Plant Reproductive Development and Structures PDF

# Bio 50: Developmental Biology ## Chapter 2. Plant Developmental Biology ### Lesson 1. Reproductive Development and Structure - The Sporophyte and Gametophyte Generation (Review) - Angiosperm Megasporogenesis and Megagametogenesis - Angiosperm Microsporogenesis and Microgametogenesis - Sperm De...

# Bio 50: Developmental Biology ## Chapter 2. Plant Developmental Biology ### Lesson 1. Reproductive Development and Structure - The Sporophyte and Gametophyte Generation (Review) - Angiosperm Megasporogenesis and Megagametogenesis - Angiosperm Microsporogenesis and Microgametogenesis - Sperm Delivery by Pollen Tube ### Learning Objectives: By the end of this lesson, you should be able to: - Describe the two stages of a plant's life cycle - Compare the process of microgametogenesis and megametogenesis/microsporogenesis and megasporogenesis in angiosperms - Identify the structures involved in the reproduction of angiosperms - Explain the process involved during fertilization of egg by the sperm in angiosperms ### The Sporophyte and Gametophyte Generation (Review) #### Life Cycle - Describes the events from the start of life to the reproduction of new life. #### Type of Life Cycle - A life cycle in which there is a dominant haploid stage - A life cycle in which the diploid stage is the dominant stage, and the haploid chromosome number is only seen for a brief time in the life cycle during sexual reproduction. #### Haploid (1n) - **Spores**: Mitosis -> **Gametophyte** -> **Gametes** -> Fusion -> **Zygote** -> Mitosis - **Meiosis** - *Alternation of generations* #### Diploid (2n) - **Sporophyte** **Figure 1. Alternation of generations between the haploid (1n) gametophyte and diploid (2n) sporophyte is shown. (credit: modification of work by Peter Coxhead)** #### Haplodiplontic - describes a life cycle in which the haploid and diploid stages alternate; also known as an alternation of generations life cycle. #### Two Distinct Generations that Alternate in plants. 1. **Gametophyte (n)** - Haploid generation; multicellular - Gives rise to the gametes, or reproductive cells, by mitosis. - It is followed in the development sequence by - 2. **Sporophyte (2n)** - Diploid generation; multicellular - Herbs, shrubs, trees, etc. ### Angiosperm Megasporogenesis and Megagametogenesis #### Megasporogenesis - The development of megaspores from the megasporocyte (or megaspore mother cell), the cell that undergoes meiosis. #### Megasporogenesis comprises three major events: 1. **Megasporocyte formation**. This takes place in the megasporangium (specifically, the nucellus) of the ovule. 2. **Meiosis to produce haploid megaspores.** The diploid (2n) megasporocyte undergoes meiosis to produce four (4) haploid (n) megaspores. 3. **Megaspore selection.** This involves the selection of the megaspore that develops into the embryo sac or megagametophyte (the female gametophyte). The other three degenerate. **Figure 2. Development of female gametophyte (embryo sac) in angiosperms in monosporic method (one megaspore gives an embryo sac).** #### Megagametogenesis - Development of the embryo sac from the megaspore - The surviving megaspore divides by mitosis three (3) times without cytokinesis to produce a huge cell with eight (8) nuclei. This multinucleate structure is then divided by membranes to form the 7-celled embryo sac. #### Cells of the Embryo Sac - The mature embryo sac contains four cell types: 1. **Antipodal cells - three (n)** - Three nuclei positions on the Chalazal end opposite the micropylar end and develop into the antipodal cells, which later degenerate. - Function is unknown. 2. **Central cell - one (2n); a dikaryotic cell** - The polar nuclei move to the equator and fuse, forming a single central cell. - Function: It directly participates in double fertilization to initiate endosperm (triploid, 3n) development. 3. **Egg cell - one (n)** - The nucleus closest to the micropylar end becomes the egg cell. - Function: It fuses with one sperm cell to give rise to the embryo. 4. **Synergid Cells (or egg apparatus) – two (2) at the micropylar end** - The two nuclei on either side of the egg nuclei, develop into synergid cells. - Function: It guides the pollen tube for successful fertilization, then disintegrates. **Figure 3. As shown in this diagram of the embryo sac in angiosperms, the ovule is covered by integuments and has an opening called a micropyle. Inside the embryo sac are three antipodal cells, two synergids, a central cell, and the egg cell.