Boonsirichai K, Guan C, Chen R, Masson P H
Laboratory of Genetics, University of Wisconsin-Madison, 445 Henry Mall, Madison, Wisconsin 53706, USA.
Annu Rev Plant Biol. 2002;53:421-47. doi: 10.1146/annurev.arplant.53.100301.135158.
The ability of plant organs to use gravity as a guide for growth, named gravitropism, has been recognized for over two centuries. This growth response to the environment contributes significantly to the upward growth of shoots and the downward growth of roots commonly observed throughout the plant kingdom. Root gravitropism has received a great deal of attention because there is a physical separation between the primary site for gravity sensing, located in the root cap, and the site of differential growth response, located in the elongation zones (EZs). Hence, this system allows identification and characterization of different phases of gravitropism, including gravity perception, signal transduction, signal transmission, and curvature response. Recent studies support some aspects of an old model for gravity sensing, which postulates that root-cap columellar amyloplasts constitute the susceptors for gravity perception. Such studies have also allowed the identification of several molecules that appear to function as second messengers in gravity signal transduction and of potential signal transducers. Auxin has been implicated as a probable component of the signal that carries the gravitropic information between the gravity-sensing cap and the gravity-responding EZs. This has allowed the identification and characterization of important molecular processes underlying auxin transport and response in plants. New molecular models can be elaborated to explain how the gravity signal transduction pathway might regulate the polarity of auxin transport in roots. Further studies are required to test these models, as well as to study the molecular mechanisms underlying a poorly characterized phase of gravitropism that is independent of an auxin gradient.
植物器官利用重力作为生长导向的能力，即向地性，已被认识两个多世纪了。这种对环境的生长反应对茎向上生长和根向下生长起着重要作用，这在整个植物界普遍可见。根的向地性受到了大量关注，因为位于根冠的重力感应主要部位与位于伸长区的差异生长反应部位之间存在物理分隔。因此，该系统有助于识别和表征向地性的不同阶段，包括重力感知、信号转导、信号传递和弯曲反应。最近的研究支持了一个关于重力感应的旧模型的某些方面，该模型假定根冠柱状淀粉体构成重力感知的感受器。这些研究还使得能够识别出几种似乎在重力信号转导中起第二信使作用的分子以及潜在的信号转导器。生长素被认为可能是在重力感应根冠和重力反应伸长区之间传递向地性信息的信号的一个组成部分。这使得能够识别和表征植物中生长素运输和反应背后的重要分子过程。可以构建新的分子模型来解释重力信号转导途径如何调节根中生长素运输的极性。还需要进一步研究来测试这些模型，以及研究向地性中一个特征不明且独立于生长素梯度的阶段的分子机制。
