Biology 120 J. Greg Doheny. Chapter 39 Plant Responses to Signals are Mediated by Plant Hormones

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1 Biology 120 J. Greg Doheny Chapter 39 Plant Responses to Signals are Mediated by Plant Hormones Plants are able to change their growth and behavior in response to external signals (ie-growing towards light, shoots growing upwards while roots grow downwards, closing stomata at night to avoid water loss etc.). These changes in plant behavior are mediated by plant hormones. Plant hormones are similar to animal hormones, except that they are simpler, and the behaviors that they modify are also simpler. Animals use more types of hormones (and more types of cytokines) than plants do, and their behaviors are more complicated. Thus, it is helpful to discuss plant hormones before discussing animal and human hormones. We will begin by discussing how plant hormones work to modify plant growth and behavior (Chapter 39 Part I) and then move on to discussing plant behaviors in more detail (Chapter 39 Part II). New Terminology Etiolation: (the etiolation response) A collective term referring to the pattern of growth a plant exhibits while still in darkness (Figure 39.2). (Long, thin, pale shoots with no photosynthetic pigments etc.) De-etiolation: (the de-etiolation response, or the greening response) when plants turn green, and start to produce leaves when exposed to light. Positive Phototropism: growth of a shoot towards the light. Negative Phototropism: growth of a shoot away from the light. Phototaxis: General term used to describe moving towards light. Chemotaxis: General term used to describe moving towards a source of a specific chemical (ie-moving towards a cell that is secreting a hormone). Abscission: Term used to describe a plant losing its leaves (ie-in the winter). Senescence: (also known as apoptosis) Refers to the programmed, deliberate death of cells (ie-death of tree leaves in fall). Senescence is more commonly used in reference to plants and plant structures, while apoptosis is usually used to refer to the programmed cell death of animal cells. Circadian Rhythms: biological patterns of behavior in plants or animals that follow a 24 hour cycle. Gravitropism: Term referring to the ability of plants to use gravity to orient proper growth of their roots and shoots. (ie-roots growing in the direction of gravity, and shoots growing in the opposite direction.) Thigmomorphogenesis: a pattern of stunted growth that plants exhibit in response to being touched or pushed against on a regular basis (ie-shorter, thicker trees growing in areas of high wind). Meristems: Basically pluripotent stem cells in plants. Many cells in adult plants are either dead or terminally differentiated (meaning they can t continue growing). Meristems are plant tissues that are capable of continuing to divide and grow. (For example, apical buds are made of meristem tissue.) 1

2 Primary Growth: Refers to growth of new plants. Primarily upward growth (shoots) or downward growth (roots). Secondary Growth: Refers to continued growth of adult plants. Usually branching, and thickening of stems; and upward growth of shoots and downward growth of roots to a lesser extent. Cork Cambium: Secondary (widening) growth of the dermal layer of an adult plant. Vascular Cambium: Secondary (widening) growth of the vascular tissue containing Xylem and Phloem. Hormone Antagonism: Hormones often occur in pairs, or pairs of sets, where one set of hormones will increase a certain behavior or effect, and the other set will have the opposite effect. We say that these hormones or sets of hormones are antagonistic to one another. Hormone Synergy: When two different hormones, each of which causes one effect when used alone, work together (in combination) to cause either a much greater effect than either one would cause alone; or work together to cause an entirely different effect (ie-auxins and cytokinins; see below). PART I Proteins are ultimately responsible for behaviors: Genes encode proteins. Cells and organisms in general change their behavior by ceasing to produce one set of proteins (that carry out one function) and starting to produce a different set (that carry out a different function). Thus, behavior can be altered by turning off one set of genes and turning on another. How genes are turned on or off: The process of turning genes on or off is called gene regulation. A DNA sequence called a Promoter is located at the start of each gene. If a gene is going to be transcribed (converted into mrna that is subsequently translated into protein) a protein complex