Control of Growth and Development Chapter 11. Developmental Processes Development includes –Growth Cell division and enlargement Morphogenesis –Developmental

Control of Growth and Development

Chapter 11

Developmental Processes

• Development includes– Growth

• Cell division and enlargement• Morphogenesis

– Developmental changes that lead to the formation of specific shapes

– Differentiation• Any process that makes cells functionally

specialized and different from one another• Often occurs through expression of genes

Growth Patterns

• Growth patterns – Determinate

• Usual pattern in animals– Cell division and differentiation result in adult organism in

animals

• “having defined limits”• In plants, dicot leaves and organs formed from

modified leaves (bud scales, bracts, sepals, stamens, carpels) show limited, determinate growth pattern

Growth Patterns

– Indeterminate• Growth of shoots and roots• Continue to grow until stopped by environmental or

internal signal

Stages in Differentiation

• First cell in developmental pathway – zygote– Totipotent

• Has capability of making all the cells in the future organism

• After three or four divisions, cells formed no longer totipotent– Cells now described as determined

• Potential to differentiate is limited


• Plant zygote is totipotent

• Plant cells may also become determined– Often reversible– Examples

• Adventitious roots form from shoot tissue• Shoots can form from roots


• Some differentiated cells can regain totipotency– Removing pieces of tissue from shoots or roots and

placing them in culture conditions can lead to formation of whole plants

• Some differentiation process occur after cell has received stimulus– Mesophyll cells produce chlorophyll only after being

illuminated– Procambial cells produce secondary walls after being

stimulated by sucrose

Gene Expression

• Processes of differentiation depend on expression of genes

• Genetic information encoded in sequence of bases in DNA

Gene Expression

• Steps in producing enzymes needed for life and growth of cell– Transcription of genetic code of DNA onto

RNA molecule• Base sequence of DNA serves as template• RNA molecule has base sequence complimentary

to DNA strand

Gene Expression

– Types of RNA synthesized• Ribosomal RNA (rRNA)

– Three separate rRNAs, in combination with several proteins, form ribosomes for making proteins

• Transfer RNA (tRNA)– Serves as decoding molecule– Translates base sequence into amino acid sequence

• Messenger RNA (mRNA)– Specifies amino acid sequences of particular proteins– Carries message from nucleus to cytoplasm where

protein is synthesized– Translates by interacting with ribosomes and rRNAs

Gene Expression

– Ribosomes bind to mRNA and move along mRNA three bases at a time while binding appropriate tRNAs

» Sequence of three mRNA bases called a codon» 64 different codons

– tRNA finds amino acid with complementary set of bases (anticodon)

– Ribosome connects amino acid of tRNA to preceding amino acid with peptide bond

Coordination of Development

• Coordination of development requires series of signals– Short-range signals

• Possibly from adjacent cells

– Long-range signals• Inform one part of plant about conditions in another part

– Environmental signals• Light, temperature, day length, water, nutrients, mechanical

disturbances such as wind and wounding by herbivores


• Signals– May stimulate new patterns of gene expression– May activate existing proteins or other cell

compounds

• Signal must be perceived by target cells– Cells must have receptor– Reception of original signal usually starts a signal

cascade• Series of events in which one signal leads to another and

another


• Original signals that coordinate growth and development are generally hormones

• Other possible signals– Transient electrical impulses– Hydraulic impulses– Environmental stimuli

• Examples: light and temperature• Can serve as initial signals


• Signal receptors are proteins– Proteins may lead to another link in cascade

of events– Cascade finally ends either in

• activation or inactivation of transcription factor (protein that stimulates reading of a particular gene)

• An enzyme that produces required differentiation of the cell

Signals Regulating Cell Cycle

• Signal allows cells to pass checkpoints in cell cycle

• Researchers Lee Hartwell, Paul Nurse, Timothy Hunt– 2001 Nobel Prize– Genes and proteins that control cell cycles of

yeast and animal cells


• Plants have similar genes and proteins– Less known about them

• cdc2 gene– Provides genetic information for enzyme

protein involved in phosphorylation reactions• Cyclin-dependent protein kinase (C-PK)

– Enzyme adds phosphate functional group to another protein

• Another enzyme catalyzes dephosphorylation– Removes phosphate from proteins


• Phosphorylation/dephosphorylation reaction seems to be key to regulating cell cycle

• C-PK works with other proteins called cyclins– C-PK must be phosphorylated before it can combine with

cyclin– Initial combination inactive– Phosphotase removes one phosphate and activates the

C-PK complex– Active form of combined proteins acts as a protein

kinase» Enzyme that is actual trigger to start the S or M

phases


– Complex is recycled in a three-step process» Complex is broken down into two parts» C-PK can be reused by having new phosphates

added to it» Cyclin protein is degraded into component amino

acids» New cyclins must be resynthesized to restart the

process

The need to regenerate C-PK and cyclin ensures that the cell must be healthy (not starved) to synthesize more DNA and divide again.

