Download foolish seedling disease

Control of Growth and
Development
Chapter 15
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
Stages in Differentiation
• Plant zygote is totipotent
• Plant cells may also become determined
– Often reversible
– Examples
• Adventitious roots form from shoot tissue
• Shoots can form from roots
Stages in Differentiation
• 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
Coordination of Development
• 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
Coordination of Development
• 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
Coordination of Development
• 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
Signals Regulating Cell Cycle
• 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
Signals Regulating Cell Cycle
• 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
Signals Regulating Cell Cycle
– 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
Discovery of Plant Hormones
– 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
Discovery of Plant Hormones
• Growth of decapitated coleoptiles
demonstrated presence of growthpromoting 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
Stem apex, young
leaves
Expanding tissues Promotes cell elongation
Developing
embryos
Effect
Roots
Initiates lateral roots
Axillary buds
Inhibits growth (apical dominance)
Cambium
Promotes xylem differentiation
Leaves, fruits
Inhibits abscission
Ovary
Promotes fruit development
Discovery of Plant Hormones
• 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
Discovery of Plant Hormones
– 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
Site of synthesis
Site of effect
Effect
Stem apex, young
leaves
Stem internode
Promotes cell division, cell elongation
Embryo
Seed
Promotes germination
Embryo (grass)
Endosperm
Promotes starch hydrolysis
Discovery of Plant Hormones
• 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
Discovery of Plant Hormones
– 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
Root apex
Site of effect
Effect
Stem apex, axillary
buds
Promotes cell division (release
of apical dominance)
Leaves
Inhibits senescence
Discovery of Plant Hormones
• 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
Discovery of Plant Hormones
– 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.
Discovery of Plant Hormones
• 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
Site of synthesis
Site of effect
Effect
Leaves
Guard cells
Closes stomata
Stem apex
Promotes dormant bud
formation
Seed coat
Inhibits seed germination
Ovule
Discovery of Plant Hormones
• 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
Discovery of Plant Hormones
– Produced by almost any wounded plant tissue
– Produced by unwounded tissues whose
growth has been restricted
– Can move by diffusion to nearby organs
Ethylene
Site of synthesis
Site of effect
Effect
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
Signals from Shoot Apex Promote
Growth
• 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
Signals from Shoot Apex Promote
Growth
• 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
Signals from Shoot Apex Promote
Growth
– 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
Signals from Shoot Apex Promote
Growth
• 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
Signals from Shoot Apex Promote
Growth
• 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
Shoot Growth in Response to
Environmental Signals
• 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
Shoot Growth in Response to
Environmental Signals
• 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+, Cland 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 nearultraviolet (black)
Cryptochromes
Intermediatewavelength
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 deetiolation 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
Photoperiodic Responses
• 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
Photoperiodic Responses
• 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
Photoperiodic Responses
• 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”
Photoperiodic Responses
– 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