Cells are regarded as the fundamental unit if life. All cells come from previously existing cells. In other words, no cell exists on its own. In the organisation of life, cells are found at lowest end of the hierarchy. Group of specialised cells then develop to form tissues, group of tissues develop to form organs while organs interact to form a system. The events leading to the formation of the first cell is a very complex one and requires detail understanding of the biology of living organism.
In tracing how the first cell was formed, the understanding of the life cycle of flowering plants which involve a change from one generation to the other is important. Two types of generations are found in the life cycle in both lower and higher plants. However, the type that dominates the life cycle depends on the nature of the plant. These two generations are a diploid sporophyte phase and a haploid gametophyte phase, comprising what is called an alternation of generations. In lower plants, the gametophytic phase predominates although the extent of its domination varies among the different types of lower plant. In ferns, mosses, fungi, and algae, the gametophytic generation predominates while in higher plants, which are our focus in this course, the sporophytic generation is more dominant. We shall look at these two generations and features that differentiate the two. We will thereafter break the discussion on this topic to five different stages in order to fully understand how the first cell was formed and how it eventually develops to form tissues.
The lifecycle of angiosperms
The prominent bodies of angiosperm trees, shrubs, perennials, and annuals as well as those of gymnosperms, ferns, sphenophytes, and lycophytes are sporophytes, having developed from fertilized egg cells (zygotes). The gametes which fused to form the zygotes, however, were produced by gametophytes, very small plant bodies, parasitic on the sporophytes in seed plants, but somewhat larger and free-living in pteridophytes (except in heterosporous species in which gametophytes when mature remain, at least in part, within the walls of the spores from which they develop).
The sporophyte in pteridophytes is dominant, and although dependent initially for its nutrition on the gametophyte, soon becomes independent. The gametophyte is much reduced in size but is freeliving and either autotrophic or saprophytic. In seed plants, the sporophyte is also dominant and initially dependent on the gametophyte, but soon becomes independent. The gametophyte is greatly reduced, however, and parasitic on the sporophyte. In angiosperms it is exceptionally small, consisting in many taxa of only seven cells and eight nuclei, and can be observed only with a microscope.
The life cycle of a vascular plant can be summarized as follows. The sporophyte produces specialized cells called sporocytes that undergo meiosis producing haploid spores. The spores germinate to form the gametophytes in which gametes are produced. The gametes fuse to form a diploid zygote from which the embryo (young sporophyte) develops.
In most pteridophytes, spores of only one size are produced. Plants of this type are described as being homosporous. Each spore has the potential to develop into a gametophyte that produces both egg cells and sperms. In contrast, the sporophytes of a few pteridophytes and all seed plants produce spores of two sizes, with different potentials, called microspores and megaspores. These plants are, thus, heterosporous. Microspores develop into gametophytes that produce sperms, and megaspores develop into gametophytes that produce egg cells (Fig. 3.1).
Fig. 3.1 Representative sexual life cycle of an
The five developmental processes involved that leads to the development of first cell and tissues are:
- Fertilization and
This is defined as the genesis of spore formation. Sporogenesis are of two types, the microsporogenesis and megasporogenesis. The microsporogenesis describes the genesis of male spore formation while megasporogenesis describes genesis of female spore formation.
In microsporogenesis, The general life cycle diagram described earlier includes a brief outline of pollen developmental stages as part of the life cycle. A diagram that is more specific excludes the surrounding anther tissue to show how a sporogenous cell can progress to a mature pollen grain. The differences that Figure 3.2 shows between dicots and monocots are generally true, but there are exceptions. The differences between pollen shed with two cells vs. pollen shed with three cells are not differences between dicots and monocots, but instead seem to be correlated with the evolutionary advancement of a family, or even with certain groups within a family. The numbers 1–8 in Figure 3.2 mark steps in the progression from diploid sporogenous cells to haploid mature pollen:
- Sporogenous cells proliferate by mitosis to a certain final number, and then each cell secretes an isolating callose sheath around itself.
- The microspore mother cells (mmc) undergo meiosis, and most monocots form a cell wall after each meiotic division, whereas most dicots do not form any internal walls yet.
- Dicots after meiosis have a tetrad of haploid microspore nuclei in a common cytoplasm, which is a coenocyte (Greek, meaning “common cell”) before “pinching off” four microspores by a furrowing process.
- Callose dissolves, releasing microspores into the fluid environment of the pollen sac. 5. Microspores enlarge, become vacuolate (i.e. deplete their food reserves and absorb water), start forming the exine, and then germinate internally to become bi-celled pollen
6. The vacuolate pollen now has a large vegetative cell and a small generative cell. 7. The generative cell migrates within the vegetative cell, food reserves begin to accumulate, and the pollen may be shed at this two-celled stage.
