Transcript Chapter 9
Chapter 9 Lecture Outline Growth and Development Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Outline Introduction Nutrients, Vitamins, and Hormones Hormonal Interactions Other Hormonal Interactions Plant Movements Photoperiodism Cytochromes and Cryptochromes A Flowering Hormone? Temperature and Growth Dormancy and Quiescence Introduction Growth - Irreversible increase in mass due to division and enlargement of cells • Determinate growth - Plant grows, stops growing and dies in one season. • Indeterminate growth - Plant or parts of plant grow and continue to be active for several to many years. Differentiation - Cells develop different forms adapted to specific functions. Development - Coordination of growth and differentiation of a single cell into tissues and organs Nutrients, Vitamins, and Hormones Nutrients - Furnish elements and energy for plant growth and maintenance • Obtained from air and soil Vitamins - Organic molecules that participate in catalyzed reactions, mostly by functioning as electron acceptors or donor • Synthesized in cell membranes and cytoplasm • Required in small amounts for normal growth and development Nutrients, Vitamins, and Hormones Hormones - Production dictated by genes. • Mostly produced in actively growing regions • Produced and active in smaller amounts than vitamins and enzymes Plant growth response results form signal transduction pathway . • Signaling molecule binds to receptor releasing messenger molecule • Eventually enzyme is produced, catalyzing reaction and responding to environmental cue Nutrients, Vitamins, and Hormones Hormones can have multiple effects. Hormones act by chemically binding to specific receptors. • Hormone-receptor association initiates effect. • Triggers series of biochemical events, including turning genes on and off – Biochemical events = Signal transduction Major types of hormones: auxins, gibberellins, cytokinins, abscisic acid, ethylene Nutrients, Vitamins, and Hormones Plant Hormones Auxins • Auxin production occurs mainly in apical meristems, buds, young leaves and actively growing parts of plants. • Similar structure to amino acid, tryptophan • Plant responses vary according to concentration, location, and other factors. • Generally, monocots less sensitive than dicots and shoots less sensitive than roots. Nutrients, Vitamins, and Hormones Plant Hormones Auxins • Some effects include: – Stimulate enlargement of cells by increasing cell wall plasticity – – Trigger production of other hormones Cause dictyosomes to increase rate of secretion – Control some phases of respiration – Influence growth – Promote cell enlargement and stem growth, cell division in cambium, initiation of roots and differentiation of cell – Delay development processes such as fruit and leaf abscission, and fruit ripening – Inhibit lateral branching Nutrients, Vitamins, and Hormones Plant Hormones Auxins • Movement of auxins from cells where they originate requires energy expenditure. – Movement is polar - Away from source – Move through parenchyma cells surrounding vascular bundles Nutrients, Vitamins, and Hormones Plant Hormones Auxins • Natural occurring growth regulators include: • o Indoleacetic acid (IAA) o Phenylacetic acid (PAA) o 4-chloroindoleacetic acid (4-chloroIAA) o Indolebutyric acid (IBA) Synthetic growth regulators include: o Naphthalene acetic acid (NAA) o 2,4-dichlorophenoxy acetic acid (2,4-D) Nutrients, Vitamins, and Hormones Plant Hormones Gibberellins (GA) • Named after a fungus that produces it (Gibberella fujikuroi) • 110 currently known gibberellins • Movement is nonpolar. • Most dicots and a few monocots grow faster with an application of GA. – Dramatically increases stem growth • Involved in same regulatory processes as auxins Effect of gibberellins on cabbage Nutrients, Vitamins, and Hormones Plant Hormones Cytokinins • Regulate cell division • Synthesized in root tips and in germinating seeds • Movement is nonpolar. • If auxin present during cell cycle, cytokinins promote cell division by speeding up progression from G2 phase to mitosis phase. • Also play role in: – – – – – Cell enlargement Differentiation of tissues Development of chloroplasts Stimulation of cotyledon growth Delay of aging in leaves Nutrients, Vitamins, and Hormones Plant Hormones Abscisic acid (ABA) • Has inhibitory effect on stimulatory