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Chapter 39

Plant Responses to Internal and External Signals

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Response to Stimuli

Plants are sensitive to a wide range of stimuli.

They elicit a response.

They use a signal transduction pathway.

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Response to Stimuli

Consider a forgotten potato in the cupboard.The eyes of the

potato (axillary buds) sprout shoots that are suited to their function. They are pale and lack broad green leaves. They lack elongated roots.

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Etiolation

These adaptations for growing in the dark are called etiolation.

The “stimulus” for growth is complete darkness.

The plant uses all energy to elongation of the stem so the leaves can open after reaching the surface.

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Etiolation

During etiolation, there is no evaporative loss of water.

Leaves would be a hindrance to the shoot passing through the soil.

There’s no need for chlorophyll--there’s no light.

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De-etiolation

When the shoot hits the light, de-etiolation occurs.

Leaves now expand.Elongation of the

stem slows.Roots elongate.

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Signal Reception

Signals are detected by receptors.

Proteins change in response to the stimulus.

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Signal Transduction

Second messengers are small, internally produced chemicals.

They transfer and amplify signals from the receptor to the other proteins causing a response.

One signal receptor protein can give rise to hundreds of specific enzymes.

In this way, 2nd messenger signal transduction leads to rapid amplification of the signal

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Response

Signal transduction leads to one or more cellular pathways being regulated.

Usually, this leads to an increase in the activity of certain enzymes.

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Response: 2 Main Mechanisms

1. Stimulating transcription of mRNA.

2. Activating existing enzyme molecules.

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1. Stimulating Transcription of mRNA

This is called transcriptional regulation.

These transcription factors bind directly to DNA molecules and control the transcription of specific genes.

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The De-etiolation Response and Phytochrome

The receptor involved in de-etiolation is a phytochrome.

The mutant tomato studied has lower levels of phytochrome.

They green less when exposed to light than normal tomatoes.

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The De-etiolation Response and Phytochrome

When the mutants were injected with phytochrome from other plants they exhibited a normal de-etiolation response when exposed to light.

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The De-etiolation Response and Phytochrome

Small amounts of light can trigger the de-etiolation response.

In the phytochrome example, small amounts of light give rise to activated phytochrome.

This gives rise to hundreds of second messenger molecules which leads to hundreds of activated enzymes.

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The Change in Phytochrome

Light causes the conformation of phytochrome to change. This leads to an increase in cGMP (2nd messenger) and

Ca2+ influx. cGMP activates protein kinases.

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If we inject the mutant tomatoes with cGMP, we get a partial de-etiolation response--even without the addition of phytochrome.

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Protein kinases are activated by cGMP and Ca2+, and can act to phosphorylate and activate other enzymes.

These can be used to stimulate or shut down transcription.

When transcription is affected, the enzymes can now synthesize proteins for chlorophyll production and other de-greening proteins.

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The mechanism by which a signal promotes a new developmental course depends on the activation of positive or negative control factors.

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Post-translational modification involves activating existing enzyme molecules.

This is where existing proteins are modified--usually via phosphorylation.

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Kinases

Often, kinases become activated by phosphorylation which activates more kinases, and so on.

Eventually, the cascades link initial stimuli to responses at the gene level where they are expressed.

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Signal Pathways

Signal pathways lead to both a turning on and off of genes.

For example, putting a potato back into the cupboard activates many phosphatases which dephosphorylate specific proteins and switch off certain pathways.

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The Idea of Signal Pathways

Classic experiments studying grass uncovered the notion of chemical messengers.

The movement of a plant shoot toward or away from a stimulus is called tropism.

The hormone pathway.

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Signal Pathways and Grass Seedlings

Phototropism is the process that directs plants toward sunlight for photosynthesis.

Grass shoots kept in the dark will grow straight up.

So will those illuminated equally on all sides.

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Signal Pathways and Grass Seedlings If illuminated from only one side, the plant will grow

toward the stimulus.

This results in differential growth on the opposite side of the stimulus.

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The Darwins’ Experiments

Observations:Plants will only bend toward the light

source if the coleoptile is present--no tip, no curve.

Covering the tip with an opaque cap prevents curving.

Covering the tip with a transparent cap or placing a cover below the tip--curving.

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The Darwins’ Experiments

Their conclusions:

The tip of the coleoptile is responsible for curvature.

Also, the curvature of the plant actually was the result of differential growth some distance below the coleoptile.

Some signal must be responsible for elongation of the coleoptile.

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Peter Boysen-Jensen

A few decades later:

He separated the tip of the coleoptile with a block of gelatin.

The cells still showed the normal growth response.

