had open or closed flowers. If the times of opening and closing were arranged in sequence, they could serve as a 39 kind of floral clock, or horologium florae , as Linnaeus called it. Figure 39.1 shows a modern representation as a 12-hour clock face. Why does the timing vary? The time at which flowers open presumably reflects the time when their insect pollinators are most active, just one example of the numer- Plant Responses ous environmental factors that a plant must sense to com- pete successfully. to Internal and This chapter focuses on the mechanisms by which flow- ering plants sense and respond to external and internal External Signals cues. At the organismal level, plants and animals respond to environmental stimuli by different means. Animals, being mobile, respond mainly by moving toward positive stimuli and away from negative stimuli. In contrast, plants are stationary and generally respond to environmental cues by adjusting their individual patterns of growth and devel- opment. For this reason, plants of the same species vary in body form much more than do animals of the same species. But just because plants do not move in the same manner as animals does not mean that plants lack sensitiv- ity. Before a plant can initiate alterations to growth pat- terns in response to environmental signals, it must first detect the change in its environment. As we will see, the molecular processes underlying plant responses are as com- plex as those used by animal cells and are often homolo- gous to them. CONCEPT 39.1 Signal transduction pathways link ᭡ Figure 39.1 Can flowers tell you the time of day? signal reception to response Plants receive specific signals and respond to them in ways KEY CONCEPTS that enhance survival and reproductive success. Consider, 39.1 Signal transduction pathways link signal for example, a forgotten potato in the back corner of a reception to response kitchen cupboard. This modified underground stem, or 39.2 Plant hormones help coordinate growth, tuber, has sprouted shoots from its “eyes” (axillary buds). development, and responses to stimuli These shoots, however, scarcely resemble those of a typical 39.3 Responses to light are critical for plant success plant. Instead of sturdy stems and broad green leaves, this 39.4 Plants respond to a wide variety of stimuli other plant has ghostly pale stems and unexpanded leaves, as well than light as short, stubby roots (Figure 39.2a) . These morphological 39.5 Plants respond to attacks by herbivores and adaptations for growing in darkness, collectively referred to pathogens as etiolation , make sense if we consider that a young po- tato plant in nature usually encounters continuous darkness O V E R V I E W when sprouting underground. Under these circumstances, Stimuli and a Stationary Life expanded leaves would be a hindrance to soil penetration and would be damaged as the shoots pushed through the Carolus Linnaeus, the father of taxonomy, was a keen nat- soil. Because the leaves are unexpanded and underground, uralist. He noted that each plant species opened and closed there is little evaporative loss of water and little requirement its flowers at a characteristic time of the day. Therefore, one for an extensive root system to replace the water lost by could estimate the time of day by observing which species transpiration. Moreover, the energy expended in producing CHAPTER 39 Plant Responses to Internal and External Signals 821 CELL CYTOPLASM WALL 1Reception 2Transduction 3 Response Relay proteins and Activation of cellular second messengers responses Receptor (a) Before exposure to light. A (b) After a week’s exposure to dark-grown potato has tall, natural daylight. The potato Hormone or spindly stems and nonexpanded plant begins to resemble a environmental leaves—morphological typical plant with broad green st imulus Plasma membrane adaptations that enable the leaves, short sturdy stems, and shoots to penetrate the soil. The long roots. This transformation roots are short, but there is little begins with the reception of ᭡ Figure 39.3 Review of a general model for signal need for water absorption light by a specific pigment, transduction pathways. As discussed in Chapter 11, a hormone or because little water is lost by phytochrome. other kind of stimulus interacting with a specific receptor protein can the shoots. trigger the sequential activation of relay proteins and also the production of second messengers that participate in the pathway. The ᭡ Figure 39.2 Light-induced de-etiolation (greening) of signal is passed along, ultimately bringing about cellular responses. In dark-grown potatoes. this diagram, the receptor is on the surface of the target cell; in other cases, the stimulus interacts with receptors inside the cell. green chlorophyll would be wasted because there is no light de-etiolation through studies of the tomato, a close relative of for photosynthesis. Instead, a potato plant growing in the the potato. The aurea mutant of tomato, which has reduced dark allocates as much energy as possible to elongating