Plant Hormones

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Plant Hormones Tropisms

• Controls of growth, development and movement

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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.

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Processes in growth

• Cell division.
• Cell enlargment.
• Cell differentiation.

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Primary growth
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Secondary growth
xylem
Phloem with bands of fibers
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Seed Germination

• Scarification
• mechanical
• chemical
• heat
• Mobilization of reserves

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Mobilization of reserves
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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

Touch Me Not
Tendrils
Mimosa
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Turgor movementMimosa pudica L. (sensitive plant)
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Pulvinus of Mimosa pudica
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Tropic responses

• Directional movements in response to a
  directional stimulus

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Growth movement
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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
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Thigmotropism
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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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Plant hormones

• First plant hormone discovered was auxin, the
  chemical responsible for photo- and gravitropic
  responses
• The chemical itself was first isolated from horse
  urine, it is indoleacetic acid

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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, i.e., 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 over 80 different gibberellin compounds in
plants but only giberrellic acid (GA3) and GA47
are often used in plant tissue culture The main
effect of gibberellins in plants is to cause stem
elongation and flowering. They are also
prominently involved in mobilization of endosperm
reserves during early embryo growth and seed
germination.
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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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• 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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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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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