called RNA Polymerase will bind to the promoter, and start making an mrna copy from the DNA template. The RNA Polymerase complex is fairly unstable, and will usually form at the promoter only to fall apart again if it is not stabilized with the help of other proteins called Transcription Factors (TFs). Transcription factors are specific proteins that bind to specific DNA sequences located near specific genes. These DNA sequences are called Enhancers. Thus, transcription from a specific gene will only take place in a certain cell if the specific transcription factors are present. (What determines whether those transcription factors will be present? Other transcription factors binding to the enhancers located near the genes that encode those transcription factors.and so on. There are thousands of different transcription factors in the human genome, for example, regulating the activity of thousands of different genes.) Genes encoding transcription factors are turned on in response to intracellular signals: If specific transcription factors are present in a cell, the gene that they act upon will be turned on. The genes encoding those transcription factors can be turned on in response to signals from other cells. There are two basic types of signals: Cytokines and Hormones. 2

3 Cytokines: Cytokines are small, intracellular signaling proteins (ie-signals that cells send to each other). These small, protein signals bind to specific receptors on the surfaces of specific cells. Generally, anything that binds to a receptor is called a ligand, and therefore a cytokine can also be called a ligand, but usually only in the context of it binding to its target receptor. Binding of the cytokine to its receptor sends a signal to the nucleus, telling certain genes to either turn on or turn off. Because cytokines can only signal cells that are expressing the proper receptor on their surfaces, the actions of cytokines tend to be more specific than the actions of hormones. Hormones: Hormones are also intracellular signaling molecules. Different types of hormones are composed of different materials, including proteins, modified lipids, or modified amino acids. Hormones that are made specifically from modified cholesterol (a lipid) are called steroid hormones. Hormones that are made of lipid are able to cross cell membranes directly, without any need for receptors. They then usually go directly to the nucleus and bind to an inactive form of a transcription factor called a nuclear receptor. Binding of a hormone to a nuclear receptor for that specific hormone will convert the nuclear receptor into an active transcription factor that can activate transcription of a specific gene. Because some hormones can cross cell membranes without the help of specific receptors, the actions of hormones tend to be more general and wide spread than the actions of cytokines. In higher organisms (like humans), hormones are sometimes produced by special large glands called endocrine glands. Plants generally don t have specialized glands that produce hormones. Signal Transduction: Many hormones pass directly through the cell plasma membrane to the nucleus, and bind to an inactive nuclear receptor, thus activating it, and turning on a gene. Simple! However, some hormones, and most cytokines can t do this. Instead, the cytokine or hormone signal received at the cell surface (when the ligand binds to its receptor) is relayed to the nucleus through a series of intermediate proteins and chemicals (collectively called second messengers ). This process of relaying messages from the surface to the nucleus is called signal transduction. The first thing that usually happens when a ligand binds to its receptor on the outer surface of the cell is that there is a conformational (allosteric) change to the receptor on the cytoplasmic side of the cell, which causes other proteins to phosphorylate it (add a phosphate group to one of the amino acids). Most proteins are activated by phosphorylation, and de-activated by dephosphorylation. The proteins that stick phosphate groups onto other proteins (ie-onto receptors) are called phosphate kinases, or simply kinases. In order to change the protein (or receptor) back into an inactive state the phosphate must be removed by other proteins collectively called phosphatases. Thus, specific proteins are activated by specific kinases and de-activated by specific phosphatases. The phosphate groups are added to amino acids that contain an OH group, like serine, threonine or tyrosine. Phosphorylation of an activated receptor sets off a cascade of other 3

