Download Plant Hormones

Plant Hormones &
Tropisms
Controls of growth,
development and movement
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.
Processes in growth
 Cell division.
 Cell enlargment.
 Cell differentiation.
Primary growth
protoderm
procambium
Leaf primordia
Apical meristem
Ground meristem
Forming axillary bud
Secondary growth
Vascular cambium
xylem
Ray parenchyma
cork
Phloem with
bands of fibers
Seed Germination
 Scarification
mechanical
chemical
heat
 Mobilization of reserves
Mobilization of reserves
Plant Responses to Environment
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Tropisms vs Taxisms +/Thigmotropism
Physical Contact.
Chemotropism
Chemicals
Thermotropism
Temperature
Traumotropism
Wounding
Electrotropism
Electricity
Skototropism
Dark
Aerotropism
Oxygen
Gravitropism
Gravity
Phototropism
light
Touch Me Not
Tendrils
Mimosa
Plants in Motion
Tropisms
Turgor movement
Mimosa pudica L. (sensitive
plant)
Pulvinus of Mimosa pudica
Tropic responses
Directional movements in
response to a directional stimulus
Growth movement
Phototropism
Photoperiodism, or the
response to change in length
of the night, that results in
flowering in long-day and
short-day plants
Geotropism
Thigmotropism
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
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Auxins
Cytokinins
Gibberellins
Ethylene (ethene)
Abscisic acid
• Other plant growth
regulators
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Brassinosteroids
Salicylic acid
Jasmonic acid
Systemin
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
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.
Auxin associated with phototropism - early experiments
demonstrate tip as receptor.
Additional responses to auxin
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Inhibits abscission - loss of leaves
flower initiation
sex determination
fruit development
Auxin Flavors:
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Indoleacetic Acid (IAA)
Phenylacetic Acid (PAA)
4-chloroindoleacetic Acid (4-chloroIAA)
Indolebutyric Acid (IBA)
Loosening of cell wall
Auxin promotes rooting
Promotes Apical dominance
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
Parthenocarpy
• Auxin induces parthenocarpy, i.e., the
formation of seedless fruits without the act of
fertilization.
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
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
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.
Function of cytokinins
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Promotes cell division.
Morphogenesis.
Lateral bud development.
Delay of senescence.
Stomatal opening.
Rapid transport in xylem stream.
Other cytokinin facts
• Cytokinins delay and
even reverse
senescence
• Release buds from
apical dominance
Auxin 
Cytokinins 
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.
Gibberellin
Gibberellins are an extensive
chemical family with over 80
different gibberellin compounds
in plants but only giberrellic acid
(GA3) and GA4+7 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.
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.
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.
Discovered in association with Foolish
disease of rice (Gibberella fujikuroi)
uninfected
infected
Found as the toxin produced by some fungi that caused rice to grow too
tall
Gibberellins are used to improve
grapes
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
Ethylene
• The smallest hormone
• A gas
• Important in seed
germination, fruit
ripening, epinasty,
abscision of leaves
• Sex expression in
cucurbits
Functions of ethylene
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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.
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