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Peter Boysen-Jensen

Using mica, there was no response.

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Frits Went

A Dutchman that modified Boysen-Jensen’s experiment to extract the chemical messenger.

He removed the coleoptile tip and placed it on an agar block.

If the messenger could diffuse into the block, then it could be substituted for the tip and placed on the decapitated coleoptile resulting in normal growth.

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Frits Went

Results: Decapitated

coleoptile--no growth.

Decapitated coleoptile + agar only--no growth.

Decapitated coleoptile + agar with hormones--growth.

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Frits Went

Also, placing the block on one side of the coleoptile or the other caused unequal growth on the side containing the block causing curvature in the opposite direction.

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Frits Went

Conclusions:The plants curved due to the higher

concentration of growth promoting chemical on the dark side of the plant.

Went named this hormone auxin.

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Went’s Model

This model doesn’t necessarily occur in all plants.

There is still an unequal distribution of auxin in a plant causing curvature.

Some plants show an increase in growth inhibitors on the light side of the plant.

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Plant Hormones

There are many different classes of plant hormones.

They all have different effects on plants.

Most are produced in very small amounts and often have profound effects on the plant.

The hormones are often amplified and acts to alter gene expression.

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3 Common Plant Hormones

Auxin-stimulate growth. Produced in the embryo, growth tissue, and meristematic tissue.

Gibberillins--produced in the apical meristems of buds and roots, young leaves and embryos.

Ethylene--promotes ripening. Opposes auxin.

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Light

Light is an important environmental factor in the growth and development of plants.

Photomorphogenesis is the effect of light on plant morphology.

The ability of a plant to perceive light allows plants to measure the passage of days and seasons.

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Circadian Rhythms

Common to all eukaryotic life, and is not governed by an known environmental factor.

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Photoperiodism

Is the physiological response of plant due to a change in the lengths of night and day--a photoperiod.

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Different Types of Plants

There are 3 general varieties of plants classified according to their light requirements for flowering:1. Short-day plants2. Long-day plants3. Day-neutral plants.

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Short-Day Plants

Respond the long nights.

A.k.a. long-night plants.

They usually flower in the late summer, fall, or winter as the light period is shorter than 14 hours.

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Long-Day Plants

Respond to short nights.

A.k.a. short-night plants.

They flower when the light period is longer than 14 hours.

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Day-Neutral Plants

These are unaffected by the light period, and flower when they reach maturity.

Tomatoes, rice, and dandelions.

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Classic Experiments

In the 1940’s scientists began experimenting with photoperiods.

They looked at the length of the night and day.

They found that short-day plants flower when days are 16 hours or shorter (nights are 8 hours or longer).

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Classic Experiments

In the short-day plants, they looked at flowering: They found that if the daytime portion of

photoperiod is broken by a brief period of darkness, there is no effect.

However, if the nighttime portion of the photoperiod is interrupted by a short period of dim light, the plant doesn’t flower.

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Classic Experiments

The opposite is true for long-day plants:When long day plants are grown in a

photoperiod of a long night, flower doesn’t occur.

However, if the long night portion of the experiment is interrupted by a brief period of dim light, flowering will occur.

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From These Experiments

Red light is most effective at interrupting the nighttime portion of the photoperiod.

Scientists have demonstrated that phytochrome is the pigment that measures the photoperiod.

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Extending the Experiments

Scientists at the USDA conducted these experiments.

Phytochrome was demonstrated to be the pigment responsible for seed germination.

From this, they were able to elucidate the flowering cycle.

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USDA Flowering Experiments

Seeds were subjected to a variety of monochromatic light.

Red and far-red light opposed each other in their germinating ability.

One induced germination, the other inhibited it.

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USDA Flowering Experiments

It was determined that the two different forms of light switched the phytochrome back and forth between two isomeric forms.

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USDA Flowering Experiments

One form caused seed germination, the other inhibited the germination response.

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USDA Flowering Experiments

The question: How do plants in nature illicit a response to light and begin germination?

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USDA Flowering Experiments

If seeds are kept in the dark, they synthesize Pr.

When seeds are illuminated with sunlight, they begin to be converted to Pfr.

The appearance of Pfr is one of the ways plants

detect sunlight.

Adequate sunlight converts Pr to Pfr and

triggers germination.

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USDA Flowering Experiments

In the flowering response, scientists were able to show the effects of the red and far red light on the flowering ability in plants.

Again, the 2 forms of light canceled each other.

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Other Stimuli

There are also a wide variety of stimuli other than light that effects plant growth.

Gravity, mechanical stimuli, and environmental stress also play a role in plant growth and development.