Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Five common hormones found in
plants
•
•
•
•
•
Auxins
Gibberellins
Cytokinins
Abscisic acid
Ethylene
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Phototropism and the
unequal distribution of
auxin
Fig. 36-16, p. 699
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
• How does auxin promote cell elongation?
• Acid growth hypothesis
• IAA increases the expression of H+-ATPase.
•
•
•
•
•
•
•
H+ are pumped into cell wall.
They activate expansins.
Ca 2+ cross-linked cellulose is broken.
H+ replaces Ca 2+
These loosen cellulose microfibrils.
Internal water drives cell elongation.
Only effective with non-lignified cells
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Where is Auxin synthesized?
• Apical meristems of shoots,
young leaves
• Seeds
• Any region of rapid cell division
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
• Auxin is involved in
–Cell elongation
–Tropisms
–Apical dominance
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
• Auxin, cont.
–Fruit development
–Stimulating root development on
stem cuttings
• Some synthetic auxins (2,4-D
and 2,4,5-T) are selective
herbicides
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Rate of Auxin transport
• Faster than simple diffusion
• Slower than phloem/xylem
• Thought to be transported in
xylem parenchyma cells
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Auxin transport
•Direction of transport is unidirectional
IAAR
No IAAR
C
A
B
No radioactive IAA found in agar when stem
segment is inverted
IAAH in cell wall, IAA- in cytoplasm
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Why is movement in a basipetal direction
• Auxin receptor transport proteins
are only found on the basal side of
the cells.
• When auxin is in the cell wall, it is
protonated (IAAH)
• So it diffuses readily into the cell
• When it is in the cytoplasm, it loses
its proton (IAA-) and needs a
protein acceptor to move it into the
cell wall again.
• Protein receptors are found on the
basal side of the cells
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
• Gibberellins
–Stem elongation
–Flowering
–Germination
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Gibberellin and
stem elongation
Fig. 36-18, p. 701
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
• Cause stem
elongation
Increased
cell elongation
& division
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Discovery by Japanese
•Foolish seedling disease
•Rice plants bolted (“foolish seedling”)
•Contained a fungus, Gibberella fujikuroi
•Scientists later isolated gibberellic acid.
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
• British and American scientists
later isolated two similar
compounds.
• Named gibberellic acid
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
What is its structure?
• Over 100 different GA
compounds
OH
O
CO
CH2
HO
CH3
OH
O
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Who has them?
• Fungi,
• All plants
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Synthesis and transport
• Made in root tips, young leaves,
seed
• Transported
• In roots via xylem
• In stems via phloem
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
How does it increase elongation
• Cell wall become extensible
without acidification
When GA is applied,
No protons are extruded
However, auxin is always present in tissues where gibberellin acts
Auxin and gibberellins may act together in bringing about cell
elongation
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Fruit growth
•Grapes treated
with GA will
produce larger
grapes.
•Long internodes
gives more space
for larger grapes
to develop.
Apples are also sprayed to form elongated apples.
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Gibberellins induce flowering in
biennuals
• Biennuals are plants that flower
only in their second year.
• Example: sugar beets
typically flower in their
second year.
• Gibberellins application can
cause plants to bolt and
flower during the first year.
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Gibberellins promote seed germination
Endosperm
E
Aleurone layer
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Biology, Seventh Edition
• Embryo
produces
GA’s stimulate
CHAPTER 36 Plant Growth and Development
Embryo uses water
and sugars to grow
Aleurone layer
Produces
α-amylase
More water
Moves in
Breaks down starch
to form glucose
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Increase in
solutes
Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Cytokinins
•
•
•
•
Cell division
Shoot formation
Delays senescence
Inhibits apical dominance
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Discovery of Kinetin at the University of
Wisconsin
• Tissue culture experiments
showed that nucleic acids
promoted undifferentiated pith
growth in tobacco
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Little background about the unique nature
of plant cells
• Plant cells are totipotent.
• What does this mean?
• Every cell has the potential to
develop into a new plant under
the right conditions.
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
• Mature cells can dedifferentiate
to become meristematic cells.
• Examples: vascular cambium
arises from parenchyma cells.
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
• Tissue culture exploits
totipotency to produce
numerous replicates of a given
cell.
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Plants, herring sperm, & coconut milk?
• Tobacco tissue culture
• Totipotent cells
• How to get differentiation?
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
•Autoclaved
.
herring sperm DNA had the
most powerful effect on cell division
Later discovered that kinetin was
causing this effect
Kinetin is a greak down product of
adenine
Adenine is a purine base of DNA
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Identified in 1950’s
OH
NH 2
HN
N
N
NH
N
Zeatin
N
N
N
Adenine
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NH
Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Who has it?
• Pathogenic bacteria
• Fungi
• All plants
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Cytokinin physiology
• Synthesis and transport
• Produced in root tips, embryos,
fruits
• Transported from root to shoot in
xylem
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Cell division
• Cytokinins activate cyclin
dependent kinases that will
cause cells to move from G2
phase into mitosis during the
cell cycle.
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Mitosis
ase
h
p
o
Pr
Metaphase
Anaphase
Tel
oph
ase
Cytokinesis
Interphase
Interphase
cytokinins
G2
(Second gap phase)
Cyclin –
dependent
Protein kinases
G1
(First gap phase)
S
(Synthesis phase)
Chromosomes duplicated
Interphase
Fig. 9.02
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Common occurrence of gall formation on
plants is caused by rapid cell division
Undifferentiated callus tissue (cell with no identity
Continue to divide)
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Caused by infection with Agrobacterium
tumefaciens
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
• Bacterium contains a plasmid
(ipt) that contains a gene that
codes for an enzyme that
makes cytokinins.
Wounded plants cells
divide to form a mass of callus
tissue called a gall
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Cytokinins promotes shoot growth when
relative proportions of cytokinin:auxin
increases
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Auxincytokinin
interactions
in tissue
culture
Fig. 36-19, p. 701
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Cytokinin delays senescence
•Delays senescence
•In transgenic tobacco
that overexpressed
Ipt gene (cytokinin
synthesis), the plants
senesced later.
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Cytokinins inhibit apical dominance
• Cytokinins promote lateral bud
growth.
• Tobacco plants that
overexpress cytokinins tend to
be bushier than wild type
plants.
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Quiz 8 Question 1
• Which of the following
compounds will promote lateral
bud formation?
• A. auxins
• B. cytokinins
• C. gibberellins
• D. phytochromes
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Question 2
• Which organs typically exhibit
gravitropism?
• A. shoots
• B. roots
• C. buds
• D. flowers
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Question 3
• If you cut a stem, invert it and
apply a radioactive IAA label to
the cut end, would you expect
to see any radioactive IAA in
the regions below the region of
application?
• A. Yes
• B. No
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Question 4.
Which of the following is thought to turn off
genes in the shoot apex during flowering?
A.
B.
C.
D.
E.
Gibberellins
Auxins
Phytochrome
A and C
B and C
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Biology, Seventh Edition
CHAPTER 36 Plant Growth and Development
Question 5. Crown gall formation is
typically due to
• A. plants producing cytokinins
• B. Bacteria producing cytokinins
inside the site of infection.
• C. plants producing auxins
• D. plants producing gibberellins
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