Braun M
Botanishes Institut, Rheinische Friedrich-Wilhelms-Universitat Bonn, Germany.
Planta. 1997 Sep;203(Suppl 1):S11-9. doi: 10.1007/pl00008098.
Unicellular tip-growing cells are excellent experimental systems in which to study gravitropism because cell extension, gravity sensing and the gravity response are all confined to the apical dome. Thus various approaches can be used to determine the distinct steps of the short gravitropic signal-transduction chain, which lacks a signal-transmission phase between the gravity-sensing cells and the competent responding target cells. Single-cell systems readily allow in-vivo observation of cellular processes during gravistimulation at 1 g, centrifugation, clinostatting and in microgravity, as well as permitting fluorescence labeling. Such diverse studies have revealed fascinating information on the mechanism of gravitropic tip growth, especially on the important role of the cytoskeleton in the positioning of the statoliths and in organizing and adjusting the Spitzenkorper. A hypothesis explaining the negative and positive gravitropism of Chara rhizoids and Chara protonemata has been put forward, which emphasizes the role of the actin cytoskeleton in the process of gravitropic tip-growth. Differences in the gravitropic responses of single-cell systems, however, reflect a diversity of gravitropic mechanisms, and represent an example of parallel evolution.
单细胞顶端生长细胞是研究向重力性的优秀实验系统，因为细胞伸长、重力感知和重力反应都局限于顶端圆顶。因此，可以采用各种方法来确定短重力信号转导链的不同步骤，该信号转导链在重力感知细胞和有反应能力的目标细胞之间缺乏信号传递阶段。单细胞系统便于在1g重力刺激、离心、回转和微重力条件下对细胞过程进行体内观察，还能进行荧光标记。这些多样的研究揭示了关于向重力性顶端生长机制的迷人信息，特别是细胞骨架在平衡石定位以及组织和调节 Spitzenkorper 中的重要作用。已经提出了一个解释轮藻假根和轮藻原丝体负向和正向向重力性的假说，该假说强调肌动蛋白细胞骨架在向重力性顶端生长过程中的作用。然而，单细胞系统向重力性反应的差异反映了向重力性机制的多样性，是平行进化的一个例子。
