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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.

According to the passage, the five well-known plant hormones are not useful in controlling the growth of crops because

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当前版本由 kisenana 更新于2015-11-05 16:25:26 感谢由 kisenana 对此题目的解答所做出的贡献。
关键词 five well-known plant hormones are not useful in controlling the growth of crops 
定位到第2段第1,2句 不同于O,5H的特点:多效性,也就是一个H有很多种作用,因为这种多效性,通过5H人工控制庄稼生长不太有用。
选项A 不是不知道H有哪些作用,而是就算知道了,它作用也是很多种的,没办法控制
选项B 正确
选项C 没有其他H就不能工作, 文章未提及
选项D 不是本回文定位内容,是一个H有很多作用,而不是每个H,对不同的植物作用不一样
选项E 这是用来解释分化成根茎叶的,不是解释为什么通过5h控制庄稼生长不太有用

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