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39 - Plant Responses
General Model for signal transduction
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Hormones: powerful
F.W. Went
(1926)
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Shaded side!
of coleoptile!
Explanation
Light rays!
Illuminated!
side of!
coleoptile!
SBM Fig. 36.15!
Promotes cell growth (mainly elongation)
Acid-growth hypothesis: auxin stimulates the proton pumps in the
plasma/cell membrane
auxin
involved in:
shoot phototropism
root gravitropism
differentiation of vascular tissue
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Apical dominance
Promote root
formation (IBA)
As herbicide (2,4-D)
(overdose for dicots)
Auxin concentration (mg/L)!
(a)!Initial!
explant!
(b) Callus!
Cell division!
with differentiation!
(c) Roots!
2.0!
(d) Shoots!
0.5!
0.4!
0.3!
1.0!
0.2!
0.1!
0!
0!
Auxin!
Cytokinin concentration (mg/L)!
Cell division!
without differentiation!
Tissue
Culture
Cytokinin!
SBM Fig. 36.18!
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effects
a. promotes cell division
b. cell enlargement
c. interacts with auxin in apical dominance
Auxin concentration (mg/L)!
(a)!Initial!
explant!
(b) Callus!
Cell division!
with differentiation!
(c) Roots!
2.0!
(d) Shoots!
0.5!
0.4!
0.3!
1.0!
0.2!
0.1!
0!
0!
Auxin!
Cytokinin concentration (mg/L)!
Cell division!
without differentiation!
Tissue
Culture:
lower
cytokinin
to get
roots
Cytokinin!
SBM Fig. 36.18!
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Slow down senescence
2 leaves
close to the
same age
Stem
elongation
Phinney s
dwarf corn
(at UCLA)
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Induce
flowering “bolting”
Others:
Break
dormancy in
buds & seeds
Lower
temperature
threshold of
growth
(b)!
Do not post photos on Internet!
Fig. 36.17b!
Required for fruit development
Thompson seedless grapes
Seed germination & starch digestion (part of malting for
beer)
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Precocious
germination due to
low levels of ABA
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Promotes cell growth
Triple response
Slowing of stem elongation
Stem thickening
Horizontal stem growth
Leaf senescence
& abscission
Fruit (not all) ripening
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Arabidopsis germinated with brassinosteroid
biosynthesis inhibitor Brz
Jasmonates
JA or SA
Striga
witchweed
help control seed germination, apical dominance, mycorrhizal
associations & adventitious root formation
10
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Florigen
Now known to involve
FT protein or RNA
Leaf -> Phloem -> SAM
induction
Can be done with 2
different species grafted
Phytochrome
Influencing seed
germination:
order of red & farred illumination
(lettuce seeds)
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Elongated
growth:
etiolation
Photoperiodism: effects on flowering
switchable
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Biological clocks: Circadian rhythms
Sleep movements
Others: dinoflagellate bioluminescence (photoperiodism)
Nastic
Movements
e.g.: sensitive
plant,
Venus fly trap
Tropisms:
Phototropism
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Tropisms: helioptropism
Sun rising!
over horizon!
Morning!
Noon!
Afternoon!
Fig. 36.11!
Tropisms: gravitropism
Thigmotropism: response to mechanical stimulus
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Methyl-salicylic acid
& salicylic acid
Phytoalexins
PR proteins
JA or SA
Jasmonic acid
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