SEXUAL REPRODUCTION IN FLOWERING PLANTS REVISION NOTES

 FLOWER – A FASCINATING ORGAN OF ANGIOSPERMS

• Flowers have been used for aesthetic, ornamental, social, religious, and cultural purposes.
• Five ornamental flowers commonly cultivated in homes and in gardens are rose, sunflower, tulip, daisy, and lavender.
• Five flowers used in social and cultural celebrations can vary but examples include marigold, chrysanthemum, lily, lotus, and peony.
• Floriculture refers to the cultivation of flowers for ornamental and decorative purposes.
• The two parts in a flower in which the two most important units of sexual reproduction develop are the stamen and pistil.

1.2 PRE-FERTILISATION: STRUCTURES AND EVENTS

• Several hormonal and structural changes occur in a plant before it starts to flower.
• These changes lead to the differentiation and development of the floral primordium.
• Inflorescences are formed which bear the floral buds and then the flowers.
• The male and female reproductive structures, the androecium and the gynoecium differentiate and develop in the flower.
• The androecium consists of a whorl of stamens representing the male reproductive organ.
• The gynoecium represents the female reproductive organ.

1.2.1 Stamen, Microsporangium and Pollen Grain

• A typical stamen has two parts: the filament and the anther.
• The filament is a long and slender stalk that is attached to the thalamus or the petal of the flower.
• The anther is the terminal structure on the filament, generally bilobed and containing pollen sacs.
• The anther is composed of four microsporangia, two in each lobe.
• The microsporangia develop into pollen sacs that are packed with pollen grains.
• The microsporangium is surrounded by four wall layers: the epidermis, endothecium, middle layers, and the tapetum.
• The tapetum is the innermost wall layer that nourishes the developing pollen grains.
• The cells of the sporogenous tissue in the anther undergo meiotic divisions to form microspore tetrads.
• Each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad.
• The process of formation of microspores from a pollen mother cell through meiosis is called microsporogenesis.
• The microspores are arranged in a cluster of four cells, the microspore tetrad.
• The microspores dissociate from each other and develop into pollen grains, which are released with the dehiscence of the anther.

POLLEN GRAINS

• Pollen grains are male gametophytes found in the anthers of flowers.
• The outer layer of the pollen grain is called the exine, which is made up of sporopollenin and has germ pores.
• The exine is hard and can withstand high temperatures and strong acids and alkalis, making pollen grains well-preserved as fossils.
• The inner layer of the pollen grain is called the intine, which is a thin and continuous layer made up of cellulose and pectin.
• Mature pollen grains contain two cells, the vegetative cell, and the generative cell.
• In over 60% of angiosperms, pollen grains are shed at the 2-celled stage. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).
• Pollen grains of many species can cause severe allergies and respiratory disorders.
• Pollen grains are rich in nutrients and are used as food supplements.
• The period for which pollen grains remain viable varies and depends on the prevailing temperature and humidity.
• Pollen grains can be stored in liquid nitrogen (-196°C) for years, similar to seed banks, in crop breeding programs.

1.2.2 The Pistil, Megasporangium (ovule), and Embryo sac

• The gynoecium is the female reproductive part of a flower, consisting of one or more pistils.
• Each pistil has three parts: the stigma, style, and ovary.
• The ovary contains ovules, which are megasporangia.
• Megasporogenesis is the process of forming megaspores from the megaspore mother cell.
• One of the four megaspores becomes functional and develops into the female gametophyte or embryo sac.
• The embryo sac is formed through free nuclear divisions and has a characteristic distribution of cells.
• The egg apparatus consists of two synergids and one egg cell.
• The antipodals are located at the chalazal end, and the large central cell contains two polar nuclei.
• The mature angiosperm embryo sac is 8-nucleate and 7-celled.

