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Chapter 39

BIOL 1030 Chapter 39: Chapter 39 Plant Responses to Internal and External Signals

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Biological Sciences
BIOL 1030
Scott Kevin

Chapter 39 Plant Responses to Internal and External Signals Lecture Outline Overview: Stimuli and a Stationary Life • At every stage in the life of a plant, sensitivity to the environment and coordination of responses are evident. • One part of a plant can send signals to other parts. • Plants can sense gravity and the direction of light. • A plant’s morphology and physiology are constantly tuned to its variable surroundings by complex interactions between environmental stimuli and internal signals. • At the organismal level, plants and animals respond to environmental stimuli by very different means. • Animals, being mobile, respond mainly by behavioral mechanisms, moving toward positive stimuli and away from negative stimuli. • Rooted in one location for life, a plant generally responds to environmental cues by adjusting its pattern of growth and development. • As a result, plants of the same species vary in body form much more than do animals of the same species. • At the cellular level, plants and all other eukaryotes are surprisingly similar in their signaling mechanisms. Concept 39.1 Signal transduction pathways link signal reception to response • All organisms, including plants, have the ability to receive specific environmental and internal signals and respond to them in ways that enhance survival and reproductive success. • Like animals, plants have cellular receptors that they use to detect important changes in their environment. • These changes may be an increase in the concentration of a growth hormone, an injury from a caterpillar munching on leaves, or a decrease 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 receptor, a molecule that is sensitive to and affected by the specific stimulus. • Upon receiving a stimulus, a receptor initiates a specific series of biochemical steps, a signal transduction pathway. • This couples reception of the stimulus to the response of the organism. • Plants are sensitive to a wide range of internal and external stimuli, and each of these initiates a specific signal transduction pathway. • Plant growth patterns vary dramatically in the presence versus the absence of light. • For example, a potato (a modified underground stem) can sprout shoots from its “eyes” (axillary buds). • These shoots are ghostly pale and have long, thin stems; unexpanded leaves; and reduced roots. • These morphological adaptations, called etiolation, are seen also in seedlings germinated in the dark and make sense for plants sprouting underground. • The shoot is supported by the surrounding soil and does not need a thick stem. • Expanded leaves would hinder soil penetration and be damaged as the shoot pushes upward. • Because little water is lost in transpiration, an extensive root system is not required. • The production of chlorophyll is unnecessary in the absence of light. • A plant growing in the dark allocates as much energy as possible to the elongation of stems to break ground before the nutrient reserves in the tuber are exhausted. • Once a shoot reaches the sunlight, its morphology and biochemistry undergo profound changes, collectively called de-etiolation, or greening. • The elongation rate of the stems slows. • The leaves expand, and the roots start to elongate. • The entire shoot begins to produce chlorophyll. • The de-etiolation response is an example of how a plant receives a signal—in this case, light—and how this reception is transduced into a response (de-etiolation). • Studies of mutants have provided valuable insights into the roles played by various molecules in the three stages of cell- signal processing: reception, transduction, and response. • Signals, whether internal or external, are first detected by receptors, proteins that change shape in response to a specific stimulus. • The receptor for de-etiolation in plants is called a phytochrome, which consists of a light-absorbing pigment attached to a specific protein. • Unlike many receptors, which are in the plasma membrane, this phytochrome is in the cytoplasm. • 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 normal de-etiolation response. • Receptors such as phytochrome are sensitive to very weak environmental and chemical signals. • For example, just a few seconds of moonlight slow stem elongation in dark-grown oak seedlings. • These weak signals are amplified by second messengers— 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 phytochrome 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 enzyme. • Light causes phytochrome to undergo a conformational change that leads to increases in levels of the second messengers’ cyclic GMP (cGMP) and Ca2+. • Changes in cGMP levels can lead to ionic changes within the cell by influencing properties of ion channels. • Cyclic GMP also activates specific protein kinases, enzymes that phosphorylate and activate other proteins. • The microinjection of cyclic GMP into aurea tomato cells induces a partial de-etiolation response, even without the addition of