After fertilization, the egg is transformed into the zygote, which embarks upon one of the most critical periods in its development as it is partitioned into cells that ultimately make up the body of the embryo. What makes the zygote so unique a cell is that it is the product of fusion of two gametes, the sperm contributing the paternal genome and the egg providing the maternal counterpart. The point in time when the egg and sperm fuse together to be woven into a new sporophyte marks the beginning of the ontogeny of the species. The phase of ontogeny concerned with progressive division of the zygote to form the embryo is known as embryogenesis, or as zygotic embryogenesis (to avoid semantic confusion with somatic embryogenesis and pollen embryogenesis). Classical histological analyses of embryo development in a large number of plants have generated an invaluable guide to the division patterns of the zygote and its immediate derivatives, and contemporary studies are beginning to provide some much needed insight into the associated cellular and molecular changes.

The history of the study of the development and physiology of embryos can be considered to have evolved in three distinct phases. Much of the early work was focused on the first few rounds of division of the zygote and on the subsequent morphogenesis of cells to give rise to the embryo.

This book is derived from my long-standing interest in the reproductive biology of vascular plants, in particular of the ferns and flowering plants. During the initial planning stages of this venture, the book was intended as a revision of my Experimental Embryogenesis in Vascular Plants (Academic Press) published in 1976, with the specific aim of describing how our present ideas in molecular and genetic biology apply to embryo development in vascular plants. After I wrote two or three chapters on angiosperm embryogenesis, I began to view the project essentially as a revision of my 1986 book, Embryogenesis in Angiosperms: A Developmental and Experimental Study (Cambridge University Press), and aborted the idea of covering embryogenesis of ferns and gymnosperms. Reflecting the perspective of current research, it began to dawn on me that molecular and genetic principles underlying embryo development in angiosperms have much in common with principles regulating the whole gamut of reproductive processes in flowering plants. This periodic reshaping and refining of the various drafts during the past five years has resulted in the present volume providing a synthetic review of molecular and cellular aspects of the familiar sequence in the reproductive biology of flowering plants, beginning with the flower and terminating with the embryo. It is thus a wholly new book and has very little to share with its predecessors.

My objective in writing this book is to explain the progress achieved toward a molecular understanding of the reproductive processes in angiosperms with particular emphasis on embryology.

As discussed in Chapter 14, the embryo responds to changing genetic and physiological pressures by initiating a series of developmental and molecular changes. These are reflected in the abrupt switch from a period of cell divisions to one of cell expansion, desiccation of cells, changes in growth hormone levels, and synthesis and accumulation of defense-related proteins and storage compounds. Storage materials are constituted primarily of an acervate complex of proteins known as storage proteins, in addition to starch and lipids. Like the endosperm storage products discussed in Chapter 12, embryo storage compounds play an important role in seedling survival by providing the source of carbon and nitrogen skeletons to the embryo during seed germination.

Many aspects of the synthesis of storage proteins during embryogenesis have been investigated using embryos of various agronomically important plants, such as bean, pea, soybean, and rapeseed. What these embryos have in common is that they represent cases in which cellular metabolism of the cotyledons is deflected from a programmed synthesis of housekeeping proteins to one concerned with the production of storage proteins. In recent years, considerable progress has been made toward understanding the regulation of the expression of storage protein genes in plant embryos.

In the reproductive biology of angiosperms, the microspore and pollen grain represent paradigms for studies on cell differentiation. As described in Chapter 4, an important reason for this is that by a program of gene expression regulated in space and time, the unicellular microspore matures into the pollen grain and embarks on a pathway leading to terminal differentiation by the production of two cells with divergent developmental potential. A second reason is that the induction of embryogenic divisions in microspores or pollen grains of certain plants by tissue culture methods makes it possible to gain insight into mechanisms involved in the deflection of a typical gametophytic program into an atypical sporophytic pathway. This chapter will focus on this alternative developmental pathway of microspores and pollen grains, which gives rise to embryoids and plantlets with the haploid or gametic number of chromosomes. This phenomenon is known as androgenesis, haploid embryogenesis, or pollen embryogenesis, but the last-mentioned term is preferred for use in this book. Irrespective of how the morphological and cytological changes converge to cause gametophytic or sporophytic types of growth in pollen grains, it seems certain that the same genetic blueprint is utilized to meet the informational demands of these transformation episodes.

