pansy

20．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 planrts 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 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 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 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.


一棵植物中的所有细胞都开始于一套完整的基因。这些细胞是怎样分化成不同的细胞从而形成根、茎、叶和果实的呢？这是因为在特定时期一种细胞中只有一小部分基因在表达或表现。这是通过植物中激素及其它调控分子等复杂的化学信息系统实现的。已经被证实的有五大激素：生长素、脱落酸、细胞分裂素、乙烯和赤霉素。现代又发现了一种新的调控分子寡聚糖素。
与特效寡聚糖素不同，这五种熟知的植物激素是多效的，也就是说每一种都对植物的生长发展有多种影响。这五种激素有太多的同期影响以至于对人为控制作物生长不是非常有效。例如生长素，刺激细胞伸长，使枝干向上生长，根向下生长，阻碍侧枝生长。生长素还能促进植物维管束的形成，侧枝的生长和乙烯的产生。
这五种熟知的植物基因的多效性与与某种动物基因的多效性类似。例如，下丘脑分泌的激素能刺激垂体前叶合成并释放多种激素。其中一种又能刺激肾上腺皮质分泌激素。这些激素对全身各个器官有特定的作用。一种刺激甲状腺，另一种刺激卵泡细胞，等等。。。换句话说就是有一种激素层级。
这种层级也可能在植物中存在。寡聚糖素是酶分解的细胞壁碎片：不同的酶分解不同的寡聚糖素。有指征说明多效的植物激素可能通过激活从细胞壁释放其它化学信息的酶来起作用。