11777 results in Plant sciences
Evolutionary Physiology of Algae and Aquatic Plants
- Edited by Mario Giordano, John Beardall, John A. Raven, Stephen C. Maberly
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Photosynthetic organisms have an enormous influence on our environment through their effects on the development of other life on Earth and the way they alter the planet's geology and geochemistry. This book takes a unique approach by examining the evolutionary history of the major groups of aquatic photoautotrophs in the context of the ecophysiological characteristics that have allowed them to adapt to the challenges of life in water and thrive under past and present environmental conditions. The important role played by aquatic photoautotrophs on a planet undergoing unprecedented anthropogenic-induced change is also highlighted, in chapters on their critical function in mitigating environmental change through their physiological processes, and on the role of algae in biotechnology. This invaluable resource will be appreciated by researchers and advanced students interested in the biodiversity and evolutionary physiology of the full range of aquatic photoautotrophs, and their interaction with the environment.
Chapter 1 - An Introduction to the Reproductive Biology of Flowering Plants
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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Summary
Reproduction is a very important stage in the life-history of a species, being essential for its survival and sustenance. Different organisms adopt different strategies as they attempt to maximize their reproductive success and produce a favourable number of new individuals. Reproduction in plants can be achieved by either vegetative or sexual means or a combination of both. The seeds and propagules produced by asexual and sexual modes of reproduction have differing implications on the perpetuation of the species. Asexual means (such as vegetative reproduction) in plants is a quicker reproductive strategy that leads to production of new individuals genetically identical to parents. However, there is a limitation of genetic variability in vegetative reproduction and this may affect the long-term survival of a species. On the other hand, reproduction by sexual means brings genetic heterogeneity in progeny resulting in their wider adaptability and better survival. Sexual reproduction in angiosperms is a complex process involving several sequential events which take place in different organs of a flower. Thus, flower is a unit of sexual reproduction in angiosperms.
Plant reproductive biology is the study of the mechanisms of both sexual and asexual reproduction in plants. It involves the study of interactions of plants with biotic factors (such as pollinators, seed dispersal agents) and abiotic components (such as soil, space, climate) in the environment. With the integration of the many aspects of ecology, reproductive biology of flowering plants is now also known as Reproductive Ecology of Flowering Plants.
Different aspects of Reproductive Biology of Flowering Plants
Study of reproductive biology of plants broadly includes observations on phenology, structural and functional floral biology, sexual system, pollination biology, mating system, pollen–pistil interactions, fertilization, embryo-endosperm development, seed formation, seed dispersal and seed recruitment. These events may also be considered as the series of steps neccessary for the formation of a perfect new sporophyte. These aspects being interconnected, each of these is discussed sequentially in the subsequent sections.
• Phenology: Phenology is the timing of recurring biological phases in response to seasonal variations. In the life-cycle of flowering plants various events such as appearance of leaves, onset of flowering, fruit initiation and seed dispersal occur in consonance with seasonal changes and are termed as phenoevents. The timing of these recurring and periodic life-cycle events plays a significant role in interaction with other species in the ecosystem.
Chapter 6 - Pollination
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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Summary
Introduction
Once a flower is fully developed, most of the plants display their sex organs to carry out reproduction. This is accomplished by opening of the flower bud which is known as floral anthesis. Opening of flower is followed by pollination, a very important step in plant reproduction. Pollination is the transfer of pollen from an anther of a flower to a stigma of the same or a different flower. The transfer of pollen grains to the conspecific (belonging to the same species) stigma is the primary step in reproduction or seed formation. Pollination also forms the basis for genetic heterogeneity in plants. Study of methods of pollination and subsequent fertilization in plants is referred to as pollination biology.
