Chapter 36B Study Guide
Responses to the Environment
Overview
•	At every stage in the life of a plant, sensitivity to the environment and coordination of responses are evident.
•	One  __________   of a plant can send signals to other  __________  .
•	Plants can sense  __________   and the  __________   of light.
•	A plant’s morphology and physiology are constantly tuned to its variable  __________   by complex interactions between environmental  __________   and internal  __________  .
•	At the organismal level, plants and animals respond to environmental stimuli by very different means.
•	Animals, being  __________  , respond mainly by behavioral mechanisms, moving toward  __________   stimuli and away from  __________   stimuli.
•	Rooted in one  __________   for life, a plant generally responds to environmental cues by adjusting its  __________   of growth and development.
•	As a result, plants of the same species vary in body  __________  much more than do animals of the same species.
•	At the cellular level, plants and all other eukaryotes are surprisingly  __________   in their signaling mechanisms.
A. Signal Transduction and Plant Responses
•	All organisms, including plants, have the ability to receive specific environmental and internal  __________   and respond to them in ways that enhance survival and reproductive success.
•	Like animals, plants have cellular __________   that they use to detect important changes in their environment.
•	These changes may be an increase in the  __________   of a growth hormone, an  __________   from a caterpillar munching on leaves, or a  __________   in day length as winter approaches.
•	In order for an internal or external stimulus to elicit a physiological response, certain cells in the organism must possess an appropriate  __________  , a molecule that is sensitive to and affected by the specific stimulus.
•	Upon receiving a  __________  , a receptor initiates a specific series of biochemical steps, a signal  __________   pathway.
•	This couples  __________   of the stimulus to the  __________   of the organism.
•	Plants are sensitive to a wide range of internal and external  __________  , and each of these initiates a specific  __________   transduction  __________  .
1. Signal transduction pathways link internal and environmental  __________   to cellular  __________  .
•	Plant growth patterns vary dramatically in the presence versus the absence of  __________  .
•	For example, a potato (a modified underground stem) can sprout shoots from its “eyes” (__________   buds).
•	These shoots are ghostly pale and have long, thin stems; unexpanded leaves; and reduced roots.
•	These morphological adaptations, called  __________  , are seen also in seedlings germinated in the dark and make sense for plants sprouting underground.
•	The shoot is supported by the surrounding  __________   and does not need a thick stem.
•	Expanded leaves would hinder soil  __________   and be damaged as the shoot pushes  __________  .
•	Because little water is lost in transpiration, an extensive  __________   system is not required.
•	The production of  __________   is unnecessary in the absence of light.
•	A plant growing in the dark allocates as much energy as possible to the  __________   of stems to break ground before the nutrient reserves in the tuber are  __________  .
•	Once a shoot reaches the sunlight, its morphology and biochemistry undergo profound changes, collectively called __________  , or greening.
•	The elongation rate of the stems  __________  .
•	The leaves  __________  , and the roots start to  __________  .
•	The entire shoot begins to produce  __________  .
•	The de-etiolation response is an example of how a plant receives a signal—in this case, __________  —and how this reception is transduced into a  __________   (de-etiolation).
•	Studies of mutants have provided valuable insights into the roles played by various molecules in the three stages of cell-signal processing: __________  , __________  , and  __________  .
•	Signals, whether internal or external, are first detected by receptors, __________   that change shape in response to a specific stimulus.
•	The receptor for de-etiolation in plants is called a __________  , which consists of a light-absorbing  __________   attached to a specific  __________  .
•	Unlike many receptors, which are in the plasma membrane, this phytochrome is in the  __________  .
•	The importance of this phytochrome was confirmed through investigations of a tomato mutant, called aurea, which greens less when exposed to light.
•	Injecting additional phytochrome into aurea leaf cells and exposing them to light produced a  __________   de-etiolation response.
•	Receptors such as phytochrome are sensitive to very weak environmental and chemical  __________  .
•	For example, just a few seconds of moonlight slow stem elongation in dark-grown oak seedlings.
•	These weak signals are amplified by  __________   __________  —small, internally produced chemicals that transfer and amplify the signal from the receptor to proteins that cause the specific response.
