Angiosperm Life Cycle: Pollen and Embryo Development PDF

Typically, the stamen is made up of a delicate filament attached to an anther composed of four microsporangia arranged in opposite pairs. Each pair of microsporangia is separated from the other by a central region of sterile tissue surrounding a vascular bundle.  The precise sequence of microspor...

Typically, the stamen is made up of a delicate filament attached to an anther composed of four microsporangia arranged in opposite pairs. Each pair of microsporangia is separated from the other by a central region of sterile tissue surrounding a vascular bundle.  The precise sequence of microsporangium development varies from species to species. In Arabidopsis, the mature anther contains archesporial cells, the cells that ultimately undergo meiosis, surrounded by four somatic layers: the epidermis, endothecium, middle layer, and tapetum. These layers are originally derived from the three layers of the floral meristem (L1, L2, and L3). The L1 layer becomes the epidermis, and the L2 layer gives rise to the archesporial cells as well as the inner surrounding layers. The central region containing the archesporial cells is called the locule.  Tetrads of four haploid microspores Callose is degraded by the activity of an enclosed within a unique callosic (β- enzyme complex (callase) secreted by the 1,3-glucan) cell wall tapetum leading to the separation of tetrads into individual microspores The development of the male gametophyte, or pollen grain, is temporally divided into two phases: microsporogenesis and microgametogenesis. During microsporogenesis, archesporial cells within the locules differentiate into microsporocytes or pollen mother cells (2n) - diploid cells capable of undergoing meiosis to produce the microspores. The microsporocytes undergo meiosis, resulting in a tetrad of haploid microspores (n) joined together at their cell walls, which are composed largely of the polysaccharide callose, a (1,3)-b-glucan. T The tapetum, a layer of secretory cells surrounding the locule, secretes the hydrolytic enzyme callase and other cell wall–degrading enzymes into the locule; this partially digests the cell walls and separates the tetrad into individual microspores. In some insect-pollinated species, pollen is normally shed as tetrads, as in the common heather (Calluna vulgaris), or in even larger assemblages termed polyads, as in Acacia. Although wild-type Arabidopsis produces individual microspores, in quartet (qrt) mutants tetrad dissolution is blocked. Nevertheless, the pollen grains of qrt mutants develop normally and are fertile. Once the microspores have formed inside the anther locules - either separated or as tetrads or polyads - the microsporogenesis phase of microgametophyte development is completed. Microgametogenesis (formation of male gametes): During microgametogenesis, the haploid microspore develops mitotically into the mature male gametophyte, composed of the vegetative (or tube) cell and two sperm cells. The polarized microspore This callose layer breaks down and the generative cell is engulfed by the vegetative cell, Prior to the first mitotic division, then undergoes a highly resulting in a unique anatomical structure: a cell within a cell (bicellular stage). the microspore expands asymmetric cell division substantially, a process associated (pollen mitosis I), giving The engulfed generative cell subsequently takes on an elongated or spindle like shape, which with cell wall biosynthesis and the rise to a large vegetative may assist in passage of the generative cell through the dynamic protoplasm of the rapidly formation of a large vacuole. In cell and a small growing pollen tube. parallel, the microspore nucleus generative cell (or male During maturation, pollen grains accumulate carbohydrate or lipid reserves to support the active migrates to the cell wall, germ cell). metabolism required for rapid germination and pollen tube growth. producing a polarized At this stage the pollen is usually released from the anther by dehiscence (opening) of the microspore. At first the generative cell anther wall, and the generative cell divides to produce the two sperm cells (pollen mitosis II) remains attached to the only after the pollen grain has landed on a stigma and a pollen tube has formed. microspore cell wall and is In many plants, however, the generative cell undergoes pollen mitosis II while still inside the enclosed by a hemispherical wall anther (tricellular stage). In either case, the production of the two sperm cells signals the end of of callose, which also serves to microgametogenesis. Although