growth chapter 11. outline introduction nutrients, vitamins, and hormones hormonal interactions ...

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 HormonesPlant 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


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


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


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)


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 auxinsEffect of gibberellins on

cabbage


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


Cytokinins cont’d.• Also play role in:

−Cell enlargement

−Differentiation of tissues

−Development of chloroplasts

−Stimulation of cotyledon growth

−Delay of aging in leaves


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


Ethylene• Produced by fruits, flowers, seeds, leaves and roots• Produced from amino acid methionine• Can trigger own production

Ethylene from apple caused abscission of holly leaves

• Used to ripen green fruits−Production

almost ceases in absence of O2

• Causes leaf abscission

Nutrients, Vitamins, and HormonesOther 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 MovementsGrowth 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

Plant Movements

Nutation

Tendril of manroot

plant


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


Movements cont’d.• Phototropism - growth movement toward or away

from light

– 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

Positive phototropism


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


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


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• Pulvini - special swellings at

base of leaf− Sensitive plant, redwood

sorrel• Turgor contact movements

not confined to leaves− Stamens of flowers

Sensitive plant

Plant Movements

Stigmas folding in bush monkey flower

Stamens folding

Plant MovementsTurgor 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 MovementsTurgor 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

growth chapter 11. outline introduction nutrients, vitamins, and hormones hormonal interactions ...

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