Flowering Plants: Sexual Reproduction

• Plants reproduce sexually and the scents and colors of flowers aid in this.
• Flowering plants exhibit sexual reproduction.
• Flowers and floral parts have many adaptations for forming fruits and seeds.

Flower - A Fascinating Organ of Angiosperms

• Human beings have had a close relationship with flowers for a long time.
• Flowers are aesthetically, ornamentally, socially, religiously, and culturally valuable.
• They are used to convey love, affection, happiness, grief, mourning, and other feelings.
• In biology, flowers are morphological and embryological marvels and the sites of sexual reproduction.
• The androecium and gynoecium are the male and female reproductive structures in a flower.
• The Androecium is a whorl of stamens representing the male reproductive organ.
• The Gynoecium represents the female reproductive organ.

Stamen, Microsporangium, and Pollen Grain

• A typical stamen consists of a filament (the long, slender stalk) and an anther (the terminal, bilobed structure).
• The proximal end of the filament attaches to the thalamus or petal.
• Stamen number and length varies across different flower species.
• A typical angiosperm anther is bilobed and dithecous, with each lobe containing two theca.
• A longitudinal groove separates the theca.
• The anther has a four-sided (tetragonal) structure with four microsporangia at the corners.
• Microsporangia develop into pollen sacs that extend lengthwise and are packed with pollen grains.
• A typical microsporangium in transverse section is surrounded by four wall layers: the epidermis, endothecium, middle layers, and tapetum.
• The outer three layers protect and aid in anther dehiscence.
• The innermost layer, the tapetum, nourishes developing pollen grains; its cells have dense cytoplasm and multiple nuclei.
• Sporogenous tissue, a group of homogenous cells, occupies the center of each microsporangium in a young anther.
• As the anther develops, sporogenous tissue cells undergo meiotic divisions (microsporogenesis) to form microspore tetrads.

Microsporogenesis

• Each cell of sporogenous tissue can give rise to a microspore tetrad, and is referred to as a potential pollen or microspore mother cell (PMC).
• Microspores are arranged in a cluster of four cells i.e. microspore tetrad.
• As anthers mature and dehydrate, microspores dissociate and develop into pollen grains.
• Thousands of microspores or pollen grains form inside each microsporangium; these are released with anther dehiscence.

Pollen Grain

• Pollen grains represent the male gametophytes.
• Pollen grains often appear as yellowish powdery deposits.
• Pollen grains are generally spherical, measuring 25-50 micrometers in diameter.
• The outer wall layer, called the exine, is made of sporopollenin which can withstand high temperatures, strong acids, and alkali.
• The exine has apertures called germ pores where sporopollenin is absent.
• Pollen grains are well-preserved as fossils due to sporopollenin.
• The inner wall of the pollen grain is called the intine; it is thin, continuous, and made of cellulose and pectin.
• The cytoplasm of the pollen grain is surrounded by a plasma membrane.
• A mature pollen grain contains a vegetative cell and a generative cell
• The vegetative cell is bigger, has abundant food reserve, and a large irregularly shaped nucleus.
• The generative cell is small, floats in the cytoplasm of the vegetative cell, and has a spindle shape with dense cytoplasm and a nucleus.
• Pollen grains are shed at two-celled stage in over 60% of angiosperms.
• Generative cell divides mitotically to form 2 male gametes before pollen grains are shed in remaining species (3-celled stage).
• Pollen grains from many species can cause allergies and bronchial afflictions like asthma and bronchitis.
• Parthenium or carrot grass, a contaminant in imported wheat, causes pollen allergy.
• Pollen grains are rich in nutrients and used as food supplements.
• It's been claimed that pollen consumption boosts athlete and racehorse performance.
• Pollen grains must land on the stigma before losing viability to bring about fertilization.
• Pollen grains vary highly in viability, depending on temperature and humidity.
• Pollen grains viability after release is 30 minutes in cereals, such as rice and wheat.
• Pollen grains viability for months in Rosaceae, Leguminoseae and Solanaceae,
• Pollen grains can be stored for years in liquid nitrogen (-196°C) and used as pollen banks for crop breeding programs.