This chapter will examine the cell and molecular biology of the embryogenic development of somatic cells of angiosperms and gymnosperms, or the phenomenon of somatic embryogenesis. Embryogenic development of somatic cells can be contrasted to the rigorously programmed development of the embryo from the zygote (zygotic embryogenesis) insofar as virtually any somatic cell of the plant body can, under certain experimental conditions, behave like a zygote and faithfully replay a developmental program leading to the production of embryolike structures while remaining innocent of sex. Thus, somatic embryogenesis provides the most clear-cut demonstration of the dictum that all plant cells except those that have undergone irreversible differentiation are totipotent and retain the developmental potential to proliferate into an adult plant. Compared to the limited number of embryos arising from gametic fusion and the difficulty of extracting them from the confines of the ovule, the enormous number of somatic cells potentially capable of embryogenic development ensures the availability of an equally staggering number of embryolike structures by simple experimental manipulations. Despite the fact that zygotic embryos and embryolike structures formed from somatic cells are identical in appearance and possess the same morphogenetic potential, to emphasize the divergent pathways through which they have evolved, the term “embryoid” is generally used to refer to the latter.

Following pollination, the pollen grain absorbs water from the stigmatic exudate and germinates to produce a pollen tube. In the life cycle of flowering plants, the pollen tube serves as a transient structural link between the end of the male gametophytic phase and the beginning of the sporophytic phase. As is well known, the pollen tube elongates in a seemingly endless extension of itself as it travels through the style to the ovary, seeks out the ovule, grows into the embryo sac, and discharges the baggage of sperm. The proportional increase in wall area that occurs during elongation of the pollen tube is by tip growth. Much of the work on the germination of pollen grains on the stigma and growth of pollen tubes in the style was considered in Chapter 7; however, this work does not tell much about the requirements for pollen germination and pollen tube growth and about the structure, growth physiology, and metabolism of pollen tubes. Indeed, with the insight gained from the knowledge of the complexity and diversity of the components of the stigmatic exudate in general, one can only wonder about the specific molecules that promote pollen germination and pollen tube growth. Therefore, it suffices to remark here that the emerging model of pollen germination on the stigma and of pollen tube growth in the style hardly does justice to the molecular mechanisms of germination and the dynamic processes of pollen tube growth.

The ability of a plant to achieve its full reproductive potential depends upon the completion of an uninterrupted cycle of sexual and asexual processes. However, there are certain instances in which physiological and genetic barriers converge to prevent completion of the component processes of sporogenesis, gametogenesis, fertilization, and embryogenesis, and thus thwart seed set. In Chapter 5 it was seen that in a wide range of plants, the arrest of normal pollen development results in male sterility. It is now well established that the molecular and cellular organization of the pollen grain and stigma provides effective recognition systems at the time of pollination for screening suitable gametes for fertilization; this theme permeated most of Chapter 7. This chapter considers the precise genetic control of cell recognition that operates in many plants and enables an individual flower to distinguish between self- and nonself-pollen grains once they land on the stigma and begin to germinate. It is now clear that the sporophytic tissues of the flower play leading roles in both the recognition and the rejection of male gametes that reinforce the outbreeding potential of the species. Although the practical importance of these phenomena has not been fully exploited, they are of great developmental and functional significance in the reproductive biology of angiosperms.

The account presented in the previous two chapters has established that the differentiation of the diploid microsporocyte proceeds in several discrete stages to produce a two- or three-celled haploid pollen grain with a complex structural and functional organization. Included in the progression of events in this multistep process are meiotic division and the evolution of cells endowed with different characteristics, form, and function. The existence of a rightly controlled series of cytological and biochemical changes associated with pollen development and maturation underlies a coordinated gene expression pattern, involving genes from both gametophytic and sporophytic generations. Although we do not yet fully understand how the genetic program operates during pollen ontogeny, we have some notion of its machinery and how the machinery might be used. This understanding has come from three independent approaches – genetic, molecular–biochemical, and gene cloning – that have been pursued from time to time during the past 50 years or so. At the genetic level, examination of the segregation of pollen characters and of spontaneous and induced mutations has indicated that a very large set of genes is expressed during male gametogenesis. Molecular–biochemical studies have included analyses of the accumulation and synthesis of nucleic acids and proteins during anther and pollen development. The main thrust of the research in recent years has been on the isolation and characterization of anther- and pollen-specific genes by differential screening of cDNA libraries from whole anthers and pollen grains.

