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Plant Reproduction by Michael Leonard, Ellie Ruess, and Allison Gregg Overview of Flowering Initiation Carefully regulated processes determine when and where flowers will form. Plants must often gain competence, or sensitivity to flowering signals, to respond to internal or external signals regulating flowering. Once palnts are competent to reproduce, a combination of factorsincluding light, temperature, and both promotive and inhibitory internal signals-determines when aflower is produced. These signals turn on genes that specify where the floral organssepals, petals, stamens, and carpelswill form. Once cells have instructions to become a specific floral organ, yet another developmental cascade leads to the three-dimensional construction of Plants Undergo Metamorphosis • Plants go through developmental changes leading to productive maturity just as many animals do. • This metamorphosis is similar to a tadpole changing to an adult frog or a caterpillar changing to a butterfly. • A plant's metamophosis leads to the production of a flower. • At germination, most plants are incapable of producing a flower, even if the all the environmental cues are optimal. • Internal development changes allow plants to obtain competence to respond to external and/or internal signals that trigger flower formation. • This transition is referred to as phase change. • Phase change can be structurally obvious or very subtle. Phase Change • Plants become reproductively competent (sensitive to flowering signals) through changes in signaling and perception. • The transition to the adult stage of development, during which reproduction is possible, is called phase change. • Plants in the adult phase of development may or may not produce reproductive structures (flowers), depending on environmental cues. Pathways Leading to Flower Production • Three genetically regulated pathways to flowering have been identified: o the light-dependent pathway o the temperature-dependent pathway o the autonomous pathway • Plants can rely primarily on one pathway, but all three pathways can be present. • The environment can promote or repress flowering, and in some cases, it can be relatively neutral. • For example, light can be a signal that long summer days have arrived in a temperate climate and that conditions are favorable for reproduction. In other cases, plants depend on light to accumulate sufficient amounts of sucrose to fuel reproduction, but flower independently of day length. • Temperature can also be used as a signal. Light-Dependent Pathway • Flowering requires much energy accumulated via photosynthesis. Thus, all plants require light for flowering, but this is distinct from the photoperiodic, or light-dependent, flowering pathway. • In the light-dependent pathway, plants use light-receptor molecules to measure the length of night. The flowering responses of plants to day length fall into several basic categories: o When the daylight becomes shorter than a critical length, flowering is initiated in short-day plants, such as the goldenrod. o When the daylight becomes longer than a critical length, flowering is initiated in long-day plants, such as the iris. o Other plants, such as snapdragons, roses, and many native to the tropics (for example, tomatoes), flower when mature regardless of day length, as long as they have received enough light for normal growth. These are referred to as day-neutral plants. • Some plants have two critical photoperiods; they will not flower if the days are too long, and they will not flower if the days are too short. These are called facultative long-day or short-day plants. The garden pea is an example of a facultative long-day plant. Light-Dependent Pathway • The information on the length of day and night is then used to signal pathways that promote or inhibit flowering. • Light receptors in the leaves trigger events that result in changes in the shoot meristem. Temperature-Dependent Pathway • Cold temperatures can accelerate or permit flowering in many species. • As with light, this ensures that plants flower at more optimal times. • The temperature-dependent pathway includes vernalization, the requirement for a period of chilling before a plant can flower. Vernalization is necessary for some seeds or plants in later stages of development. • The phenomenon of vernalization was discovered by the Russian scientist Lysenko while trying to solve the problem of winter wheat rotting in the fields. Because winter wheat could not flower without a period of chilling, Lysenko chilled the seeds and then successfully planted them in the spring. Lysenko erroneously concluded that he had converted one species, winter wheat, to another, spring wheat, by simply altering the environment. Autonomous Pathway • The autonomous pathway leads to flowering independent of environmental cues, except for basic nutrition. • Plants integrate information about position in regulating flowering, and both promoters and inhibitors of flowering are important. • Presumably, this was the first pathway to evolve. • Day-neutral plants often depend primarily on the autonomous pathway, which allows plants to "count" and "remember." • For example, a field of day-neutral tobacco will produce a uniform number of nodes before flowering. If the shoots of these plants are removed at different positions, axillary buds will grow out and produce the same number of nodes as the removed portion of the shoot. At a certain point in development, the shoots become committed or determined to flower. The upper axillary buds of flowering tobacco will remember their position when rooted or grafted. The terminal shoot tip becomes florally determined about four nodes before it initiates a flower. In some other species, this commitment is less stable and/or occurs later. • Shoots know where they are and at some point "remember" that information because inhibitory signals are sent from the roots. Identity Genes • The three flowering pathways lead to an adult meristem becoming a floral meristem by either activating or repressing the inhibition of floral meristem identity genes. • Two of the key floral meristem identity genes are LEAFY and APETALA1. These genes establish the meristem as a flower meristem. They turn on floral organ identity genes. Formation of Floral Meristems and Floral Organs • The floral organ identity genes define four concentric whorls, moving inward in the floral meristem, as sepal, petal, stamen, and carpel. • The scientists Meyerowitz and Coen proposed a model, called the ABC model, to explain how three classes of floral organ identity genes could specify four distinct organ types. The ABC model proposes that three classes of organ identity genes (A, B, and C) specify the floral organs in the four floral whorls. By studying mutants, the researchers have determined the following: 1. Class A genes alone specify the sepals. 