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Read and study the chapter in the textbook.
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Come prepared for discussion and activities in class.

2. Chapter 40: Plant sensory systems, signals and responses

3. Introduction
Plants receive and must respond appropriately to a wide variety of environmental stimuli.
Plants can sense and respond to information about light, gravity, pressure, and wounds.
Plants have the equivalent of a sense of smell and can perceive certain airborne molecules.
Plants have a sense of taste, because their roots can detect nutrients in soil.

4. Information Processing in Plants
Plants gather, process, and respond to the information they monitor in a three-step process:
Sensory cells receive an external signal and change it into an intracellular signal.
The sensory cells then send a signal to target cells in other parts of the plant body.
Target cells receive the signal and change their activity to produce an appropriate response.

5. External stimulus 1. Sensory cell on sensory cell perceives external
Figure 37.1
External stimulus
on sensory cell
1. Sensory cell
perceives external
stimulus and
transduces the
information to an
internal signal.
Internal
signal
2. A cell–cell
signal released by
the sensory cell
travels throughout
the body.
Cell–cell
signal
Figure 37.1 In Many Cases, Cell-Cell Signals Link a Stimulus and a Response.
3. Target cells
receive the cell–cell
signal and change
activity.
Internal
signal
© 2017 Pearson Education, Inc.

6. How Do Cells Receive and Process an External Signal?
When a stimulus is received by a sensory cell, the information it carries has to be changed into a form that is meaningful to the cell.
Signal receptor proteins change shape in response to an environmental stimulus.
Environmental stimuli include the detection of a particular wavelength of light, the application of pressure, or the binding of a particular type of molecule.

7. How Do Cells Receive and Process an External Signal?
When a signal receptor changes shape in response to a stimulus, the information changes form from an external signal to an intracellular signal.
The intracellular signal primes the sensory cell for action.
How does information from an activated sensory cell get to a target cell?
Usually, a hormone is transported to the target cells, where it causes a physiological response.

8. How Do Cells Respond to Cell–Cell Signals?
Signaling molecules elicit a response only if the cell has an appropriate receptor.
Some signal receptors exist inside the cell, where they respond to signaling molecules that can diffuse through the plasma membrane.
Most signal receptors are located in the plasma membrane, where they respond to external stimuli or bind to signaling molecules that do not cross the membrane, in a process called signal transduction.

9. Blue Light: The Phototropic Response
Plants sense and respond to a specific, narrow range of wavelengths in the visible spectrum.
Charles Darwin and his son Francis experimented with coleoptiles, modified leaves that form a sheath protecting emerging grass shoots.
They germinated seeds in the dark and placed the young, straight coleoptiles next to a light source.
The coleoptiles grew toward the light due to elongation of cells on the shaded side of the plant.

10. Blue Light: The Phototropic Response
Directed movement in response to light is phototropism.
The Darwins found that plants bend only toward light that includes blue wavelengths.
These wavelengths are important for photosynthesis.

11. (a) Shoots bend toward full-spectrum light.
Figure 37.3
(a) Shoots bend toward full-spectrum light.
(b) Shoots bend specifically toward blue light.
Figure 37.3 Experimental Evidence Suggests that Plants Sense Specific Wavelengths of Light.
© 2017 Pearson Education, Inc.

12. Phototropins as Blue-Light Receptors
Biologists knew that the blue-light receptor must be a pigment that absorbs certain wavelengths of light.
A key breakthrough came in the early 1990s.
Researchers identified a membrane protein in the tips of emerging shoots that gains a phosphate group in response to blue light.
They hypothesized that the membrane protein becomes activated when it is phosphorylated in response to blue light, triggering the phototrophic response.

13. Blue Light Triggers an Array of Responses
Phototropins trigger signal transduction cascades that lead to two other responses:
Chloroplasts move within leaf cells to positions that optimize light absorption.
Stomata open to allow carbon dioxide to diffuse into cells as blue light triggers photosynthesis.
Other blue-light receptors help control stem elongation and flower production.

14. Signals That Promote Flowering
Flower formation begins when an apical meristem stops making energy-harvesting stems and leaves and begins to produce the modified stems and leaves that form flowers.
The signals that promote flowering are based on the number of hours of light and dark in the day.
The switch that triggers flowering shows red/far-red photoreversibility.

15. Signals That Promote Flowering
Photoperiodism is any response by an organism that is based on photoperiod the relative lengths of day and night.
In plants, the ability to measure photoperiod allows individuals to respond to seasonal changes in climate.
This allows plants to flower when pollinators are available and resources for producing seeds are abundant.

16. Responding to Changes in Photoperiod
Plants fall into three categories:
​Long-day plants bloom in midsummer when days are longer than a certain length.
​Short-day plants bloom in the spring or fall when days are shorter than a certain length.
​Day-neutral plants flower without regard to photoperiod.
Interrupting the night period with a light flash changes the flowering response, triggering the phytochrome switch.

17. Gravity: The Gravitropic Response
Gravitropism is the ability to move or grow in response to gravity.
Roots respond to gravity by growing down.
Shoots respond to gravity by growing up.
The ends of root tips have a protective root cap .
The Darwins found that a root stops responding to gravity if the root cap is removed.
Gravity sensing occurs in cells containing starch granules and located at the center of the root cap.