effects of other hormones • Synthesized in plastids from carotenoid pigments • Movement is nonpolar. • Common in fleshy fruits - Prevents seeds from germinating while still on plant • Helps leaves respond to excessive water loss – Interferes with transport or retention of potassium ions in guard cells, causing stomata to close Nutrients, Vitamins, and Hormones Plant Hormones Ethylene • Produced by fruits, flowers, seeds, leaves and roots • Produced from amino acid methionine • Can trigger its own production • Used to ripen green fruits – Production almost ceases in absence of oxygen. • Causes leaf abscission Ethylene from apple caused abscission of holly leaves Nutrients, Vitamins, and Hormones Other Hormones or Related Compounds Oligosaccharins • Released from cell walls by enzymes - Influence cell differentiation, reproduction, and growth in plants – Produce effects at concentrations 1000x less than auxins – Effects are highly specific and responses are same in all species. Brassinosteroids • Bind to receptors on cell surfaces • Affect apical dominance, gravitropism, seed germination Hormonal Interactions Apical dominance - Suppression of growth of lateral (axillary) buds • Believed to be brought about by auxin-like inhibitor in terminal bud • Strong in trees with conical shapes – Pines, spruces, firs • Weak in trees that branch more often – Elms, ashes, willows • If cytokinins applied in appropriate concentration to axillary buds, they will begin to grow, even in presence of terminal bud. Hormonal Interactions Senescence - Breakdown of cell components and membranes, eventually leading to death of cell • Some studies have suggested certain plants produce a senescence “factor.” • Not certain of precise mechanisms involved Other hormonal interactions • Root and shoot development in tissue culture regulated by auxins and cytokinins. • Seed germination regulated by gibberellins and ABA. Plant Movements Growth movements - Result from varying growth rates in different parts of an organ • Movements resulting primarily from internal stimuli: – – Nutations - Spiraling movements not visible to eye Nodding movements Side-to-side oscillations o In bent hypocotyl of bean - Facilitates progress of plant through soil Nutation Plant Movements Growth Movements Movements resulting primarily from internal stimuli: • Twining movements - Visible spiraling in growth – – Stems of flowering plants Morning glory Tendrils • Contraction movements – Contractile roots that pull roots deeper • Nastic movements - Non- directional • Epinasty - Permanent downward bending Tendril of manroot plant Plant Movements Growth Movements Movements resulting from external stimuli: • Tropisms - Permanent movements resulting from external stimuli – Growth of a plant toward or away from a stimulus – Can be divided into three phases: o Initial perception - Organ receives greater stimulus on one side. o Transduction - One or more hormones become(s) unevenly distributed across organ. o Asymmetric growth - Result of greater cell elongation on one side Plant Movements Growth Movements Movements resulting from external stimuli: • Phototropism - Growth movement toward or away from light – Positive phototropism - Toward light o – Negative phototropism - Away from light o – Shoots Roots either insensitive or negatively phototrophic. Auxin migrates away from light, and accumulates in greater amounts on opposite side, promoting greater elongation of cells on dark side. Plant Movements Growth Movements Movements resulting from external stimuli: • Gravitropism - Growth responses to stimulus of gravity – – – Primary roots - Positively gravitropic Shoots - Negatively gravitropic Gravity may be perceived by amyloplasts in root cap, by proteins on outside of plasma membrane, or by whole protoplast. o Auxin causes cell elongation that produces curvature of root. Negative gravitropism Plant Movements Growth Movements Movements resulting from external stimuli: • Other Tropisms: – Thigmotropism - Contact with solid object o Twining – Chemotropism - Chemicals o Germination of pollen grains – Thermotropism - Temperature o Horizontal stems when cold in some weeds – Traumotropism - Wounding – Electrotropism - Electricity – Skototropism - Dark – Aerotropism - Oxygen Plant Movements Turgor movements - Result from changes in internal water pressures and often