Pistil, Megasporangium (Ovule), and Embryo Sac

• The gynoecium is the female reproductive part consisting of one pistil (monocarpellary) or multiple pistils (multicarpellary).
• Pistils can be fused together (syncarpous) or free (apocarpous).
• A pistil has three parts: stigma, style, and ovary.
• The stigma receives pollen grains.
• The style is the slender part beneath the stigma.
• The ovary is the basal, bulged part with an ovarian cavity (locule) containing the placenta.
• Megasporangia, commonly called ovules, arise from the placenta.
• The number of ovules in the ovary ranges from one (wheat, paddy, mango) to many (papaya, water melon, orchids).

Megasporangium

• The ovule is a small structure attached to the placenta by the funicle.
• The body of the ovule fuses with the funicle at the hilum, marking the junction.
• Each ovule has one or two protective envelopes named integuments, that encircle the nucellus except at the micropyle.
• Chalaza is at the base of the ovule.
• Nucellus is the mass of cells enclosed within the integuments, that has abundant reserve food materials.
• The embryo sac, or female gametophyte, is located in the nucellus.
• An ovule generally contains a single embryo sac, formed from a megaspore.

Megasporogenesis

• Megasporogenesis is the formation of megaspores from the megaspore mother cell.
• Ovules typically differentiate a single megaspore mother cell (MMC) in the micropylar region of the nucellus.
• The MMC undergoes meiotic division to produce four megaspores.

Female Gametophyte

• In flowering plants, typically one megaspore is functional; the others degenerate.
• The functional megaspore develops into the female gametophyte (embryo sac), and is termed monosporic development.
• The nucleus of the functional megaspore divides mitotically to form two nuclei which move to opposite poles, creating the 2-nucleate embryo sac.
• Two more sequential mitotic nuclear divisions result in the 4-nucleate and then 8-nucleate stages of the embryo sac.
• These mitotic divisions are not immediately followed by cell wall formation i.e they are strictly free nuclear.
• Cell walls are laid down after the 8-nucleate stage, organizing a typical female gametophyte or embryo sac.
• Six of the eight nuclei are surrounded by cell walls and organized into cells; the remaining two nuclei, called polar nuclei, are located below the egg apparatus in the large central cell.
• Three cells group at the micropylar end to form the egg apparatus which consists of two synergids and one egg cell.
• Synergids have filiform apparatus, that play a role in guiding pollen tubes into the synergid.
• Three cells at the chalazal end are called antipodals.
• The large central cell contains two polar nuclei.
• Angiosperm embryo sac at maturity is 7-celled, though 8-nucleate.

Pollination

• As male and female gametes are non-motile, pollination is used to bring them together for fertilization.
• Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.

Kinds of Pollination

• Autogamy: Pollination occurs within the same flower; pollen grains transfer from the anther to the stigma of the same flower.
• Autogamy requires synchronization of pollen release and stigma receptivity, along with close proximity of the anthers and stigma.
• Viola (common pansy), Oxalis, and Commelina produce chasmogamous flowers- exposed anthers and stigma.
• Viola (common pansy), Oxalis, and Commelina produce cleistogamous flowers that do not open at all.
• Cleistogamous flowers are invariably autogamous, with assured seed-set.
• Geitonogamy: Pollen grains transfer from the anther to the stigma of another flower on the same plant
• While functionally cross-pollination involving a pollinating agent, Geitonogamy is genetically similar to autogamy.
• Xenogamy: Pollen grains transfer from the anther to the stigma of a different plant.
• Xenogamy is the only type of pollination that brings genetically different types of pollen grains to the stigma.
• Plants use abiotic (wind and water) and biotic (animals) agents to cross-pollinate.
• Wind and water are abiotic agents for which pollen grains must contact the stigma by chance
• Flowers produce enormous pollen grains compared to the number of ovules.

