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Plant Hormones 101
MUPGRET Workshop
What are hormones?

“a group of naturally occurring organic
compounds that influence
physiological processes at low
concentrations.”
From: Plant Hormones: Biosynthesis, Signal Transduction, Action!
2005. Peter Davies, ed.

“a substance that is transferred from
one part of an organism to another.”
From: Went and Thimann. 1937. Phytohormones.
Hormones

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Can be synthesized locally or
transported to their site of action.
They differ from mammalian hormones
in this respect.
The first hormone


Auxin
Fits the original definition of a
hormone.

Transported from site of synthesis to site
of action.
Sachs

Hypothesized root forming and
flowering forming compounds move
through the plants and cause
morphological changes in 1880.
Darwin
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First to observe phototropism, the
bending of a plant to light.
Also in 1880.
Hypothesized that redistribution of a
chemical within the young seedling
caused it to bend toward the light.
Auxin



Darwin’s compound that caused
phototrophism.
Isolated several years later by
extracting the chemicals that diffused
from cut coleoptiles into blocks of agar.
More about this compound tomorrow
when Dr. Hagen talks.
Classical Plant Hormones
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Auxin
Gibberellins
Cytokinins
Abscisic Acid
Ethylene
New generation plant
hormones

Brassinosteriods
Salicylic acid
Peptides

Polyamines
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http://www.planthormones.info/Index.htm
Auxin
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IAA Structure
Indole-3-acetic acid
(IAA) and its
conjugates.
Synthesized from
tryptophan or
indole in leaf
primodia, young
leaves and
developing seed.
Auxin Transport



In vascular cambium and procambial
strands.
Possibly between epidermal cells.
Transport to root may occur through
phloem.
Auxin Stimulates


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Cell enlargement
Stem growth
Cell division in
cambium
Phloem and xylem
differentiation

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Root initiation
Root branching
Phototropism
Floral organ growth
Auxin functions to



Delay leaf senescence
Promote apical dominance
Delay fruit ripening
Auxin mutant
Brachytic 2
(br2) mutant in
maize.
Gibberellin



Family of 125
compounds.
Gibberellin acid
(GA3)is the most
common.
Synthesized in
young shoots and
developing seed.
Gibberellins


Chloroplast is the initial site of
synthesis.
Transported in phloem and xylem.
Gibberellin promotes


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Stem elongation
Cell division and elongation in stems
Germination if cold or light treatment is
required.
Enzyme production, eg. α-amylase.
Fruit set
Gibberellin mutant
•Dwarf 8 (D8) mutant in
maize.
•The same gene in wheat
is responsible for the
Green Revolution.
•Introduction of this gene
into cultivated wheat
earned Norman Borlaug
the Nobel Prize.
Cytokinins

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Adenine
derivatives.
Zeatin is the most
common.
Synthesized in root
tips and developing
seed.
Transported in
xylem.
Cytokinin promotes
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Cell division if auxin is present.
Photomorphogenesis, eg. Crown gall
formation
Lateral bud growth
Leaf expansion by cell enlargement
Stomatal opening
Chloroplast development
Cytokinin delays

Leaf senescence
Ethylene
H2C=CH2

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Chemical Structure

Gas
Synthesized from
methionine
Most tissues can
synthesize
ethylene in
response to stress.
Ethylene

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Transport occurs by diffusion.
Not absolutely required for growth.

Mutants with non-functional ethylene
gene develop normally.
Ethylene causes

The triple response in dark

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Reduced stem elongation
Stem thickening
Lateral growth
Ethylene stimulates
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Defense response to wounding or
disease
Release from dormancy
Shoot growth and differentiation
Root growth and differentiation
Adventitious root formation
Leaf and fruit abscission
Ethylene stimulates
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Flower opening
Fruit ripening
Abscisic Acid
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Synthesized from
glyceraldehyde-3phosphate through
carotenoid
pathway.
Synthesized in
roots, mature
leaves, and seeds.
Abscisic Acid
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Synthesis increases in response to
drought.
Transported from roots in xylem.
Transported from shoots in phloem.
ABA stimulates
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Stomatal closure
Root growth under water stress.
Storage protein synthesis in seeds.
Breaking dormancy
Defense response
ABA inhibits

Shoot growth under water stress.
Abscisic Acid Mutant
viviparous 5 (vp5) mutant in maize.
Polyamines
H
N
NH2 

H2N
Spermidine structure


Aliphatic amines.
Putrescine, spermidine
and spermine are most
common.
Effective at low
concentrations.
Mutants have abberant
development.
Brassinosteroids
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Brassinolide structure
Sixty steroidal
compounds.
Effective at low
concentrations.
Brassinosteriods stimulate
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Cell division
Cell wall loosening
Vascular differentiation
Ethylene biosynthesis
Brassinosteroids
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Required for fertility
Inhibit root development and growth
Jasmonates
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Methyl jasmonate
Jasmonic acid is
most common
Methyl esters
Scented
Jasmonate function
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Induce tuberization
Important in plant defense
Inhibit growth
Inhibit germination
Promote senescence
Promote pigmentation
Salicylic Acid
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Synthesized from
phenylalanine.
Promotes
production of
pathogenesis
related proteins.
Can reverse effects
of ABA in some
cases.
Signal Peptides
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Small molecules that can be
transported throughout the plant and
effect development.
Involved in defense response
Help determine cell fate
Involved in self-incompatibility
Involved in nodule formation in
legumes