Baluska F, Hasenstein K H
Institute of Botany, Slovak Academy of Sciences, Bratislava, Slovakia.
Planta. 1997;203(Suppl):S69-78. doi: 10.1007/pl00008117.
We have critically evaluated the possible functions of the plant cytoskeleton in root gravisensing and graviresponse and discussed the evidence that microtubules (MTs) and actin microfilaments (MFs) do not control differential cell growth during bending of roots. On the other hand, MF and MT networks are envisaged to participate in gravisensing because of the mechanical properties of the cytoskeletal structures that interconnect plant cell organelles with the plasma membrane. In restrained gravisensing, forces are suggested to be transmitted to membranes because large-scale gravity-dependent repositioning of organelles is effectively prevented due to the cytoskeleton-mediated anchorage of their envelopes at the plasma membrane. From the cytoskeletal point of view, we can also envisage an unrestrained gravity sensing when cytoskeletal tethers are not strong enough to preserve the tight control over distribution of organelles and the latter, if heavy enough, are allowed to sediment towards the physical bottom of cells. This situation obviously occurs in root cap statocytes because these uniquely organized cells are depleted of prominent actin MF bundles, endoplasmic MT arrays, and ER elements in their internal cytoplasm. Nevertheless, indirect evidence clearly indicates that sedimented root cap statoliths are enmeshed within fine but dynamic MF networks and that their behaviour is obviously under, at least partial, cytoskeletal control. The actomyosin-enriched domain among and around amyloplasts is proposed to increase the perception of gravity due to the grouping effect of sedimenting statoliths. Cytoskeletal links between myosin-rich statoliths, and cell peripheries well equipped with dense cortical MTs, membrane-associated cytoskeleton, as well as with ER elements, would allow efficient restrained gravisensing only at the statocyte cell cortex. As a consequence of cytoskeletal depletion in the internal statocyte cytoplasm and bulk sedimentation of large amyloplasts, restrained gravisensing is spatially restricted to the bottom of the statocyte irrespective of whether roots are vertical or horizontal. This spatial aspect allows for efficient gravisensing via amplification of gravity-induced impacts on the cellular architecture, a phenomenon which is unique to root cap statocytes.
我们严格评估了植物细胞骨架在根的重力感应和重力反应中的可能功能，并讨论了微管（MTs）和肌动蛋白微丝（MFs）在根弯曲过程中不控制细胞差异生长的证据。另一方面，由于细胞骨架结构的机械特性，MF和MT网络被设想参与重力感应，这些结构将植物细胞器与质膜相互连接。在受限重力感应中，由于细胞骨架介导的细胞器包膜在质膜上的锚定有效地阻止了细胞器大规模的重力依赖性重新定位，因此力被认为会传递到膜上。从细胞骨架的角度来看，当细胞骨架系链不够强壮，无法保持对细胞器分布的严格控制，而如果细胞器足够重，允许它们向细胞的物理底部沉降时，我们也可以设想一种不受限的重力感应。这种情况显然发生在根冠平衡细胞中，因为这些独特组织的细胞内部细胞质中缺乏突出的肌动蛋白MF束、内质网MT阵列和内质网元件。然而，间接证据清楚地表明，沉降的根冠平衡石被包裹在精细但动态的MF网络中，并且它们的行为显然至少部分受细胞骨架控制。淀粉体之间和周围富含肌动球蛋白的区域被认为由于沉降平衡石的聚集效应而增加了对重力的感知。富含肌球蛋白的平衡石与配备有密集皮质MTs、膜相关细胞骨架以及内质网元件的细胞周边之间的细胞骨架连接，将仅在平衡细胞的细胞皮层实现有效的受限重力感应。由于平衡细胞内部细胞质中的细胞骨架耗竭以及大型淀粉体的大量沉降，受限重力感应在空间上被限制在平衡细胞的底部而不管根是垂直还是水平。这种空间方面允许通过放大重力对细胞结构的影响来实现有效的重力感应，这是根冠平衡细胞独有的现象。