initiated by contact with objects outside of plant • Leaf movement by pulvini – Pulvini - Special swellings at base of leaf o Sensitive plant, redwood sorrel – Turgor contact movements are not confined to leaves. o Many flowers exhibit movements of stamens and other parts, facilitating pollination. Sensitive plant Plant Movements Turgor movements “Sleep” movements Circadian rhythms • Regular daily cycles – Leaves or petals fold in regular daily cycles. o Members of the legume family, prayer plants • Turgor movements, and stimuli of light and temperature involved. • Controlled by a biological “clock” on approximately 24 hours cycles – Appear to be controlled internally Circadian rhythm in prayer plant Plant Movements Turgor movements Solar tracking – Heliotropism-Leaves often twist on their petioles in response to illumination and become perpendicularly oriented to light source. • Blades oriented at right angles to sun. Water conservation movements • Bulliform cells - Special thin-walled cells in leaves of many grasses that lose turgor and cause leaves to roll up or fold during periods of insufficient water Plant Movements Taxes (taxic movement) - Movement that involves entire plant or reproductive cells. • In several groups of plants and fungi, but not in flowering plants • Cell or organism, moves by flagella or cilia toward or away from stimulus. – Chemotaxic - Chemicals o Sperm in ferns swim toward chemical produced by female reproductive structures. – Phototaxic - Light – Aerotaxic - Oxygen concentrations Photoperiodism Photoperiodism - Length of day (night) directly related to onset of flowering. • Short-day plants - Will not flower unless day length is shorter than a critical period – Asters, poinsettias, ragweed, sorghums, strawberries • Long-day plants - Will not flower unless periods of light are longer than a critical period – Beets, larkspur, lettuce, potatoes, spinach, wheat Photoperiodism Intermediate-day plants - Will not flower if days too short, or too long • Several grasses Day-neutral plants - Will flower under any daylength, provided there is minimum amount of light necessary for normal growth • Tropical plants, beans, carnations, cotton, roses, tomatoes Vegetative activities affected by phototropisms: • Dormancy of buds • Germination of seeds Prepares plants for seasons Phytochromes and Cryptochromes Phytochromes - Pigments that control photoperiodism • Pale blue proteinaceous pigments that absorb light • Mostly in meristematic tissues • Two stable forms: – – – Pr - Absorbs red light Pfr - Absorbs far-red light When either form absorbs light - Converted to other form Phytochromes and Cryptochromes Phytochromes • Play role in other plant responses: – Plant development, changes in plastids, production of anthocyanins, and detection of shading Cryptochromes - Blue, light-sensitive pigments that play a role in circadian rhythms and interact with phytochromes to control reactions to light A Flowering Hormone? Flowering Locus T (FT)-gene produced by Arabidopsis thaliana associated with flowering • When phytochrome senses a short photoperiod, it represses CONSTANS gene • When phytochrome senses a long photoperiod, it activates FT • FT is transported from leaves, to shoot apical meristem, where it stimulates flowering. Temperature and Growth Each plant species has optimum temperature for growth and minimum temperature below which growth will not occur. • Thermoperiod - Optimum night and day temperatures • Optimum temperatures may change with growth stage of plant. • Lower night temperatures often result in higher sugar content and in greater root growth. • Growth of many field crops is roughly proportional to prevailing temperatures. Dormancy and Quiescence Dormancy - Period of growth inactivity in seeds, buds, bulbs, and other plant organs even when temperature, water, or day length would typically cause growth Quiescence - State in which seed cannot germinate unless environmental conditions normally required for growth are present After-ripening - Factors that control change from dormancy to germination in seeds Stratification - Artificially breaking dormancy Review Introduction Nutrients, Vitamins, and Hormones Hormonal Interactions Other Hormonal Interactions Plant Movements Photoperiodism Cytochromes and Cryptochromes A Flowering Hormone? Temperature and Growth Dormancy and Quiescence