•	In the de-etiolation response, each activated  __________   may give rise to hundreds of molecules of a second messenger, each of which may lead to the activation of hundreds of molecules of a specific  __________  .
•	Ultimately, a signal transduction pathway leads to the  __________   of one or more cellular activities.
•	In most cases, these responses to stimulation involve the increased activity of certain  __________  .
•	This occurs through two mechanisms: by stimulating  __________   of mRNA for the enzyme or by  __________   existing enzyme molecules (post-translational modification).
•	In transcriptional regulation, transcription  __________   bind directly to specific regions of DNA and control the transcription of specific __________  .
•	In the case of phytochrome-induced de-etiolation, several transcription factors are activated by  __________  , some through the cyclic GMP pathway, while activation of others requires Ca2+.
•	The mechanism by which a signal promotes a new developmental course may depend on the activation of positive transcription factors (proteins that  __________   transcription of specific genes) or negative transcription factors (proteins that  __________   transcription).
•	During post-translational modifications of proteins, the activities of existing  __________   are modified.
•	In most cases, these modifications involve  __________  , the addition of a phosphate group onto the protein by a protein kinase.
•	Many second messengers, such as cyclic GMP, and some receptors, including some phytochromes, activate protein  __________   directly.
•	One protein kinase can phosphorylate other protein kinases, creating a kinase  __________  , finally leading to phosphorylation of  __________   factors and impacting gene expression.
•	Thus, they regulate the synthesis of new  __________  , usually by turning specific genes on and off.
•	Signal pathways must also have a means for turning  __________   once the initial signal is no longer present.
•	Protein phosphatases, enzymes that  __________   specific proteins, are involved in these “switch off” processes.
•	At any given moment, the activities of a cell depend on the balance of activity of many types of protein  __________   and protein  __________  .
•	During the de-etiolation response, a variety of proteins are either synthesized or activated.
•	These include enzymes that function in photosynthesis directly or that supply the chemical precursors for  __________   production.
•	Others affect the levels of plant  __________   that regulate growth.
•	For example, the levels of two hormones (auxin and brassinosteroids) that enhance stem elongation will  __________   following phytochrome activation—hence, the reduction in stem elongation that accompanies de-etiolation.
•	The loss of leaves each autumn is an adaptation that keeps deciduous trees from  __________   during winter when roots cannot absorb water from the frozen ground.
•	Before leaves abscise, many essential elements are salvaged from the dying leaves and stored in stem  __________   cells.
•	These nutrients are recycled back to developing  __________   the following spring.
•	When an autumn leaf falls, the breaking point is an  __________   layer near the base of the petiole.
•	The parenchyma cells here have very thin walls, and there are no fiber cells around the vascular tissue.
•	The abscission layer is further weakened when enzymes hydrolyze polysaccharides in the cell walls.
•	The weight of the leaf, with the help of the wind, causes a separation within the abscission layer.
•	A change in the balance of  __________   and  __________   controls abscission.
•	An aged leaf produces less and less  __________  , and this makes the cells of the abscission layer more sensitive to  __________  .
•	As the influence of ethylene prevails, the cells in the abscission layer produce enzymes that digest the  __________   and other components of cell walls.
•	The consumption of ripe fruits by  __________  helps disperse the seeds of flowering plants.
•	Immature fruits are tart, hard, and green but become edible at the time of seed maturation, triggered by a burst of  __________   production.
•	Enzymatic breakdown of cell wall components softens the fruit, and conversion of starches and acids to  __________   makes the fruit sweet.
•	The production of new scents and colors helps advertise fruits’ ripeness to animals, which eat the fruits and disperse the  __________  .
B. Plant Responses to Light
•	Light is an especially important factor in the lives of plants.
•	In addition to being required for  __________  , light also cues many key events in plant  __________  and  __________  .
•	These effects of light on plant morphology are what plant biologists call  __________  .
•	Light reception is also important in allowing plants to  __________   the passage of days and seasons.
•	Plants detect the presence, direction, intensity, and wavelength of light.
•	For example, the measure of the  __________   spectrum of photosynthesis has two peaks, one in the red and one in the blue.
•	These match the absorption peaks of  __________  .
•	Action spectra can be useful in the study of any process that depends on  __________  .
