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Topic 9 Plant Biology Part II Why? We love plants 9.3 Growth in plants Nature of science: Developments in scientific research follow improvements in analysis and deduction—improvements in analytical techniques allowing the detection of trace amounts of substances has led to advances in the understanding of plant hormones and their effect on gene expression. (1.8) Understandings • Undifferentiated cells in the meristems of plants allow indeterminate growth • Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves • Plant hormones control growth in the shoot apex • Plant shoots respond to the environment by tropisms • Auxin efflux pumps can set up concentration gradients of auxin in plant tissue • Auxin influences cell growth rates by changing the pattern of genes 9.3 Growth in plants Applications and Skills • Application: Micropropagation of plants using tissue from the shoot apex, nutrient agar gels, and growth hormones • Application: Use of micropropagation for rapid bulking up of new varieties, production of virus-free strains of existing varieties and propagation of orchids and other rare species 9.4 Reproduction in plants Nature of science: Paradigm shift—more than 85% of the world’s 250,000 species of flowering plant depend on pollinators for reproduction. This knowledge has led to protecting entire ecosystems rather than individual species. (2.3) Understandings • Flowering involves a change in gene expression in the shoot apex • The switch to flowering is a response to the length of light and dark periods in many plants • Success in plant reproduction depends on pollination, fertilization, and seed dispersal • Most flowering plants use mutualistic relationships with pollinators in sexual reproduction 9.4 Reproduction in plants Applications and Skills • Application: Methods used to induce short-day plants to flower out of season • Skill: Drawing internal structure of seeds • Skill: Drawing of half-views of animal-pollinated flowers • Skill: Design of experiments to test hypotheses about factors affecting germination Meristems • There are two types of meristems 1. Apical meristems 2. Lateral meristems • Apical meristems: primary meristems • Lateral meristems: cambium • Meristems generate new cells for growth of the plant throughout the plant’s life • This property is termed indeterminate growth • Two main types of indeterminate growth: 1. Primary growth 2. Secondary growth Primary Growth • Primary growth allows roots to extend throughout the soil and shoots to increase exposure to light and carbon dioxide • This is performed by the apical meristems located at the tips of roots and in the buds of shoots • Apical meristems are found in all flowering plants and enable the plant to grow in length Secondary Growth • Performed by lateral meristems and is only present in dicots • Growth in thickness = secondary growth • Caused by activity of lateral meristems called the vascular cambium and cork cambium • Extend along the length of roots and stems • Vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem • Cork cambium replaces the epidermis with periderm, which is thicker and tougher • Mitosis and cell division in the shoot apex (apical meristem) provide cells needed for extension of the stem and development of leaves • Some cells always remain in the meristem and continue through the cell cycle • With each division, one cell remains in the meristem while the other increases in size and differentiates as it is pushed away from the meristem region • These cells are displaced to the edge of meristem Things apical meristem can give rise to: • Protoderm (epidermis); Procambium (vascular tissue); ground meristem (pith) • Cells at edge stop dividing and undergo growth and differentiation to become either stem or leaf tissue • Leaves are initiated as small bumps at the side of the apical dome and are called leaf primordia Growth and Plant Hormones • Plant development involves extensive coordination among individual cells within a plant • This requires communication within the plant • Many factors affect plant development and growth Environmental factors such as day length and water availability Receptors which allow the plant to detect certain environmental factors Genetic makeup of plant Hormones which are chemical messengers Plant Hormones • Plant hormones have varying effects depending on the receptor’s location in the plant • There is often a great deal of interaction between the different hormones to bring about the most appropriate response • In plants, a hormone may be produced throughout the plant, not just in an individual organ Auxin and Phototropism • Tropism is the growth response that results