Considering the fact that plants are immobile, most angiosperm species have to rely on external agents for the transfer of pollen from anther to stigma. Pollination services are rendered by various biotic and abiotic agencies and their presence is essential for optimal reproductive performance of a flowering plant. In case of biotic pollination, the plant attracts a particular type of pollinator and when the same pollinator visits the next flower there are chances that its pollen is carried to another flower of the same species. In exchange for this service, the pollinator gets access to the food (pollen and nectar) offered by the flowers. Thus, by visiting a particular type of flower, the pollinator gets important food resources and the plant gets pollinated in return. Several groups of insects, birds, bats and other animals are dependent on plants for their nutritional needs, especially during their breeding seasons. Such mutually beneficial relationship between the angiosperms and the pollinators has led to co-evolution and adaptations among these groups over millions of years. Plants and pollinators have co-evolved and undergone changes in their physical forms to increase the chances of successful interaction.
An array of floral features like color, form, nectar and scent exhibited by angiosperms is associated with different modes of pollination. Such correlation between floral features and pollinating agency is referred to as pollination syndrome. The pollination services provided by biotic and abiotic agents are not only essential for fertilization but are also required to maximize dispersal distances and increase genetic variability in plants.
Chapter 8 - Self-Incompatibility
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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Summary
Introduction
The pistil of a flower is exposed to all types of pollen grains in the atmosphere irrespective of whether they belong to the same species or not. However, mere landing of pollen on the stigma is not enough to effect fertilization. As we learnt in the last chapter, there are cellular interactions or cross talk that take place between the pollen and the pistil before successful fertilization. These specific interactions between pollen and pistil facilitate selection of the right type of pollen grains by the pistil and limit fertilization between incompatible gametes.
The inability of a functional male gamete and female gamete to fuse with each other and achieve fertilization is termed as sexual incompatibility. Sexual incompatibility may be interspecific or intraspecific. Following pollination, the ability of a pistil (or stigma) to reject pollen grains from other species is termed as inter-specific incompatibility. This type of incompatibility prevents the formation of inter-specific hybrids and maintains the identity of a biological species. The inter-specific incompatibility is controlled by several genes and is also referred to as heterogenic incompatibility. Interestingly, in nature there are several incidences where pistil carrying functional female gametes are unable to set fruits even when pollinated by viable and fertile self-pollen grains. Scientific investigation have established that the failure of fruit set in these plants is due to genetic factors which impose a physiological barrier to self-fertilization. This phenomenon of failure of a male gamete and a female gamete to achieve self-fertilization is termed as intra-specific incompatibility or more specifically, self-incompatibility (SI). In other words, self-incompatibility is the inability of a fertile hermaphrodite plant to set seeds when self-pollinated. The term self-incompatibility was first coined by Stout (1917); it allows flowering plants to avoid inbreeding and involves genetic mechanisms which prevent self-fertilization and promote out-crossing.
In a self-incompatible plant, whenever its own pollen grains reach stigma either pollen germination or pollen tube growth is terminated which results in failure of seed-set. Yet, there are incidences where self-pollen are able to germinate, and self-pollen tubes are even able to penetrate the ovules. In these cases either fertilization fails to occur, or if at all occurs, the zygote gets aborted after syngamy. This type of SI is called Late Acting Self-incompatibility (LSI).
Chapter 12 - Seed
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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Summary
Introduction
The protective seed habit is a significant feature in the evolutionary success of angiosperms. The seed, encloses an undeveloped miniature plant ‘the embryo’ and acts as a functional unit which links the successive generations. Developmentally, seed is a fertilized ovule and, a typical angiospermous seed consists of an embryo, some storage tissue (mostly endosperm) and a seed-coat. While the embryo and the endosperm are the products of double fertilization, the seed-coat develops from the integument/s of the ovule. Embryos accomplish their early development before seed germination, protected by the surrounding seed coats and sustaining on the stored food in the endosperm. Protection provided to embryo by seed coat increases its chances of survival, and establishment of subsequent generations. Generally, seeds develop as discrete units attached to the inside of the fruit wall through a stalk called the funiculus. However, in many plants, seeds are associated with some other structures that help in their dispersal. In such cases, a single entity of the seed and the structure assisting in dispersal are together described as dispersal units or diaspores. For example, in the members of Asteraceae, the outer integument of the ovule is completely fused with the ovary wall and the diaspore is called a cypsella. Some other examples of diaspores are the seeds with the elaiosomes, achene (dry indehiscent fruits), and caryopsis (fruit type seen in grasses).
