Seed germination - Howard University > Plant Biotechnology

Seed Germination: Signal Response
Dr. Caren Chang
Dept of Cell Biology and Molecular Genetics
University of Maryland, College Park
[email protected]
301-405-1643 (office phone)
Appropriate responses to internal and external signals are critical
for plant growth, development and survival
Lecture Outline:
1. What are seeds and how do they germinate?
2. Responding to plant hormones
3. Growth in light vs. dark
4. Responding to the environment: Tropisms
5. Dealing with environmental stress
What is a seed?
Is it alive?
What’s inside?
What is germination and
how does the seed germinate?
Ginkgo seed
Stages in seed development
1. The zygote develops and the endosperm
tissue proliferates through cell divisions
and tissue differentiation.
2. Cell division stops and storage
compounds accumulate.
3. The seeds dehydrate (losing up to 90%
of their water) and become “quiescent”.
Metabolism halts. The seed coat
becomes hard and impermeable to
water. The seed detaches from the
mother plant as a self-contained system
complete with embryo and food source,
ready for travel.
Lotus seeds
Seed germination
Germination occurs when the seeds are rehydrated. As
the seeds take up water, the following happens:
• The cells expand
• The seed’s food storage supplies become hydrated
• Inhibitors of germination are washed out
• Hydrated enzymes become active
• The seed increases its metabolic activities to produce energy
for the growth process
Seed Germination
This movie of seed germination was made by Professor Roger
Hangarter (at Indiana University) using time-lapse photography.
Germination of an Arabidopsis seedling
In the lab, we can germinate seeds on
solid agar media
When grown on agar plates,
we can see the roots
Breaking dormancy
• Seeds of many species are dormant, requiring additional
signals before germination can occur. They might have a
mechanical barrier to water or a physiological "block” that
prevents them from germinating.
• Some seeds will be not germinate until they undergo “afterripening”, which triggers a series of biochemical events,
allowing the embryo to become competent to germinate. For
example, some temperate species will not germinate until they
have been exposed to very cold temperature.
• Other seeds must pass through an animal's digestive tract
and become scarified before they can imbibe water and
become metabolically active.
• Still others must pass through fire. This is common in biomes
where annual fires are part of the normal climate.
The seeds of many species of plants, deciduous and evergreen trees
and shrubs along with some perennials and herbs will not germinate
without a pre-treatment known as stratification, which helps to
remove moisture barriers and physiological blocks.
Stratification simulates the natural conditions that a seed undergoes
before germinating.
Depending on the species, seeds will not break dormancy under too
wet or too dry growing conditions, imminent winter conditions, or
high temperatures. For seeds that require a development period, dry
storage often suffices.
Lecture Outline:
1. What are seeds and how do they germinate?
2. Responding to plant hormones
3. Responding to light vs. dark
4. Responding to the environment: Tropisms
5. Dealing with environmental stress
Plant hormones
Abscisic Acid (ABA)
ABA controls many plant processes including embryo maturation,
germination, cell division and elongation, and adaptive responses to
environmental stresses such as drought, salinity, cold, pathogen attack
and UV radiation.
• ABA induces seed dormancy. For instance, ABA is produced in
abundance in the late summer/early fall. The seeds become dormant
and cannot germinate even if they are dispersed in autumn. During
the winter, the abscisic acid is degraded. By spring the abscisic acid is
gone and the seeds can germinate.
• Abscisic acid degrades during stratification. When the seeds are then
placed in a warm greenhouse (or in an incubator), the seeds assume
that spring has come and begin to sprout.
What might happen if seeds do not have ABA?
Vivipary: precocious germination
Is ABA a positive or
negative regulator of
seed germination?
EXPT: Germinating Arabidopsis seeds
on ABA
How would you design an experiment to
demonstrate the effect of ABA on seed
In the experiment that you will begin today, you will
sow seeds on agar growth media containing three
different concentrations of ABA (a dose response)
Lecture Outline:
1. What are seeds and how do they germinate?
2. Responding to plant hormones
3. Growth in light vs. dark
4. Responding to the environment: Tropisms
5. Dealing with environmental stress
What happens in the DARK?
Another time-lapse movie by Roger Hangarter
This short movie shows Arabidopsis thaliana seeds
germinating in darkness on a vertical agar surface
and growing over a period of a few days (1 image
captured each hour). Arabidopsis seeds are very
small, less than 1 mm long. Typical of most seeds,
the root emerges first. Once the root has emerged,
the shoot begins to grow.
How does this growth differ from
germination in the light?
Germination in darkness
Etiolation: Growth in the absence of light
When germination occurs in darkness (such as under the soil), the
following occur:
• Root growth slows down
• Shoot elongation accelerates (The shoot becomes long and
• The stem has an apical hook with cotyledons closed together
• Chlorophyll production is suppressed, so the plant is not green
• Leaf development is suppressed
Germination in the Light
Photomorphogenesis: developmental effects
that are controlled by light
Once the seedling emerges into the light, the plant undergoes
the following dramatic changes :
The apical hook opens
The cotyledons separate and expand in size
Shoot elongation slows down
Root growth increases
Leaves develop
The chloroplasts develop and produce chlorophyll, so the
plants are green
Light vs. Dark
Two days in the life of germinating sunflower seedlings
EXPT: Germinating Arabidopsis seeds
in the light vs. dark
Lecture Outline:
1. What are seeds and how do they germinate?
2. Responding to plant hormones
3. Growth in light vs. dark
4. Responding to the environment: Tropisms
5. Dealing with environmental stress
What is tropism?