8. In some groups the generative cell divides mitotically and produces two sperm cells (becomes tri-celled) before the vegetative cell completes its engorgement with food reserves
Anther and pollen development
In the angiosperms an important embryological character,one often treated as a standard morphological character, is the number of microsporangia per anther Microsporangia are typically tubular in shape and occur in pairs, which coalesce during development by the breakdown of the cell layers between them Each pair of microsporangia is termed a theca. The great majority of angiosperm species have anthers composed of two thecae, termed bithecal or tetrasporangiate (Figure 3.3A), which is the ancestral condition However, some angiosperm taxa, such as the
Malvaceae, have a derived anther type with only one theca, termed monothecal or bisporangiate
Fig. 3.2 General diagram of pollen development in dicots and monocots;
stages 1–8 show sporogenous cell to pollen grain angiosperm.
Fig. 3.3 Anther types in the angiosperms A. Dithecal B. Monothecal
Anther Wall Development
A cross section of an anther reveals a division between the internal microsporangium, the cells of which undergo meiosis, and an outer anther wall The development of the anther wall has provided some useful embryological features A mature anther wall consists of few to several layers of cells The outermost cell layer (just inside the epidermis) is termed the endothecium, which typically consists of enlarged cells with secondary wall thickenings functioning in anther dehiscence The secondary wall thickenings function by providing tensile force that pulls back the anther walls from the line or region of dehiscence The innermost cell layer is termed the tapetum, which consists of metabolically active cells that function in the development of pollen grains Additional wall layers, termed middle layers, may occur between the endothecium and tapetum Both the total number of wall layers and their developmental origin define various anther wall types Early in development an anther contains two layers of cells, an outer epidermis and an inner layer of primary parietal cells Cells of the primary parietal layer divide tangentially (parallel to the outer surface) to give rise to two layers of cells, secondary parietal cells.
Another embryological character concerns the development of the tapetum, with two basic types defined (Figure 3.4). In some angiosperms the tapetum remains intact with no breakdown of cell walls This tapetal type is called secretory (or glandular) because of the implication that compounds are secreted into the locule of the anther that function in pollen development In other angiosperm taxa the tapetal cell walls break down, with release of the cytoplasm of the tapetal cells into the locule This latter tapetal type is called amoeboid (plasmodial or periplasmodial) because the cytoplasmic contents surround developing pollen grains like an amoeba surrounds food Subtypes of the secretory and amoeboid tapetal types have been proposed by some, based on fine developmental differences.
Fig. 3.4 Tapetum types
A final embryological character dealing with the anther wall is endothecial anatomy. Two basic types of endothecial cells have been defined based on the structure of the secondary wall thickenings A girdling endothecium is one in which the secondary wall thickenings form rings with cross bridges between them (Figure 3.5) A spiral endothecium is one in which the secondary wall thickenings are spiral or helical in shape.
Fig. 3.5. A girdling anther endothecium type
Symbols: en =endothecium; ep=epidermis;
gtc=glandular tapetal cell; mi=microspore; ml=middle
Development of microspores from microsporocytes is termed microsporogenesis There are two basic types of microsporogenesis as determined by the timing of cytokinesis, which is the formation of a plasma membrane and cell wall that divides one cell into two (Figure 3.6A). If cytokinesis occurs after meiosis I, then microsporogenesis is successive (Figure 3.6B) Successive microsporogenesis results in two cells after meiosis I and four cells after meiosis II If cytokinesis does not occur until after meiosis II, then microsporogenesis is simultaneous (Figure 3.6C) Simultaneous microsporogenesis results in cell formation only after meiosis II.
Fig. 3.6. Microsporogenesis. Diagram showing two major
types, simultaneous and successive
The formation of megaspores in the ovule has parallels to microspore production in the pollen sac because meiosis occurs only in a special cell (megaspore mother cell), which subdivides itself into four megaspores, each with a nucleus containing half the number of chromosomes that were present in the parent cell. The stages of meiosis in megasporogenesis reportedly progress similarly to meiosis during microsporogenesis (Bennett, 1977).
