Chapter 39 Plant Responses to Internal and External Signals

Chapter 39 Plant Responses to Internal and External Signals...

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Chapter 39 Plant Responses to Internal and External Signals Lecture Outline 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. o Only minute quantities of hormones are necessary to induce substantial change in an organism. o Hormone concentration or rate of transport can change in response to environmental stimuli. o 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. o Plants grow toward light, and if you rotate a plant, it will reorient its growth until its leaves again face the light. o Any growth response that results in curvature of whole plant organs toward or away from stimuli is called a tropism. o 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. o 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. o 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. o This response stopped if the tip was removed or covered with an opaque cap (but not a transparent cap). o 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.
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o 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. o 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,
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Chapter 39 Plant Responses to Internal and External Signals...

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