1.2.3 Pollination

• Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.
• External agents are often used to achieve pollination in flowering plants.
• Possible external agents include insects, birds, bats, wind, water, and even humans.
• Pollination can be divided into three types: autogamy, geitonogamy, and xenogamy.
• Autogamy occurs within the same flower, while geitonogamy occurs between flowers of the same plant.
• Xenogamy involves the transfer of pollen grains between flowers of different plants.
• Cleistogamous flowers are a type of autogamous flower that does not open and produce assured seed-set even in the absence of pollinators.
• Xenogamy is the only type of pollination that brings genetically different types of pollen grains to the stigma.

Agents of Pollination :

• Plants use wind, water, and animals as pollinating agents.
• The majority of plants use biotic agents for pollination, but a small proportion uses abiotic agents.
• Pollination by wind is more common amongst abiotic pollinations and requires light, non-sticky pollen grains.
• Pollination by water is rare in flowering plants and is limited to about 30 genera, mostly monocotyledons.
• Both wind and water-pollinated flowers are not very colorful and do not produce nectar.
• The majority of flowering plants use a range of animals as pollinating agents, particularly bees.
• Insect-pollinated flowers are often large, colorful, fragrant, and rich in nectar.
• Flowers pollinated by flies and beetles secrete foul odors to attract these animals.
• Flowers provide rewards to the animals in the form of nectar and pollen grains.
• Animal visitors to the flowers get a coating of pollen grains, which are generally sticky in animal-pollinated flowers, and bring about pollination when they come in contact with the stigma.
• In some species, floral rewards are in providing safe places to lay eggs.
• Examples of such relationships include the Amorphophallus flower and the Yucca plant.
• The Yucca plant and a species of moth 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.
• To observe which animals visit different plants, one needs to patiently observe the flowers over a few days and at different times of the day.
• It's crucial to see if any of the visitors come in contact with the anthers and the stigma as only such visitors can bring about pollination.
• Many insects may consume pollen or nectar without bringing about pollination. Such floral visitors are referred to as pollen/nectar robbers.
• It may or may not be possible to identify the pollinators, but the observations are interesting.

Outbreeding Devices :

• The majority of flowering plants produce hermaphrodite flowers, which can lead to inbreeding depression through self-pollination.
• Plants have evolved various mechanisms to discourage self-pollination and encourage cross-pollination.
• One such mechanism is asynchronous pollen release and stigma receptivity, where either the pollen is released before the stigma is receptive or vice versa.
• Another mechanism is to place the anther and stigma at different positions to prevent self-pollination within the same flower.
• Self-incompatibility is a genetic mechanism that prevents self-pollen from fertilizing the ovules by inhibiting pollen germination or tube growth.
• Production of unisexual flowers can also prevent self-pollination, for example, having male and female flowers on different plants (dioecy) or on the same plant (monoecy).

Pollen-pistil Interaction : 

• Pollination may result in the transfer of incompatible pollen.
• The pistil has the ability to recognize compatible pollen and reject incompatible pollen.
• The recognition process is mediated by chemical interactions between the pollen and pistil components.
• After compatible pollination, the pollen grain germinates on the stigma and produces a pollen tube that grows through the stigma and style.
• The pollen tube reaches the ovary and enters the ovule through the micropyle.
• Pollen-pistil interaction involves pollen recognition and promotion or inhibition of pollen.
• Understanding pollen-pistil interaction can help plant breeders manipulate pollination to obtain desired hybrids.
• Pollen germination can be studied by dusting pollen on a glass slide containing a sugar solution and observing it under a microscope.
• Breeders cross different species and genera to produce commercially superior varieties.
• Artificial hybridization is a major approach to crop improvement.
• Emasculation and bagging techniques are used to prevent contamination from unwanted pollen.
• Emasculation involves removing anthers from the flower bud of a bisexual flower.
• Bagging involves covering emasculated flowers with a bag to prevent contamination.
• Pollination is carried out using desired pollen on the stigma of bagged flowers.
• Unisexual flowers do not need emasculation; female flower buds are bagged before they open.
• Pollination is carried out when the stigma becomes receptive, and the flower is rebagged.