Here, one must acknowledge that all structures and processes associated with reproduction in angiosperms are directly responsible for the formation of seed. Seeds perform a wide variety of functions including dispersal, perennation (surviving seasons of stress such as winter), dormancy (a state of arrested development), and most importantly, perpetuation of a plant species.
A huge variation in the size, shape, color, seed coat, weight and dispersal mechanism can be observed among angiospermous seeds. The smallest known seeds are those of orchids which are about 85 micrometers in size and weigh about 0.8 micrograms, thus appearing similar to dust particles. Double coconut or Lodoicea maldivica has the largest (nearly 0.5 meter) and the heaviest (weighing up to 25 kg) known seeds in the world (Fig. 12.1). The size of the seed in a plant depends on the size of the embryo and also on whether the seed at maturity is endospermous or non-endospermous. Seeds in Orchids are small as the endosperm formation is completely suppressed, and also, the embryo is highly reduced.
Image Sources
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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Foreword
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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Summary
Plants in general and flowering plants (angiosperms) in particular are the essential components for sustenance of life of all non-photosynthetic organisms on our planet. Plants reproduce by asexual as well as sexual means. Asexual reproduction is not congenial for long-term sustenance and evolutionary processes of the species because of genetic uniformity of the progeny. Sexual reproduction which permits genetic recombination is the dominant mode. Although Angiosperms were the last to evolve as land plants, they soon became the most successful and dominant group amongst land plants. Their success is largely due to the mode of their reproduction through the evolution of the flower and the consequent advantages it brought in. For human beings, flowering plants provide most of their essential needs – food, fibres, shelter, medicines, clean air and water. Reproduction is the basis for sustenance of any species. Thus, understanding reproductive biology of flowering plants is important not only from the fundamental point of view but also for their manipulation for human welfare. Reproductive biology of angiosperms is more complex when compared to other groups of plants because of the involvement of the flower. The progress in understanding the structural and functional aspects of reproduction has been very slow.
Initial studies on reproductive biology of angiosperms were largely confined to examining embryological details using fixed and sectioned materials. Enormous data accumulated over the years on the developmental details of the pollen grains, ovules and female gametophyte, double fertilization, embryo and endosperm, seed and fruit development. These advances were taught to the undergraduate and postgraduate students under the title embryology of angiosperms as a part of their curriculum. Following the development of electron microscopy and histochemistry, embryological details were further elaborated by using these techniques. Development of aseptic culture techniques broadened scope for experimental studies on embryological processes leading to a slow but steady understanding of the functional details of embryological structures. These developments were incorporated in some of the books of embryology under a chapter on experimental embryology. However, there was hardly any integrated account of embryological processes in relation to the structure with their function. Pre-fertilization aspects of reproductive biology covering the details of pollen, pistil, and pollen–pistil interactions, which are unique to angiosperms and play a critical role in their successful evolution, were the last to enter the field of embryology of angiosperm.
Reproductive Biology of Angiosperms
- Concepts and Laboratory Methods
- Yash Mangla, Priyanka Khanduri, Charu Khosla Gupta
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Reproductive Biology of Angiosperms: Concepts and Laboratory Methods will cater to the needs of undergraduate and graduate students pursuing core and elective courses in life sciences, botany, and plant sciences. The book is designed according to the syllabi followed in major Indian universities. It provides the latest and detailed description of structures and processes involved in reproduction in higher plants. The inclusion of colour photographs and illustrations will be an effective visual aid to help readers. Interesting and significant findings of the latest research taking place in the field of reproductive biology are also provided in boxes. At the end of each chapter, the methodology of hands-on exercises is presented for the implementation and practice of theoretical concepts.