4 proteins being turned on by phosphorylation to relay the message to the nucleus, ultimately activating transcription of a set of transcription factor proteins that will activate transcription of other genes (Figures 39.3 and 39.4). Not all secondary messengers are proteins, however. Cyclic GMP (cgmp) and Cyclic AMP (camp), both derivatives of the nucleic acids, can also act as secondary messengers; and intracellular levels of calcium also help to relay messages to the nucleus. That s how cytokines and hormones work to modify behaviors. Plants use mostly hormones (called plant hormones ), and relatively few cytokines. Mammals use both (as we ll discuss later). Experimental Proof that Plant Hormones Modify Plant Growth and Behavior (Figure 39.5, 39.6 and 39.23): The coleoptile (monocot cotelydon wrapped around a shoot, emerging from a seed) of a grass seed will bend towards sunlight. 1. In 1880, Darwin discovered that you could prevent it from bending towards the light if you covered the tip, but not the stem of the coleoptile. Concluded that some form of signal was being sent by the tip cells to the shoot cells. 2. In 1931, another scientist (Boysen-Jensen) put either a permeable or an impermeable barrier between the tip of the coleoptile and the shaft. Found that the coleoptile would still bend towards the light if liquid (and thus macromolecules) were allowed to pass through the barrier. Concluded that the signal must be a macromolecule contained in the liquid. 3. In 1926, another scientist (Went) cut off the tip of a coleoptile that had been exposed to light, put it onto an agar block that absorbed whatever fluids were coming out of it, and then put the block onto truncated shoots to see what would happen (Figure 39.6, note the use of two types of negative controls in this experiment!). He found that whichever side of the shoot you put the block of agar onto would grow longer, thus causing the bend to occur in the opposite direction. Concluded that the macromolecule (the hormone) causes elongation of plant cells. Thus, when a plant bends towards the light, it does so by sending this hormone to the side of the plant that is opposite to the light. (This hormone was subsequently discovered to be Auxin, a plant hormone that causes elongation of cells, but not cell division. Much of plant growth is achieved by elongating existing cells rather than growing new cells.) Much of plant growth and plant movement happens through elongation of existing cells, rather than addition of new cells (Figure 35.13): In adult plants, only meristem tissue is capable of division. Meristem tissue is only located at the tips of shoots or the bottoms of roots. Thus, a growing root cell can be divided into four sections: 1. A root cap which does not divide, but instead protects the dividing meristem tissue underneath, 2. A zone of cell division, containing dividing meristems, 3. A zone of elongation containing cells that are elongating with the help of plant hormones, but which are neither dividing nor differentiating, and 4. A zone of differentiation where cells are differentiating (ie-into root hairs of the epidermis), rather than elongating or dividing. 4

5 Examples of Plant Hormones and What They Do (Table 39.1) 1. Auxins: A class of plant hormone that stimulates cell elongation in stems and roots. Auxin does this by stimulating active transport of hydrogen ions outside of the cell, towards the cell wall (Figure 39.8). The acid conditions activate an enzyme called Expansin, that works at low ph. Activated expansin breaks the hydrogen bonds between cellulose fibers, thus loosening the cell wall. With the cell wall loosened, and turgor pressure lowered the cell is free to absorb more water, causing it to swell and elongate (Figure 39.8). Many herbicides are actually artificial auxins. Most weeds are eudicots, and eudicots are more sensitive to auxin overdoses than monocots such as cerials (corn, wheat etc.). So, spraying fields of corn or wheat with artificial auxins will kill weeds. Interestingly, auxins only cause cell elongation at low concentration. At very high concentrations, auxins induce the production of ethylene, a gas that promotes plant senescence (dormancy or death). Thus, at low concentrations auxins promote cell elongation, and at high concentrations they promote plant dormancy or death (which explains why they can be used as herbicides). 2. Cytokinins: A class of plant hormone that stimulates cell division, but only in combination with auxins. Alone they have no effect. 