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Hormones 101
Duane W. Greene
University of Massachusetts
What are Plant Hormones?
• Plant hormones are organic compounds,
produced by the plant that In low
concentration (10-3 M) regulate or direct
growth and development in a plant.
Plant Growth Regulators
• Plant growth regulators are very similar to
plant hormones except PGRs may or may not
be produced by the plant. Many are synthetic
or not universally found in all plants.
• Some plant hormones are also considered
PGRs (GA4+7, GA3 , ABA, ACC).
Plant Hormones
• PGRs act as signal givers or directors of
growth and development.
• In general, they are not directly involved in the
processes that they direct.
• One could look at PGRs as the first domino in
a series of event that ultimately results in a
physiological response.
• Another way of looking at this that they are the
trigger that initiates a physiological action.
Plant hormones signal to initiate a
response which is then passed on and
results in a response.
PGR
Response
Send signals that lead to gene expression,
gene activation and activation and/or
synthesis of enzymes
Major Classes of Hormones
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Auxins
Gibberellins
Cytokinins
Abscisic Acid
Ethylene
(Brassinosteroids)
(Jasmonates)
Auxins
• This is the first hormone that was isolated
and identified- 1930’s.
• Indole-3-acetic acid is the primary
endogenous auxin.
• While limited in the number of field
applications, auxin probably has the
greatest influence on how you grow and
manage your trees.
Auxins control or influence:
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Apical Dominance
Phototropism
Geotropism
Abscission: leaves, fruit and flowers
Cell elongation
Cambial activity
Root formation
Increase flowering
Auxin induced ethylene production
Apical Dominance
Apical Dominance
Leaf Abscission
Fruit Abscission
Auxins present in the king fruit
suppress the auxins coming from the
lateral fruit.
This reduction on auxin triggers the
production of enzymes that weaken
and destroy the abscission zone.
We suspect that the reduction in
auxin is carbohydrate (CHO)
mediated. The greater the CHO
deficit the more abscission.
Phototropism
Geotropism
Geotropism
Gibberellins (GA)
• Stem Elongation
• Stimulate seed germination
• Flowering
– Promotion
– Inhibition
• Stimulate pollen tube growth
• Elongates fruit
• Improves fruit finish
Stem Elongation
• Generally vigorous vegetative
growth is undesirable for tree
fruit.
• Inhibitors of GA biosynthesis
are used to retard growth and
make trees more efficient,
productive and carry higher
quality fruit.
Inhibition of Flowering
• GA inhibits flower bud formation in tree fruit.
• Seeds in developing fruit are rich sources of
GA.
• Since trees generally set more fruit than is
desirable, attempts are made to cause a
large number of fruit containing seeds to
abscise early in the season so that flower bud
formation for the following year will not be
inhibited or at least inhibited minimally.
• Inhibition may occur early.
Elongation of Fruit
Cytokinins
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Cell division and cell growth
Deferral of senescence
Apical dominance, promote lateral bud break
Cell differentiation
Nutrient mobilization
Morphogenesis
Fruit elongation
Cell Division
• Cytokinins were discovered in the late
1940s because they caused cell division.
• The cytokinins were given their name
because they did promote cytokinesis (cell
division).
Cytokinins inhibit scenesence
• Cytokinins delay and defer
senescence by regulating enzymes
that encourage cytokinin synthesis
and inhibit their degradation.
Apical Dominance
• Cytokinins interact with auxin to regulate
apical dominance.
• Sugars are also invovled.
Abscisic Acid (ABA)
• BA was discovered in large part because
it caused leaf abscission in cotton.
• However, ABA only plays a secondary role
in abscission. Both auxins and ethylene
are more prominent hormones in
regulation of abscission.
Abscisic Acid
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Controls stomatal movement
Inhibition of vegetative growth
Promotes abscission
Involved in seed and seed dormancy
Produced in stressed plants
Stomatal Movement & Transpiration
• ABA plays a pivotal role in the regulation
of stomatal movement water loss.
• In response to water stress ABA increase
in the leaves which results in cations
moving out of the guard cells esp. K+.
• The lower osmotic pressure in the guard
cell results in osmotic loss of water
causing the stomates to close.
Ethylene
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Advances fruit ripening
Promotes senescence
Enhances flowering in pome fruit
Causes epinasty
Frequently produced as result of auxin
application
• Auxin induced ethylene production
Ethylene the Hormone
• Ethylene was ignored as a legitimate plant
hormone for many years.
• Scientist could not understand how a small
gas molecule could be a hormone
• It was not until ethylene could be detected
a very low rate with a gas chromatograph
that its importance as recognized.
Ripening of Fruit
• When fruit ripen they give off large
amounts of ethylene.
• Efforts are made either inhibit ethylene
production (ReTain) or block its response
(SmartFresh, Harvista).
• Ethylene management is a key to extend
storage life.
Ethylene Promotes Abscission
• Ethylene along with auxins are key
hormones that regulate leaf abscission.
• Ethylene production is increased in
stressed plants.
• This in turn results in a decrease in auxin
content which can then lead to abscission
Auxin Induced Ethylene Production
• Auxins stimulate ethylene production.
Frequently, responses in a plant that
appear ethylene-like are actually triggered
by auxin.
• Examples:
– Epinasty of leaves following NAA application
– Leaf curling following 2,4-D application
Jasmonates
• Produced in response to wounding as a
natural defense compound.
• The can be growth retardants and retaerd
root and coleoptile growth
• Promote senescence.
• Most common are methyl jasmonate and
jasmonic acid.
Brassinosteroids
• Steroids that promote and are necessary
for normal stem elongation.
• Plants that lack this hormone are dwarfed
and this can be reversed by providing
brassinosteriods.
Summary
• Plant hormones are the organic compound
produced by the plant that regulate the
growth and development of the plant.
• They control all aspect of growth and
development of aa plant.
• They should be considered signal givers
that control the growth in an ordered
sequenced series of events.