• Directional growth movement in response to an external stimulus
What are some examples?
For example, roots monitor their surroundings and react to gravity and
touch stimulation, because they need to anchor the plant in the soil and
supply the plant with water and nutrients.
Why do plants have tropisms?
Plants must monitor environmental conditions to optimize their growth and
survive. Plants have the ability to detect favorable growth conditions, and can
alter their growth to move toward those favorable conditions.
towards (or away from) light
oriented relative to gravity
• THIGMOTROPISM: growth in
response to touch
towards water
Phototropism in tomato seedlings
This movie shows tomato seedlings that have been germinated in the dark. After 4 days of
growth in the dark, a dim blue light is turned on and a movie starts recording the action.
Images were captured every 20 min and are played back at 6 frames per sec.
Why did the plants exhibit a curvature to the left?
Phototropism in corn seedlings
This movie shows a similar experiment using corn seedlings. Images were
captured every 20 min and are played back at 6 frames per second.
Gravitropism in Arabidopsis
This movie shows the negative gravitropic
response of Arabidopsis flower stems
after placing the plant on its side. In just
over 1 hour, the stems reorient
themselves back to vertical. The images
for this movie were captured every 15
min in darkness using infrared lighting.
Frames are played back at 6 frames per
Light and gravity
Corn seedlings again. After 3 days of growth in darkness, a single light bulb over
the center of the pot was turned on. The plants were imaged every 10 minutes
for 18 hours. For the first 14 hours the seedlings appear to be worshiping the light
as they maintain phototropic curvature. Shown at 12 frames per second.
Why do the seedlings straighten near the end of this movie?
Nutation -Thigmotropism
Morning glory vines photographed every 10 minutes
What about ROOTS? This movie by Roger Hangarter shows a
root of corn (maize)
This movie by Professor Simon Gilroy (at University of Wisconsin at
Madison) shows the root of an Arabidopsis seedling hitting a glass barrier
(12 hours)
What do you predict will happen when the root hits the glass?
How do tropisms occur?
Phototropism is a blue-light-dependent response controlled by the action
of specific blue light photoreceptors called phototropins.
Gravitropism is dependent on the presence of starch-filled plastids
(amyloplasts) in specialized cells. When the orientation of the cells
changes, the mass of the starch-filled plastids causes them to sink to the
lower end of the cell. The tumbling of the amyloplasts triggers differential
growth that causes curvature to develop.
The differential growth in tropisms is due to the plant hormone auxin.
Auxin stimulates cell division and cell elongation.
Where is auxin located?
• Shoots and roots have opposite responses to
phototropism and gravitropism
• Shoots and roots have opposite responses to
What if there is NO GRAVITY such as in
outer space?
Plants in outer space
Seeds germinated on Space Shuttle mission 91 (STS-91)
The WAICO experiment (= Waving and Coiling of Arabidopsis Roots at
Different g-levels) could help scientists figure out how to grow crops in
space for future astronauts during long space voyages. Scientists are
growing two types of Arabidopsis seeds in both zero gravity and Earth
gravity during the study.
Arabidopsis seedlings tilted at 45
degrees on hard agar surface.
(Top) Wild type seedlings showing
more root waving.
(Bottom) Mutant strain showing more
root coiling.
EXPT: Germinating seeds against gravity
Lecture Outline:
1. What are seeds and how do they germinate?
2. Responding to plant hormones
3. Growth in light vs. dark
4. Responding to the environment: Tropisms
5. Dealing with environmental stress
Salinity in soil around the world
• High salinity in soil is a common environmental
problem in irrigated lands and in arid and semiarid regions, yielding poor crop production.
• The problem is often associated with high water
tables, caused by a lack of natural drainage
• Low precipitation is the major factor causing high
soil salinity worldwide.
• The primary cause of man-made soil salinity is
the salt that is brought in with irrigation water.
Salt stress in plants
A high salt level interferes with seed germination, because
water and nutrients cannot move into the plant roots. Salinity
acts like drought on plants, preventing roots from performing
their osmotic activity where water and nutrients move from an
area of low concentration into an area of high concentration.
As salinity levels increase, the stress on germinating seedlings
increases. Salinity can have a toxic effect on plants because
of the high concentration of certain salts in the soil. Salinity
prevents the plants from taking up the proper balance of
nutrients they require for healthy growth.
Salt stress
EXPT: Germinating seeds under high
salt conditions
Arabidopsis thaliana
The life cycle is short--about 6 weeks from germination to
seed maturation.
Seed production is prolific and the
plant is easily cultivated in restricted space.
Self-fertilizing, but can also be out-crossed
by hand.
Relatively small genome (1.5 MB), completely sequenced
Extensive genetic and physical maps of all 5 chromosomes
A large number of mutant lines and genomic resources is
available - Mutants are available in nearly every gene
Genetic transformation is simple using Agrobacterium
Extensive databases for gene expression analyses,
multinational projects, etc.
From the website of Dr. Simon Gilroy (University of Wisconsin at Madison)

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