Preface
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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Summary
The inception of interest in understanding mechanisms of plant reproduction is as old as inception of interest in biology. The seminal work and critical observations by Charles Darwin can be regarded as a foundation for establishing a wide interest in pollinators and reproductive biology of angiosperms as a formal subject. In the last few decades, systematic field investigations, advancement of microscopy tools, and molecular techniques have taken the reproductive biology of angiosperms to a new zenith. The scope of the subject is no longer limited to just studying embryo-endosperm development and taxonomic studies but is extended to study the effect of climate change, evolution, conservation of threatened taxa, raising commercial plantations and orchards, pollinator management, seed development, population biology, phyto-geography, and much more. The reproductive biological studies are also closely linked with the understanding of, physiology, genetics and epigenetics of plants.
For a thorough understanding of the subject, a textbook summarizing the basic concepts of plant reproduction integrated with current research, is the need of the hour for both students and instructors. The aim of the present book is to provide a comprehensive account of basic concepts and recent developments in the field of reproductive biology of flowering plants with essential practical exercises. The book extensively covers all the topics from structure of a flower to seed dispersal and presents the concepts with accompanying color photographs and illustrations wherever necessary, to enhance the level of a student's perception. The new, advanced and interesting information is also provided in a box format in each chapter to reinforce learning. An elaborate glossary and questions are provided with each chapter for quick revision and concept enhancement. Boxes summarizing differences between two terms/concepts which students otherwise usually find difficult to comprehend have also been furnished in the book. This book is a blend of theoretical concepts and details of hands-on exercises in the field and laboratory. Methods for field observations, sample observation tables, and suggestions for plant materials to be used for classroom studies/demonstrations pertaining to each concept have also been provided. In addition, the observation sections under practicals are supplemented with the photographs.
Chapter 11 - Polyembryony and Apomixis
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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Summary
Introduction
Reproduction is the ultimate goal of every life-form on earth. Accordingly, flowering plants have evolved diverse and versatile strategies to ensure their reproductive success. Broadly, reproduction in higher plants can be divided into two types: sexual and asexual reproduction. Sexual reproduction in vascular plants is complex wherein, multicellular haploid and diploid generations alternate. The diploid sporophyte undergoes meiosis to produce haploid gametes which undergo fusion or syngamy to give rise to seed, the next sporophytic generation. On the other hand, asexual reproduction in plants occurs when a plant produces offspring without meiosis and syngamy. New individuals produced through asexual reproduction are genetically identical to the mother plant. Both sexual and asexual reproduction, have distinct advantages for natural plant populations. Sexual reproduction introduces genetic variability in a population and thus increases the adaptability of species to changing environments. By contrast, asexual reproduction eliminates the cost and the complexity associated with biparental sexual reproduction, and also fixes the genotype of mother plant as offsprings produced are clonal.
When vascular plants reproduce asexually, new individuals may be produced from somatic cells or somatic structures (vegetative reproduction) or through seeds that are produced without fertilization (apomixis or agamospermy). Vegetative reproduction occurs through propagules like bulbils, suckers, and tubers, which are generated from vegetative parts of a plant. Apomixis (away from mixing), is the formation of an embryo and seed from an unreduced gametophyte or sporophyte. Thus, apomixis leads to the formation of a seed without the processes of meiosis (apomeiosis), and fertilization (nuclear fusion). The discovery of apomixis in higher plants is attributed to the observation of a solitary female plant of an Australian species Alchornea ilicifolia (syn. Caelebogyne ilicifolia) by Smith (1841). This female tree would constantly form seeds at the Royal Botanic Gardens in England without any pollen donor around. The term apomixis was introduced by Winkler (1908) to denote “substitution of sexual reproduction by an asexual reproduction process without nuclear and cell fusion”. This led to the use of term apomixis to describe all forms of asexual reproduction in plants (including vegetative reproduction), but this generalization is no longer accepted.