Hormones

• Chemicals produced by plant – promote or inhibit growth or differentiation of plant

cells – coordinate development in different parts of plant

• Many agricultural uses• Chemicals are diffusible but often influence

same cells that produced them• Sometimes referred to as growth regulators

Hormones

• Examples of plant hormones or growth regulators– Auxins– Gibberellins– Cytokinins– Abscisic acid– Ethylene

Discovery of Plant Hormones

• Discovered by studying plant developmental processes

• Auxin– Discovered during studies of growth of grass

coleoptiles– Charles Darwin and son Francis

• Experiments demonstrated that coleoptile tip was necessary for elongation of shaft

• Coleoptile could also be induced to bend toward light phototropism

• Coleoptile, when placed on its side, would bend upward gravitropism


– N. Cholodny and Frits Went• 1920s• Cut off coleoptile tip• Placed it on small block of agar so that diffusible

substance could move into agar• Agar block placed on decapitated coleoptile

– Shaft resumed growth– Showed agar had received chemicals from tip that

stimulated growth of coleoptile– Agar placed on side of coleoptile, caused uneven growth– Coleoptile bent away from side that received agar


• Growth of decapitated coleoptiles demonstrated presence of growth- promoting substance

• Substance named auxin– Substance that acted like an auxin was first

purified from urine– Identified as indoleacetic acid– Same substance was later found in plant

extracts

Auxin

Site of synthesis Site of effect Effect

Stem apex, young leaves

Expanding tissues Promotes cell elongation

Roots Initiates lateral roots

Axillary buds Inhibits growth (apical dominance)

Cambium Promotes xylem differentiation

Leaves, fruits Inhibits abscission

Developing embryos

Ovary Promotes fruit development


• Gibberellins– Group of similar plant hormones that stimulate

elongation of stem internodes– Discovered in Japan by plant physiologists

studying rice disease (bakanae, or foolish seedling disease) caused by fungus Gibberella fujikuroi

– Infected seedlings grew faster than uninfected ones but died before could produce grain


– Physiologists showed that chemicals produced by fungus stimulated the growth of the rice seedlings

– Later found that same chemicals are made in low amounts in young leaves and transported throughout plant in phloem

Gibberellins


Stem apex, young leaves

Stem internode Promotes cell division, cell elongation

Embryo Seed Promotes germination

Embryo (grass) Endosperm Promotes starch hydrolysis


• Cytokinin– Discovered during experiments designed to

define conditions needed for culturing plant tissues

– Sterilized pieces of stem, leaf, and root tissue placed in flask with nutrient medium

– Medium contained carbon source, nutrient minerals, and certain vitamins


– Hormones added to induce cells to divide– In medium containing only auxin, cells enlarge without

dividing– Studies showed that hormone found in solutions of

boiled DNA and in coconut milk (liquid endosperm) were necessary

• active ingredients were cytokinins, modified forms of adenine

– Plant cells divided and grew rapidly in nutrient medium containing both auxin and cytokinin

Cytokinin

Site of synthesis

Site of effect Effect

Root apex Stem apex, axillary buds Promotes cell division (release of apical dominance)

Leaves Inhibits senescence


• Further experiments with auxin and cytokinin showed they also influence development– 10 times more auxin than cytokinin added to

cell culture growth undifferentiated forms amorphous mass called a callus

– Auxin concentration increased further or when nutrient concentration in medium reduced callus produces roots


– Increase in cytokinin concentration callus becomes green and compact and produces shoots

Precise response of plant tissues to different auxin and cytokinin concentrations depends on the plant species and other growth conditions.


