Plant Hormones

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Plant Hormones Tropisms
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Plant hormones

• Hormone was first used to describe substances
  in animals
• a substance produced in a gland that circulates
  in the blood and has an effect far away from the
  site of production
• In plants used to mean a compound that acts at
  low concentrations to affect growth and
  development.
• Affect division, elongation and differentiation

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Primary growth
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Secondary growth
xylem
Phloem with bands of fibers
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Plant Responses to Environment

• Tropisms vs Taxisms /-
• Thigmotropism? Physical Contact.
• Chemotropism? Chemicals
• Thermotropism? Temperature
• Traumotropism? Wounding
• Electrotropism? Electricity
• Skototropism? Dark
• Aerotropism? Oxygen
• Gravitropism? Gravity
• Phototropism? light
• Plants in Motion
• Tropisms

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Tropic responses

• Directional movements in response to a
  directional stimulus

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Thigmotropism
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Thigmotropic Response Turgor movement in Mimosa
pudica
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Phototropism
Photoperiodism, or the response to change in
length of the night, that results in flowering in
long-day and short-day plants
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Geotropism/Gravitropism
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Plant hormones

• Five plant hormones known by the mid 1960s, new
  compounds called plant growth regulators
• Signal molecules produced at specific locations.
• Occur in low concentrations.
• Cause altered processes in target cells at other
  locations.

• The five hormones
• Auxins
• Cytokinins
• Gibberellins
• Ethylene (ethene)
• Abscisic acid

• Other plant growth regulators
• Brassinosteroids
• Salicylic acid
• Jasmonic acid
• Systemin

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Summary of Functions of Major Plant Hormones Summary of Functions of Major Plant Hormones Summary of Functions of Major Plant Hormones
Hormone Function Location
Auxins (IAA) stem elongation apical dominance root formation produced in shoot apical meristem
Cytokinins cell division differentiation produced in roots
Gibberellins (GA) stem internode elongation seed germination produced in apical portion of root shoot
Ethylene abscission fruit ripening produced in leaves, stems young fruits
Abscisic Acid suppression of bud growth stomatal opening leaf senescence mature leaves, fruits root caps
most horticultural/ agricultural applications most horticultural/ agricultural applications most horticultural/ agricultural applications
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Auxin
Auxins primarily stimulate cell elongation
Auxins also have many secondary actions root
initiation, vascular differentiation, tropic
responses, apical dominance and the development
of auxiliary buds, flowers and fruits. Auxins
are synthesized in the stem and root apices and
transported through the plant axis. Auxins are
often most effective in eliciting their effects
when combined with cytokinins.
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Auxin associated with phototropism - early
experiments demonstrate tip as receptor.
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Additional responses to auxin

• Inhibits abscission - loss of leaves
• flower initiation
• sex determination
• fruit development
• Auxin Flavors
• Indoleacetic Acid (IAA)
• Phenylacetic Acid (PAA)
• 4-chloroindoleacetic Acid (4-chloroIAA)
• Indolebutyric Acid (IBA)

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Loosening of cell wall
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Auxin promotes rooting
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Promotes Apical dominance
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Auxin as a weed killer

• Many synthetic auxins are used as selective weed
  killers and herbicides. 2, 4 - D (2, 4 - dichloro
  phenoxy acetic acid) is used to destroy broad
  leaved weeds. It does not affect mature
  monocotyledonous plants. Causes a plant to grow
  itself to death
• More readily absorbed by broad-leaved plants
• Most often the weed of Weed and Feed lawn
  fertilizers

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Parthenocarpy

• Auxin induces parthenocarpy, the formation of
  seedless fruits without the act of fertilization.

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Control of abscission by auxin
Formation of an abscission layer at the base of
petiole or pedicel results in shedding of leaves,
flowers or fruits. But auxins inhibit abscission,
as they prevent the formation of abscission
layer. Auxin Spray Prevents Premature Fruit
Abscission and Increase in Yield. a) Auxin
Sprayed b) Auxin not Sprayed
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The infamous side of auxin

• Active ingredient in Agent Orange
• Chemicals with auxin activity sprayed (together
  with kerosene) on forests in Viet Nam to cause
  leaf drop (and fire)
• The chemical process used to make the auxins also
  made dioxin, an extremely toxic compound

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Cytokinins
Cytokinins are able to stimulate cell division
and induce shoot bud formation in tissue culture.
They usually act as antagonists to auxins.
Morphogenesis. Lateral bud development. Delay
of senescence. Stomatal opening. Rapid transport
in xylem stream.
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Function of cytokinins

• Promotes cell division.
• Morphogenesis.
• Lateral bud development.
• Delay of senescence.
• Stomatal opening.
• Rapid transport in xylem stream.