Chapter 13 - Plant Germline Transformation
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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Summary
Introduction
Plant breeding is an age-old practice of genetic improvement of plants so that they become more useful to humans. It involves combining selected parental plants to obtain the next generation with an improved genetic potential of disease-resistance, stress-tolerance or better yields. The process includes manual crosses or controlled pollination followed by an artificial selection of progeny. Plant breeding, for its enormous benefits to mankind is widely practiced. However, the process is labor intensive and it takes many years for the integration of the required gene and development of a desired progeny. Another major limitation of conventional breeding is that the gene transfer can be achieved only in genetically related species/genera. Even in conspecific plants, the incompatibility of crosses becomes a major barrier. In the last few decades, these limitations of plant breeding have largely been overcome by modern plant genetic engineering/transformation techniques which allow insertion of foreign genes from one organism into another organism. Introduction of foreign genes is relatively less time consuming and does not require recipient and donor organisms to be genetically related to each other. For example, Bt-cotton which was created by genetically altering the cotton genome using genes from the soil bacterium Bacillus thuringiensis (Pursell & Perlak 2004).
The technique of developing transgenic plants has become an integral part of crop improvement programs across the world (Eapen 2011). Although, transgenic approach for crop improvement has several advantages over conventional breeding, it suffers from some drawbacks as well. The transformation techniques used are expensive, genotype-dependent and involve time taking procedures such as in vitro culture and regeneration of explants. Therefore, alternate methods of quick and easy transformation are being developed. One such approach which offers several advantages is transformation of germ cells of plants instead of somatic cells/tissues. Germ cells of plants include sperm cells in the pollen grain (male germ cell) and egg cell in the female gametophyte (female germ cell). The process of introduction of desired genes into germ cells is known as Germline Transformation. This method of transformation has the potential to produce genetically modified plants within less time as it bypasses some tedious steps of in vitro regeneration. The genetically transformed germ cells can also be used directly for fertilization to recover transgenics without tissue culture and the procedure is known as in planta transformation.
Contents
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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Chapter 10 - Zygotic Embryogenesis
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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Summary
Introduction
Zygote is a unique cell from which the life cycle of angiosperms begins. It is a product of fertilization between the sperm and the egg cell. Zygote undergoes organized divisions and cell-specifications to give rise to an embryo, a young sporophyte. The process of development of an embryo from a zygote is known as embryogenesis. A particular pattern, form, and polarity exists during the development of embryo which is the outcome of several cellular, molecular and genetic mechanisms. These programmed changes enable the embryo to form the future sporophyte as a unique entity. The present chapter deals with the embryogenesis in angiosperms, encompassing embryo patterning, genetics and physiology involved. Embryos can also develop from somatic cells under in vitro conditions and resulting embryos are known as somatic embryos and the process as somatic embryogenesis. In this chapter, the term embryogenesis will be used for only zygotic embryogenesis.
Structure of the Embryo
Typical mature embryos in both monocots and dicots are similar in the basic design as they share similar embryogeny, at least up to a particular stage (i.e., octant, as will be discussed in Section 10.4); after which ontogenetic differences appear between them. A typical dicot embryo comprises of an embryonal axis with two cotyledons attached to it laterally. The part of embryonal axis above the level of cotyledons is known as the epicotyl; which terminates into the embryonic shoot (also called plumule). The part of embryonal axis below the level of cotyledons is known as the hypocotyl; which gives rise to an embryonic root (also called radicle) (Fig. 10.1 A). A typical monocot embryo differs from a dicot embryo in having only one cotyledon attached to the embryonal axis (Fig. 10.1 B).
The embryos in cereals like Zea mays, Triticum sp., Oryza sp. need special mention because of additional embryonic organs associated with them namely, scutellum, coleoptile and coleorhiza (Fig. 10.1 C). Scutellum is thought to be a seed leaf or a single massive cotyledon in cereals, which fully covers the embryo. Lateral to the scutellum, a short embryonal axis is attached which is divided into an epicotyl (above the level of scutellum) and a hypocotyl (below the level of scutellum). The epicotyl comprises of several young leaves covered by a sheath called coleoptile.
