All the cells in a particular plant start out with the same complement of genes. How then can these cells differentiate and form structures as different as roots, stems, leaves, and fruits? The answer is that only a small subset of the genes in a particular kind of cell are expressed, or turned on, at a given time. This is accomplished by a complex system of chemical messengers that in plants include hormones and other regulatory molecules. Five major hormones have been identified: auxin, abscisic acid, cytokinin, ethylene, and gibberellin. Studies of plants have now identified a new class of regulatory molecules called oligosaccharins.
Unlike the oligosaccharins, the five well-known plant hormones are pleiotropic rather than specific ; that is, each has more than one effect on the growth and development of plants. The five have so many simultaneous effects that they are not very useful in artificially controlling the growth of crops. Auxin, for instance, stimulates the rate of cell elongation, causes shoots to grow up and roots to grow down, and inhibits the growth of lateral shoots. Auxin also causes the plant to develop a vascular system, to form lateral roots, and to produce ethylene.
The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animals. For example, hormones from the hypothalamus in the brain stimulate the anterior lobe of the pituitary gland to synthesize and release many different hormones, one of which stimulates the release of hormones from the adrenal cortex. These hormones have specific effects on target organs all over the body. One hormone stimulates the thyroid gland, for example, another the ovarian follicle cells, and so forth. In other words, there is a hierarchy of hormones.
Such a hierarchy may also exist in plants. Oligosaccharins are fragments of the cell wall released by enzymes: different enzymes release different oligosaccharins. There are indications that pleiotropic plant hormones may actually function by activating the enzymes that release these other, more specific chemical messengers from the cell wall.
The passage suggests that, unlike the pleiotropic hormones, oligosaccharins could be used effectively to
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推理题，问的是O的作用，且题目中有O和5H的对比，那么找有他们对比的地方
定位到第二段
这里说的是不同于O，5H的特点：多效性，因为多效性，通过5H人工控制庄稼生长不太有用。
那么题目问的是O，反过来说，O是专一的，可以有效控制庄稼生长
选项A trace the passage of chemicals through the walls of cells未提及
选项B O可以被有效地用于 pinpoint 其他植物荷尔蒙得功能，文章未提及
选项C 正确
选择D O可以被有效地用于 alter the complement of genes 文章未提及，直说了O和5H可以控制基因表达，没有说可以改变机经组成
选项E 改变5H 的effect 未提及
原文没有提到的内容竟然可以这样意淫吗？？？？？five hormones不能用与控制，oligo就可以吗？不一定吧
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2022-10-29 23:33:41
妈的我是瞎子
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2022-08-03 10:53:18
Unlike the oligosaccharins, 。。。。。。。 The five have so many simultaneous effects that they are not very useful in artificially controlling the growth of crops.
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2021-01-01 11:16:36
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