2. Class A and class B genes together specify the petals. 3. Class B and class C genes together specify the stamens. 4. Class C genes alone specify the carpels. Formation of Floral Meristems and Floral Organs • The beauty of the ABC model is that it is entirely testable by making different combinations of floral organ identity mutants. • Each class of genes is expressed in two whorls, yielding four different combinations of the gene products. When any one class is missing, atypical floral organs occur in predictable positions. • This is actually only the beginning of the making of a flower. These organ identity genes are transcription factors that turn on many more genes that will actually give rise to the three-dimensional flower. Other genes "paint" the petals-that is, complex biochemical pathways lead to the accumulation of anthocyanin pigments in vacuoles. These pigments can be orange, red, or purple, and the actual color is influenced by pH as well. ABC Model for Floral Organ Specification ABC Model for Floral Organ Specification The Formation of Gametes • The ovule within the carpel has origins more ancient than the angiosperms. • Floral parts are modified leaves, and within the ovule is the female gametophyte. • This next generation develops from placental tissue in the ovary. • A megaspore mother cell develops and meiotically gives rise to the embryo sac. • Usually, two layers of integument tissue form around this embryo sac and will become the seed coat. • Genes responsible for initiating the integuments have been identified. • Some genes also affect leaf structure. Evolution of the Flower • Successful pollination in many angiosperms depends on pollinators such as insects, birds, and animals. These animals perform the same functions in plant reproduction as plants otherwise do for themselves • Mutations in either partner can block reproduction • Floral form and structure has coevolved with pollinators • The diversity of angiosperms is partly due to the evolution of a large variety of floral phenotypes that may enhance the effectiveness of pollination Reproductive Structure of Plants • Calyx- usually consists of the outermost part of the "whorl". Consists of flattened appendages called sepals, which protect the flower as a bud • Petals = Corolla • Corolla and Calyx are sterile, but they attract pollinators and increase chances of reproductive success • Androecium- collective term for the stamen (male structure) of the flower. Most angiosperm have stamen whose filaments ("stalks") are slender and threadlike. The anther, which store the microsporangia ("pollen") are attached in buds • Carpel- term for ovary (holds the ovule (future seed of new plant), the stigma (pollen receiver), and style (necklike connection from stigma to ovary) • Gynoecium- collective term for female parts of flower, including carpel, and the ovule Formation of Angiosperm Gametes • Reproductive success depends on the gametes (egg and sperm) found in the embryo sacs and the pollen grains of flowers • Pollen grains and the embryo sac both are produced in separate specialized structures of the angiosperm flower • Separate male and female gametes, but usually occur in the same flower Pollen Formation • Pollen grains from in the two pollen sacs located in the anther • Each sac contains specialized chambers in which mother cells are enclosed and protected • Mother cells undergo meiosis and become haploid pollen grains which later separate • Each pollen grain contains a generative cell, which will eventually divide to produce two sperm cells Embryo Sac Formation • Eggs develop in the ovules of the angiosperm flower • Within each ovule is a megaspore mother cell • Each megaspore undergoes meiosis to produce 4 haploid megaspores, but in most plants, only one of these usually survives • The remaining megaspore undergoes repeated mitosis to produce 8 haploid nuclei enclosed within a 7-celled embryo sac Pollination • The first step in uniting the sperm with the egg to get pollen germinating on the stigma and growing toward the embryo sac • Pollen may be carried to the flower by wind, or animals, or may originate within the original flower itself Pollination in Early Seed Plants and Pollination by Wind • Early seed plants were pollinated passively by the wind • Like present-day conifers, great quantities of pollen were shed and blown about, occasionally reaching ovules of the same species • Individual plants grew closely together to make this system work • Many angiosperms are also pollinated this way such as oaks, birches, cottonwoods, grasses, sedges and nettles • The flowers of these plants are small, greenish, and odorless, and their corollas are reduced or absent • Many wind-pollinated plants have stamen- and carpelcontaining flowers separated to promote outcrossing Pollination by Bees & Insects • Flowers that bees characteristically visit and thus pollinate tend to be blue or yellow, and have stripes, or dots that indicate the location of the nectaries • Most bees visit flowers to obtain the pollen for food, but they also help pollinate the plant • Butterflies and moths also help pollinate flowers often Pollination by Birds and Other Animals • Hummingbirds and sunbirds commonly help pollinate plants • They tend to visit plants with