phytochrome. • Changes in cytosolic Ca2+ levels also play an important role in phytochrome signal transduction. • The concentration of Ca2+ is generally very low in the cytoplasm. • Phytochrome activation can open Ca2+ channels and lead to transient 100-fold increases in cytosolic Ca2+. • Ultimately, a signal transduction pathway leads to the regulation of one or more cellular activities. • In most cases, these responses to stimulation involve the increased activity of certain enzymes. • This occurs through two mechanisms: by stimulating transcription of mRNA for the enzyme or by activating existing enzyme molecules (post-translational modification). • In transcriptional regulation, transcription factors bind directly to specific regions of DNA and control the transcription of specific genes. • In the case of phytochrome-induced de-etiolation, several transcription factors are activated by phosphorylation, 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 increase transcription of specific genes) or negative transcription factors (proteins that decrease transcription). • During post-translational modifications of proteins, the activities of existing proteins are modified. • In most cases, these modifications involve phosphorylation, 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 kinases directly. • One protein kinase can phosphorylate other protein kinases, creating a kinase cascade, finally leading to phosphorylation of transcription factors and impacting gene expression. • Thus, they regulate the synthesis of new proteins, usually by turning specific genes on and off. • Signal pathways must also have a means for turning off once the initial signal is no longer present. • Protein phosphatases, enzymes that dephosphorylate 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 kinases and protein phosphatases. • 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 chlorophyll production. • Others affect the levels of plant hormones that regulate growth. • For example, the levels of two hormones (auxin and brassinosteroids) that enhance stem elongation will decrease following phytochrome activation—hence, the reduction in stem elongation that accompanies de- etiolation. Concept 39.2 Plant hormones help coordinate growth, development, and responses to stimuli • The word hormone is derived from a Greek verb meaning “to excite.” • Found in all multicellular organisms, hormones are chemical signals that are produced in one part of the body, transported to other parts, bind to specific receptors, and trigger responses in target cells and tissues. • Only minute quantities of hormones are necessary to induce substantial change in an organism. • Hormone concentration or rate of transport can change in response to environmental stimuli. • Often the response of a plant is governed by the interaction of two or more hormones. Research on how plants grow toward light led to the discovery of plant hormones. • The concept of chemical messengers in plants emerged from a series of classic experiments on how stems respond to light. • Plants grow toward light, and if you rotate a plant, it will reorient its growth until its leaves again face the light. • Any growth response that results in curvature of whole plant organs toward or away from stimuli is called a tropism. • The growth of a shoot toward light is called positive phototropism; growth away from light is negative phototropism. • Much of what is known about phototropism has been learned from studies of grass seedlings, particularly oats. • The shoot of a grass seedling is enclosed in a sheath called the coleoptile, which grows straight upward if kept in the dark or illuminated uniformly from all sides. • If it is illuminated from one side, it will curve toward the light as a result of differential growth of cells on opposite sides of the coleoptile. • The cells on the darker side elongate faster than the cells on the brighter side. • In the late 19th century, Charles Darwin and his son Francis observed that a grass seedling bent toward light only if the tip of the coleoptile was present. • This response stopped if the tip was removed or covered with an opaque cap (but not a transparent cap). • While the tip was responsible for sensing light, the actual growth response occurred some distance below the tip, leading the Darwins to postulate that some signal was transmitted from the tip downward. • Later, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance. • He separated the tip from the remainder of the coleoptile by a block of gelatin, preventing cellular contact, but allowing chemicals to pass. • These seedlings were phototropic. • However, if the tip was segregated from the lower coleoptile by an impermeable barrier, no phototropic response occurred. • In 1926, Frits Went extracted the chemical messenger for phototropism, naming it auxin. • Modifying the Boysen-Jensen experiment, he placed excised tips on agar blocks, collecting the hormone. • If an agar block with this substance was centered on a coleoptile without a tip, the plant grew straight upward. • If the block was placed on one side, the plant began to bend away from the agar block. • The classical hypothesis for what causes grass coleoptiles