in curvatures of whole plant organs toward or away from stimuli • Growth of a shoot toward light is called positive phototropism and gravitropism is growth in response to gravitational forces • Phototropism was discovered from studies of grass seedlings (oats) • Think photo= light, tropism= growth thus growth toward light • Shoot of a sprouting grass seedling is enclosed in a sheath called the coleoptile • Coleoptile grows straight up if kept in dark or lit uniformly from all sides • If illuminated from one side, it grows toward light as a result of a differential growth of cells on opposite sides of the coleoptile • Cells on darker side elongate faster than cells on the brighter side Phototropism Auxin and Phototropism • Both phototropism and gravitropism are auxin dependent • First, light is absorbed by photoreceptors • Proteins within these called phototrophins absorb the light • When phototrophins absorb a light of a certain wavelength they will change conformation and will bind to receptors and induce transcription of certain genes • These genes most likely code for glycoproteins in the plasma membrane of the cells in the stem that transport auxin • These glycoproteins are called PIN3 proteins • The type and position of PIN3 proteins is variable to the where auxin is needed • Example: light is shining on the tip of a plant stem more intensely on one side vs the other • Phototrophins will detect the light on one side of the stem and auxin will be transported laterally to the side receiving LESS light • Thus there is a higher concentration of auxin on the shaded side • This will cause greater growth of the cells on the shadier side which will cause the stem to curve toward the light allowing the leaves to receive more light Auxin and Phototrophism Auxin • Auxin – any chemical substance that promotes elongation • It was purified by Kenneth Thimann at Cal Tech and determined to be indoleacetic acid (IAA) • Auxins are found in the embryo of seeds, the meristems of apical buds, and young leaves • IAA is produced by all cells in the stem on the side towards the light • It is transported into the nuclei of cells on the side of the stem opposite the light • Caused by distribution of auxin transport proteins (auxin efflux carriers) in the cell. Synthesized by apical meristem of a shoot • The auxin and a receptor in the nuclei form a complex that activates a proton (hydrogen ion) pump • Proton pumps play a major role in growth response of cells to auxin. Auxin stimulates plasma membrane’s proton pumps • Proton pump moves hydrogen ions into the spaces of the cell wall • Pumping of H+ increases voltage across membrane and lowers the pH in cell wall which activates enzymes called expansins that break the cross-links (hydrogen bonds) between cellulose microfibril. Increased ion uptake, uptake of water, increased turgor, increased cell wall plasticity • This results in the elongation of the cells on the side away from the light • Auxin also alters gene expression and causes new protein production Auxins have many functions within a plant • Stimulation of cell division in most meristematic tissues • Differentiation of xylem and phloem • Development of lateral roots in tissue culture • Suppression of lateral bud growth when present in the apical bud • Stimulation of growth of flower parts • Induction of fruit production without pollination Auxin and Gravitropism • The upward and downward growth of a plant depends on gravity • If a root is placed on its side, gravity will cause organelles called statoliths to accumulate on the lower side of the cells • PIN3 proteins will direct auxin transport to the bottom cells • High amounts of auxin in the roots will inhibit cell elongation • The top cells elongate faster than the bottom cells causing the roots the bend down • So… this is opposite of how auxin works in the stem Application: Micropropagation of plants • Micropropagation is an in vivo procedure that produces large numbers of identical plants. Much faster and takes less space than traditional methods of propagation 1. Cells from the shoot apex are cultured on nutrient agar. (derived from plant with some desirable feature). Depends on totipotency (ability of a cell to divide and produce all cells in an organism) of plant tissues 2. Growth hormones are used to achieve maximum growth and quantity of particular types of plants (auxin and cytokinin- plant growth hormones, phytohormones, that promote cell division in plant roots and shoots) 3. Rare and endangered species can be maintained Micropropagation 4. Used in the case of orchids, which have very small seeds, to grow plants more reliably in sterile cultures 5. Must maintain pathogen-free environment when culturing