Freshly harvested seeds usually
have primary dormancy
Primary
versus Typical release – “after-ripening”,
sometimes followed by light,
secondary temperature, smoke substances
like butenolide and compounds
like nitric oxide
seed
dormancy Secondary dormancy can be
induced in seeds after dispersal
(e.g. primary dormancy broken,
but e.g. soil too dry)
After Ripening
• http://www.seedbiology.de/afterripening.asp
• Several weeks storage at room temperature of
seeds often releases dormancy
• BUT – even after-ripened seed often does not
show complete germination, needs other
signals
• E.g. Bidens pilosa – shows 20% germination
after after-ripening, 90% after red light
Example
of After-
Ripening
Summary of PD in Arabidopsis –
note complex regulation – you will
see this again…..
Arabidopsis shows “Physiological dormancy”
(PD), but there are other types as well!
Morphological dormancy (MD)
Types of Morpho-physiological dormancy (MPD)
dormancy
Physical dormancy (PY)
Combinational dormancy (PY+PD)
PD non-deep – most common.
Just need after-ripening (any
temperature) for a couple of
weeks or cold (e.g. 0-10 °C) or
warm (above 15 °C) treatment
Type 1 -
Physiological
dormancy PD deep – Often need 3-4
(PD) months of cold or warm before
germinate.
PD non-deep
• Temperature most important factor
• Three main types; with time:
• 1 - temperature range for germination widens
from low to higher (e.g. Arabidopsis)
• 2 – temperature range for germination widens
from high to lower temperature (Helianthus)
• 3 – temperature range of germination widens
• Note, ability of GA and light to release
dormancy increases with time
• Temperature range for
germination increases
during dormancy release
as a continuum from
low to high.
• Example: Arabidopsis
thaliana
• Temperature range for
germination increases
during dormancy release
as a continuum from
high to low.
• Example: Helianthus
annuus
• Temperature range for
germination increases
during dormancy release
as a continuum from
medium to high and
low.
• Example: Aster
ptarmacoides
Suppression of germination by
exposure to high temperatures
One interesting In Arabidopsis, high temperature
effect - stimulates ABA synthesis and
thermoinhibition represses GA synthesis
Why do you think plants have
this???
Thermoinhibition
• Tagetes minuata is a “local”
example (actually Mexican)
Phytochrome B in endosperm?
• In Arabidopsis seedlings, temperature variations are detected
by a protein called phytochrome B
• Acs as a brake on plant growth in seedlings - increase of 1 to
2°C tends to inactivate phytochrome B, making it less effective
at preventing growth.
• “We found that thermo-inhibition in Arabidopsis is not autonomously controlled
by the embryo but implemented by the endosperm, revealing a new essential
function for this tissue,” explains Urszula Piskurewicz, researcher at the
Department of Plant Sciences of the UNIGE Faculty of Science, and first author of
the study. ‘”In other words, in the absence of endosperm, the embryo within the
seed would not perceive that the temperatures are too high and would begin its
germination, which would be fatal.”
Other Factors Breaking Dormancy
• Light (textbook page 438) – a Phytochrome
response (e.g. lettuce, Arabidopsis)
• ROS e.g. NO
• Smoke
• Nitrates
ROS (my interest!)
• ROS are produced during storage of seeds
• ROS can cause “carbonylation”, introduction of
CO into molecules
• This may knock out dormancy inhibiting
proteins, make storage proteins more easily
broken down, or initiate “redox signalling”
Hormones
and
Physiological
Dormancy
Induces dormancy and
maintains in imbibed
seeds after shedding
ABA Deficiency during seed
development prevents
primary dormancy
Over-expression of ABA
biosynthesis genes
increases seed dormancy
and delays germination
“variations in ABA levels in imbibed seeds appear clearly correlated with
seed dormancy. A decrease in ABA levels is observed in both dormant
and non-dormant seeds, however dormant seeds maintain higher ABA
levels”
Boron
deficiency
can block
ABA
synthesis
and cause
vivipary
Poa alpina
subsp.
