Download Angiosperms VII

Angiosperms VII
Internal Control of
Development:
The Plant Growth Regulators
The Growth Regulators
• Often referred to as “hormones”
– adapted from animal physiology
– animal hormones are produced in one
place (gland) and exert an effect some
other location
– plant “hormones” don’t always work like
that
– “growth regulator” better descriptive term
Five Basic Groups
•
•
•
•
•
Auxins
Cytokinins
Ethylene
Abscisic Acid
Gibberellins
Early Work on Phototropism
• Charles Darwin and his son Francis
published early work on the
problem:
– “Power of Movement in Plants” (1880)
– Worked with oat (Avena) Coleoptiles
– Demonstrated importance of the “tip”
in the plant response
Early Darwin Experiments
Discovery of Auxin
• Fritz Went (1928)
– Found that he could collect the
substance responsible for the
bending response
– Developed the Avena Coleoptile
Curvature Test (first bioassay)
– Went called the unknown substance
AUXIN
Coleoptile Curvature Test
What is Auxin?
• German workers in the 1930’s
identified Went’s auxin as indoleacetic acid (IAA)
– Lucky discovery based on study of
urine (so the story goes) in pregnant
women
Formula of IAA
Effects of IAA In Phototropism
• Causes local elongation of cells on the
shaded side
• This unequal elongation causes the
bending of the stem (coleoptile) toward
the light
• Mechanism of action is called the acid
growth hypothesis
Acid Growth Hypothesis
• IAA triggers H+ ion transport which
lowers the pH
• This drop in pH loosens cell wall
structure probably via proteins
(expansins) which “break” bonds holding
the cellulose microfibrils
• This creates a “loosening” of the cell wall
structure so turgor pressure can
“expand” the cells
• The effect is quite rapid
Cell Wall Expansion
Natural vs. Synthetic Auxins
• IAA (indole-acetic acid) is the naturally
occurring growth regulator
• Synthetic auxins
– substances which will cause bending the
Avena Coleoptile Curvature Test
– examples include: indole-butyric acid (IBA),
naphthalene acid acid (NAA), 2,4-D and
2,4,5-T
AUXIN RESPONSES
• Apical Dominance
– IAA produced in the shoot apex inhibits the
development of lateral (axillary) buds
– However, it may be that high IAA
concentrations stimulate ethylene
production which actually inhibits the bud
development
– Concept used frequently in horticulture
(creation of “bushy” shrubs)
Other Auxin Responses (cont.)
• Abscission
– Actively growing leaves and fruits produce
large amounts of auxin (IAA) which is
transported to the stem
– This inhibits abscission of leaves and fruits
– Environmental or age changes stimulate
production of ethylene which stimulates
production of abscission zone forming
enzymes
Other Auxin Responses (cont.)
• Differentiation of
Vascular Tissue
– Auxin + gibberellins
and/or various
concentrations of
sucrose can
stimulate
development of
xylem/phloem
(either or both)
Other Auxin Responses (cont.)
• Fruit Development
– Seeds (result of fertilization) are a source of auxin,
which in turn stimulates the formation (not
ripening) of the fruit
– May form parthenocarpic fruits (tomato and
cucumber)
Other Auxin Responses (cont.)
• Adventitious Root Formation
– Several synthetic auxins (especially IBA)
are used commercially to stimulate root
development in “cuttings”
– Some plants produce enough IAA in the
shoot or leaves to stimulate the root
formation in a cutting without additional
hormone
Other Auxin Responses (cont.)
• Weed Killers and
Defoliants
– 2,4-D and 2,4,5-T
effective against
dicots
– manufacture as part
of “Agent Orange”
produced toxic
trace molecule
dioxin (a
carcinogen)
Cytokinins
• Discovered in the 1940’s in attempting
plant tissue culture
• Found that coconut milk stimulated cell
division
• Trail led to “old herring sperm DNA”
• Isolated “kinetin” and dubbed the group
of growth regulators cytokinins (after
cytokinesis in cell division)
Cytokinins (cont.)
• Zeatin the first naturally-occurring
cytokinin to be isolated
• Most cytokinins are produced in roots,
but also in seeds, fruits and young
leaves
• Effects include:
– stimulation of cell division
– retard senescence in leaf tissues (once
used as a bioassay)
Cytokinins (cont.)
– with IAA, stimulate
formation of either
roots or shoot
• HIGH IAA, low
cytokinin = ROOTS
• HIGH CYTOKININ,
low IAA = SHOOTS
Cytokinins (cont.)
– generally a “juvenile” hormone =
keeps things young
– used commercially to keep cut
foliage “green and fresh”, but NOT
for human consumption (a
suspected carcinogen because of
its nucleotide structure)
Strange Observations?
• Burning of “illuminating gas” in Europe
in 1800s caused trees near street lights
to become defoliated on one side
• Oranges can cause rapid ripening of
bananas (don’t store them together)
• The ancient Chinese burned incense in
special huts to ripen fruit
• “One bad apple can spoil a whole
bushel”
Ethylene
• A gas (H2C=CH2), unusual for a growth
regulator
• Produced in most CLIMATERIC fruits
like apples, oranges, tomatoes,
bananas
• Used widely in the commercial fruit
industry (here in Omaha)
– The apples you buy in March were
probably picked in September
Other Ethylene Responses
• Promotes flowering in some plants like
mangos and pineapples
– Some growers may actually set fires near crops
• May induce senescence in some flowers
(orchids)
• Generally promotes leaf and fruit
abscission
• Also involved in in monocot sex expression
flowers, stem elongation (shaking response
inhibits normal elongation), waterlogging
effects (epinasty)
Abscisic Acid (ABA)
• First extracted from dormant buds and
called dormin
• Later, found to be chemically identical
with another compound called abscisic
acid (unfortunate choice since it is not
involved in abscission)
• Involved in closure of stomata (guard
cells) by stimulating loss of K+ ions
(followed by water loss and closure)
Gibberellins
• Discovered by E. Kurosawa studying
“foolish seedling” disease of rice
• Fungus, Gibberella fujikuroi, found to be
disease agent
• Could induce symptoms (stem
elongation) from fungus extract
• Later, found same substances in plants
themselves
Effects of Gibberellins
• Growth of Intact
Plants
– elongation AND
cell division
throughout the
plant (unlike auxin)
– overcomes genetic
dwarfing
Gibberellins and Mendel
• One of the 7 pairs of traits that Mendel studied in
peas as he worked out the basic rules of
inheritance was dwarf-tall.
• The recessive gene - today called le - turns out to
encode an enzyme that is defective in enabling the
plant to synthesize GA.
• The dominant gene, Le, encodes a functioning
enzyme permitting normal GA synthesis and
making the "tall" phenotype.
• Seed
Germination
(grasses)
– produced by the
embryo and
stimulates the
aleurone layer to
synthesize
amylases
Effects of
Gibberellins (cont.)
Starch Digestion in Seeds
Treated with 1 ppm GA
Treated with 1 ppb
Treated with water
Effects of Gibberellins (cont.)
– Used in production of sugarcane
(increases biomass)
– Mechanism of action not involved in
cell wall acidification
• May overcome light or cold
requirements for seed germination
• used in brewing industry to help
germinate barley and produce the
“malt”
Effects of Gibberellins (cont.)
• Flowering/Bolting
of Biennials
– can substitute for
“winter” cold
period for bolting
(flowering) in
rosette biennials
Effects of Gibberellins (cont.)
• Fruit Formation
– used to
produce larger
fruits in open
clusters in
Thompson
seedless
grapes