• Abscisic acid– More associated with suspension of growth

rather than stimulation– Discovered almost simultaneously by two

different groups• F. Addicott at University of California

– Discovered compound that promoted abscission

• P.F. Wareing and his coworkers in Wales– Found compound that was associated with dormancy of

woody shoots in winter

Abscisic Acid


Leaves Guard cells Closes stomata

Stem apex Promotes dormant bud formation

Ovule Seed coat Inhibits seed germination


• Ethylene – Associated with inhibition and modification of

growth

– C2H4

– Gas at normal temperatures and pressures– Discovered when the gas, produced by oil or

kerosene heaters in greenhouses, stimulated senescence of flowers and caused lemons and oranges to ripen


– Produced by almost any wounded plant tissue– Produced by unwounded tissues whose

growth has been restricted– Can move by diffusion to nearby organs

Ethylene


Wounded tissues, aged tissues

Stem Inhibits cell elongation

Leaves Promotes senescence

Fruits Promotes ripening

Signals from Shoot Apex Promote Growth

• Auxin and gibberellins normally produced by shoot apical meristems, young leaves, developing fruits and seeds

• Actively transported down stem toward roots from shoot apices and young leaves

• Stimulate primary growth of stem

• Major effect of auxin is increase in plasticity of cell wall


• Hypotheses for plasticity increase by auxin– Acid-growth hypothesis

• Suggests main effect of auxin is to cause cells to secrete acid (H+ ions, protons)

• Acid stimulates changes in plasticity

– Induced gene expression hypothesis• Suggests auxin works by inducing the expression

of genes that make growth-promoting proteins


• May be special proteins that affect cell wall’s plasticity– Activities may promote acidic conditions– One protein induced by auxin stimulates

activity of plasma membrane proton pump which acidifies cell wall

– Protein called expansin increases plasticity at pH values less than 6.0


– Another protein with enzymatic activity (transglycosylation) promotes plasticity and growth

• Breaks carbohydrate chains • Reforms carbohydrate chains in configuration that

can result in more extended cell wall


• Gibberellins also needed for elongation of stem internodes

• Auxin and gibberellins– Stimulates cell division in vascular cambium

• Auxin stimulates growth of secondary xylem• Gibberellins stimulate growth of secondary phloem


• Auxin– Tends to inhibit activity of axillary meristems

near apical meristem• Restricts formation of shoot branches• Phenomenon called apical dominance

– Helps coordinate root growth with shoot growth

Cytokinin Coordinates Shoot with Root Growth

• Found in embryos and endosperm– Stimulates cell cycle

• Possibly promotes cyclin synthesis

• Plays similar role in mature plants• Produced in roots and transported to

shoots in xylem sap– Presence of cytokinin signals to shoots the

presence of healthy roots

• Delays senescence

Shoot Growth in Response to Environmental Signals

• Gibberellin– Bolting (internode growth) of plants with

rosette morphology• Induced by environment by longer days or cold

temperatures• Can be stimulated by spraying plant with

gibberellin• Observations suggest that rosette plants are

deficient in gibberellin and they bolt when environmental signal stimulates them to produce gibberellin


• Abscisic acid– Accumulates in shoots of perennial plants– Environmental stimulus

• Shorter days or temperature decrease

– Stimulates the formation of dormant bud at each shoot apical meristem


• Ethylene – Slows growth of stems and roots when

produced by wounded cells or by organs meeting physical obstacle

– Stem or root growing under influence of ethylene

• Short and stumpy because formed from short, round cells

Seed Development and Germination

• Abscisic acid– Plays role in formation of viable seeds

• Induces formation of large amounts of certain proteins thought to store materials needed for use by embryo when it germinates

– Associated with dormancy of some seeds• Accumulates in seed coat during development• In presence of abscisic acid, embryo does not germinate• Seed requires long period under cool, wet conditions before

it can germinate– Conditions stimulate breakdown of abscisic acid

Seed Development and Germination

• Gibberellin– Promotes germination in many types of seeds

• Possibly causes increased concentration of one of cell wall-loosening enzymes

• Possibly causes reorientation of microtubules and microfibrils so fewer microfibrils oppose cell elongation

– Promotes metabolic breakdown of storage materials

• Made by germinating embryo• Represent signal from embryo to endosperm

announcing need for nutrients

Stimulation of Senescence

• Ethylene– Triggers expression of genes leading to

synthesis of enzymes that begin process of senescence

• Chlorophyllases and proteases

– Mechanism by which ethylene induces synthesis of enzymes is not well understood

– A specific protein receptor that recognizes and binds to ethylene has been found