Vivipara
When
might
vivipary be
advantage
ous?
Gibbs
• After “after ripening” (and sometimes
during), GA treatment often releases
dormancy
• Dormant seeds maintain a high ABA/GA ratio
(e.g. adding GA promotes ABA synthesis)
• Signals that promote germination (e.g. light
and temperature) promote GA synthesis and
inhibit GA degradation
DOG1 (delay of germination)
• The dog1 mutant is completely non-dormant
•
DOG1 interacts in a
complex way with ABA
and Gibbs to control
dormancy, but details
are only just now being
worked out
Transcriptome data have been used to identify more than 12,000 and
17,000 mRNAs in the mature dry seeds of Arabidopsis and rice,
respectively
“Recent studies have functionally
characterized increasing
numbers of diverse RBPs and
shown that they participate in
seed development and
performance, providing
significant insight into the role of
RBP–mRNA interactions in seed
processes. …recent research
progress (shows) RBPs have
crucial roles in RNA metabolism
and affect seed development,
dormancy, and germination.”
How can a dry seed do stuff?
Type 4 - Physical
dormancy (PY)
• Water-impermeable layers of
“palisade” cells in the seed or fruit
coat
• Seeds dormant until covering layer
permeable
• Examples: high temperatures, widely
fluctuating temperatures, fire, drying,
freezing/thawing and passage
through the digestive tracts of
animals.
In seed technology,
mechanical or chemical
scarification can break PY
Physical dormancy.
dormancy
(PY) (cont) Once PY is broken, i.e. the
seed or fruit coat becomes
permeable to water, the
seeds easily germinate
Unlike PD-seeds, which may re-
enter (secondary) dormancy after
primary dormancy is broken (once
the coat broken cannot be fixed!)
Physical
The mechanism for PY-break must
dormancy be fine-tuned to the environment.
(PY) (cont)
Can you think of the advantages of
PY dormancy in the field???
This palisade layer(s) prevent water uptake by their physical
arrangement as a tissue and the chemical coatings/impregnates of
the cells (heavy lignification, suberin-cutin matrix, waxes)
Evolution of dormancy
• PD - most common dormancy class
• PY is rarer, adaptation to specialized life
strategies or habitats (but not uncommon in
RSA?)
• PY not found in gymnosperms, and PY is only
evident in one Monocot family (Cannaceae)
Dormancy is complex!!!
Seed Technology
• See www.seedbiology.de for more details
• Two examples: “pelleting” and synseeds
“Pelleting”
Artificial Seeds
Germination
Triggered by imbibition of water
after possible dormancy
mechanisms have been released
by appropriate triggers
Embryo grows and often radicle
Germination breaks the testa / endosperm
Characterized by the loss of
desiccation tolerance
Cell elongation is enough for
radicle protrusion
Some say cell division is needed
Germination before a seed can truly be said to
have germinated
Note, some damage often happens
as a seed is hydrated (this is called
“imbibitional” damage)
One-step seed germination
• E.g. Brassica and pea seeds:
• N.B. Seeds non-endospermic
• Germination comprises testa rupture and
initial radicle elongation
Two-step seed
germination
• E.g. Lepidium (cress)
• Rupture of the testa
and endosperm are
separate events
• ABA can inhibit
endosperm rupture,
but not testa rupture,
inhibitory effect of
ABA is counteracted
by GA
Role of aleurone layer in grasses
Role of aleurone layer
• Surrounds the endosperm tissue of grass seed
• Germination - GA makes aleurone cells release
enzymes for starch hydrolysis (supplies sugars for
root growth)
• ABA can counter this, keeping the seed dormant.
• GA is used in brewing, specifically in the production
of barley malt where treatment ensures that a batch
of barley seeds will germinate evenly
• Undergoes PCD, see next lecture
