layer) nourishes the developing pollen grains. Cells of the tapetum possess dense cytoplasm and generally have more than one nucleus.
-    When the anther is young, a group of compactly arranged homogenous cells (sporogenous tissue) occupies the centre of each microsporangium.
Microsporogenesis: (For figures see TB page: 22,23)
-    As the anther develops, each cells of sporogenous tissue undergo meiotic divisions to form microspore tetrads. Each one is a potential pollen (microspore mother cell).
-    The formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis.
-    The microspores are arranged in a cluster of four cells (microspore tetrad).
-    As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.
-    Inside each microsporangium thousands of pollen grains are formed that are released with the dehiscence of anther.
Pollen grain (male gametophyte):
-    Generally spherical. 25-50 mm in diameter. Cytoplasm is surrounded by a plasma membrane.
-    A pollen grain has a two-layered wall- exine and intine.
o Exine: The hard outer layer. Made up of sporopollenin (highly resistant organic material). It can withstand high temperature and strong acids and alkali. Enzymes cannot degrade sporopollenin.
Exine has apertures called germ pores where sporopollenin is absent.
Pollen grains are well preserved as fossils due to the presence of sporopollenin. Exine exhibits a fascinating array of patterns and designs.
o Intine: The inner wall. It is a thin and continuous layer made up of cellulose and pectin.
-    A matured pollen grain contains 2 cells-
o Vegetative cell: is bigger, has abundant food reserve and a large irregularly shaped nucleus.
o Generative cell: is small and floats in the cytoplasm of the vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus.
-    In over 60% of angiosperms, pollen grains are shed at the 2-celled stage. In others, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
-    When once they are shed, pollen grains have to land on the stigma before they lose viability. The period for which pollen grains remain viable is variable and depends on the prevailing temperature and humidity.
-    Viability of pollen grains of some cereals (rice, wheat etc) is 30 minutes. Some members of Leguminoseae, Rosaceae & Solanaceae have viability for months.
-    Pollen grains of some plants (e.g. Parthenium or carrot grass) are allergic for some people. It leads to chronic respiratory disorders – asthma, bronchitis, etc.
-    Pollen grains are rich in nutrients. Pollen tablets are used as food supplements. Pollen consumption (as tablets & syrups) increases performance of athletes and race horses.
-    It is possible to store pollen grains of a large number of species for years in liquid nitrogen (-1960C). Such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programmes.
Gynoecium (Pistil) (For figures see TB page: 25)
-    It represents the female reproductive part of the flower.
-    It may consist of a single pistil (monocarpellary) or more than one pistil (multicarpellary).
-    When there are more than one, the pistils may be fused together (syncarpous) or may be free (apocarpous).
-    Each pistil has three parts:
o Stigma: It is a landing platform for pollen grains.
o Style: It is an elongated slender part beneath the stigma.
o Ovary: It is the basal bulged part of the pistil. Inside the ovary is the ovarian cavity (locule) in which the placenta is located. Arising from the placenta are the ovules (megasporangia). The number of ovules in an ovary may be one (wheat, paddy, mango etc) to many (papaya, water melon, orchids etc).
Megasporangium (Ovule): (For figures see TB page: 26)
-    It is a small structure attached to the placenta by means of a stalk (funicle). The junction where the body of ovule and funicle fuse is called hilum.
-    Each ovule has one or two protective envelopes called integuments. Integuments encircle the ovule except at the tip where a small opening (micropyle) is organized.
-    Opposite the micropylar end is the chalaza (basal part of the ovule).
-    Enclosed within the integuments, there is a mass of cells called nucellus. Its cells contain reserve food materials.
-    Located in the nucellus is the embryo sac (female gametophyte). An ovule generally has a single embryo sac formed from a megaspore through meiosis.
-    The formation of megaspores from the megaspore mother cell (MMC) is called megasporogenesis.
-    Ovules generally differentiate a single megaspore mother cell in the micropylar region of the nucellus. It is a large cell containing dense cytoplasm and a prominent nucleus.
-    The MMC undergoes meiotic division. It results in the production of 4 megaspores.
Female gametophyte (embryo sac):
-    In a majority of flowering plants, one of the megaspores is functional while the other three degenerate.
-    The functional megaspore develops into the female gametophyte. This method of embryo sac formation from a single megaspore is termed monosporic development.
Formation of the embryo sac:
-    The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac.
-    Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac.
-    These divisions are strictly free nuclear, i.e. nuclear divisions are not followed immediately by cell wall formation.
-    After the 8-nucleate stage, cell walls are laid down leading to the organization of the typical female gametophyte or embryo sac.
