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 the place of hypothalamic hormones in the hormonal hierarchies of animals is similar to the place of which of the following in plants?
问的是在动物层级结构地位中 的这些荷尔蒙 和植物中的什么是差不多，具有可比性的
定位到第三段第一句 植物中的5好跟 动物中的荷尔蒙是差不多的，说明是荷尔蒙的对比，相似
The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animals.
1. hierarchy of hormone in animal:hormones from the hypothalamus->anterior lobe->many different hormones->hormones from the adrenal cortex 说明在这个等级关系中，hormones from the hypothalamus是处在最高的位置，有它刺激产生后面一系列的东西 2. hierarchy in plant：pleiotropic plant hormones->enzyme->cell wall(Oligosaccharins), 在这一等级中处于最高位置的是pleiotropic plant hormones，地位与hormones from the hypothalamus在动物中相似 因此选D
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.那五个啥不是没这种针对的功能吗？
hierarchy是说荷尔蒙起作用的方式是层级，是一层层传导的。here 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.题里面的pleiotropic plant hormones就是相似的hypothalamic hormones
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