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Chapters 36, 37 & 38 Lecture 11 Regulation of Plant Growth Dr. Chris Faulkes Plant Growth Aims: • To introduce the minerals required by plants • To introduce plant hormones • To examine the life cycle of a plant Plant Growth Aims: • To introduce the minerals required by plants • To introduce plant hormones • To examine the life cycle of a plant These lecture aims form part of the knowledge required for learning outcomes 1 and 3: Describe the structure, diversity and reproductions of selected plants and animal groups (LOC1) and Describe mechanisms for the life processes (LOC3). Plant Growth Essential reading • page 780-782 (Chapter section 36.1) • pages 796-801 (Chapter section 37.1) • pages 818-824 (Chapter section 38.1) • 36.1 How Do Plants Acquire Nutrients? 36.1 How Do Plants Acquire Nutrients? Nutrients: the major ingredients for macromolecules: carbon, hydrogen, oxygen, nitrogen. Carbon enters the living world from CO2 in the atmosphere through fixation by photosynthesis. Hydrogen and oxygen enter as water. These three elements are in plentiful supply. Nitrogen is in relatively short supply. 36.1 How Do Plants Acquire Nutrients? Mineral nutrients required include sulfur for proteins and phosphorus for nucleic acids. Chlorophyll has magnesium, many compounds require iron. In the soil, these and other minerals are dissolved in the soil solution. Plants take up most minerals from the soil solution in ionic form. 36.1 How Do Plants Acquire Nutrients? Autotrophs: make their own organic molecules from simple inorganic nutrients. Plants, some protists, and some bacteria, are autotrophs. Heterotrophs required pre-formed organic molecules and depend directly or indirectly on autotrophs for food. 36.1 How Do Plants Acquire Nutrients? Most autotrophs are photosynthesizers; light is the source of energy to synthesize organic compounds. Some are chemolithotrophs, using energy from reduced inorganic compounds such as H2S. All are bacteria. 36.1 How Do Plants Acquire Nutrients? Plants cannot move around to obtain nutrients, but they can extend themselves by growth. As roots grow through the soil, they mine it for new sources of minerals and water. Growth of leaves helps plant obtain more sunlight and CO2. 36.2 What Mineral Nutrients Do Plants Require? Two categories of essential elements: • Macronutrients: need at least 1 g per kg of dry plant matter. • Micronutrients: need less than 100 mg per kg. Table 36.1 Mineral Elements Required by Plants 37.1 How Does Plant Development Proceed? • 37.1 How Does Plant Development Proceed? Plant development is regulated in complex ways. Four factors regulate growth: • Environmental cues • Receptors such as photoreceptors • Hormones • The plant s genome 37.1 How Does Plant Development Proceed? Much recent progress in understanding plant growth and development has come from studies of Arabidopsis thaliana. It is used as a model organism— it is small, matures quickly, it s genome is small and has been fully sequenced. Mutants provide insights into mechanisms of hormones and receptors. 37.1 How Does Plant Development Proceed? Hormones: regulatory chemicals that act at low concentrations at sites often quite distant from where they were produced. Each plant hormone is produced in many cells, and has multiple roles. Interactions can be complex. Table 37.1 Plant Growth Hormones 37.1 How Does Plant Development Proceed? Photoreceptors are involved in many developmental processes. They are pigments (molecules that absorb light) associated with proteins. Light acts directly on photoreceptors, which regulate processes of development. 37.1 How Does Plant Development Proceed? Plants make use of signal transduction pathways—sequences of biochemical reactions by which a cell responds to a stimulus. Protein kinase cascades amplify responses to signals just as they do in other organisms. 37.1 How Does Plant Development Proceed? Seeds are dormant—the cells do not divide, expand, or differentiate. As the seed begins to germinate, it takes up water. The growing embryo obtains chemical building blocks by digesting the food stored in the seed. Germination is completed when the radicle (embryonic root) emerges. Now called a seedling. 