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7/30/2019 08- Germination I
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Peter Toorop
Germination I
Imbibition, activation
and reserve
mobilization
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Imbibition water potential
membranes
leakage
osmotic stress
Energy production respiration mitochondria
electron transport pathway
DNA RNA protein DNA repair
DNA replication
transcription and translation
Reserve mobilization starch
oil
Protein
hemicelluloses
Outline
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Definition of germination
Germination begins with water
uptake by the seed (imbibition)
and ends with the start of
elongation of the embryonic
axis, usually the radicle.
Bewley and Black (1994)
Seeds: physiology of
development and germination
Bewley (1997)
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Imbibition curve triphasic
Coffea arabicaDa Silva et al (2004)
Solanum lycocarpumPinto et al (2007)
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Water potential
Water potential represents energy status of water
Pure water = 0 by convention
Solutes < 0 negative
cell = + c + p
= osmotic potential, determined
by dissolved solutes negative
c = matric component negative
p = pressure potential positive
In dry seeds c is more important than In imbibed seeds
is more important than c
c
p
cell
cell
cell
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Membranes
Water flows from high to low :In soil, seeds imbibe as a result ofhigh soil and low cell
Cell membranes have selective permeability Water flows freely across the membrane water channels
Solutes need to be transported actively ion channels
During imbibition membranes need to restore beforebecoming functional
Initial electrolyte leakage occurs
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Phase transition
During imbibitionmembranes changefrom gel phase toliquid crystallinephase
Electrolyte leakageoccurs in the gelphase
Early duringimbibition seeds leakelectrolytes
Crowe et al (1989)
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Electrolyte leakage
Sacand et al (2001)
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Osmotic stress
of the environment < 0
Saline soil
PEG
Hilhorst and Downie (1996)
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Imbibition water potential
membranes
leakage
osmotic stress
Energy production respiration mitochondria
electron transport pathway
DNA RNA protein DNA repair
DNA replication
transcription and translation
Reserve mobilization starch
oil
Protein
hemicelluloses
Outline
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Respiration
Pattern of oxygen consumption
by embryos follows three
phases (1-3), parallel to water
uptake
Storage tissues show a fourth
phase (4) associated with
senescenceO2consumption
Imbibition time
1
2
3
3
4
radicleprotrusion
After Bewley and Black (1994)
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Respiration
In excised embryos oxygen uptake follows water uptake
Logan et al (2001)
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Mitochondria
Mitochondrial structure is restored in embryos of maize seeds
during imbibition
0h 24h 48h
Logan et al (2001)
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Oxidative phosphorylation
reaction energy release
O2 + NADH + H+ H2O + NAD+ 220 kJ mol-1
Released energy is used to generate a proton gradient across the
mitochondrial membrane
Cytochrome C is the terminal oxidase
Proton gradient is used to generate ATP
Alternative oxidation AOX bypasses cytC overflow of energy
no ATP production downstream of ubiquinone
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Electron transport pathway
Millenaar and Lambers (2003)
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Mitochondrial activity cotyledons
Morohashi and Bewley (1980)
succinate
NADH
malate
-ketoglutarate
Respiratory rate in
pea cotyledons
during early
imbibition
Phase I: substrates
are succinate and
NADH
Phase II: substratesare malate and -
ketoglutarate
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Mitochondrial activity axes
Maize axes show cytC oxidase activity early during imbibition
This generates 3 ATP for every substrate molecule
AOX pathway may play a role in storage tissues
excess ATP production negative feedback
metabolic shutdown
no reserve mobilization
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Imbibition water potential
membranes
leakage
osmotic stress
Energy production
respiration mitochondria
electron transport pathway
DNA RNA protein DNA repair
DNA replication
transcription and translation Reserve mobilization
starch
oil
Protein
hemicelluloses
Outline
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DNA repair
DNA synthesis in maize embryos
open circles: thymidine incorporation
closed circles: total DNA
Early DNA synthesis:
repair
Late DNA synthesis:
post-germination replication
Zlatanova et al (1987)
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Transcription and translation
Early protein synthesis doesnot depend on de novotranscription during the first2-3h of imbibition
Stored mRNA declinesduring the first 2h ofimbibition of radishembryonic axes (B); afterthat de novo transcriptionincreasingly contributes toprotein synthesis
After Delseny et al (1977)
proteinsynthesis
0 2 4 6 8
Imbibition time (h)
A
B
cordycepin
water
cordycepin
mR
NA
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Transcription and translation
Transcription is requiredfor seedling development
Inhibition of transcriptiononly delays radicleprotrusion
Translation is required forradicle protrusion
Rajjou et al (2004)
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Transcription and translation
Inhibition of transcription inhibitssynthesis of
enzymes for reserve mobilization
proteins for metabolic activity
Inhibition of transcription inducessynthesis of
storage proteins
HSP70, HSP101
Rajjou et al (2004)
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Imbibition water potential
membranes
leakage
osmotic stress
Energy production
respiration mitochondria
electron transport pathway
DNA RNA protein DNA repair
DNA replication
transcription and translation Reserve mobilization
starch
oil
Protein
hemicelluloses
Outline
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Reserve mobilization starch
monocotyledonous spec
endosperm depletion
starch hydrolysis
alpha-amylase
beta-amylase
de-branching enzymes
alpha-glucosidase
hemicelluloses
sucrose synthesis and
transport to embryo
pericarp/testa
starchy endosperm
aleurone
scutellum
embryonic axis coleoptile + leaves
coleorhiza + radicle
after Ross Koning
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Reserve mobilization oil bodies
dicotyledonous spec oily
TAG glycerol + 3 fatty acids
glycerol
phosphorylation DHAP
G3P
glycolytic pathway
FA
-oxidation acetyl-CoA
pericarp/testa
embryo
after Ross Koning
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Reserve mobilization protein bodies
dicotyledonous spec endospermic
endopeptidases
aminopeptidases
carboxypeptidases
after Ross Koning
pericarp/testa
embryo
endosperm
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Nonogaki et al (2000)
LeMAN2
LeMAN1
http://www.lsbu.ac.uk/water/hygua.html
Reserve mobilization
dicotyledonous spec endospermic
endosperm depletion
cellulose
hemicelluloses
endo-enzymes
exo-enzymes
Toorop (1998) Photo: Adriaan van Aelst
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Germination
Germination begins with water
uptake by the seed (imbibition)
and ends with the start of
elongation of the embryonicaxis, usually the radicle.
Bewley and Black (1994)
Seeds: physiology of
development and germination
Bewley (1997)
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