the majority of flowering plants produce bicellular pollen, many separate the generative cell important food crop plants such as rice, wheat and maize produce short-lived, tricellular pollen from the vegetative cell. grains  Depending on the species, tapetal cells may either remain at the periphery of the locule (as they do in Arabidopsis) or become amoeboid and migrate into the locule, intermingling with the developing microspores. In both cases tapetal cells perform a secretory function and eventually undergo programmed cell death, releasing their contents into the locule. Because of the essential role of tapetal cells in supplying enzymes, nutrients, and cell wall constituents to the developing pollen grains, defects in the tapetum usually cause abnormal pollen development and decreased fertility. Pollen Development  Single Celled Pollen grain or Microspore First mitotic division Rich in food, irregular shaped nucleus Two-celled stage pollen grain Float in cytoplasm of vegetative cell (Vegetative cell and Generative cell) Although the majority of flowering plants produce bicellular pollen, many important food crop plants such as rice, wheat and maize produce short-lived, tricellular pollen grains  The pollen wall and its coatings isolate and protect the male gametophyte and its precious cargo, and mediate the complex communication with the stigma surface. Sporopollenin is one of the most resistant biopolymers known and is a complex containing fatty acids and phenylpropanoids. The exine is not evenly distributed over the pollen grain surface and regions with reduced thickness or that lack sporopollenin can form apertures that are used as sites for pollen tube emergence. Exine (outer layer) + Intine (inner layer)= Sporoderm Gynoecium: refers to the entire or collective female reproductive part. located in the center of most flowers. can be composed of one or more carpels. Carpel: carpel is the units or building block of the gynoecium. a modified leaf that contains, ovary (swollen base of the carpel that contains the ovules), style (a stalk connecting the stigma to the ovary or tube-like structure that grows pollen tubes) and, stigma (the sticky part that receives pollen), responsible for receiving pollen and producing seeds. Pistil: a pistil can be made up of one carpel (Simple Pistil) or multiple carpels that are fused together (Compound Pistil) The Arabidopsis gynoecium is an important model system for studying ovule development The gynoecium of Arabidopsis, as in many members of the Brassicaceae (mustard family), consists of two fused carpels, referred to as valves, separated by a medial partition called the septum. The edges of valves and the septum are joined at a strip of tissue called the replum, which plays an important role in the dehiscence of the dry fruit. In each carpel there are two strips of placental tissue associated with the septum on either side of the gynoecium. Placenta: Ovule primordia arise in a specialized ovary tissue called the placenta. The locations of placental tissue vary among different plant groups, and include the marginal, parietal, axile, basal, and free-central types of placentation. The type of placentation within the ovary determines the positions and arrangement of the seeds within the fruit.  The structure or parts of ovule Integuments: The outer layer of the ovule that protects the nuclleus. Nucellus is typically protected by two integument layers: inner and outer. Nucellus: The nucellus is a mass of diploid cells in the center of the ovule that's made up of parenchyma cells. The nucellus is a nutritive tissue that surrounds the embryo sac and provides nourishment to the developing embryo and also protection. The nucellus can vary in shape and size (number of layers), which can be characteristic of a plant species. Female gametophyte: The central core of the ovule, which is also known as the embryo sac in angiosperms. It is a seven-celled reproductive structure that contains egg cells. Micropyle: A small opening at the top of the ovule or integuments leave a pore that allows pollen tube to enters the ovule. Chalaza: The basal part of the ovule, where the integuments and nucellus join. Funicle: A small stalk that connects the ovule to the placenta. Hilum: The point of attachment where the funicle and the body of the ovule fuse.  