Seed Germination: Signal Response

Dr. Caren ChangDept of Cell Biology and Molecular GeneticsUniversity of Maryland, College Parkcarenc@umd.edu301-405-1643 (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 development1. 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 “after-ripening”, 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.

StratificationThe 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 germination?

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?

From: http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

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 spindly) • The stem has an apical hook with cotyledons closed together• Chlorophyll production is suppressed, so the plant is not green• Leaf development is suppressed

WHY?

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

From: http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

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.

Tropism

Tropisms

• PHOTOTROPISM: growth towards (or away from) light

• GRAVITROPISM: growth oriented relative to gravity

• THIGMOTROPISM: growth in response to touch

• HYDROTROPISM: growth towards water

Phototropism in tomato seedlingsThis 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.

From: http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

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.

From: http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

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 second.

http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

Gravitropism in Arabidopsis

Light and gravityCorn 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.

From: http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

Why do the seedlings straighten near the end of this movie?

Nutation -Thigmotropism

http://sciencetrack.blogspot.com/2007/07/twining-motion-of-vines.html

Morning glory vines photographed every 10 minutes

What about ROOTS? This movie by Roger Hangarter shows a root of corn (maize)

http://www.botany.wisc.edu/gilroy/Site/Movies.html

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?

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.

How do tropisms occur?

Where is auxin located?

• Shoots and roots have opposite responses to phototropism and gravitropism

• Shoots and roots have opposite responses to auxin

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 semi-arid regions, yielding poor crop production.

• The problem is often associated with high water tables, caused by a lack of natural drainage underground.

• 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 plantsA 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 tumefaciens

• Extensive databases for gene expression analyses, multinational projects, etc.

From the website of Dr. Simon Gilroy (University of Wisconsin at Madison)http://www.youtube.com/watch?feature=player_popout&v=oMCvGrKyTWI