meristematic tissue 6. Has been used to develop virus-free strains of existing plants. This allows international exchange of plant materials 7. Can be used in conjunction with gene transfer creating GMO plants 8. Micropropagation is very expensive Micropropagation 9.4 Reproduction in plants 9.4 Reproduction in plants Nature of science: Paradigm shift—more than 85% of the world’s 250,000 species of flowering plant depend on pollinators for reproduction. This knowledge has led to protecting entire ecosystems rather than individual species. (2.3) Understandings • Flowering involves a change in gene expression in the shoot apex • The switch to flowering is a response to the length of light and dark periods in many plants • Success in plant reproduction depends on pollination, fertilization, and seed dispersal • Most flowering plants use mutualistic relationships with pollinators in sexual reproduction 9.4 Reproduction in plants Applications and Skills • Application: Methods used to induce short-day plants to flower out of season • Skill: Drawing internal structure of seeds • Skill: Drawing of half-views of animal-pollinated flowers • Skill: Design of experiments to test hypotheses about factors affecting germination Flower Structure Skill: Drawing of half-views of animal-pollinated flowers • Review guide: Lamium album, Plum flower, Azalea Reproductive parts of flowering plants • Flowers are specialized shoots bearing the reproductive organs of the angiosperm sporophyte (2N stage… do you remember what 2N is?) • Female gametes are contained in ovules in the ovaries of the flower • Pollen grains are produced by stamens contain the male gametes • The 4 kinds of flower organs, in sequence from the outside to the inside of the flower are the sepals, petals, stamens, and carpels Stamen • Stamen are the male reproductive organ consisting of a stalk called a filament and a terminal structure called the anther • Think staMEN MEN MALE • The male gametophytes (1N stage) are sperm-producing structures called pollen grains which form within the pollen sacs of anthers Carpel • Carpel is the female reproductive organ and has ovaries at its base and a slender neck called the style • At the top of the style is a sticky structure called the stigma that serves as a landing platform for pollen • Within the ovary are one or more ovules • The female gametophytes are eggproducing structures called embryo sacs which form within the ovules in ovaries Sepals and Petals • Non-reproductive structures • Sepals enclose and protect the floral bud before it opens and are usually green and more leaf-like in appearance than the other floral organs • Petals are more brightly colored than sepals and advertise the flower to insects and other animal pollinators Fertilization • Fertilization definition- the formation of a zygote by the union of a male gamete with a female gamete inside the ovule • For the egg within the embryo sac to be fertilized the male and female gametophytes must meet and their gametes must fuse Pollination • Pollination definition- the transfer of pollen from an anther to a stigma • Pollination occurs when pollen, released from anthers and carried by wind or animals, lands on a stigma (not necessarily on the same flower or plant) • Each pollen grain produces a structure called a pollen tube, which grows down into the ovary via the carpel and discharges sperm nucleus into the embryo sac resulting in fertilization of the egg • The pollen tube completes its growth by entering an opening (micropyle) at the bottom of the ovary • The sperm moves from the tube to combine with the egg of the ovule to form a zygote Pollen Mutualism regarding pollination • 85% of the world’s 250,000 species of flowering plants depend on insects or other animal pollinators for reproduction • Appears that the first angiosperms were pollinated by insects. Convincing fossil evidence that angiosperms and insects coevolved • Most flowering plants use mutualistic relationships with pollinators in sexual reproduction Mutualism • Plant benefits- flowers are pollinated so that they form seeds and pass on their genes • Pollinators benefit- obtains nectar which is a source of energy and pollen, which is a source of protein • Co-evolution often entails developing a mutualistic relationship for pollination with one specific species of insect • Example: Vanilla orchid pollinated by a species of Melipona bee. Advantage: insect will transfer pollen from flower to flower of the species and not to other species • Orchid mimics bee and has scent to attract males How does pollinator find flower? • Flowers can employ various means to attract their vector • Red flowers are conspicuous for birds • Yellow and orange flowers