18. Figure 37.12
Root
cap
Figure Gravity Sensing Occurs in the Root Cap.
25 µm
© 2017 Pearson Education, Inc.

19. How Do Plants Respond to Wind and Touch?
Plants respond to mechanical stresses such as wind and touch.
Growth responses produce stout, stiff stems.
Directional growth moves plants toward or away from contact with an object.
Movement responses make a Venus flytrap snap shut.
A large suite of genes may be transcribed in response to touch or other mechanical stimuli.
The protein products of these genes act to stiffen cell walls, resulting in shorter and stockier plants.

20. Untouched (control) Touched Figure 37.15
Figure Plant Growth Changes in Response to Wind or Touch.
© 2017 Pearson Education, Inc.

21. Movement Responses
Thigmotropism: a plant’s movement in response to touch, may be moderately fast.
A plant tendril makes contact with an object and responds by wrapping around the item one or more times per hour.
A Venus flytrap captures insects by closing quickly when an insect lands on its leaves.
Such a rapid, nondirectional, thigmonastic movement occurs when a touch-receptor cell transduces a mechanical signal to an electrical signal.

22. Figure 37.16
Figure Thigmotropism Is Directional Movement in Response to Touch.
Tendril
© 2017 Pearson Education, Inc.

23. Youth, Maturity, and Aging: The Growth Responses
Controlling growth in response to changes in age or environmental conditions is one of the most basic aspects of information processing in plants.
Six major hormones act as growth regulators in plants:
Auxins
Cytokinins
Gibberellins
ABA
Brassinosteroids
Ethylene

24. Cytokinins and Cell Division
Cytokinins promote cell division.
Cytokinins are synthesized in root tips, young fruits, growing buds, and other developing organs.
Most cytokinins are synthesized in the apical meristems of roots and transported up the shoot system through xylem.
Biologists add cytokinins to plant cells in culture to stimulate cell division.

25. Gibberellins and ABA: Growth and Dormancy
Plants grow when temperature and soil moisture are suitable and become dormant under conditions of cold and drought.
Two hormones initiate and halt growth as environmental conditions change:
​Gibberellins stimulate plant growth
​Abscisic acid or ABA inhibits growth
The two hormones interact like start and stop signals

26. ABA Closes Guard Cells in Stomata
A major challenge to land plants is replacing water lost when stomata are open.
Stomata open in response to blue light, allowing gas exchange during photosynthesis.
When stomata are open, water is lost; if roots cannot replace water lost at the leaves, then stomata close.
ABA is involved in the mechanism of stomatal closing.
Applying ABA to the exterior of stomata causes them to close within seconds.

27. Ethylene and Senescence
Senescence, the regulated process of aging and death, is triggered by interacting hormones.
The gaseous hormone ethylene is strongly associated with three aspects of senescence:
Fruit ripening
Flower fading
Leaf abscission
Ethylene also influences plant growth and is a stress hormone induced by drought.

28. Ethylene and Fruit Ripening
Ethylene induces the production of enzymes required for fruit ripening and an increase in cellular respiration, furnishing ATP.
During fruit ripening, starch is converted to sugar; cell walls are degraded; chlorophyll is broken down; and pigments and aromas that signal ripeness are produced.
These activities prepare fruit for seed dispersal.
Fruit growers manipulate ethylene levels to control fruit ripening or to prolong the life of the fruit.

29. Figure 37.23
Figure Ethylene Speeds Ripening and Other Aspects of Senescence.
+ Ethylene
© 2017 Pearson Education, Inc.

30. An Overview of Plant Growth Regulators
There are two key observations about plant growth regulators:
A single hormone may affect many different target tissues, carrying a similar message to a variety of tissues and organs.
Several different hormones may affect the same response as they interact with one another.
Cross-talk is the interaction between signaling pathways triggered by different hormones, integrating information from many sensory cells and signals.

31. Pathogens and Herbivores: The Defense Responses
Plants are subject to attack by a variety of pathogens, including viruses, bacteria, and fungi.
Plant roots are susceptible to nematode attack.
Plants have physical defenses:
The waxy cuticle covering epidermal cells is an effective barrier to many pathogens.
Thorns, spines, and trichomes protect leaves and stems from damage by herbivores.

32. How Do Plants Sense and Respond to Pathogens?
When attacked by a pathogen, plants mount a defense called the hypersensitive response (HR).
HR causes the rapid and localized death of cells surrounding the site of infection, starving the pathogen.
The hypersensitive response is a cascade of signals:
Stomata close, keeping out pathogens.
Toxins targeting the pathogen are produced.
Cell walls are reinforced to prevent pathogen movement.
Cells in the infected area rapidly die by suicide.

33. How Do Plants Sense and Respond to Herbivore Attack?
Many plant seeds and storage organs contain proteinase inhibitors that block proteinase enzymes in the mouths and stomachs of herbivores.
When an herbivore ingests a large dose of a proteinase inhibitor, it gets sick.
Herbivores learn to detect and avoid plant tissues containing high concentrations of these proteins.

34. Pheromones Recruit Help from Wasps
Parasitoids are free living as adults but parasitic as larvae.
Parasitoids must kill their host to complete development.
As a result, parasitoid attacks limit the amount of damage that herbivores do to plants.
Plants produce parasitoid wasp-attractant pheromones in response to attack by caterpillars.
Pheromones are chemical messengers synthesized by an individual and released into the environment to elicit a response from a different individual.