large amounts of nectar because birds will not revisit plants if they do not find enough food for themselves • Flowers producing such large amounts of nectar have no advantage in being visited by insects because it could obtain enough food at one plant and it would not cross-pollinate • Red is highly visible to birds, though insects cannot see it, so birds will be very attracted to a red plant that insects will bypass. It is also seen in red fruits dispersed by birds • Bats, rodents and monkeys also help disperse pollen Self-Pollination • No outcrossing involved • Most self-pollinating angiosperms have small, inconspicuous flowers that tend to shed pollen directly onto the stigma • There are two basic reasons for the frequent occurrence of self-pollinated angiosperms 1. It is advantageous under certain circumstances because it does not rely on pollinators, and expend less energy trying to attract an animal or insect 2. Self-pollinating plants produce progenies that are more uniform than those that outcrossed, Because meiosis is involved, recombination still takes place, and the offspring will not be an exact copy of the parent. However, such progenies will likely contain high proportions of individuals that are well-adapted to their habitat.Self-pollination in normally outcrossing species tends to produce large numbers of ill-adapted individuals because it brings together bad recessive alleles, but some of these combinations may be advantageous in particular habitats, and it could be advantageous for the plant to continue self-pollinating indefinitely. Factors That Promote Outcrossing • One strategy to promote outcrossing is to separate stamen and pistils to avoid self-pollination. It could be separated by flower of the plant, called monoecious, ("one house") or entirely by plant. Plants that have only ovules or only pollen are called dioecious • If ovules and pollen both occur on every flower of a species of plant, they are called dichogamous. These plants can promote outcrossing by maturing the male and female reproductive parts at different times. If the stamens mature first in these plants, the flower is effectively staminate at that time. Once the pollen is shed and the stigma becomes receptive, the flower is essentially pistillate. Self-Incompatibility • Even when the stamen mature at the same time, genetic self-incompatibility can prevent self-pollination • Self-incompatibility results when the pollen and stigma recognize each other as related and pollen tube growth is blocked • Self-incompatibility is controlled by the S locus allele • Two types of self- incompatibility: Gametophytic depends on haploid S locus of pollen and diploid S locus of stigma. If either of the S alleles in the stigma matches the pollen S alleles pollen tube growth stops before it reaches the sac • Sporophytic self-incompatibility: both S alleles of the pollen are important; if the alleles in the stigma match either of the pollen parent alleles, the haploid pollen will not germinate Fertilization double fertilization-fertilization process used in angiosperms where two sperm cells are released. angiosperms-flowering plants -Two sperm cells are released. One sperm cell fertilizes the egg and makes a zygote, while the other sperm eventually creates endosperm. -The endosperm will nourish the zygote. -Hence, the egg is fertilized and also has the endosperm to live off of. Fertilization Asexual Reproduction Types of asexual reproduction:Vegetative reproduction and Apomixis -Vegetative Reproductionnew plants cloned from parts of the adult plant. Types of Vegetative Reproduction: -runners-thin stems that run from the parent plant to a new location along surface of ground, the tip of the stem eventually forms roots and becomes a new plant. Asexual Reproduction -rhizomes-underground stems, infiltrate ground around parent plant; create a new shoot to become new plant. Ex. grasses, potatoes, and ginger -suckers-roots or plants produce suckers, new plants sprout of the parent roots. Ex. apple trees, blackberries, cherry trees Asexual Reproduction -adventitious plantlets-the leaves are reproductive, new plants are made fromt he tissue in the notches of the parent leaves -Apomixis-parent plant makes seeds that are genetically thesame as the parent plant. Plant Tissue Culture -Plants can also be cloned by using plant cells or tissues and growing it in a nutrient medium with growth hormones, this is a form of asexual reproduction -These individual cells can develop into entire plants -The cells can also changed into protoplasts. -protoplasts-plant cell enclosed only by its cell membrane, the cell wall has been removed by enzymes Plant Tissue Culture -Protoplasts of different plants can be bound together to form a hybrid. These protoplasts then can grow into whole plants. -They form hybrids that otherwise wouldn't have happened. -Protoplasts allow another option of genetically engineering plants. Life Span of Plants -Plants are either annuals, biennials, or perennials. -Annual plants-live for only one growing season. Most crops are annuals. Biennial plants-have two year life cycles. They store energy their first year, and flower and use the stored energy the second year. The energy is kept underground in the roots. -All biennial plants flower only once before they die, though they usually do not flower until they are 3 years or older. ex. Carrots, cabbage, and beets. Perennial Plants-continuously grow for many years. Energy is often stored in a large root system. ex. trees and prarie wildflowers Abscission -Abscission-the process where leaves or petals are shed -Areas that take up too much nutrients can be disposed of. Ex. When flowers lose their petals after being pollinated. Abscission -Abscission occurs in the abscission zone. -abscission zone-where the leaf or petal breaks off -two protective layers form at the abscission zone; a protective layer on the stem side, and a seperation layer on the discarded leaf or petal. -Enzymes help further seperate the cells; wind, rain, or other environmental factors will cause the leaf to finally fall Thank You! ☺