Chapter 7 - Pollen–Pistil Interactions and Fertilization
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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Summary
Introduction
In the previous chapter, we learnt that pollination results in transferring of pollen grains from the anther to the stigma of a flower. Such a transfer or landing of pollen grains initiates a series of events that involve a continuous exchange of signals between the haploid pollen and the diploid maternal tissue of the pistil. These events and interactions which include pollen selection or rejection, pollen hydration, pollen tube growth, its nourishment and entry into ovules and embryo sac are recognized as the pollen–pistil interactions, (Herrero & Hormaza 1996; Shivanna 2003; Lora et al. 2016). The pollen–pistil interactions result in screening and selection of conspecific/homospecific pollen (of same species) from the heterospecific pollen (pollen of other species) ensuring fertilization only between the conspecific male and female gametes.
In a pistil, while stigma is the landing platform and recipient of the pollen, the style is the conduit for the transfer of non-motile male gametes to the embryo sac seated in the ovules with the help of pollen tubes. The process of delivery of non-motile sperm to the egg via a pollen tube is known as Siphonogamy. It is regarded as a key innovation in the course of evolution of angiosperms that has allowed flowering plants to carry out sexual reproduction on land without the need for water. Flowering plants have an elaborate screening process for selecting the right pollen and the pollen tubes. This system works at different levels in the pistil. The selection of pollen starts at the stigma itself which allows only the compatible pollen grains to germinate while the incompatible ones are rejected. The selected pollen grains then germinate and put forth pollen tubes which grow in the style, where again a competition takes place to select the best mate. The pollen tubes travel at a rate specific to each species to ultimately reach the ovule. A study by Williams (2008) covering about 130 seed plant families and 717 taxa suggested that the time interval between pollination and fertilization ranges between 15 minutes to >12 months in angiosperms. This time interval is referred to as the fertilization interval.
Chapter 2 - The Flower
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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Summary
Introduction
Angiosperms possess a vast diversity of flowers which serve various purposes for the different groups of living beings, including humans. Due to their color, fragrance, and beauty flowers have always occupied a special place in human lives. Flowers are considered sacred across most cultures and have inspired much artistic expression. Apart from their aesthetic value, flowers possess myriad medicinal properties that further enhance their value to humans. Describing from a botanist's perspective though, the flower is a unit of reproduction in angiosperms. A flower may be defined as a modified determinate shoot system with four distinct whorls, viz. calyx, corolla, androecium and gynoecium arranged on a receptacle. Outer whorls, calyx and corolla are leaf-like structures which are not directly involved in reproduction. The two inner whorls, the androecium and the gynoecium harbor the reproductive organs of the flower and are the ones involved in reproduction. Flowering plants exhibit enormous diversity in size, shape, color, symmetry and the other morphological features (Fig. 2.1). This diversity in floral forms plays a huge role in ensuring pollinator services by different groups of pollinators. The diverse forms of flowers are accompanied by an array of mating strategies and sexual systems in angiosperms.
The timing of flowering in plants is critical for their reproductive success as both late and premature flowering can limit proper seed development. Plants also attempt to realize their reproductive potential by synchronizing their flowering to match pollinator availability. Floral induction is promoted by distinct environmental cues such as photo-period, vernalization and endogenous regulators like phytohormones. These signalling cues are perceived in the leaves and the shoot apical meristem (SAM) for induction of flowering. Plants use genetic machinery to control all events starting from induction of flower to development of different whorls. Research in the last few decades has identified numerous genes which are involved in floral induction, floral meristem formation, and floral organ development. Genes which control floral organ development are called floral organ identity genes. These genes belong to the MADS box gene family and are also known as homeotic genes. The functioning of these genes is explained by the ABCDE model of flower development. This chapter gives an outline of the organization of a flower, sexual system seen in angiosperms and a summary of the components that play important role in the floral induction and floral organ development.
