Introduction 1. INTRODUCTION

Transcription

1 1. INTRODUCTION Early in their evolution, plants have acquired a life cycle that alternates between a multicellular haploid organism, the gametophyte and a multicellular diploid organism, the sporophyte. Highly evolved angiosperms of plant kingdom have reduced male and female gametophytes. The functions of the gametophytes are the production of the gametes and their union. In angiosperms, the pollen grain is the male gametophyte and the embryo sac is the female gametophyte and their production is known as micro and macro sprogenesis and gametogenesis respectively. The angiosperm gametophytes are essential for the reproductive process. During sexual reproduction in angiosperms, the male gametophyte is transferred from the anther to the stigma, whereupon it forms a pollen tube that grows great distances through the stylar tissues to deliver its two sperm cells to the female gametophyte which is situated within the ovule. One sperm cell fertilizes the egg cell and the second sperm cell fuses with the central cell. After fertilization ovary develop into fruit, the ovule gives rise to a seed; the zygote into embryo, central cell into endosperm, and the integuments into seed coat (Maheshwari, 1950). The female gametophyte is critical in many steps of the angiosperm reproductive process, including pollen tube guidance, fertilization, the induction of seed development, and maternal control of seed development after fertilization. Gametophytes and sporophytes differ morphologically and functionally. The major function of diploid sporophyte generation is to produce haploid spores, which are the products of meiosis. Spores undergo cell proliferation and differentiation to develop into gametophytes. The major function of gametophyte generation is to produce haploid gametes. The fusion of egg and Ph.D Thesis 97

2 sperm gives rise to the zygote, which is the beginning of diploid sporophyte generation, thereby completing the life cycle (Gifford and Foster, 1989). The mature anther has four pollen sacs (tetrasporangiate) and half anther has two pollen sacs as in Malvaceae. The pollen sacs (microsporangia) produce the male gametophyte (pollen grains) at maturity and released out to fertilize the egg cell of female gametophyte. The anther wall comprises the epidermis followed by the endothecium, middle layer/s and tapetum. The epidermal cells may persist or collapse. The endothecial cells, in the majority of angiosperms, usually develop fibrous thickenings which are absent in the region of dehiscence. The fibrous thickenings may be absent in some cleistogamous flowers like Vallisnera. The middle layer collapse due to expansion of the endothecium and tapetum. The tapetum is the innermost layer of the anther wall surrounding the sporogenous tissue, plays an important role in pollen development. The tapetum is dual in origin. Periasamy and Kandasamy (1981) reported trimorphic tapetum in Annona squamosa. The tapetum may become two to four nucleate or sometimes the number may goes up to 16. The tapetum is of two types, glandular and amoeboid. Sometimes there is an intermediate type between glandular and amoeboid tapetum. The tapetum is physiologically the most active layer of the anther wall and acts as molecular filter which nourishes the developing pollen grains. The tapetum also secretes sporopollenin, a constituent of pollen wall. The enzyme responsible for the degradation of the callose wall is also considered to be derived from the tapetal periplasmodium. The tapetum synthesizes and releases fibro-granular proteins and carotenoid-containing lipid droplets which enter the exine of pollen and are stored in bacula. They are extra cellular and capable of recognizing stigma compatibility. The carotenoid containing lipid granules spread over the exine and helps in pollination and are describe as pollen kit. Ph.D Thesis 98

3 The male gametophyte also referred to as the pollen grain or microgametophyte or male germ unit (recent concept-dumas et al., 1984) develops within the anther and is composed of two sperm cells encased within a vegetative cell (McCormick, 1993, 2004). The sperm cell 1 or SUN (Sperm cell Unassociated with vegetative nucleus) contains more number of plastids and less number of mitochondria (Plumbago zeylanica) and B chromosome in sperm cell 1 of Zea mays is known as true sperm cell since it is fusing with egg cell. Hence the fertilization in angiosperms is preferential. The sperm cell 2 or SVN (Sperm cell associated with Vegetative Nucleus) contains more mitochondria and less plastids forced to fuse with the central cell of the female gametophyte. The female gametophyte, also referred to as the embryo sac or megagametophyte, develops within the ovule. The most common female gametophyte form consists of seven cells and four different cell types: three antipodal cells, two synergid cells, one egg cell, and one central cell (Maheshwari, 1950). Later in recent years the concept of female germ unit has been developed which includes two synergid cells, one egg cell and the central cell (Dumas et al., 1984). The egg cell is called as female gamete 1 which fuses with the sperm cell 1 result in the formation of zygote (syngamy). The central cell is the female gamete 2 fuses with the sperm cell 2 result in primary endosperm cell (PEC) (triple fusion). The syngamy and triple fusion together known as double fertilization which is unique to angiosperms. In view of rapid advancement of biological and biophysical techniques, the study of ovule development has attracted much attention (Vijayaraghavan et al., 1988; Huang and Russell, 1992; Russell, 1992; Reiser and Fischer, 1993). Genetic studies also suggest that female gametophyte development depends on the activities of many female gametophyte expressed genes (Drews et al., 1998). During megasporogenesis, the diploid megaspore mother cell undergoes meiosis and gives rise to four haploid megaspores. Angiosperms exhibit three main patterns of megasporogenesis, referred to as monosporic, bisporic, and Ph.D Thesis 99