3. Gibberelllins: A class of plant hormone that stimulates elongation of stems only. Also promotes germination of seeds; and growth of fruit. Many fruit trees and grape vines are sprayed with gibberellins to promote growth of the fruit. (ie-thompson seedless grapes are sprayed with gibberellin to promote enlargement of the grapes.) 4. Abscisic Acid: Is antagonistic to the growth hormones like auxins. Abscisic Acid promotes seed and tissue dormancy, promotes stromal closure (to withstand drought conditions), and (like the name says) promotes leaf abscission. 5. Ethylene: Is a plant hormone in the form of a gas that is released from plant tissues. It is antagonistic to the plant growth hormones. Ethylene inhibits root elongation, encourages senescence, leaf abscission, and ripening of fruit. (Fruit ripening is caused by the activation of amylase enzymes that break the starches stored in fruits down into sweet sugars like glucose and sucrose.) Because ethylene is a gas, it can be spread from one plant to another. Thus, if you have an old (rotting) banana sitting in a bowl of fruit next to a green banana, the ethylene from the old banana will hasten the ripening of the green banana. Therefore, if you want to keep green bananas fresh longer, keep them away from old bananas. Shipping containers used to transport fruits are usually well ventilated to prevent buildup of ethylene gas, thus preventing premature ripening of the fruit. Ethylene also promotes the breakdown of the green photosynthetic pigments (but not the yellow and orange accessory pigments) in leaves, prior to leaf abscission in the fall. The green photosynthetic pigments are broken down and fed back into the tree trunk where they can be readily converted back into the chlorophylls the following spring. 5

6 PART II PLANT HORMONAL RESPONSES TO LIGHT AND TEMPERATURE You have learned how hormones and cytokines can signal cells to change their gene expression patterns, and how changes to gene expression patterns can change plant behavior. You also learned that cytokines are able to activate genes by stimulating receptors on the cell surface, and that the activation message is relayed from the cell surface to the nucleus via signal transduction. In Chapter 10, you also learned that certain plant pigments are stimulated by light. Here we will learn that plants also have receptors that combine these two abilities. They have receptors called Phytocromes that are stimulated by light the way light harvesting complexes are, but respond to the stimulation the way cytokine receptors do (by transmitting a gene activation signal to the nucleus). In some cases the photoreceptors are stimulated by blue light, and in some cases they are stimulated by red light (recall the absorbance spectrum of plants, Figure 10.10). Receptors sensitive to blue light are critical for phototropism and growth. Receptors sensitive to red light are responsible for germination of seeds and blossoming of flowers. Interestingly, while the red receptors can be turned on by red light, stimulating germination of seeds or blossoming of flowers, they can be turned off again by far-red light (wavelengths bordering on infrared), discouraging seed germination and flower blossoming. PHOTORECEPTORS Blue-Light Photoreceptors (Figure 39.16): Blue-light photoreceptors are stimulated by light in the blue wavelength, and mediate phototropism by inducing production of auxins (See experiment, Figure 39.16). Phytochromes (Red-Light Photoreceptors; Figure 39.18): Phytochromes are stimulated by red light, and encourage seeds to germinate by inducing production of gibberellins and other hormones. Interestingly, while phytochrome receptors are turned on by red light, they can be turned off again by far-red light (see Experiment, Figures and 39.19). The presumed purpose of this is to allow seeds to germinate in direct sunlight, but not germinate while in the shade of another tree. A canopy of green leaves will absorb most of the red light, but not the far-red light. Thus, seeds will not germinate ( hatch ) in the shade of another large tree. They would rather wait to see if they re blown or moved to a more sunny location. CIRCADIAN RHYTHMS We ve learned that certain hormones can be induced in response to cues from the environment (light, cold, mechanical pressure.) However, the flow of many hormones is not dependent on stimulation from the environment. Some hormones ebb and flow according to a 24 hour cycle, called a Circadian Rhythm, regardless of what is happening in the outside world. Life has been evolving on earth for 2.5 billion years, mostly near the equator where there is a constant 24 hour day vs. night light cycle. As a result, many of the biological functions mediated by hormones 6

