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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 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?
选项 A、Plant cell walls.
B、The complement of genes in each plant cell.
C、A subset of a plant cell’ s gene complement .
D、The five major hormones.
E、The oligosaccharides.
答案D
解析 动物体内荷尔蒙等级中的下丘脑，相当于植物体内的哪种物质?首先看第三段叙述：下丘脑释放的荷尔蒙刺激垂体前叶合成，释放多种荷尔蒙。其中之一刺激肾上腺分泌荷尔蒙，对全身许多器官有特定作用(如甲状腺和卵泡细胞)。可见，下丘脑在这个等级制中是起最高等级作用的，决定了后面一系列荷尔蒙的释放。我们看最后一段，哪种物质在植物体内有类似作用：多向性荷尔蒙激发酶释放其他的专一化学传送物质，可见在此等级制中较高地位的是多向性荷尔蒙。∴D正确，五种主要荷尔蒙。
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