Limbach Christoph, Hauslage Jens, Schäfer Claudia, Braun Markus
Gravitationsbiologie, Institut für Molekulare Physiologie und Biotechnologie der Pflanzen, Universität Bonn, Germany.
Plant Physiol. 2005 Oct;139(2):1030-40. doi: 10.1104/pp.105.068106. Epub 2005 Sep 23.
Early processes underlying plant gravity sensing were investigated in rhizoids of Chara globularis under microgravity conditions provided by parabolic flights of the A300-Zero-G aircraft and of sounding rockets. By applying centrifugal forces during the microgravity phases of sounding rocket flights, lateral accelerations of 0.14 g, but not of 0.05 g, resulted in a displacement of statoliths. Settling of statoliths onto the subapical plasma membrane initiated the gravitropic response. Since actin controls the positioning of statoliths and restricts sedimentation of statoliths in these cells, it can be calculated that lateral actomyosin forces in a range of 2 x 10(-14) n act on statoliths to keep them in place. These forces represent the threshold value that has to be exceeded by any lateral acceleration stimulus for statolith sedimentation and gravisensing to occur. When rhizoids were gravistimulated during parabolic plane flights, the curvature angles of the flight samples, whose sedimented statoliths became weightless for 22 s during the 31 microgravity phases, were not different from those of in-flight 1g controls. However, in ground control experiments, curvature responses were drastically reduced when the contact of statoliths with the plasma membrane was intermittently interrupted by inverting gravistimulated cells for less than 10 s. Increasing the weight of sedimented statoliths by lateral centrifugation did not enhance the gravitropic response. These results provide evidence that graviperception in characean rhizoids requires contact of statoliths with membrane-bound receptor molecules rather than pressure or tension exerted by the weight of statoliths.
在由A300-Zero-G飞机和探空火箭的抛物线飞行所提供的微重力条件下,对球形轮藻假根中植物重力感应的早期过程进行了研究。在探空火箭飞行的微重力阶段施加离心力,0.14 g的横向加速度会导致平衡石移位,但0.05 g的横向加速度则不会。平衡石沉降到根尖下质膜上会引发向重力性反应。由于肌动蛋白控制着平衡石的定位并限制其在这些细胞中的沉降,因此可以计算出,作用在平衡石上使其保持原位的横向肌动球蛋白力在2×10⁻¹⁴牛的范围内。这些力代表了任何横向加速度刺激要使平衡石沉降和重力感应发生所必须超过的阈值。当在抛物线飞行过程中对假根进行重力刺激时,飞行样本的弯曲角度与飞行中的1g对照样本的弯曲角度并无差异,在31个微重力阶段中,其沉降的平衡石有22秒处于失重状态。然而,在地面对照实验中,当通过将受重力刺激的细胞倒置不到10秒来间歇性中断平衡石与质膜的接触时,弯曲反应会大幅降低。通过横向离心增加沉降平衡石的重量并不会增强向重力性反应。这些结果证明,轮藻假根中的重力感知需要平衡石与膜结合受体分子接触,而不是平衡石重量所施加的压力或拉力。