Chapter 36A Study Guide
Plant Hormones
Plant Responses to Hormones
•	The word  __________   is derived from a Greek verb meaning “to excite.”
•	Found in all multicellular organisms, hormones are  __________   signals that are  __________  in one part of the body, __________  to other parts, __________  to specific receptors, and trigger  __________   in target cells and tissues.
•	Only minute  __________   of hormones are necessary to induce substantial change in an organism.
•	Hormone  __________   or rate of  __________   can change in response to environmental stimuli.
•	Often the response of a plant is governed by the interaction of two or more  __________  .
1. 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  __________  .
•	Plants shoots grow  __________   light, and if you rotate a plant, it will reorient its  __________   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  __________  .
•	The growth of a shoot toward light is called positive  __________  ; growth away from light is __________   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  __________  , which grows straight upward if kept in the dark or illuminated  __________   from all sides.
•	If it is illuminated from one side, it will curve  __________   the light as a result of differential  __________   of cells on opposite sides of the coleoptile.
•	The cells on the darker side elongate __________   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 __________   of the coleoptile was present.
•	This response stopped if the tip was removed or covered with an  __________   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  __________   was transmitted from the tip downward.
•	Later, Peter Boysen-Jensen demonstrated that the signal was a mobile  __________   substance.
•	He separated the tip from the remainder of the coleoptile by a block of gelatin, preventing cellular contact, but allowing  __________   to pass.
•	These seedlings were phototropic.
•	However, if the tip was segregated from the lower coleoptile by an __________   barrier, no phototropic response occurred.
•	In 1926, Frits Went extracted the chemical messenger for phototropism, naming it  __________  .
•	Modifying the Boysen-Jensen experiment, he placed excised tips on agar blocks, collecting the  __________  .
•	If an agar block with this substance was centered on a coleoptile without a tip, the plant grew  __________   upward.
•	If the block was placed on one side, the plant began to bend  __________   from the agar block.
•	The classical  __________   for what causes grass coleoptiles to grow  __________   light, based on the previous research, is that an  __________   distribution of auxin moving  __________  from the coleoptile tip causes cells on the  __________   side to elongate faster than cells on the  __________   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  __________  side of a stem.
2. Plant hormones help coordinate growth, development, and responses to environmental stimuli.
•	In general, plant hormones control plant growth and development by affecting the  __________  , __________  , and  __________   of cells.
•	Some hormones also mediate shorter-term physiological responses of plants to environmental stimuli.
•	Each hormone has  __________   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  __________   against pathogens are probably plant hormones as well.
•	Plant hormones tend to be relatively  __________   molecules that are  __________   from cell to cell across cell  __________  , a pathway that blocks the movement of large molecules.
•	Plant hormones are produced at very low  __________  .
•	Signal transduction pathways  __________   the hormonal signal many-fold and connect it to a cell’s specific  __________  .
•	These include altering the expression of  __________  , affecting the  __________   of existing enzymes, or changing the properties of __________  .
•	Response to a hormone usually depends not so much on its absolute concentration as on its  __________   concentration compared to other hormones.
•	It is hormonal __________  , rather than hormones acting in isolation, that control growth and development of the plants.
•	The term __________   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  __________   acid, or IAA.
•	In growing shoots, auxin is transported unidirectionally, from the shoot apex  __________   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  __________  , but slower than  __________   in the phloem.
•	Auxin seems to be transported directly through  __________   tissue, from one cell to the next.
•	This unidirectional transport of auxin is called  __________   transport, and has nothing to do with gravity.
•	Auxin travels  __________   if a stem or coleoptile is placed upside down.
•	The polarity of auxin transport is due to the polar distribution of auxin transport  __________   in the cells.
•	Concentrated at the  __________   end of the cells, auxin transporters move the hormone out of the cell and into the  __________   end of the neighboring cell.
•	Although auxin affects several aspects of plant development, one of its chief functions is to stimulate the  __________   of cells in young shoots.
•	The apical meristem of a  __________   is a major site of auxin  __________  .
•	As auxin moves from the apex down to the region of cell elongation, the hormone  __________   cell growth, binding to a receptor in the plasma membrane.
•	Auxin stimulates cell growth only over a certain  __________   range, from about 10−8 to 10−4 M.
•	At higher concentrations, auxins may  __________   cell elongation, probably by inducing production of  __________  , a hormone that generally acts as an inhibitor of elongation.
•	According to the  __________   growth hypothesis, in a shoot’s region of elongation, auxin stimulates plasma membrane  __________   pumps, increasing the voltage across the membrane and  __________   the pH in the cell wall.
