08- Germination I

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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membranes

leakage

osmotic stress

Energy production respiration mitochondria



DNA replication

transcription and translation


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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membranes

leakage

osmotic stress

Energy production

respiration mitochondria



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 is requiredfor seedling development

Inhibition of transcriptiononly delays radicleprotrusion

Translation is required forradicle protrusion

Rajjou et al (2004)


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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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membranes

leakage

osmotic stress

Energy production

respiration mitochondria



DNA replication

transcription and translation Reserve mobilization

starch

oil

Protein

hemicelluloses

Outline


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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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08- Germination I