•	A close correspondence between an  __________   spectrum of a plant response and the  __________   spectrum of a purified pigment suggests that the  __________   may be the photoreceptor involved in mediating the response.
•	Action spectra reveal that  __________   and  __________   light are the most important colors regulating a plant’s photomorphogenesis.
•	These observations led researchers to two major classes of light receptors: a heterogeneous group of  __________  -light photoreceptors and a family of photoreceptors called  __________   that absorb mostly red light.
1. Blue-light photoreceptors are a heterogeneous group of pigments.
•	The action spectra of many plant processes demonstrate that blue light is effective in initiating diverse responses.
•	The biochemical identity of the blue-light photoreceptor was so elusive that they were called  __________  .
•	In the 1990s, molecular biologists analyzing Arabidopsis mutants found three completely different types of pigments that detect blue light.
•	These are cryptochromes (for the inhibition of hypocotyl elongation), phototropin (for phototropism), and a carotenoid-based photoreceptor called zeaxanthin (for stomatal opening).
2. Phytochromes function as photoreceptors in many plant responses to light.
•	__________   were discovered from studies of seed germination.
•	Because of their limited food resources, successful sprouting of many types of small seeds, such as lettuce, requires that they germinate only when conditions, especially  __________   conditions, are near optimal.
•	Such seeds often remain dormant for many years until light conditions change.
•	For example, the death of a shading tree or the plowing of a field may create a favorable light environment.
•	In the 1930s, scientists at the U.S. Department of Agriculture determined the  __________   spectrum for light-induced germination of lettuce seeds.
•	They exposed water-swollen seeds to a few minutes of monochromatic light of various  __________   and stored them in the dark for two days and recorded the number of seeds that had germinated under each light regimen.
•	While red light (660 nm) __________   germination, far-red light (730 nm) __________   it and the response depended on the  __________   flash of light.
•	The photoreceptor responsible for these opposing effects of red and far-red light is a  __________  .
•	It consists of a protein covalently bonded to a nonprotein part that functions as a  __________  , the light-absorbing part of the molecule.
•	The chromophore is  __________   and reverts back and forth between two isomeric forms with one (Pr) absorbing  __________   light and becoming (Pfr), and the other (Pfr) absorbing  __________  light and becoming (Pr).
•	This interconversion between isomers acts as a switching mechanism that controls various light-induced events in the life of the plant.
•	The Pfr form  __________   many of the plant’s developmental responses to light.
•	Exposure to far-red light  __________   the germination response.
•	Plants synthesize phytochrome as Pr, and if seeds are kept in the dark, the pigment remains almost entirely in the Pr form.
•	If the seeds are illuminated with sunlight, the phytochrome is exposed to  __________   light (along with other wavelengths), and much of the Pr is  __________   to Pfr, triggering germination.
•	The phytochrome system also provides plants with information about the quality of light.
•	During the day, with the mix of both red and far-red radiation, the Pr <=>Pfr photoreversion reaches a dynamic  __________  .
•	Plants can use the  __________   of these two forms to monitor and adapt to changes in light conditions.
•	For example, changes in this equilibrium might be used by a tree that requires high light intensity as a way to assess appropriate growth strategies.
•	If other trees shade this tree, its phytochrome ratio will shift in  __________   of Pr because the canopy screens out more  __________  light than far-red light.
•	The tree could use this information to indicate that it should allocate resources to growing taller.
•	If the target tree is in direct sunlight, then the proportion of Pfr will __________  , which stimulates branching and inhibits  __________   growth.
3. Biological clocks control circadian rhythms in plants and other eukaryotes.
•	Many plant processes, such as transpiration and synthesis of certain enzymes, oscillate during the  __________  .
•	This is often in response to changes in light levels, temperature, and relative humidity that accompany the 24-hour cycle of day and night.
•	Even under constant conditions in a growth chamber, many physiological processes in plants, such as opening and closing of stomata and the production of photosynthetic enzymes, continue to oscillate with a frequency of about  __________   hours.
•	For example, many legumes lower their leaves in the evening and raise them in the morning.
•	These movements  __________   even if plants are kept in constant light or constant darkness.
