Plant Hormones
Plant Hormones
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There are five major types of plant
hormones:
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Gibberelins
Cytokinins
Ethylene
Abcisic Acid
Auxins
The structure and function of each type
of hormone will be described
Gibberellins
Overview
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Gibberellins (GAs) regulate and
influence:
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cell elongation
seed germination
dormancy
flowering
sex expression
enzyme induction
leaf and fruit senescence.
Germination
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Signal starch hydrolysis through
inducing the synthesis of the enzyme
α-amylase in the aleurone cells
Gibberellins produced in the scutellum
diffuse to the aleurone cells where
they stimulate the secretion α-amylase
α-Amylase then hydrolyses starch into
glucose
Gibberellins cause higher levels of
transcription of the gene coding for the
α-amylase enzyme
Gibberellins: Chemical Structure
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Gibberelins have complex ring structures
Typically contain carboxylic acid groups
Many specific gibberelins exist
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Numeric naming system (i.e. GA#)
May be classified into two structural
types:
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C-19 Gibberelins (19 carbon)
C-20 Gibberelins (20 carbon)
Gibberellins: Chemical Structure
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Type 1: 19 Carbon Gibberelins
Gibberellins: Chemical Structure
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Type 2: 20 Carbon Gibberelins
Cytokinins
Cytokinins
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Found in a variety of plants and have many
functions
Synthesized in meristematic tissues in roots
and transported to aboveground organs
Regulate growth and development of tissue
primarily by promoting cell division
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Involved in germination, shoot differentiation, leaf
senescence
Interacts with other plant hormones for some
functions
Cytokinins Function
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Regulates apical dominance and lateral
root initiation
Slows down senescence (plant aging) and
chlorophyll degradation in aging leaves
Regulates growth of dicot seedlings in the
dark (in combination with ethylene)
Involved in development of sex organs
and male sterility
Synthesized in meristematic tissues in
roots and transported to aboveground
organs
Cytokinins
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Cytokinins contain adenine:
Two structure types:
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Isoprenoid
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Isoprene structural units:
Aromatic
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Contain aromatic groups
Cytokinins: Isoprenoid
Isoprene units
adenine
Cytokinins: Aromatic
adenine
Aromatic group
Cytokinins: Aromatic
Aromatic group
adenine
Ethylene
Ethylene
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Universally produced by all plants
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Angiosperms, Gymnosperms, Ferns, Mosses,
Liverworts
Also found in some fungi, yeast and
bacteria
Important roles in:
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Abscission
Germination
Senescence
Stress
response to pathogens
Ethylene and Fruit Ripening
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Helps fruits go through color
change, softening of walls,
conversion of starch to sugar
Ethylene is produced in low
amounts throughout plant life
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some “climacteric” plants
have sudden peaks in
ethylene synthesis which
signals ripening changes
Ethylene gas is sprayed on
fruit crops to ripen at same
time
Ethylene and Stress
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Some stress situations trigger ethylene
production
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exposure to heat/cold
physical damage
attack by fungal or bacterial pathogens
flooding that limits oxygen
Similar to Abscisic acid’s stress response
Growth and Messaging
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Ethylene and growth
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Promotes root growth and root hair growth
Can cause asymmetric growth in stems and
leaves
Ethylene regulates seedlings’ horizontal growth
& apical hook formation…
= “Triple response” of seedlings grown in dark
Can act as second messenger
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Auxin, cytokinin can cause ethylene production
in seedlings
Ethylene’s “triple response”
Apical hook formation
Ethylene: Chemical Structure
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Ethylene is a very small, simple molecule
compared to other plant hormones
Two carbons sharing a double bond
Ethylene is a gas at room temperature
Abscisic Acid
Abscisic Acid (ABA)
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Found universally in plants and algae
Many functions! Important roles in:
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plant development
bud & seed dormancy
Germination
cell division
leaf senescence
Abscission
cellular response to stress
Abscisic Acid
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Acts as a general inhibitor of growth
and metabolism
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Inhibits growth in hypocotyls, epicotyls,
leaves, coleoptiles
Seed dormancy
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ABA promotes seed dormancy so plant
seeds can withstand desiccation
ABA as a Stress Hormone
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ABA increases with various
environmental or biological plant
stresses
 Excess heat, pests, excess salt
and/or dehydration
Wilted plants have high levels of
ABA
In a drought, ABA increases in
some plants, causing the stomata
to close, preventing water loss
ABA can also produces osmolytes
that protect cell membranes from
dehydration
Abscisic Acid Chemical Structure
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Abscisic acid is a carboxylic acid
Carboxylic acid
Auxins
It’s All in the Name
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“Auxins” from the
Greek word
αυξανω = "I grow
or increase".