4 tetrasporic. The three types differ mainly based on cell plate formation during meiotic divisions, thus determining the number of meiotic products that contribute to the formation of the mature female gametophyte. In the monosporic pattern, both meiotic divisions are accompanied by cell plate formation, resulting in four one-nucleate megaspores. Subsequently, three megaspores, generally the micropylar-most megaspores undergo cell death. In the bisporic pattern, cell plates form after meiosis I but not in meiosis II. The result is two uni-nucleate megaspores, one of which degenerates. In the tetrasporic pattern, cell plates fail to form after both meiotic divisions, resulting in one four-nucleate megaspore. Thus, these three patterns give rise to a single functional megaspore that contains one (monosporic), two (bisporic), or four (tetrasporic) meiotic nuclei. The monosporic pattern is the most common form and is represented as Polygonum pattern (Maheshwari, 1950; Willemse and Went, 1984; Haig, 1990; Huang and Russell, 1992). Female gametophyte development occurs over two phases referred to as megasporogenesis and megagametogenesis. More than 15 different patterns of female gametophyte development have been described. The different patterns arise mainly from variations in cytokinesis during meiosis, number and pattern of mitotic divisions, and pattern of cellularization. The developmental pattern exhibited by most species is referred to as the Polygonum type because it was first described in Polygonum divaricatum (Strasburger, 1879; Maheshwari, 1950). The Polygonum-type female gametophyte is exhibited by more than 70% of flowering plants and is the pattern found in many economically and biologically important groups, including Brassicaceae (e.g., Arabidopsis, Capsella, Brassica), Gramineae (e.g., maize, rice, wheat), Malvaceae (e.g., cotton), Leguminoseae (e.g., beans, soybean), and Solanaceae (e.g., tobacco, tomato, potato, petunia) (Maheshwari, 1950; Willemse and Went, 1984; Haig, 1990; Huang and Russell, 1992). During megagametogenesis, the functional megaspore gives rise to the mature female gametophyte. Initially, the megaspore undergoes one or more Ph.D Thesis 100

5 rounds of mitosis without cytokinesis, resulting in a multinucleate coenocyte. Subsequently, cell walls form around these nuclei, resulting in a cellularized female gametophyte. For example, in the Polygonum-type pattern a single nucleus undergoes two rounds of mitosis, producing a four-nucleate cell with two nuclei at each pole. During a third mitosis, phragmoplasts and cell plates form between sister and nonsister nuclei, and soon thereafter, the female gametophyte cells become completely surrounded by cell walls. During cellularization, two nuclei, one from each pole (the polar nuclei), migrate toward the center of the developing female gametophyte and fuse together either before or upon fertilization of the central cell. These events result in an 8- nuclaeated or seven-celled structure consisting of three antipodal cells, one central cell with two nuclei, two synergid cells, and one egg cell. The monosporic, Polygonum type of female gametophyte is typically a seven-celled structure at maturity. However, this structure may be modified by cell death or cell proliferation events in various species. For example, in Arabidopsis, the antipodal cells undergo cell death immediately before fertilization, whereas in grasses (e.g., maize), the antipodal cells proliferate (Haig, 1990; Huang and Russell, 1992; Drews et al., 1998). Throughout development, the female gametophyte exhibits a polarity along its chalazal-micropylar axis. During Polygonum-type megasporogenesis, the chalazal-most megaspore survives and the other three megaspores undergo cell death. During cell differentiation, the nuclei at the micropylar end become specified to develop into the egg cell, the micropylar polar nucleus, and the synergid cells; the chalazal nuclei develop into the three antipodal cells and the chalazal polar nucleus. Furthermore, all of the cells within the female gametophyte differentiate into polar structures. For example, in many species, the egg cell's nucleus is located toward the chalazal end and its vacuole occupies the micropylar end; by contrast, the synergid and central cells have the opposite polarity (Willemse and Went, 1984; Huang and Russell, 1992; Christensen et al., 1997). Ph.D Thesis 101

6 Hence in the present study, to know the development of anther wall, microsporogenesis, microgametogensis, megasporogenesis and megametogenesis in Ionidium suffruticosum Ging. is undertaken by using histochemical techniques to localize the insoluble polysaccharides, proteins and RNA during various stages of development. Ph.D Thesis 102