Growth Chapter 11

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 single cell into tissues and organs

Nutrients, Vitamins, and Hormones Nutrients - substances that furnish elements and energy to produce organic molecules Obtained from air and soil Vitamins - organic molecules of varied structure that participate in catalyzed reactions, mostly as electron acceptor 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 Some effects of vitamins similar to those of hormones -> can be difficult to distinguish Growth Regulators - compounds that affect plant development similar to those of naturally produced hormones and vitamins

Nutrients, Vitamins, and Hormones Hormones can have multiple effects Hormones chemically bind 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 Produced 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 cont’d. 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

Nutrients, Vitamins, and Hormones Plant Hormones Auxins cont’d. Effects cont’d.: 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 cont’d. Movement of auxins from cells where they originate requires energy expenditure Movement is polar - away from source Move through parenchyma cells surrounding vascular bundles Several Forms: Indoleacetic acid (IAA) Phenylacetic acid (PAA) 4-chloroindoleacetic acid (4-chloroIAA) Indolebutyric acid (IBA)

Nutrients, Vitamins, and Hormones Plant Hormones Gibberellins (GA) Named after fungus that produces it (Gibberella fujikuroi) 110 currently known gibberellins Movement is nonpolar Most dicots and 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

Nutrients, Vitamins, and Hormones Plant Hormones Cytokinins cont’d. 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) 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 H2O loss Interferes with transport or retention of K+ 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 own production Used to ripen green fruits Production almost ceases in absence of O2 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 highly specific and responses same in all species Brassinosteroids Have GA-like effects on plant stem elongation Known from legumes and few other plants

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 -> begin to grow, even in presence of terminal bud

Hormonal Interactions Senescence - breakdown of cell components and membranes, eventually leading to death of cell 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 GA and ABA

Plant Movements Growth Movements - result from varying growth rates in different parts of organ Movements resulting primarily from internal stimuli: Nutations - spiraling movements not visible to eye Nodding Movements - side-to-side oscillations In bent hypocotyl of bean - facilitates progress of plant through soil

Plant Movements Growth Movements Movements cont.’d 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 Nutation Tendril of manroot plant

Plant Movements Growth Movements Movements cont’d. Tropisms - permanent movements resulting from external stimuli Growth of plant toward or away from stimulus Can be divided into 3 phases: Initial Perception - organ receives greater stimulus on one side Transduction - one or more hormones become(s) unevenly distributed across organ Asymmetric Growth - result of greater cell elongation on one side

Plant Movements Growth Movements Movements cont’d. Phototropism - growth movement toward or away from light Positive phototropism Positive phototropism - toward light (Ex. shoots) Negative Phototropism - away from light (Ex. roots) 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 cont’d. Gravitropism - growth responses to stimulus of gravity 1° roots - positively gravitropic Shoots - negatively gravitropic Gravity perceived by amyloplasts in root cap, by proteins on outside of plasma membrane, by whole protoplast, or by mitochondria and dictyosomes Auxin causes cell elongation producing curvature of root Negative gravitropism

Plant Movements Growth Movements Movements cont’d. Other Tropisms: Thigmotropism - contact with solid object Twining Chemotropism - chemicals Germination of pollen grains Thermotropism - temperature Horizontal stems when cold in some weeds

Plant Movements Growth Movements Movements cont’d. Other Tropisms cont’d.: Traumotropism - wounding Electrotropism - electricity Skototropism - dark Aerotropism - O2

Plant Movements Turgor Movements - result from changes in internal H2O pressures and often initiated by contact with objects outside of plant Sensitive plant Pulvini - special swellings at base of leaf Sensitive plant, redwood sorrel Turgor contact movements not confined to leaves Stamens of flowers

Stigmas folding in bush monkey flower Plant Movements Stigmas folding in bush monkey flower Stamens folding

Plant Movements Turgor movements “Sleep” movements - Circadian rhythms Regular daily cycles Leaves or petals fold in regular daily cycles Members of legume family, prayer plants Turgor movements, and stimuli of light and temperature involved Controlled by biological “clock” on approx. 24 hours cycles Controlled internally Circadian rhythm in prayer plant

Plant Movements Turgor movements Solar Tracking - leaves often twist on their petioles in response to illumination and become perpendicularly oriented to light source Blades oriented at right angles to sun H2O 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 H2O

Plant Movements Leaf of grass folding due to bulliform cells Bulliform cells in rolled leaf

Plant Movements Taxes (Taxic Movement) - movement involving 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 Sperm in ferns swim toward chemical produced by female reproductive structures Phototaxic - light Aerotaxic - O2 concentrations

Photoperiodism Photoperiodism - length of day (night) directly related to onset of flowering Short-day Plants - will not flower unless day length shorter than critical period Asters, poinsettias, ragweed, sorghums, strawberries Long-day Plants - will not flower unless periods of light longer than 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 day-length, provided 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 controlling 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 cont’d. Play role in other plant responses: Plant development, changes in plastids, production of anthocyanins, and detection of shading Cryptochromes - blue, light-sensitive pigments playing role in circadian rhythms and interact with phytochromes to control reactions to light

A Flowering Hormone? Florigen - floral stimulus transported from leaf to apical meristems where flower buds initiated mRNA of gene, Flowering Locus T (FT), transported from leaves, where photoperiod is perceived, to shoot apical meristem, where transition from vegetative to flowering state occurs

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 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 present After-ripening – factors 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