Pollination By Agents

• Pollination is more common by wind among abiotic pollination.
• Wind-pollinated pollen grains must be light and non-sticky for wind transport.
• Stamens are well-exposed (for wind dispersal) and stigmas are large and often feathery (to catch airborne pollen).
• Wind-pollinated flowers often have a single ovule and numerous flowers packed into an inflorescence, such as corn.
• Water pollination is quite rare, limited to about 30 genera (mostly monocotyledons).
• Water is a common way to transport the male gametes among lower plant groups such as algae, bryophytes and pteridophytes.
• Vallisneria and Hydrilla are pollinated by water in fresh water, while Zostera pollinated in marine environments.
• Most aquatic plants pollinate via emergent flowers (above the water), and wind/insects pollinate these.
• The female flower in Vallisneria reaches the surface by a long stalk with pollen grains released onto the surface and carried passively to the stigma.
• Seagrasses pollinate with submerged female flowers releasing pollen grains inside the water to reach the stigma.
• Most wind and water pollinated flowers are not colorful and don't produce nectar.

Animal Agents

• Most flowering plants use animals as pollinating agents (bees, butterflies, flies, beetles, wasps, ants, moths, birds, and bats).
• Insects, particularly bees, are dominant biotic pollinators.
• Some primates (lemurs), arboreal rodents, and reptiles (gecko lizards and garden lizards) have also been reported as pollinators in some species.
• Insect-pollinated flowers are large, colourful, fragrant, and nectar-rich.
• Smaller flowers are clustered into inflorescences to increase visibility.
• Animals are attracted by color and/or fragrance..
• Fly or beetle-pollinated flowers secrete foul odors.
• Nectar and pollen grains are typical floral rewards to sustain animal visits and flower come in contact with anthers and stigma.
• Some species create safe places for laying eggs with floral rewards, such as Amorphophallus.

Outbreeding Devices

• Majority of flowering plants produce hermaphrodite flowers and pollen grains are likely to contact the stigma of the same flower, resulting in inbreeding depression.
• Flowering plants have developed mechanisms to discourage self-pollination and encourage cross-pollination.
• Pollen release and stigma receptivity are not synchronized in some species, while in others, anther and stigma are in different positions which prevents autogamy.
• Self-incompatibility is a genetic mechanism preventing self-pollen from fertilizing ovules.
• Production of unisexual flowers prevents self-pollination.
• Male and female flowers on the same plant (monoecious) prevent autogamy, but not geitonogamy (castor and maize).
• For plants such as papaya, which has male and female flowers on different plants (dioecy), prevents both autogamy and geitonogamy.

Pollen-Pistil Interaction

• Pistils can recognize compatible pollen and undergo post-pollination events.
• Pistils reject the pollen if it is the wrong type, and prevent pollen germination or pollen tube growth in the style.
• The pistil's pollen recognition involves continuous dialogue between pollen grain and pistil, mediated by chemical components.
• Pollen grains germinate on the stigma, forming a pollen tube and grows through stigma and style reaching the ovary.
• Pollen tubes carry the 2 male gametes and reaches ovary and enters ovule.
• In some plants, the generative cell divides to create 2 male gametes, while some already have a 3 celled condition
• The filiform apparatus at the micropylar part guides the pollen tube's entry into synergids.
• Pollen-pistil interaction is pollen recognition followed by pollen promotion or inhibition.
• Understanding this interaction can help create desired hybrids, even in incompatible pollinations.
• One can easily study pollen germination.
• Artificial hybridization is used to make desired pollen for pollination, and stigma is protected with emasculation and bagging.
• Anthers are removed before they dehisce on female parent bearing bisexual flowers, followed by covering with a bag- emasculation.
• The bagged stigma is dusted with mature pollen from the make plant; they are the rebagged.
• If the female parent produces unisexual flowers, there is no need for emasculation; female flower buds are bagged before flowers open.
• Pollination is carried our with desired pollen and then rebagged.