-    6 of the 8 nuclei are surrounded by cell walls and organized into cells. Remaining 2 nuclei (polar nuclei) are situated below the egg apparatus in the large central cell.
Distribution of the cells within the embryo sac:
-    3 cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus consists of 2 synergids and one egg cell.
-    The synergids have special cellular thickenings at the micropylar tip called filiform apparatus. It helps to guide the pollen tubes into the synergid.
-    Three cells are at the chalazal end and are called the antipodals. The large central cell has two polar nuclei. Thus, a typical mature angiosperm embryo sac is 8-nucleate and 7-celled.
-    It is the transfer of pollen grains from the anther to the stigma of a pistil.
-    Some external agents help the plants for pollination.
Depending on the source of pollen, pollination is 3 types.
a.    Autogamy: In this, pollen grains transfer from the anther to the stigma of the same flower.
Complete autogamy is rare in flowers with exposed anthers and stigma. Autogamy in such flowers requires synchrony in pollen release and stigma receptivity. Also, the anthers and stigma should lie close to each other to enable self-pollination.
Plants like Viola (common pansy), Oxalis & Commelina produce 2 types of flowers:
·   Chasmogamous flowers: They are similar to flowers of other species with exposed anthers and stigma.
·   Cleistogamous flowers: They do not open at all.  Anthers & stigma lie close to each other. They are autogamous as there is no chance of cross-pollination. When anthers dehisce in the flower buds, pollen grains come in contact with the stigma for pollination. Cleistogamous flowers produce assured seed-set even in the absence of pollinators.
b.   Geitonogamy: In this, pollen grains transfer from the anther to the stigma of another flower of the same plant. It is functionally cross-pollination involving a pollinating agent. But it is genetically similar to autogamy since the pollen grains come from the same plant.
c.    Xenogamy: In this, pollen grains transfer from anther to the stigma of a different plant. This brings genetically different types of pollen grains to the stigma.
Agents of Pollination
1. Abiotic agents (wind & water) of pollination
Pollination by wind (anemophily):
-    More common abiotic agent.
-    Ways for effective pollination:
o  The flowers produce enormous amount of pollen.
o  The pollen grains are light and non-sticky so that they can be transported in wind currents.
o  They often possess well-exposed stamens (for easy dispersion of pollens into wind currents).
o  Large, feathery stigma to trap air-borne pollen grains.
-    Wind pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence.
-    E.g. Corncob – the tassels are the stigma and style which wave in the wind to trap pollen grains. Wind-pollination is quite common in grasses.
Pollination by water (hydrophily):
-    It is quite rare. It is limited to about 30 genera, mostly monocotyledons. E.g. Vallisneria & Hydrilla (fresh water), Zostera (marine sea-grasses) etc.
-    As against this, water is a regular mode of transport for the male gametes among the lower plants. It is believed, particularly for some bryophytes & pteridophytes, that their distribution is limited because of the need for water for the transport of male gametes and fertilisation.
-    In Vallisneria, the female flower reaches the surface of water by the long stalk and the male flowers or pollen grains are released on to the surface of water. They are carried by water currents and reach the female flowers.
-    In sea grasses, female flowers remain submerged in water. Pollen grains are long and ribbon like. They are carried inside the water and reach the stigma.
-    The pollen grains of most of the water-pollinated species have a mucilaginous covering to protect from wetting.
-    Not all aquatic plants use hydrophily. In most of aquatic plants (water hyacinth, water lily etc), the flowers emerge above the level of water for entomophily or anemophily.
-    Wind and water pollinated flowers are not very colourful and do not produce nectar.
2. Biotic agents (animals) of pollination
-    Majority of flowering plants use animals as pollinating agents. E.g. Bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and humming birds) bats, some primates (lemurs), arboreal (tree-dwelling) rodents, reptiles (gecko lizard & garden lizard) etc.
-    Pollination by insects (Entomophily), particularly bees is more common.
-    Often flowers of animal pollinated plants are specifically adapted for a particular species of animal.
-    Features of insect-pollinated flowers:
o  Large, colourful, fragrant and rich in nectar. Nectar & pollen grains are the floral rewards for pollination.
o  When the flowers are small, they form inflorescence to make them visible.
o  The flowers pollinated by flies and beetles secrete foul odours to attract these animals.
o  The pollen grains are generally sticky.
-    When the animal comes in contact with the anthers and the stigma, its body gets a coating of pollen grains. When it comes in contact with the stigma, it results in pollination.
-    Some plants provide safe places as floral reward to lay eggs.