7 follow a 24 hour biological clock. For example, plants raise their leaves during the day to collect sunlight, and lower them at night as a function of cyclical hormone activity (Figure 39.20). Interestingly, if you remove the plant from light its leaves will still do this, even in the absence of light. Similarly, its leaves will follow this up/down cycle if you keep it in light 24 hours a day. However, eventually the leaves will not raise or lower with a 24 hour period. They may adopt anything from a 21 to a 27 hour period. This deviation from the 24 hour cycle can be quickly corrected again by putting the plant back into a 24 hour light/dark cycle. The same is true for human sleeping patterns and many other biological functions in the absence, or constant presence of light. PHOTOPERIODISM Photoperiodism refers to the response of plants to different growing seasons, and different lengths of day vs. night. (ie-plants becoming dormant in the winter and coming back to life in the summer. Other plants will only bloom in the summer.) The response of plants to seasonal changes in light is called Photoperiodism. Specifically, certain flowering angiosperms are conditioned to blossom as soon as days (and daylight) have reached a certain length in the spring. Presumably this is related to the length of the growing season, and the time when insects will start becoming available to pollenate them. Three types of flowering angiosperms are known: 1. Short-day plants, 2. Long day plants, and 3. Day-neutral plants that will bloom regardless of the length of the day. LONG-DAY vs. SHORT-DAY PLANTS Originally, researchers noticed that some plants will bloom only if the day is shorter than 12 hours. They named these short-day plants. Other plants only bloomed if the day was longer than 12 hours. They named these long-day plants. Eventually, however, they discovered that it is not the length of daylight that is important for blooming, but the length of the night-time dormancy period (Figure 39.21). It is not the number of sunlight hours that is critical to plant flowering, but the number of hours the plant sits in uninterrupted darkness. Thus, it would have been better to name these plants long-night and short-night plants, rather than short-day and long-day plants, but the older naming convention still persists. For example, if a long-day (short-night) plant is left in darkness for 16 hours it will not bloom, but if it is only left in darkness for eight hours it will bloom. However, if you leave it in darkness for 16 hours, but interrupt the darkness with a brief flash of light after 8 hours of darkness, it will bloom. This means that the plant interpreted the brief flash of light as a whole day. Thus, a brief flash of light, interrupting a long night will activate a short-night plant (Figure 39.21b). Furthermore, the short-night plant can be activated by a brief flash of red light only. Full spectrum white light is not necessary. Interestingly, this effect can be reversed if the flash of red light is followed by a flash of far-red light (Figure 39.22), strongly suggesting that this day/night response pattern is mediated by phytochrome photoreceptors. This process is exploited by industrial flower growers, who grow flowers in green houses, giving them flashes of artificial light during the night to cause blooming out of season. Short-day plants generally flower in the late summer. Examples include Chrysanthemums and Poinsettias. 7

8 Long-day plants flower in the spring and early summer. Examples include Irises. Day-neutral plants are unaffected by the photoperiod. Examples include tomatoes and Dandelions. VERNALIZATION Another cue that plants take from their environment when deciding when to bloom is the temperature. Many plants will not bloom unless they have been exposed to a cold temperature for a while. (Essentially, they wait until they re sure there has been a winter season before coming to life.) The process of delaying blooming until after a prolonged cold period is called vernalization. GRAVITROPISM Plants are able to use gravity to sense which direction is up and which is down in order to send their roots towards the ground and their shoots and leaves up towards the Sun. This process is called gravitropism, and it is mediated by dense grains of starch, called Statoliths, inside apical bud and root cells. Statoliths fall to the bottom of cells, indicating which direction is down, and allowing the plant to orient its roots and shoots accordingly (Figure 39.24). THIGMOMORPHOGENESIS A plant that is repeatedly touched or pressed on when it is young will grow up to be stunted (like a Bonsai Tree). This response is mediated by the hormone ethylene, and the shorter aspect and thicker stems and trunks that are characteristic of thigmomorphogenesis are thought to be an adaptation against being blown over when growing in a windy environment. PRACTICE QUESTIONS: Short Answer Questions: 1. Name for a DNA sequence located near a gene, to which Transcription Factors bind, and increase the transcription rate from the gene? 2. Name for the class of proteins known to bind to specific gene enhancers? 