Chapter 38 Plant Growth and
Regulation
Biology 102
Tri-County Technical College
Pendleton, SC
Seed Dormancy
• Seed is dormant if all developmental activity
within has been suspended
– Cells inside do NOT divide, expand, or differentiate
• Insures survival through unfavorable conditions
and results in germination when conditions are
favorable
• Adaptations include: long cold periods, internal
clock, need for fire/heat, light and/or dark, annual
seeds can skip a year, moisture (cypress)
Dormancy, cont.
• For embryo to begin developing, dormancy must
be broken by physical mechanisms or leeching of
inhibitors by water
– Exposure to light, mechanical abrasion, fire
• As seed germinates (begins to develop), it first
imbibes (takes up) water
• Growing embryo must obtain monomers for its
development by digesting polysaccharides, fats,
and proteins stored in cotyledon(s) or endosperm
Dormancy, cont.
• Release of GIBBERELLINS signals seeds to
break dormancy and germinate
• Imbibed water stimulates gibberellin release (as
does some environmental cues)
• In cereal grains, gibberellins stimulate germination
and support growth by stimulating synthesis of αamylase
– Will digest stored starch making it available to embryo
and seedling
Cereal Grass Visual
Mono/Dicot Germination
• First step is imbibition (absorption of water) for
seed germination in many plants
– Hydration causes seed to swell and ruptures seed coat
– Triggers metabolic changes in embryo that cause it to
resume growth
– Storage materials of endosperm/cotyledon(s) digested
by enzymes and nutrients transferred to growing
regions of embryo
• Radicle (embryonic root) emerges from seed
Germination, cont.
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Shoot tip breaks through soil surface
In many dicots, hook forms in the hypocotyl
Growth pushes hypocotyl above ground
Light stimulates hypocotyl to straighten,
raising cotyledons and epicotyl
• Epicotyl then spreads first leaves which
become green and begin photosynthesis
Germination, cont.
• Germination may follow different methods
depending on plant species
• In peas, hook forms in epicotyl and shoot tip lifted
by elongation of epicotyl and straightening of
hook
• Cotyledons of peas stay in the ground
• In monocots, coleoptile pushes through soil and
shoot tip grows up through tunnel of tubular
coleoptile
I want to see…
More, I want to see more…
The Hormones are Flowing…
• Hormone is regulatory compound that acts at very
low [ ] at sites distant from where it is produce
• Each plant hormone typically plays multiple
regulatory roles
• AUXIN (indoleacetic acid, IAA) promotes
elongation of young developing shoots or
coleoptiles
• Affects secondary growth by inducing vascular
cambium cell division & differentiation of
secondary xylem
Hormones, cont.
• Auxin promotes formation of adventitious
roots
• Promotes fruit growth in many plants
• Used in herbicides (2,4-D synthetic auxin
for dicots)
• Apical meristem is major site of
production
Hormones III
• CYTOKININS are modified forms of
adenine that stimulate cytokinesis
• Affect cell division and differentiation
• Influence apical dominance
• Serve as anti-aging hormones
• Manufactured in the roots
Hormones, IV
• GIBBERELLINS are produced primarily in roots
and young leaves
• Stimulate growth in leaves/stems; have little effect
on roots
• Stimulate cell division & elongation in stems
(perhaps in conjunction with auxin)
• Causes BOLTING-rapid growth of floral stems
which elevates flowers
• Fruit development controlled by
gibberellins/auxins (Thompson seedless grapes)
• Release causes seeds to break dormancy and
germinate
Hormones, V
• ABSCISIC ACID (ABA) produced in terminal
bud; prepares plant for winter by suspending both
primary/secondary growth
• Directs leaf primordia to develop scales to protect
dormant buds
• Inhibits cell division in vascular cambium
• Helps in seed dormancy
• Also stress hormone (closing stomata in times of
water stress
Hormones, VI
• ETHYLENE is gaseous hormone that
diffuses through air spaces between plant
cells
• High auxin [ ]s induce release of ethylene
which acts as growth inhibitor
• Senescence (aging) at cellular, organ, &
whole plant level affected by ethylene
– Important in fruit ripening and leaf abscission
Hormones, VII
• BRASSINOSTEROIDS promote
elongation of stems/pollen tubes
• Promote vascular tissue differentiation
• JASMONATES trigger defenses against
pathogens and herbivores
• OLIGOSACCHARINS trigger defenses
against pathogens
– Limit effects of high auxin [ ]s
– Regulate cell differentiation
Hormones, VIII
• SALICYLIC ACID triggers resistance to
pathogens
• SYSTEMIN causes jasmonate production
in response to tissue damage
• IO #4 “Explain the probable mechanism by
which gibberellins trigger seed germination
was covered in detail in latter part of IO #1.
Enough said……
Tropism
• Tropism is growth response that results in
curvatures of whole plant organs toward or away
from the stimuli
• Mechanism is differential rate of cell elongation
on opposite sides of the organ
• Two stimuli that most profoundly affect plant
growth are LIGHT and GRAVITY
– Phototropism and Gravitropism
• Should mention thigmotropism
Role of Auxin
• Phototropism: cells on darker side of grass
coleoptile elongate faster than cells on bright side
due to asymmetric distribution of auxins moving
down from shoot tip
– May be different in other organs
• Shoot tip is site of photoreceptioin that triggers
growth response
• PR sensitive to blue light is present in shoot tip
– Believed to be yellow pigment related to riboflavin
– Same receptor may be involved in other plant responses
to light
Role of Auxin, cont.