37.1 How Does Plant Development Proceed? Imbibition, or uptake of water, is the first step in seed germination. A seed s water potential is very negative; water will enter if the seed coat is permeable. Expanding seeds exert tremendous force. Enzymes are activated with hydration, RNA and proteins are synthesized and respiration increases. Initial growth is by expansion of pre-formed cells. Figure 37.1 Patterns of Early Shoot Development Early seedling development varies in monocots and eudicots 37.1 How Does Plant Development Proceed? Formation of flowers may be initiated when plant reaches a certain size or age. Some plants flower at certain times of the year; plant must be able to distinguish seasons. Light absorption by photoreceptors is the first step in measuring time. Hormone signals then trigger flowering. Hormones also control growth of pollen tube, fertilization, and fruit and seed development. 37.1 How Does Plant Development Proceed? Some plants are perennials; they continue to grow year after year. Annuals complete life cycle in a single year, then senesce (deteriorate due to aging) and die. Senescence is controlled by hormones such as ethylene. In some perennials, leaves senesce and fall at the end of the growing season. Leaf fall is regulated by interplay of ethylene and auxin. 37.1 How Does Plant Development Proceed? Annuals must cope with year to year variation in rainfall and other factors. Seeds may remain dormant in unfavorable years. Other seeds germinate at specific times of year to ensure correct conditions. Dormancy can also ensure that seeds only germinate in specific environments. 37.1 How Does Plant Development Proceed? Until the seedling can photosynthesize, it depends on food reserves in the cotyledons or endosperm. Starch, lipids, and proteins must be broken down by enzymes into monomers that can enter the embryo s cells. 38.1 How Do Angiosperms Reproduce Sexually? • 38.1 How Do Angiosperms Reproduce Sexually? In angiosperms, flowers contain the sex organs. Nearly all reproduce sexually, many reproduce asexually as well. Sexual reproduction produces new gene combinations and diverse phenotypes. Asexual reproduction produces clones of genetically identical individuals. 38.1 How Do Angiosperms Reproduce Sexually? In agriculture, both are important. Many annual crops are grown from seeds (e.g., the grains (grasses), soybean, etc. Other crops such as strawberries, potatoes, and bananas, are produced asexually. Navel oranges are seedless, and must be propagated asexually. All descend from one mutant tree. 38.1 How Do Angiosperms Reproduce Sexually? A complete flower has four groups of organs (modified leaves): carpels, stamens, petals, sepals. Carpels are female sex organs. A pistil is composed of one or more carpels. Base of pistil is the ovary, containing ovules, each of which contain a megasporangium. Female gametophyte develops in the megasporangium. 38.1 How Do Angiosperms Reproduce Sexually? Stamens are the male sex organs. Each has a filament with a two-lobed anther, containing four fused microsporangia. The male gametophyte develops here. 38.1 How Do Angiosperms Reproduce Sexually? Petals make up the corolla. They are often colored to attract pollinators. Sepals make up the calyx. All the flower parts are attached to the stem tip or receptacle. 38.1 How Do Angiosperms Reproduce Sexually? Angiosperms have alternation of generations: multicellular diploid generation alternates with a multicellular haploid generation. The diploid sporophyte generation produces flowers. The flowers produce haploid spores. Spores develop into the haploid gametophyte generation. Figure 38.1 Development of Gametophytes and Nuclear Fusion 38.1 How Do Angiosperms Reproduce Sexually? Megagametophytes (female) develop in megasporangia, called embryo sacs. Microgametophytes (male) develop in microsproangia, called pollen grains. 38.1 How Do Angiosperms Reproduce Sexually? Within an ovule, a megasporocyte undergoes meiosis to produce four haploid megaspores—all but one degenerates. The surviving megaspore undergoes three mitotic divisions to produce eight haploid nuclei. Cell wall formation leads to a gametophyte with seven cells. 38.1 How Do Angiosperms Reproduce Sexually? At one end of the gametophyte are three cells—egg and two synergids. Synergids attract the pollen tube and receive the sperm nuclei. Three antipodal cells at the opposite end usually degenerate. The central cell has two polar nuclei, together they combine with one sperm nucleus. 38.1 How Do Angiosperms Reproduce Sexually? In the anther, a microsporocyte undergoes meiosis to produce four haploid microspores. Each develops a spore wall and divides mitotically to form two haploid cells in each pollen grain (microgametophyte). The two cells are the tube cell and generative cell. 38.1 How Do Angiosperms Reproduce Sexually? Pollination: transfer of pollen from anther to stigma. Many mechanisms have evolved for pollen transport. In some plants, such as peas, self pollination occurs before the flower opens, resulting in self-fertilization. When pollen is transferred to a different individual, it is called cross-pollination. 