3. Name for the DNA sequence located at the start of a gene, where the RNA Polymerase II complex binds and begins transcription? 4. What are hormones made of, and what do they bind to in or on their target cells? (2 points) 5. What are cytokines made of, and what do they bind to in or on their target cells? (2 points) 6. Term used to describe the transmission of a signal received at the cell surface to the nucleus, through a series of intermediate chemicals or proteins. 7. Name one secondary signal transduction messenger (a second messenger ). (Hint: one derived from a nucleic acid that is also used as an energy intermediate.) 8. General term for a protein that puts phosphate groups onto other proteins (thus activating them)? 9. General term for a protein that removes phosphate groups from other proteins (thus inactivating them)? 10. Name one amino acid that can be phosphorylated. 11. What is the collective term used to describe the pattern of growth that plants exhibit while growing in the dark (thin roots, pale shoots lacking photosynthetic pigments etc.)? 8

9 12. Term used to describe the behavior of a plant shoot growing towards the light. 13. General term used to describe movement towards the source of a chemical (ie-a cell secreting a hormone)? 14. A term used to describe deliberate, programmed cell death (2 possible answers). 15. Term used to describe a periodic pattern of biological behavior that follows a 24 hour cycle (ie-sleeping vs. waking hours etc). 16. Term used to describe exposing certain flowering plants to cold temperatures before they will flower. (ie-certain plants will not flower unless they have been exposed to a cold period first.) 17. Term used to describe a pattern of stunted growth that results when plants are touched or pushed on regularly (ie-shorter, thicker trees growing in areas of high wind). 18. What do you call plant tissue that is capable of dividing and/or growing, even in adult plants? 19. Are most weeds monocots or eudicots? 20. Artificial auxins are sometimes used as herbicides ( weed killers ). Are eudicots or monocots more sensitive to an auxin overdose? 21. Which plant hormone is sprayed on seedless grapes to promote enlargement of the grapes? 22. Are plant blue-light receptors responsible for phototropism or seed-germination? 23. Are plant phytochromes responsible for phototropism or seed-germination? 24. Will red light activate or inhibit seed germination? 25. Will far-red light activate or inhibit seed germination? 26. Give one example of a short-day flower. 27. Give one example of a long-day flower. 28. Give one example of a day-neutral flower. Plant Hormone Exercise: What will happen to plant cells in the presence of the following combinations of Plant Hormones: 1. Auxin (alone, in low concentration) in a root. 2. High concentrations of gibberellin in a seed. 3. Abscisic Acid in leaves. 4. Auxin (alone, in low concentration) in a stem. 5. Auxin (low concentration) and cytokinin in a root. 6. Auxin in high concentration in a root. 7. Gibberellin in a fertilized flower. 8. High concentrations of both gibberellin and auxin in a seed. 9. High concentrations of gibberellin and low concentrations of auxin in a seed. 10. High concentrations of abscisic acid and low concentrations of gibberellin in a seed. 11. High concentrations of ethylene in an old banana sitting in a bowl of fresh fruit. Phytochromes, Seed Germination and Flower Blooming: 1. Will seeds germinate if you expose them to blue light? 2. Will seeds germinate if you expose them to red light? 3. Will seeds germinate if you expose them to red light and then blue light? 4. Will seeds germinate if you expose them to blue light, red light, and then far-red light? 9

