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  3. All of the cells in a particular plant start out with the same complement of genes. How then can these cells differentiate and f

All of the cells in a particular plant start out with the same complement of genes. How then can these cells differentiate and f

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问题     All of 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 sys- tern of chemical messengers that in plants include hormones and other regulatory molecules. Five major hormones have been identified: auxin, abscisic acid, cytokinin, ethylene, and gibberel- lin. Studies of plants have now identified a new class of regulatory molecules called oligosaccharide.
    Unlike the oligosaccharide, 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 has 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 animal. 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 thy- roid 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. Oligosaccharides are fragments of the cell wall released by enzymes: different enzymes release different oligosaccharides. There are indications that pleiotropic plant hormones may ac- tually function by activating the enzymes that release these other, more specific chemical messengers from the cell wall.
The author mentions specific effects that auxin has on plant development in order to illustrate the
选项 A、point that some of the effects of plant hormones can be harmful.
B、way in which hormones are produced by plants.
C、hierarchical nature of the functioning of plant hormones.
D、differences among the best-known plant hormones.
E、concept of pleiotropy as it is exhibited by plant hormones.
答案E
解析 作者提到生长素在植物发育中的特殊作用，是为了描述：A．指出一些植物荷尔蒙有害。无。B．荷尔蒙被选出的方式。无。C．植物荷尔蒙等级制特点。这一点在第四段才提出。D．已知荷尔蒙的区别。文中五种一直是在一起作为整体被作者提到，没有指出它们之间有区别。E．按照多向性在植物荷尔蒙中的表现给出这个观念。正确。第二段就是指出五种已知荷尔蒙是多样性的，和低聚糖不同，生长素是其中一个例子。
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