•	Such physiological cycles with a frequency of about 24 hours that are not directly paced by any known environmental variable are called  __________   __________  .
•	These rhythms are ubiquitous features of eukaryotic life.
•	Because organisms continue their rhythms even when placed in the deepest mine shafts or when orbited in satellites, they do not appear to be triggered by some subtle but pervasive environmental  __________  .
•	All research thus far indicates that the oscillator for circadian rhythms is  __________   (internal).
•	This internal clock, however, is  __________   (set) to a period of precisely 24 hours by daily signals from the  __________  .
•	If an organism is kept in a constant environment, its circadian rhythms deviate from a 24-hour period to free-running periods ranging from 21 to 27 hours.
•	Deviations of the free-running period from 24 hours does not mean that the biological clocks drift erratically, but that they are not  __________   with the outside world.
•	In considering biological clocks, we need to distinguish between the  __________   (clock) and the rhythmic __________   it controls.
•	For example, if we were to restrain the leaves of a bean plant so they cannot move, they will rush to the appropriate position for that time of day when we  __________   them.
•	We can interfere with a biological rhythm, but the  __________   goes right on ticking off the time.
•	A leading hypothesis for the molecular mechanisms underlying biological timekeeping is that it depends on synthesis of a  __________   that regulates its own production through  __________   control.
•	This protein may be a transcription factor that inhibits  __________   of the gene that encodes for the transcription factor itself.
•	The concentration of this transcription factor may accumulate during the first half of the circadian cycle and decline during the second half due to self-inhibition of its own  __________  .
4. Light entrains the biological clock.
•	Because the free running period of many circadian rhythms is greater than or less than the 24-hour daily cycle, they eventually become desynchronized with the natural environment when denied environmental  __________  .
•	Humans experience this type of desynchronization when we cross several times zones in an airplane, leading to the phenomenon we call  __________   __________  .
•	Eventually, our circadian rhythms become  __________   with the external environment.
•	Plants are also capable of reestablishing (__________  ) their circadian synchronization.
•	Both phytochrome and blue-light photoreceptors can entrain  __________   rhythms of plants.
•	The  __________   system involves turning cellular responses off and on by means of the Pr <=> Pfr switch.
•	In darkness, the phytochrome ratio shifts gradually in favor of the Pr form, in part from  __________   of new Pr molecules and, in some species, by slow biochemical  __________   of Pfr to Pr.
•	When the sun rises, the Pfr level suddenly increases by rapid  __________   of Pr.
•	This sudden increase in Pfr each day at dawn resets the biological __________  .
5. Photoperiodism synchronizes many plant responses to changes of season.
•	The appropriate appearance of seasonal events is of critical importance in the life cycles of most plants.
•	These seasonal events include seed  __________  , flowering, and the onset and breaking of bud  __________  .
•	The environmental stimulus that plants use most often to detect the time of year is the  __________  , the relative lengths of night and day.
•	A physiological response to photoperiod, such as flowering, is called photoperiodism.
•	One of the earliest clues to how plants detect the progress of the seasons came from a mutant variety of tobacco studied by W. W. Garner and H. A. Allard in 1920.
•	This variety, Maryland Mammoth, does not flower in summer as normal tobacco plants do, but in winter.
•	In light-regulated chambers, they discovered that this variety would flower only if the day length was 14 hours or shorter, which explained why it would not flower during the longer days of the summer.
•	Garner and Allard termed the Maryland Mammoth a -day plant, because it required a light period shorter than a critical length to flower.
•	Other examples include chrysanthemums, poinsettias, and some soybean varieties.
•	__________  -day plants will only flower when the light period is longer than a critical number of hours.
•	Examples include spinach, iris, and many cereals.
•	Day- __________   plants will flower when they reach a certain stage of maturity, regardless of day length.
•	Examples include tomatoes, rice, and dandelions.
•	In the 1940s, researchers discovered that it is actually  __________   length, not day length, that controls flowering and other responses to photoperiod.
•	Research demonstrated that the cocklebur, a short-day plant, would flower if the daytime period was broken by brief exposures to  __________  , but not if the nighttime period was broken by a few minutes of dim light.
•	Short-day plants are actually long- __________   plants, requiring a minimum length of uninterrupted darkness.
