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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