•	Lowering the pH activates  __________   enzymes that break the cross-links between cellulose microfibrils and other cell wall constituents, __________   the wall.
•	Increasing the membrane  __________   enhances ion uptake into the cell, which causes the  __________   uptake of water.
•	Uptake of water increases  __________  and elongates the loose-walled cell.
•	Auxin also alters  __________   expression rapidly, causing cells in the region of elongation to produce new  __________   within minutes.
•	Some of these proteins are short-lived  __________   factors that repress or activate the expression of other genes.
•	Auxin  __________   a sustained growth response of making the additional cytoplasm and wall material required by elongation.
•	Auxins are used commercially in the  __________   propagation of plants by cuttings.
•	Treating a detached leaf or stem with rooting powder containing  __________   often causes adventitious  __________   to form near the cut surface.
•	Auxin is also involved in the branching of roots.
•	One Arabidopsis mutant that exhibits extreme proliferation of  __________   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  __________  .
•	Monocots, such as maize or turfgrass, can rapidly  __________   these synthetic auxins.
•	However, dicots cannot and die from a hormonal  __________  .
•	Spraying cereal fields or turf with 2,4-D eliminates dicot (broadleaf) __________   such as dandelions.
•	Auxin also affects secondary growth by inducing cell division in the  __________   __________  and by influencing the growth of secondary xylem.
•	Developing seeds synthesize auxin, which promotes the growth of  __________  .
•	Synthetic auxins sprayed on tomato vines induce development of  __________   tomatoes because the synthetic auxins substitute for the auxin normally synthesized by the developing seeds.
•	__________   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  __________  .
•	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  __________   and signal  __________  .
•	Cytokinins are produced in actively  __________   tissues, particularly in roots, embryos, and fruits.
•	Cytokinins produced in the  __________   reach their target tissues by moving up the plant in the  __________  sap.
•	Cytokinins interact with  __________   to stimulate cell division and differentiation.
•	In the absence of cytokinins, a piece of parenchyma tissue grows large, but the cells do not  __________  .
•	In the presence of cytokinins and auxins, the cells divide, while cytokinins alone have  __________   effect.
•	If the ratio of cytokinins and auxins is at a specific level, then the mass of growing cells, called a  __________  , remains undifferentiated.
•	If cytokinin levels are  __________  , shoot buds form from the callus.
•	If auxin levels are  __________  , roots form.
•	Cytokinins, auxins, and other factors interact in the control of  __________   __________  , the ability of the terminal bud to suppress the development of __________   buds.
•	Until recently, the leading hypothesis for the role of hormones in apical dominance—the direct inhibition hypothesis—proposed that auxin and cytokinin act  __________   in regulating axillary bud growth.
•	Auxin levels would  __________   axillary bud growth, while cytokinins would  __________   growth.
•	Many observations are consistent with the direct inhibition hypothesis.
•	If the  __________   bud, the primary source of auxin, is removed, the inhibition of  __________   buds is removed and the plant becomes bushier.
•	This can be inhibited by adding  __________   to the cut surface.
•	The direct inhibition hypothesis predicts that removing the primary source of auxin should lead to a  __________   in auxin levels in the axillary buds.
•	However, experimental removal of the terminal shoot (decapitation) has not demonstrated this.
•	In fact, auxin levels actually __________   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  __________   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  __________   much longer than otherwise.
•	Cytokinins also slow deterioration of leaves on intact plants.
•	Florists use  __________   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  __________  .
•	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  __________   concentrations.
•	__________   and  __________   are major sites of gibberellin production.
•	Gibberellins  __________   growth in both leaves and stems but have little effect on __________   growth.
•	In stems, gibberellins stimulate cell  __________   and cell  __________  .
•	One hypothesis proposes that gibberellins stimulate cell wall–loosening  __________   that facilitate the penetration of expansin  __________   into the cell well.
•	Thus, in a growing stem, auxin, by acidifying the cell wall and  __________   expansins, and gibberellins, by facilitating the  __________   of expansins, act in concert to promote elongation.
•	The effects of gibberellins in enhancing stem elongation are evident when certain  __________   varieties of plants are treated with gibberellins.
•	After treatment with gibberellins, dwarf pea plants grow to  __________   height.
•	However, if gibberellins are applied to normal plants, there is often  __________   response, perhaps because these plants are already producing the optimal dose of the hormone.
•	The most dramatic example of gibberellin-induced stem elongation is  __________  , the rapid formation of the floral stalk.
•	In their vegetative state, some plants develop in a  __________   form with a body low to the ground with short internodes.
•	As the plant switches to  __________   growth, a surge of gibberellins induces internodes to elongate rapidly, which  __________   the floral buds that develop at the tips of the stems.