They were the first
of the major plant
hormones to be
discovered.
Overview
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essential for cell growth
affects both cell division and cellular
expansion.
may promote axial elongation (as in
shoots), lateral expansion (as in root
swelling), or isodiametric expansion
(as in fruit growth)
auxin-promoted cellular expansion
occurs in the absence of cell division.
auxin-promoted cell division and cell
expansion may be closely sequenced
within the same tissue (root initiation,
fruit growth)
Important Functions
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coordination of many growth and
behavioral processes in the plant
life cycle
stimulate or inhibit the expression
of specific genes.
coordinate development at all levels
in plants, from the cellular level
through organs and ultimately the
whole plant.
Master Hormone
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indole-3-acetic acid (IAA).
the most important member of the
auxin family
the most potent native auxin
generates the majority of auxin
effects in intact plants
Working Together
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patterns of active transport are
complex
typically act in concert with, or in
opposition to other plant hormones
auxins and other plant hormones
nearly always interact to determine
patterns of plant development.
Auxin Shared Functions
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stimulates cell elongation by
stimulating wall loosening factors,
such as elastins, to loosen cell walls
(with gibberellins)
stimulates cell division (with
cytokinins)
applied to callus, rooting can be
generated (with cytokinin)
xylem tissues can be generated
(with cytokinins)
More Auxin Shared Functions
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promotes femaleness in dioecious
flowers (with ethylene)
inhibits or promotes leaf and fruit
abscission (with ethylene)
stimulate cell division in the
cambium andin tissue culture (with
cytokinins)
Auxin Functions
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Stimulate cell elongation
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stimulate differentiation of phloem and
xylem
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Stimulate root initiation on stem cuttings
and lateral root development in tissue
culture
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mediate the tropistic response of bending
in response to gravity and light
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suppresses growth of lateral buds
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delay leaf senescence
More Auxin Functions
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can induce fruit setting and growth in some
plants
involved in assimilate movement toward
auxin, possibly by an effect on phloem
transport
delay fruit ripening
promote flowering in Bromeliads
stimulate growth of flower parts
stimulate the production of ethylene at high
concentrations
inhibit growth by closing the stoma during
water stress.
Auxins: Chemical Structure
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Many naturally occurring auxins exist,
along with many synthetic auxins used
in agriculture
Most naturally occurring auxins contain
an indole ring group or a phenyl group
Auxins (natural and synthetic) are
carboxylic acids
Halides are also seen in both natural
and synthetic auxins
Naturally Occurring Auxins
Carboxylic acid
=IAA, the most
important
member of the
auxin family
Naturally Occurring Auxins
Synthetic Auxins
Synthetic Auxins
Ether linkage
halogens
Sources
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Wikipedia, Auxin, 2010, http://en.wikipedia.org/wiki/Auxin
Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston:
Benjamin-Cummings Company, 2001.
Delker, C., Raschke, A. and Quint, M., 2008, Auxin dynamics: the
dazzling complexity of a small molecule’s message, Planta, vol 227,
929-941.
Gibberellins: A Short History, from http://www.plant-hormones.info,
the home since 2003 of a website developed by the now-closed Long
Ashton Research Station
Wikipedia, Gibberellin, 2010, http://en.wikipedia.org/wiki/Gibberellin
Koning, Ross E. 1994. Auxins. Plant Physiology Information Website.
http://plantphys.info/plant_physiology/auxin.shtml. (4-7-2010).
Litwak, G. 2005. Plant hormones. Elsevier Academic Press: San Diego,
CA.
Raghavan, V. 1997. Molecular embryology of flowering plants.
Cambridge University Press. New York, NY.
Srivastava, LM. 2002. Plant growth and development: hormones and
environment. Elsevier Science: San Diego, CA.
http://www.plant-hormones.info/auxins.htm the home since 2003 of a
website developed by the now-closed Long Ashton Research Station
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Photo credits:
http://humankinetics.files.wordpress.com/2009/0
7/fresh-fruit.jpg
http://www.nature.com/emboj/journal/v22/n6/th
umbs/7595043f4.jpg
http://plantphys.info/plant_physiology/images/tri
pleresponse.gif
http://farm4.static.flickr.com/3657/3513022448_
e7bb1c305e_m.jpg
http://www.hiltonpond.org/images/FreezeHackber
ry01.jpg