The Macaulay Institute for Soil Research, Craigiebuckler, AB9 2QJ, Aberdeen, UK.
Planta. 1983 Jun;158(1):76-81. doi: 10.1007/BF00395406.
Photographic observations on germinating seedlings of Lepidium sativum L., Cucumis sativus L., and Helianthus annuus L. showed that the hypocotyl hook is not present in the seed but forms during the early stages of growth. Evidence that gravity plays a major role in inducing curvature of the hypocotyl, and in maintaining the hook once it has been formed, was obtained from clinostat experiments, from the use of morphactin to remove geotropic sensitivity and from inversion of seedlings to change the direction of the geostimulus. In L. sativum and H. annuus gravity perception seemed to be the only mechanism responsible for hook formation. In C. sativus hook formation was additionally aided by the mode of emergence of the cotyledons from the seed coat but gravity played an indirect role in regulating such emergence. Further evidence that hook formation is linked to a georesponse was derived from a comparison of hypocotyl development in wild-type Arabidopsis thaliana seedlings with that of an ageotropic mutant, hook formation being found to occur only in the wild type. Hook formation and maintenance is discussed in terms of contrasting geosensitivity between the apical and basal ends of the hypocotyl and it is suggested that light-induced hook opening is a reversal to a condition of uniformly negative georesponse throughout the hypocotyl.
对发芽的荠（Lepidium sativum L.）、黄瓜（Cucumis sativus L.）和向日葵（Helianthus annuus L.）幼苗的摄影观察表明，下胚轴钩不是种子中存在的，而是在生长的早期阶段形成的。来自回旋仪实验、使用形态素去除向地性敏感性以及倒置幼苗以改变地刺激方向的证据表明，重力在诱导下胚轴弯曲和维持已经形成的钩方面起着主要作用。在荠和向日葵中，重力感知似乎是形成钩的唯一机制。在黄瓜中，钩的形成除了受到子叶从种皮中出现的方式的辅助外，重力还间接地调节了这种出现。进一步的证据表明，钩的形成与向地性反应有关，这是通过比较野生型拟南芥（Arabidopsis thaliana）幼苗和非向地突变体的下胚轴发育得出的，只有在野生型中才发现钩的形成。根据下胚轴顶端和基部之间的向地性敏感性的对比，讨论了钩的形成和维持的问题，并提出了光诱导的钩张开是下胚轴整个向地性反应均匀为负的一种逆转。