are noticed by bees • Heavily scented flowers can be located at night • If wind pollinated, often have inconspicuous, odorless flower parts Fertilization • Actually a double fertilization • One of the two sperms produced by the pollen grain combines with the egg • The other combines with two polar nuclei within the ovary to produce a triploid (3N) endosperm • The endosperm has the function of storing nutrients for the young plant Reproductive Adaptations: Fertilized ovules develop into seeds • Include reduced gametophyte, advent of the seed, and the evolution of pollen • A seed consists of a sporophyte embryo packaged along with a food supply within a protective tough coat • Ovaries containing fertilized ovules develop into fruits Skill: Drawing internal structure of seeds Bean Seed • Parts of the seed include: 1. Seed coat called the testa 2. Embryo root or radicle 3. Embryo shoot or epicotyl which has at its tip 4. Plumule which consists of the shoot tip with a pair of miniature leaves 5. Cotyledons • The cotyledons of the bean plant are fleshy before the seed germinates because they absorbed food from the endosperm when the seed developed • The endosperm is a food-storing tissue of the seed • Hypocotyl is part of the developing stem External Structure For this class you will be expected to DRAW internal AND external structures of a seed!! Hilum- scar on seed where it was attached to seed vessel Internal Structure Internal Structure Embryotic Development • The zygote gives rise to an embryo, and as the embryo grows, the ovule that contains it develops into a seed • The entire ovary, meanwhile, develops into a fruit containing one or more seeds • Fruits, carried by wind or by animals, help disperse seeds some distance from their source plants • So short and sweet… the function of fruit is to disperse seeds Germination • As a seed matures, it dehydrates and enters a phase referred to as dormancy, a condition of extremely low metabolic rate and a suspension of growth and development • Conditions required to break dormancy vary between plant species • Some seeds germinate as soon as they are in a suitable environment • Others remain dormant and will not germinate, even if sown in a favorable place, until favorable environmental cue causes them to break dormancy Breaking dormancy • Desert plants germinate only after a substantial rain • Where natural fires are common, many seeds require intense heat to break dormancy • Where winters are harsh, seeds may require extended exposure to cold • Some require light for germination • Some seeds have coats that must be weakened by chemical attack as they pass through an animal’s digestive tract General Factors Needed for Seed Germination 1. Water to rehydrate the dry tissues of the seed 2. Oxygen for aerobic cell respiration • Some seeds respire anaerobically if oxygen is not available but ethanol produced in anaerobic respiration usually reaches toxic levels 3. Temperature suitable for germination • Since germination involves enzymes, temperature cannot be too low (low collision of enzyme’s active site with substrate) or too high (denaturation of enzyme). Seeds stay dormant when temps are too high or too low Metabolic events in germination 1. 2. 3. 4. 5. 6. Seed absorbs water Gibberellic acid is produced by the embryo Gibberellic acid stimulates production of amylase Amylase catalyzes the breakdown of starch to maltose Maltose diffuses to embryo Maltose is used for energy production and growth Skill: Design of experiments to test hypotheses about factors affecting germination • Be aware of safety • Some of the factors you should consider- temperature, plant hormones, salt concentrations, light, wavelengths of light, water levels, possible toxins (heavy metals, pesticides) • Independent, dependent (length of shoot or root at X days), controlled variables (temp., oxygen, water, # seeds/dish/container, amount of time, method of measurement) • Hypothesis • Materials and procedures • Data collected and what method of analysis (t-test, regression, etc) Metabolic events during germination • Imbibition- the uptake of water causes the seed to expand and rupture its coat and also triggers metabolic changes in the embryo that enable it to resume growth • Gibberellin production- the plant hormone, gibberellin, is synthesized in the cotyledons of the seed. • Gibberellin stimulates the production of the digestive enzyme, amylase, which catalyzes the digestion of starch into maltose in the food stores of the seed More metabolic events • Maltose (like any good disaccharide used for transport) is transported from the food stores to the growth regions of the seedling, including the embryo root and the embryo shoot • Maltose