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1、智课网GMAT备考资料GMATOG阅读文章英译汉（17）-智课教育Allthecellsinaparticularplantstartoutwiththesamecomplementofgenes.Howthencanthesecellsdifferentiateandformstructuresasdifferentasroots,stems,leaves,andfruits?Theansweristhatonlyasmallsubsetofthegenesinaparticularkindofcellareexpressed,orturnedon,atagiventime.Thisis
2、accomplishedbyacomplexsystemofchemicalmessengersthatinplantsincludehormonesandotherregulatorymolecules.Fivemajorhormoneshavebeenidentified:auxin,abscisicacid,cytokinin,ethylene,andgibberellin,studiesofplanrtshavenowidentifiedanewclassofregulatorymoleculescalledoligosaccharins.Unliketheoligosacchar
3、ins,thefivewell-knownplanthormonesarepleiotropicratherthanspecific;thatis,eachhasmorethanoneeffectonthegrowthanddevelopmentofplants.Thefivehavesomanysimultaneouseffectsthattheyarenotveryusefulinartificiallycontrollingthegrowthofcrops.Auxin,forinstance,stimulatestherateofcellelongation,causesthepla
4、nttodevelopavascularsystem,toformlateralroots,andtoproduceethylene.Thepleiotropyofthefivewell-studiedplanthormonesissomewhatanalogoustothatofcertainhormonesinanimals.Forexample,hormonesfromthehypothalamusinthebrainstimulatetheanteriorlobeofthepituitaryglandtosynthesizeandreleasemanydifferenthormon
5、es,oneofwhichstimulatesthereleaseofhormoneshavespecificeffectsontargetorgansalloverthebody.Onehormonestimulatesthethyroidgland,forexample,anothertheovarianfolliclecells,andsoforth.Inotherwords,thereisahierarchyofhormones.Suchahierarchymayalsoexistinplants.Oligosaccharinsarefragmentsofthecellwallre
6、leasedbyenzymesreleasedifferentoligosaccharins.Thereareindicationsthatpleiotropicplanthormonesmayactuallyfunctionbyactivatingtheenzymesthatreleasetheseother,morespecificchemicalmessengersfromthecellwall.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
7、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