1.3 DOUBLE FERTILISATION

• After entering the synergid, the pollen tube releases two male gametes.
• One male gamete fuses with the egg cell to form the zygote, resulting in syngamy.
• The other male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus (PEN).
• The fusion of the three haploid nuclei is termed triple fusion.
• Since both syngamy and triple fusion occur in the embryo sac, it is termed double fertilisation.
• The central cell becomes the primary endosperm cell (PEC) and develops into the endosperm.
• The zygote develops into an embryo.

1.4 POST-FERTILISATION: STRUCTURES AND EVENTS

1.4.1 Endosperm

• Endosperm development precedes embryo development.
• Primary endosperm cell (PEC) divides repeatedly to form a triploid endosperm tissue.
• The endosperm tissue is filled with reserve food materials used for the nutrition of the developing embryo.
• In the most common type of endosperm development, the PEC undergoes successive nuclear divisions to give rise to free nuclei.
• This stage of endosperm development is called free-nuclear endosperm.
• Subsequently, cell wall formation occurs and the endosperm becomes cellular.
• The number of free nuclei formed before cellularisation varies greatly.
• Endosperm may either be completely consumed by the developing embryo (e.g., pea, groundnut, beans) before seed maturation or it may persist in the mature seed (e.g. castor and coconut) and be used up during seed germination.
• Coconut water from tender coconut is made up of thousands of nuclei and is an example of a free-nuclear endosperm.
• The surrounding white kernel is the cellular endosperm.
• The persistence of endosperm in cereals such as wheat, rice, and maize varies.

1.4.2 Embryo

• The embryo develops at the micropylar end of the embryo sac where the zygote is situated.
• Most zygotes divide only after certain amount of endosperm is formed to provide assured nutrition to the developing embryo.
• Early stages of embryo development are similar in both monocotyledons and dicotyledons.
• A dicotyledonous embryo consists of an embryonal axis and two cotyledons.
• Monocotyledonous embryo possess only one cotyledon called scutellum that is situated towards one side (lateral) of the embryonal axis.
• Soaking seeds of wheat, maize, peas, chickpeas, groundnut overnight and splitting them open can help in observing the various parts of the embryo and the seed.

1.4.3 Seed 

• Seeds in angiosperms consist of seed coats, cotyledons, and an embryo axis, and may be albuminous or non-albuminous.
• Fruits develop from ovaries and can contain fewer or more seeds than the number of ovules, depending on factors such as fertilization success and competition among seeds.
• False fruits develop from both the ovary and other floral parts, such as the thalamus.
• Parthenocarpic fruits can develop without fertilization, and seedless varieties can be induced by applying growth hormones.
• Seeds offer several advantages to plants, such as dependable reproduction and better adaptive strategies for dispersal, as well as nourishment and protection for young seedlings.
• Seeds are important for agriculture as they can be stored and used as food or for planting in the next season.
• Some flowering plants can produce enormous numbers of seeds, such as orchid fruits, parasitic species such as Orobanche and Striga, and the Ficus tree.

1.5 APOMIXIS AND POLYEMBRYONY

• Some flowering plants can produce seeds without fertilization, through a process called apomixis.
• Apomixis is a form of asexual reproduction that mimics sexual reproduction.
• Apomictic seeds can develop from a diploid egg cell or from nucellar cells surrounding the embryo sac.
• In some species, each ovule can contain many embryos, a phenomenon known as polyembryony.
• Apomictic embryos are genetically identical and can be called clones.
• Hybrid seeds have to be produced every year and are expensive, but if hybrids are made into apomicts, farmers can use the same hybrid seeds year after year.
• Active research is being conducted to understand the genetics of apomixis and transfer apomictic genes into hybrid varieties.