2. 考研
  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

admin2020-08-17  14
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 system of chemical messengers that in plants include hormones and other regulatory molecules. Five major hormones have been identified: auxin, abscisic acid, cyto-kinin, 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 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 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, why are the five well-known plant hormones not useful in artificially controlling the growth of crops?
选项
答案Because they are pleiotropic and each has various effects on plants.
解析 事实细节题。根据the five well-known plant hormones和not useful in artificially controlling the growth of crops定位到第二段前两句。这两句提到，这五种著名植物激素具有多效性，也就是说，每一种激素对植物的生长发育都有不止一种的影响。这五种激素同时产生的效应太多，在人工控制作物生长方面就用处不大。由此可知，这五种激素在人工控制作物生长方面作用不大的原因是其多效性。
转载请注明原文地址:https://kaotiyun.com/show/98ra777K
本试题收录于： 翻译硕士（翻译硕士英语）题库专业硕士分类
0

翻译硕士（翻译硕士英语）

专业硕士

相关试题推荐
随机试题
最新回复(0)