Stimulation of Senescence

– Ripening of fruit stimulated by ethylene– May involve

• Conversion of starch or organic acids to sugars• Softening of cell walls to form a fleshy fruit• Rupturing of cell membrane with resulting loss of

cell fluid to form dry fruit

– Overripe fruit is potent source of ethylene• Promote ripening of adjacent fruits

• CO2 inhibits effect of ethylene

Stress Signals

• Abscisic acid– Plays role in control of photosynthetic system

under stress• Under drought conditions, wilted mesophyll cells

synthesize and excrete abscisic acid• Abscisic acid diffuses into guard cells• Receptor recognizes hormone and releases K+, Cl-

and H2O which closes stomata

Stress Signals

• Upon detection of attack of plant by fungus or fungus-like protists – Plant may produce H2O2 (hydrogen peroxide)

• Thought to act as antibiotic

– Plant may produce enzymes that break down fungal cell walls

• Plant cells around infection site may die• Release tannins• Fungus has trouble spreading through dead cells

to new, live cells

Stress Signals

• Systemic acquired resistance– If plant survives infection by virus, bacterium, or

fungus, this infection may make plant less susceptible to later invasion into other parts of its body by same pathogen

– Mechanism not fully understood• Involves transmission of signal from infected organ to new

leaves, probably through phloem• Induction of several types of antibiotic proteins in new leaves• Does not involve antibodies of type that appear in animals

Stress Signals

– Compounds possibly involved in systemic acquired resistance

• Salicylic acid– Compound related to aspirin

• Jasmonic acid• Hydrogen peroxide• Systemin

– Produced in response to infections– Required for establishing systemic resistance– May be first polypeptide hormone discovered in plants

Light and Plant Development

• Light – most important environmental factor influencing plant development

• Response to light– Major way plants adapt to surroundings

Light and Plant Development

• Plants “sense” three colors of light that correspond to at least three distinct light receptors

Light color Receptor

Red light Phytochromes

Blue and near-ultraviolet (black)

Cryptochromes

Intermediate-wavelength ultraviolet light (UVB radiation from sun)

Currently unnamed receptor

Red/Far-Red Response

• Wavelengths seem to act as on/off switch

• Responses governed by switch– Growth of seedlings

• Seedling grown in dark is etiolated (long and light yellow)

• Exposure of seedling to red light starts de-etiolation process

• Process is retarded if red signal is followed immediately by far-red light

Red/Far-Red Response

– Structure and function of phytochromes• Type of protein containing a pigment molecule

related to heme (O2 carrying molecule in animal blood cells)

• Pr - inactive• Pfr – active• Irradiating phytochrome with red and then far/red

light is equivalent of turning a switch on and then off

• Function of phytochromes not certain– May attach phosphate to other proteins

Photoperiodic Responses

• Plants have system that measures lengths of days and nights

• System called photoperiodism

• Protoperiodism used to time flowering in many plants

• Two major groups of plants– Long-day plants– Short-day plants


• Day-neutral plants – Flowering is not referenced to length of day

• Long-day plants– Begin flowering sometime between January

and June– Flower when days get longer than

characteristic day length


• Short-day plants– Begin flowering between July and December– Flower when days become shorter than

characteristic day length

• Experiments have shown – length of uninterrupted night is most important

part of signal– phytochrome is receptor by which short-day

plants perceive light


• Biological clocks– Known in all eukaryotic organisms– Generally reset every day in response to light

exposure– Endogenous

• Can continue to operate in continuous darkness• “coming from within”


– Can be observed as daily rhythms or patterns• Mimosa leaflets open during day and close at night• Nyctinastic movement

– Caused by transport of ions and resulting change in turgor pressures of cells on opposite side of petiole

– Other responses controlled by photoperiodism probably through interaction between light and endogenous clock

• Dormancy• Senescence in the fall• Resumption of growth in the spring

Plant Responses to Light

• Responses are complex

• Some responses involve exposure to blue part of spectrum– Phototropism– Induction of enzymes that synthesize red

pigments (anthocyanins) in skins of fruits such as apples and plums

Plant Responses to Light

• Other responses require exposure to more than one color of light– Some plants require exposure to both red and blue

light in order to form red pigments– Either color forms some pigment but both colors are

required for full response– Synthesis of chlorophyll

• Brief exposure to red light will not turn etiolated leaves green• Chlorophyll synthesis requires longer exposure to red or blue

light

Documents

Control of Growth and Development Chapter 11. Developmental Processes Development includes –Growth Cell division and enlargement Morphogenesis –Developmental