The adrenal cortex is a component of the hypothalamic-pituitary-adrenal (HPA) axis. Each region secretes its own set of hormones. According to the passage, the five well-known plant hormones are not useful in controlling the growth of crops because_ A.it is not known exactly what functions the hormones perform B.each hormone has various effects on plants C.none of the hormones can function without the others D.each hormone has different effects on different kinds of plants E.The adrenal gland consists of the adrenal cortex that is composed of glandular tissue and the adrenal medulla that is composed of nervous tissue. There are indications that pleiotropic plant hormones may actually function by activating the (40) enzymes that release these other, more specific chemical messengers from the cell wall. Oligosac- charins are fragments of the cell wall released by enzymes: different enzymes release different oligosac- charins. (35) Such a hierarchy may also exist in plants. In other words, there is a hierarchy of hormones. One hormone stimulates the thyroid gland, for example, another the ovarian follicle cells, and so forth. These hormones have specific effects on target organs all over the body. 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 (30) of hormones from the adrenal cortex. The pleiotropy of the five well-studied plant (25) hormones is somewhat analogous to that of certain hormones in animal. Auxin also causes the plant to develop a vascular system, to form. Auxin, for instance, stimulates the rate of cell elongation, causes shoots to (20) grow up and roots to grow down, and inhibits the growth of lateral shoots. The five has so many simultaneous effects that they are not very useful in artificially controlling the growth of crops. Unlike the oligosaccharins, the five well-known plant hormones are pleiotropic rather than specific, that is, (15) each has more than one effect on the growth and devel- opment of plants. Studies of plants have now identified a new class of regulatory molecules called oligosaccharins. Five major hormones have been identified: (10) auxin, abscisic acid, cytokinin, ethylene, and gibberel- lin. structures as different as roots, stems, leaves, and fruits? The answer is that only a (5) small subset of the genes in a particular kind of cell are expressed, or turned on, at a given timE.This is accom- plished by a complex system of chemical messengers that in plants include hormones and other regulatory molecules. How then can these cells differentiate and form.
未分类题 All of the cells in a particular plant start out with the same complement of genes.
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