E.g. Amorphophallus (it has the tallest flower of about 6 feet). A species of moth and the plant Yucca cannot complete their life cycles without each other. The moth deposits its eggs in the locule of the ovary and the flower gets pollinated by the moth. The larvae of the moth come out of the eggs as the seeds start developing.
-    Many insects consume pollen or nectar without bringing about pollination. They are called pollen/nectar robbers.
Outbreeding Devices:
Majority of flowering plants produces hermaphrodite flowers can undergo self-pollination. Continued self-pollination results in inbreeding depression.
To avoid self pollination and encourage cross-pollination, there are some devices in plants:
a. Avoiding synchronization: In some species, pollen release and stigma receptivity are not synchronized. Either the pollen is released before the stigma becomes receptive or stigma becomes receptive before the release of pollen. It prevents autogamy.
b.Arrangement of anther & stigma at different positions: This prevents autogamy.
c. Self-incompatibility: It is a genetic mechanism to prevent self-pollen (from the same flower or other flowers of the same plant) from fertilization by inhibiting pollen germination or pollen tube growth in the pistil.
d.Production of unisexual flowers: If male & female flowers are present on the same plant (i.e., monoecious, e.g. castor & maize), it prevents autogamy but not geitonogamy. In dioecious plants (e.g. papaya), male and female flowers are present on different plants (dioecy). This prevents both autogamy and geitonogamy.
Pollen-pistil Interaction: (For figures see TB page: 32)
-    It is a dynamic process involving pollen recognition followed by promotion or inhibition of the pollen.
-    This interaction takes place through the chemical components produced by them.
-    If the pollen is compatible (right type), the pistil accepts it and promotes post-pollination events. The pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores. The contents of the pollen grain move into the pollen tube. Pollen tube grows through the tissues of the stigma and style and reaches the ovary.
-    If the pollen is incompatible (wrong type), the pistil rejects the pollen by preventing pollen germination on the stigma or the pollen tube growth in the style.
-    In some plants, pollen grains are shed at 2-celled condition (a vegetative cell & a generative cell). In such plants, the generative cell divides and forms the two male gametes during the growth of pollen tube in the stigma.
-    In plants which shed pollen in the 3-celled condition, pollen tubes carry 2 male gametes from the beginning. Pollen tube, after reaching the ovary, enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus. The filiform apparatus present at the micropylar part of the synergids guides the entry of pollen tube.
-    A plant breeder can manipulate pollen-pistil interaction, even in incompatible pollinations, to get desired hybrids.
Artificial hybridisation:
-    It is one of the major approaches of crop improvement programme.
-    In this, desired pollen grains are used for pollination. This is achieved by emasculation & bagging techniques.
-    Emasculation is the removal of anthers (using forceps) from the bisexual flower bud of female parent before the anther dehisces.
-    Emasculated flowers are covered with a suitable bag (made up of butter paper) to prevent contamination of its stigma with unwanted pollen. This is called bagging.
-    When the stigma attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma. Then the flowers are rebagged and allowed to develop the fruits.
-    If the female parent produces unisexual flowers, there is no need for emasculation. The female flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the desired pollen and the flower rebagged.
(For figures see TB page: 34)
-    After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid. One of the male gametes moves towards the egg cell and fuses with its nucleus (syngamy). This forms the zygote (a diploid cell).
-    The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN). As this involves the fusion of three haploid nuclei it is called triple fusion.
-    Since 2 types of fusions (syngamy & triple fusion) take place in an embryo sac it is called double fertilisation. It is an event unique to flowering plants.
-    The central cell after triple fusion becomes the primary endosperm cell (PEC) and develops into the endosperm while the zygote develops into an embryo.
Post-fertilisation events: Endosperm & embryo development, maturation of ovule(s) into seed(s) & ovary into fruit.
Endosperm development:
-    The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue.
-    Endosperm cells are filled with reserve food materials. They are used for the nutrition of the developing embryo.
-    In common endosperm development, the PEN undergoes successive nuclear divisions to give rise to free nuclei. This stage is called free-nuclear endosperm. The number of free nuclei varies greatly.
-    Then the endosperm becomes cellular due to the cell wall formation. The tender coconut water is a free-nuclear endosperm (made up of thousands of nuclei) and the surrounding white kernel is the cellular endosperm.
Embryo development: (For figures see TB page: 35)
-    Embryo develops at the micropylar end of the embryo sac where the zygote is situated.
-    Most zygotes divide only after the formation of certain amount of endosperm. This is an adaptation to provide nutrition to the developing embryo.