** ### Angiosperm Microsporogenesis and Microgametogenesis #### Microsporeogenesis - The development of microspores from microsporocytes (or microspore mother cells), the cell that undergoes meiosis. - Takes place in all the four microsporangia (specifically, the pollen sac) of the anther of the stamen. Each of these diploid (2n) microsporocytes inside each pollen sac undergo meiosis to produce four haploid (n) microspores. #### Tapetum - The innermost cell layer in the anther which surrounds the microspores. - Provides nutrition to the developing microspores and contributes key components to the pollen wall. **Figure 3. Shown is (a) a cross section of an anther at two developmental stages. The immature anther (top) contains four microsporangia, or pollen sacs. Each microsporangium contains hundreds of microspore mother cells that will four pollen grains. The tapetum supports the development and maturation of the pollen grains. Upon maturation of the pollen (bottom), the pollen sac walls split open and the pollen grains (male gametophytes) are released. (b) In these scanning electron micrographs, pollen sacs are ready to burst, releasing their grains. (credit b: modification of work by Robert R. Wise; scale-bar data from Matt Russell).** #### Microgametogenesis - Comprises events that lead to the progressive development of the microspores into mature pollen. - The microspore initially occurs in tetrads. Then, these are released from the tetrad are now called pollen grains. The nucleus goes through mitosis which results in the formation of two unequal cells, a large vegetative (tube) cell and a small generative cell, each with a haploid nucleus. The generative cell is engulfed by the vegetative cell, resulting in a unique "cell within a cell" structure. Later the generative cell will divide via mitosis to give rise to two (2) sperm nuclei. **Figure 4. A binucleate angiosperm pollen grain containing a generative nucleus and a tube nucleus. After the pollen grain germinates into a pollen tube, the generative nucleus divides into two sperm nuclei. Because the generative nucleus and sperm nuclei contain cytoplasmic sheaths, they are often referred to as cells in some textbooks. The tube nucleus controls the growth of the pollen tube as grows down the style and into the ovary of a flower. Eventually it penetrates the micropyle of an ovule and releases its two sperm into the 8-nucleate embryo sac. During double fertilization, one sperm fuse with the egg nucleus to form a zygote. The other sperm fuses with the two polar nuclei inside the endosperm mother cell to form the endosperm. In corn, this process must occur for each grain that forms. Even more astonishing is the growth of separate pollen tubes down each strand of silk (styles).** #### Upon maturity, the microsporangia burst, releasing the pollen grains from the anther. The pollen grain is a haploid cell with two layers: 1. **Exine**: - Outer layer; present throughout the pollen grain except for a small area (germ pore) where the pollen tube emerges after pollination. - Contains *sporopollenin* - a complex waterproofing substance supplied by the tapetal cells; it allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage. 2. **Intine** - Inner layer. **Figure 5. Pollen develops from the microspore mother cells. The mature pollen grain is composed of two cells: the pollen tube cell and the generative cell, which is inside the tube cell. The pollen grain has two coverings: an inner layer (intine) and an outer layer (exine). The inset scanning electron micrograph shows Arabidopsis lyrata pollen grains. (credit "pollen micrograph": modification of work by Robert R. Wise; scale-bar data from Matt Russell)** #### Sperm Delivery by Pollen Tube - When a pollen grain lands on a suitable stigma, it absorbs H2O and germinates, producing a pollen tube. The pollen tube is an extension of the cytoplasm of the pollen grains' tube cell. - As a pollen tube elongates, no cell division occurs. The pollen tube grows via tip growth. - As the pollen tube elongates through the style, the nucleus of the generative cell divides by mitosis and produces two sperm nuclei, which remain inside the tube cell. The tube nucleus leads ahead of the two sperm as the tip of the pollen tube grows toward the micropyle in response to chemical attractants produced by the synergids. - The arrival of the pollen tube initiates the death of one of the two synergids, thereby providing a passageway into the embryo sac. The tube nucleus and the two sperm are then discharged from the pollen tube into one of the synergid cell and travel towards the egg to fertilize it. **Figure 6. Sexual reproduction in angiosperms.**