The vast majority of angiosperms exhibit Polygonum-type embryo sac development The development of the female gametophyte or embryo sac, is more complex and more diverse than that of the male gametophyte. According to one classification scheme, there are more than 15 different patterns of embryo sac development in angiosperms. The most common pattern was first described in the genus Polygonum (“knotweed”) and is therefore called the Polygonum type of embryo sac. The archesporial cell within the nucellus differentiates into the megaspore mother cell, the cell that undergoes meiosis. The cell that will differentiate into the megaspore mother cell is clearly visible in the primordial nucellus because of its large size, large nucleus, and dense cytoplasm.  The beige areas represent cytoplasm, the white areas represent vacuoles, and the purple circles represent nuclei. The chalazal pole is depicted up and the micropylar pole down. The central cell nucleus is formed by fusion of the polar nuclei. In the Polygonum type of embryo Four of the nuclei then migrate to the The three cells at the chalazal end of sac, meiosis of the diploid chalazal pole, and the other four migrate to the embryo sac are termed the megaspore mother cell produces the micropylar pole. antipodal cells. four haploid megaspores. Three of the nuclei at each pole undergo In many other species, including Three of the megaspores, usually cellularization, while the remaining two Arabidopsis, the antipodal cells those at the micropylar end of the nuclei, called polar nuclei, migrate toward degenerate prior to fertilization, which nucellus, subsequently undergo the central region of the embryo sac, which suggests they do not play an essential programmed cell death, leaving also contains a large vacuole. role in fertilization. only one functional megaspore. The cytoplasm and the two polar nuclei The egg cell (the female gamete that Functional megaspores then develop their own plasma membrane and combines with a sperm cell to form the undergo three rounds of free cell wall, giving rise to a large binucleate zygote) and the two synergid cells are nuclear mitotic divisions (mitoses cell. located at the micropylar end of the without cytokinesis) to produce a The fully cellularized embryo sac embryo sac and are collectively syncytium - a multinucleate cell represents the mature female gametophyte referred to as the egg apparatus. formed by nuclear divisions. The or embryo sac. The large binucleate cell in the middle result is an eight-nucleate, At maturity, the Polygonum-type embryo of the embryo sac is called the central immature embryo sac. sac consists of seven cells and eight cell. nuclei.  Ultrastructural studies have shown that the antipodal cells contain large membrane invaginations, perhaps indicative of a role in nutritional exchange or hormonal signaling. However, antipodal cells are absent in the order Nymphaeales, which includes the water lilies, as well as in members of the evening primrose family (Onagraceae). As a result, these two plant groups have only four-nucleate embryo sacs at maturity. In contrast, in members of the grass family (Poaceae) the antipodal cells proliferate, so they may play a role in fertilization in the grasses. An additional feature is the presence of a filiform apparatus at the extreme micropylar end of each synergid. The filiform apparatus consists of a convoluted, thickened cell wall that increases the surface area of the plasma membrane. Synergid cells are involved in the final stages of pollen tube attraction, the discharge of pollen tube contents into the embryo sac, and gamete fusion. In Arabidopsis, the two polar nuclei of the central cell fuse to form a single diploid nucleus prior to fusion with the sperm cell. During double fertilization in the Polygonum-type embryo sac, one sperm cell fuses with the egg to produce the zygote, and the other fuses with the central cell to produce the triploid primary endosperm cell, which divides mitotically to give rise to the nutritive endosperm of the seed. Because different types of embryo sacs contain different numbers of polar nuclei, the ploidy level of the endosperm ranges from 2N in Oenothera to 15N in Peperomia.  Embryo sac development involves hormonal signaling between sporophytic and gametophytic generations Three hormones - auxin, cytokinin, and brassinosteroids - have been implicated in the regulation of various stages of female gametophyte development in Arabidopsis. Auxin is having role as a cell fate determinant in female gametophytes. Cytokinins synthesized in the chalazal region of the nucellus have been implicated in megasporogenesis. Triple mutants lacking functional AHK receptors, which are required for the cytokinin response, fail to develop functional megaspores. Brassinosteroids have been shown to be required for the initiation of mitotic divisions by the megaspore. In other words, brassinosteroid biosynthesis inside the embryo sac is required for the initiation of megagametophyte development.  