to grow toward light, based on the previous research, is that an asymmetrical distribution of auxin moving down from the coleoptile tip causes cells on the dark side to elongate faster than cells on the brighter side. • However, studies of phototropism by organs other than grass coleoptiles provide less support for this idea. • There is, however, an asymmetrical distribution of certain substances that may act as growth inhibitors, with these substances more concentrated on the lighted side of a stem. Plant hormones help coordinate growth, development, and responses to environmental stimuli. • In general, plant hormones control plant growth and development by affecting the division, elongation, and differentiation of cells. • Some hormones also mediate shorter-term physiological responses of plants to environmental stimuli. • Each hormone has multiple effects, depending on its site of action, its concentration, and the developmental stage of the plant. • Some of the major classes of plant hormones include auxin, cytokinins, gibberellins, brassinosteroids, abscisic acid, and ethylene. • Many molecules that function in plant defenses against pathogens are probably plant hormones as well. • Plant hormones tend to be relatively small molecules that are transported from cell to cell across cell walls, a pathway that blocks the movement of large molecules. • Plant hormones are produced at very low concentrations. • Signal transduction pathways amplify the hormonal signal many-fold and connect it to a cell’s specific responses. • These include altering the expression of genes, affecting the activity of existing enzymes, or changing the properties of membranes. • Response to a hormone usually depends not so much on its absolute concentration as on its relative concentration compared to other hormones. • It is hormonal balance, rather than hormones acting in isolation, that control growth and development of the plants. • The term auxin is used for any chemical substance that promotes the elongation of coleoptiles, although auxins actually have multiple functions in both monocots and dicots. • The natural auxin occurring in plants is indoleacetic acid, or IAA. • In growing shoots, auxin is transported unidirectionally, from the shoot apex down to the base. • The speed at which auxin is transported down the stem from the shoot apex is about 10 mm/hr, a rate that is too fast for diffusion, but slower than translocation in the phloem. • Auxin seems to be transported directly through parenchyma tissue, from one cell to the next. • This unidirectional transport of auxin is called polar transport, and has nothing to do with gravity. • Auxin travels upward if a stem or coleoptile is placed upside down. • The polarity of auxin transport is due to the polar distribution of auxin transport protein in the cells. • Concentrated at the basal end of the cells, auxin transporters move the hormone out of the cell and into the apical end of the neighboring cell. • Although auxin affects several aspects of plant development, one of its chief functions is to stimulate the elongation of cells in young shoots. • The apical meristem of a shoot is a major site of auxin synthesis. • As auxin moves from the apex down to the region of cell elongation, the hormone stimulates cell growth, binding to a receptor in the plasma membrane. • Auxin stimulates cell growth only over a certain concentration range, from about 10?8 to 10?4 M. • At higher concentrations, auxins may inhibit cell elongation, probably by inducing production of ethylene, a hormone that generally acts as an inhibitor of elongation. • According to the acid growth hypothesis, in a shoot’s region of elongation, auxin stimulates plasma membrane proton pumps, increasing the voltage across the membrane and lowering the pH in the cell wall. • Lowering the pH activates expansin enzymes that break the cross-links between cellulose microfibrils and other cell wall constituents, loosening the wall. • Increasing the membrane potential enhances ion uptake into the cell, which causes the osmotic uptake of water. • Uptake of water increases turgor and elongates the loose- walled cell. • Auxin also alters gene expression rapidly, causing cells in the region of elongation to produce new proteins within minutes. • Some of these proteins are short-lived transcription factors that repress or activate the expression of other genes. • Auxin stimulates a sustained growth response of making the additional cytoplasm and wall material required by elongation. • Auxins are used commercially in the vegetative propagation of plants by cuttings. • Treating a detached leaf or stem with rooting powder containing auxin often causes adventitious roots to form near the cut surface. • Auxin is also involved in the branching of roots. • One Arabidopsis mutant that exhibits extreme proliferation of lateral roots has an auxin concentration 17-fold higher than normal. • Synthetic auxins, such as 2,4-dinitrophenol (2,4-D), are widely used as selective herbicides. • Monocots, such as maize or turfgrass, can rapidly inactivate these synthetic auxins. • However, dicots cannot and die from a hormonal overdose. • Spraying cereal fields or turf with 2,4-D eliminates