Transcript Chapter 45
Plant Responses to the Environment
Chapter 45
Plant Hormones
• Hormones – chemicals secreted by cells,
transported to other cells where they exert
effect
• Released in response to an environmental
stimuli
• Promote growth, development, aging
Six Plant Hormones
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Auxins
Gibberellins
Cytokinins
Ethylene
Abscisic acid
Florigens
Auxins
• Promote or inhibit elongation in target cells
– Shoot – high conc. causes elongation
– Root – low levels stimulate elongation, high conc.
inhibit
• Synthetic auxin (2,4 D) is used to kill dicots
• Commercially used to promote root formation
in plant cuttings, stimulate fruit development,
delay fruit fall
Gibberellins
• Primarily in plant shoots
• Promote stem elongation by increasing cell
elongation and division
• Stimulate bud sprouting, flowering, fruit
production and development, seed
germination
• Produced in the shoot apical meristem, young
leaves, plant embryos
Cytokinins
• Promote cell division
• Synthesized in root apical meristem
• Inhibit formation of root branches, cause
nutrients to be transported to leaves,
stimulating chlorophyll production and
delaying aging
• Commercially - sprayed on cut flowers to
keep them fresh
Ethylene
• Gas
• Produced in plant tissues, released in response to
a range of environmental stimuli
• Stress hormone – produced in response to
wounding, flooding, drought, extreme temp.
• Stimulates weak celled abscission layers leaves, petals, fruit drop off at appropriate times
• Commercially – used to ripen fruit
Abscisic Acid
• Synthesized in tissues throughout the plant
• Helps plants to withstand unfavorable
environmental conditions
• Causes stomata to close when water is scarce
• Promotes root growth, inhibits stem growth in
dry conditions.
• Helps maintain dormancy
Florigens
• Synthesized in leaves
• Control flowering in response to environment
• Discovered in 2007
Hormones regulate plant life cycles
• Hormones are produced in response to an
environmental stimulus
• These hormones may influence the activity of
genes (activate, repress)
Maintaining Dormancy of Seeds
• In temperate zones, seeds remain dormant
until spring
• Cold weather reduces abscisic acid, preparing
plant for spring germination
• In desert plants, some seeds have high levels
of abscisic acid in their coats. It may be
washed away by rain
• Grasslands, chaparral, forest – require fire for
germination
Gibberellin Stimulates Germination
• -Abscisic acid and +Gibberellin = germination
• Gibberellin is produced by the embryo
• Enzymes break down starch for energy
Auxin controls Orientation
• Light and gravity help the seedling figure which way
is up
• Auxin controls phototrophism – growth towards light,
in shoots
• Gravitropism – growth to/away from gravity, in
shoots and roots
• Gravirtopism and phototrophism work together to
cause shoot to grow upward
Auxin mediates Gravitropism
• Vertical stem – auxin distributed evenly
• Horizontal stem – position detected and auxin
distributed to lower side of stem.
• Lower cells elongate, bending stem upwards
(-gravitropism)
• When stem is vertical auxin is evenly distributed
Gravitropism
shoot
Auxin, produced in the shoot
tip, is distributed evenly
across the shoot and root as
it travels downward
seed
Auxin from the shoot
tip travels down and
collects in the root tip
root
(a) The shoot and root are oriented
vertically
Gravitropism
Auxin is transported to the lower side of the
shoot, where it stimulates cell elongation
and causes the stem to bend upward
Auxin is transported to the lower side of the
root, where it inhibits cell elongation and
causes the root to bend downward
(b) The shoot and root are oriented
horizontally
Negative Gravitropism
Positive Gravitropism
Auxin mediates Phototropism
• Auxin accumulates in
the side of the shoot
that is away from the
light.
• Cells elongate and
bend towards the light
Animation: Hormone Characteristics
Animation: Hormone Transport and Activity
Auxin mediates Root Elongation
• Toward gravity
• If root is horizontal, they sense gravity and
cause auxin transport to lower side
• Lower side cells elongate, causing root to
grow towards gravity
• How do they sense gravity? Statoliths –
starch filled plastids settle into the lower part
of the cell
Statoliths May Be Gravity Detectors
root
cell in
root cap
nucleus
statoliths
Plant responds to Environment
• When shoot or root push against soil,
ethylene is given off.
• Elongation slows and cells become thicker
and stronger
• More able to force their way through soil
• Dicots – ethylene causes formation of hook in
forming shoot
Thimotropism
• Directional movement or
growth in response to touch
• Cell elongation on contact
side is inhibited, tendrils
grow
• Etheylene may be produced
by cells touching the object
Shoot and Root Branching
• Controlled by Auxin and Cytokinin
• Growth of shoot must be balanced by root growth
• Water, mineral, anchorage of plant
• Stems – auxin inhibits later bud growth to form
branches, cytokinin promotes this growth
• Root – cytokinin stimulates root branching, auxin
promotes this growth
Apical Dominance
• Pinching back the tip of a
plant causes bushy
growth because apical
meristems release auxin
which suppresses bud
development into
branches.