its levels of phytochrome, greens less than wild-type tomatoes stems. This adaptation enables the shoots to break ground when exposed to light. ( Aurea is Latin for “gold.” In the ab- before the nutrient reserves in the tuber are exhausted. sence of chlorophyll, the yellow and orange accessory pig- The etiolation response is one example of how a plant’s ments called carotenoids are more obvious.) Researchers morphology and physiology are tuned to its surroundings produced a normal de-etiolation response in individual aurea by complex interactions between environmental and in- leaf cells by injecting phytochrome from other plants and ternal signals. then exposing the cells to light. Such experiments indi- When a shoot reaches light, the plant undergoes profound cated that phytochrome functions in light detection dur- changes, collectively called de-etiolation (informally known ing de-etiolation. as greening). Stem elongation slows; leaves expand; roots elon- gate; and the shoot produces chlorophyll. In short, it begins to Transduction resemble a typical plant (Figure 39.2b) . In this section, we will Receptors can be sensitive to very weak environmental or use this de-etiolation response as an example of how a plant chemical signals. Some de-etiolation responses are triggered by cell’s reception of a signal—in this case, light—is transduced extremely low levels of light, in certain cases as little as the into a response (greening). Along the way, we will explore how equivalent of a few seconds of moonlight. The transduction of studies of mutants provide insights into the molecular details these extremely weak signals involves second messengers — of the stages of cell signal processing: reception, transduction, small molecules and ions in the cell that amplify the signal and response (Figure 39.3) . and transfer it from the receptor to other proteins that carry out the response (Figure 39.4) . In Chapter 11, we discussed Reception several kinds of second messengers (see Figures 11.12 and Signals are first detected by receptors, proteins that undergo 11.14). Here, we examine the particular roles of two types of ϩ changes in shape in response to a specific stimulus. The recep- second messengers in de-etiolation: calcium ions (Ca 2 ) and tor involved in de-etiolation is a type of phytochrome , a mem- cyclic GMP (cGMP). ϩ ber of a class of photoreceptors that we’ll discuss more fully Changes in cytosolic Ca 2 levels play an important role later in the chapter. Unlike most receptors, which are built in phytochrome signal transduction. The concentration of ϩ Ϫ into the plasma membrane, the type of phytochrome that cytosolic Ca 2 is generally very low (about 10 7 M), but ϩ functions in de-etiolation is located in the cytoplasm. Re- phytochrome activation leads to the opening of Ca 2 searchers demonstrated the requirement for phytochrome in channels and a transient 100-fold increase in cytosolic 822 UNIT SIX Plant Form and Function 1 Reception 2 Transduction 3 Response Transcription CYTOPLASM factor 1 NUCLEUS Specific cGMP Plasma protein P membrane kinase 1 Second messenger activated produced Transcription factor 2 Phytochrome activated 2 One pathway uses cGMP as a by light second messenger that activates P Cell a protein kinase. The other wall pathway increases the cytosolic Specific level of Ca2+ , which activates protein a different protein kinase. kinase 2 activated Transcription Light 3 Both pathways Translation lead to expression of genes for proteins 1 The light signal is that function in the detected by the de-etiolation response. De-etiolation phytochrome receptor, Ca 2+ channel (greening) which then activates opened response at least two signal proteins transduction pathways. Ca 2+ ᭡ Figure 39.4 An example of signal transduction in plants: the role of phytochrome in the de-etiolation (greening) response. MAKE CONNECTIONS Which panel in Figure 11.18 (p. 222) best exemplifies the phytochrome-dependent signal transduction pathway during de-etiolation? Explain. 2ϩ Ca levels. In response to light, phytochrome undergoes a Post-Translational Modification of Preexisting Proteins change in shape that leads to the activation of guanylyl cy- In most signal transduction pathways, preexisting proteins clase, an enzyme that produces the second messenger cyclic ϩ are modified by the phosphorylation of specific amino acids, GMP. Both Ca 2 and cGMP must be produced for a com- which alters the protein’s hydrophobicity and activity. Many plete de-etiolation response. The injection of cGMP into ϩ second messengers, including cGMP and Ca 2 , activate pro- aurea tomato leaf cells, for example, induces only a partial tein kinases directly. Often, one protein kinase will phospho- de-etiolation response. rylate another protein kinase, which then phosphorylates another, and so on (see Figure 11.10). Such kinase cascades Response may link initial stimuli to responses at the level of gene ex- Ultimately, second messengers regulate one or more cellular pression, usually via the phosphorylation of transcription activities.