dicot (broadleaf) weeds such as dandelions. • Auxin also affects secondary growth by inducing cell division in the vascular cambium and by influencing the growth of secondary xylem. • Developing seeds synthesize auxin, which promotes the growth of fruit. • Synthetic auxins sprayed on tomato vines induce development of seedless tomatoes because the synthetic auxins substitute for the auxin normally synthesized by the developing seeds. • Cytokinins stimulate cytokinesis, or cell division. • They were originally discovered in the 1940s by Johannes van Overbeek, who found that he could stimulate the growth of plant embryos by adding coconut milk to his culture medium. • A decade later, Folke Skoog and Carlos O. Miller induced cultured tobacco cells to divide by adding degraded samples of DNA. • The active ingredients in both were modified forms of adenine, one of the components of nucleic acids. • These growth regulators were named cytokinins because they stimulate cytokinesis. • The most common naturally occurring cytokinin is zeatin, named from the maize (Zea mays) in which it was found. • Much remains to be learned about cytokinin synthesis and signal transduction. • Cytokinins are produced in actively growing tissues, particularly in roots, embryos, and fruits. • Cytokinins produced in the root reach their target tissues by moving up the plant in the xylem sap. • Cytokinins interact with auxins to stimulate cell division and differentiation. • In the absence of cytokinins, a piece of parenchyma tissue grows large, but the cells do not divide. • In the presence of cytokinins and auxins, the cells divide, while cytokinins alone have no effect. • If the ratio of cytokinins and auxins is at a specific level, then the mass of growing cells, called a callus, remains undifferentiated. • If cytokinin levels are raised, shoot buds form from the callus. • If auxin levels are raised, roots form. • Cytokinins, auxins, and other factors interact in the control of apical dominance, the ability of the terminal bud to suppress the development of axillary buds. • Until recently, the leading hypothesis for the role of hormones in apical dominance—the direct inhibition hypothesis—proposed that auxin and cytokinin act antagonistically in regulating axillary bud growth. • Auxin levels would inhibit axillary bud growth, while cytokinins would stimulate growth. • Many observations are consistent with the direct inhibition hypothesis. • If the terminal bud, the primary source of auxin, is removed, the inhibition of axillary buds is removed and the plant becomes bushier. • This can be inhibited by adding auxins to the cut surface. • The direct inhibition hypothesis predicts that removing the primary source of auxin should lead to a decrease in auxin levels in the axillary buds. • However, experimental removal of the terminal shoot (decapitation) has not demonstrated this. • In fact, auxin levels actually increase in the axillary buds of decapitated plants. • Further research is necessary to uncover all pieces of this puzzle. • Cytokinins retard the aging of some plant organs. • They inhibit protein breakdown by stimulating RNA and protein synthesis and by mobilizing nutrients from surrounding tissues. • Leaves removed from a plant and dipped in a cytokinin solution stay green much longer than otherwise. • Cytokinins also slow deterioration of leaves on intact plants. • Florists use cytokinin sprays to keep cut flowers fresh. • A century ago, farmers in Asia noticed that some rice seedlings grew so tall and spindly that they toppled over before they could mature and flower. • In 1926, E. Kurosawa discovered that a fungus in the genus Gibberella causes this “foolish seedling disease.” • The fungus induced hyperelongation of rice stems by secreting a chemical, given the name gibberellin. • In the 1950s, researchers discovered that plants also make gibberellins. Researchers have identified more than 100 different natural gibberellins. • Typically each plant produces a much smaller number. • Foolish rice seedlings, it seems, suffer from an overdose of growth regulators normally found in lower concentrations. • Roots and leaves are major sites of gibberellin production. • Gibberellins stimulate growth in both leaves and stems but have little effect on root growth. • In stems, gibberellins stimulate cell elongation and cell division. • One hypothesis proposes that gibberellins stimulate cell wall– loosening enzymes that facilitate the penetration of expansin proteins into the cell well. • Thus, in a growing stem, auxin, by acidifying the cell wall and activating expansins, and gibberellins, by facilitating the penetration of expansins, act in concert to promote elongation. • The effects of gibberellins in enhancing stem elongation are evident when certain dwarf varieties of plants are treated with gibberellins. • After treatment with gibberellins, dwarf pea plants grow to normal height. • However, if gibberellins are applied to normal plants, there is often no response, perhaps because these plants are already producing the optimal dose of the hormone. • The most dramatic example of gibberellin-induced stem elongation is bolting, the rapid formation of the floral stalk. • In their