Staehelin L A, Zheng H Q, Yoder T L, Smith J D, Todd P
Department of Molecular Biology, University of Colorado, Boulder, USA.
Gravit Space Biol Bull. 2000 Jun;13(2):95-100.
A hundred years of research has not produced a clear understanding of the mechanism that transduces the energy associated with the sedimentation of starch-filled amyloplast statoliths in root cap columella cells into a growth response. Most models postulate that the statoliths interact with microfilaments (MF) to transmit signals to the plasma membrane (or ER), or that sedimentation onto these organelles produces the signals. However, no direct evidence for statolith-MF links has been reported, and no asymmetric structures of columella cells have been identified that might explain how a root turned by 90 degrees knows which side is up. To address these and other questions, we have (1) quantitatively examined the effects of microgravity on the size, number, and spatial distribution of statoliths; (2) re-evaluated the ultrastructure of columella cells in high-pressure frozen/freeze-substituted roots; and (3) followed the sedimentation dynamics of statolith movements in reoriented root tips. The findings have led to the formulation of a new model for the gravity-sensing apparatus of roots, which envisages the cytoplasm pervaded by an actin-based cytoskeletal network. This network is denser in the ER-devoid central region of the cell than in the ER-rich cell cortex and is coupled to receptors in the plasma membrane. Statolith sedimentation is postulated to disrupt the network and its links to receptors in some regions of the cell cortex, while allowing them to reform in other regions and thereby produce a directional signal.
根冠中柱细胞内充满淀粉的造粉体沉降所伴随的能量，是如何转化为生长响应的。多数模型假定，造粉体与微丝（MF）相互作用，将信号传递至质膜（或内质网），或者造粉体沉降到这些细胞器上会产生信号。然而，尚无关于造粉体与微丝联系的直接证据被报道，并且也未发现中柱细胞的不对称结构，这种结构或许能解释根旋转90度后如何知晓哪一侧朝上。为解决这些及其他问题，我们（1）定量研究了微重力对造粉体大小、数量及空间分布的影响；（2）重新评估了高压冷冻/冷冻置换根中中柱细胞的超微结构；（3）追踪了重新定向根尖中造粉体运动的沉降动力学。这些发现促成了一种新的根重力感应装置模型的形成，该模型设想细胞质被基于肌动蛋白的细胞骨架网络所贯穿。此网络在细胞缺乏内质网的中央区域比在内质网丰富的细胞皮层更密集，并与质膜中的受体相连。据推测，造粉体沉降会破坏细胞皮层某些区域的网络及其与受体的连接，同时允许它们在其他区域重新形成，从而产生定向信号。