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Other cytokinin facts
Auxin ?

• Cytokinins delay and even reverse senescence
• Release buds from apical dominance

Cytokinins ?
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Interaction of cytokinin and auxin in tobacco
callus tissue
High cytokinin to auxin ratio causes
differentiation of shoots. A low ratio of
cytokinin to auxin causes root formation.
Intermediate cytokinin to auxin ratio causes
formation of roots as well as shoots. Intermediate
cytokinin to low auxin causes growth of large
amount of callus.
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Gibberellin

• Gibberellins are an extensive chemical family
  with gt80 compounds in plants
• The main effect of gibberellins in plants is to
  cause stem elongation and flowering.
• Also prominently involved in mobilization of
  endosperm reserves during early embryo growth and
  seed germination.
• Gibberellin Signal Transduction

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Gibberellins

• Now known to be essential for stem elongation
• Dwarf plant varieties often lack gibberellins
• Gibberellins are involved in seed germination
• gibberellins will induce genes to make enzymes
  that break down starch
• Promotion of flowering.

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Seed Germination caused by Mobilization of
reserves

• Scarification
• mechanical
• chemical
• heat

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• Gibberellins are involved in bolting of rosette
  plants

Gibberellin induces stem elongation in rosette
plants. Cabbage is a rosette plant with profuse
leaf growth and retarded internodal length. Just
prior to flowering, internodes elongate
enormously. This is called bolting. Bolting needs
either long days or cold nights. When a cabbage
head is kept under warm nights, it retains its
rosette habit. Bolting can be induced
artificially by the application of gibberellins
under normal conditions.
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Discovered in association with Foolish disease of
rice (Gibberella fujikuroi)
infected
uninfected
Found as the toxin produced by some fungi that
caused rice to grow too tall
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Gibberellins are used to improve grapes
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EK2.E.1 Timing and coordination of specific
events are necessary for the normal development
of an organism, and these events are regulated by
a variety of mechanisms.

• b. Induction of transcription factors during
  development results in sequential gene
  expression.
• 3. Temperature and the availability of water
  determine seed germination in most plants.

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Abscisic acid (ABA)

• Incorrectly named, not related to abscission,
  slows plant growth
• Important in drought stress and other stresses
• Causes stomatal closure
• Prevents premature germination of seeds (enhances
  dormancy)
• Changes gene expression patterns

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Ethylene

• The smallest hormone
• A gas
• Important in seed germination, fruit ripening,
  epinasty, abscision of leaves
• Sex expression in cucurbits

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Functions of ethylene

• Gaseous in form.
• Rapid diffusion.
• Affects adjacent individuals.
• Fruit ripening.
• Senescence and abscission.
• Interference with auxin transport.
• Inhibition of stem elongation
• Positive feedback mechanisms amplify responses in
  organisms. Amplification occurs when the stimulus
  is further activated which, initiates an
  additional response that produces system change.

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EK2.C.2 Organisms respond to changes in their
external environments.

• a. Organisms respond to changes in their
  environment through behavioral and physiological
  mechanisms.
• Photoperiodism and phototropism in plants

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EK 2.E.2 Timing and coordination of
physiological events are regulated by multiple
mechanisms.

• a. In plants, physiological events involve
  interactions between environmental stimuli and
  internal molecular signals.
• 1. Phototropism, or the response to the presence
  of light
• 2. Photoperiodism, or the response to change in
  length of the night, that results in flowering in
  long-day and short-day plants

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EK 2.E.2 Timing and coordination of
physiological events are regulated by multiple
mechanisms.

• b. Responses to information and communication of
  information are vital to natural selection.
• 1. In phototropism in plants, changes in the
  light source lead to differential growth,
  resulting in maximum exposure of leaves to light
  for photosynthesis.
• 2. In photoperiodism in plants, changes in the
  length of night regulate flowering and
  preparation for winter.
• Mammalian Circadian Rhythms

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Resources

• Plant Hormones Info
• Plant Hormones, Nutrition Transport
• Tropism Animation
• Auxin in Cell Walls
• Plant Responses to Environmental Challenges
  Signaling between Plants and Pathogens
• Growth/Hormones
• Plants in Motion
• Auxin Animation
• Transpiration Lesson
• How Hormones Protect Seed Development in Peas
  Virtual lab
• Herbicide Mechanisms Animations