•	Cocklebur is actually unresponsive to day length, but it requires at least 8 hours of continuous darkness to flower.
•	Similarly, long-day plans are actually short- __________   plants.
•	A long-day plant grown on photoperiods of long nights that would not normally induce flowering will flower if the period of continuous darkness is interrupted by a few minutes of  __________  .
•	Long-day and short-day plants are distinguished not by an absolute night length but by whether the critical night lengths sets a  __________   (long-day plants) or  __________   (short-day plants) number of hours of  __________   required for flowering.
•	In both cases, the actual number of hours in the  __________   night length is specific to each species of plant.
•	While the critical factor is night length, the terms “long-day” and “short-day” are embedded firmly in the jargon of plant physiology.
•	__________  light is the most effective color in interrupting the nighttime portion of the photoperiod.
•	Action spectra and photoreversibility experiments show that  __________   is the active pigment.
•	If a flash of  __________   light during the dark period is followed immediately by a flash of far-red light, then the plant detects no interruption of night length, demonstrating red/far-red  __________  .
•	Plants measure night length very accurately.
•	Some short-day plants will not flower if night is even one minute shorter than the critical length.
•	Some plants species always flower on the same day each year.
•	Humans can exploit the photoperiodic control of flowering to produce flowers “out of season.”
•	By punctuating each long night with a flash of light, the floriculture industry can induce chrysanthemums, normally a short-day plant that blooms in fall, to delay their blooming until Mother’s Day in May.
•	The plants interpret this as not one long night, but two short  __________  .
•	While some plants require only a single exposure to the appropriate photoperiod to begin flowering, others require several successive days of the appropriate photoperiod.
•	Other plants respond to photoperiod only if pretreated by another environmental stimulus.
•	For example, winter wheat will not flower unless it has been exposed to several weeks of temperatures below 10oC (called  __________  ) before exposure to the appropriate photoperiod.
•	While buds produce flowers, it is  __________   that detect photoperiod and trigger flowering.
•	If even a single leaf receives the appropriate  __________  , all buds on a plant can be induced to flower, even if they have not experienced this signal.
•	Plants lacking  __________   will not flower, even if exposed to the appropriate photoperiod.
•	The flowering signal, not yet chemically identified, is called  __________  , and it may be a hormone or some change in the relative concentrations of two or more hormones.
•	Whatever combination of environmental cues (such as photoperiod or vernalization) and internal signals (such as hormones) is necessary for flowering to occur, the outcome is the transition of a bud’s  __________   from a vegetative state to a flowering state.
•	This requires that  __________  - __________   genes that induce the bud to form a flower must be switched on.
•	Then  __________  - __________   genes that specify the spatial organization of floral organs—sepals, petals, stamens, and carpels—are activated in the appropriate regions of the meristem.
•	Identification of the signal transduction pathways that link external cues to the gene changes required for flowering are active areas of research.
D. Plant Responses to Environmental Stimuli Other Than Light
•	Because of their  __________  , plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms.
•	While light is one important environmental cue, other environmental stimuli also influence plant development and physiology.
1. Plants respond to environmental stimuli through a combination of developmental and physiological mechanisms.
•	Both the roots and shoots of plants respond to gravity, or  __________  , although in diametrically different ways.
•	Roots demonstrate  __________   gravitropism, and shoots exhibit  __________  gravitropism.
•	Gravitropism ensures that the root grows in the soil and that the shoot reaches sunlight regardless of how a seed happens to be  __________   when it lands.
•	__________   plays a major role in gravitropic responses.
•	Plants may tell up from down by the settling of  __________  , specialized plastids containing dense starch grains, to the lower portions of cells.
•	In one hypothesis, the aggregation of statoliths at low points in cells of the root cap triggers the redistribution of calcium, which in turn causes lateral transport of  __________   within the root.
•	The high concentrations of auxin on the  __________   side of the zone of elongation inhibits cell elongation, slowing growth on that side and curving the root  __________  .
•	Experiments with Arabidopsis and tobacco mutants have demonstrated the importance of “falling statoliths” in root gravitropism, but these have also indicated that other factors or organelles may be involved.
