Bakrim Ahmed, Maria Annick, Sayah Fouad, Lafont René, Takvorian Najat
UPMC, Univ. Paris 06, Laboratoire de Biochimie Structurale et Fonctionnelle des Protéines, CNRS FRE 2852, Case Courrier 29, 7 Quai Saint Bernard, Paris Cedex 05, France.
Plant Physiol Biochem. 2008 Oct;46(10):844-54. doi: 10.1016/j.plaphy.2008.06.002. Epub 2008 Jun 13.
Many plant species produce phytoecdysteroids (PEs: i.e. analogues of insect steroid hormones). There is increasing evidence that PEs are used as a chemical defence by plants against non-adapted insects and nematodes. PEs are good candidates for the development of an environmentally safe approach to crop protection. Most crop species do not accumulate PEs. However, many arguments support the idea that most, if not all, plant species have the genetic ability to produce PEs, but the biosynthetic pathway is not active. A better understanding of the PE biosynthetic pathway and its regulation is consequently necessary. Spinach is one of the very few crop plants which produce large amounts of PEs, of which 20-hydroxyecdysone is the major component. Labeling experiments with radiolabeled precursor (mevalonic acid), putative ecdysteroid intermediates and 20-hydroxyecdysone itself have allowed investigation of PE biosynthesis and transport during spinach development. Biosynthesis takes place in older leaf sets ("sources"), but not in the young developing ones, which in contrast accumulate (acting as "sinks") the PEs produced by the older leaves. PEs are thus continuously redistributed within the developing plant, as its leaf set number increases. The biosynthetic pathway has been analyzed using excised leaves and various labeled precursors, and a preferential sequence of the last steps has been established. Although they do not produce PEs, apical leaf sets are nevertheless able to perform several putative terminal steps of PE biosynthesis. The regulatory mechanisms of PE synthesis appear to involve a direct negative feedback of 20-hydroxyecdysone (the major PE in spinach) on its own synthesis; thus, a sustained synthesis in older leaves requires that they can export the PE they produce.
许多植物物种都会产生植物蜕皮甾类化合物（PEs：即昆虫类固醇激素的类似物）。越来越多的证据表明，PEs被植物用作针对非适应性昆虫和线虫的化学防御物质。PEs是开发环境安全的作物保护方法的理想选择。大多数作物物种不会积累PEs。然而，许多观点支持这样一种看法，即大多数（如果不是全部）植物物种具有产生PEs的遗传能力，但生物合成途径并不活跃。因此，有必要更好地了解PE生物合成途径及其调控机制。菠菜是极少数能大量产生PEs的作物之一，其中20-羟基蜕皮酮是主要成分。利用放射性标记的前体（甲羟戊酸）、假定的蜕皮甾类中间体和20-羟基蜕皮酮本身进行的标记实验，使得人们能够研究菠菜发育过程中PE的生物合成和运输。生物合成发生在较老的叶组（“源”）中，而不是在正在发育的幼叶中，相反，幼叶会积累（作为“库”）较老叶片产生的PEs。因此，随着叶组数的增加，PEs在发育中的植物体内不断重新分布。利用离体叶片和各种标记前体对生物合成途径进行了分析，并确定了最后几步的优先顺序。尽管顶端叶组不产生PEs，但它们仍然能够进行PE生物合成的几个假定的终端步骤。PE合成的调控机制似乎涉及20-羟基蜕皮酮（菠菜中的主要PE）对其自身合成的直接负反馈；因此，较老叶片中的持续合成要求它们能够输出所产生的PE。