Pollination in Flowering Plants Pollination in angiosperms is the process of transferring pollen grains from the anther of the stamen, the male organ of the flower, to the stigma of the pistil, the female organ of the flower. Self-pollination, or selfing: pollen and the stigma belong to the same individual sporophyte e.g. Arabidopsis thaliana and rice. Cross pollination or outcrossing: male parent and the female parent are separate sporophytic individuals, pollen might travel great distances before landing on a suitable stigma, pollen grains are dispersed by wind, insects, birds, and mammals. Successful pollination depends on several factors, including ambient temperature, timing, and the receptivity of the stigma of a compatible flower. Many pollen grains can tolerate desiccation and high temperatures during their journey to the stigma. However, some pollen grains, such as those of tomato, are damaged by heat. Understanding how some pollen grains tolerate periods of high temperature will help ensure our food supply as the global climate changes. Delivery of sperm cells to the female gametophyte by the pollen tube occurs in six phases Adhesion and hydration of a pollen grain on a compatible flower depend on recognition between pollen and stigma surfaces Pollen grains physically adhere to the stigma papillar cells, probably due to biophysical and chemical interactions between pollen proteins and lipids and stigma surface proteins. Pollen grains adhere poorly to stigmas of plants of other families.  Angiosperm reproduction is highly selective. Female tissues are able to discriminate among diverse pollen grains, accepting those from the appropriate species and rejecting others from unrelated species. Flowers have either wet or dry stigmas. The surface cells of wet stigmas release a viscous mixture of proteins, lipids, and polysaccharides; the surface cells of dry stigmas, such as those found in the Brassicaceae, are covered by a cell wall, cuticle, and protein pellicle. Whereas pollen grains become hydrated by default on wet stigmas, the hydration process on dry stigmas is highly regulated. After landing on the stigma, lipids and proteins from the pollen coat flow out onto the stigma and mingle with materials from papillar cells to form the “foot,” a structure that attaches the pollen grain firmly to the tip of the papillar cell. During this process, the lipids in the foot are thought to reorganize, creating a capillary system through which water and ions can flow from the stigma to the pollen grain. In support of the role of lipids in pollen hydration, Arabidopsis mutants with defects in long-chain lipid metabolism produced pollen without a pollen coat, and these pollen grains failed to hydrate on the stigma. This defect could be rescued by either high humidity or the application of lipids to the stigma, both of which allowed the pollen grain to hydrate and form a pollen tube. The mechanism of water movement from the papillar cell into the foot is still unclear. In principle, water could either diffuse out of the papillar cell via plasma membrane aquaporin channels or be secreted by vesicular exocytosis. In favor of a secretory mechanism, pollen grains fail to hydrate on pistils with a mutation in a gene that is required for the normal exocytosis of Golgi vesicles.  Ca2+-triggered polarization of the pollen grain precedes tube formation During hydration, the pollen grain becomes physiologically activated. Calcium ion influx into the vegetative cell triggers reorganization of the cytoskeleton and causes the cell to become physiologically and ultrastructurally polarized. The source of the Ca2+ is unknown but may be either the cytoplasm or the cell wall of the papillar cell. In addition to water and Ca2+ , the stigma may supply a variety of other factors that promote pollen germination as well, but thus far these appear to be species-specific. Cytosolic Ca2+ concentration increases at the future germination site soon after hydration, and remains elevated until tube emergence. Both actin microfilaments and secretory vesicles accumulate below the germination pore or aperture, and the vegetative nucleus or tube nucleus and two sperm cells migrates to the germinating pollen tube. Pollen tubes grow by tip growth. Receptor-like kinases are thought to regulate the ROP1 GTPase switch, a master regulator of tip growth. Pollen tube tip growth in the pistil is directed by both physical (e.g. pistil architecture) and chemical cues (some specific proteins).  