•	Mutants lacking statoliths have a slower response than wild-type plants.
•	One possibility is that the entire cell helps the root sense gravity by mechanically pulling on proteins that tether the protoplast to the cell wall, __________   proteins on the “up” side and  __________   proteins on the “down side.”
•	Other dense organelles may also contribute to gravitropism by distorting the cytoskeleton.
•	Plants can change form in response to mechanical perturbations.
•	Such  __________   may be seen when comparing a short, stocky tree growing on a windy mountain ridge with a taller, more slender member of the same species growing in a more sheltered location.
•	Because plants are very sensitive to mechanical stress, researchers have found that even measuring the length of a leaf with a ruler alters its subsequent growth.
•	Rubbing the stems of young plants a few times results in plants that are  __________   than controls.
•	Mechanical stimulation activates a signal transduction pathway that increases cytoplasmic calcium, which mediates the activity of specific genes, including some that encode for proteins that affect cell wall properties.
•	Some plant species have become, over the course of their evolution, “touch specialists.”
•	For example, most vines and other climbing plants have tendrils that grow straight until they touch something.
•	Contact stimulates a coiling response, __________  , caused by differential growth of cells on opposite sides of the tendril.
•	This allows a vine to take advantage of whatever mechanical support it comes across as it climbs upward toward a forest canopy.
•	Some touch specialists undergo rapid leaf movements in response to mechanical stimulation.
•	For example, when the compound leaf of a Mimosa plant is touched, it collapses and leaflets fold together.
•	This occurs when pulvini, motor organs at the joints of leaves, become  __________   from a loss of potassium and subsequent loss of water by  __________  .
•	It takes about ten minutes for the cells to regain their  __________   and restore the “unstimulated” form of the leaf.
•	The folding of Mimosa leaves may reduce surface area in response to strong winds, thus reducing water loss. Collapse of the leaves exposes thorns on the stem, which may serve to deter  __________  .
•	One remarkable feature of rapid leaf movement is that signals are transmitted from leaflet to leaflet via action  __________  .
•	Traveling at about a centimeter per second through the leaf, these electrical impulses resemble nervous-system messages in animals, although the action potentials of plants are thousands of times slower.
•	Action potentials, which have been discovered in many species of algae and plants, may be widely used as a form of internal communication.
•	In the carnivorous Venus flytrap, stimulation of sensory hairs in the trap results in an action potential that travels to the cells that close the trap.
•	Occasionally, factors in the environment change severely enough to have an adverse effect on a plant’s survival, growth, and reproduction.
•	These environmental  __________   can devastate crop yields.
•	In natural ecosystems, plants that cannot tolerate environmental stress will either succumb or be outcompeted by other plants, and they will become locally  __________  .
•	Thus, environmental stresses, both  __________   and  __________  , are important in determining the geographic range of plants.
•	On a bright, warm, dry day, a plant may be stressed by a  __________   deficit because it is losing water by transpiration faster than water can be restored by uptake from the soil.
•	Prolonged drought can stress or even kill crops and plants in natural ecosystems.
•	Plants have control systems that enable them to cope with less extreme water  __________  .
•	Much of the plant’s response to a water deficit helps the plant conserve water by reducing  __________  .
•	As the water deficit in a leaf rises, guard cells lose __________   and the stomata  __________  .
•	A water deficit also stimulates increased synthesis and release of  __________   acid in a leaf, which also signals  __________   cells to close stomata.
•	Because cell expansion is a  __________  -dependent process, a water deficit will inhibit the growth of young leaves.
•	As plants wilt, their leaves may roll into a shape that reduces transpiration by exposing less leaf surface to dry air and wind.
•	These responses also reduce  __________  .
•	Root growth also responds to water deficit.
•	During a drought, the soil usually dries from the  __________   down.
•	This inhibits the growth of  __________   roots, partly because cells cannot maintain the turgor required for  __________  .
•	Deeper roots surrounded by soil that is still moist continue to  __________  , and the root system proliferates in a way that maximizes exposure to soil  __________  .
•	Plants in flooded soils (or overwatered houseplants) can  __________   because the soil lacks the air spaces that provide  __________  for cellular  __________   in the roots.