vegetative state, some plants develop in a rosette form with a body low to the ground with short internodes. • As the plant switches to reproductive growth, a surge of gibberellins induces internodes to elongate rapidly, which elevates the floral buds that develop at the tips of the stems. • In many plants, both auxin and gibberellins must be present for fruit to set. • Spraying of gibberellin during fruit development is used to make the individual grapes grow larger and to make the internodes of the grape bunch elongate. • This enhances air circulation between the grapes and makes it harder for yeast and other microorganisms to infect the fruits. • The embryo of a seed is a rich source of gibberellins. • After hydration of the seed, the release of gibberellins from the embryo signals the seed to break dormancy and germinate. • Some seeds that require special environmental conditions to germinate, such as exposure to light or cold temperatures, will break dormancy if they are treated with gibberellins. • Gibberellins support the growth of cereal seedlings by stimulating the synthesis of digestive enzymes that mobilize stored nutrients. • First isolated from Brassica pollen in 1979, brassinosteroids are steroids chemically similar to cholesterol and the sex hormones of animals. • Brassinosteroids induce cell elongation and division in stem segments and seedlings at concentrations as low as 10?12 M. • They also retard leaf abscission and promote xylem differentiation. • Their effects are so qualitatively similar to those of auxin that it took several years for plant physiologists to accept brassinosteroids as nonauxin hormones. • Joann Chory and her colleagues provided evidence from molecular biology that brassinosteroids were plant hormones. • An Arabidopsis mutant that has morphological features similar to light-grown plants even when grown in the dark lacks brassinosteroids. • This mutation affects a gene that normally codes for an enzyme similar to one involved in steroid synthesis in mammalian cells. • Abscisic acid (ABA) was discovered independently in the 1960s by one research group studying bud dormancy and another investigating leaf abscission (the dropping of autumn leaves). • Ironically, ABA is no longer thought to play a primary role in either bud dormancy or leaf abscission, but it is an important plant hormone with a variety of functions. • ABA generally slows down growth. • Often ABA antagonizes the actions of the growth hormones— auxins, cytokinins, and gibberellins. • It is the ratio of ABA to one or more growth hormones that determines the final physiological outcome. • One major affect of ABA on plants is seed dormancy. • The levels of ABA may increase 100-fold during seed maturation, leading to inhibition of germination and the production of special proteins that help seeds withstand the extreme dehydration that accompanies maturation. • Seed dormancy has great survival value because it ensures that the seed will germinate only when there are optimal conditions of light, temperature, and moisture. • Many types of dormant seeds will germinate when ABA is removed or inactivated. • For example, the seeds of some desert plants break dormancy only when heavy rains wash ABA out of the seed. • Other seeds require light or prolonged exposure to cold to trigger the inactivation of ABA. • A maize mutant that has seeds that germinate while still on the cob lacks a functional transcription factor required for ABA to induce expression of certain genes. • ABA is the primary internal signal that enables plants to withstand drought. • When a plant begins to wilt, ABA accumulates in leaves and causes stomata to close rapidly, reducing transpiration and preventing further water loss. • ABA causes an increase in the opening of outwardly directed potassium channels in the plasma membrane of guard cells, leading to a massive loss of potassium. • The accompanying osmotic loss of water leads to a reduction in guard cell turgor, and the stomata close. • In some cases, water shortages in the root system can lead to the transport of ABA from roots to leaves, functioning as an “early warning system.” • Mutants that are prone to wilting are often deficient in ABA production. • In 1901, Dimitry Neljubow demonstrated that the gas ethylene was the active factor that caused leaves to drop from trees that were near leaking gas mains. • Plants produce ethylene in response to stresses such as drought, flooding, mechanical pressure, injury, and infection. • Ethylene production also occurs during fruit ripening and during programmed cell death. • Ethylene is also produced in response to high concentrations of externally applied auxins. • Ethylene instigates a seedling to perform a growth maneuver called the triple response that enables a seedling to circumvent an obstacle as it grows through soil. • Ethylene production is induced by mechanical stress on the stem tip. • In the triple response, stem elongation slows, the stem thickens, and curvature causes the stem to start growing horizontally. • As the stem continues to grow horizontally, its tip touches upward intermittently. • If the probes continue to detect a solid object above, then another pulse of ethylene is