The carpel is the gross morphological part of the flower concerned with female sporogenesis and gametogenesis and is the homologue of the stamen. In its simplest form, the carpel is a leaflike organ composed of the ovary, style, and stigma, although various kinds of fusions between carpels have produced a complex organ in the modern flower. As is well known, along its margins the ovary encloses ovules, which become seeds in the mature fruit. In its role of sheltering the progenitors of seeds, the carpel determines the extent of physical and chemical influences that regulate the transformation of ovules into seeds. Tissues constituting the wall of the carpel serve as supporting structures when the ovary becomes the fruit.

The ovule is functionally a megasporangium because it defines the structural unit of the carpel in which sporogenesis and subsequent differentiation of the female gamete, or the egg, take place. In the context of the reproductive biology of angiosperms, the functional unit of the plant that produces the egg for fusion with the sperm is the megagametophyte or the female gametophyte. The megagametophyte is a highly reduced group of cells dependent upon the sporophytic plant for its nurture and nutrition; it is generally equated with the embryo sac.

In the life cycle of plants, fertilization is invariably associated with sexual reproduction and therefore represents the genetic switch that initiates the diploid, or the sporophytic, phase of development. The history of research into fertilization in angiosperms may be said to have begun with the discovery of syngamy, or the actual fusion of the male and female gametes, by Strasburger (1884) and of double fertilization by Nawaschin (1898) and Guignard (1899). The latter two investigators called attention to the streamlining of the male gametophyte's economy by providing light microscopic evidence for the fusion of one sperm with the egg cell to form the diploid zygote and of the other sperm with the polar fusion nucleus of the central cell to generate the triploid endosperm. In spite of an abundance of studies on the development of the male and female gametophytes and on postfertilization events in angiosperms in the years following these discoveries, relatively few investigators have ventured into the area of fertilization research. This has led to the unenviable situation that, as a biological process, fertilization in angiosperms has remained as an unknown black box. The paucity of information on fertilization is undoubtedly related to the fact that events leading to the egg–sperm encounter take place in the privileged interior of the multicellular ovule, where, hidden from the eye, they are not prone to experimental assault and to the technical difficulties that have so far prevented the development of an exovulo in vitro fertilization system.

BOTANY AND DISTRIBUTION

Several sections have been described under the genus Dioscorea of family Dioscoreaceae. The main food yams have been grouped as follows:

Section Enantiophyllum

This is the largest section with respect to number of species and food importance (Degras 1993). Members may be further grouped in terms of geography as: Asian - Oceanian species, e.g. D. alata L. (water yam, greater yam, white yam), D. glabra Roxb., D. nummularia Lam., D. transversa Br.; Sino-Japanese species (or species complex), e.g. D. japonica Thumb, (igname de Chine, Chinese yam), D. opposita Thumb., and African species or species complex, e.g. D. cayenensis Lam. (yellow yam), D. rotundata Poir. (white Guinea yam, white yam).

Section Lasiophyton

D. pentaphylla L., D. hispida Dennsdest, D. dumetorum (Knuth) Pax (bitter yam)

Section Opsophyton

D. bulbifera L. (aerial yam)

Section Combilium

D. esculenta (Lour.) Burk. (Chinese yam, lesser yam)

Section Macrogynodium

D. trifida L. (cush-cush yam)

The many species of yams (Dioscorea sp.) have various unique or peculiar characteristics that distinguish them from each other. The principal food species have been described in a series of monographs (Martin 1974a, 1974b, 1976; Martin and Degras 1978a, 1978b; Martin and Sadik 1977). Generally the yam plant comprises a shoot portion made up of a vine with branches, leaves and sometimes bulbils in the axils of the leaves, fibrous roots and an underground storage organ, the tuber. The vine twines clockwise or anticlockwise depending on the species.