The typical situation in an ovule is that a single cell, which could be considered a 1-celled archesporium, enlarges and becomes the megaspore mother cell, and then undergoes meiosis. Normal meiosis produces four cells, or at least four nuclei, so a mechanism must exist to reduce the four meiotic products down to one, in order to end up with a single megaspore. Because it and the female gametophye it will become after internal germination are destined to remain in the ovule to nurture the embryo, one is as many as a normal ovule can support. More than a century of observations on thousands of species have demonstrated three possible ways that plants handle the problem of going from four to one. These are the three types of megasporogenesis, which are shown diagrammatically in Figure 3.7. The monosporic type is most common, in which meiotic cytokinesis is successive, producing four megaspores. Three of these megaspores degenerate, and one, which is almost always the deepest (chalazalmost) one, lives on to become the functional megaspore. The bisporic type differs because one of the two cells formed by cytokinesis of the first meiotic division degenerates; after the second meiotic division no cytokinesis occurs,
Fig. 3.7. Three pathways of megasporogenesis. A. Monosporic, producing a uninucleate functional megaspore after three others degenerate. B. Bisporic, with binucleate dyad forming functional megaspore when second dyad degenerates after first meiosis and no cytokinesis occurs after second meiosis. C. Tetrasporic, producing a tetranucleate functional megaspore after meiosis when no cytokinesis or degeneration occurs.
and the resulting functional megaspore is binucleate; the term bisporic refers to the two nuclei that would have been in separate spores if a wall had formed. Meiosis in the tetrasporic type lacks cytokinesis and all four haploid nuclei remain within the 4-nucleate megaspore; tetrasporic refers to the nuclei that would have been in four separate spores if cytokinesis occurred after each meiotic division. The bisporic and tetrasporic pathways to megaspore formation are not common, but asexamples one can point to the onion (bisporic) and the cultivated lilies and their relatives (tetrasporic).
The genesis of gamete development.
Development of pollen grains (male gametophytes) from microspores is called microgametogenesis, technically beginning with the first mitotic division of the single microspore nucleus. Microspores develop into pollen grains which upon germination develop into male gametophytes, commonly called microgametophytes. One embryological character concerning microgametogenesis is the number of nuclei present in the pollen grain at the time of anthesis, or flower maturation (Figure 3.8). Most angiosperms have pollen grains that are binucleate (Figure 3.8), containing one tube cell/nucleus and one generative cell/nucleus. The generative cell divides to form two sperm cells each comprising a nucleus and some surrounding cytoplasm only after pollen tube formation. In many angiosperm taxa, however, the pollen at anthesis is trinucleate (Figure 3.8), caused by division of the generative cell prior to pollen release
It was long assumed that the two sperm cells were essentially identical and that it was a matter of chance which one fused with the egg cell. Recent research has shown, however, that the sperm cells in some taxa differ in size and content of cytoplasmic organelles such as
plastids and mitochondria, and that preferential
|fertilization may occur in species in which the sperm cells|
|are different (Russell, 1984, 1985; Knox et al., 1993). Sperm cells of some other species, for example, Nicotiana tabacum (tobacco), are approximately the same size and have similar distributions of cytoplasmic organelles. Consequently, they are characterized as being isomorphic (Yu et al., 1992). Prior to fertilization||Fig 3.8 .Microgametogenesis Pollen nucleus number at anthesis|
Meiosis also occurs in megasporocytes, one of which is contained in each of the developing ovules (Fig. 3.9). The megasporocyte is enclosed in a vegetative tissue, the nucellus, bounded by one or two ovular integuments. Most commonly, in angiosperms, as in conifers, meiosis results in the formation of a linear tetrad of megaspores, oriented in a plane parallel to the long axis of the ovule (Fig. 3.9a,b). Three of these spores degenerate and the remaining megaspore (Fig. 3.9c) develops into the female or megagametophyte, called in angiosperms the embryo sac. Three mitotic divisions within this cell result in eight nuclei (Fig. 3.9d–f). As these nuclear divisions are occurring, the original cell expands and elongates, and four of the nuclei migrate to each end of the developing embryo sac. At the micropylar end (the end of the embryo adjacent to the micropyle in the ovule) (Fig. 3.9g) one nucleus becomes at least partially enclosed by a cell wall and functions as the egg cell, while two others differentiate into synergids. These cells, in contact with the egg cell, are distinctive in possessing a filiform apparatus (Fig. 3.9), a much-branched system of haustoria that extends from the synergid walls into the surrounding cytoplasm (Jensen, 1965; Jensen and Fisher, 1968).
Fig. 3.9. Diagrams representing stages in the development of the megagametophyte in Lilium. (a, b) Meiosis results in the formation of a linear tetrad of megaspores, three of which abort leaving the single functional megaspore (c). (d–f) A series of three mitotic divisions results in eight nuclei. (g) Three nuclei migrate to each end and two migrate to the center of the developing gametophyte. The three at the micropylar end develop into the egg cell and two synergids. Those at the opposite end (the chalazal
end) differentiate as antipodal cells, and the central pair function as polar nuclei.