Lateral bud sprouting
• Auxin is transported from the stem to the root,
decreasing in concentration.
• Cytokinin is transported from the root to the stem,
decreasing in concentration.
• Lateral buds closes to shoot receive enough auxin to inhibit
growth, very little cytokinin = remain dormant
• Lower lateral buds receive less auxin and more cytokinin =
stimulate to grow into branches
Root branch formation
• Auxin, transported down from the stem
stimulates branch roots to form
• Cytokinin produced in the root apical
meristem inhibits root branching and is
transported from the root towards the shoot
• Roots closer to the shoot develop branch
roots
• Gradient of hormones keeps size of root and
shoot in balance
Gradient of Auxin and Cytokinin
auxin
high
shoot tip
Lateral buds are inhibited
by high auxin levels
Lateral buds develop into
branches (optimal ratio of
auxin to cytokinin)
Branch roots
develop
(optimal ratio
of cytokinin to
auxin)
high
cytokinin
Branch roots are inhibited
by high cytokinin levels
root tip
Response to Light and Dark
• Timing of flowering and seed production is
crucial
• Environmental clues like water and
temperature are unpredictable
• Day length is very reliable
• Shortening vs. lengthening days
It’s about the amount of darkness
• Day-neutral plants – flower independently of day
length
• Roses, tomatoes, cucumbers, corn
• Long-day plants – flower when uninterrupted dark is
shorter than species-specific duration
• Iris, lettuce, spinach, hollyhocks
• Short-day plants – flower when uninterrupted dark is
longer than species-specific duration
• Cockleburs, chrysanthemums, asters, potato, goldenrod
• It is really more about the amount of darkness than light
The Effects of Darkness on Flowering
day-neutral plant
(rose)
short-day plant
(chrysanthemum)
long-day plant
(iris)
long night
night day
short night
interrupted night
Phytochrome
• Plants measure darkness using a biological
clock that isn’t well understood
• Each time the phytochrome molecule is
exposed to light, the clock resets to 0
• If a plant needs 8 hours of dark to flower but
is interrupted with a flash of light at 4 hours,
the clock will reset
• Light changes the shape of the phytochrome
molecule
The Light-Sensitive Phytochrome Pigment
absorbs
red light
absorbs
far-red light
conversion in light
Pr
(inactive)
both forms are
present in daylight
conversion in dark
Pfr
(active)
Pfr stimulates
or inhibits a
response
Author Animation: Phytochrome
Author Animation: Morning Glory
Animation: Seedling Elongation
Florigen Stimulates Flowering
• Leaves produce florigen in response to the
biological clock
• Transported on phloem to the apical
meristem where it activates genes that are
responsible for flowering
Coordination of Seeds and Fruit
• Auxin and gibberellin
promote growth of
ovary
• Apply to fruit and they
grow larger and looser
Ethylene
• Unripe fruit is green, bitter
• Ripened fruit turns color &
attracts animals
• Ethylene gas stimulates
ripening, as they ripen fruits
give off ethylene gas to
stimulate ripening of
adjacent fruit
• Bananas & tomatoes are
picked & shipped green
Senscence
• Genetically programmed series of events that
prepare the plant for winter
• Ethylene production increases
• Auxin and cytokinin production decreases
• Starches and chlorophyll are broken down and
stored in the stem and roots
The Abscission Layer
• A layer located where the fruit or
leaf join the stem
• Ethylene promotes breakdown of
this layer
• Leaves/fruit drop at correct time
• Can also be triggered by stress
Plant Communication
• Plants summon insect bodyguards
• When attacked by caterpillars, corn releases
chemicals, stimulated by volicitin (in caterpillar
saliva)
• Parasitic wasps are attracted to chemical, lay
eggs in caterpillar
• Lima beans, attacked by spider mites release
chemical that attracts carnivorous mite that preys
on spider mite
A Chemical Cry for Help
2 Volicitin and
leaf damage
cause the plant to
synthesize and
release volatile
chemicals
1 A caterpillar chews on a
corn leaf, leaving traces of
saliva that contains volicitin
3 The released
chemicals attract
female parasitic wasps
4 The wasps lay their
eggs on the caterpillar,
which will provide food
for their larvae
Animation: Chemical Messengers
Plant Defense
• Some plants, when damaged by insects,
produce a signaling molecule that moves
through the plant.
• The plant then makes a distasteful chemical
• Radishes and caterpillars
Warning the Neighbors
• Healthy plants sense chemicals released by
neighbors that have been wounded by
insects.
• Salicylic acid methyl salicylate (volitile)
• Neighbors boost their defenses
Mimosa
• Thigmotropism - sensitive to touch
• Stimulated by electric signals conducted through
motor cells at the base of each leaf
Carnivorous Plants
• Sundew - movement of trapped insects
triggers thigmotropism in the hairs, secrete
sticky goop, smothering the insect
• Bladderwort – trapdoor is sprung by insect,
opens inward suddenly, sucking insect into
the bladder where it is digested.
A Sundew and Its Insect Prey
The Bladderwort Snares Tiny Aquatic Organisms
Author Animation: Venus Fly Trap