Chapter 5 - The Ovule and Female Gametophyte
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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Summary
Introduction
Sexual reproduction involves delivery of sperm cells, via the pollen tube, to the egg cell present in the embryo sac, where fertilization occurs and the new sporophyte is formed (Dumas & Mogensen 1993). While the formation of male gametophyte (pollen grains) takes place within the anther, the female gametophyte (embryo sac or megagametophyte) develops within the ovule. Thus, the ovule can be defined as a specialized sporophytic structure within which development of female gametophyte or mega-gametophyte takes place. Ovule is the site for delimitation of megasporocyte, production of a functional megaspore (megasporogenesis) and eventually formation of embryo sac (megagametogenesis). The embryo sac harbors the female gamete or the egg, which subsequently gets fertilized by the male gamete to form an embryo. In angiosperms, apart from the female gametophyte and egg cell development, important reproductive events such as pollen tube attraction and guidance, double fertilization, and embryo and endosperm development all occur within the ovule. The ultimate result of all these events is the formation of seed and therefore, the ovule is also considered the developmental precursor or progenitor of the seed.
Among angiosperms, different modes of female gametophyte ontogeny are seen, leading to different types of female gametophytes. The cells of female gametophyte are very peculiar in their ultrastructure and with the help of electron microscopy, great details of these cells are known. In various species, besides the typical parts of ovule, there are many specialized structures associated with the ovules which aid in pollen tube guidance and facilitate fertilization. All these aspects of structure and development of the angiosperm ovule, female gametophyte and their types have been discussed in the present chapter. The chapter also includes exceptions to these developmental patterns and details of extra ovular structures.
Basic Structure of Ovule
In general, ovules among angiosperms are fundamentally similar in their basic structure, consisting of three major tissues: a nucellus, protective coat(s) or integument(s), and a funiculus. Besides these, a typical ovule also consists of a micropyle, a chalaza and its vascular supply (Fig. 5.1). In angiosperms, the ovules remain enclosed in the ovary, and a stalk like structure through which ovules remain attached to the ovary wall or placenta is known as the funiculus.
Chapter 3 - Brief Historical Account on Transformation of Classical Embryology to Integrated Reproductive Biology
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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- 05 January 2024
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- 05 January 2023, pp 25-39
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Summary
The inception of interest in plant reproduction is as old as the inception of interest in biology. The science of sexual plant reproduction is more than 400 years old. During all these years there has been an accumulation of information which has greatly enriched our understanding of plant reproduction. Our current knowledge of plant reproduction is a result of continuous efforts of scientists world-wide that transformed it from an observational investigation to an important field of experimental science. This chapter provides a summary of important mile stones in the history of reproductive biology of flowering plants. To maintain conciseness, only significant contributors across the world including India have been mentioned in the chapter without undermining the importance of others whose ideas and concepts have shaped the science of plant reproduction today.
Early Discoveries
The long and venerable history of studies in plant reproduction dates back to seventeenth century with the ideas of European naturalists Rudolf Jacob Camerarius in Germany and Nehemiah Grew in England. Though, Grew first proposed the idea of sexual processes occurring in plants for generation of seeds, to Camerarius must go the principal credit for experimentally establishing the existence of plant sexuality. N. Grew, in an address to the Royal Society of London in 1676, had expressed the view that the stamens are the male organs of a flower and the pollen act as vegetable sperm. He is credited for documenting stamens as the male sex organ of plants in his book The Anatomy of Plants (1682). The experiments of Camemarius were primarily based on the removal of stamens and styles along with isolation of female plants in dioecious species. He provided evidence for the inevitability of both sex organs in seed formation. His publications On the Sex of Plants (1694) and Botanical Works (1697); are landmarks in the history of botany.
In the history of plant reproduction, Adam Zalužanský is a little-known botanist. He was a professor at the University of Prague and his book Methodi herbariae libri tres was published in 1592 and 1604. This book includes a chapter De sexu plantarum in which, almost a century before the work of Camerarius, sexuality of plants was suggested.