generated, and the stem continues its horizontal progress. • If upward probes detect no solid object, then ethylene production decreases, and the stem resumes its normal upward growth. • It is ethylene, not the physical obstruction per se, that induces the stem to grow horizontally. • Normal seedlings growing free of all physical impediments will undergo the triple response if ethylene is applied. • Arabidopsis mutants with abnormal triple responses have been used to investigate the signal transduction pathways leading to this response. • Ethylene-insensitive (ein) mutants fail to undergo the triple response after exposure to ethylene. • Some lack a functional ethylene receptor. • Other mutants undergo the triple response in the absence of physical obstacles. • Some mutants (eto) produce ethylene at 20 times the normal rate. • Other mutants, called constitutive triple-response (ctr) mutants, undergo the triple response in air but do not respond to inhibitors of ethylene synthesis. • Ethylene signal transduction is permanently turned on even though there is no ethylene present. • The various ethylene signal-transduction mutants can be distinguished by their different responses to experimental treatments. • The affected gene in ctr mutants codes for a protein kinase. • Because this mutation activates the ethylene response, this suggests that the normal kinase product of the wild-type allele is a negative regulator of ethylene signal transduction. • One hypothesis proposes that binding of the hormone ethylene to a receptor leads to inactivation of the kinase and inactivation of this negative regulator allows synthesis of the proteins required for the triple response. • The cells, organs, and plants that are genetically programmed to die on a particular schedule do not simply shut down their cellular machinery and await death. • Rather, during programmed cell death, called apoptosis, there is active expression of new genes, which produce enzymes that break down many chemical components, including chlorophyll, DNA, RNA, proteins, and membrane lipids. • A burst of ethylene productions is associated with apoptosis whether it occurs during the shedding of leaves in autumn, the death of an annual plant after flowering, or as the final step in the differentiation of a xylem vessel element. • The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating 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 parenchyma cells. • These nutrients are recycled back to developing leaves the following spring. • When an autumn leaf falls, the breaking point is an abscission 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 ethylene and auxin controls abscission. • An aged leaf produces less and less auxin, and this makes the cells of the abscission layer more sensitive to ethylene. • As the influence of ethylene prevails, the cells in the abscission layer produce enzymes that digest the cellulose and other components of cell walls. • The consumption of ripe fruits by animals 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 ethylene production. • Enzymatic breakdown of cell wall components softens the fruit, and conversion of starches and acids to sugars makes the fruit sweet. • The production of new scents and colors helps advertise fruits’ ripeness to animals, which eat the fruits and disperse the seeds. • A chain reaction occurs during ripening: ethylene triggers ripening and ripening, in turn, triggers even more ethylene production—a rare example of positive feedback on physiology. • Because ethylene is a gas, the signal to ripen even spreads from fruit to fruit. • Fruits can be ripened quickly by storing the fruit in a plastic bag, accumulating ethylene gas, or by enhancing ethylene levels in commercial production. • Alternatively, to prevent premature ripening, apples are stored in bins flushed with carbon dioxide, which prevents ethylene from accumulating and inhibits the synthesis of new ethylene. • Genetic engineering of ethylene signal transduction pathways has potentially important commercial applications after harvest. • For example, molecular biologists have blocked the transcription of one of the genes required for ethylene synthesis in tomato plants. • These tomato fruits are picked while green and are induced to ripen on demand when ethylene gas is added. • Plant responses often involve interactions of many hormones and their signal transduction pathways. • The study of hormone interactions can be a complex problem. • For example, flooding of deepwater rice leads to a 50-fold increase in internal ethylene and a rapid increase in stem elongation. • Flooding also leads to an increase in sensitivity to GA that is mediated by a decrease in ABA levels. • Thus, stem elongation is the result of interaction among three hormones and their signal transduction chains. • Imagine that you are a molecular biologist assigned the task of genetically engineering a rice plant that will grow faster when submerged. • What is the best molecular target for genetic manipulation? Is it an enzyme that inactivates ABA, an ethylene receptor, or an enzyme that produces more GA? • Many plant biologists are promoting a systems-b
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