•	In many plants, both auxin and gibberellins must be present for  __________   to set.
•	Spraying of gibberellin during fruit development is used to make the individual grapes grow  __________   and to make the internodes of the grape bunch  __________  .
•	This enhances air circulation between the grapes and makes it harder for yeast and other microorganisms to infect the fruits.
•	The  __________   of a seed is a rich source of gibberellins.
•	After hydration of the seed, the release of  __________   from the embryo signals the seed to break  __________   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 support the growth of cereal seedlings by stimulating the  __________   of digestive enzymes that mobilize stored nutrients.
•	First isolated from Brassica pollen in 1979, __________   are steroids chemically similar to cholesterol and the sex hormones of animals.
•	Brassinosteroids induce cell  __________   and  __________   in stem segments and seedlings at concentrations as low as 10−12 M.
•	They also retard leaf  __________   and promote  __________  differentiation.
•	Their effects are so qualitatively similar to those of  __________  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  __________  .
•	An Arabidopsis mutant that has morphological features similar to light-grown plants even when grown in the dark lacks  __________  .
•	This mutation affects a gene that normally codes for an enzyme similar to one involved in steroid synthesis in mammalian cells.
•	__________   __________   (ABA) was discovered independently in the 1960s by one research group studying bud   __________  and another investigating leaf  __________   (the dropping of autumn leaves).
•	Ironically, ABA is no longer thought to play a primary role in either bud  __________   or leaf  __________  , but it is an important plant hormone with a variety of functions.
•	ABA generally  __________   down growth.
•	Often ABA antagonizes the actions of the growth  __________  —auxins, cytokinins, and gibberellins.
•	It is the  __________   of ABA to one or more growth hormones that determines the final physiological outcome.
•	One major affect of ABA on plants is seed  __________  .
•	The levels of ABA may  __________   100-fold during seed maturation, leading to inhibition of germination and the production of special proteins that help seeds withstand the extreme  __________   that accompanies maturation.
•	Seed dormancy has great survival value because it ensures that the seed will germinate only when there are  __________   conditions of light, temperature, and moisture.
•	Many types of dormant seeds will germinate when ABA is  __________   or inactivated.
•	For example, the seeds of some desert plants break dormancy only when heavy rains wash ABA out of the  __________  .
•	Other seeds require light or prolonged exposure to cold to trigger the  __________   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  __________   in leaves and causes stomata to  __________   rapidly, reducing  __________   and preventing further  __________   loss.
•	ABA causes an  __________   in the opening of outwardly directed potassium channels in the plasma membrane of  __________   cells, leading to a massive loss of potassium.
•	The accompanying  __________   loss of water leads to a reduction in guard cell  __________  , and the stomata  __________  .
•	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  __________  .
•	In 1901, Dimitry Neljubow demonstrated that the gas  __________   was the active factor that caused leaves to drop from trees that were near leaking gas mains.
•	Plants produce ethylene in response to  __________   such as drought, flooding, mechanical pressure, injury, and infection.
•	Ethylene production also occurs during fruit  __________  and during programmed cell  __________  .
•	Ethylene is also produced in response to high concentrations of externally applied  __________  .
•	Ethylene instigates a seedling to perform a growth maneuver called the  __________   response that enables a seedling to circumvent an obstacle as it grows through soil.
•	Ethylene production is induced by  __________   stress on the stem tip.
•	In the triple response, stem elongation  __________  , the stem  __________  , and curvature causes the stem to start  __________  horizontally.
•	As the stem continues to grow horizontally, its tip touches upward intermittently.
•	If the probes continue to detect a solid object  __________  , then another pulse of  __________   is generated, and the stem  __________  its horizontal progress.
•	If upward probes detect no solid object, then ethylene production  __________  , and the stem resumes its normal  __________   growth.
•	It is  __________  , 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  __________  .
•	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  __________  , 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  __________   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  __________   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  __________  .
•	A chain reaction occurs during ripening: ethylene triggers  __________   and ripening, in turn, triggers even more  __________   production—a rare example of  __________   feedback on physiology.
•	Because ethylene is a __________  , the signal to ripen even spreads from fruit to fruit.
•	Fruits can be ripened quickly by storing the fruit in a plastic bag, accumulating  __________   gas, or by enhancing ethylene levels in commercial production.
•	Alternatively, to prevent premature ripening, apples are stored in bins flushed with  __________   __________  , which prevents ethylene from  __________   and inhibits the synthesis of new  __________  .
•	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  __________   in tomato plants.
•	These tomato fruits are picked while  __________   and are induced to ripen on demand when  __________   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  __________   hormones and their signal transduction chains.