-    Though the seeds differ greatly, the embryogeny (early embryonic developments) is similar in monocotyledons & dicotyledons. (For figures see TB page: 34).
-    The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.
Dicotyledonous embryo
-    It has an embryonal axis and 2 cotyledons. The portion of embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule (stem tip). The cylindrical portion below the level of cotyledons is hypocotyl that terminates with the radicle (root tip). The root tip is covered with a root cap.
Monocotyledonous embryo
-    They possess only one cotyledon.
-    In the grass family the cotyledon is called scutellum. It is situated lateral to the embryonal axis. At its lower end, the embryonal axis has the radicle and root cap enclosed in coleorrhiza (an undifferentiated sheath).
-    Portion of embryonal axis above the level of attachment of scutellum is the epicotyl. It has a shoot apex and a few leaf primordia enclosed in coleoptile (a hollow foliar structure).
Seed (For figures see TB page: 37)
-    Seed is the final product of sexual reproduction. It is the fertilized ovule formed inside fruits.
-    It consists of seed coat(s), cotyledon(s) & an embryo axis.
-    The cotyledons are simple, generally thick and swollen due to storage food (as in legumes).
-    Mature seeds may be non-albuminous or albuminous.
o Non-albuminous seeds: have no residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut, beans).
o Albuminous seeds: retain a part of endosperm as it is not completely used up during embryo development (e.g., wheat, maize, barley, castor, coconut, sunflower).
-    Occasionally, in some seeds (black pepper, beet etc) remnants of nucellus are also persistent. It is called perisperm.
-    Integuments of ovules harden as tough protective seed coats. It has a small pore (micropyle) through which O2 & water enter into the seed during germination.
-    As the seed matures, its water content is reduced and seeds become dry (10-15 % moisture by mass). The general metabolic activity of the embryo slows down. The embryo may enter a state of inactivity (dormancy). If favourable conditions are available (adequate moisture, oxygen and suitable temperature), they germinate.
-    The ovary develops into a fruit. Transformation of ovules into seeds and ovary into fruit proceeds simultaneously.
-    The wall of ovary develops into pericarp (wall of fruit).
-    The fruits may be fleshy (e.g. guava, orange, mango, etc.) or may be dry (e.g. groundnut, mustard, etc).
-    Many fruits have mechanisms for dispersal of seeds.
-    Fruits are 2 types:
o True fruits: In most plants, the fruit develops only from the ovary and other floral parts degenerate and fall off. They called true fruits.
o False fruits: In this, the thalamus also contributes to fruit formation. E.g. apple, strawberry, cashew etc.
-    In some species fruits develop without fertilisation. Such fruits are called parthenocarpic fruits. E.g. Banana.
-    Parthenocarpy can be induced through the application of growth hormones and such fruits are seedless.
Advantages of seeds:
·   Since pollination and fertilisation are independent of water, seed formation is more dependable.
·   Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonize in other areas.
·   They have food reserves. So young seedlings are nourished until they are capable of photosynthesis.
·   The hard seed coat protects the young embryo.
·   Being products of sexual reproduction, they generate new genetic combinations leading to variations.
·   Dehydration and dormancy of mature seeds are crucial for storage of seeds. It can be used as food throughout the year and also to raise crop in the next season.
Viability of seeds after dispersal:
-    In a few species the seeds lose viability within a few months. Seeds of many species live for several years.
-    Some seeds can remain alive for hundreds of years. The oldest is that of a lupine (Lupinus arcticus) excavated from Arctic Tundra. The seed germinated and flowered after an estimated record of 10,000 years of dormancy.
-    2000 years old viable seed is of the date palm (Phoenix dactylifera) discovered during the archeological excavation at King Herod’s palace near the Dead Sea.
-    Apomixis is the production of seeds without fertilisation. E.g. Some species of Asteraceae and grasses.
-    Apomixis is a form of asexual reproduction that mimics sexual reproduction.
-    Development of apomictic seeds: In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilisation. In many species (e.g. many Citrus & Mango varieties) some of the nucellar cells surrounding the embryo sac divide, protrude into the embryo sac and develop into the embryos. In such species each ovule contains many embryos. Occurrence of more than one embryo in a seed is called polyembryony.
Importance of apomixis in hybrid seed industry
-    Hybrid seeds have to be produced every year. If the seeds collected from hybrids are sown, the plants in the progeny will segregate and lose hybrid characters.
-    Production of hybrid seeds is costly. Hence the cost of hybrid seeds is also expensive for the farmers.
-    If the hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. Then the farmers can keep on using the hybrid seeds to raise new crop year after year.

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