plant hormones ap biology – lahs. what are hormones? chemical signals that coordinate the various...
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Plant Hormones
AP Biology – LAHS
What are Hormones? Chemical signals
that coordinate the various parts of an organism
Chemicals are made in one region and are target for some other region of the organism
The Discovery of Plant HormonesPlant hormones were discovered as
scientists were studying how it is that plants grow towards light
Phototropism – growth of a shoot towards light
Darwin’s Experiments with Phototropism Coleoptile – term for the sheath that encloses
a grass seedling Studied growth of the coleoptile in different
conditions Darkness – grew straight Illuminated uniformly from all sides – grew straight Illuminated from one side only – grew towards the
lighted side
Question: What Causes the Coleoptile to Bend towards Light?
Hypothesis: cells on the darker side of the coleoptile elongate faster than those on the lighted side. This causes the coleoptile to bend toward light.
How does this happen?
Darwin’s IdeasThe part of the coleoptile responsible for
sensing light is the TIP.Growth response that caused curvature
of the coleoptile was BELOW the tip.Hypothesis: some signal was
transmitted downward from the tip
Diagrams of Experiments
Testing Darwin’s Hypothesis Peter Boysen-Jensen Tip was separated from the coleoptile Control treatment: A gelatin block separated
the tip form the lower parts of the plant The gelatin block allowed the plant to be cut as it
would be in the experimental treatment, but still allowed the chemicals from the tip to pass down
Resulted in curvature as normal
Testing Darwin’s HypothesisExperimental treatment:
An impermeable barrier was placed between the coleoptile tips and the lower parts of the plants
Prevented the chemicals made at the tip from moving down the plant
Result: Curvature growth did not occur
Diagrams of Experiments
Went’s Experiments Extracted the chemical messenger from the coleoptile
tip Removed the tip and allowed it to diffuse onto a piece of agar
Removed and discarded growing tip from other coleoptile seedlings
Placed the agar block evenly centered onto the “decapitated” seedlings They grew straight
Placed the agar block Uncentered onto other decapitated seedlings Their growth caused them to curve AWAY from the side with
the agar block
Went’s Experiments
Went’s ConclusionsThe chemical in the tip stimulated
growth as it passed down the coleoptileThe coleoptile curved toward light
because of a HIGHER concentration of the growth-promoting chemical on the DARKER side of the coleoptile
Went named the chemical messenger that he studied AUXIN
Tropisms Growth responses that result in curvatures of
whole plant organs toward or away from some stimulus.
Mechanism Elongation of cells on the OPPOSITE side of the
organ region that is receiving the stimulus Stimulii
Gravity Light Touch
How Does Auxin Stimulate Growth? Causes cell walls to become “looser” and
more malleable. Then they can be expanded/elongated
Plant Hormones - Auxin IAA (indoleacetic acid)
Found: Meristems of apical buds
Major Functions: stimulation of stem elongation Root growth, differentiation,
branching Apical dominance
Growth of a stem occurs only at the tip unless the tip is cut off
Absence of auxin from tip will allow lateral buds to emerge
This is why we prune Actively transorted from
cell to cell in a specific direction
Plant Hormones - Auxin (IAA) cont.
Found Embryos within seeds
Major Functions Stimulate growth of
fruit from ovary Influences responses
to light & gravity
Plant Hormones - CytokininsFound
In actively growing tissues Produced in roots, transported
elsewhere Major function:
Stimulate cytokinesis (cell division) Work with auxins to control plant
growth Plant tissue treated with auxin w/o
cytokiinin – cells will grow but not divide
Control of apical dominance – supports lateral buds (weakens apical dominance)
Anti-aging hormones Delays senescence (aging) of leaves Slow deterioration of leaves – used by
florists
Plant Hormones - Gibberellins Found in:
Apical meristems; young leaves/embryos
Major function: Simulates growth in leaf and stem Stem bolting – rapid elongation Fruit growth
Grapes are sprayed with gib to cause them to grow larger and further apart
Germination of seeds After water is imbibed, gibberellins
are released in embryo to break from dormancy
Inhibition of aging leaves
Plant Hormones – Abscisic Acid Found in:
Leaves, stems, roots Seeds, green fruit
Major function: Slow down growth Dormancy for overwintering
Suspends primary and secondary growth Promotes abscision of leaves (falling off)
In seeds – inhibits growth until ABA can be overcome or diminished by favorable conditions
Heavy rain may wash out ABA Light may degrade Increased gib to ABA ratio may determine germination growth
Stress hormone When a plant wilts, ABA accumulates causing stomata to close
Plant Hormones - Ethylene Found in
Tissues of ripening fruit Nodes of stems Ageing leaves and flowers
Major functions Changes of ovary to become fruit
Degradation of cell walls; softening
Dropping from plant Leaf abscission
Loss of leaves to prevent water loss
Tissue at base of petiole dies Senescence (aging)
Autumn leaves; withering flowers
Tropisms Growth responses that result in curvatures of
whole plant organs toward or away from some stimulus.