conversion- maltose is converted to glucose, which is either used in aerobic cell respiration as a source of energy, or is used to synthesize cellulose or other substances needed for growth • Photosynthesis kicks in as soon as the leaves of the seedling have reached light and have opened. Photosynthesis supplies the seedling with food and the food stores of the seed are no longer needed 9.4.2 Control of flowering • Plants can detect not only the presence of light but also its direction, intensity, and wavelength (color) • Two major classes of light receptors 1. Blue-light photoreceptors 2. Phytochromes- photoreceptors that absorb mostly red light Phytochromes • Consists of two identical proteins joined to form one functional molecule • Each of these proteins has two domains 1. Photoreceptor- covalently bonded to a nonprotein pigment or chromophore 2. Kinase- photoreceptor domains interact with kinase domains to link light reception to cellular responses triggered by the kinase • Chromophore of a phytochrome is photoreversible meaning it can be converted back and forth between 2 isomers • Pr is inactive absorbs red light of wavelength 660 nm or white light (400-700 nm) • Pfr absorbs far-red light 730 nm • When Pr absorbs red it is converted to Pfr • When Pfr absorbs far-red light, it is converted back to Pr • In normal sunlight: Pr rapidly converted to Pfr • Pr is more stable than Pfr, so in darkness Pfr very gradually changes into Pr Phytochrome Swing 9.4.1 Control of Flowering • When a plant has roots, stems, and leaves only vegetative phase • When a plant produces flowers reproductive phase • This change occurs when meristems in the shoot start to produce parts of flowers instead of leaves • This involves a change to the pattern of gene expression in cells produced by the meristem • In some plants, the trigger is a particular day length • Experiments done in growth chambers where the length of light and dark periods are controlled showed that it is the length of darkness that matters, not the length of daylight Long-day (short night) vs. short-day (long night) plants • Long-day plants: flower in summer when nights have become short enough (asters, lettuce, spinach, and potatoes) • Short-day plants: flower in autumn, when nights have become long enough (chrysanthemums, poinsettias, and Christmas cactus) • Pfr is the active form and binds to receptor proteins in cytoplasm. Pr does not bind. • In long-day plants, large enough amounts of Pfr remain at end of short nights to bind to the receptor, which then promotes transcription of genes needed for flowering. Pfr is a promoter in these plants • In short-day plants, the receptor inhibits the transcription of genes needed for flowering when Pfr binds to it. Pfr is an inhibitor in these plants • However, at the end of long nights, very little Pfr remains, so the inhibition fails and the plant flowers Application: Methods used to induce shortday plants to flower out of season • Example: Poinsettias, Chrysanthemum • Can maintain plants in pots in greenhouses with blinds • When the nights are not long enough to induce flowering, the blinds are closed to extend the nights artificially • Some growers will cover individual plants with black cloth for 12-15 hours a day until the flower buds begin to show color Detecting Plant Hormones • Hormones in plants, like hormones in animals, are regulators of gene transcription (transcription factors) • Concentrations at which plant hormones are active can be as low as picograms/gram of plant tissue (1 million millionth of a gram) • Five groups of hormones which are very diverse and require different extraction methods Methods- ELISA • ELISA (Enzyme linked immunosorbent assays)- an antibody is synthesized to the hormone (which acts as the antigen) • A primary antibody will bind to a sample of phloem or tissue that has been absorbed onto a plastic well • A secondary antibody that has an enzyme linked to it will bind to the primary antibody/hormone complex • The substrate of the enzyme is added so that there is a color change • The color is read and interpreted • If you inject an antigen (hormone) into an animal (goat, rabbit, mouse) the animal will make antibodies from B lymphocytes which can be cultured and antibodies extracted. These antibodies are used to detect the “antigen” Methods- Gas/Liquid Chromatography • Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition Methods- Microarray • Microarray analysis detects changes in the pattern of gene expression • Begins by isolating RNA samples from cells and synthesizing DNA copies • Hybridizing cDNAs linked to fluorescent dyes to small surface with millions of DNA probes • Laser light makes the fluorescent dyes give off light