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Ch39WordLectureOutline - CHAPTER 39 PLANT RESPONSES TO...

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CHAPTER 39 PLANT RESPONSES TO INTERNAL AND EXTERNAL SIGNALS Introduction 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. 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. A. Signal Transduction and Plant Responses 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.
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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. 1. Signal-transduction pathways link internal and environmental signals to cellular responses. Plant growth patterns vary dramatically in the presence versus the absence of light. For example, a potato (a modified underground stem) can sprout shots from its “eyes” (axillary buds). These shoots are ghostly pale, have long and thin stems, unexpanded leaves, and reduced roots. These morphological adaptations, seen also in seedlings germinated in the dark, 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.
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