Chapter 9 - Endosperm
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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- 05 January 2023, pp 289-323
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Introduction
Strasburger's work in Monotropa identified the embryo as the product of fertilization of egg cell by the one of the male gametes. The mystery around the fate of the second male gamete discharged by the pollen tube was resolved through the discovery of double fertilization by Nawaschin (1898). It is a common knowledge now, that one of the male gametes undergoes fusion with the nucleus of the egg cell and the other fuses with the two polar nuclei of the central cell. The fusion of three nuclei in the latter is known as triple fusion which was also referred to as vegetative fertilization by Strasburger in 1900. Triple fusion results in a triploid nucleus known as Primary Endosperm Nucleus (PEN), which divides and forms the endosperm. Thus, double fertilization initiates development of embryo and endosperm. Discovery of triple fusion led to questions like what is endosperm and what role does it play? The term endosperm means ‘with-in the seed,’ i.e., a tissue that develops inside a seed. A plethora of studies have established that endosperm is the nutritive tissue for a growing embryo inside a seed. The two tissues are closely connected in their growth within a seed, reflecting the importance of the embryo-endosperm relationship. Recent investigations show that failure of endosperm formation leads to the abortion of the developing embryo which establishes that embryo development is regulated by endosperm.
Formation of the PEN is a well-organized event which is preceded by several ultrastructural changes in the central cell. The PEN follows different developmental pathways forming the basis for classifying the types of endosperm. The PEN and the endosperm cells are mostly triploid but the ploidy level may vary with the type of female gametophyte from which a central cell develops. In most angiosperm families, the endosperm is short-lived and the developing embryos consume the endosperm completely before germination. This leaves mature seeds without any endosperm and such type of seeds are known as non-endospermous or ex-albuminous seeds, e.g., Cucurbita, pea, and beans. In other angiosperms, endosperms act as a storage tissue and persist in mature seeds. Such seeds where endosperm is present at maturity are known as endospermous or albuminous seeds, e.g., cereals, coconut, and castor bean.
Chapter 4 - The Anther and Male Gametophyte
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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- 05 January 2024
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- 05 January 2023, pp 40-118
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Introduction
Life cycle of an angiosperm is characterized by alternation of generation between a diploid sporophyte and a haploid gametophyte. Unlike lower plants, gametophytic generation in angiosperms is much shorter and dependent on sporophytic generation for its development. Gametophyte develops from the cells of a sporophyte in preparation for reproduction. The gametophytic cells undergo meiotic division and produce haploid gametes within the specialized structures of a flower. While the male gametophyte develops within the anther, the female gametophyte develops within the ovule. Pollen grain is the male gametophyte in flowering plants and contains the two male gametes (also called the sperm cells). Pollen grains are also involved in the formation of pollen tubes to facilitate the movement of sperm cells for fertilization with female gametes.
The male reproductive organ in flowering plants is the stamen. It consists of two morphologically distinct parts, the anther and the filament (Fig. 4.1 A). Filament is an entirely sporophytic structure which anchors the stamen to the flower. It also contains vascular tissue for transporting water and nutrients. The anther on the other hand contains both sporophytic and gametophytic tissues that are responsible for producing and releasing pollen grains. Anther development is a perfectly timed and orchestrated event which follows different pathways in different groups of angiosperms. Development of pollen grains (male gametophyte) takes place within the anther and is divided into two phases. It begins with the meiosis in the microspore mother cells to produce four haploid microspores, each of which later develops into a pollen grain and the process is called as microsporogenesis. This is followed by a second phase of pollen development where the formation of two sperm cells takes place and the process is known as microgametogenesis.
Pollen development includes participation of various sporophytic cells of the anther and the associated molecules. Pollen grains vary immensely in size, shape and surface characteristics among different plant species. At maturity, the pollen grains are surrounded by an elaborate cell wall which consists of a thin inner wall known as the intine, and an outer thicker wall called the exine. The shape and the external features of the exine are highly variable, and often used to distinguish pollen grains produced by different species.
Color Plates
- Yash Mangla, University of Delhi, Priyanka Khanduri, University of Calcutta, Charu Khosla Gupta, University of Delhi
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- Reproductive Biology of Angiosperms
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- 05 January 2024
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- 05 January 2023, pp 473-504
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