The function of the filiform apparatus is unclear, but it may be a transfer structure, similar in function to the highly branched wall ingrowths of transfer cells. The egg cell and the two synergids may be homologous with the archegonia in the megagametophytes of gymnosperms. Three of the nuclei at the other end of the developing embryo sac, the chalazal end, differentiate, with associated cytoplasm, as antipodal cells. The remaining (fourth) nucleus at each end migrates to the center of the developing embryo sac. These two nuclei, the polar nuclei, and the cytoplasm remaining after wall formation around the antipodal cells, the synergids, and the egg cell, are contained within the central cell. The three antipodal cells, plus the central cell with its polar nuclei, two synergids and the egg cell comprise the mature female gametophyte (Fig.3.9g). In megagametogenesis of the type just described the embryo sac develops from a single spore and, thus, is referred to as monosporic. There are several variations in embryo sac development, the next best known probably being the tetrasporic type, often designated as the Fritillaria type in which, following meiosis, there is no degeneration of megaspores, all four becoming incorporated into the embryo sac.
Pollination is the transfer of pollen grains from the anther of the male flower to the stigma of the same flower or another flower of the same species. In other words two types of pollination exists; the self-pollination and the cross-pollination. Self-pollination is when the pollen of a plant fertilise the stigma of one of the flowers on the same plant It is cross-pollination when the pollen o a plant is transferred to the stigma of another plant of the same species. In both types, plants of the same species are involved.
Following pollination, and upon germination of pollen grains on the stigma (Fig. 18.14a–c), pollen tubes will grow down through the style into the locules of the carpels and through the ovular micropyles. As a pollen tube approaches the embryo sac, one of the synergids begins to deteriorate, in preparation for entrance of the tip of the pollen tube. Upon entry, the two sperm cells are released into the synergid, the plasma membrane of which has disintegrated (Jensen and Fisher, 1968). Upon entry into the embryo sac (female gametophyte), the leading sperm cells of the pair (Fig. 18.10), associated with the tube nucleus, will fuse preferentially with the polar nuclei forming the triploid endosperm nucleus. The trailing sperm cell fuses preferentially with the egg cell (Zhang and Russell, 1999), forming the diploid zygote, completing the process of double fertilization. At this stage the ovule consists of the embryo sac, enclosed by the nucellus, and one or two integuments. It is attached to the wall of the carpel by a stalk, the funiculus. At least one primary vascular bundle, which differentiated from provascular tissue prior to formation of endosperm, serves the ovule. This strand, which enters the funiculus from vasculature in the carpel wall (or sometimes several strands and their branches), extends to the chalazal end of the ovule through the outer integument
Embryogenesis refers to the genesis (the beginning) and development of the embryo within the seed. Embryogenesis initiates plant development. It transforms a single-celled zygote into a multicellular, microscopic, embryonic plant. Embryogenesis occurs within the embryo sac of the ovule while the ovule and associated structures develop into the seed. Embryogenesis and endosperm development typically occur in parallel with seed development, and the embryo is part of the seed. Endosperm may also be part of the mature seed, but in some species the endosperm disappears before seed development is completed. Embryogenesis and seed development are highly ordered, integrated processes, both of which are initiated by double fertilization. Embryogenesis also establishes the primary meristems. Meristems can be considered to be cell factories in which the ongoing processes of cell division, expansion, and differentiation generate the plant body. The vegetative phase of development begins with embryogenesis, but development continues throughout the life of a plant. Understanding how growth, cell differentiation, and pattern formation are regulated at the cellular, biochemical, and molecular levels is the ultimate goal of developmental biologists.
Embryogenesis establishes the two basic developmental patterns that persist and can easily be seen in the adult plant:
- The apical–basal axial developmental pattern.
- The radial pattern of tissues found in stems and roots.
In axial patterning, almost all plants exhibit an axial polarity in which the tissues and organs are arrayed in a precise order along a linear, or polarized, axis. The shoot apical meristem is at one end of the axis, the root apical meristem at the other. In the embryo and seedling, one or two cotyledons are attached just below the shoot apical meristem. Next in this linear array is the hypocotyl, followed by the root, the root apical meristem, and the root cap. This axial pattern is established during embryogenesis.