38.1 How Do Angiosperms Reproduce Sexually? In many species, wind transports pollen. The flowers have sticky or featherlike stigmas, and pollen grains are produced in great numbers. In some aquatic plants, pollen is carried by water. Many species are pollinated by animals, including insects, birds, and bats. 38.1 How Do Angiosperms Reproduce Sexually? Some plants reject pollen from their own flowers—self-incompatibility. This promotes outcrossing between different genotypes. The S gene is responsible for selfincompatibility. It has many alleles. If S allele in pollen matches either S allele in pistil, the pollen grain fails to grow. Figure 38.3 Self-Incompatibility 38.1 How Do Angiosperms Reproduce Sexually? The stigmas of most plants are exposed to pollen of many species. Strong cell–cell signaling between pollen and stigma from the same species binds the pollen to the stigma. Foreign pollen drops off or fails to germinate. 38.1 How Do Angiosperms Reproduce Sexually? Germination of the pollen grain involves the growth of the pollen tube. The pollen tube grows through the style to reach the ovule. Downward growth is guided by a chemical signal released by the synergids. 38.1 How Do Angiosperms Reproduce Sexually? The pollen tube grows through the megasporangium and reaches the embryo sac. The generative cell undergoes one mitotic division to produce two haploid sperm cells. Both sperm cells enter a synergid, which degenerates and releases the sperm cells. 38.1 How Do Angiosperms Reproduce Sexually? Double fertilization: • One sperm cell fuses with the egg cell, forming a diploid zygote. This divides mitotically to produce the sporophyte embryo. • The other sperm cell fuses with the two polar nuclei in the central cell, forming a triploid nucleus. This divides by mitosis to form the nutritive endosperm. Figure 38.5 Double Fertilization 38.1 How Do Angiosperms Reproduce Sexually? First mitotic division of the zygote results in asymmetric division of the cytoplasm. One daughter cell becomes the embryo, the other becomes a supporting structure, the suspensor. Asymmetric division establishes polarity and the longitudinal axis of the new plant. Figure 38.6 Early Development of a Eudicot 38.1 How Do Angiosperms Reproduce Sexually? In eudicots, the embryo develops into a heart stage as the cotyledons begin to form. Further elongation gives rise to the torpedo stage. The shoot apex forms between the cotyledons; the root apex forms at the other end. They contain meristematic cells. 38.1 How Do Angiosperms Reproduce Sexually? The endosperm accumulates starch, lipids, and proteins. In some species the cotyledons absorb nutrients from the endosperm and become much larger than the embryo. In other species, the cotyledons remain thin, and use nutrients from the endosperm during germination. Figure 38.7 Variety in Angiosperm Seeds 38.1 How Do Angiosperms Reproduce Sexually? In later stages of development, the seed loses water and becomes dormant. Integuments: tissues surrounding the megasporangium—develop into the seed coat. The carpel becomes the wall of the fruit that surrounds the seed. Plant Growth Check out • 36.1 RECAP, page 782 • 36.1 CHAPTER SUMMARY, page 793 • 37.1 RECAP, page 801 • 37.1 CHAPTER SUMMARY, page 815, see web/CD activities 37.1 and 37.2 • 38.1 RECAP, page 825, question 1 only • 38.1 CHAPTER SUMMARY, page 834, see web/CD activity 38.1 Plant Growth Self quiz • page 794: Chapter 36 question 1 • page 815: Chapter 37 questions 1-3 • page 835: Chapter 38 questions 1-5 For Discussion • page 816: Chapter 37 questions 1-2 Plant Growth Key terms: absicic acid, annual, anther, antipodal cells, autotrophs, auxin, calyx, carpels, chemolithotrophs, corolla, cotyledon, diploid, dormancy, embryo, endosperm, ethylene, flower, fruit, gametophyte, germination, gibberellin, haploid, heterotrophs, hormones, imbibition, integuments, macronutrients, megagametophtres, megasporangium (pl. megasporangia), microenvironment, microgametophytes, micronutrients, microsporangium (pl. microsporangania), ovule, perennial, petals, photoreceptors, pistil, pollen, pollination, radicle, seed coat, seedling, senescence, sepals, sessile, signal transduction pathways, stamens, stigma, suspensor, synergids