Double Fertilization

• Following synergid entry, the pollen tube releases two male gametes into the synergid cytoplasm.
• One gamete move towards egg cell for syngamy, resulting in the diploid zygote.
• The other gamete moves to the two polar nuclei of the central cell and fuses to form the triploid primary endosperm nucleus (PEN).
• Since the embryo sac has 2 fusions (syngamy and triple fusion), the process is called double fertilization and is unique to flowering plants.
• The central cell becomes primary endosperm cell (PEC) after triple fusion and develops into endosperm.
• The zygote develops into an embryo.

Post-Fertilization: Structures and Events

• The events following double fertilization, including endosperm and embryo development and maturation of ovules into seeds and ovary into fruit, are known as post-fertilization events.
• Endosperm development comes comes before embryo development.

Endosperm

• Repeated divisions of the primary endosperm cell (PEC) form a triploid endosperm tissue, filled with food reserves for the developing embryo.
• Free nuclear endosperm (PEN undergoing nuclear divisions) and cellular endosperm ( cell wall formation) are two types of endosperm development.
• Coconut water is an example of free-nuclear endosperm, while the surrounding white kernel is cellular endosperm.
• The developing embryo either completely consumes all endosperm, such as in pea, groundnut and beans; or persists at seed maturation and be used during seed germination (e.g. castor and coconut).

Embryo

• The embryo typically starts cell division after a certain amount of endosperm is formed.
• Early stages of embryo development (embryogeny) are the same for all seeds, yet they may vary greatly.
• The zygote develops proembryo and later in globular, heart-shapes, and mature embryo.

Dicotyledonous Embryo

• Consists of an embryonal axis and two cotyledons.
• The portion of the embryonal axis above the level of cotyledons is known as the epicotyl, which terminates with the plumule or stem tip.
• The cylindrical portion below the level of cotyledons is the hypocotyl, terminating at its lower end with the radicle or root tip, which has a root cap.

Monocotyledonous Embryo

• Possess only one cotyledon. In the grass family, the cotyledon is called the scutellum, located on the lateral side of the embryonal axis.
• The embryonal axis at its lower end has the radicle and root cap enclosed in a coleorrhiza, an undifferentiated sheath.
• The epicotyl is the portion of the embryonal axis above the scutellum, which has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure-coleoptile.

Seed

• In angiosperms, the seed is final product of sexual reproduction i.e fertilized ovule is the seed.
• Seeds are composed of seed coat(s), cotyledon(s), and an embryo axis.
• Storage tissues that are often thich and swollen (as in legumes) are from cotyledons .
• Mature seeds are either non-albuminous (no residual endosperm) or albuminous (part of endosperm remains).
• Integuments of ovules form protective seed coats, and the micropyle remains as a pore assisting O2 and H2O entry for germination.
• As the seed matures, water content reduces and it becomes dry, lowering its metabolic activity.
• The embryo may enter dormancy, or germinate is there are favorable resources.

Fruits

• Fruits develop from transformation of ovary after the ovules transform into seeds.
• The ovary wall forms the fruit wall called pericarp.
• The pericarp may be either fleshy or dry.
• Fruit develops dispersion strategies .
• Thalamus may form the fruit in only a few species such as strawberry.
• Fruits are normally from fertilization results, yet parthenocarpic fruits are without results.

Apomixis and Polyembryony

• Apomixis a special process in Asteraceae and grasses evolved the make seeds without fertilization.
• Diploid egg cell form without reduction division and form the embryo sans fertilization. Nucellar cells may surround and divide and protrude into to form embryo sac to turn into embryos.
• This event where each ovule contains much embryo is referred to as polyembryony.
• Hybrid varieties have increased yield due to hybrids always produced, yet progeny do often segregate and aren't the same hybrid.
• Because of apomixis' import to keep desired traits and in the hybrid source, there are many studies.