10 5. Will seeds germinate if you expose them to far-red light, red light, and then blue light? 6. Will seeds germinate if you expose them to far-red light, red light, and then far-red light again? 7. Will short-day (long night) plant bloom if you expose them to a flash of white light eight hours into a 16 hour night time period? 8. Will a long-day (short night) plant bloom if you expose it to a flash of white light eight hours into a 16 hour night period? 9. Will a long-day (short night) plant bloom if you expose it to a flash of white light four hours into an eight hour night period? (Why or why not? Explain your answer.) 10. Will a long-day (short night) plant bloom if you expose it to a flash of far-red light eight hours into a 16 hour night time period? 11. Will a long-day (short night) plant bloom if you expose it to a flash of far-red light, followed by a flash of red light eight hours into a 16 hour night time period? Description Questions: Describe the following terms. 1. Etiolation (in plants) 2. Signal Transduction 3. Second messenger 4. Ligand 5. Positive Phototropism 6. Phototaxis 7. Chemotaxis 8. Apoptosis 9. Circadian Rhythm 10. Gravitropism (in plants) 11. Vernalization (in plants) 12. Thigmomorphogenesis (in plants) 13. Meristem 14. Zone of Elongation (in a plant roots) 15. Photoperiod 16. Vernalization 17. Short-day plant 18. Long-day plant 19. Day-neutral plant Essay Questions: 1. Describe the Ethylene-Induced Triple Response in plants, and what plants use it for. (20 points) 2. Briefly explain the mechanism by which the plant hormone Auxin causes cells to elongate. (20 points) Hint: you can study for this question by looking at Figure 39.8 of your textbook. 10

11 3. What is the presumed purpose of having phytochromes be turned on by red light, but turned off by far-red light? (10 points) 4. Why are shipping containers that are used to transport fruit usually kept well ventilated (or alternatively, pumped full of carbon dioxide)? (5 points) 5. Why do leaves change colour from green to orange in the fall? (5 points) 6. Another name for a cytokine that binds to a cell-surface receptor. (A general term for something that binds to a receptor; 5 points) 7. What evidence exists to suggest that the long day (short night) plant blooming phenomenon is mediated by phytochrome receptors? (20 points) 8. Outline the experimental evidence in support of the theory that coleoptile phototaxis is mediated by soluble plant hormones. What two negative controls were used in the experiments? (20 points) Extended Matching Inventory: Match the term to the definition. A. Abscission B. Apoptosis C. Auxin D. Chemotaxis E. Circadian F. Cytokine G. Enhancer H. Etiolation I. Expansin J. Gravitropism K. Hormone L. Meristems M. Phototropism N. Phytochrome O. Promoter P. Statolith Q. Thigmomorphogenesis R. Transcription Factor S. Vernalization 1. Term referring to plants using gravity as a cue to direct growth of roots (downwards) and shoots (upwards). 2. An enzyme that promotes cell elongation through acidification of the cell wall. 3. Name of an enzyme that loosens cellulose fibers in cell walls (in response to acidification by auxin). 4. A photoreceptor that is activated by red light, but de-activated by far-red light. 5. Name for plant tissue that is capable of cell division and growth even in adult plants. 6. A DNA sequence located near a gene, that can increase the rate of transcription from the gene, provided a specific type of protein is bound to it. 7. A part of a gene that determines whether a gene will be transcribed or not. (Determines whether a gene will be turned on or off.) 8. A biological macromolecule that is turned on by red light and turned off by far-red light. 9. An intracellular signal made of protein, which cannot cross a cell membrane without the help of Receptor Mediated Endocytosis (RME). 10. Refers to a plant dropping its leaves in the fall. 11. An intracellular signal, originally derived from lipids, that can cross cell membranes without the help of receptors. 12. A pattern of plant growth characteristic of plants growing in darkness. 11

12 13. A pattern of plant growth that results when the plants are touched or pushed on regularly (ie-shorter, thicker trees growing in areas of high wind.) 14. Deliberate, genetically-programmed cell death. 15. A type of periodic behavior (or rhythm) that follows a 24 hour cycle. 16. Exposing flowering plants to cold temperatures as a pre-requisite for blooming. 17. A small, intracellular compartment filled with dense starch grains that settle to the bottom of cells, allowing plants to vary their growth patterns in response to gravity. 18. Term referring to movement or growth towards the source of a chemical (ie-a cell secreting hormones, and having other cells grow towards it.) 19. Term referring to plant leaves and shoots growing towards sunlight. 20. A special protein that binds to an enhancer, and increases the rate of transcription from a gene. J. Greg Doheny