Plant Responses to Internal and External Signals Chapter 39 Plant Responses to Internal and External Signals travismulthaupt.com
Response to Stimuli Plants are sensitive to a wide range of stimuli. They elicit a response. They use a signal transduction pathway. travismulthaupt.com
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. travismulthaupt.com
Signal Reception Signals are detected by receptors. Proteins change in response to the stimulus. travismulthaupt.com
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 travismulthaupt.com
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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. travismulthaupt.com
Response: 2 Main Mechanisms 1. Stimulating transcription of mRNA. 2. Activating existing enzyme molecules. travismulthaupt.com
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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. travismulthaupt.com
39.2 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. travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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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. travismulthaupt.com
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. travismulthaupt.com
Peter Boysen-Jensen Using mica, there was no response. travismulthaupt.com
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. travismulthaupt.com
Frits Went Results: Decapitated coleoptile--no growth. Decapitated coleoptile + agar only--no growth. Decapitated coleoptile + agar with hormones--growth. travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
39.3 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. travismulthaupt.com
Circadian Rhythms Common to all eukaryotic life, and is not governed by an known environmental factor. travismulthaupt.com
Photoperiodism Is the physiological response of plant due to a change in the lengths of night and day--a photoperiod. travismulthaupt.com
Different Types of Plants There are 3 general varieties of plants classified according to their light requirements for flowering: 1. Short-day plants 2. Long-day plants 3. Day-neutral plants. travismulthaupt.com
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. travismulthaupt.com
Long-Day Plants Respond to short nights. A.k.a. short-night plants. They flower when the light period is longer than 14 hours. travismulthaupt.com
Day-Neutral Plants These are unaffected by the light period, and flower when they reach maturity. Tomatoes, rice, and dandelions. travismulthaupt.com
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). travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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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. travismulthaupt.com
USDA Flowering Experiments One form caused seed germination, the other inhibited the germination response. travismulthaupt.com
USDA Flowering Experiments The question: How do plants in nature illicit a response to light and begin germination? travismulthaupt.com
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. travismulthaupt.com
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. travismulthaupt.com
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39.5 Respond to attacks T G bozeman travismulthaupt.com