Double fertilization occurs in three distinct stages When the pollen tube senses chemical attractants secreted by the synergids, the tube grows through the micropyle, penetrates the embryo sac, and enters one of the synergid cells. Based on live imaging of fluorescently tagged sperm cells, sperm cell behavior in Arabidopsis can be divided into three stages: 1. First once inside the synergid, the pollen tube stops growing and the pollen tube bursts within a few seconds of entering the synergid, either during or just prior to the breakdown of the receptive synergid cell. 2. Second, the two sperm cells remain stationary at a boundary region between the egg cell and the central cell for approximately 7 min. 3. Third, one sperm fuses with the egg and the other fuses with the central cell, completing double fertilization. This sperm surface protein apparently facilitates the fusion of the male and female gamete  Embryogenesis: The Origins of Polarity The term embryogenesis describe the process by which a single cell is transformed into a multicellular entity having a characteristic, but typically rudimentary, organization. In most seed plants, embryogenesis takes place within the ovule, a specialized structure formed within the carpels of the flower. During embryogenesis, groups of cells become functionally specialized to form epidermal, cortical, and vascular tissues. Certain groups of cells, known as apical meristems, are established at the growing points of the shoot and root and enable the elaboration of additional tissues and organs during subsequent vegetative growth. In addition, a number of physiological changes occur to enable the embryo to withstand long periods of dormancy and harsh environmental conditions. Embryogenesis differs between eudicots and monocots, but also features common fundamental processes.  Arabidopsis embryogenesis Due to the relatively small size of the Arabidopsis embryo, the pattern of cell division by which it arises are relatively simple and easily followed. Five stages, each of which is linked to the shape of the embryo, are widely recognized: 1. Zygotic stage: The first stage of the diploid life cycle commences with the fusion of the haploid egg and sperm to form the single-celled zygote. Polarized growth of this cell, followed by an asymmetric transverse division, gives rise to a small apical cell and an elongated basal cell. 2. Globular stage: The apical cell undergoes a series of divisions to generate a spherical eight-cell (octant) globular embryo exhibiting radial symmetry. Additional cell divisions increase the number of cells in the globular embryo and create the outer layer, the protoderm, which later becomes the epidermis. 3. Heart stage: Focused cell division in two regions occurs on either side of the future shoot apical meristem to form the two cotyledons, giving the embryo bilateral symmetry. 4. Torpedo stage: Cell elongation and cellular differentiation processes occur throughout the embryonic axis. Visible distinctions between the adaxial and abaxial tissues of the cotyledons become apparent. 5. Mature stage: Toward the end of embryogenesis, the embryo and seed lose water and become metabolically inactive as they enter dormancy. Storage compounds accumulate in the cells at the mature stage.  The stages of Arabidopsis embryogenesis are characterized by precise patterns of cell division. After the first Two-cell embryo Eight-cell embryo Mid-globular stage, which has developed a distinct protoderm division of the (surface layer). zygote, forms the apical and basal cells. Early heart stage Late heart stage Torpedo stage Mature embryo Apical or Terminal cell or Embryonal initial cell: Following the asymmetric division of the zygote, the Small and cytoplasmically densed, smaller, apical daughter cell divides to form an give rise to the neearly the entire embryo, and ultimately the mature plant eight-cell embryo consisting of two tiers of four cells Basal or Suspensor initial cell: Longer and large central vacuole, each. series of transverse divisions produces the filamentous suspensor The upper tier gives rise to the SAM and most of the flanking cotyledon primordia. The lower tier produces the hypocotyl (embryonic stem) and some of the cotyledons, the embryonic root, and the upper cells of the root apical meristem. The basal daughter cell produces a single file of cells that make up the filamentous suspensor (6-10 cells). The uppermost cell of the suspensor becomes the hypophysis (blue), which is part of the embryo. The hypophysis divides to form the quiescent center and the stem cells (initials) that form the root cap.