Axial polarity is established very early in embryogenesis. In fact, the zygote itself becomes polarized and elongates approximately threefold before its first division. The apical end of the zygote is densely cytoplasmic, but the basal half of the cell contains a large central vacuole (Figure 3.91). The first division of the zygote is asymmetric and occurs at right angles to its long axis. This division creates two cells—an apical and a basal cell—that have very different fates. The smaller, apical daughter cell receives more cytoplasm than the larger, basal cell, which inherits the large zygotic vacuole. Almost all of the structures of the embryo, and ultimately the mature plant, are derived from the smaller apical cell. Two vertical divisions and one horizontal division of the apical cell generate the eight-celled (octant) globular embryo. The basal cell also divides, but all of its divisions are horizontal, at right angles to the long axis. The result is a filament of six to nine cells known as the suspensor that attaches the embryo to the vascular system of the plant. Only one of the basal cell derivatives contributes to the embryo.
|The basal cell derivative nearest the embryo is known as the hypophysis (plural hypophyses), and it forms the columella, or central part of the root cap, and an essential part of the root apical meristem known as the quiescent center. As the embryo continues to grow and reaches the heart stage, its axial polarity becomes more distinct, and three axial regions can readily be recognized:
Fig 3.91. Arabidopsis ovule containing the embryo sac at about 4 hours after double fertilization. The zygote exhibits a marked polarization. The terminal half of the zygote has dense cytoplasm and a single large nucleus, while a large central vacuole occupies the basal half of the cell. At this stage, the embryo sac surrounding the zygote also contains 4 endosperm nuclei.
In radial patterning, different tissues are organized in a precise pattern within plant organs. In stems and roots the tissues are arranged in a radial pattern extending from the outside of a stem or a root into its center. In a typical cross section of a root, for example, three concentric rings of tissues are arrayed along a radial axis: An outermost layer of epidermal cells (the epidermis) covers a cylinder of cortical tissue (the cortex), which in turn overlies the vascular cylinder (the endodermis, pericycle, phloem, and xylem)(Fig. 3.92). The protoderm is the meristem that gives rise to the epidermis, the ground meristem produces the future cortex and endodermis, and the procambium is the meristem that gives rise to the primary vascular tissue and vascular cambium.
Fig 3.92. The radial pattern of tissues found in plant organs can be observed in a crosssection of the root. This crosssection of an Arabidopsis root was taken approximately 1 mm back from the root tip, a region in which the different issues have formed
The Arabidopsis pattern of embryogenesis has been studied extensively and is the one we will present here, but keep in mind that angiosperms exhibit many different patterns of embryonic development, and this is only one type. The most important stages of embryogenesis in Arabidopsis, and many other angiosperms, are these:
- The globular stage embryo. After the first zygotic division, the apical cell undergoes a series of highly ordered divisions, generating an eight-cell (octant) globular embryo by 30 hours after fertilization. Additional precise cell divisions increase the number of cells in the sphere
- The heart stage embryo. This stage forms through rapid cell divisions in two regions on either side of the future shoot apex. These two regions produce outgrowths that later will give rise to the cotyledons and give the embryo bilateral symmetry
- The torpedo stage embryo. This stage forms as a result of cell elongation throughout the embryo axis and further development of the cotyledons
- The maturation stage embryo. Toward the end of embryogenesis, the embryo and seed lose water and become metabolically quiescent as they enter dormancy
Fig 3.93 . The apical–basal organization of plant tissues and organs is established very early in embryogenesis. This diagram illustrates how the organs of the early Arabidopsis seedling
originate from specific regions of the embryo. (From Willemsen et al. 1998.)
The radial pattern of tissue differentiation is first observed in the octant embryo (Figure 16.6). As cell division continues in the globular embryo, transverse divisions divide thelower tier of cells radially into three regions. These regions will become the radially arranged tissues of the root and stem axes. The outermost cells form a one-cell-thick surface layer, known as the protoderm. The protoderm covers both halves of the embryo and will generate the epidermis.
Cells that will become the ground meristem underlie the protoderm. The ground meristem gives rise to the cortex and, in the root and hypocotyl, it will also produce the endodermis. The procambium is the inner core of elongated cells that will generate the vascular tissues and, in the root, the pericycle
Fig 3.94. The radial tissue patterns are also established during embryogenesis. This drawing illustrates the origin of the different tissues and organs from embryonic regions in Arabidopsis embryogenesis. The gray lines between the torpedo and seedling stages indicate the regions of the embryo tha t give rise to various regions of the seedling. The expanded regions represent boundaries where developmental fate is somewhat flexible. (After Van Den Berg et al. 1995.)
THE END OF CRP 201 LECTURE NOTE 3.