•	Some plants are adapted to very wet habitats.
•	Mangroves, inhabitants of coastal marshes, produce aerial roots that provide access to  __________  .
•	Less specialized plants in waterlogged soils may produce ethylene in the roots, causing some cortical cells to undergo  __________  , which creates air tubes that function as “snorkels.”
•	An excess of sodium chloride or other salts in the soil threaten plants for two reasons.
•	First, by lowering the water  __________   of the soil, plants can  __________   water to the environment rather than  __________   it.
•	Second, sodium and certain other ions are toxic to plants when their concentrations are relatively high.
•	The selectively permeable membranes of root cells impede the uptake of most harmful ions, but this aggravates the problem of acquiring water.
•	Some plants produce compatible solutes, organic compounds that keep the water  __________   of the cell more negative than that of the soil, without the toxic quantities of salt.
•	Still, most plants cannot survive salt stress for long.
•	The exceptions are  __________  , salt-tolerant plants with adaptations such as salt glands that pump salts out across the leaf epidermis.
•	Excessive heat can harm and eventually kill a plant by  __________   its enzymes and damaging its metabolism.
•	Transpiration helps dissipate excess heat through evaporative  __________  , but at the cost of possibly causing a water  __________   in many plants.
•	__________   stomata to preserve water sacrifices evaporative cooling.
•	Most plants have a backup response that enables them to survive heat stress.
•	Above a certain temperature—about 40°C for most plants in temperate regions—plant cells begin to synthesize relatively large quantities of __________  - __________   proteins.
•	Some heat-shock proteins are identical to chaperone proteins, which function in unstressed cells as temporary scaffolds that help other proteins fold into their functional shapes.
•	Similarly, heat-shock proteins may embrace enzymes and other proteins and help  __________   denaturation.
•	One problem that plants face when the temperature of the environment falls is a change in the fluidity of cell  __________  .
•	When the temperature becomes too cool, lipids are locked into crystalline structures and membranes lose their fluidity, which adversely affects solute transport and the functions of other membrane proteins.
•	One solution is to alter  __________   composition in the membranes, increasing the proportion of unsaturated fatty acids, which have shapes that keep membranes fluid at  __________   temperatures.
•	This response requires several hours to days, which is one reason rapid chilling is generally more stressful than  __________   seasonal cooling.
•	Freezing is a more severe version of cold stress.
•	At subfreezing temperatures, __________   forms in the cell walls and intercellular spaces of most plants.
•	__________   in the cytosol depress its freezing point.
•	This lowers the extracellular water potential, causing water to  __________   the cytoplasm and, therefore, causing  __________  .
•	The resulting increase in the concentration of  __________   ions in the cytoplasm is also harmful and can lead to cell death.
•	Plants native to regions where winters are cold have special adaptations that enable them to cope with freezing stress.
•	This may involve an overall resistance to dehydration.
•	In other cases, the cells of many frost-tolerant species increase their cytoplasmic levels of specific solutes, such as __________  , which are better tolerated at high concentrations and which help reduce water  __________   from the cell during extracellular freezing.
E. Plant Defense: Responses to Herbivores and Pathogens
•	Plants do not exist in isolation, but interact with many other species in their communities.
•	Some of these interspecific interactions—for example, associations with fungi in mycorrhizae or with insect pollinators—are mutually  __________  .
•	Most interactions that plants have with other organisms are not  __________   to the plant.
•	As primary producers, plants are at the base of most food webs and are subject to attack by a wide variety of plant-eating (__________  ) animals.
•	Plants are also subject to attacks by pathogenic viruses, bacteria, and fungi.
1. Plants deter herbivores with both physical and chemical defenses.
•	Herbivory is a  __________   that plants face in any ecosystem.
•	Plants counter excess herbivory with both  __________   defenses, such as thorns, and  __________   defenses, such as the production of distasteful or toxic compounds.
•	For example, some plants produce an unusual amino acid, canavanine, which resembles arginine.
•	If an insect eats a plant containing canavanine, canavanine is incorporated into the insect’s proteins in place of arginine.
•	Because canavanine is different enough from arginine to adversely affect the conformation and, hence, the function of the proteins, the insect  __________  .