Mechanism Elongation of cells on the OPPOSITE side of the
organ region that is receiving the stimulus Stimulii
Gravity Light Touch
Tropisms - PhototropsmPhototropism:
response to light Achieved through
auxin When all sides
equally lit, straight growth
When stem unequally lit, differential growth
Tropisms - Gravitropism Also geotropism Response to gravity
be stems & roots Gibberellins & Auxin
involved (relative concentrations)
Tropisms - Gravitropism If stem is horizontal: auxin at apical meristem moves down and
concentrates on lower side – stem bends upwards If root is horizontal, auxin produced at apical meristem moves up
roots and concentrates on lower side – inhibits growth in roots Special starch-storing plastics (staloliths) settle at lower ends of
cells to influence auxin movement
Tropisms - Thigmotropism Response to touch Seen in climbing
vines, venus fly trap, etc.
Photoperiodism
Response of plants to changes in the photoperiod (relative length of day/night)
A plant maintains circadian rhythm: internal clock that measures length of day/night
PhytochromesChemicals that function as
photoreceptors in plants and allow plants to “measure” photoperiod
Phytochromes The name given to the photoreceptor that is
responsible for the reversible effects of red and far-red light is phytochrome Phytochrome = a light absorbing protein 2 forms
Pr = absorbs red light Pfr = absorbs far red light
The two forms are photoreversible When Pr is exposed to red, it becomes Pfr When Pfr is exposed to far red, it becomes Pr
Phytochrome
Phytochrome Pr is form of photochrome synthesized in plant
cells. Pr synthesized in leaves. Pr and Pfr in equilibrium during daylight. Pr ->
Pfr since red light present in sunlight. Pr accumulates at night. No sunlight for Pr -> Pfr.
Pfr breaks down faster. Cells continue to make Pr at night.
Daybreak, light rapidly converts to accumulated Pr into Pfr. Equilibrium again.
Phytochromes
The Pr <-> Pfr interconversion acts as a switching mechanism that controls various events in the life of a plant.
Phytochromes Red light - 660nm Wavelength of light that is most effective at
interrupting the critical night length of a short day (long night) plant. Exposure at night will cause the plant NOT to flower
HOWEVER, if this light briefly interrupts the night of a long day (short night) plant, the plant will flower The red flash will shorten the plants perception of
night length
PhytochromesThe shortening of night length can be
negated by providing a flash of light at 730nm wavelength. This is called the far-red part of the
spectrum
Phytochrome Night length is responsible for resetting the
circadian rhythm clock If daylight is interrupted with dark there is no effect If dark is interrupted with flashes of red or far-red
the clock can be affected Red-light shortens night length
Because it converts Pr to Pfr – which would not normally accumulate at night
Far-red light restores – as though night was not broken
Because far red light flashes convert Pfr back to Pr
PhytochromePlants synthesize phytochrome as Pr If left in the dark, nothing happens to
this pigment If the pigment is illuminated with
sunlight, Pr changes to PfrThus the plant can detect the presence
of sunlight
Phytochrome Pr = Pfr during daylight If shade of larger trees were to block sunlight
from a smaller tree, the radiation most blocked by canopy is red (not far red)
Pigments in the plant would be converted to Pr
This cue would stimulate the plant to grow taller.
Phytochrome If ample sunlight were
available, the reverse would happen –
Pfr proportions would increase and the plant would “sense” that it was in sun.
It would be cued to branch and vertical growth would be inhibited
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