• Gravitropism: even in dark, auxin moves
to lower side of tipped-over shoot
• Auxin moves downward in response to
gravitational stimuli
• Higher auxin [ ] causes more rapid growth
on lower side
• Tip curves upward
Role of Auxin, cont.
• Apical Dominance is [ ] of growth at tip of
plant shoot where terminal bud partially
inhibits axillary bud growth
• Cytokinins and auxin contribute to apical
dominance through antagonistic mechanism
• Auxin from terminal bud restrains axillary
bud growth causing the shoot to lengthen
• Cytokinins (from roots) stimulate axillary
bud growth
Apical Dominance, cont.
• Auxin CANNOT suppress axillary bud growth
once it begins
• Lower buds grow before higher ones since they
are closer to cytokinin source than auxin source
• Auxin stimulates lateral root growth while
cytokinins restrain it
• This stimulation-inhibition action balances plant
growth since > in root system would signal plant
to produce more shoots
Apical Dominance, III
• Auxin and cytokinins indirectly change [ ]
of ethylene
• Levels of different nutrients in bud may also
affect response to auxin/cytokinins
• Gibberellins also contribute to apical bud
dominance
• Brassinosteriods are required for normal
growth and development
Leaf Abscission
• High [ ]s of auxin induce release of ethylene
which acts as growth inhibitor
• Senescence (aging) in plants occurs at cellular,
organ, or whole plant level
– Ethylene plays important role at each level
– Xylem vessel elements/cork cells that die before
becoming fully functional
– Leaf fall in autumn
– Withering of flowers
– Death of annuals after flowering
Leaf Abscission, cont.
• Mechanics controlled by change in balance of
ethylene and auxin
• Auxin decrease makes cells in abscission layer
more sensitive to ethylene
• Cells then produce more ethylene which inhibits
auxin production
• Ethylene induces synthesis of enzymes that digest
polysaccharides in cell walls further weakening
the abscission layer
• Two most important stimuli for leaf abscission are
shortening days and cooler temperatures
Leaf Abscission Visual
Leaf Abscission Visual II
Fruit Ripening
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Ethylene triggers senescence during fruit ripening
Aging cells then release more ethylene
Breakdown of cell walls and loss of chlorophyll
Signal to ripen then spreads from fruit to fruit
since ethylene is a gas
• Use of ethylene is single most important use of a
plant hormone in agriculture and commerce
Parthenocarpy
• Fruit development normally depends on
prior fertilization of the egg (ovum)
• In many species, treatment of unfertilized
ovary with auxin or gibberrellins causes
parthenocarpy
• Fruit formation without fertilization
• Dandelions, seedless grapes, and cultivated
bananas
More of Ethylene
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Role of ethylene in leaf abscission covered (IO 6)
Ethylene can be produced in all parts of plant
Called the senescence hormone
As fruit ripens, loses chlorophyll & cell walls
break down
• Ethylene promotes both processes and causes
more ethylene to be produced (apple & barrel)
• Also associated with apical hook of eudicots,
inhibition of stem elongation in general, and
causing stem to lose sensitivity to gravitropic
stimulation
Abscisic Acid & Stress
• Called stress hormone because it accumulates
when plants deprived of water & possible role in
maintaining winter dormancy of buds
• Is most common inhibitor of seed germination
• Causes stomata to close and prevents stomatal
opening normally caused by light
• Both processes involve ion channels in plasma
membrane of guard cells
Abscisic Acid, cont.
• First response of guard cell to abscisic acid is
opening of calcium channels and entry of calcium
into cell
• This calcium causes cell’s vacuole to release
calcium too
• Increased [ ] of calcium leads to opening of
potassium channels
• Release of K+ ions and of water causes guard cells
to sag together
• Results in closing of the stomata
Photoreceptors
• Light-its presence or absence, intensity, color, and
duration-provides cues to various conditions
• Light regulates many aspects of plant development
• Seed gemination to shoot elongation to flowering
to etiolation
• Photoreceptors interpret light, its duration, and its
wavelength distribution
• Five phytochromes mediate effects of red and dim
blue light
Photoreceptors, cont.
• They are bluish proteins (pigments) found in
CYTOSOL of plant cells
• Phytochromes help plants measure the length of
darkness in a photoperiod
• Phytochrome is protein containing a chromophore
(light-absorbing component) responsible for a
plant’s response to photoperiod
• Phytochromes alternate between 2 photoreversible
forms
– Pr (P red) and Pfr (P far red)
Photoreceptors, cont.
• Plants synthesize Pr and if kept in dark, it remains
• If illuminated, some Pr converted to Pfr
• Pfr triggers many plant responses to light (seed
germination)
• Shift in equilibrium indicates relative amounts of
red light and far-red light present in sunlight
• Shifts in ratio may causes changes such as
increased growth
Moving right on along…
• Far red converted back to red after sunset
• Three or more blue-light receptors mediate effects
of higher-intensity light
• Cryptochromes are yellow photoreceptor
pigments that absorb in blue/ultraviolet
• Affect some of same developmental processes as
phytochromes (seedling development/flowering)
• Cryptochromes located in plant nucleus
• Phototropin (yellow protein) appears to be
photoreceptor for phototropism