•	Some plants even recruit predatory animals that help defend the plant against specific herbivores.
2. Plants use multiple lines of defense against pathogens.
•	A plant’s first line of defense against infection is the physical barrier of the plant’s “skin,” the  __________   of the primary plant body and the  __________  of the secondary plant body.
•	However, viruses, bacteria, and the spores and hyphae of fungi can enter the plant through injuries or through natural openings in the epidermis, such as  __________  .
•	Once a pathogen invades, the plant mounts a chemical attack as a second line of defense that kills the pathogens and prevents their spread from the site of infection.
•	Plants are generally resistant to most pathogens.
•	Plants have an innate ability to recognize invading pathogens and to mount successful defenses.
•	In a converse manner, successful pathogens cause  __________   because they are able to evade recognition or suppress host defense mechanisms.
•	Those few pathogens against which a plant has little specific defense are said to be virulent.
•	Avirulent pathogens gain enough access to their host to  __________   themselves without severely damaging or killing the plant.
•	Specific resistance to a plant disease is based on what is called gene-for-gene recognition.
•	This involves recognition of pathogen-derived molecules by the protein products of specific plant disease resistance (R) genes.
•	There are many pathogens, and plants have many R genes.
•	An R protein usually recognizes only a single corresponding pathogen molecule that is encoded by an avirulence (Avr) gene.
•	Many Avr proteins play an active role in pathogenesis and are thought to redirect host metabolism to the advantage of the pathogen.
•	The simplest biochemical model for gene-for-gene recognition is the  __________-   __________  model.
•	The R protein functions as a specific receptor protein that triggers resistance on binding to the correct corresponding Avr protein.
•	If the host lacks the R gene that counteracts the pathogen’s Avr gene, the pathogen can invade and kill the plant.
•	The “guard” hypothesis for gene-for-gene recognition proposes that R proteins function as a surveillance system to detect changes in protein activity or conformation induced by Avr proteins.
•	Regardless of the mechanism, recognition of pathogen-derived molecules by R proteins triggers a  __________   transduction pathway leading to a defense response in the infected plant tissue.
•	This defense includes an enhancement of the  __________   response at the site of infection and a  __________   response of the whole plant.
•	Even if a plant is infected by a virulent strain of a pathogen—one for which that particular plant has no genetic resistance—the plant is able to mount a localized chemical attack in response to molecular signals released from cells damaged by infection.
•	Molecules called elicitors, often cellulose fragments called  __________   released by cell wall damage, induce the production of antimicrobial compounds called phytoalexins.
•	Infection also activates genes that produce PR proteins (for pathogenesis-related).
•	Some of these are antimicrobial and attack bacterial  __________   walls.
•	Others spread “news” of the infection to nearby cells.
•	Infection also stimulates cross-linking of molecules in the cell wall and deposition of  __________  .
•	This sets up a local barricade that slows spread of the pathogen to other parts of the plant.
•	If the pathogen is avirulent based on an R-Avr match, the localized defense response is more vigorous and is called a hypersensitive response (HR).
•	There is an enhanced production of phytoalexins and PR proteins, and the “sealing” response that contains the infection is more effective.
•	After cells at the site of infection mount their chemical defense and seal off the area, they destroy themselves.
•	These areas are visible as lesions on a leaf or other infected organ, but the leaf or organ will survive, and its defense response will help protect the rest of the plant.
•	Part of the hypersensitive response includes production of chemical signals that spread throughout the plant, stimulating production of phytoalexins and PR proteins.
•	This response, called  __________   __________   __________   (SAR), is nonspecific, providing protection against a diversity of pathogens for days.
•	A good candidate for one of the hormones responsible for activating SAR is  __________   acid.
•	A modified form of this compound, acetylsalicylic acid, is the active ingredient in  __________  .
•	Centuries before aspirin was sold as a pain reliever, some cultures had learned that chewing the bark of a willow tree (Salix) would lessen the pain of a toothache or headache.
•	In plants, salicylic acid also appears to have medicinal value, but only through the stimulation of the systemic acquired resistance system.
•	Plant biologists investigating disease resistance and other evolutionary adaptations of plants are getting to the heart of how a plant responds to internal and external  __________  .